A multi-scale model for the modulation and rectification

1 ABSTRACT 5 We introduce the modulation of the ITCZ equations (M-ITCZ) which describe 6 the multi-scale dynamics of the ITCZ on one day to one month time scales in 7 which mesoscale convectively coupled Rossby waves in the ITCZ are modulated 8 by a large scale gravity wave which is also generated by convection. Westward 9 propagating disturbances are observed to cause ITCZ breakup over the course 10 of a few days, and the M-ITCZ meso/planetary scale coupled waves provide 11 a mechanism for this interaction, thereby providing a framework to study the 12 modulation and rectification of the Hadley circulation over long zonal length 13 scales in the ITCZ. 15 system and generate a Hadley circulation consistent with the observed winds. 16 Zonally localized heating creates a wind response throughout the tropics which 17 is carried by a pair of zonally propagating gravity bores driving mean easterlies 18 at the base and mean westerlies at the top of the troposphere. The bores carry 19 low temperature/upward velocity perturbations to the west of the heating and 20 high temperature and downward velocity perturbations to the east making the 21 westward propagating branch favorable to convective triggering and the eastward 22 propagating branch favorable to convective suppression. 23 The mesoscale dynamics of the M-ITCZ describe convectively forced, nonlin-24 ear Rossby waves propagating in the zonal winds created by the planetary scale 25 gravity wave. We suggest that convective coupling slows the westward propa-26 gating gravity wave, thereby creating a coupled gravity/Rossby wave which is 27 2 similar to the westward propagating disturbances seen in the ITCZ.