Variability in the subtropical-tropical cells and its effect on near-surface temperature of the equatorial Pacific: a model study

A set of experiments utilizing different implementations of the global ORCA-LIM model with horizontal resolutions of 2°, 0.5° and 0.25° is used to investigate tropical and extra-tropical influences on equatorial Pacific SST variability at interannual to decadal time scales. The model experiments use a bulk forcing methodology building on the global forcing data set for 1958 to 2000 developed by Large and Yeager (2004) that is based on a blend of atmospheric reanalysis data and satellite products. Whereas representation of the mean structure and transports of the (sub-) tropical Pacific current fields is much improved with the enhanced horizontal resolution, there is only little difference in the simulation of the interannual variability in the equatorial regime between the 0.5° and 0.25° model versions, with both solutions capturing the observed SST variability in the Niño3-region. The question of remotely forced oceanic contributions to the equatorial variability, in particular, the role of low-frequency changes in the transports of the Subtropical Cells (STCs), is addressed by a sequence of perturbation experiments using different combinations of fluxes. The solutions show the near-surface temperature variability to be governed by wind-driven changes in the Equatorial Undercurrent. The relative contributions of equatorial and off-equatorial atmospheric forcing differ between interannual and longer, (multi-) decadal timescales: for the latter there is a significant impact of changes in the equatorward transport of subtropical thermocline water associated with the lower branches of the STCs, related to variations in the off-equatorial trade winds. A conspicuous feature of the STC variability is that the equatorward transports in the interior and along the western boundary partially compensate each other at both decadal and interannual time scales, with the strongest transport extrema occurring during El Niño episodes. The behaviour is rationalized in terms of a wobbling in the poleward extents of the tropical gyres, which is manifested also in a meridional shifting of the bifurcation latitudes of the North and South Equatorial Current systems

The Agulhas Leakage: Role of Mesoscale Processes and Impact on the Atlantic Meridional Overturning Circulation

The Agulhas region around South Africa is a key region of global climate and climate change. Under present climate conditions the Agulhas leakage from the Indian to the Atlantic Ocean feeds the bulk of the upper limb of the meridional overturning circulation (MOC) in the Atlantic Ocean, highly affected by the nonlinear constituents of the Agulhas Current system.To examine the role of the mesoscale processes in the mean flow in the Agulhas system, particularly in regard to the Agulhas leakage and its effect on the Atlantic MOC, an innovative ocean modeling program has been set up that utilizes new global model components and methodologies developed in international cooperation (DRAKKAR) based on a framework of the European model system NEMO. The model configuration involves a high-resolution grid of the greater Agulhas region nested into a coarse-resolution global ocean –sea-ice model forced by atmospheric conditions of the period 1958 –2004. Due to an effective “two-way” nesting approach this system for the first time allows to unravel, how the explicitly simulated mesoscale variability in the Agulhas dynamics feeds back to the global ocean.There is vast range of mesoscale –mean flow interactions in the Agulhas region. In the South East Madagascar Current offshore eddies do lead to different modes of the current extension, one favoring cyclonic flow into the Mozambique Channel, the other anticyclonic eddies drifting towards southwest. Eddies generated in the central Mozambique Channel introduce strong perturbations into the western boundary current systems off the African coast by triggering Natal Pulses, causing offshore displacements of the Agulhas Current which then lead to strong changes in the volume transport of the Agulhas Current and eventually to upstream retroflections of the current back into the Indian Ocean. The barotropic nature of the interplay with Mozambique eddies and Natal Pulses also affects the Agulhas Undercurrent leading to strong fluctuations similar to observed ones, raising the question what portion of the AgulhasUndercurrent is a coherent flow throughout the South Indian Ocean and what portion is virtually generated by passing Natal Pulses.The sequence of model experiments demonstrates that upstream perturbations have a vital effect on the mesoscale dynamics in the Agulhas retroflection area. A comparison of the reference model with a sensitivity experiment not including the Mozambique eddies shows that they are not only triggering the shedding of Agulhas rings but also lead to more realistic eddy structures in the Cape Basin and beyond. However, the presence of these upstream perturbations does not alter the mean Agulhas leakage, i.e, the net volume transport from the Indian to the Atlantic Ocean.The magnitude of the Agulhas leakage is quantitatively strongly dependent on the representation of Agulhas rings and other associated mesoscale processes in the retroflection area; there is a strong difference in the interoceanic transport between the high-resolution, nested model and the coarser, non-eddying model, the latter leading to higher, unrealistic transport values. While in the time-mean the bulk of this difference is modifying the horizontal circulation of the subtropical super-gyre rather than the Atlantic MOC, the mesoscale dynamics of the Agulhas regime appear as an important source of decadal variability in the MOC: An isolation of the effect of the mesoscale demonstrated that the Agulhas leakage acts as the source of low-frequency undulations in thermocline depth, a signal carried across the South Atlantic by Rossby waves and into the North Atlantic by wave processes along the American continental slope. The resulting signal in MOC transport gradually diminishes from south to north, but has an amplitude in the tropical Atlantic of comparable magnitude to the effect of subarctic deep water formation processes discussed in previous studies. It is evident that a proper representation of the mesoscale processes it vital for the correct interpretation of variations of the upper ocean transport across the equator, and even at subtropical latitudes in the North Atlantic where current monitoring efforts aim at a quantification of inter-annual MOC variations

Characteristics and robustness of Agulhas leakage estimates: an inter-comparison study of Lagrangian methods

The inflow of relatively warm and salty water from the Indian Ocean into the South Atlantic via Agulhas leakage is important for the global overturning circulation and the global climate. In this study, we analyse the robustness of Agulhas leakage estimates as well as the thermohaline property modifications of Agulhas leakage south of Africa. Lagrangian experiments with both the newly developed tool Parcels and the well established tool Ariane were performed to simulate Agulhas leakage in the eddy-rich ocean–sea-ice model INALT20 (1/20∘ horizontal resolution) forced by the JRA55-do atmospheric boundary conditions. The average transport, its variability, trend and the transit time from the Agulhas Current to the Cape Basin of Agulhas leakage is simulated comparably with both Lagrangian tools, emphasizing the robustness of our method. Different designs of the Lagrangian experiment alter in particular the total transport of Agulhas leakage by up to 2 Sv, but the variability and trend of the transport are similar across these estimates. During the transit from the Agulhas Current at 32∘ S to the Cape Basin, a cooling and freshening of Agulhas leakage waters occurs especially at the location of the Agulhas Retroflection, resulting in a density increase as the thermal effect dominates. Beyond the strong air–sea exchange around South Africa, Agulhas leakage warms and salinifies the water masses below the thermocline in the South Atlantic

Sea surface slope as a proxy for Agulhas Current strength

The linear relation between the strength of the Agulhas Current at nominal latitude 34°S and the gradient in sea level height anomaly across the current is investigated in a 1/10° resolution regional numerical ocean model. Our results show that the strength of the current can be estimated with reasonable accuracy using altimeter data, once it has been calibrated using in-situ transport measurements. Three years of transport measurements provide a calibration with worst-case correlation R = 0.78. In that case the errors in proxy transport have a standard deviation of 9.8 Sv, compared to a 20.2 Sv standard deviation of the transport time series itself. From these results we conclude that the design of the Agulhas Current Timeseries (ACT) experiment, a three-year deployment of moorings across the Agulhas Current and along a TOPEX/Jason altimeter ground track, will likely produce a good quality multi-decadal time series of Agulhas Current strength

Anthropogenic impact on Agulhas leakage

Recent work suggests that changes of the Southern Hemisphere (SH) winds led to an increase in Agulhas leakage and a corresponding salinification of the Atlantic. Climate model projections for the 21st century predict a progressive southward migration and intensification of the SH westerlies. The potential effects on the ocean circulation of such an anthropogenic trend in wind stress are studied here with a high-resolution ocean model forced by a step-function change in SH wind stress that involves a 7% increase in westerlies strength and a 2° shift in the zero wind stress curl. The model simulation suggests a rapid dynamic adjustment of Agulhas leakage by 4.5 Sv, about a third of its original value, after a few years. The change in leakage is reflected in a concomitant change in the transport of the South Atlantic subtropical gyre, but leads only to a small increase in the Atlantic Meridional Overturning Circulation (AMOC) of O(1 Sv) after three decades. A main effect of the increasing inflow of Indian Ocean waters with potential long-term ramifications for the AMOC is the salinification and densification of upper-thermocline waters in the South Atlantic, which extends into the North Atlantic within the first three decades

Relating Agulhas leakage to the Agulhas Current retroflection location

The relation between the Agulhas Current retroflection location and the magnitude of Agulhas leakage, the transport of water from the Indian to the Atlantic Ocean, is investigated in a high-resolution numerical ocean model. Sudden eastward retreats of the Agulhas Current retroflection loop are linearly related to the shedding of Agulhas rings, where larger retreats generate larger rings. Using numerical Lagrangian floats a 37 year time series of the magnitude of Agulhas leakage in the model is constructed. The time series exhibits large amounts of variability, both on weekly and annual time scales. A linear relation is found between the magnitude of Agulhas leakage and the location of the Agulhas Current retroflection, both binned to three month averages. In the relation, a more westward location of the Agulhas Current retroflection corresponds to an increased transport from the Indian Ocean to the Atlantic Ocean. When this relation is used in a linear regression and applied to almost 20 years of altimetry data, it yields a best estimate of the mean magnitude of Agulhas leakage of 13.2 Sv. The early retroflection of 2000, when Agulhas leakage was probably halved, can be identified using the regression

Modes of the southern extension of the East Madagascar Current

Data sets from satellite observations and a nested high-resolution model are used to study a source region of the Agulhas Current. Altimeter-derived geostrophic surface currents are averaged over varying periods, providing evidence of the persistence of flow patterns in the extension of the southern branch of the East Madagascar Current (SEMC). South of Madagascar, the SEMC separates into one branch toward the Agulhas Current and into a second branch retroflecting and connecting to the Subtropical Indian Ocean Countercurrent (SICC). Good agreement is found between long-term mean patterns of observational and model dynamic heights. Two basic modes are identified in the SEMC extension, with anticyclonic motion favoring retroflection in the northern Mozambique Basin when the extension is in a southwestward direction and cyclonic motion occurring in the case of the SEMC flowing westward along the southern Madagascar slope. A cross-correlation sequence between model SEMC transports and the modal changes in the extension region displays a correlation at about 1-month lag which agrees with eddy propagation time from the SEMC to the outflow region. Mean model SEMC transports are determined using floats released at 21 degrees S, and the contribution of the SEMC to the SICC is obtained using floats injected at 55 degrees E with the model running backward. Almost half of the SEMC volume transport contributes to the Agulhas system, and about 40% of SICC water originates from the SEMC