OS24A-01
Western PIRATA upper ocean circulation and water masses
Shipboard PIRATA ADCP and CTD data collected along 38°W from 1999 to 2005 provide seven meridional sections of simultaneous mass and direct velocity fields from the South American coast to 15°N. The Equatorial Undercurrent (EUC) depicts a multiple-core structure in all cruises with two main cores above and below the the 20°C, respectively. High salinity and high dissolved oxygen associated with one of the EUC cores strongly suggest that the thermocline is ventilated by both South and North Atlantic subtropical waters, the Subtropical-Tropical Cells (STC). Therefore, the EUC structure at 38°W shows to be much more complex then thought, consequently it is a key-region on subtropical-tropical exchange, and therefore on climate issues. Thermocline ventilation also occurs in the North Equatorial Undercurrent (NEUC). However, whether this subtropical water is from South or North Hemisphere is still inconclusive. The PIRATA data also provide direct measurements of the northern branch of the wind driven North Equatorial Coutercurrent (nNECC), which lies between 8°N and 14°N, and carries both North and South Atlantic waters. The two NECC cores are 4° apart from each other during boreal winter-spring (FEB1999, MAR2000, APR2002) but connected during summer cruises (JUL2003, JUL2004, and JUL2005). Inter-annual to decadal variability in the Subtropical South and North Atlantic subduction regions affect the circulation in the tropical region through the STCs. Ocean circulation is one of the dynamical mechanisms determining the patterns of the tropical Sea Surface Temperature (SST). Small changes in the SST field is able to trigger atmospheric convective process, which is closely connected with the climate over South America and Africa continents.
OS24A-02
The PIRATA South Western Extension
This note presents the Southwestern extension of the original PIRATA array (hereafter referred to as the PIRATA SWE), along with the analysis of its first year-and-half data time series. The PIRATA SWE is composed of three ATLAS buoys between 19°S, 35°W and 8°S, 30°W, along a ground track of Jason altimetry satellite. The three buoys of the PIRATA SWE were deployed during August 2005 and have been transmitting via both ARGOS and the Brazilian SCD satellites ever since. The data collected in near-real time are available via internet at both NOAA-PMEL and INPE-CPTEC webpages. The first-and-half year of PIRATA SWE data is compared with numerical simulation results using GFDL's Modular Ocean Model (MOM) at CPTEC-INPE. The results show good agreement with PIRATA's vertical profiles of temperature on the upper ocean, but with increasing warm model bias with depth. The observed time series of temperature at the PIRATA SWE sites also reveals intense intraseasonal variability, indicating that the region might be subjected to strong ocean eddy activity. Indeed, complementary ADCP data analysis reveals the presence of strong eddies in the region. The question of model resolution to resolve the observed eddies in the region will also be discussed.
OS24A-03
The Role of the Eddies in the Brazil-Malvinas Frontal System: a Numerical Study
The eddy rich Brazil-Malvinas Frontal System (BMFS) is a region where significant water mass subduction and transformation is taking place and mesoscale activity is known to have an important role in the rate at which these processes occur. An eddy-resolving (10km) multi-year solution for the South Western Atlantic is presented. Its eddy kinetic energy level and Brazil-malvinas confluence seasonal cycle are realistic in regard to altimetric observations. In this context the solution is analyzed for its upper ocean heat balance. The eddies are shown to affect the heat budget in two different ways: through quasi-horizontal eddy fluxes that redistribute heat across the BMFS as a consequence of its baroclinic instability; through vertical eddy fluxes associated with secondary circulations around fronts that are actively strained. While the former are adequately resolved with a 10km mesh grid size, the latter strongly increase with horizontal resolution as revealed by a sensitivity study. Implications for the water mass subduction/transformation are discussed.
OS24A-04 INVITED
The South Atlantic Valve Regulating THC Stability
Abrupt climate change is often related to a collapse of the thermohaline circulation (THC). However, the THC in most IPCC-class ocean-atmosphere models do not fully collapse under global warming. Only unrealistically large fresh water perturbations in the North Atlantic can trigger a collapse, but the circulation often recovers afterward. This indicates that multiple equilibria in these models do not exist. In this talk the inflow in the South Atlantic is considered as regulating the fresh water budget of the Atlantic Ocean and the existence of multiple equilibria of the THC. The compensating or amplifying effects of the azonal circulation in the South Atlantic and coupled ocean-atmosphere feed backs in the Atlantic region are further discussed. Results from intermediate complexity and state-of-the-art coupled ocean-atmosphere models are used to illustrate these processes. The results motivate a South Atlantic observing system to monitor changes in the THC.
OS24A-05
Impacts of no Agulhas leakage on the Tropical Atlantic in a coupled Ocean-Atmosphere simulation
The filaments and rings at the retroflection of the Agulhas Current constitute the mechanism for the transport of subtropical Indian Ocean waters into the Atlantic. Paleo-oceanographic records indicate that the extinction and reappearance of species of foraminifera in the SA in the last glacial period seem to be associated with the closing and reopening of the connection between the subtropical gyres of the South Indian and Atlantic Oceans. These records even suggest that there have been a common start-up of the NADW formation, the reappearance of the species in the SE Atlantic and a poleward displacement of the zero wind stress curl line some 10K years ago. This has motivated us to use a coupled ocean-atmosphere model to investigate the impact of shutting off the Agulhas leakage on the the South Atlantic circulation and on the Tropical Atlantic variability. Our experiments show that the resulting temperature and salinity anomalies would increase the stability of the Equatorial Atlantic mixed layer, which, in consequence, would inhibit the upwelling in the cold tongue region. Unable to reach the surface, after reaching the eastern side of the basin, the anomalous signal transported by the Equatorial Undercurrent would spread away from the Equator in the thermocline and upwells in the Guinea and Beguella Domes.
OS24A-06
Origin and Circulation of Water Masses in the Atlantic Basin
In this study we present initial results from a lagrangian investigation of South Atlantic—Tropical Atlantic interaction in a numerical model. The model used for this work is the Modular Ocean Model (MOM) version 4 coupled to the FMS Sea Ice Simulator, both from the Geophysical Fluid Dynamics Laboratory (GFDL). Global oceans were considered (90°N to 90°S), and for the vertical resolution 50 levels were adopted, 23 of them in the first 225 m, spaced by 10 m. An eddy-resolving grid was set for the Tropical Atlantic, with 1/4 degree for latitudinal and longitudinal resolution between 10°N, 10°S and 60°W, 12°E, decreasing uniformly to about 3 degrees out of this area. The model was integrated for 50 years from Levitus climatology, forced with climatological fields from the dataset used for CORE (Coordinate Ocean-ice Reference Experiments) developed by Large and Yeager (2004). Results show that the model is able to successfully reproduce with detail the wind and thermohaline circulation in the Atlantic basin, allowing the determination of the origin of the different cores of the Equatorial Undercurrent (EUC).
OS24A-07
The Meridional Heat Transport in the South Atlantic Ocean
Fourteen temperature sections collected between July 2002 and May 2006 are analyzed to obtain estimates of the meridional heat transport variability of the South Atlantic Ocean. A methodology proposed by Baringer and Garzoli (2007) is used to calculate the heat transport from temperature data obtained from high-density XBT profiles taken along transects from Cape Town, South Africa to Buenos Aires, Argentina. Salinity is estimated from Argo profiles and CTD casts for each XBT temperature observation using statistical relationships between temperature, latitude, longitude and salinity computed along constant depth surfaces. Full-depth temperature/salinity profiles are obtained by extending the profiles to the bottom of the ocean using deep climatological data. The meridional transport is then determined by using the standard geostrophic method, applying NCEP-derived Ekman transports, and requiring that the salt flux through the Bering Straits is conserved. The results from the analysis indicate a mean meridional heat transport of 0.54 PW (PW = 1015 Watts) with a standard deviation of 0.11 PW. The geostrophic component of the heat flux has a marked annual cycle following the variability of the Brazil Malvinas Confluence Front, and the geostrophic annual cycle is 180° out of phase with the annual cycle observed in the Ekman fluxes. As a result, the total heat flux shows significant interannual variability with only a small annual cycle. Uncertainties due to different wind products and locations of the sections are independent of the methodology used.