Speasker: Philip Tuckman, MIT

Title: The Monsoon’s Impact on ENSO Seasonality

Abstract: The Monsoon’s Impact on ENSO Seasonality  The two primary modes of variability in the tropics are the seasonal cycle and ENSO. The seasonal cycle induces meridional migrations of the ITCZ, leading to seasonal concentrations of precipitation in the form of monsoons. ENSO, an atmosphere-ocean coupled mode associated with zonal anomalies in SST and wind, acts mostly on interannual timescales but is phase-locked to the seasonal cycle with events peaking in northern winter. Here, we explore the interaction of these two phenomena in reanalysis data and a coupled aqua-planet setting. In a simulation without land, and therefore without monsoons, an ENSO-like mode exists on interannual timescales but is not phase-locked with the seasonal cycle. However, in a simulation with a land mass north of the equator in the Indian basin (to represent Asia), a monsoon forms and ENSO peaks during northern winter, just as in observations. We interpret these findings by introducing the concept of a “monsoonal mode,” a zonally propagating energy anomaly which is triggered by heating over land. During northern summer, energetic air from land is advected by westerlies and suppresses surface fluxes in the West Pacific. These suppressed fluxes cause the West Pacific to be warmer during northern fall than during northern spring. Finally, this seasonal asymmetry in heat content in the West Pacific leads to a tendency for ENSO events to peak during northern winter.

Bio: Philip Tuckman is a graduate student in the Program in Atmospheres, Oceans and Climate at MIT and has been working in Professor John Marshall lab's since Fall 2020. His main interest focuses on changes in the global climate. Before coming to MIT, he received a BS in Physics from Yale.