Atmospheric Equatorial Wave Observations During the 2019 MISO-BOB Field Campaign

The focus of the 2019 MISO-BOB field campaign conducted from May 27th to July 31st during the southwest monsoons season of the Bay of Bengal (BOB) was the capture and studies of Monsoon Intra-Seasonal Oscillations (MISO) events. To this end, a comprehensive array of ground measurement stations was deployed in Sri Lanka, Maldives, and Seychelles augment ship-based campaigns. During the period of June 1st to July 31st, the frequency of radiosonde launches at WMO sites in Maldives and Sri Lanka was significantly increased. Two MISO-BOB ocean cruises aboard R/V Sally Ride in the Bay of Bengal launched radiosondes at 6 (sometimes 3) hour intervals. While MISO events were rare during the observation period, a few intense weather anomalies of significant interest were observed that might have implications in air-sea interaction studies. A noteworthy anomaly was recorded on July 18th in Sri Lanka, where unusual rainfall (>200 mm) was reported in the Southern and Central highland areas of the nation accompanied by gale force winds (70-80 km/hr) that lasted for several hours. At first it was hypothesized that the event was tied to the breakdown of a high-altitude baroclinic Kelvin wave propagating in the area during the experimental period, but further analysis proves that this was not the case. High-resolution radiosonde data from the land sites accompanied by satellite data from TRMM and ERA 5 show the presence of an Equatorial Rossby wave propagating at a low altitude and an easterly propagating precipitation system on July 18 over Sri Lanka. Interestingly, a baroclinic Kelvin wave was present during the observational period, but it was not related to the atmospheric event that took place in Sri Lanka. Satellite and land site data combined with equatorial wave theory provided an overview of the Equatorial Rossby wave that led to the precipitation system related that caused the event on July 18, 2019. (This work was supported by ONR Grant # N00014-17-1-2334)