The General Circulation of the Atmosphere

Weather with different scales

What we need to know for today Pressure gradient force: from High to Low pressure Coriolis force (effect): Results from the rotation of the planet. Maximum at the poles and no effect at the equator. Acts perpendicular to the direction of motion: changes the direction of the wind but not the wind magnitude. In the NH deflects the wind to the right. In the SH deflects the wind to the left. Winds aloft Balance between the pressure force and the Coriolis force. The wind is parallel to the isobars. Surface winds Balance between the pressure gradient force, the Coriolis force and the air friction. The wind crosses the isobars (from High to Low pressure).

Average Wind Structure The direction and the magnitude of the winds at a given location can vary significantly during the day, and from day to day. The general circulation (GC) refers to the average (the prevailing) winds on a global scale (around the world). The GC of the atmosphere is the result of the uneven heating of the Earth’s surface. It is impacted by the Earths rotation. The GC transports and redistributes energy from one region to another (warm air towards the poles and cold air towards the equator).

The Single Cell Model This is a very simplified model based on the following three assumptions: The Earth’s surface is uniformly covered with water (no differential heating of the land and the oceans) The sun is always directly over the equator (no seasonal variations of the winds). The Earth does not rotate. No Coriolis effect. The only active force is the pressure gradient force.

Thermal circulations Due to uneven heating of the surface. Example: South area heats up, North area cools Warmer southern air aloft moves north towards low pressure It then cools and sinks Surface pressure to the North increases Surface wind from N to S The surface air warms up and rises. The process continues

The Hadley Cell It is driven by the uneven heating of the Earth’s surface by the sun - thermally direct cell: warm air rises, cold air sinks. One Hadley cell in each hemisphere. The equator is warmer than the poles. Warm moist air at the equator rises upwards It expands, cools, and saturates, the water vapor condenses and forms clouds. It creates low surface pressure in the tropics. At the poles we have cool, dry, sinking air that creates high surface pressure in the polar region. The PGF (pressure gradient force) drives the surface winds from the poles towards the equator. The winds aloft close the cell by blowing from the equator towards the poles.

The one cell model does not work! It is obviously wrong: predicts northern prevailing winds everywhere in the NH What is wrong with the model? It is too simple! The rotation of the Earth will deflect the winds to the right in the Northern hemisphere and to the left in the Southern hemisphere. This will result in surface winds blowing: From the East (easterlies) in the NH From the East (easterlies) in the SH This will result in winds aloft blowing: From the West (westerlies) in the NH From the West (westerlies) in the SH

Observing global winds Intertropical convergence zone Observing global winds from space

Winds Aloft Warm air above the equator and cold air above the polar regions Higher pressure at the equator, lower pressure both to the north and to the south of the equator The pressure gradient force is towards the poles, sets the air in motion The Coriolis force NH: to the right SH: to the left The wind turns right in the NH and left in the SH, becomes parallel to the isobars Westerly winds aloft in both the NH and SH. Easterly winds at the surface in both the NH and SH.

The Three Cell Model Keep two of the assumptions, relax the third: The Earth is covered with a continuous ocean The sun is always directly over the equator The Earth rotates -> Coriolis force!

Three cell model: the Hadley cell (0-30 deg) Thermally direct cell: warm air rises, cool air sinks Intertropical Convergence Zone (ITCZ) A.k.a. equatorial doldrums Warm air, weak PGF, light winds, cumulus clouds and thunderstorms Air rises up to the tropopause, then laterally toward the poles Deflected east due to the CF Winds aloft in NH: from southwest Subtropical highs (anticyclones) Equatorial air cools, sinks, warms up, clear skies -> major deserts Air converges (follow the meridians on a globe) – high surface pressure Horse latitudes: small PG, weak horizontal winds -> sailors get stuck Surface winds in NH: from the northeast (Trade winds)

Three cell model: the Ferrel cell (30-60 deg) Thermally indirect cell: cool air rises and warm air sinks Some of the sinking air in the horse latitudes heads toward the pole Deflected east by the CF Surface winds in NH: from the southwest (westerlies) At the polar front the westerlies encounter cold air moving down from the poles Air is forced to rise, some of it returns to the horse latitudes, completing the Ferrel cell, the rest heads for the pole Upper air winds in the Ferrel cell: from the northeast. William Ferrel William Ferrell

Three cell model: the polar cell (60-90 deg) It is a Hadley type of circulation. Surface winds: from the north east (polar easterlies) Upper winds in NH: from the southwest Summary: two major areas of Low pressure (ITCZ and subpolar low), and two of High pressure (poles and subtropical highs)

The converging/diverging regions ITCZ (Intertropical Convergence Zone ) - Equator Low surface pressure with small PG and weak horizontal winds. Upward motion of warm moist air. Results in convective cloud towers Subtropical highs (the horse latitudes) – 30N; 30S High surface pressure The upper air is sinking, warms up and the relative humidity is very low. Weak winds, clear sky, dry climate – large deserts at these latitudes. Subpolar lows (polar front) – 60N, 60S A converging zone at the surface. Air moves up and results in strong storms. Weak winds Polar highs – 90N, 90S

Winds and pressure in the real world Semi-permanent highs and lows: persist throughout the year, correspond to converging/diverging upper air masses. Bermuda, Pacific highs; Icelandic, Aleutian lows Seasonal highs and lows (continents heat/cool faster) Winter: Siberian high, Canadian high Summer (thermal lows): Southwest US, Iran January July Subtropical highs Subtropical highs

The General Circulation and Precipitation Paterns Converging surface flows: Low surface pressure Uprising air Heavy precipitation Diverging surface flows: High surface pressure Sinking air Dry climate

Winds and Pressure Systems Aloft The wind system aloft differs from the surface wind system. It is close to a geostrophic flow. There is no significant friction with the ground. The three cell model does not work that well in the middle latitudes. The winds aloft are stronger than on the ground. In the winter the gradients are bigger -> the winds are stronger. July January