PREFERRED LATITUDES OF THE INTERTROPICAL CONVERGENCE ZONE

The latitude preference of the intertropical convergence zone (ITCZ) i s examined on the basis of observations, theory, and a modeling analys is. Observations show that convection is enhanced at latitudes of abou t 4-degrees to 10-degrees relative to the equator, even in regions whe re the sea surface temperature (SST) is maximum on the equator. Both l inear shallow-water theory and a moist primitive equation model sugges t a new explanation for the off-equatorial latitude preference of the ITCZ that requires neither the existence of zonally propagating distur bances nor an off-equatorial maximum in SST. The shallow-water theory indicates that a finite-width, zonally oriented, midtropospheric heat source (i.e., an ITCZ) produces the greatest local low-level convergen ce when placed a finite distance away from the equator. This result su ggests that an ITCZ is most likely to be supported via low-level conve rgence of moist energy when located at these ''preferred'' latitudes a way from die equator. For a plausible range of heating widths and damp ing parameters, the theoretically predicted latitude is approximately equal to the observed position(s) of the ITCZ(s). Analysis with an axi ally symmetric, moist, primitive equation model indicates that when th e latent heating field is allowed to be determined internally, a posit ive feedback develops between the midtropospheric latent heating and t he low-level convergence, with the effect of enhancing the organizatio n of convection at latitudes of about 4-degrees to 12-degrees. Numeric al experiments show that 1) two peaks in convective precipitation deve lop straddling the equator when the SST maximum is located on the equa tor; 2) steady ITCZ-like structures form only when the SST maximum is located away from the equator; and 3) peaks in convection can develop away from the maximum in SST, with a particular preference for latitud es of about 4-degrees to 12-degrees-, even in the (''cold'') hemispher e without the SST maximum. The relationship between this mechanism and earlier theories is discussed, as are implications for the coupled oc ean-atmosphere system and the roles played by midlevel latent heating and SST gradients in forcing the low-level atmospheric circulation in the tropics.