Revisiting the General Circulation of the Tropics

Includes bibliographical references

PhD;2024;Centre for Atmospheric and Oceanic Sciences

This thesis examines some aspects of the tropical general circulation. First, we consider the zonal momentum budget of the tropical upper troposphere. Our motivation is the zonal inhomogeneity of the Earth that presents the scope for interesting dynamics masked by the zonal averaging. Using reanalysis data, we show that during summer, climatological stationary Rossby waves over the Asian monsoon longitudes converge westerly momentum into the tropics and are the primary contributors to the eddy momentum flux. During winter, anomalous westerly winds over the tropical East Pacific allow extratropical waves to propagate into the deep tropics, where they tend to break and decelerate the flow. When integrated over all longitudes, eddies from these two regions sum constructively in summer and destructively in winter, always yielding a net positive momentum forcing that balances the mean flow contribution. Such a view of the tropical circulation as a function of longitude is relevant in the context of climate change, wherein changing precipitation patterns induce shifts in regional climates, and warming-related changes may not be similar across longitudes. We investigate the effects of anthropogenic climate change on the momentum budget using historical and warming simulations from the CMIP6 archive. In summer, stationary eddy circulations in the Asian monsoon zone weaken in the upper troposphere (UT) but strengthen in the lower stratosphere (LS). Greater upward mass flux from the troposphere forces a stronger divergence and a more intense circulation in the LS following a Sverdrup vorticity balance. This strengthening of summertime tropical and subtropical stationary waves in the LS is observed over all longitudes and is verified in an idealized general circulation model experiment. In winter, eddy westerlies over the East Pacific longitudes decrease in strength due to a weakening of the subtropical stationary waves. This causes a significant decrease in the propagation of extratropical waves into this region and a reduction in eddy potential vorticity fluxes associated with these waves. Potential implications of these changes in the context of the zonal mean flow and regional circulations are discussed. Next, given the presence of longitudinally inhomogeneous systems in the tropics, we focus our attention on the interaction of the Hadley circulation with tropical waves. To this end, we use an aquaplanet GCM forced by fixed zonally symmetric sea-surface temperatures (SSTs). We vary SST forcing from a present-day Earth-like warm-equator cold-poles configuration to one that is globally uniform. This approach allows us to control the generation of midlatitude waves since they are sensitive to the imposed temperature gradient. A traditional Hadley Cell (HC)-like flow is observed in all experiments along with the poleward transport of heat and angular momentum. In simulations with non-zero SST gradients, latent heat released from organized convection near the equator sets up a deep tropical cell; midlatitude baroclinic Rossby waves flux heat and angular momentum poleward, reinforcing the thermally direct circulation. As the imposed SST gradient is weakened, the HC transitions from a thermally and eddy-driven regime to one that’s completely eddy-driven. When the SST is globally uniform, equatorial waves concentrate precipitation in the tropics and facilitate the lower-level convergence necessary for the ascending branch of the HC. Midlatitude Rossby waves become very weak, but upper-level baroclinicity generates waves that cause equatorward transport of heat and poleward transport of momentum. Moreover, these upper-level waves induce a circulation that opposes the time-mean HC, thus highlighting the role of tropical waves in driving a traditional overturning flow for uniform SSTs. In all cases, anomalies associated with the tropical waves closely resemble those that sum to give the upper-level zonal mean divergent outflow. In all, through their ability to modulate tropical rainfall and the related latent heating, equatorial waves cause considerable hemispheric asymmetry in the HC and impart synoptic and intraseasonal variability to the tropical overturning circulation. Finally, we consider the episodic propagation of extratropical waves into the tropics. Numerous observational studies suggest extratropical wave influence on the Indian summer monsoon rainfall. However, classical theory precludes such interactions due to critical layer absorption in the strong upper-level easterlies characteristic of the monsoon flow. We reconcile this disconnect using Rossby wave ray tracing and show that stationary Rossby waves generated in the subtropical Atlantic can propagate toward India, following a great circle path. The results suggest that northerlies are crucial in guiding Rossby waves over India, challenging conventional understanding. Applying the results to Large-scale Extreme Rainfall Events over India shows that the composited wave activity flux and geopotential height anomalies during such events follow the ray path very closely, thereby lending theoretical credibility to the observations.