Marine Atmospheric Boundary Layer structure during an episode of cold front passage over the coastal region of the Southwestern Atlantic Ocean

Studying the ocean-atmosphere interaction processes is essential to understand and to characterize the marine atmospheric boundary layer (MABL) and its effects over the weather and climate of the coastal regions and the deep ocean. This work is part of an initiative to better understand the effects of the Southwestern Atlantic Ocean on the coastal regions off the south of Brazil.

Among the peculiarities of the coupled ocean-atmosphere system of the southern coast of Brazil, the effects in the atmosphere caused by the thermal horizontal contrasts occurring between the water masses transported by the Brazilian Coastal Current (BCC) and the Brazil Current (BC) during winter are basically unknown to the present.

We guess that the MABL along across-shore transects in the southern coast of Brazil during winter may be modulated by the presence of the strong horizontal thermal contrasts between these currents.

The first in situ observations made over the coupled ocean-atmosphere system at the synoptic scale at the southern region of Brazil were recently taken during the research cruize ACEx (Atlantic Ocean Carbon Experiment)/SIMTECO (Integrated System for Monitoring the Weather, the Climate and the Ocean), from 11 to 21 June 2012 onbord the Research Vessel Cruzeiro do Sul.

Satellite images were previously analyzed in order to direct the vessel towards the maximum thermal contrasts between the BC and the BCC and that this study was 0.09 ºC/km. The objective of the present study is to describe the behavior of the MABL from oceanographic and atmospheric radiosondes measurements taken between the region of the deep ocean and the coastal region of Brazil at latitude 29 ºS and estimate the sensible and latent heat fluxes from data station automatic the vessel and sea surface temperature (SST) thermosalinograph following the bulk formulas described in Fairall et al. (1996).

The characterization of atmospheric synoptic system was performed using the reanalysis data provided by NCEP Climate Forecast System (CFS) version 2 at 0.5 º grid spacing and temporal resolution of six hours. Along the transect, which duration about 36 hours, the passage of a cold front and a case of cold thermal advection associated with the incursion of mass post-frontal air was observed. To identify the cold front objective criteria that determine the passage of synoptic systems by increased atmospheric pressure, decrease in air temperature and changes in the wind meridional component at 925 hPa were applied.

The height of the top of MABL was established through the virtual potential abrupt change in temperature (Θ_v) and specific humidity (q) and the estimated position of maximum vertical slope of Θ_v. Based on the average behavior of the vertical profile of Θ_v e q obtained in the prefrontal condition and the warm waters (mean SST of 21.2 ºC) originating in BC there was a well-developed mixed layer with height above 800 m. Have the post-frontal and traversing waters of BCC (SST average 19.2 ºC) the vertical structure was more stratified, which indicates the transition to a more stable layer and vertical mixing condition can be observed only in the first 200 m. The air temperature was lower than the SST during the journey, also resulting in a flow of heat from the ocean to the atmosphere. CB on the average latent heat flux was 143.9 W.m^-2 while the cold side this average was 119.1 Wm^-2. The results point to a strong contribution of the effects of ocean currents along the front oceanographic BC/BCC added the passage of transient large scale system in the modulation of MABL.