Estimating the energy released from a mesoscale oceanic eddy to the atmosphere from in situ vertical profiles of scalars

Mesoscale oceanic eddies play an important role to a number of oceanic processes, including its circulation and the transport of properties, such as heat, momentum and salt. They are also relevant to the air-sea exchange, as their peculiar surface characteristics may favor their location as a preferential site for the exchange of heat between the two mediums. In fact, a significant portion of the total poleward energy transport may be provided by such structures, but it is quite difficult to estimate properly the total amount of heat energy transported by a given eddy.

INTERCONF is a scientific program that aims at studying both oceanic and atmospheric dynamics at the Southwestern Atlantic Ocean, with a special focus at the Brazil-Malvinas Confluence region, as well as higher-latitude environments. As part of this program, in November 2009, a Brazilian research vessel crossed a warm oceanic eddy shed by the Brazil Current in the Brazil-Malvinas Confluence region. A total of 14 atmospheric and 26 oceanic profiles were obtained during the cruise, sampling waters located at both the northern (warm) and southern (cold) sides of the confluence, as well as within the eddy itself. The atmospheric profiles were obtained by radiosondes, that provided information on pressure, temperature, humidity, wind speed and direction. The oceanic profiles were taken by expendable bathy-termographs which provided information on water temperature with respect to depth. In the present study, the total energy released from the warm eddy sampled in the cruise is estimated in both its latent and sensible forms. To do that, the atmospheric specific humidity and temperature perturbations above the eddy with respect to their values outside the mesoscale structure are compared and integrated vertically across the marine atmospheric boundary layer. Such determination demands a careful analysis of typical meridional sea surface temperature gradients, to guarantee that the estimates are not being affected by any modes of variability not associated with the oceanic eddy. Similarly, thermal and moisture advections need to be accounted for and a methodology to do so is presented herein.

Results indicate that a total of 1.6 × E17 J are released from the eddy's surface as latent heat, while the atmosphere above this oceanic structure absorbs 1.28 × E16 J as sensible heat, summing up to an energy release of 1.32 × E17 J. This energy is associated to the specific humidity and temperatures sampled during the cruise. When that value is compared to the eddy surface flux estimates, it indicates that the mean residence time for the air above the eddy is close to 1.3 days. This residence time regards only to the heat flux at the ocean-atmosphere interface. Considering a typical lifetime of 3 months for a mesoscale oceanic eddy in the Brazil-Malvinas Confluence region, and assuming that its structure is preserved along such period, the total energy transferred to the atmosphere from the eddy surface is estimated at 9.3 × E18 J.