Clima

Rossby waves

Long, continuous mid-latitude waves around the globe (planetary waves) seen on the 500 mb chart; carry warm air poleward and cold air equatorward and are essential to each hemisphere's energy balance. Fewer, longer waves in winter (about 3-4); more, shorter waves in summer (about 5-7).

Jet streams

Narrow, concentrated bands of fast-moving air in the upper troposphere, about 30,000-40,000 ft up. Polar and subtropical jets; strongest in the cold season. The polar jet is the most important for upper-air maps and marks the storm track.

Planetary vorticity

Spin due to the Earth's rotation, proportional to the Coriolis effect: f = 2 Omega sin(latitude). Zero at the equator, maximum at the poles; increases with latitude.

Relative vorticity

Spin from the air's own motion. Two parts: curvature + shear. Positive = counterclockwise (cyclonic, low pressure/trough) in the Northern Hemisphere; negative = clockwise (anticyclonic, high pressure/ridge).

Meridional flow

Upper-level flow with large north-south swings (big troughs and ridges). Greatest change between positive and negative vorticity.

Zonal flow

Flat upper-level flow, west to east, parallel to lines of latitude. Small change between positive and negative vorticity.

Baroclinicity

Intermixing of steep temperature gradients over a small region. A baroclinic zone interacting with an upper-level trough brings stormy weather.

Surface current

Ocean current driven by surface winds that extends about 100 m below the surface.

Ocean gyre

Large circular motion of surface currents within an ocean basin. Clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere.

Ekman spiral

Water below the surface doesn't move with the surface wind because of friction; Coriolis turns each deeper layer. Net water transport is 90 degrees to the right of the wind (Northern Hemisphere). Helps cause upwelling and downwelling.

Upwelling

Movement of colder, more saline, nutrient-rich water toward the surface. Occurs along coasts when Ekman transport moves water away from shore, and where two currents diverge. Example: the Humboldt Current off Peru.

Downwelling

Surface water sinks. Occurs when wind pushes water against a coastline and where two surface currents converge. Supports high ocean temperatures.

Thermohaline

Deep-ocean flow driven by temperature and salinity differences (density). Cold, salty water sinks. The thermohaline circulation (Stommel conveyor) links all ocean basins and moves heat around the globe.

El Nino

Originally the yearly warming of the ocean off the west coast of South America around Christmas. Now an unusually extreme rise in sea surface temperatures in the central/eastern equatorial Pacific: weak trade winds, warm water shifts east, upwelling decreases off South America. Negative Southern Oscillation Index.

Southern Oscillation

Seesaw in sea-level pressure between the eastern and western equatorial Pacific. Linked to El Nino/La Nina as ENSO. Index = Tahiti pressure minus Darwin pressure, standardized (0 neutral, + La Nina, - El Nino).

Teleconnection

Recurring, persistent, large-scale pattern of pressure and circulation anomalies spanning vast areas (seesaws in sea-level pressure). Impacts temperature, precipitation, storm track, and jet stream location/strength.

Pacific Decadal Oscillation

North Pacific pattern similar to El Nino but more layered. Positive (warm): warm water off North America, cold off Japan/central North Pacific. Negative (cold): the reverse. Cycles over decades.

Atlantic Multidecadal Oscillation

Cycling of warm and cold North Atlantic surface water over about 15-30 years (per class slides). Warm phase: high SSTs, more hurricanes, Midwest droughts. Cold phase: less severe Midwest moisture deficit.

North Atlantic Oscillation

North-south pressure seesaw between the Icelandic low and the Bermuda-Azores high. Positive: strong westerlies, warmer eastern North America. Negative: weaker jet, blocking, cooler eastern North America, cold and snowy East Coast. Can flip quickly.

Arctic Oscillation

Pressure seesaw between the Arctic and mid-latitudes over the Atlantic and Pacific. Positive: low Arctic pressure, strong jet, cold air stays north. Negative: high Arctic pressure, weak wavy jet dips south, colder North America and Europe.

Pacific North American oscillation

Pressure pattern between the Aleutian low and the high over Hawaii/the Rockies. Positive: warmer western Canada/western US, cooler south-central and southeast US, tends to go with El Nino. Negative: reverse, tends to go with La Nina.

Climate model

Mathematical or software-based method that simulates interactions between air, water, and land. Ranges from simple energy balance models to complex general circulation models (GCMs).

Positive feedback loop

A change causes further changes of the same kind, amplifying the original change. Example: ice-albedo (warming melts ice, albedo drops, more absorbed sunlight, more warming).

Negative feedback loop

A change triggers a response that reduces or dampens the original change.

Energy balance model

Model of how energy gained and lost sets surface temperature. Zero-dimensional (global slab, pole to pole) or one-dimensional (latitude only). Strengths: simple, fast, shows variable interactions. Weaknesses: no seasonal variation, latitude-only transfer, global scale only.

Temporal resolution

How finely a model resolves time; the time step is how far ahead each model run steps. Smaller time steps need more computing power but may be more accurate.

Spatial resolution

How finely a model resolves space (like pixels on a screen). Finer resolution means more grid boxes per unit area and more detail, but needs more computing power.

Swamp model

Coupled atmosphere-ocean model in which water has a different albedo but cannot store heat. (Compare slab model: constant depth, can store heat.)

Squall line thunderstorms

Line of thunderstorms along a front or dry line in the mid-latitudes; a common source of tornadoes in North America.

Tornado alley

Midwest/Great Plains region most affected by tornadoes in North America; peak in the late afternoon.

Dixie alley

Southern US region heavily affected by tornadoes; they can occur at any time of day.

Pineapple express

Storm systems and moisture plume originating near Hawaii that bring heavy precipitation to the West Coast and build the Sierra Nevada snowpack.

Foehn winds

Warm, dry winds on the leeward side of a mountain range, created when air is forced over the mountains and descends. Air cools at the moist rate going up and warms at the dry rate coming down, so it ends warmer than it started.

Chinook winds

Foehn winds off the Rockies moving east into the Great Plains; called 'snow eaters' because they melt snowpack quickly.

Cold air damming

Cold, dense air trapped against the leeward (eastern, in the US) side of a mountain range; leads to more clouds and precipitation.

Loop current

Warm branch of the Gulf Stream in the Gulf of Mexico; helps hurricanes strengthen.

California current

Cold current along the Pacific coast of North America, tied to the Pacific high; cools the coast.

Gulf stream

Warm current along the eastern seaboard of North America; in winter helps fuel nor'easters.

Labrador current

Cold current flowing south along the east coast of Canada and New England; can cause coastal fog.

Continental high pressure

Cold, dense high-pressure air mass that moves into the Midwest in winter when the polar jet dips south (about 53% of winters); brings extreme cold, such as January 1994.

Alberta clipper

Fast-moving winter low (also called a Colorado wave cyclone) that comes down the leeward side of the Rockies; brings limited snow.

Lake effect snow

Heavy snow on the leeward side of the Great Lakes when cold air flows over still-warm, unfrozen lakes; occurs late fall to early winter.

Great Lakes

Moderate temperatures for lakeside cities, produce lake effect snow on the leeward side, and support fruit growing (cherries, grapes).