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

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

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

On multidecadal and quasi-decadal North Atlantic variability

Observed sea surface temperatures (SSTs) in the North Atlantic from 1958 through 2000, as well as data from an ocean model simulation driven with the atmospheric variability observed during the same period, are examined using multichannel singular spectrum analysis. The two leading oscillatory modes are associated with a multidecadal and a quasi-decadal period. The former is connected to a basinwide uniform SST pattern and changes in the deep North Atlantic meridional overturning circulation. The quasi-decadal mode involves a tripolar SST anomaly pattern forced by atmospheric variability with a spatial structure resembling that of the North Atlantic Oscillation (NAO). The upper ocean’s dynamical response to this NAO variability provides an instantaneous positive feedback to the SST pattern, while a delayed negative feedback is due to shallow overturning circulation anomalies

Modeling the fate of methane hydrates under global warming

Large amounts of methane hydrate locked up within marine sediments are vulnerable to climate change. Changes in bottom water temperatures may lead to their destabilization and the release of methane into the water column or even the atmosphere. In a multimodel approach, the possible impact of destabilizing methane hydrates onto global climate within the next century is evaluated. The focus is set on changing bottom water temperatures to infer the response of the global methane hydrate inventory to future climate change. Present and future bottom water temperatures are evaluated by the combined use of hindcast high-resolution ocean circulation simulations and climate modeling for the next century. The changing global hydrate inventory is computed using the parameterized transfer function recently proposed by Wallmann et al. (2012). We find that the present-day world's total marine methane hydrate inventory is estimated to be 1146Gt of methane carbon. Within the next 100years this global inventory may be reduced by ∼0.03% (releasing ∼473Mt methane from the seafloor). Compared to the present-day annual emissions of anthropogenic methane, the amount of methane released from melting hydrates by 2100 is small and will not have a major impact on the global climate. On a regional scale, ocean bottom warming over the next 100years will result in a relatively large decrease in the methane hydrate deposits, with the Arctic and Blake Ridge region, offshore South Carolina, being most affected