Seasonal Sensitivity of the Eddy-Driven Jet to Tropospheric Heating in an Idealized Atmospheric General Circulation Model

October 28, 2015

Marie McGraw

Committee: Libby Barnes (advisor), Thomas Birner, Karan Venayagamoorthy (Civil and Environmental Engineering)

Abstract

A dry dynamical core is used to investigate the seasonal sensitivity of the circulation to two idealized thermal forcings: a tropical upper tropospheric forcing, and a polar lower tropospheric forcing. The circulation is modified using a set of perpetual simulations to simulate each month of the year, while the thermal forcings are held constant. The circulation responses to tropical warming and polar warming are studied separately, and then the response to the simultaneously applied forcings is analyzed. Finally, the seasonality of the internal variability of the circulation is explored as a possible mechanism to explain the seasonality of the responses. The primary results of these experiments are: 1) There is a seasonal sensitivity in the circulation response to both the tropical and polar forcings. 2) The jet position response to each forcing is greatest in the transition seasons, and the jet speed response exhibits a seasonal sensitivity to both forcings although the seasonal sensitivities are not the same. 3) The circulation response is nonlinear in the transition seasons, but approximately linear in the summer and winter months. 4) The internal variability of the unforced circulation exhibits a seasonal sensitivity that may partly explain the seasonal sensitivity of the forced response. The seasonality of the internal variability of daily MERRA reanalysis data is compared to that of the model, demonstrating that the broad conclusions drawn from this idealized modeling study may be useful for understanding the jet response to anthropogenic forcing.