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Climate Belts. II. the Sea Level Pressure Distribution

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Climate Belts. II. the Sea Level Pressure Distribution I. The surface wind circulation - climate belts. As we have discussed earlier, atmospheric motion is driven by the uneven horizontal distribution of net incoming radiation. On a global scale, the most outstanding part of this uneven distribution is its latitudinal dependence (Fig. 1). The atmosphere (and oceans) respond to this imbalance by attempting to move heat from the tropics and subtropics, where insolation surpasses the infrared terrestrial radiation going out to space, to the middle and high latitudes, where there is a net radiative loss of heat. Convection, the vertical process of heat transport, and advection, the horizontal process of heat and moisture transport, work together to accomplish this goal. It seems logical then to expect that the mean atmospheric motion (so called general circulation) will go from equator to pole in both hemispheres. Could this be the case? Lets begin by observing the horizontal distribution of winds near the surface. This is shown in Fig. 2 for January and July. The most outstanding feature about this distribution is its zonally banded structure - that is, it has the pattern of east-west elongated features aligned along latitude circles. As we shall discuss hereafter, these belts define different climate and weather regimes. Marching from equator to pole we find the following characteristic wind belts: Trade wind belts: In the tropics, on both sides of the equator, lies a wide region where winds blow from east to west (easterlies) with a slight equatorward tilt. This region is named the trade wind belt, because of the steadiness of the air flow here. In the days when ships relied on sail power for locomotion these winds made for reliable travel westward. Intertropical Convergence Zone (ITCZ): The trade winds from the Northern and Southern Hemispheres converge into a narrow belt close to the equator, nowadays generally referred to as the Intertropical Convergence Zone (ITCZ). The convergence of the trade winds results in rising motion of the colliding air masses (to obey the law of mass continuity - see last Monday's lecture). This region is also known as the doldrums, where the weather is generally cloudy and periods of light winds are frequently interrupted by squalls and hard rain, making for a troubled and uncertain sea voyage. Midlatitude westerlies: North and south of the trade wind belt (in the Northern and Southern Hemispheres, respectively) lie regions where winds tend to blow fromwest to east (westerlies), and are therefore referred to as the westerly wind belts. Here the winds are highly variable and unsteady, especially so during winter. This fact is not seen in our charts, clearly demonstrating that it is not enough to look at the mean. In these regions, during wintertime, midlatitude storms and their frontal systems travel from west to east bringing frequent changes in weather. Further discussion of this phenomenon appears below (baroclinic instability). Subtropics: Between the trade wind regions lie the subtropics - regions of divergence and subsidence, where sunny weather with little clouds and no rain prevails. In the days of sailing these latitudes were referred to as the horse latitudes because winding up in these latitudes meant serious delays in the voyage. It is said that the horses that were carried along to be used on land had to be thrown off board to lighten the loads or that, according to others, the sailors had to resort to horse meat after all other food rations were depleted in the slow voyage. Polar easterlies: Poleward from the westerly wind belt, winds with a generally easterly component prevail. The air here is cold, dry and stable, especially during winter, and is accompanied by subsidence from above. Polar front: The convergence zone between polar easterlies and midlatitude westerlies is referred to as the polar front. It separates between the cold (and dry) polar air, and the relatively warm (and more humid) midlatitude air. The polar front can be thought of as the average expression of the transient frontal systems that move along with midlatitude cyclones (see last Monday's lecture). The surface wind distribution is not perfectly zonal. Clear difference between continents and ocean are evident in both panels of Fig. 2. Winds are much weaker over land (friction over land is larger than over the oceans, and high variability is another reasons for a low average). Along the coasts, winds deviate quite a bit from zonal symmetry. More on that in the next section. II. The sea level pressure distribution - monsoons. As we saw in last Monday's lecture, the winds are related to the pressure distribution via the geostrophic balance and, at the surface, via the additional influence of friction. The global sea level pressure distribution for January and July (Fig. 3) is indeed consistent with the wind fields we saw above (Fig. 2). The zonal wind belts are matched by the more or less zonal distribution of sea level pressure. The breakdown in zonal symmetry is however much more evident here. What is causing the asymmetry? The surface of Earth is far from homogenous in its composition. Land masses interrupt large ocean basins, and the land itself is intersected by massive mountain ranges. The cover (vegetation, snow and ice, soil moisture) of the land surface varies from region to region, partly because of climate itself. All these zonal (and meridional) asymmetries contribute to the asymmetry of the circulation for both thermal and mechanical reasons. Consider for example the difference in thermal properties between land and ocean surfaces. On the time scales of a day, these gave rise to the sea breeze circulation (last Monday's lecture). On seasonal time scales they give rise to many of the zonal asymmetries we see in Fig. 3. The difference in the heat absorbing properties of continent and ocean - continents warming and cooling faster - is apparent throughout the seasonal cycle. In winter the continents are colder than the surrounding oceans at the same latitude, and in summer they are warmer (see Fig. 4). These temperature differences express themselves as pressure differences (Fig. 3). The continent-ocean pressure differences (Fig. 2) drive the development of regional wind circulations, which are much different than the winds dominating that latitude zone. Such regional winds are particularly noticeable in the tropics. As with the daily breeze cycle, air will flow from where the surface is cold to where the surface is warm, thereby creatingseasonal circulation systems known as monsoons (derived from an Arabic word denoting season). The most famous monsoon circulation is that over India. In summer, southern Asia and the Indian subcontinent are warmed up by the sun, and low pressure develops over the continent (Fig. 3, July). A large system of ocean-to-land winds forms around India (Fig. 2, July) just as with the sea breeze. However, unlike the sea breeze circulation, monsoon winds are subjected to the Coriolis force which divert the winds from flowing directly from ocean to land. It is surface friction that is causing deviation from geostrophic balance, and convergence into the warm land mass. Over the oceans, divergence occurs. Aloft, the situation is reversed, and the air diverges from the land to the ocean. The cycle of winds is closed by the rising and sinking of air over land and ocean, respectively. Convergence into land and the uplift it generates brings large amounts of moisture and some of the largest rainfalls known on Earth (the uplift is also aided by the air flowing against the Himalayas and the Tibetan Plateau). In winter the land is colder than the surrounding ocean, and surface winds diverge from the land to the ocean. The dry continental winds and the subsidence over the subcontinent, suppress clouds and rainfall over land. However, further east over Southeast Asia and northern Australia (centered on the equator and ~130°E, see Fig. 2, January), the circulation induced by the contrast in winter temperature between Asia and the Indian Ocean brings convergence, massive convection and enhanced rainfall. Monsoons occur in other regions of the world: The Central and North American monsoon brings summer rainfall to Mexico and the US Southwest. In the winter, the American monsoon migrates to South America, where it is summer. Monsoon circulation also affects the African continent. Differences between land and ocean are also seen in the westerly wind belt (midlatitudes). Here the deepest winter "lows" are located over the warmer oceans, analogous with the night half of the sea breeze cycle. In summer the situation reverses, and the pressure over land is lower than over the ocean. The thermal effects of continents and oceans affects climate extremes such as the difference in temperature between winter and summer, or the average difference between daily minimum and maximum temperatures in different seasons. In Fig 5, for example, we plot the climatological difference between January and July temperature (a rough measure of the amplitude of the seasonal cycle). In part, this figure depicts the fact that amplitude of the annual cycle increases poleward. However it also clearly depicts a continent-ocean contrast: the largest differences at every latitude are over land. Notice also that over the massive continental regions of the Northern Hemisphere (Asia in particular) the largest differences occur over the eastern sides of the continent. The further poleward we go, the larger the difference. This reflects the extent of the moderating effect exerted by the oceans on the western sides of the continents within the westerly wind belt. III. The upper troposphere. A note on upper level data representation: In meteorology, upper level variables are represented on pressure rather than on height surfaces. The fundamental force on an equal-pressure surface is the height gradient multiplied by the constant of gravity (a quantity referred to as the geopotential).
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