On the relationship between satellite‐retrieved surface temperature fronts and chlorophyll a in the western South Atlantic

Abstract

The time‐space distribution of chlorophyll a in the southwestern Atlantic is examined using 6 years (1998–2003) of sea surface color images from Sea‐viewing Wide Field of View Sensor (SeaWiFS). Chlorophyll a (chl a) distribution is confronted with sea surface temperature (SST) fronts retrieved from satellite imagery. Histogram analysis of the color, SST, and SST gradient data sets provides a simple procedure for pixel classification from which eight biophysical regions in the SWA are identified, including three new regions with regard to Longhurst (1998) work: Patagonian Shelf Break (PSB), Brazil Current Overshoot, and Zapiola Rise region. In the PSB region, coastal‐trapped waves are suggested as a possible mechanism leading to the intraseasonal frequencies observed in SST and chl a. Mesoscale activity associated with the Brazil Current Front and, in particular, eddies drifting southward is probably responsible for the high chl a values observed throughout the Brazil Current Overshoot region. The Zapiola Rise is characterized by a local minimum in SST gradient magnitudes and shows chl a maximum values in February, 3 months later than the austral spring bloom of the surroundings. Significant interannual variability is present in the color imagery. In the PSB, springs and summers with high chl a concentrations seem associated with stronger local northerly wind speed, and possible mechanisms are discussed. Finally, the Brazil‐Malvinas front is detected using both SST gradient and SeaWiFS images. The time‐averaged position of the front at 54.2°W is estimated at 38.9°S and its alongshore migration of about 300 km.

Variability and 20-Year Trends in Satellite-Derived Surface Chlorophyll Concentrations in Large Marine Ecosystems around South and Western Central America

“…The circulation is characterized by the northward flow of the Malvinas Current carrying cold nutrient-rich and relatively fresh water of subantarctic origin, and the southward flow of warmer nutrientpoor and saltier waters from the Brazil Current Palma et al, 2008;Matano et al, 2010). The strong frontal zone that separates these distinct water masses presents high mesoscale variability in the BMC region (e.g., Saraceno et al, 2004), which has in turn a strong effect on species distribution (Brandini et al, 2000). The encounter of shelf waters of subantarctic and subtropical origin close to the mouth of the Río de la Plata near 32 • S (Figure 1) also leads to a cross-shelf front known as the Subtropical Shelf Front (STSF), which appears to be an extension of the transition observed at the BMC over the shelf .…”

Malvinas-slope water intrusions on the northern Patagonia continental shelf

“…Thus, shelf open-ocean interactions between the MC and the outer shelf off northern Patagonia do not appear to be dominated by mesoscale eddies, as further north. This portion of the slope region presents intense chlorophyll blooms (Podestá, 1997;Longhurst, 1998;Saraceno et al, 2005;Romero et al, 2006;Signorini et al, 2006) and is important for the life cycle of a variety of species throughout the water column and in the benthic domains (Podestá, 1989;Bertolotti et al, 1996;Sanchez and Ciechomski, 1995;Rodhouse et al, 2001;Acha et al, 2004;Bogazzi et al, 2005). It has recently been suggested that downstream divergence of slope currents flowing in the direction of coastal waves, such as the Malvinas Current, caused by bottom friction and lateral diffusion, is compensated by upwelling near the shelf break .…”

Analysis of SeaWiFS imagery over the southwestern Atlantic Ocean during the March 2002 R/V IOFFE cruise

“…Over the slope the MC presented chl-a between 0.2-0.9 mgm -3 , while in the inner shelf the values varied from 0.2 to 2.5 mgm -3 . These higher chl-a in the inner shelf can be associated to the shelf fronts created by strong vertical mixing that is promoted by very energetic tidal currents in the region 13,14 . An interesting coccolithophore bloom is evidenced along the Patagonian shelf break in red color, with chl-a between 4 -7 mgm -3 .…”