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The development of upper‐tropospheric geopotential height anomaly in the Western Hemisphere during MJO convective initiations
Author(s) -
Sakaeda Naoko,
Roundy Paul E.
Publication year - 2015
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.2696
Subject(s) - geopotential height , madden–julian oscillation , troposphere , middle latitudes , anomaly (physics) , geology , climatology , convection , atmospheric sciences , atmospheric circulation , meteorology , geography , precipitation , physics , condensed matter physics
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 W estern H emisphere ( WH ) prior to and during the convective initiation of the M adden– J ulian O scillation ( MJO ) over the I ndian 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 R ossby ridges over the east P acific basin. As the easterly wind continues to amplify and propagate farther eastward, a negative upper‐tropospheric geopotential height anomaly develops over tropical S outh A merica and couples with the easterly wind anomaly, forming a structure consistent with theoretical K elvin waves. This study shows that the development of the negative geopotential height anomaly over S outh A merica 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 H emisphere, suggesting that the interaction with midlatitude circulation is a part of the dynamics driving the formation of upper‐tropospheric K elvin wave structure over the WH. During some MJO convective initiation events, the upper‐tropospheric zonal wind anomaly is positive over the east P acific basin and the formation of K elvin wave structure is weak or absent over S outh A merica. Those events subsequently develop weaker MJO convective envelopes over the I ndian basin, apparently due to the absence of upper‐tropospheric divergence induced by the incoming K elvin wave circulation that enhances convective updraughts over the region of initiating convection. The results suggest that the formation of the upper‐tropospheric K elvin wave over the WH partly determines the subsequent development of MJO convection over the Indian basin.