Eastward Disturbances in the Tropical Pacific: NH Extratropical Forcing and Impacts on the Shallow Meridional Circulation

Eastward disturbances, defined here as eastward propagating tropical convective systems organized at synoptic scales, play an important role in the Intertropical Convergence Zone (ITCZ). This study aims to characterize eastward disturbances in the tropical Pacific during boreal summer (DJF) and spring (MAM) when the Northern Hemisphere (NH) Rossby wave activity is strong. We applied empirical orthogonal functions to the eastward 2.5 to 20-day band brightness temperature and the first principal component is regressed against dynamic fields from MERRA2 at different lag days. Our results suggest that NH Rossby waves force eastward tropical disturbances during DJF and MAM but via different mechanisms. During DJF, extratropical NH Rossby waves insert positive potential vorticity into the Tropics that modulates short-lived eastward disturbances located at 15°N, just north the ITCZ, with a phase speed of 10 m/s. During MAM, Kelvin waves initiated in the west Pacific propagate at 15 m/s at 7°N, along the ITCZ axis, and interact with NH Rossby waves, which induce upper-level divergence in the Kelvin waves. The shallow meridional circulation (SMC) in the east Pacific ITCZ is also studied. During DJF, the short-lived eastward waves do not impact the SMC, but the deep circulation in the northern part of the ITCZ strengthens. During MAM, shallow convection ahead of the Kelvin wave is observed and enhances the SMC in the southern part of the ITCZ. Therefore, our results distinguish between two types of eastward disturbances in the tropical Pacific during DJF and MAM, with different physical mechanisms and impacts in the central and east Pacific ITCZ.