Atmospheric and Ocean Circulation

Ocean Circulation via Heating by the Sun

Surface layer: drives atmospheric circulation, weather and climate. Winds drive ocean surface currents and waves in the top 1000 m. Deep layer: Atmospheric cooling and evaporation drives deep ocean currents between 1,000m and 10,000m deep. The overturning circulation.

Ocean Circulation via Gravitational Attraction

Gravity (plus inertia) from the gravitational pull of the mood and sun forces ocean tides and tidal currents.

Ocean Circulation via Tectonic Activity

Displacements of large volumes of fluid via earthquakes (tsunamis)

Pycnocline

A density boundary separating the surface of the ocean and the deep zone.

Heat Budget of Earth

The amount of incoming solar radiation reflected and absorbed by the Earth and the amount of Earth radiation radiating out into space helping balance the inputs and outputs and achieving approximate thermal equilibrium.

Short-Wavelength Solar Radiation

Radiation emitted by the sun, about 30% is reflected back into space by reflective surfaces, the remaining 70% is absorbed by the land, oceans, and atmosphere.

Longer-Wavelength Radiation

Radiation emitted by Earth into the atmosphere and space, helping balance the incoming SWR.

Seasons

Occur on Earth because its axis is tilted 23.5 degrees away from perpendicular and the tilt remains fixed as Earth moves around the sun.

Incoming SWR Varies with Latitude

Due to Earth’s spherical shape. Toward the poles, sunlight strikes Earth at a shallow angle and is spread over a large area, more sunlight is also reflected by the atmosphere and Earth’s surface. Toward the equator, sunlight strikes the Earth at a steep angle, delivering more heat per unit area.

Earth’s Heat Gains and Losses

Toward the equator, there is a net surplus of heat, with incoming short wave radiation greater than outgoing longer wave radiation, resulting in net heat gain. Toward the poles, there is a net deficit of heat, with incoming short wave radiation lesser that outgoing longer wave radiation, resulting in net heat loss.

Convection Currents

Air subject to heat becomes less dense and rises and air subject to cooling becomes more dense and sinks,. Rising air is replenished from the surface.

A Simple Model of Atmospheric Circulation

Surface winds blow down the pressure gradient from high to low pressure. The uneven heat budget with latitude results in a latitudinal heat transfer with warm air rising at the equator and cool air sinking at the poles. This is an atmospheric convection current.

Coriolis Effect

The rotation of the Earth on its axis deflects moving fluid toward the right in the Northern Hemisphere and the left in the Southern Hemisphere, resulting in curved paths. Because the surface of the Earth rotates faster at the equator than at the poles.

Hadley Cells

0-30 degrees of latitude. Warm, moist air converges and rises in the tropics, causing cloud formation and rainfall. Cool, dry air flows poleward at high altitude, it is deflected by the Coriolis effect to blow west to east, Upon reaching 30 degrees of latitude, the air has cooled sufficiently to sink. Air then flows back toward the equator, deflected east by the Coriolis effect, creating the trade winds.

Ferrel Cells and Polar Cells

30-60 and 60-90 degrees of latitude. In each hemisphere, on the poleward side of the Hadley cells, are a second pair of circulation cells known as the Ferrel Cells, where the surface winds are the westerlies. The third major region of atmospheric circulation cells, the Polar cells, lie on the poleward side of the Ferrel cells, where the surface winds are the polar easterlies.

Ekman Transport

Wind blowing across the ocean surface affects surface water, which drags on the water immediately beneath, setting it in motion. Internal friction reduces wind’s effect with depth, and the Coriolis effect shifts each successively deeper layer further to the right in NH and left in SH. The net result is an Ekman spiral, the average flow over the full depth of the spiral is Ekman transport, which occurs 90 degrees to the wind direction.

Geostrophic Gyres

Ekman transport piles water toward central regions of gyres, producing a broad mound of water. The interaction of the pressure gradient and Coriolis force causes water to flow in a circular pattern around this mound. Gyres flow clockwise in the NH and counter clockwise in the SH.

Gyres

Large surface currents moving in circular circuits along the periphery of major ocean basins.

Western Boundary Currents

Narrow, warm, deep, swift currents transporting hundreds of kilometers per day. Forming from a sharp boundary with a coastal circulation system; little or no coastal upwelling; waters tend to be depleted in nutrients and unproductive; waters derived from trade-wind belts

Eastern Boundary Currents

Broad, cold, shallow, slow currents transporting only tens of kilometers per day. Forming from diffuse boundaries separating from coastal currents; coastal upwelling common; waters derived from mid-latitudes.

Coastal Upwelling

Caused by the upward slope of the continental shelf and wind blowing from the North. Resulting in Ekman Transport to the west and raising of the thermocline as cold, nutrient dense water rises to replace the surface water that is pushed away from the coast.

Coastal Downwelling

Caused by the upward slope of the continental shelf and wind blowing from the South. Resulting in Ekman Transport to the east and lowering of the thermocline as warm water is forced down.

Equatorial Upwelling

Cold, nutrient dense water rises to replace the evaporating hot water at the surface, facilitated by Southeast trade winds.

Transverse Boundary Currents

Currents which flow from East to West and West to East, linking to Eastern and Western boundary currents.

Waves

Move energy across the sea surface, we can describe them based on wavelength, period, height, and velocity. Created because something (like the wind) displaces the ocean surface and the restoring force (like gravity) overshoots.

Deep Water Waves

Speed depends on wavelength. c = sqrt(g lambda/2 pi) and lambda/d < 2. Where d = water depth, lambda = wavelength, g = 9.8 ms², and pi = 3.14159.

Shallow Water Waves

Speed depends on water depth. c = sqrt(gd) and lambda/d > 20. Where d = water depth, and lambda = wavelength.

Wind wave development depends on…

Wind speed, wind duration, and wind fetch.

Wind Fetch

The length of water over which a given wind has blown without obstruction.

Tsunami

Generated when a large volume of water is displaced (such as by an underwater earthquake or landslide). Due to their huge wavelength (>150 km) compared to seafloor depth (<5000 m), they behave as a shallow water wave.

El Nino Southern Oscillation (ENSO)

A regional-scale, anomalous weather event that results from coupled atmosphere-ocean processes in the tropical Pacific.

ENSO Neutral Phase

Southwest trade winds push surface water west to maintain the West Pacific Warm Pool near Papua New Guinea, with a low thermocline in the west and a high thermocline in the east due to coastal upwelling.

ENSO El Nino Conditions

Southwest trade winds are weakened or reversed, allowing a warm pool to form in the East near South America, with a lower thermocline in the east and higher in the west than neutral conditions.

ENSO La Nina Conditions

Southwest trade winds are strengthened, exacerbating the West Pacific Warm Pool near Papua New Guinea, with a lower thermocline in the west and a higher thermocline in the east than in neutral conditions.

Walker Circulation

The east-west circulation of the atmosphere above the tropical Pacific.

Thermohaline Circulation

The “great ocean conveyor belt” driven by differences in temperature and salinity (and therefore density) of various water masses. Drives the movement of seawater in the bottom 90% of the ocean. Propelled by surface processes (cooling/heat loss, evaporation) that increase the density of surface water, producing deep waters that sink.

The Equilibrium Theory of Tides

Explains tides based on celestial mechanics (the gravity of the moon and sun, and their motion relative to one another). Examines the balance and effects of forces that allow our planet to orbit around the sun, or the moon to orbit Earth. Because of its nearness to Earth, our moon has greater influence on the tides than the sun.

The Dynamic Theory of Tides

Explains tides based on celestial mechanics and the characteristics of fluid motion. Treats the tide as a wave and takes into account seabed contours and inertia.

Amphidromic Points

Nodes at the center of ocean basins; these are no-tide points.

Tides

Periodic changes in ocean surface height. Forced waves formed by gravity and inertia.