On the vorticity statistics in the Atlantic and East Pacific ITCZ

Abstract

We investigate the small scale dynamical controls on the Intertropical convergence zone (ITCZ) in the Atlantic and East Pacific basins using 5 year, global km-scale coupled atmosphere-ocean-land simulations. To this end, using an ITCZ-based coordinate system, we develop a composited view of the zonal mean statistics of potential vorticity (PV) during Boreal Summer (JJA) at geopotential heights of 0.3, 1.5 and 4 km. The ITCZ-based coordinate system is defined locally at each longitude, such that the ITCZ latitude — identified as the latitude where the column water vapor is a maximum — constitutes the origin. The zonal, 5-year JJA mean latitudes of the Atlantic and East Pacific ITCZ determined based on this definition are 7.8°N and 10.8°N, respectively. The thus obtained composited PV profiles are robust with low inter-annual variability. The PV profiles exhibit a similar structure in both the basins of interest: a gradual increase in the PV values with latitude, followed by a sharp increase in the PV values in the ITCZ due to latent heating. The necessary conditions for the instability of the zonal flow are met, as the sign of the meridional gradient of PV is reversed at the ITCZ. The magnitudes of PV statistics in and around the Atlantic ITCZ are slightly smaller than those of the East Pacific ITCZ. The differences in PV values in the basins can be explained using one of the processes governing the vertical vorticity in the ITCZ, i.e., vortex stretching due to convergence. The vortex stretching term is proportional to the Coriolis parameter, i.e., for a given convergence rate in the ITCZ, more northern ITCZ latitudes could experience greater jumps in vertical vorticities. We also find that at geopotential heights of 0.3 and 1.5 km, the monthly mean relative vertical vorticity in the ITCZ increases as the ITCZ moves to the North.