The role of surface dynamics in boreal spring submonthly eastern Pacific ITCZ shifts

Abstract:

The latitudinal location of the intertropical convergence zone (ITCZ) over the eastern Pacific Ocean changes on time scales of days to weeks during boreal spring. This study illustrates the critical role of near-surface dynamics in setting the stage for rapid boreal spring eastern Pacific Ocean ITCZ shifts using high-resolution QuikSCAT wind, ERA5 reanalysis wind and geopotential, and TRMM precipitation data. The climatological equatorial meridional surface winds dip down near zero in conjunction with the climatological double ITCZ during February–April so that any significant meridional wind variability alters the interhemispheric transport of moisture, energy, momentum, and thus, ITCZ position. There are several distinct dynamical features that appear in the days leading up to the two leading types of ITCZ events, nITCZ (Northern Hemisphere) and dITCZ (double). Southerly and northerly equatorial flows intensify over a zonally wide eastern Pacific region preceding nITCZ and dITCZ events, respectively. In addition, there is a southeast to northwest-oriented southeasterly flow region spanning the tropical-subtropical Pacific that strengthens before dITCZ events. Furthermore, anomalous extratropical north Pacific cyclones and anticyclones develop leading up to nITCZ and dITCZ events, respectively. A meridional momentum budget analysis reveals that both linear and nonlinear dynamics are important to the intensification of cross-equatorial flow associated with nITCZ and dITCZ events. We suggest that more process-level understanding of observed nITCZ and dITCZ events is needed to solidify ITCZ theory and to mitigate climate model ITCZ biases.