Observing Atmospheric Waves' Impacts on Troposphere down to Surface during ASIRI-RAWI Campaign

The atmosphere of the equatorial Indian Ocean is a hotbed of phenomena that drive the world's weather and climate variability, phenomena that include Walker and Hadley circulations, trade winds, quasi-biennial oscillation, monsoons, and a host of subseasonal oscillations. Air-sea interactions strongly affect this variability, and thus fluxes across the air-sea interface and underlying oceanic circulation have been central to studying it. An aspect that has received much attention is the subseasonal variability, which includes the Madden-Julian Oscillation (MJO) and equatorial Kelvin and Rossby waves in the atmosphere and ocean. To this end, two Departmental Research Initiatives of the Office of Naval Research are in progress: Air-Sea Interactions in Northern Indian Ocean (ASIRI, 2013-2017) and North Arabian Sea Circulation autonomously researched (NASCar, 2015-2019). As a part of these programs, a dedicated field experiment was conducted to study the atmospheric subseasonal variability, dubbed ASIRI-RAWI (Remote Sensing of Atmospheric Waves and Instabilities; see also http://ceees.nd.edu/research-facilities/projects/asiri-rawi). ASIRI-RAWI investigated the spatiotemporal structure of the troposphere and lower stratosphere as well as their effects on the air-sea interface. During the 6-week field campaign in February and March 2015, a suite of instruments was deployed at three locations (Seychelles, Sri Lanka, and Singapore). The ground-level instrumentation included meteorological flux towers as well as remote sensing Doppler Lidars, microwave radiometers, and ceilometers. Multiple radiosondes were also released daily at each site. This array of instruments, probing from the stratosphere to surface layer, captured a swath of space-time scales and provided measurements of how the surface layer was impacted by waves and dynamics aloft.

Due to the weak phase of MJO during the experimental period, other features of subseasonal variability were accented. The most notable were equatorial Kelvin waves propagating in the stratosphere–troposphere interface. Kelvin waves, particularly convectively coupled waves, have a role in MJO dynamics, as observed in past studies including DYNAMO field campaign. Beyond their impacts on in-phase MJO, Kelvin waves modulate the equatorial atmosphere via influence on stratospheric quasi-biennial oscillation and stratosphere–troposphere exchange. The observed atmospheric waves impacted convective activity and wind burst events during ASIRI-RAWI campaign and so a closer look at these dynamic intraseasonal phenomena is warranted.

2-3 daily radiosonde releases at each site and NCEP Reanalysis data captured the planetary waves' characteristics, especially a 14-16 day period. The Kelvin wave, during its maximum eastward phase, forced a quasi-biweekly perturbation of underlying layers, evident in the spatiotemporal evolution of the troposphere observed by radiosondes. Interactions of the descending westerly disturbances with shear layers (instabilities) and tropospheric air masses (mixing) were observed and found to play key roles in transport and variability of atmospheric momentum, heat, and moisture. The descent culminated in hypothesized “periodic ground strikes” of momentum at experimental sites, and we have studied to what extent this biweekly oscillation, originating in lower stratosphere, affected the surface layer. In this paper, a variety of observed subseasonal phenomena will be presented, together with possible dynamical explanations.