The Role of Ocean Stratification in the Propagation of Intraseasonal Oscillations

Intraseasonal Madden-Julian Oscillation (MJO) atmospheric forcing exerts a profound influence on the near ocean surface layer of the tropics through the coupled air-sea system that in turn affects the structure, development and propagation of the mesoscale convective systems that are part of the MJO. Models suggest that an accurate depiction of the upper ocean stratification in the MC is necessary to correctly reproduce intraseasonal variability. Nonetheless, the dynamics and time scales of the processes and characteristics at the air-sea interface during MJO events are still not well understood.
One large gap in our understanding is the role of upper ocean salinity in MJO variability. Since salinity controls stratification throughout much of the MC it can play a critical role in the complex coupling of the air-sea system during MJO events. In many MC regions, a salt-stratified but isothermal “barrier layer” can exist that traps fluxes of heat, freshwater, and momentum to a thin surface layer. Climatological variations in the thickness of the barrier layer during MJO events are known to drive SST anomalies that influence the coupled air-sea system. Similarly, few studies of the diurnal-intraseasonal interaction within the MC have considered the role played by salinity in setting the diurnal ocean stratification. Yet the global maximum in sea surface salinity diurnal amplitude lies within the MC region.
The main aim of the proposed effort is to understand the mechanisms responsible for upper ocean stratification variability in the MC, with a particular attention on near surface salinity stratification and how this influences the structure and propagation of MJO convection and winds. High-resolution ship-board measurements of the upper ocean temperature and salinity will be obtained using a portable underway CTD (uCTD) system. The data will provide distinct case studies of the ocean conditions during MJO events, that will be examined in concert with remotely-sensed and other in situ datasets, with science objectives to (1) determine the characteristics and the time and space scales of upper ocean salinity variability of importance to MJO variability; (2) identify the main forcing mechanisms that control that salinity variability; and (3) establish connection with the intraseasonal MJO atmospheric phenomena and relationship to the propagation characteristics (speed, intensity, MJO phase, geographical location etc) of the MJO across the MC region.
Relevance to Competition: The proposed research is a contribution to the CVP – Observing and Understanding Processes Affecting the Propagation of Intraseasonal Oscillations in the Maritime Continent Region. The MJO is the dominant mode of intraseasonal variability in the global tropics influencing the monsoon systems, convective patterns, cyclogenesis and triggering of El Niño events. The MC plays a special role in the behavior of the MJO, critically affecting the propagation speed and intensity of the convective systems in ways that are not yet fully understood. The proposed research aims to provide new information on the processes that control upper ocean stratification in the MC region and so enable a better representation and prediction in models of the characteristics, structure and evolution of the MJO propagation in the MC. The project directly addresses the key NOAA long-term goal of improving scientific understanding of the Earth’s climate variability.