Rainfall in the Tropics
Earlier this week we looked at the radiation budget of the Earth, that is the difference between the incoming energy from the Sun and the outgoing infra-red energy lost to space. The most striking feature of this budget is that, in the tropical regions of the Earth there is a surplus of energy whilst outside the Tropics there is a deficit.
The excess of energy within the Tropics has a fundamental impact on the types of weather systems that form in this part of the world; with more energy available to the atmosphere it is no surprise that tropical weather systems are more intense than those in the extra-tropics. The most obvious way that this can be seen is by looking at rainfall amounts. It rains far more in the Tropics than elsewhere on the globe.
One way that the large amounts of energy from the Sun entering the Tropics is manifested is in evaporating water from the ocean surface, and from the soil and vegetation over land. This increases the amount of water vapour in the atmosphere available to form clouds and thus rain as it condenses. The map in figure 1 shows the global mean precipitation rate (in mm per day). Although there is some heavy rain outside the Tropics, mainly over the western North Pacific and North Atlantic, it is clear that all of the really wet parts of the Earth are in the Tropics. Much of this rain in the Tropics falls in intense thunderstorms.
Figure 1. Annual average precipitation rate (in mm per day). This estimate is based on information from rain gauges over land and satellite measurements over the oceans.© 2010 UCAR
There is a lot of variation in rainfall amounts within the tropical belt. In general it is found that the heaviest rain occurs over the warmest oceans, the Indian and West Pacific Oceans, and also over land masses like equatorial Africa, south-east Asia, northern South America and the islands of Indonesia and Papua New Guinea. Another feature of rainfall in the Tropics, not shown on figure 1 as it is an annual average, is that the heaviest rain at any given location recorded in the summer months, emphasising the link between rainfall rates and the energy from coming from the Sun.
Over the tropical Pacific Ocean, apart from the band of very heavy rain just to the north of the equator, there is a clear decrease in rainfall rates from the west, which is very wet, to the east which is rather dry. This change in rainfall rates across the ocean is directly related to the temperature of the ocean surface; the west Pacific is very warm with the surface water often exceeding 29°C, whilst the east Pacific is rather cool (at least for the Tropics) at around 24°C (see figure 2). This is an indication of strong ‘coupling’ between the atmosphere and the ocean in the Tropics, with rainfall rates being very closely related, or coupled, to the temperature of the ocean beneath.
Figure 2: Annual mean sea surface temperature (degrees C), averaged from 1971-2000. Note the very warm water in the tropical west Pacific and the cold “tongue” along the equator in the east Pacific.© US Climate Prediction Center