The effect of variation in sea-surface temperature and its meridional gradient on the equatorial and off-equatorial ITCZ in an Aquaplanet General Circulation model

The factors that control the strength of the ITCZ (Inter Tropical Convergence Zone) in an aquaplanet GCM (General Circulation Model) have been investigated. The strength of the equatorial ITCZ was found to increase rapidly with increase in meridional gradient of SST. On the other hand, the strength of the off-equatorial ITCZ does not increase rapidly with increase in meridional gradient of SST. This unusual difference in behavior between off-equatorial and equatorial ITCZ has been interpreted with a diagnostic model. The diagnostic model is based on budgets of moisture and dry static energy in the ITCZ. The diagnostic model indicates that the variations in the strength of the ITCZ are related to changes in the net energy convergence and vertical moist static stability. It was found that the net energy convergence in the off-equatorial ITCZ increases much less rapidly with meridional SST gradient than the equatorial counterpart. This difference in the behavior of net energy convergence is related to the surface wind speed which in the off-equatorial ITCZ simulation is largely insensitive to changes in the meridional SST gradient. Thus the primary difference between the equatorial and off-equatorial ITCZ is on account of the fact that wind speeds were lower in the former (on account of the constraint that zonal wind has to be zero at the equator). The impact of increasing the SST maximum on the strength of the ITCZ has also been studied. It was found that the strength of ITCZ increases with an increase in SST maximum. This increase in the strength of the ITCZ with the maximum SST is governed by the increase in boundary layer specific humidity and its impact on vertical moist static stability.