Department of Meteorology, University of Hawaii
Bureau of Meteorology Research Centre, Melbourne, Victoria, Australia
A coupled atmosphere-ocean-coastline model driven by solar radiation forcing is advanced to understand essential physics determining the annual cycle of the intertropical convergence zone (ITCZ)-equatorial cold tongue (ECT) complex and associated climate asymmetry. Without land effect, the aqua-planet climate is axially symmetric and stable to air-sea interaction. Land serving as an oceanic eastern boundary supports east-west climate asymmetry (or an ECT) due to amplification of zonal sea surface temperature (SST) gradient by unstable air-sea interaction and a counter stabilization by zonal differential surface buoyancy flux. Formation of latitudinal climate asymmetry requires the presence of the east-west asymmetry but does not necessarily require unstable air-sea interaction. A stable latitudinal symmetric climate with an ECT can be converted into latitudinally asymmetric by antisymmetric solar forcing (due to annual variation of the solar declination angle). The antisymmetric solar forcing drives and invisible monsoon which breaks latitudinal climate asymmetry and regulates annual variation of the ECT. The latter become essentially a phase-delayed annual cycle of SST in the ITCZ-free hemisphere. The interaction between the meridional SST gradients associated with the ECT and the ITCZ convection provides a self-maintenance mechanism for ITCZ to linger in one hemisphere, either the northern or southern, depending on initial conditions. Critical factors involved in the self-maintenance mechanism include (i) the annual phase-delay between the ECT and the ITCZ-free hemisphere, (ii) the meridional SST gradient-enhanced moisture convergence into the ITCZ, and (iii) the off-equatorial heating-induced remarkable asymmetry in the monsoon and associated evaporation cooling. The formation of this bistable state asymmetric climate requires a delicate balance between the counter effects of the self-maintenance and antisymmetric solar forcing. We find that the annual cycle of insolation may play critical roles in converting bistable state asymmetry into a preferred latitudinal climate asymmetry. The present annual variation in the earth-sun distance favors ITCZ staying north of the equator by compelling the ECT into an in-phase variation with the Southern Hemisphere SST. The antisymmetric forcing can dramatically amplify the weak latitudinal asymmetry induced by a tilted eastern ocean boundary under annual mean solar forcing found previously by Philander et al..
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