Modeling the Variability of the Greater Agulhas Current System (original) (raw)
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Ocean/atmosphere heat fluxes within the Agulhas Retroflection region
Journal of Geophysical Research, 1988
The southern section of the Agulhas cycle is a characteristic feature of northern western boundary current system exhibits unique hemisphere currents and has been documented in characteristics as regards ocean/atmosphere heat detail for the Gulf Stream [Bunker, 1976]. flux processes. The Agulhas Retroflection Southern hemisphere western boundary currents region's high heat flux core from 37øS to 41øS, have received little attention as regards air/sea 16 øE to 22 øE does not demonstrate a distinct interaction, most likely due to a paucity of annual cycle of turbulent heat fluxes (latent and measurements. A short reference is made to the sensible) as is characteristic of its northern Agulhas and Brazil currents by H6flich [1984] in hemisphere counterparts. Rather, a weak semi-his review of the climatology of the South annual heat flux cycle is found with maximum Atlantic Ocean. average losses during winter and summer (200 and This study focuses on the southern reaches of 211 W/m 2) and minimum losses during spring and the Agulhas Current (AC) system located south of autumn (185 and 162 W/m2). Upstream where the Africa. Volume flux of 130 x 106 m3/s in this Agulhas Current is closer to land, winter heat area [Jacobs and Georgi, 1977] suggests that the losses exceed those of summer, but the AC system is the major western boundary current differences are small. This behavior contrasts of the southern hemisphere [LutJeharms and with that encountered at the poleward ends of Gordon, 1987]. Within this extensive ocean area northern hemisphere western boundary currents the AC flows poleward toward the southeast where winter heat fluxes are several times those Atlantic Ocean with the continental shelf as its of summer. The main reason for this difference northern boundary. The AC usually retroflects is persistent westerly and southwesterly wind back into the southwest Indian Ocean between 15øE flow over the Agulhas Retroflection region and 22øE and follows the northern edge of the throughout the year which ensures that cold, Subtropical Convergence (STC) (Figure 1). Eddies unsaturated maritime air repeatedly forces loss and rings, shed along the western margins of the of heat from the ocean's surface. Spatial heat Agulhas Retroflection, introduce variable amounts flux gradients associated with the Agulhas-of warm water into the southeast Atlantic Ocean Subtropical Convergence surface temperature front [LutJeharms, 1981; Gordon, 1985; Walker, 1986]. are more pronounced in summer than in winter, Gordon [1986] has proposed that the global indicating that cyclogenesis locally may be less oceanic circulation cell is completed by exchange seasonally dependent than in the northern of thermocline water between the southwest Indian hemisphere situation. Average oceanic cooling and southeast Atlantic oceans, a process rates in the core region of the Retroflection, accomplished primarily through ring shedding. based on net heat flux calculations and a mixed Thus Agulhas rings may play a decisive climatic surface layer of 75 m, range from 1.35øC/month role through their contribution to global oceanic during winter to 0.25 øC/month during summer. heat transfer [Gordon, 1985]. Interannual variability in ocean/atmosphere heat Once formed, Agulhas rings lose heat and water fluxes within the Agulhas Retroflection region to the atmosphere which alters their mixed layer often exceeds the variability illustrated by the characteristics of temperature, salinity, and annual cycle. West of the Agulhas Retroflection density. Knowledge of local heat flux processes core region, interannual sea surface temperature is crucial to an understanding of the climatic (SST) anomalies are more influential in the implications of this interocean heat transfer. generation of heat flux anomalies by virtue of Furthermore, oceanic heat losses induce their large temporal variability. This high SST atmospheric modifications via the marine boundary variability is primarily attributed to layer which influence local weather and climate interannual changes in flux of Agulhas Current patterns [Jury and Walker, 1988] (also M.S.J. water into the southeast Atlantic Ocean. Oceanic Harrison et al., Circulation differences between heat loss within this warm water zone is an one wet and one dry month of October over the important modifying influence to both ocean and interior of South Africa, and M.S.J. Harrison and atmosphere, thus meriting further research. N.D. Walker, Circulation differences between one 10 ø-30øE. Results have Copyright 1988 by the American Geophysical Union. revealed that heat flux behavior of the southern reaches of the AC system is unusual in comparison Paper number 88JC03112. with that of northern hemisphere western boundary 0148-0227/88/88JC-03112505.00 currents. 15,473 15,474 Walker and Mey' Heat Fluxes of Agulhas Retroflection Region
Surface heat fluxes and marine boundary layer modification in the Agulhas Retroflection region
Journal of Geophysical Research, 1990
The Subtropical Convergence Agulhas Retroflection Cruise, from February 12 to March 4, 1987, provided an opportunity for studying variability of surface heat fluxes and marine boundary layer modification within an ocean area where few atmospheric measurements have been made. From 3-hourly surface observations it has been ascertained that surface heat flux processes are enhanced under cold air outbreak, postfrontal synoptic conditions and over warmer ocean areas. A maximum turbulent heat loss of 828 W/m 2, 4 times the climatological value, was found. Over the cooler Subtropical Convergence waters and over the Agulhas Bank, heat fluxes were comparatively low. Two transect lines were performed across the Subtropical Convergence-Agulhas SST front. In both cases, air temperatures, dew point temperatures, and particularly wind speeds increased over the warmer water owing to enhanced vertical mixing resulting from oceanic heat losses. The second transect revealed marked vertical shear of the zonal wind. The essential elements for cyclogenesis, namely, low static stability and strong low level baroclinicity, were found to be characteristic of the region. 1. INTRODUCTION The Agulhas Current is classically thought to be derived from the East Madagascar and Mozambique currents [Paech, 1926] and reaches its full velocity and momentum flux stature near 33 øS, 27 øE (East London) [Harris et al., 1978]. The current core separates from the Agulhas Bank, near 20*E, after which it retroflects south and eastward. This retroflection occurs in two main spatial modes: the more common at 19 øE + 1.5 ø and the less well known at 13 øE + 1.0 ø [Harris et al, 1978]. The Agulhas Retroflection region (ARR), with its associated features, is intensely dynamic and is considered important to global oceanographic heat exchange [Gordon, 1986] and to the atmospheric heat exchange of the southern hemisphere [Walker and Mey, 1988]. The Subtropical Convergence and Agulhas Retroflection Cruise (SCAR. C) was vndertaken on the research vessel S•I. •lgulhas from February 12, 1987, to March 4, 1987. Main objectives were to study the general circulation and biological processes within the region, focusing on Agulhas ring dynamics, interaction of Agulhas and Subantarctic waters at the Subtropical Convergence (STC) and the overlying air masses. In February 1987 the Agulhas Current retroflection was situated near 40 øS, 20 øE, and five mesoscale eddies or rings in various stages of decay were identified [Lutjeltarms, 1987] (Figure 1). These eddies were well mixed and characterized by a low level of biological activity in comparison 1Now at Department of Physical Oceanography, University of Cape with the STC-ARR sea surface Town, Rondebosch, South Africa.
Wind-Driven Cross-Equatorial Flow in the Indian Ocean
Journal of Physical Oceanography, 2012
Meridional velocity, mass, and heat transport in the equatorial oceans are difficult to estimate because of the nonapplicability of the geostrophic balance. For this purpose a steady-state model is utilized in the equatorial Indian Ocean using NCEP wind stress and temperature and salinity data from the World Ocean Atlas 2005 (WOA05) and Argo. The results show a Somali Current flowing to the south during the winter monsoon carrying −11.5 ± 1.3 Sv (1 Sv ≡ 106 m3 s−1) and −12.3 ± 0.3 Sv from WOA05 and Argo, respectively. In the summer monsoon the Somali Current reverses to the north transporting 16.8 ± 1.2 Sv and 19.8 ± 0.6 Sv in the WOA05 and Argo results. Transitional periods are considered together and in consequence, there is not a clear Somali Current present in this period. Model results fit with in situ measurements made around the region, although Argo data results are quite more realistic than WOA05 data results.
Variability and sources of the southeastern Atlantic circulation
Journal of Marine Research, 1996
The 1992-1993 Benguela Sources and Transport (BEST) time series provide a quantitative view of the Benguela Current transport and the eddy field crossing 3OS, as well as an estimate of the relation between its barotropic and baroclinic components. This is done by a simultaneous analysis of the BEST data derived from inverted echo sounders, pressure sensors, current meter moorings, CTD, and ADCP stations. The analysis of the time series indicates that the annual mean baroclinic transport of the Benguela Current is 13 Sv with a total transport of 16 Sv. Through the combination of instruments the total baroclinic plus barotropic transport of the upper 2600 m was obtained without making any assumption about the level of no motion. Results from this calculation corroborated the assumption that 1000 m as a level of no motion could be used as a fairly good approximation.
Journal of Physical Oceanography, 2004
An ocean general circulation model (OGCM) is used to investigate the low-frequency (period longer than 90 days) rectification of atmospheric intraseasonal variability (10-90-day periods) in zonal surface current and transport of the equatorial Indian Ocean. A hierarchy of OGCM solutions is found in an actual tropical Indian Ocean basin for the period of 1988-2001. To help to identify and isolate nonlinear processes, a linear continuously stratified model and a 4-layer intermediate ocean model are also used. Results from the OGCM solutions suggest 1 2 that intraseasonal atmospheric forcing acts to weaken the equatorial seasonal surface currents. Amplitudes of the spring and autumn eastward surface jets, the Wyrtki jets (WJ), and the westward surface current during January-March are reduced by as much as 15-25 cm s Ϫ1 by intraseasonal atmospheric forcing, and strengths of the rectification exhibit a significant interannual variability. Important processes that cause the low-frequency rectification are asymmetric response of mixed layer depth to easterly and westerly winds, entrainment, and upwelling of momentum. During spring and autumn, the westerly (easterly) phase of an intraseasonal event enhances (weakens or even reverses) the seasonal westerly winds, increases (decreases) equatorial convergence and entrainment, and thus deepens (thins) the mixed layer. A net, westward current is generated over an event mean because easterly wind acts on a thinner surface mixed layer whereas westerly wind acts on a thicker one. In contrast, during January-March when the seasonal winds are equatorial easterlies, surface currents are westward and equatorial undercurrents (EUC) develop. The rectified surface currents are eastward, which reduces the westward surface flow. This eastward rectification results largely from the vertical advection and entrainment of the EUC. The seasonal-to-interannual variability of the rectified surface flow is determined primarily from the seasonal cycle and interannual variability of the background state. Seasonal-to-interannual variability of the intraseasonal wind forcing also contributes. The rectified low-frequency zonal volume (heat) transports integrated over the entire water column along the Indian Ocean equator are persistently eastward with an amplitude of 0-15 ϫ 10 6 m 3 s Ϫ1 (0-1.2 pW). This is because westerly winds generate equatorial downwelling, advecting the surface eastward momentum downward and giving an eastward subsurface current. Easterly winds cause equatorial upwelling and produce an eastward pressure gradient force that drives an eastward subsurface current. This eastward subsurface current is advected upward by upwelling. The mean effect over an intraseasonal event at the equator is to increase the eastward transport in the water column. In the layers above the thermocline, the rectified zonal volume (heat) transports are in the same direction as the rectified surface currents. Results from this paper may have important implications for understanding climate variability because modification of WJ strength and transports can affect the SST and heat storage in the equatorial Indian Ocean warm pool.
Modeling the Wind-Driven Variability of the South Indian Ocean
Journal of Physical Oceanography, 1999
This article describes the results of numerical experiments carried out with a general circulation ocean model to investigate the effect of the seasonal cycle of the wind forcing on the Agulhas transport. Two cases are described. The first was initialized with temperature and salinity values obtained by horizontally averaging Levitus climatology. The second experiment was designed to isolate the spatial and temporal structure of the barotropic mode. The model, therefore, was initialized with constant values of temperature and salinity. Both experiments were started from rest, forced at their surface with Hellerman and Rosenstein wind stress climatology, and spun up until dynamical equilibrium. According to the experiments there are two distinct modes of variability in the south Indian Ocean. These modes appear to be separated by the topographic ridges that run south of Madagascar. On the western side of the basin there is a dominant mode with a maximum during spring-summer and a minimum during fall-winter. East of Madagascar there is a marked decrease of the circulation in fall and relative maximums during late summer and late winter. The midlatitude time variability, east of 45ЊE, appears to be dominated by advection and wave propagation. West of 45ЊE there is dominance by local wind forcing. A comparison between baroclinic and barotropic experiments indicates that although their annual mean structure is markedly different, their monthly anomalies, south of 30ЊS, are quite similar. This result, which agrees with previous theoretical and experimental studies, indicates that the seasonal adjustment in the south Indian Ocean is mostly accomplished by the westward propagation of barotropic planetary waves. This propagation is inhibited by the bottom topography of the Madagascar Ridge and Southwest Indian Ridge (ϳ45ЊE). These topographic features appear to isolate the Agulhas Current in the western region from the large-scale gyre farther east at seasonal timescales.
Impacts of Agulhas Leakage on the Tropical Atlantic Western Boundary Systems
Journal of Climate
The influx of warmer and saltier Indian Ocean waters into the Atlantic—the Agulhas leakage—is now recognized to play an important role in the global thermohaline circulation and climate. In this study the results of a ⅞° simulation with the Hybrid Coordinate Ocean Model, which exhibit an augmentation in the Agulhas leakage, is investigated. This increase in the leakage ought to have an impact on the meridional oceanic volume and heat transports in the Atlantic Ocean. Significant linear trends found in the integrated transport at 20°, 15°, and 5°S correlate well with decadal fluctuations of the Agulhas leakage. The augmented transport also seems to be related to an increase in the latent heat flux observed along the northeastern coastline of Brazil since 2003. This study shows that the precipitation on the Brazilian coast has been increasing since 2005, at the same location and with the same regime shift observed for the latent heat flux and the volume transport. This suggests that t...
Climate Dynamics, 2011
In this paper we use a coupled ocean-atmosphere model to investigate the impact of the interruption of Agulhas leakage of Indian ocean water on the Tropical Atlantic, a region where strong coupled ocean atmosphere interactions occur. The effect of a shut down of leakage of Indian ocean water is isolated from the effect of a collapse of the MOC. In our experiments, the ocean model is forced with boundary conditions in the southeastern corner of the domain that correspond to no interocean exchange of Indian ocean water into the Atlantic. The southern boundary condition is taken from the Levitus data and ensures a MOC in the Atlantic. Within this configuration, instead of warm and salty Indian ocean water temperature (cold) and salinity (fresh) anomalies of southern ocean origin propagate into the South Atlantic and eventually reach the equatorial region, mainly in the thermocline. This set up mimics the closure of the "warm water path" in favor of the "cold water path". As part of the atmospheric response, there is a northward shift of the Intertropical Convergence Zone (ITCZ). The changes in Trade Winds lead to reduced Ekman pumping in the equatorial region. This leads to a freshening and warming of the surface waters along the equator. Especially in the Cold Tongue region, the cold and fresh subsurface anomalies do not reach the surface due to the reduced upwelling. The anomaly signals are transported by the Equatorial undercurrent and spread away from the Equator within the thermocline. Part of the anomaly eventually reaches the Tropical North Atlantic, where it affects the Guinea Dome.
Mesoscale perturbations control inter-ocean exchange south of Africa
Geophysical Research Letters, 2008
1] The quantification of inter-ocean leakage from the South Indian to the South Atlantic Ocean is an important measure for the role of the Agulhas system in the global thermohaline circulation. To explore the specific role of mesoscale variability (such as Agulhas rings and Mozambique eddies) in this process a high-resolution model (based on NEMO-ORCA) for the Agulhas region has been set up. It is nested into a global coarse-resolution model. The high-resolution nest captures all salient features of the greater Agulhas region, including the upstream perturbations of the Agulhas Current and Natal Pulses along the African coast. A comparison of the inter-ocean exchange in the high-resolution nest with its coarse resolution counterpart reveals that the latter significantly over-estimates the amount of water flowing into the Atlantic Ocean, demonstrating the need to explicitly simulate the mesoscale features. A sensitivity experiment that excludes the upstream perturbations revealed no difference in the amount of inter-ocean exchange. However, the realistic representation of Agulhas rings and their drift path into the South Atlantic depends on the simulation of those upstream perturbations.
Dynamics of Intermediate Water Circulation in the Subtropical South Atlantic
Journal of Physical Oceanography, 2000
The circulation of the low-salinity Antarctic Intermediate Water in the South Atlantic and the associated dynamical processes are studied, using recent and historical hydrographic profiles, Lagrangian and Eulerian current measurements as well as wind stress observations. The circulation pattern inferred for the Antarctic Intermediate Water supports the hypothesis of an anticyclonic basinwide recirculation of the intermediate water in the subtropics. The eastward current of the intermediate anticyclone is fed mainly by water recirculated in the Brazil Current and by the Malvinas Current. An additional source region is the Polar Frontal zone of the South Atlantic. The transport in the meandering eastward current ranges from 6 to 26 Sv (Sv ϵ 10 6 m 3 s Ϫ1 ). The transport of the comparably uniform westward flow of the gyre varies between 10 and 30 Sv. Both transports vary with longitude. At the western boundary near 28ЊS, in the Santos Bifurcation, the westward current splits into two branches. About three-quarters of the 19 Sv at 40ЊW go south as an intermediate western boundary current. The remaining quarter flows northward along the western boundary. Simulations with a simple model of the ventilated thermocline reveal that the wind-driven subtropical gyre has a vertical extent of over 1200 m. The transports derived from the simulations suggest that about 90% of the transport in the westward branch of the intermediate gyre and about 50% of the transport in the eastward branch can be attributed to the wind-driven circulation. The structure of the simulated gyre deviates from observations to some extent. The discrepancies between the simulations and the observations are most likely caused by the interoceanic exchange south of Africa, the dynamics of the boundary currents, the nonlinearity, and the seasonal variability of the wind field. A simulation with an inflow/outflow condition for the eastern boundary reduces the transport deviations in the eastward current to about 20%. The results support the hypothesis that the wind field is of major importance for the subtropical circulation of Antarctic Intermediate Water followed by the interoceanic exchange. The simulations suggest that the westward transport in the subtropical gyre undergoes seasonal variations. The transports and the structure of the intermediate subtropical gyre from the Parallel Ocean Climate Model (Semtner-Chervin model) agree better with observations.