Downstream Influence of Sumatra Island on DYNAMO Weather Events

The Dynamics of the Madden-Julian Oscillation (MJO) or DYNAMO field campaign was conducted during the period 1 October 2011 – 31 March 2012. An array of atmosphere and ocean measurement platforms was situated in the central Indian Ocean during DYNAMO with the objective of investigating processes associated with the initiation of the MJO. During the special observing period, 1 October – 30 November, when both atmospheric sounding sites and research ships were in full operation, two prominent MJOs passed through the DYNAMO experimental domain.

Throughout much of the October-November period, the low-level flow over the Indian Ocean was westerly, consistent with the seasonal-mean flow during this period. However, during certain periods of the MJO passages, the flow over Sumatra and the surrounding region was easterly such that the eastern portion of the equatorial Indian Ocean was downstream of the island. Sumatra, which is oriented in a northwest-southeast direction, straddles the equator extending from approximately 5 N to 6 S.

Observations from DYNAMO indicate that Sumatra exerted at least two potentially important impacts on weather events over the DYNAMO intensive observational array, which was ~2500 km to its west. First, during the easterly flow regimes, diurnally generated precipitation features over Sumatra moved westward of the island maintaining coherence over distances up to several thousand kilometers as they approached the DYNAMO array. In October, there was a prominent two-day signal in the precipitation over the DYNAMO array. The extent to which this two-day signal is related to the diurnal signal from Sumatra is being explored.

Second, wake vortices at the northern and southern ends of Sumatra (cyclonic in both hemispheres) appeared at times during easterly flow, had considerable longevity, and moved well to the west of the island influencing the weather downstream. Both the diurnally generated disturbances and the wake vortices will be discussed in the presentation