Dynamics of the ITCZ–Equatorial Cold Tongue Complex and Causes of the Latitudinal Climate Asymmetry*

A coupled atmosphere–ocean–coastline model driven by solar radiation is advanced to understand the essential physics determining the annual cycle of the intertropical convergence zone (ITCZ)–equatorial cold tongue (ECT) complex and associated latitudinal climate asymmetry. With a thermocline depth similar to that of the western Pacific, the aquaplanet climate is latitudinal symmetric and stable. The presence of an oceanic eastern boundary supports an east–west asymmetric climate and an ECT due to unstable air–sea interaction and counter stabilization provided by zonal differential surface buoyancy flux. Formation of latitudinal climate asymmetry requires the presence of the ECT. The antisymmetric solar forcing due to annual variation of the solar declination angle can convert a stable latitudinal symmetric climate into a bistable-state latitudinal asymmetric climate by changing trade winds, which in turn control annual variations of the ECT. The ECT then interacts with ITCZ, providing a self-maintenance mechanism for ITCZ to linger in one hemisphere, either the northern or southern, depending on initial conditions. The establishment of the bistable-state asymmetry requires a delicate balance between counter effects of the antisymmetric solar forcing and self-maintenance. Two factors are critical for the latter: (i) The annual variation of ECT follows the SST of the ITCZ-free hemisphere and the meridional SST gradients between the ECT and ITCZ sustain moisture convergence, which prolongs residence of the ITCZ in summer hemisphere. (ii) The latent heat released in the ITCZ produces remarkable asymmetry in Hadley circulation and trades between the two hemispheres, and the stronger evaporation cooling in the ITCZ-free hemisphere delays and weakens the warming and convection development in that hemisphere. The annual cycle of insolation due to the earth–sun distance variation may convert the bistable-state asymmetry into a preferred latitudinal asymmetric climate. The earth’s present orbit (with a minimum distance in December solstices) favors ITCZ staying north of the equator by compelling the ECT into a delayed in-phase variation with the Southern Hemisphere SST. With annual-mean solar forcing a tilted eastern boundary can support a weak preferred latitudinal asymmetry. Inclusion of the annual variation of insolation can dramatically amplify the asymmetry in the mean climate through the self-maintenance mechanism.