The Role of Tropical Synoptic-Scale Disturbances in Modulating the Strength of East Pacific Hadley Circulation

Abstract

The east Pacific (EP), dominated by synoptic scale convective activity, presents a unique environment to explore the question of how synoptic scale disturbances can modulate the strength of mean meridional circulation. The role of mixed Rossby-Gravity (MRG) waves in modulating the east Pacific Hadley circulation (EPHC) strength is explored during boreal summer season using ERA5 reanalysis data. Composite analysis of MRG activity for five strong and five weak EPHC seasons identified based on a mass stream function based metric reveal that strong EPHC seasons are associated with pronounced MRG activity, whereas the MRG activity is weak during weak EPHC seasons. Although the sea surface temperature (SST) background state over EP is not favorable for thermally driven deep convection, low-level convergence induced by synoptic scale disturbances such as the MRG waves can trigger deep convection over the region, and in turn influence the EPHC strength. The question of whether boundary forced convergence can have an impact on the EPHC strength is further investigated using an atmospheric mixed layer model. Surface convergence driven by meridional SST gradients are not found to be significantly different during strong and weak EPHC seasons, implying the dominant role of MRG in modulating the strength of EPHC. The study also reveals a new possible mechanism via which the El Niño Southern Oscillation (ENSO) influences the strength of EPHC—ENSO induced changes in the mean background state modulates the spatio-temporal characteristics of MRG waves which in turn affect the low-level convergence over the region and impacts the strength of EPHC.