Dynamics of the South East Atlantic Ocean

Principal Investigator:

Gustavo Goni

Project Description:

Historical hydrographic data was used to compute the mean upper layer thickness and reduced gravity in the South Atlantic. These parameters along with TOPEX/POSEIDON (T/P) sea height anomaly (SHA) are used to derive the upper layer thickness of the ocean. This product was used to identify and track anticyclonic (warm) rings in the region.

The upper layer and ring dynamics can be studied using the common two layer representation of the vertical structure of the ocean. The upper layer thickness, which goes from the surface to the depth of the thermocline (10C) can be estimated using altimeter-derived sea height anomalies (Goni et al, 1996):

(1)

where:

h₁(x,y,t) is the altimeter-derived upper layer thickness,

is the mean upper layer thickness,

is the upper layer thickness anomaly,

A(x,y) is a constant derived from the reduced gravity,

SHA(x,y,t) is the altimeter-derived sea height anomaly and,

C(x,y) is proportional to the barotropic contribution.

Figure 1 shows (a) the mean upper layer thickness from historical hydrographic data, (b) A, the inverse of the reduced gravity and (c) the reduced gravity.

During August 1997 near real time T/P SHA data was used to identify several anticyclonic rings in the region. The upper panel of figure 2 shows the SHA corresponding to August 26, 1997. The positive anomalies are associated with anticyclonic circulation. The alternation of positive and negative values at 38S between 15 and 25W is due to the oscillation of the Agulhas Retroflection and shedding of rings. The lower panel of figure 2 shows the upper layer thickness derived using (1). Several anticyclonic rings, identified by their larger upper layer thickness, can be detected in the Agulhas ring corridor.

In September 1997, as part of the Benguela Current Experiment, a cruise on the R/V Seward Johnson guided by the maps shown in figure 2 identified and surveyed three rings. In figure 3 the crusie track from this experiment is overlain on figure 2.

The upper layer thickness map corresponding to September 16, 1997, is shown in figure 4. The cruise tracks (blue line), hydrography-derived depth of the 10C isotherm (red contours), altimeter groundtracks (dots), and bathymetry (black contours) are superimposed in the figure. The altimeter-derived locations of the rings are remarkably similar to those found using hydrographic data. The maximum values of upper layer thickness are approximately 10% smaller in the altimeter-derived field. This might be due because the altimeter did not sample the rings through their center, or because of attenuation of the sea height anomaly signal due to the T/P 10 day smapling period. Similar upper layer thickness maps are constructed every day and used to track these three rings back in time. Figure 5 is a plot of the tracks each of these three rings took from shedding through the fall of 1997.