The ITCZ, easterly wave generation, equatorial westerlies: A common solution for some fundamental questions in the tropics

There are a number of fundamental questions in tropical meteorology and climate that remain unanswered: What determines the mean location of the ITCZ? Why is convection strongest when the ITCZ is off the equator and why is this convection associated with sub-geostrophic westerlies? What is the formation process of easterly waves and why do they have a particular frequency and scale? It will be shown that each of these questions possesses a common forcing agent: the existence of cross-equatorial pressure gradients (CEPGs) set up by SST distributions about the equator. The latitude of the mean ITCZ, located on the poleward side of the symmetrically (inertially) unstable region, is determined by the strength of the CEPG and occurs where the cross-equatorial advection of absolute vorticity is matched by vortex tube stretching. In addition, within the mean ITCZ there is an “inertial heating oscillator” possessing the scale and frequency of easterly waves that matches the inertial frequency of the latitude of the mean convection. The equatorial westerlies are a consequence of the symmetric or inertial instability and not due to Coriolis turning. They enhance the curl of the wind stress, shallowing the thermocline and produce climatically sensitive regions. These processes occur in all oceans except the South Atlantic in the austral summer and the North Indian Ocean during the boreal summer. The latter region, central to modulations of the South Asian summer monsoon, possesses the strongest cross-equatorial pressure gradient on the planet and will be discussed separately.