IDD20730 Australian Government Bureau of Meteorology Northern Territory Darwin RSMC - Australia

El Nino-Southern Oscillation Update
The 30-day Southern Oscillation Index [SOI] to 27 June was -4. Contributing pressure anomalies were +0.4 hPa at Darwin and -0.1 hPa at Tahiti. The official monthly SOI for May was -5, and its 5-month running mean [centred on March] was +6.

The 2008/09 southern hemisphere summer period showed many La Nina like characteristics, but these have eased over recent months, with the climate system now portraying an ENSO-neutral state. The SOI has remained mostly within one standard deviation of the long-term mean since early March. Enhanced easterly low-level winds over the near-equatorial western Pacific were a persistent feature during late 2008 and early 2009 but averaged over the last month or so, have been weaker than average in the eastern Pacific. However, the sea-surface temperatures [SST] of the tropical Pacific have recently shown a basin-wide warming trend. Most ENSO forecasting models captured this warming and suggest further warming in the Pacific, implying a greater than average possibility of an El Nino event developing during the southern hemisphere winter/spring period. See the Bureau's "ENSO Wrap-Up" at www.bom.gov.au/climate/enso/ which includes a compilation of ENSO dynamic model predictions.

Intra-Seasonal Patterns
The first broadscale active phase of the Madden-Julian Oscillation [MJO] of the 2008/09 southern hemisphere's summer monsoon season became evident in the central Indian Ocean around mid-November. Subsequent active phases appeared to develop about the central Indian Ocean or western Indonesia around mid-December, mid/late January, late February and early/mid April. Not all events during the period retained a coherent MJO structure while propagating eastward to the Pacific basin. See http://www.bom.gov.au/bmrc/clfor/cfstaff/matw/maproom/OLR_modes/h.6.ALL.EQ.html.

A renewed pulse of enhanced convection associated with a weak MJO event, the first of the 2009 northern hemisphere summer monsoon season, developed during early/mid May in the Indian Ocean and moved east over the Maritime Continent and north toward the Indian sub-continent. Time-longitude plots of outgoing long-wave radiation [OLR - a proxy for cloudiness] show this signal to have a nearly continuous link to the previous MJO event over the Maritime Continent in April. [See http://www.bom.gov.au/bmrc/clfor/cfstaff/matw/maproom/ OLR_modes/h.6.ALL.EQ.html]. The onset of the monsoon over India took place on 25 May, about 5 days earlier than the long-term average date. Convection then became below average over most of the tropical Indian Ocean during early June, consistent with the inactive MJO over that region. Enhanced convection was then seen to develop in the north Indian Ocean and Arabian Sea during the past fortnight which may be attributed to a weak but slowly developing MJO signal. As a result, the northern hemisphere monsoon may remain active, initially over the longitude range of the western and central Indian Ocean, with a tendency for eastward propagation. It seems unlikely that the focus of enhanced tropical weather would reach the longitudes of the Maritime Continent [including northern Australia] within the next one to two weeks. The impacts on northern Australia, of an active MJO event over the Indian Ocean region, is typically enhanced southeasterly trade wind flow.

* Darwin RSMC chart area extends from 40S to 40N, 70E to 180
The Weekly Tropical Climate Note is updated each Tuesday by about 0330 UTC.
http://www.bom.gov.au/climate/tropnote/tropnote.shtml
For more information please contact climate.tropical@bom.gov.au