WHY THE ITCZ IS MOSTLY NORTH OF THE EQUATOR

Although the distribution of sunshine is symmetrical about the equator , the earth's climate is not. Climatic asymmetries are prominent in th e eastern tropical Pacific and Atlantic Oceans where the regions of ma ximum sea surface temperature, convective cloud cover, and rainfall ar e north of the equator. This is the result of two sets of factors: int eractions between the ocean and atmosphere that are capable of convert ing symmetry into asymmetry, and the geometries of the continents that determine in which longitudes the interactions are effective and in w hich hemisphere the warmest waters and the intertropical convergence z one are located. The ocean-atmosphere interactions are most effective where the thermocline is shallow because the winds can readily affect sea surface temperatures in such regions. The thermocline happens to s hoal in the eastern equatorial Pacific and Atlantic, but not in the ea stern Indian Ocean, because easterly trade winds prevail over the trop ical Atlantic and Pacific whereas monsoons, with a far larger meridion al component, are dominant over the Indian Ocean. That is how the glob al distribution of the continents, by determining the large-scale wind patterns, causes climatic asymmetries to be prominent in some bands o f longitude but not others. The explanation for asymmetries that favor the Northern rather than Southern Hemisphere with the warmest waters and the ITCZ involves the details of the local coastal geometries: the bulge of western Africa to the north of the Gulf of Guinea and the sl ope of the western coast of the Americas relative to meridians. Low-le vel stratus clouds over cold waters are crucial to the maintenance of the asymmetries.