El Niño and Tropical Heat

The Big Picture

The story of El Niño begins in the tropical Pacific.

1. The Pacific is big. At the equator, it extends from Ecuador in the east to the west coast of Sumatra in the west, a distance of about 176 degrees of longitude, or about halfway around the earth.
2. So much sunlight (solar heat) reaches the Pacific, that it provides a large portion of the heat needed to drive the circulation of the atmosphere. The tropical Pacific ocean and atmosphere are a giant heat engine that converts solar energy into kinetic energy of the winds.
3. Thus any small changes in the interaction of the ocean and atmosphere in the Pacific must have important consequences.
4. We call these perturbations in the air-sea interaction of the Pacific El Niño.

The Details

Let's begin our study of the interaction of the ocean and atmosphere in the Pacific with a look at the heat budget of the region.

1. The Pacific is heated by incoming solar radiation (sunlight), called insolation. Look at this plot of the average surface insolation calculated by the Centre for Medium Range Weather Forecasting (ECMWF) using 40 years of meteorological data. Notice that the heating exceeds 250 W/m² over much of the equatorial Pacific.
   
   
2. The heat gained by the Pacific must be balanced by a heat loss of the same size, otherwise the Pacific would warm or cool. Heat gain must equal heat loss averaged over a year or so.
   
   
3. The ocean loses heat by:
   1. Evaporation,
   2. Emitting infrared radiation (energy),
   3. Direct conduction to the atmosphere, or
   4. Transporting heat away by currents.
      
      
4. In the Pacific, evaporation, called latent heat flux, and infrared radiation (energy) dominate.
   1. Look at these plots of latent heat flux and
   2. infrared heat flux from the ECMWF.
   3. Notice that evaporation loss is greatest, and infrared is smaller, and that together they account for a net loss of about 150 W/m². The net heat flux is shown in this map from the ECMWF. Net heat flux is downward, and varies from 30 to 150 Watts per square meter along the equator, with the highest heating in the east.
   4. Currents carry most of the remainder, mostly from the eastern tropical Pacific to the west and to slightly higher latitudes.
      
      
5. Currents are very important.
   1. Look at these maps of mean surface temperature in January and July.
   2. Notice that the eastern equatorial Pacific is much cooler than the western equatorial Pacific, although the eastern area gets more net heat.
   3. Trade winds blowing westward along the equator cause upwelling on the equator. In the east, the upwelling brings up cold water, which is close to the surface. In the west, the upwelling brings up warm water because cold water is much deeper below the surface. The heat gained in the east is partly lost by mixing with cold upwelled water, and partly transported to the west by equatorial currents.
      
      Average January temperatures as a function of depth along the Equator in the Pacific. Click on the image for a zoom.
      Map from Pacific Marine Environmental Laboratory of NOAA, Seattle.
      
      
6. The latent and infrared heat lost by the tropical Pacific warms the atmosphere.
   • The infrared radiation (energy) is mostly absorbed by water vapor in the air, water vapor being a greenhouse. And the water vapor comes from the ocean when heat is lost through evaporation.
   • Water vapor evaporated from the sea is carried by the trade winds into the Inter Tropical Convergence Zone (ITCZ). In the convergence zone the rises and cools, water vapor condenses, releasing the latent heat, and falls as rain. Each kilogram of rain releases 2.5 million joules of heat. The heat warms the air relative to other air at the same height in the atmosphere, which causes convection, which draws in more water vapor from the trade winds, which drives the trade winds and the Hadley circulation of the atmosphere.
     
     
7. Where is the rain? How much is available to drive the atmosphere? What happens in the atmosphere?
   
   Global map of rain rate averaged averaged over the 25-year period from 1979 to 2004. The band of heavy rain just north of the equator is the Intertropical Convergence Zone. Click on the map for a zoom. From Japanese 25-year Reanalysis of atmospheric data.
   
   Rain releases latent heat, warming the atmosphere. Here is where the atmosphere is heated.
   
   Global map of heating of the atmosphere from the surface to space (column integrated heating) averaged over the 25-year period from 1979 to 2004. Notice that areas of heavy rain coincide with areas of high heating. The atmosphere is heated mostly by the release of latent heat by rain. Click on the map for a zoom. From Japanese 25-year Reanalysis of atmospheric data.
   

Revised on: 15 August, 2008