The Equivalent Barotropic Structure of Waves in the Tropical Atmosphere in the Western Hemisphere

Tropical waves are generally considered to have a baroclinic structure. However, analysis of ERA-Interim and NOAA OLR data for the period 1979-2010 shows that in the equatorial and Northern Hemisphere near-equatorial regions in the tropical Western Hemisphere (WH), westward-and eastward-moving transients, with zonal wavenumbers 2-10 and periods of 2-30 days, have little tilt in the vertical and can be said to be equivalent barotropic. The westward-moving transients in the equatorial region have large projections onto the westward mixed Rossby-gravity (WMRG) wave and those in the near-equatorial region project onto the gravest Rossby wave and also the WMRG. The eastward-moving transients have large projections onto the Doppler-shifted eastward-moving versions of these waves. To examine how such equivalent barotropic structures are possible in the tropics, terms in the vorticity equation are analyzed. It is deduced that waves must have westward intrinsic phase speeds and can exist in the WH with its large westerly vertical shear. Throughout the depth, the advection of vorticity by the zonal flow and the beta term are large and nearly cancel. In the upper troposphere the zonal advection by the strong westerly flow wins and the residual is partially balanced by vortex shrinking associated with divergence above a region of ascent. Below the region of ascent the beta term wins and is partially balanced by vortex stretching associated with the convergence. An equivalent barotropic structure is therefore maintained in a similar manner to higher latitudes. The regions of ascent are usually associated with deep convection and, consistently, WH waves directly connected to tropical convection are also found to be equivalent barotropic.