Moistening Processes for the October-November 2011 MJO Events (Invited)

The mechanisms for tropospheric moistening during the developing phase of the MJO over the Indian Ocean are yet to be fully understood. Multiple processes may be at play: horizontal advection of moisture, moistening by evolving cloud populations, changes in the boundary layer in response to SST warming, etc. Unraveling the key mechanisms has been a major challenge; however, it is expected that data from the CINDY/DYNAMO/AMIE field campaign will provide new insights on this problem. Sounding data from the field campaign are used along with radar observations to address this issue for the October and November 2011 MJO events. The MJO signal for both the October and November MJOs was strongest in the northern sounding quadrilateral, so attention is focused on results for that array. During the first half of October, large-scale subsidence and horizontal advection of dry air over the northern array was offset by shallow cumulus moistening, much like the trade-wind regime. Throughout this period, the SST increased, with a superimposed diurnal cycle, and the atmospheric mixed layer deepened to over 700 m. Then in mid-October, abrupt moistening of the mid-troposphere occurred as cloudiness increased over the array. Evidence from the moisture budget points to an important role of hydrometeor storage and/or advection into the domain between 15 and 17 October when considerable moistening occurred. In addition, zonal advection of moisture by easterlies above 700 hPa and meridional advection by southerlies throughout much of the column moistened the atmosphere over the northern array in mid-October. A similar sequence of events occurred prior to the active phase of the November MJO, including further evidence of the role of hydrometeor storage just past mid-month, although there were also important differences. Easterly advection of moisture from the maritime continent was more prominent for the November MJO than in October. SST increased during the build-up phase, although there was a higher-amplitude diurnal cycle in November due to weaker surface winds. The moisture budget also shows evidence of moistening by shallow cumulus and congestus, with a prominent diurnal cycle. Work is underway to compare these sounding-based findings with radar observations during the campaign.