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  • Author or Editor: Deirdre A. Byrne x
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George R. Halliwell Jr., Peter Cornillon, and Deirdre A. Byrne

Abstract

Sea surface temperature (Ts) maps of the region from 59.5° to 75.5°W, 22.5° to 33.5°N containing the western North Atlantic Subtropical Convergence Zone (STCZ) were derived from AVHRR/2 images. The 7- year mean annual cycle was removed and the maps were filtered in space and time to represent anomaly variability with wavelengths ≥ 220 km and periods ≥ 50 days. Warm and cold anomaly features were observed cast of 71°W between 26° and 32°N that propagated westward at 3–4 km day−1 and that occasionally exceeded ±1°C in amplitude. They are generally strong and persistent from fall to spring and are only marginally detectable during summer. During 1981–82, 1982–83, and 1985–86, individual features could be followed through the entire fall-spring interval. During 1983–84,1986–87,and 1987–88,they could typically be followed for 2–4 months, and during 1984–85, for only 1–2 months. The features were anisotropic during all fall-spring intervals except 1986–87, and they had characteristic wavelengths of ∼800 km in the minor axis direction and periods of ∼200 days. Local forcing by synoptic atmospheric variability alone could not amount for the existence of these features. Anomaly features propagated westward in a manner consistent with theoretical zonal dispersion properties of first-mode baroclinic Rossby waves, suggesting that the anomalies may be coupled to a field of wavelike eddies. Since the anomalies were confined to the zonal hand of large mean meridional Ts gradients associated with the STCZ, where meridional eddy currents are relatively effective at forcing anomalies these eddy currents could be largely responsible for their existence. In one case, however, the influence of eddies an vertical heat flux at the mixed layer base appeared to be important. The relatively strong and persistent 1985–86 anomaly features appeared during a several-day interval at the onset of relatively stormy fall weather and (presumably) rapid mixed-layer deepening.

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Deirdre A. Byrne, Arnold L. Gordon, and William F. Haxby

Abstract

Warm core rings formed in the, Agulhas Retroflection transfer water from the Indian Ocean to the South Atlantic. In an attempt to measure the strength of this exchange, a combination of satellite altimeter and hydrographic data are used to examine Agulhas eddy paths and decay rates in the South Atlantic. Because the surface dynamic height of a warm core eddy is higher than surrounding waters, the rings are visible in satellite altimeter measurements. Over 20 Agulhas eddies have been tracked from maps of anomalous sea surface height (SSH) derived from the Geosat Exact Repeat Mission (ERM) dataset. The correlation (r2) of dynamic height referenced to 2000 dbar and anomaly SSH for one coincidentally sampled area is 97% within an Agulhas eddy, dropping to a fraction of that outside of it, indicating that the SSH anomaly signal is a reliable measure for strong features like Agulhas eddies.

The sizes and distribution of the Agulhas eddies in the ERM record compare favorably with those in recent hydrographic records from the area. Individual eddy tracks from the ERM show the influence of topography, with slowed translation over area of steep relief. The eddies tracked take a generally WNW course across the South Atlantic, propelled by the mean flow and internal dynamics. While propagating westward, Agulhas eddies decay in amplitude with an e-folding distance of O(1700–3000 km) alongtrack. As they approach the western boundary of the South Atlantic, at 40°W, the eddies have O(10%) of their initial amplitude remaining.

This study finds the residence time of an Agulhas eddy in the South Atlantic to be 3–4 years. On average, the authors find six eddies per year form by the retroflection that enter the South Atlantic. The 20 eddies tracked therefore represent 50%–60% of the population that would have been extant during the ERM. The Agulhas eddies appear to contribute a minimum of 5 × 106 m3 s−1 to the Indian-South Atlantic water mass transfer, with a corresponding energy flux on the order of 1017 J.

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Vladimir M. Kamenkovich, Yurii P. Leonov, Dmitrii A. Nechaev, Deirdre A. Byrne, and Arnold L. Gordon

Abstract

A series of numerical experiments with a two-layer primitive equation model is presented to study the dynamics of Agulhas eddies. The main goal of the paper is to examine the influence of an underwater meridional ridge (modeled after the Walvis Ridge) on an Agulhas eddy hitting it. First, the propagation of an eddy of the specified vertical structure over a flat bottom is considered, varying the initial eddy horizontal scale from 40 to 120 km. Unlike small nonlinear eddies, large nonlinear eddies (on the scale of Agulhas eddies) do not rapidly evolve into a compensated state (no motion in the lower layer). Second, the influence of a ridge on eddies of differing vertical structures having a specified intensity in the upper layer and a prescribed horizontal scale is analyzed. Significantly baroclinic eddies can cross the Walvis Ridge, but barotropic or near-barotropic ones cannot.

The evolution of eddies crossing the ridge is compared with that of initially identical eddies moving over a flat bottom and with field observations. Eddies in our model tend toward the compensated state, with a motionless lower layer, when they cross a steep ridge. This tendency appears largely independent of the initial state of the eddy. Eddies crossing the ridge, show an intensification just before the eddy center encounters the ridge, expressed as a deepening of the thermocline depth and a heightening of the sea surface elevation. This effect is large enough [O(10 cm)] that it should be noticeable in altimeter records such as the one from the Topex-Poseidon satellite. The translational speed and direction of model eddies agree with observations, even in the absence of externally prescribed large-scale currents or friction; model eddies averaged 4.6 km day−1 and moved westward.

The modeled eddies proved an effective transport for passive tracers; tracers initially located near the center of the eddy were transported with practically no losses. The influence of the ridge leads to the substantial increase of the transported tracers. Model eddies show a realistic e-folding scale for amplitude decay of 2680 km. This long scale, combined with the tracer transport, indicates that Agulhas eddies, which cross the Walvis Ridge, are capable of carrying their observed thermal and salinity anomalies far into the South Atlantic subtropical gyre.

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