Zonal variability of equatorial flow, cross-equatorial pressure gradients and a personal view of the influence of Jim Holton on tropical meteorology

In the early 1980s, Jim Holton and I discussed both the influence of variability of zonal flow in the equatorial regions and regions of strong cross-equatorial pressure gradients that exist in different parts of the tropics. The existence of equatorial westerlies in the upper troposphere allowed extratropical waves to penetrate deeply into low latitudes simply because of the preclusion of a continuous critical latitude around the equator, leading to the concept of the “westerly duct”. The existence of a zonally asymmetric basic state was later shown to have a strong influence on equatorial wave propagation, modifying group speed and local accumulations of “wave energy”. Both of these factors were later shown to be important to equatorial processes ranging from the modification of both fast (sub-seasonal) and slow (seasonal and interannual) teleconnection patterns, to the genesis of hurricanes. Strong variability in the magnitude of a cross-equatorial pressure gradient throughout the troposphere raises the possibility of inertial instability associated with cross-equatorial flow. Theoretical and modeling studies were used to explain why the Hadley Circulation is strongest in solsticial seasons and why the ascending region of the cell is displaced equatorward of the warmest sea-surface temperature and lowest sea-level pressure. Subsequent studies showed that the linear criterion for inertial instability was rarely reached in the lower troposphere in regions of moderate cross-equatorial pressure gradient but often in the upper troposphere. On the other hand, nonlinear effects in regions of strongest equatorial pressure gradient (e.g., Indian Ocean) were shown to cause rapid and large-scale modifications of the monsoon circulation. This paper is dedicated to the important influence that Jim Holton has had on the field of tropical meteorology and on the careers and lives of many who have worked with him.