2. Zonally Asymmetric Features of the Tropics

This chapter covers the zonally asymmetric climatology of the tropics. Seasonal or monthly mean weather maps are particularly important in the tropics. The following example illustrates this point. Over the North American continent, the January mean sea level isobar map consists of a large continental anticyclone extending southwards from the Arctic and Canada towards the southern United States. This monthly mean pattern does not reveal any of the migrating polar front cyclones that cause much of the weather there during the winter season. The reason for this is that climatological charts in middle latitudes do not display the transient disturbances. A similar exercise carried out over the global tropical belt shows that daily as well as monthly mean charts both carry much the same information. The subtropical highs, the equatorial troughs, the monsoon troughs, the trades of the two hemispheres are common in both the daily and the monthly mean charts. Another way of expressing this is that climatological means carry much of the variance of the total motion field in the tropics. Thus an understanding of the maintenance of the time averaged zonally asymmetric features of the tropics is important.