De. Waliser et Rcj. Somerville, PREFERRED LATITUDES OF THE INTERTROPICAL CONVERGENCE ZONE, Journal of the atmospheric sciences, 51(12), 1994, pp. 1619-1639
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.