Climate and Water Resource Case Study

Overview of Climate Change
Greenhouse Effect and Climate Change
What is the world doing about climate change?
Investigating Regional and Local Projected Climate Change
Consequences of Projected Climate Change
Chapter 7 title
Chapter 8 title

Chapter 4 - C. Intertropical Convergence Zone


When geologists use the term "convergence zone", they are discussing the region where two tectonic plates are colliding, with one plate sliding beneath the other. The result is geological turbulence: fault zones that produce earthquakes, and generated heat that gives rise to explosive volcanoes. When meteorologists use the term "convergence zone", they are describing a phenomenon in the atmosphere which works in an analogous fashion but is referring to air masses colliding. Near the equator, warm air rises and colder air moves in beneath it. As the warm air rises, it forms huge bands of clouds and thunderstorms over the ocean, an area called the Intertropical Convergence Zone, or ITCZ. A picture of the ITCZ is bounded by a white box in the figure to the left.

Sea Surface Temperatures and the Atmosphere

            In the short- and long-term models, the pattern of increased sea-surface temperatures along the equatorial Central and Eastern Pacific suggests a greater tendency for El Niño-like conditions with sea-surface temperatures steadily warming in the Central and Eastern equatorial Pacific Ocean. Increased precipitation in the aforementioned region is indicative of both the short- and long-term December, January, and February warming patterns seen in the model results (Figure 17). Anomalies (or difference from the average tempeature) of sea-surface temperatures in the Niño-3 region (defined as 5o N to 5o S and 150o W to 90o W) are useful in measuring the strength of El Niños and La Niñas. The value of the Niño-3 sea-surface temperature anomaly index over time from the Hadley model (Figure 19) shows a general warming trend from present day through 2099. Over this interval and in this region, the average sea-surface temperature increases by roughly 3o C (5.4o F). It should be remembered that the characteristics or behavior of the El Niño Southern Oscillation could change with the overall warming of the atmosphere and ocean. If so, this would complicate and create uncertainty about the details of future climate conditions in the Pacific.

Figure 19. Monthly SST anomaly time series for the period 1900-2099. The El Niño 3 region is defined as 5 oN to 5 oS and 150 oW to 90 oW, with the base years for the climatology defined as 1950-1979. The thick red line is the 10-year running average. Units are in oC.

            Regarding the short and long-term model results in terms of the natural Pacific climate variability and El Niño Southern Oscillation, it is useful to discuss how the atmosphere and ocean interact and thus impact climate. Both the Intertropical Convergence Zone and the El Niño Southern Oscillation are consequences of the atmosphere and ocean systems interacting, play a role in climate variability, and thus may be impacted by climate change.

            The air-sea interactions are important to the transfer of energy and mass between the oceans and atmosphere. Both atmospheric and ocean circulation are driven by heating at low latitudes (near the equator) and high latitude cooling (near the poles). Heat is absorbed by the planetary surface at low latitudes and is transferred poleward in both the northern and southern hemispheres. The equatorial region is a barrier to the exchange of materials between the the northern and southern hemisphere atmosphere. It is also a barrier to water, salt, and heat exchange at the ocean’s surface. The large solar radiation input (due to the equator being closer to the sun) to the tropics leads mainly to heating of the ocean surface. In turn, the air above the surface ocean is heated, expands, and becomes less dense. This reduction in density causes the air to rise owing to convection. A region of low pressure develops because the mass of the overlying atmosphere in the tropics is reduced. The “void” left by the rising, warm, and moist air is replaced by air that moves toward the equator from higher latitudes. This region is called the Intertropical Convergence Zone. It is the zone along which the trade wind systems of the Northern and Southern Hemispheres meet (Figure 20).

Figure 20. Longitudinal cross section through the lower 20 kilometers of Earth’s atmosphere from the polar region to the equator showing general vertical air circulation patterns.

            Heating of the tropical ocean surface causes evaporation of water. The water vapor rises and cools at higher elevations in the atmosphere, leading to the formation of clouds. Thus, the region of the Intertropical Convergence Zone is characterized by cloudiness and heavy precipitation. The zone of the Intertropical Convergence Zone is particularly intensely developed in the western Pacific. Here a warm water pool of surface water is found with mean temperatures of about 31o C! The average water temperature in Hawaii is around 24o C. On average, the Intertropical Convergence Zone is also a region of intense atmospheric convection and the wettest part of the tropics. This surface ocean warm pool and the associated zone of heavy rainfall are important to the dynamics of El Niño-Southern Oscillation events. The Intertropical Convergence Zone shifts seasonally. During the northern hemisphere summer, the ITCZ shifts northward as the Asian continent is warmed more than the adjacent ocean (Figure 21a). The warm continental air rises, and air is drawn from the ocean toward the land. The zone of heavy rainfall expands northwestward from Indonesian into India and Southeast Asia. Southerly winds blowing from the oceans toward India and Southeast Asia are dominant at this time (Figure 21a). This is the time of the Southwest Monsoon (May to September). During the northern hemisphere winter, the reverse is true when the air above the Asian continent becomes very cold (Figure 21b). An intense high-pressure region develops in the atmosphere above continental Asia. The flow of air is from the continent toward the ocean. This is the time of the Northeast Monsoon (November to March).

Figure 21. The average prevailing wind directions at Earth’s surface during the Northern hemispheric summer (a, July) and winter (b, January). The solid black line shows the Intertropical Convergence Zone position and its change with season. 

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