Measurements of Sea Surface Height Variability in the Eastern South Atlantic from Pressure Sensor–Equipped Inverted Echo Sounders: Baroclinic and Barotropic Components

Abstract

Variability in sea surface height (SSH) can be decomposed into two contributions: one from changes in mass in the water column (barotropic) and the other from purely steric changes (baroclinic). Both contributions can be determined from data recorded by a pressure sensor-equipped inverted echo sounder (PIES). PIES data from the Agulhas South Atlantic Thermohaline Experiment (ASTTEX) were used, collected in the Cape Basin off South Africa, along 1000 km of an eddy corridor where Agulhas eddies carry cores of warm, salty Indian Ocean waters into the South Atlantic. The paper presents in detail the method used to convert PIES measurements into barotropic, baroclinic, and total SSH, and discusses the error budget. The baroclinic contribution is geopotential height (reference 4500 dbar), which can be determined from the measured vertical acoustic travel time together with a lookup curve based on the regional hydrography. The main error source is scatter about this curve that depends on the extent to which water masses advecting along each geopotential streamline may derive from different ocean regions. The barotropic contribution can be determined from the bottom pressure measurements of the mass variation in the water column and has an uncertainty due to sensor calibration drift in two years corresponding to 1-cm water column height. The barotropic component accounts for 20% of the overall SSH variance and 47% during large signal intervals exceeding 15 cm. PIES data demonstrate via the two measurements that barotropic and baroclinic contributions may work independently or in concert in different mesoscale eddies. The combined structure need not be equivalent barotropic. In particular, deep barotropic eddies exhibit mesoscale spatiotemporal scales and may or may not be systematically tilted or aligned in space or time relative to baroclinic eddies.

Transformation of an Agulhas eddy near the continental slope

“…They interact with each other, merge, split, and change shape, as seen in hydrographic and satellite data, directvelocity measurements, model simulations, and pressuresensor equipped inverted echo sounder data (Arhan et al, 1999;Richardson et al, 2003;Boebel et al, 2003;Matano and Beier, 2003;Baker-Yeboah, 2008). Interestingly, an Agulhas ring is often accompanied by a cyclonic eddy in the Cape Basin, and when propagating together these counterrotating eddies support a mutual advection or "dipoleadvection" (Baker-Yeboah, 2008). Cyclonic eddies are shed into the South Atlantic south of Africa and are often instrumental in the formation of Agulhas rings .…”

“…Previous studies by Boebel et al (2003) suggest that cyclones can form within the Cape Basin along the African shelf, inshore of the Agulhas Current, and in the subantarctic region south of Africa, and they are generally small, 120 km in diameter, compared to Agulhas rings. Recent studies by Baker-Yeboah (2008) reveal that cyclonic eddies can have comparable scales to Agulhas rings and that they can exist along the continental slope of southern Africa in association with Agulhas rings.…”

“…The continental slope and mean depth give a similar topographic beta effect of 2.5×10 14 (m s) −1 associated with the western coast of southern Africa, which is similar to that reported in Smith and O'Brien (1983) and in Sanson and van Heijst (2000). We use observations presented in Baker-Yeboah (2008) in conjunction with numerical simulations to show that eddy-slope interaction provides a dynamical link between the slope waters, shelf waters, and the open ocean in the Benguela upwelling regime.…”

“…Correspondence to: S. Baker-Yeboah (sbaker@mit.edu) Agulhas rings in the Agulhas Basin near the formation region off of South Africa can have mixed barotropicbaroclinic velocity structure (van Aken et al, 2003), as can some of those in the Cape Basin beyond the Agulhas ridge system (Baker-Yeboah, 2008). After formation, these eddies continue to adjust as they interact with local topography and other eddies, e.g., Agulhas rings can split as they interact with the Erica seamount (Arhan et al, 1999;Baker-Yeboah, 2008) or with cyclonic eddies (Baker-Yeboah, 2008).…”

“…After formation, these eddies continue to adjust as they interact with local topography and other eddies, e.g., Agulhas rings can split as they interact with the Erica seamount (Arhan et al, 1999;Baker-Yeboah, 2008) or with cyclonic eddies (Baker-Yeboah, 2008). In addition, Agulhas rings can interact with the western continental slope along southern Africa.…”