WHOI PO

Abstract:

Earth system models have severe rainfall biases over the tropical Pacific Ocean, with a zonal rainfall band in the Southern Hemisphere that is rarely seen in observations and too broad of a rainfall band in the Northern Hemisphere. These rainfall errors have an outsized influence on global climate, contributing to uncertainty in our understanding and prediction of El Niño-Southern Oscillation and future weather patterns. Tropical rainfall is primarily driven by moisture convergence in the atmospheric boundary layer, which is a combination of moist, positively buoyant air and horizontal wind convergence. In this talk, I focus on the role of horizontal wind convergence and aim to better understand the monthly to seasonal variability of the horizontal momentum budget over the tropical east Pacific Ocean. The marine boundary layer momentum budget is dominated by the pressure gradient force (PGF), the vertical mixing of horizontal momentum, and the Coriolis force. The PGF reinforces rainfall by accelerating air towards low pressure while the vertical mixing of horizontal momentum typically counteracts the PGF. On the other hand, the Coriolis force can work with or against the PGF in the east Pacific because rainfall is typically favored off the equator, which creates a delicate balance between these three terms. I analyze the vertical structure of the PGF, Coriolis force, and vertical momentum mixing in the ERA5 reanalysis during years when significant rainfall is observed in the Southern Hemisphere, which is a relatively rare occurrence in observations but is commonplace in Earth system models.