Vegetation Cope With Dry Climate North India

Equator

Figure 1.14. How the monsoon rains move north then south of the equator during the year, following the zone where the sun is directly overhead.

Northern winter wwwwwww

Equator even more tightly, so there is no chance of rain falling from it. These bands of descending air, north and south of the Equator, tend to give desert climates with hardly any rainfall. Hence the same mechanism that produced very wet climates along the equator also produces arid climates to the north and south.

The ITCZ does not just stay static. It wavers north and south during the year, because the earth is tilted relative to the sun (this giving winter and summer, as explained earlier). So the highest point of sun in the sky, relative to your point of view on the ground, moves north and south of the equator. Thus, the strongest zone of solar heating is north or south of equator, at different points during the year (Figure 1.14).

The band of rising air near the equator (the ITCZ) follows this zone of greatest heating. In the northern summer, it is slightly north of the equator—although its precise position depends on the layout of land and sea surfaces that can help to drag it either slightly farther south or farther north. In the southern summer, the ITCZ moves to the south. During spring or autumn, it moves between these two extremes, usually crossing the equator itself at these times of the year. Each time the ITCZ passes over the equator, there is an increase in rainfall there—so equatorial rainforest climates have two peaks in rainfall each year (Figure 1.15). However, because they at least get the edge of the ITCZ throughout the year these equatorial locations tend to be quite rainy all the time; the seasonal peak just makes them extra-rainy! Farther away north or south from the equator (Figure 1.16) towards the edges of the tropics,

Figure 1.15.

Two seasonal rainfall peaks at the equator.

Figure 1.15.

Two seasonal rainfall peaks at the equator.

NORTHERN WINTER

NORTHERN WINTER

NORTHERN SPRING

NORTHERN SPRING

NORTHERN SUMMER

Figure 1.16.

The passage of peak rains (shaded areas) from south to north of the equator with the seasons. In autumn, the rains will pass over the equator once again on their way down south, giving the year's second rainy peak on the equator (not shown).

the summer "monsoon" is caused by the arrival of the ITCZ as the sun's summer heating pulls it north (into the northern hemisphere) and then south (into the southern hemisphere). In these places the dry descending air is replaced for a few months by the equatorial climate. In satellite images one can see a "green wave'' traveling up through northern Africa in early summer, as the vegetation starts to grow again with the rains.

In India the summer monsoon is especially strong because the mountains of the Himalayan Plateau heat up and feed rising air straight into a belt of upper-level winds known as the jet stream. This pulls up more air to replace itself from lower altitudes, dragging the ITCZ especially far north in this region during the summer, way up into northern India. The pulling effect of the Himalayas on the ITCZ also means that it gives rain to other mid-latitude areas such as Japan and Korea, that would otherwise be much too far north to see an effect from the monsoon.

In winter, when the ITCZ has gone south, there is a "winter monsoon'' wind traveling from the north in Asia. In most areas this is dry and cold, but it can carry rain-bearing winds from temperate latitudes if it sucks in some air that has traveled over a moist sea surface.

Winds off the oceans transport water vapor, so areas that get ocean winds tend to be wet. Britain's notoriously damp climate results from it sitting in the path of a belt of eastward-blowing winds off the Atlantic (these are paradoxically known as "westerlies", an old mariner's term, because they blow from the west). Similarly damp climates occur on western edges of land masses that receive other belts of westerly winds which follow the curving ocean gyre currents: for example, New Zealand, southernmost Chile in South America, and the area around Vancouver and Seattle in North America. Such ocean-dominated climates tend to have about the same amount of rain in any month of the year (Figure 1.17a*).

If we travel just slightly closer to the equator, these moist climates begin to lose their summer rain. The moist eastward-blowing ocean winds can only reach these areas during part of the year, the winter (Figure 1.18). This characteristic pattern of wet winters and hot dry summers (Figure 1.17b) is known as the Mediterranean climate: named after the area around the Mediterranean Sea at the southern end of Europe, which provides the archetype of this pattern. Other examples of this same climate occur in southwestern and southeastern Australia, southernmost Africa, south-central Chile and California: all are known for their wine-making and olive cultivation. Mediterranean climates are all on the western side of continents, just beyond the outer edge of the tropical desert belt. The summer dryness is caused by the descending desert-making air of the ITCZ extending a little farther outwards, as the summer sun's heating pulls the whole ITCZ away from the equator (Figure 1.17). This outer belt of descending air pushes away the moist westerly winds of the ocean, resulting in dryness all summer long until it is time for the ITCZ to move back equatorwards again, and the moist westerly winds can get access once again. In effect, the desert climate extends out over these areas for a few months during summer, before it retreats again in winter. But if the ocean is cold,

* See also color section.

Indian Dry Climate
Figure 1.17. A satellite image of the density of vegetation across northernmost South America. By contrast to the green-ness of the Amazon Basin to the east, the coastal strip of Peru is barren of vegetation due to an extremely dry climate. Source: Ned Nikolov, from NOAA NDVI imagery.

WINTER

Moist winds off

the Atlantic *

->

bring rain

->

"--

----

->

Descending

desert air stays

over North Africa —___

Mediterranean climate. In winter, moist westerly winds cross southern Europe and bring rain. In summer, rain-bearing winds are pushed away by descending air from the "desert belt" of North Africa.

Figure 1.18. The

Mediterranean climate. In winter, moist westerly winds cross southern Europe and bring rain. In summer, rain-bearing winds are pushed away by descending air from the "desert belt" of North Africa.

SUMMER

Northern

_ V.

Europe stays ' -moist

Belt of descending desert air moves

North to cover Southern Europe

colder than the air, there may be an arid belt along the coast (Figure 1.19). For example, such desert belts occur close to ocean upwelling areas off Peru (Figures 1.17 and 1.20* and Namibia, where winds pulling the surface water away draw up cold deep water to the surface. How does this cause aridity? Because to get rain, there needs to be a cooling effect on already-moist air causing water droplets to condense out to cloud and then raindrops. If the air actually warms as it moves over land, the water vapor is held more tightly in the warmer air and cannot condense out. As an additional influence, over the cold sea surface where water up-wells, the cooling of air above tends to cause sinking within the atmosphere. This too makes rain unlikely, because strong upwards convection is necessary for producing rain.

Another factor which influences which places get rain, and which do not, is topography. If a mountain range is in the path of moist winds coming in off the sea, the stream of air must rise up the windward slope if it is to get over the mountains. This causes the air to cool, and as it cools water droplets condense out to form clouds and then either rain or snow. Thus, the leading edge of a mountain range is usually rainy: a phenomenon known as the orographic effect. By the time the wind reaches over to the far side of the mountain range, it has already lost most of its water vapor, squeezed out as rain on the windward slope. And now descending, it warms and holds onto its remaining water vapor more tightly again—so there is less rain on this leeward side. In fact, this water-squeezing effect can alter the climate for hundreds of kilometers downwind of a mountain range, giving an extensive dry area.

Desert climate

Cooled air moving inland Air warms, humidity falls

Cooled air moving inland Air warms, humidity falls

Figure 1.19. (Top) Where cold seawater wells up off the coast, air cools and then is warmed as it passes over land. This prevents rainfall, bringing about a coastal desert. In addition to this, the cold sea surface may prevent upwards movement in the atmosphere, likewise supressing rain formation. (Bottom) Coastal deserts form in this way.

It is almost as if the mountain is casting a shadow of dryness across the landscape— hence this is termed the "rainshadow effect". Mountain ranges that have a noticeable rainshadow—with a dry zone just inland—include the Cascades in the western USA, and the Great Dividing Range in eastern Australia (Figure 1.21).

The basic climate system explained in this chapter is the blank canvas on which we will now paint a complex picture of the ways in which plants both respond to and actually modify their environment. In Chapter 2 the broad patterns in vegetation produced by this background of climate will be described, and in Chapter 3 the ways in which vegetation can move in response to changes in this background. Chapter 4 will deal with the ways in which plants both respond to and produce their own local climate, the microclimate. Chapters 5 and 6 cross over to how vegetation itself can

Figure 1.20. A view off the coast of Peru. Cool seawater welling up nutrients from the deep supports a very active marine ecosystem, which feeds the abundant sea birds. Desert cliffs on the coast are also influenced by the the cool water suppressing the formation of rain clouds. Source: Axel Kleidon.

Figure 1.20. A view off the coast of Peru. Cool seawater welling up nutrients from the deep supports a very active marine ecosystem, which feeds the abundant sea birds. Desert cliffs on the coast are also influenced by the the cool water suppressing the formation of rain clouds. Source: Axel Kleidon.

help to make climate on the broader scale, over hundreds and thousands of kilometers. Chapters 7 and 8 will deal with some other important ways in which plants both modify and respond to their natural environment, in terms of the carbon-containing gases in the atmosphere.

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  • christin
    How does the vegetation cope with dry climate of north western indiaexplain?
    12 months ago

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