The development of upper-tropospheric geopotential height anomaly in the Western Hemisphere during MJO convective initiations

This study uses geopotential height budget and composite analysis of reanalysis data to examine the driving dynamics and structural evolution of intraseasonal upper-tropospheric circulation over the Western Hemisphere (WH) prior to and during the convective initiation of the Madden-Julian Oscillation (MJO) over the Indian basin. Previous studies by the authors showed that, during most of MJO convective initiation events over the Indian basin, upper-tropospheric equatorial easterly wind anomaly initially develops in association with subtropical and midlatitude Rossby ridges over the east Pacific basin. As the easterly wind continues to amplify and propagate farther eastward, a negative upper-tropospheric geopotential height anomaly develops over tropical South America and couples with the easterly wind anomaly, forming a structure consistent with theoretical Kelvin waves. This study shows that the development of the negative geopotential height anomaly over South America is driven by net tropospheric adiabatic cooling induced by updraughts. The convection associated with the updraughts is triggered by the intrusion of a midlatitude wave train from the Southern Hemisphere, suggesting that the interaction with midlatitude circulation is a part of the dynamics driving the formation of upper-tropospheric Kelvin wave structure over the WH. During some MJO convective initiation events, the upper-tropospheric zonal wind anomaly is positive over the east Pacific basin and the formation of Kelvin wave structure is weak or absent over South America. Those events subsequently develop weaker MJO convective envelopes over the Indian basin, apparently due to the absence of upper-tropospheric divergence induced by the incoming Kelvin wave circulation that enhances convective updraughts over the region of initiating convection. The results suggest that the formation of the upper-tropospheric Kelvin wave over the WH partly determines the subsequent development of MJO convection over the Indian basin.