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159 Cards in this Set
- Front
- Back
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Thunderstorms – ordinary cell – Formation
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warm, humid air rises in conditionally unstable atmosphere where vertical wind shear is weak.
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Thunderstorms – ordinary cell – duration/weather
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Short-lived, dissipate in less than an hour. Rarely produce severe weather
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Thunderstorms – life cycle
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growth (cumulus), maturity, dissipation (decay)
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Thunderstorms – ordinary cell- cumulus stage
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Rising parcel of warm, humid air condenses into single or cluster of cumulus clouds. The formation of the clouds causes condensation of water vapor into water/ice, releasing large quantities of latent heat. Heat keeps the rising air inside the cloud warmer (less dense) than the air surrounding it.
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Thunderstorms – ordinary cell – mature stage
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Downdraft marks the mature stage. Downdraft and updraft within mature storm constitutes the cell. The storm is most intense during mature stage. Updraft and downdraft strongest in the middle of the cloud, creating severe turbulence. Lightning and thunder are present.
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Thunderstorms – ordinary cell – dissipation
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Dissipation occurs when the downdraft falls into the updraft, which cuts off the storm's fuel supply.
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Thunderstorms – multicell – formation
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Thunderstorms that contain a number of cells, each in a different stage of development. Form in a region of moderate-to-strong vertical wind speed shear, which causes the cell inside the storm to tilt in such a way that the updraft actually rides up and over the downdraft. Tilted shape causes new cells to form as old ones die out.
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Thunderstorms – multicell – complexes
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Squall line (a long line of thunderstorms that form along or out ahead of a frontal boundary) and Mesoscale Convective Complex (a large circular cluster of thunderstorms).
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Thunderstorms – multicell – severity
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Depends on convection and longevity – stronger convection and longer duration = greater chances of the thunderstorm becoming severe.
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Thunderstorms – muilticell – gust front
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Created when the cold downdraft reaches the earth's surface and it pushes outward in all directions, producing a strong gust, represents the leading edge of the cold outflowing air.
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Thunderstorms – multicell – gust front clouds
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Shelf cloud (also called an arcus cloud, forms as wam, moist air rises along gust front), roll clouds (an elongated ominous-looking cloud just behind the gust front, spin around a horizontal axis).
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Thunderstorms – multicell – downbursts
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Downdraft beneath an intense thunderstorm may become localized so that it hits the ground and spreads horizontally in a radial burst of wind, called downbursts. A downburst with winds extending only 4 km or less is termed a microburst. Winds as high as 146 knots.
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Thunderstorms – multicell – squall line
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Multicell thunderstorms formed as a line of thunderstorms directly along a cold front (hundreds of kilometers), or form in the warm air 100 to 300 km out ahead of the cold front (prefrontal squall-line thunderstorms).
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Thunderstorms – multicell – prefrontal squall line storms
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The largest and most severe type of squall line, with huge thunderstorms causing severe weather over much of its length.
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Thunderstorms – multicell – bow echo
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A bow-shaped squall line on radar that results from strong winds pushing the squall line outward in a bow shape.
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Thunderstorms – multicell – derecho
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Windstorm that forms when the straight-line winds extend for several hundred kilometers along the squall line's path, typicall form in the early evening and last throughout the night.
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Thunderstorms – multicell – MCC
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Under conditions favorable for convection, a number of individual multicell thunderstorms can grow in size and organize into a large circular convective weather system called Mesoscale Convective Complexes (MCCs). As much as 1000 times larger than an individual ordinary cell thunderstorm, can cover an entire state, an area in excess of 100,000 square kilometers
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Thunderstorms – multicell – MCC duration
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Individual thunderstorms work together to generate a long-lasting weather system (>12 hours), moves slowly (< 20 knots). The circulation of the MCCs supports the growth of new thunderstorms as well as a region of widespread precipitation.
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Thunderstorms – multicell – MCC formation
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Tend to form during the summer, where the upper-level winds are weak, often beneath a ridge of high pressure. If a weak cold front should stall beneath the ridge, surface heating and moisture may generate thunderstorms on the cool side of the front.
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Thunderstorms – multicell – MMC peak intensity
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Maximum intensity in the early morning hours, which is partly due to the fact that the low-level jet reaches its maximum strength late at night or in the early morning.
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Thunderstorms – supercell – formation
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Strong vertical wind shear (both speed and direction shear) causes thunderstorms to form such that outflow of cold air from the downdraft never undercuts the updraft. Wind shear may be strong enough to create horizontal spin, which, when tilted into the updraft, causes rotation. Rotating aspectcan lead to tornadoes.
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Thunderstorms – supercell – definition
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A large, long-lasting thunderstorm with a single violently rotating updraft.
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Thunderstorms – supercell – types
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Classic (CL) (heavy rain, large hail, high surface winds, and most tornadoes), High Precipitation (HP) (heavy precipitation and large hail, which appears to fall in the center of the storm, extreme downdrafts, flash flooding), Low Precipitation (LP) (little precipitation).
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Thunderstorms – supercell – mesocyclone
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The rotating air column on the south side of the storm, usually 5 to 10 kilometers across
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Thunderstorms – supercell – precipitation pattern
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No precipitation in rotating updraft – too strong, leads to rain-free base beneath the updraft. Strong southwesterly winds aloft blow precipitation northeastward. Large hails fall just north of the updraft, and the heaviest rain occurs just north of the falling hails, with lighter rain in the northeast quadrant of the storm. If low-level humid air is drawn into the updraft, a rotating cloud, called a wall cloud, may descend from the base of the storm.
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Thunderstorms – supercell – more likely where?
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(a) the winds aloft are strong and change direction from southerly at the surface to more westerly aloft and (b) a low-level jet exists just above the earth's surface.
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Thunderstorms – supercell – why one big storm?
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A shallow inversion separates warm, moist conditionally unstable air below from dry, cold conditionally unstable air above. This lid prevents many small thunderstorms from forming, and saves up energy for one big storm.
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Thunderstorms – supercell – tornado relation
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Strong wind shear near the ground causes air to spin about a horizontal axis, which can be taken up by an updraft into the cloud. Changes of wind direction from southerly to westerly intensifies storm rotation and sets the stage for tornado.
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Thunderstorms – supercell – direction of movement
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Most storms move roughly in the direction of the winds in the middle troposphere; most supercell storms move about 30 degrees to the right to the mean wind in the middle troposphere.
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Tornadoes – definition
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A rapidly rotating column of air that blows around a small area of intense low pressure with a circulation that reaches the ground. A tornado's circulation is present on the ground either as a funnel-shaped cloud or as a swirling cloud of dust and debris.
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Tornadoes – rotation direction
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When viewed from above, the majority of North American tornadoes rotate counterclockwise about their central core of low pressure.
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Tornadoes – size, speed, duration
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Diameter between 100 and 600 m, although some are just a few meters wide and others have diameters exceeding 1600 m, move from the southwest to the northeast at speeds between between 20 and 40 knots, last only a few minutes and have an average path length of about 7 km, but can travel for hundreds of kilometers and exist for many hours.
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Tornadoes – stages
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Major tornadoes evolve through five stages: (1) dust-whirl stage; (2) organizing stage; (3) mature stage; (4) shrinking stage; and (5) decay stage.
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Tornadoes – “outbreak”
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When a large number of tornadoes (typically 6 or more) form over a particular region. Tornado families are different tornadoes spawned by the same thunderstorm. They often are the result of a single, long-lived supercell thunderstorm.
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Tornadoes – prevalence
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US is tops –no country experiences more tornadoes than the US, averages more than 1000 annually.
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Tornadoes – where in the US?
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The Central Plains region is most susceptible to tornadoes; it often provides proper atmospheric setting for the thunderstorms that spawn tornadoes: warm, humid surface air is overlain by cooler, drier air aloft, producing a conditionally unstable atmosphere.
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Tornadoes – seasonal distribution in US
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During winter, tornadoes concentrate over the southern Gulf states. In spring, tornadoes become more prevalent form the southern Atlantic states westward into the southern Great Plains. In summer, tornado activity tends to be concentrated from the northern plains eastward to New York State.
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Tornadoes – wind speeds
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Most powerful have winds up to 220 knots, and most tornadoes less than 125 knots. When a tornado is approaching from the southwest, its strongest winds are on its southeast side.
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Tornadoes – multi-vortex
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Tornadoes that contain smaller whirls that rotate within them; these smaller whirls are called suction vortices.
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Tornadoes – formation in supercells
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Starts out as vortex tubes generated by wind direction shear. Updraft of developing thunderstorm picks up and draws it into the storm to form mesocyclone. When rear-flank downdraft hits the ground, it may sweep around mesocyclone, causing it to shrink horizontally and stretch vertically and form a tornado vortex.
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Tornadoes – non-supercell
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May occur with intense multicell storms as well as with ordinary cell thunderstorms. Tornadoes that form along a gust front are commonly called gustnadoes. Rather weak and short-lived tornadoes that occur with rapidly building cumulus congestus clouds are called landspouts.
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Tornadoes – waterspout
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A rotating column of air that is connected to a cumuliform cloud over a large body of water. Tornadic waterspout is a tornado moved from land into water. Fairweather waterspouts may form over water, especially warm, tropical water.
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Hurricanes – definition
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An intense storm of tropical origin, with sustained winds exceeding 64 knots (74 mi/hr), which forms over the warm northern Atlantic and eastern North Pacific oceans.
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Hurricanes – anatomy
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Eye, eyewall, spiral rain bands, anticyclone flow at top
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Hurricanes – eye
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The area of broken clouds at the center. Within the eye, winds are light and clouds are mainly broken. The surface air pressure is very low.
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Hurricanes – eye – why no clouds in the middle?
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In the vigorous convective clouds of the eyewall, the air warms due to the release of large quantities of latent heat, producing slightly higher pressures aloft, which initiate downward air motion within the eye. As the air descends, it warms by compression, leading to warm air and the absence of convective clouds in the eye of the storm.
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Hurricanes – eye wall
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Adjacent to the eye is the eye wall, a ring of intense thunderstorms around the storm's center that may extend upward to almost 18 km above sea level. Area of heaviest precipitation and the strongest winds.
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Hurricanes – spiral rain bands
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Clouds align themselves into spiraling bands, called spiral rain bands, that swirl in toward the storm's center, wrapping themselves around the eye. Surface winds increase in speed as they blow counterclockwise and inward toward this center. (In the SH, the winds blow clockwise around the center).
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Hurricanes – anticyclone flow of air at top
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Near the top of the clouds, the relatively dry air, having lost much of its moisture, flows outward away from the center, producing anticyclonic flow of air several hundred kilometers from the eye. As this outflow reaches the storm's periphery, it begins to sink and warm, inducing clear skies.
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Hurricanes – key structural features
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Boundary layer inflow, eyewall, cirrus shield, rainbands, and upper tropospheric outflow. Eye present in stronger storms.
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Hurricanes – direction of rotation
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tropical cyclones spin counter-clockwise in the NH and clockwise in the SH, with corresponding variations in their spiral rainband structure
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Hurricanes – boundary layer
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Only approximately isothermal inside strong pressure gradient/high wind region (about 1.25° radius from storm center). Outside this region, near-surface air in boundary layer cools more than expected from adiabatic expansion – hurricane boundary layer is neither isothermal nor isentropic between about 1.25° and 3.0° radius. Well defined jet structures in the storm boundary layer.
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Hurricanes – formation – key points
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Tropical waters, light winds, high humidity in a deep layer extending up through the troposphere.
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Hurricanes – formation – surface water temp
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Warm, typically 26.5°C (80°F) or greater, over a large area to a depth of 60m.
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Hurricanes – formation – “triggers”
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Hurricanes require a "trigger" to start air converging. Waves along ITCZ (trigger low pressure to start convection), pre-existing atmospheric disturbance.
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Hurricanes – formation – latitude
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Rotation needs coriolis force, therefore hurricanes form in tropical regions between 5° and 20° latitude.
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Hurricanes – conditional instability
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Hurricanes do not form under trade wind inversion and where upper-level winds are strong.
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Hurricanes – developing storm
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Cluster of thunderstorms rotate around a low pressure. Release of latent heat warms the air aloft and causes divergence.
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Hurricanes – dissipation
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Poleward movement leads to the decay of the system as it encounters strongly sheared environment and cooler waters (or land) of the midlatitudes.
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Hurricanes – extratropically transitioning cyclones
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When tropical cyclones move out of the tropics into mid latitudes. The transport of moist warm tropical air to high latitudes provides an atypically large energy reservoir for a warm season midlatitude cyclone.
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Hurricanes – ET and planetary wave patterns
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The energy reservoir provided by the moist warm tropical air at high latitudes can amplify midlatitude planetary wave pattern, causing intense midlatitude cyclogenesis far from the location of the ET event. It can also feed back onto the longwave pattern, modifying interactions between other tropical cyclones and troughs.
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Hurricanes – ET occurrence
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Indian Ocean, rare, 10% of TCs. North Atlantic, more common, 46% of TCs, Western North Pacific, somewhat rare, 27%, western South Pacific somewhat rare, 33%. Note that in each basin, the likelihood of ET increases through the storm season, with the peak in ET frequency lagging the peak month for TC activity.
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Hurricanes – ET structural changes to the storm
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Peak low-level winds reduced, maximum winds now occur aloft, and symmetric cloud structures become asymmetric with strong winds equatorward and a large expanse of heavy precipitation to the poleward side.
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Ozone in the earth's atmosphere – where
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Stratosphere: Absorbs part of the ultraviolet component of solar radition (that would damage biological systems). Urban areas near the ground: Can cause respiratory problems when present in large amounts.
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Ozone – production in the stratosphere
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Two-step process: 1. ultraviolet sunlight breaks apart an oxygen molecule to form two separate oxygen atoms.2. each atom then undergoes a binding collision with another oxygen molecule to form an ozone molecule. In the overall process, three oxygen molecules plus sunlight react to form two ozone molecules.
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Ozone – stratospheric chemistry in pure-oxygen atmosphere
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Chemical theory consisting of four major reactions (proposed by Sydney Chapman in 1930).
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Ozone – pure oxygen R1
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R1 = photodissociation of oxygen = O2 + hv --> O + O (hv = photon), wavelength must be < 240 nm
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Ozone – pure oxygen R2
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R2 = three-body reaction btw O, O2 and another molecule, M= O + O2 + M --> O3 + M identity of M unimportant, only needs to remove energy
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Ozone – pure oxygen R3
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R3 = photodissociation of O3 = O3 + hv --> O2 + O, only efficient at wavelengths < 300 nm
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Ozone – pure oxygen R4
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R4 = O and O3 react (chemically active “odd” oxygen) to produce stable, long-lived O2 = O + O3 --> O2 + O2
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Chlorofluorocarbons – key points
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Used as refrigerants and propellants in spray cans. Most abundant CFCs CF2CL2 and CFCl3. Long atmospheric lifetimes, allows mixing over the vertical depth of the atmosphere. CFC abundance grew at 5-6% per year during the late 20th century. Abundance is now shrinking in response to the ban on new CFC production.
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CFCs and stratospheric ozone
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At altitudes above 30 km, photodissociation of CFCs releases chlorine atoms. Chlorine destroys ozone and atomic oxygen (“odd oxygen”) via a “catalytic cycle.” Net effect of the cycle is: O + O3 --> O2 + O2.
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Catalytic cycle – what is it
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The atom or molecule that initiates the cycle (the “catalyst”) is recreated at the end of the cycle, therefore, catalyst is available to go through the cycle many times (until it is removed by some other process). Allows gases that exist in very small quantities to exert a major influence on the atmospheric ozone abundance.
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Ozone – what and where?
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A gas that is naturally present in the atmosphere. Contains three atoms of oxygen (O3). 10% of atmospheric ozone is in the troposphere, 90% in the stratosphere, primarily between the top of the troposphere and about 50 kilometers (31 miles) altitude (ozone layer).
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Tropospheric oxidants – what are they?
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in descending order of importance: The hydroxyl radical OH, nitrate radical NO3, oxygen atom O(3p), Peroxy and hydroperoxy radicals HO2 and RO2 (where R is an alkyl), Hydrogen peroxide H2O2.
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Tropospheric oxidants – hydroxyl radical
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OH: A short-lived free radical, most effective scavenger in the troposphere. It is the main oxidant for CO, CH4 and higher hydrocarbons, H2S (hydrogen sulfide) and SO2 (sulfur dioxide).
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Tropospheric oxidants – nitrate radical
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NO3: At night, takes over from hydroxyl as dominant oxidant in the atmosphere: hydroxyl is formed by photolysis and its concentration peaks during daytime while NO3 does not survive sunlight.
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Tropospheric oxidants – oxygen atom
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O(3p): This exited state of the oxygen atom has the ability to oxidize unsaturated hydrocarbons and other gases containing a double bond such as CS2 and COS in the upper troposphere.
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Tropospheric oxidants – peroxy and hydroperoxy radicals
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HO2 and RO2 (where R is an alkyl): intertwined with hydroxyl in the oxidation cycle. They are not as efficient as hydroxyl, but react with themselves to form H2O2, an important oxidant in cloud droplets.
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Tropospheric oxidants – hydrogen peroxide
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H2O2: This strong acid reacts very efficiently in cloud droplets and oxidizes a number of trace gases, in particular sulfur dioxide. Highly soluble, it also accounts for a large part of the excess acidity in rain.
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Tropospheric oxidants and trace species
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Oxidants determine the lifetime and the abundance of trace species, act as a atmospheric regulators. The reverse is also true: the abundance of trace species regulate the oxidizing capacity of the atmosphere, since an increase in the emission of a given pollutant reduces the abundance of its principal oxidant. Can cause positive feedback that increases other pollutants.
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Troposphere – primary pollutants
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An undesirable atom or molecule introduced directly into the atmosphere by some form of human activity. CO, oxides of nitrogen, hydrocarbons.
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Primary pollutants – CO
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carbon monoxide. A byproduct of inefficiency in burning fossil fuels (gasoline, natural gas, coal). A problem in indoor air quality.
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Primary pollutants – oxides of nitrogen
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Esp. NO. From high temperature combustion. (N2 and O2 thermally decompose and some of the resulting N and O form NO.)
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Primary pollutants – hydrocarbons
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A large family of molecules that comprise oil-based fuels, released into air via incomplete combustion. Example: C8H18 (octane).
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Aerosol – sources
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Biological, oceans, smoke, solid earth, volcanoes anthropogenic, in-situ
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Aerosol – biological sources
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Solid and liquid particles, which include seeds, pollen, spores, and fragments of animals and plants, are usually 1-250 μm in diameter. Bacteria, algae, protozoa, fungi, and viruses are generally < 1 μm in diameter.
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Aerosol – oceanic sources
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One of the most important sources of atmospheric aerosols. Major mechanism for ejecting ocean materials into the air is bubble bursting.
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Aerosol – smoke source
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Smoke from forest fires is a major source of atmospheric aerosols. Small smoke particles (primarily organic compounds and elemental carbon) and fly ash are injected directly into the air.
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Aerosol – solid earth source
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The transfer of particles to the atmosphere from the Earth’s surface is caused by winds and atmospheric turbulence. On the global scale, semiarid regions and deserts are the main source of particles from the Earth’s surface.
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Aerosol – volcanic source
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Volcanoes inject gases and particles into the atmosphere. The large particles have short residence times, but the small particles (produced primarily by gas-to-particle (g-to-p) conversion of SO2) can be transported globally, particularly if they reach high altitudes.
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Aerosol – anthropogenic sources
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The global input of particles into the atmosphere from anthropogenic activities is ~20% (by mass) of that from natural sources. The main anthropogenic sources of aerosols are dust from roads, wind erosion of tilled land, biomass burning, fuel combustion, and industrial processes.
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Aerosol – in situ formation
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In situ condensation of gases (i.e., g-to-p conversion). Three major families of chemical species are involved in g-to-p conversion: sulfur, nitrogen, and organic and carbonaceous materials.
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Aerosol – effects on radiative forcing
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direct effect and indirect effects
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Aerosol – direct effect on RF – definition
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The direct effect is the mechanism by which aerosols scatter and absorb shortwave and longwave radiation, thereby altering the radiative balance of the Earth-atmosphere system.
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Aerosol – direct effect on RF – key parameters
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Aerosol optical properties (the single scattering albedo, specific extinction coefficient, and the scattering phase function), which vary as a function of wavelength and relative humidity, and the atmosphereic loading and geographic distribution of the aerosols in the horizontal and vertical, which vary as a function of time.
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Aerosol – direct effects on RF – positive and negative direct RF effects
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Scattering aerosols = net negative direct RF. Partially absorbing aerosols = negative top-of-the-atmosphere (TOA) direct RF over dark surfaces such as oceans or dark forest surfaces, but a positive TOA RF over bright surfaces such as desert, snow and ice, or if the aerosol is above cloud.
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Aerosol – direct effect on RF – consequences
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Both positive and negative TOA direct RF mechanisms reduce the shortwave irradiance at the surface. The longwave direct RF is only substantial if the aerosol particles are large and occur in considerable concentrations at higher altitudes.
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Aerosol – indirect effects
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The mechanism by which aerosols modify the amount, lifetime, and microphysical properties of clouds. Altering the microphysical properties alters the radiative properties.
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Aerosol – indirect effects on RF – key parameters
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Key parameters for determining the indirect effect are the effectiveness of an aerosol particle to act as a cloud condensation nucleus, which is a function of the size, chemical composition, mixing state and ambient environment.
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Aerosol – indirect effects on RF – cloud droplet size effect
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Microphysically-induced effect on cloud droplet number concentration and hence cloud droplet size (liquid water content held fixed). Has been called variously the ‘first indirect effect’, the ‘cloud albedo effect’, or the ‘Twomey effect’.
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Aerosol – indirect effects on RF – liquid water content, cloud height, lifetime of clouds
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Microphysically-induced effect on the liquid water content, cloud height, and lifetime of clouds has been called variously as the ‘second indirect effect’, the ‘cloud lifetime effect’, or the ‘Albrecht effect’.
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Aerosol – indirect effects on RF – semi-direct effect
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Mechanism by which absorption of shortwave by tropospheric aerosols leads to heating of the troposphere that in turn changes the relative humidity and the stability of the troposphere and thereby influences cloud formation and lifetime.
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Teleconection – definition
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Long-distance climatic links. A teleconnection pattern refers to a recurring and persistent, large-scale pattern of pressure and circulation anomalies that spans vast geographical areas. Teleconnection patterns are also referred to as preferred modes of low-frequency (or long time scale) variability.
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North Atlantic Oscillation – what is it?
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Teleconnection pattern: continual oscillation in the difference in atmospheric pressure between the Iceland Low and the Azores High.
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NAO – effects
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Most important factor affecting wintertime climatic conditions over the northern Atlantic Ocean and the Nordic Seas. A strong NAO (positive NAO index) leads to strong westerlies, and warm wet winters in north-west Europe.
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NAO – periodicity
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In the atmosphere, the NAO does not seem to have an obvious periodicity; it is a mix of periodicities. In the ocean, the variability associated with the NAO has a roughly decadal time-scale.
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Equatorial currents – major components
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Westward-flowing North and South Equatorial Currents, one or more eastward-flowing Counter-Currents (surface and subsurface), and eastward-flowing Equatorial Undercurrent, which is generally centered on the Equator. Flow in the North and South Equatorial Currents is partly driven by the Trade Winds and is partly geostrophic flow.
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Equatorial currents – difference between Pacific and Atlantic
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Best developed in the Pacific Ocean, b/c surface waters are under the influence of Trade Winds over great distances. In the Atlantic, equatorial circulation affected by the shape of the ocean basin and, indirectly, by the effect of the continental masses on the ITCZ. In the Indian Ocean, the circulation is monsoonal, most resembling that in the other tropical oceans in the northern winter.
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Equatorial currents – ITCZ, divergence, convergence
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ITCZ generally displaced north of the Equator, so that the South-East Trade Winds blow across it. As a result, divergence of surface waters, and upwelling, occur just south of the Equator. There is a convergence of surface water at about 4˚N.
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Equatorial currents – sea surface and thermocline
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The prevailing easterly winds over the tropical ocean cause the sea-surface to slope up (and the thermocline to slope down) towards the west. As a result, there is an eastward horizontal pressure gradient force and the Equatorial Counter-Current(s) flow(s) down this gradient towards the east in zones of small westward wind stress (the Doldrums).
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Equatorial currents – equatorial undercurrents
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The eastward horizontal pressure gradient produced by easterly winds also drives the Equatorial Undercurrent, which flows in the thermocline below the mixed surface layer. Fast,wider than it is thick. Aligned along the Equator, although it may have long-wavelength undulations; Coriolis force turns it equatorwards if diverted. Significant volume transport, particularly in the Pacific.
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Equatorial currents – upwelling
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In Pacific and Atlantic, areas of upwelling occur just south of the Equator, in association with the South Equatorial Current. Coastal upwelling along eastern boundaries -- either year-round or seasonal -- as a result of the Trade Winds blowing along the shore.
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Equatorial currents – ITCZ and upwelling
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Surface divergence and upwelling may occur below the ITCZ because it is a region of low pressure and cyclonic winds.
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Monsoonal circulation – definition
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Winds over Indian Ocean change seasonally as a result of the differential heating of the ocean and the Asian landmass. North-East Monsoon (northern winter): dry, cool winds from Asia; South-West Monsoon: stronger, winds carry moisture from the Arabian Sea to the Indian subcontinent.
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Monsoonal circulation – sea surface slope
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Winds over the equatorial zone change over the course of the year, so the direction of the sea-surface slope along the Equator changes. As a result, in the Indian Ocean the Equatorial Undercurrent is only a seasonal feature of the circulation.
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Antarctic Circumpolar Current – definition and features
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Major current feature of the Southern Ocean. Deep with large volume transport. Influence of westerly wind stress is balanced mainly by frictional forces generated by the interaction of the ACC with the sea-floor topography.
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Antarctic Circumpolar Current – Antarctic Polar Frontal Zone
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Region where strongest currents in the ACC flow along fronts in the Antarctic Polar Frontal Zone. Current jets often form meanders and eddies. surface water converges and sink.
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Antarctic Circumpolar Current – Antarctic Divergence
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Between the Antarctic Circumpolar Current and the Antarctic Polar Current, region of upwelling.
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Madden-Julian Oscillation
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Identified in early 1970s, surface pressure and atmospheric winds tend cycle at many tropical locations over periods ranging from 30 to 60 days. These variations tie to alternation of broad active and inactive tropical rainfall in both the Northern and Southern hemispheres: a broad area of active cloud and rainfall propagates eastwards around the equator at intervals of between about 30 to 60 days. Rainfall in the near-equatorial regions of the Indian and Pacific Oceans also show a strong association with the disturbance.
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MJO – atmospheric component
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An oscillation propagating eastwards from the Maritime Continent around the equator at about 5 m/s; atmospheric MJO period of roughly 30-60 days. Spatial scale of atmospheric MJO: a local wavelength of roughly 12,000-20,000 km. Best developed in the region from the (southern) Indian Ocean eastward across Australia to the western Pacific Ocean in austral summer. The atmospheric signature is evident in the surface pressure, the lower and upper tropospheric wind strength and proxies for deep convection.
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MJO – oceanic component
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An oscillation with a period of 60-75 days. The oceanic signature evident in the sea surface temperature (SST), mixed layer depth, surface latent heat flux, and surface wind stress fields.
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ENSO – definition
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A coupled atmospheric-oceanic process caused by recurring redistributions of heat and atmospheric momentum in the Equatorial Pacific.
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ENSO – cycle and extent
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Irregular, two to seven-year, cycle and large areal extent (the entire tropical Pacific), global impacts.
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ENSO – El Nino
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The oceanic component of ENSO, characterized by weakening of the trade winds and warming of SSTs in the Equatorial Pacific. Events typically last 9-15 months.
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ENSO – La Nina
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Associated with stronger than normal trade winds and the anomalously cool SST. Less extreme anomaly than El Niño but lasts longer, one to three years. El Niño to La Niña transitions much faster than La Niña to El Niño transitions.
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ENSO – Southern Oscillation
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The atmospheric component of ENSO, involves a seesawing of surface pressure across the Equatorial Pacific and occurring in concert with El Niño. It is not certain whether El Niño causes the Southern Oscillation or vice–versa, they could both rely on and modulate each other. The Southern Oscillation Index (SOI) is the normalized difference in pressure between Darwin, Australia and Tahiti, French Polynesia.
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ENSO – signature
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Evident in the SST, mixed layer depth, upper ocean currents, and surface wind stress fields, and shift in the Walker Circulation.
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Deep circulation – other terms
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(i) Abyssal circulation; (ii) Thermohaline circulation; (iii) Meridional overturning circulation; and (iv) Global conveyor.
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Deep circulation – key concepts, drivers
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Currents below the level of the Ekman spiral driven primarily by differences in density: the densest water will move downhill along the bottom of the ocean. Below a kilometer to depths of 4 km – 5 km. Cold, potential temperature less than 4°C. Water mass formed when cold, dense water sinks from the surface to great depths at high latitudes. It spreads out from these regions to fill the ocean basins. Deep mixing eventually pulls the water up through the thermocline over large areas of the ocean. It is this upwelling that drives the deep circulation.
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Deep circulation – wind and mixing
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Deep circulation is also driven by wind, tidal mixing is important. The wind cools the surface and evaporates water, which determines where deep convection occurs, it produces turbulence in the deep ocean which mixes cold water upward.
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Deep circulation – temperature and salinity
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Temperature and salinity determine the density of sea water, and bottom water in the ocean basins is both cold and salty. Most deep water in the world’s ocean basins originates in the North Atlantic, where warm, salty water of the Gulf Stream cools at high latitudes, resulting in very cold, very salty, sea water. This water then sinks, initiating Meridional Overturning Circulation (MOC).
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Drake Passage Effect
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The latitude band of Drake Passage, between South America and Antarctica, is the only oceanic band that circles the Earth without encountering meridional topographic barriers to a depth of about 2500 m. Because no net meridional geostrophic flow can be sustained, the flow is directly forced by the westerly wind, driving a northward Ekman transport in the ocean’s surface layer of about 30 Sv. The maximum zonal westerly wind stress occurs roughly at 50ºS. Hence south of this area surface divergence leads to upwelling, while north of it there is surface convergence.
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Estuary – definition
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Estuary is a semienclosed body of water having a free connection with the open sea and within which seawater is measurably diluted with freshwater deriving from land drainage.
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Estuary – highly stratified – characteristics
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Develop where rivers discharge into seas with a low tidal range (less than 2 m, e.g. The Mediterranean and Black Sea). Less dense (more buoyant) river water flows over the surface of the underlying denser seawater, forming “salt wedge” that penetrates and thins up-river. Low freshwater discharge = salt wedge further up the estuary. Stable halocline, sharp salinity and density gradients between freshwater and seawater. Shear stresses at the interface between the flowing river water and the salt wedge generate internal waves, which inject small quantities of salt water into the overlying freshwater, making it brackish.
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Estuary – partially mixed – characteristics
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Develop where rivers discharge into sea with a moderate tidal range (about 2-4 m). Water mass moves up and down the estuary with the tides. Friction between the water and the estuary bed causes turbulence which mixes the water column more effectively, the halocline is less well defined, and the stratification is weaker. Wide variety of these types of estuaries based on local characteristics. Spring tides = enhanced mixing, reduced stratification. Neap tides = enhanced stratification.
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Estuary – partially mixed – null point
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The point where the depth at which there is no net landward or seeward movement of water coincides with the bed of the channel – no net movement of water at channel bed (landward meets seaward flow). Moves up and down the estuary with the tides, and over a greater distance during spring than during neap tides.
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Estuary – well mixed – characteristics
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When tidal range is large (larger than 4 m), a well-mixed estuaries result when tidal currents are strong relative to river flow and the whole water column is mixed, so that salinity hardly varies with depth at all.
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Estuary – what determines estuarine type?
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The ratio of tidal flow to river flow: low ratio = highly stratified salt wedge estuary. As the ratio increases the estuary becomes less stratifed, until reach well-mixed state.
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Estuary – tidal flow ratio data
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salt wedge: ≤ 1, partially mixed: 10-103, Well mixed: ≥ 103
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Arctic ocean – characteristics
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Commonly referred to as an ocean, some geographers refer to it as an extension of the Atlantic Ocean. No agreement on its boundaries. Usually has a limited connection the Pacific Ocean through Bering Straits, to the North Atlantic through the Fram Strait. Using the Bering and Fram Straits as bounds, has an area of about 9.5 million sq km. Not well studied or understood. About a third of it is shallow, <200 m deep. The mean surface flow in the Canadian Basin is clockwise, with flow out of the Arctic through the Fram Straits east of Greenland. Water leaving the Fram Straits at the surface is replaced by warmer Atlantic water at depth.
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Arctic ocean – influence of ice
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Ice cover over much of Arctic ocean 3 to 5 m thick means normal processes controlling the air-ocean exchange of heat and momentum are very different. High albedo, nearly all of the energy from the sun that is absorbed is used to heat and/or melt the ice cover. Little water to evaporate, so average latent heat flux is smaller than the sensible heat lost; unlike other areas, where latent heat loss is several times that of sensible heat loss.
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Arctic ocean – ice movement
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The ice sheets are in continual movement, converge to form pressure whose keels extend some 10 to 20 m below the surface, and diverge, leaving narrow leads (poynas), 10 to 1000 m wide. Open water or thin ice leads small percent of surface of the central Arctic Ocean in winter, but heat flux from the ocean to the atmosphere larger than that through the adjacent ice sheets. In midsummer, the percentage of open water may be as high as 20%.
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Arctic ocean – Atlantic warm water
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There is a layer of relatively warm water (3º above freezing) at a depth of about 300 m, from the Atlantic, and it fills much of the Arctic Ocean at about this depth. This water can't reach the surface due to a layer of colder but less saline water.
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Arctic ocean – fresh water layer
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Several sources for fresh water layer: fresh-water runoff from land, ice cover inhibits evaporation so Arctic ocean gets more precipitation per year than it loses in evaporation, and the low-salinity Bering Sea via the Bering Straits. Together these contribute to a lens of low-saline water, on the order of 100 m thick, which covers the surface of the Arctic Ocean.
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Climate system – definition
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The “long-term” statistics of all quantities required to specify the state of the atmosphere. The precise meaning of “long-term” can differ from one context to another. The concept of climate includes a mean state and measures of temporal variability over a wide range of time scales. Climate may refer to spatial scales from local to global.
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Climate system – components
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Components of climate system include: the atmosphere, hydrosphere, cryosphere, lithosphere, and biosphere. The climate system is the totality of these components interacting and exchanging energy and substance with each other.
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Climate system – components – atmosphere
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Comparatively thin film of a gaseous mixture which is distributed almost uniformly over the surface of the earth.
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Climate system – components – hydrosphere
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All water in the liquid phase distributed on the earth. It includes the oceans, interior seas, lakes, rivers, and subterranean waters.
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Climate system – components – cryosphere
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The large masses of snow and ice of the earth's surface. It includes the extended ice fields of Greenland and Antarctica, other continental glaciers and snow fields, sea ice, and permafrost.
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Climate system – components – lithosphere
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The continents whose topography affects air motions, and the ocean floor.
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Climate system – components – biosphere
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The terrestrial vegetation, the continental fauna, and the flora and fauna of the oceans.
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Climate change mechanisms
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Changes in the Solar Constant (very likely on time scales of hundreds of millions to billions of years, significance is controversial over century to decadal time scales), Continental drift and associated changes in ocean currents (on time scales of tens to hundreds of millions of years), Changes in the shape of the Earth’s orbit around the sun and in the direction of the rotation axis in space (on time scales of tens of thousands to a hundred thousand years), Changes in the planetary albedo (on all time scales, including recent decades), Changes in the magnitude of the greenhouse effect (on all time scales, including recent decades)
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Carbon cycle – definition
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The biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth.
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Carbon cycle – reservoirs
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Plants; the terrestrial biosphere, which is usually defined to include fresh water systems and non-living organic material, such as soil carbon; oceans, including dissolved inorganic carbon and living and non-living marine biota; sediments including fossil fuels.
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Carbon cycle – carbon budget
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The balance of the exchanges (incomes and losses) of carbon between the carbon reservoirs or between one specific loop (e.g., atmosphere ↔ biosphere) of the carbon cycle. An examination of the carbon budget of a pool or reservoir can provide information about whether the pool or reservoir is functioning as a source or sink for carbon dioxide.
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Carbon cycle – oceanic carbon dioxide
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The ocean gets a disproportionate share of the carbon dioxide available to the ocean-atmosphere system. The main reason is that carbon dioxide readily reacts with water to make soluble species of ions, “bicarbonate” (formula: HCO3-), rather than trying to fit between the water molecules as a gas. Additional reasons are existence of carbon pumps.
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Carbon cycle – carbon pumps – physical
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The exchange of carbon between the atmosphere and the ocean through physical mixing of the ocean, i.e. vertical deep mixing, gives the ocean a lot more carbon than if equilibrium with the surface ocean were the only mechanism.
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Carbon cycle – carbon pumps – biological
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Removes carbon dioxide from the surface water of the ocean, changing it into living matter and distributing it to the deeper water layers, where it is out of contact with the atmosphere. Thus, when the ocean shares carbon dioxide with the atmosphere, it does so by not only simply taking on carbon dioxide into solution but also by incorporating the carbon dioxide into living organisms.
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