The Relationship of MJO Convective Onset to Large-Scale Upper-Tropospheric Dynamic and Thermodynamic Structures

Friday, 19 December 2014
Scott Powell and Robert Houze, University of Washington, Seattle, WA, United States
ERA-Interim reanalysis reveals large-scale (LS) fields of anomalous upper-tropospheric divergence and associated vertical motion that are detectable as early as mid-September 2011 over the Indian Ocean during DYNAMO/AMIE. The LS anomalous motions circumnavigate the globe several times, and the arrival of 150 hPa divergence anomalies and 300 hPa upward motion anomalies coincides with onset of widespread, deep, mesoscale convection over the Indian Ocean. Such motions are linked to at least four, and possibly five, separate MJO events between October 2011 and March 2012. Convergence is present at 150 to 300 hPa during convectively suppressed periods over the central Indian Ocean, and it is associated with subsidence above 500 hPa that at times exceeds the expected downward motion response to clear-sky radiative cooling. At Gan Island, the LS upward motion anomaly arrives as much as 2 days prior to onset of a LS convective event. Divergence anomalies appear within a day of the upward motion anomalies. Positive temperature anomalies arrive prior to the arrival of LS divergence aloft, indicating that the temperature anomalies do not destabilize the troposphere prior to MJO convective onset. We show further that changes in relative humidity at 300 hPa are strongly (weakly) influenced by changes in specific humidity (temperature). The anomalous vertical motions may influence the specific humidity variability. LS convective events can develop after LS subsidence moves out of the Indian Ocean as follows: Subsidence suppresses convection through drying, and convection flourishes when the subsidence propagates out of the area. Model simulations in a latitudinally wide WRF channel can replicate some of the LS structures of eastward propgating divergence and vertical motion even if domain boundaries are located in mid-latitudes; however such signals appear only during the October case. Modeled LS vertical motion and divergence is illustrated using a variety of boundary conditions. Additionally, the ability to replicate LS vertical motions in a highly idealized model with limited physics is demonstrated.