Global Atmospheric & Oceanic Circulation PDF

Global Atmospheric & Oceanic Circulation  Global wind patterns – a system of low and high pressure systems beginning with a low pressure cell at the equator is the catalyst for the prevailing global wind patterns aro...

Global Atmospheric & Oceanic Circulation  Global wind patterns – a system of low and high pressure systems beginning with a low pressure cell at the equator is the catalyst for the prevailing global wind patterns around the world. Hadley Cell – the circulation cell between the tropical low pressure zone and subtropical high pressure zone – is the key to understanding wind patterns Air rises due to intense heating by the sun over Equator forming a Low pressure system at surface General over equator As the air rises it condenses and precipitates Circulation of (leading to heavy rain and clouds in the tropics) High level air is pulled back to Earth’s surface Atmosphere leading to a high pressure, dry air cell sinking in the suptropical zone. 30°N & S air sinks to surface Sinking air → Subtropical high pressure belts converge with Earth’s surface and air returns to the equator to replace the rising air This is the ITCZ and forms the “Trade” winds Trades deflected by Coriolis force, from NE in northern hemisphere, SE in southern hemisphere Hadley Cell  Poleward side of subtropical high pressure belt – the air that does not return to the equator Coriolis force turns diverging air → forms the westerly winds Midlatitudes 30-60° N & S – Midlatitude prevailing winds are "Westerlies" Weather systems move generally west-to-east Poleward Over poles cold air sinks and forms high cell pressure Polar easterlies develop, carrying cold air Polar easterlies intermittent in northern hemisphere Where cold air meets warm air is Polar Front Midlatitudes & Polar Regions Global Circulation in an Ideal World with no land Real World Global Circulation (ITCZ) January Sun on southern hemispere ITCZ swings south July Sun on northern hemisphere ITCZ swings north, esp. over Asia Monsoon - Seasonal wind shift over Asia Most of the year air is rushing from Russia to the warm, lifting air over the Indian and Eastern Pacific Ocean. But, for a short period in the summer, the sharp summer heat of Russia leads to a reversal of winds, where the warm, humid air over the Indian ocean sweeps across India and SE Asia. When the warm, wet air hits the Himalayas heavy rain hits South and Southeast Asia – this is Monsoon season Monsoon means: Reversal of wind in Arabic Summer – intense heating over Asia creates thermal low ITCZ is pulled north into Asiatic low (25°N) Moist, onshore winds bring precipitation to South & SE Asia  Winter – Asia cools rapidly, creating high pressure Called Siberian High ITCZ retreats far into southern hemisphere Cool, dry offshore wind dominates South & SE Asia  Diverging air drives trade winds & westerlies Subtropical 4 persistent STH in southern hemisphere High (STH) In northern hemisphere, 2 major STH: Hawaiian High (Pacific Ocean) Azores High (Atlantic Ocean) Pressure STH intensify and grow in summer b/c cooler over oceans Belts Eastern side of STH (SW US, NW Africa) drier Western side of STH (SE US, SE China) moister  In northern hemisphere, land cold in winter Higher pressure over land, lower pressure over ocean Higher North Pacific dominated by Aleutian Low North Atlantic dominated by Icelandic Low Latitude In southern hemisphere, Antarctica Circulation always cold Always high pressure over Antarctica Seas around Antarctica low pressure, rough sailing July similar to January Winter at Higher Latitudes January views from above poles show: Persistent Antarctic High Icelandic Low Aleutian Low  Warm air in tropics → high pressure aloft Winds Aloft: Cold air over poles → low pressure aloft AKA upper winds, Isobaric surfaces – pressure levels upper-level winds Winds Aloft Coriolis force turns wind aloft toward east Called upper-level westerlies, from 30° to polar low Strongest in midlatitudes 35- 55°N & S Also, over equator and tropics, weak easterlies  Jet stream – narrow, intense flow within westerlies Where pressure & temp gradient is strongest Jet Streams & Polar May reach 250 mph at 30,000-40,000 ft altitude Front Most poleward jet stream called polar jet 2nd jet stream called subtropical jet – intermittent  Disturbances in Jet Stream A – Smooth zonal flow B – Undulations begin C – Warm ridges & cold A B troughs develop D – Cold air cut off in upper- level lows C D Growth of Disturbances near polar jet Warm air pulled north into ridge strengthens high pressure Clockwise rotation brings more warmth into high, more cold into low in east Temp & pressure gradients between high & low increase Higher winds More disturbance Oceanic Circulation Oceanic temp structure: Low & mid latitudes, layer of warm water near top Called mixed layer Below warm layer, Thermocline → temp drops fast Below thermocline, deep ocean → very cold Base of deep ocean near freezing No warm layer or thermocline in polar regions  Persistent, horizontal movement of ocean water At surface, driven by friction from winds In two large oceans, gyres, circular currents, Ocean turn under STHs Currents Equatorial currents move east-to-west Western edge of gyres, strong poleward currents Called Western Boundary Currents, e.g Gulf Stream Eastern edge of gyres, Eastern Boundary Currents Toward Equator, upwelling brings cool water to surface Earth’s Ocean Currents Notable Western Boundary Currents: Gulf Stream → North Atlantic Drift, Kuroshio Current Notable Eastern Boundary Currents: Humboldt Current, Benguela Current, California C.  Begins in high latitudes of South & North Atlantic Dense, cold, salty water sinks to ocean depths North Atlantic water goes south of Africa Thermohaline Travels deep in ocean Joins water from South Atlantic Circulation Eventually, water upwells into Indian & Pacific Oc. Some makes its way back to the Atlantic Thermohaline circulation importance: Delivers CO2 to ocean depths, blunting climate change Helps drive Gulf Stream, moving heat toward pole Thermohaline Circulation  Much of global circulation transfers heat from low to high latitudes – by both air and water Hadley cells bring heat from Equator areas to subtropics Jet stream disturbances bring heat further Heat & toward poles Moisture Western boundary currents also bring heat toward poles Transport Thermohaline circulation big part of this Tropical Cyclones bring a lot of latent heat toward poles