Air-sea exchanges at the ocean mesoscale: a driver of the ocean circulation?

Exchanges of heat, water and momentum at the air-sea interface are key to understanding, both oceanic and atmospheric circulations, and their representation in models is crucial to a range of issues: from short-range weather prediction (through an effect on the Atmospheric Boundary Layer (ABL)) up to the climate scale (e.g. intense localized air-sea fluxes in the North Atlantic couple the Gulf Stream and Meridional Overturning Circulation to the atmosphere).

Air-sea interactions on the ocean-basin scale (1000s of km) have been extensively investigated. With the increasing quality of satellite observations of winds and Sea Surface Temperature (SST), it has become apparent that vigorous air-sea interactions also occur on the scale of ocean mesoscale eddies (50-100 km). Ocean eddies populate the global ocean, with thousands of them found at anytime. Observations show that the ABL adjusts to the large SST anomalies associated with ocean eddies. Although details of the adjustment mechanism are still debated, it appears that mesoscale SST anomalies modulate the intensity of the turbulent mixing and modify the strength of the surface winds (i.e. air-sea fluxes of momentum). For example, warm core eddies are associated with a more intense turbulent mixing and stronger surface winds. Through this mechanism, the ocean imprints its turbulent structure onto the global marine ABL and drives a rich atmospheric variability on scales of 100's km.

Air-sea interactions at the ocean mesoscale are a new frontier in our understanding of the coupled system. Up to now, most studies of the Eddy-ABL interaction have focused on the quasi-linear relationship between SST and wind(-stress) anomalies and impacts on the atmospheric circulation. Impacts on oceans have received much less attention. The goal of the proposed work is to understand and to quantify the impact of the Eddy-ABL interaction on the ocean state: is it a fundamental mechanism that needs to be accounted for in the analysis and design of climate models?

The new high-resolution coupled simulation (ORCA012/N512) developed at the Met Office offers a fantastic opportunity to investigate this topic. Preliminary analysis showed that the model captures the SST-wind relationship with the observed intensity, providing a solid basis to explore new ground.

In this PhD project, the student will 1) Quantify the effect of oceanic eddies on air-sea fluxes of heat and moisture, 2) Clarify the mechanisms and quantify the impact on the ocean circulation, and 3) develop and test a parameterization of the Eddy-ABL interaction for models where it is absent.