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General Circulation of the Atmosphere Tropical heating drives Hadley cell circulation Warm wet air rises along the equator Transfers water vapor from tropical oceans to higher latitudes Transfers heat from low to high latitudes The Hadley Cell Along equator, strong solar heating causes air to expand upward and diverge to poles Creates a zone of low pressure at the equator called Equatorial low Intertropical Convergence Zone (ITCZ) The upward motions that dominate the region favor formation of heavy rainfall ITCZ is rainiest latitude zone on Earth Rains 200 days a year – aka. doldrums General Circulation Air parcels rise creating low pressure Heat and expand Become humid Transfer heat (sensible & latent) to poles Transfer of moisture towards poles In mid latitudes Dry air sinks creating high pressure Air flows away from high pressure General Circulation Hadley cell circulation creates trade winds Dry trade winds move from subtropics to tropics and pick up moisture Trade winds from both hemispheres converge in the ITCZ Trade winds warm and rise Contribute to low pressure and high rainfall in the ITCZ Monsoons – Circulation in ITCZ ITCZ shifts with seasons Circulation driven by solar heating Circulation affected by seasonal heat transfer between tropical ocean and land Heat capacity and thermal inertia of land < water Summer Monsoon Air over land heats and rises drawing moist air in from tropical oceans Winter Monsoon Air over land cools and sinks drawing dry air in over the tropical oceans Circulation at Mid & High Latitudes Sinking air from Hadley circulation creates high pressure in subtropics Circulation modified Coriolis effect Monsoons Flow of cold air from high latitudes Coriolis Effect Air moving from high to low pressure is deflected by Earth’s rotation Clockwise rotation in the northern hemisphere Counterclockwise rotation in the southern hemisphere Ocean Circulation? Circulation in the troposphere is caused by atmospheric pressure gradients Result from vertical or horizontal temperature differences Temperature variations caused by latitudinal differences in solar heating Ocean surfaces are heated by incoming surface radiation Do the oceans circulate for the same reason as the atmosphere? No! 90% of solar radiation that penetrates oceans absorbed in upper 100 m Warm water at surface is less dense than the colder water below Water column is inherently stable Very little vertical mixing Water has a high heat capacity Lots of heat required for a small change in temperature Lateral temperature and salinity differences are small over large areas Ocean Circulation Ultimately driven by solar energy Distribution of solar energy drives global winds Latitudinal wind belts produce ocean currents Determine circulation patterns in upper ocean Distribution of surface ocean temperatures strongly influence density structure Density structure of oceans drives deep ocean circulation Negative feedback Surface temperature gradients drive circulation Net effect is to move warm water to poles and cold water towards tropics Heat Transfer in Oceans Heating occurs in upper ocean Vertical mixing is minimal Average mixed layer depth ~100 m Heat transfer from equator to pole by ocean currents Oceans redistribute about half as much heat at the atmosphere Surface Currents Surface circulation driven by winds As a result of friction, winds drag ocean surface Water movement confined to upper ~100 m Although well-developed currents ~1-2 km Examples, Gulf Stream, Kuroshiro Current Coriolis effect influences ocean currents Water deflected to right in N. hemisphere Water deflected to left in S. hemisphere Eckman Spiral Eckman theory predicts 1) surface currents will flow at 45° to the surface wind path 2) flow will be reversed at ~100 m below the surface 3) flow at depth will be considerably reduced in speed Few observations of true Eckman Spiral Surface flow <45°, but still to an angle Eckman Transport Observations confirm net transport of surface water is at a right angle to wind direction Net movement of water referred to as Eckman Transport Gyre Circulation Wind driven and large scale Sea level in center 2 m higher than edge Eckman transport producing convergence Circulation extends to 600-1000 m Volume of water moved is 100 x transport of all Earth’s rivers Flow towards equator balanced by flow toward pole on westward margin In Atlantic, by Gulf Stream and North Atlantic Drift Downwelling In areas of convergence Surface water piles up in center of gyre Sea level in the center of gyre increases Surface layer of water thickens Accumulation of water causes it to sink Process known as downwelling Equatorial Divergence Areas of the ocean where divergence of surface currents occurs Equatorial divergence (e.g., Atlantic) In N. hemisphere, NW trades result in westward flowing N. equatorial current Eckman transport moves water to N In S. hemisphere, SW trades result in westward flowing S. equatorial current Eckman transport moves water to S Divergence occurs along the equator Equatorial Upwelling As surface water diverges, sea level falls, surface layer thins and cold water “upwells” Eckman Transport Along Coasts Winds along a coast may result in Eckman transport that moves water towards or away from the coast Divergence from easterly winds and southward moving currents SW coast of N. America W coast of N. Africa Divergence from northward moving currents West coasts of S. America and S. Africa Coastal Upwelling Coastal divergence results in upwelling as cold water rises to replace surface water Geostrophic Currents Eckman transport from wind-driven currents piles water up in gyre center Gravity pulls water down slope Slope is opposite to Coriolis effect Net effect is flow 90° to slope Result is a geostrophic current Geostrophic currents push water in the same direction as the wind-driven flow Boundary Currents Gyre circulation pushes water to the west Flow of water around gyres is asymmetric In the western part of gyre water is confined to a narrow fast-moving flow Western boundary current In the eastern part of gyre flow is diffuse, spread out and slow Eastern boundary current Eastern currents tend to be divergent Eckman transport away from continent Gulf Stream Western boundary current in Atlantic Narrow, fast-moving from Cuba to Cape Hatteras Decreases speed across N. Atlantic Flow broadens and slows becoming N. Atlantic Drift Movement to the south along the Canary Current is very slow, shallow and broad Deep Ocean Circulation Driven by differences in density Density of seawater is a function of Water temperature Salinity Quantity of dissolved salts • Chlorine • Sodium • Magnesium • Calcium • Potassium Thermohaline Circulation Deep ocean circulation depends on temperature (thermo) & salinity (hals) Controls seawater density Density increases as: • Salinity increases • Temperature decreases Horizontal density changes small Vertical changes not quite as small Water column is stable Densest water on bottom Flow of water in deep ocean is slow However, still important in shaping Earth’s climate Vertical Structure of Ocean Surface mixed layer Interacts with atmosphere Exchanges kinetic energy (wind, friction) and heat Typically well mixed (20-100 m) Vertical Structure of Ocean Pychnocline (~1 km) Zone of transition between surface and deep water Characterized by rapid increase in density Some regions density change due to salinity changes – halocline Most regions density change due to temperature change – thermocline Steep density gradient stabilizes layer Bottom Water Formation Deep-ocean circulation begins with production of dense (cold and/or salty) water at high latitudes Ice formation in Polar oceans excludes salt Combination of cold water and high salinity produces very dense water Dense water sinks and flows down the slopes of the basin towards equator Antarctic Bottom Water (AABW) Weddell Sea major site of AABW formation AABW circles Antarctica and flow northward as deepest layer in Atlantic, Pacific and Indian Ocean basins AABW flow extensive 45°N in Atlantic 50°N in Pacific 10,000 km at 0.03-0.06 km h-1; 250 y North Atlantic Deep Water (NADW) Coastal Greenland (Labrador Sea) site of NADW formation NADW comprises about 50% of the deep water to worlds oceans NADW in the Labrador Sea sinks directly into the western Atlantic NADW forms in Norwegian Basins Sinks and is dammed behind sills • Between Greenland and Iceland and Iceland and the British Isles NADW periodically spills over sills into the North Atlantic Deep Atlantic Water Masses Deep Atlantic water comes from high latitude N. Atlantic, Southern Ocean and at shallower depth, the Mediterranean Sea AABW and NADW Interact NADW flowing south in the Atlantic joins the Antarctic Circumpolar Current NADW and AABW combine Spin around Antarctica Eventually branch off into the Pacific, Indian and Atlantic ocean basins Ocean Circulation Surface water at high latitudes forms deep water Deep water sinks and flows at depth throughout the major ocean basins Deep water upwells to replace the surface water that sinks in polar regions Surface waters must flow to high latitudes to replace water sinking in polar regions Idealized circulation – Thermohaline Conveyer Belt Thermohaline Conveyor Belt NADW sinks, flows south to ACC and branches into Indian and Pacific Basins Upwelling brings cold water to surface where it eventually returns to N. Atlantic Ocean Circulation and Climate Warm surface waters move from equator to poles transferring heat pole-ward and into the deep oceans Oceans vast reservoir of heat Water heats and cools slowly Pools of water warmer than normal heat the atmosphere Pools of water colder than normal cool the atmosphere Timescale of months to years Time needed for heating/cooling of water Ocean Circulation and Climate On long timescales, average ocean temperature affects climate Most water is in deep ocean Average temperature of ocean is a function of Process of bottom-water formation Transport of water around ocean basins Deep water recycle times is ~1000 y Thermohaline circulation moderates climate over time periods of ~ 1000 y Ice on Earth Important component of climate system Ice properties are different from water, air and land Two important factors affecting climate High albedo Latent heat stored in ice Sea Ice Salt rejection during sea ice formation Important for bottom water formation Sea ice stops atmosphere from interacting with surface mixed layer Sea Ice Distribution Most sea ice in Southern Ocean Enormous amount form and melt each season Average thickness ~1 m Landmasses in Arctic prevent sea ice movement Arctic sea ice persists for 4-5 years Reach thickness of 4 m in central Arctic and 1 m on margins Glacial Ice Mountain glaciers Equatorial high altitude or polar lower altitude Few km long, 100’s m wide and 100’s m thick Glacial Ice Continental ice sheets Large ice cube Existing ice sheets Antarctica and Greenland • ~3% of Earth’s surface or 11% of land surface • 32 million km3 (= 70 m of sea level)
