Description: 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.

Description: 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.

Description: NEMO is a fluid dynamics code used for oceanographic research. Within the TERAFLOP Workbench in cooperation with the Leibniz-Institut für Meereswissenschaften (IFM-GEOMAR) in Kiel a performance assessment and improvement campaign was carried out, ranging from MPI to memory addressing in solvers. At The High Performance Computing Center Stuttgart (HLRS) tests were made using a large configuration of SX nodes running NEMO at 2.1 Teraflop/s. The improved code is running the test case 29% faster on 512 SX-8 CPUs.

Description: Predicting the evolution of climate over decadal timescales requires a quantitative understanding of the dynamics that govern the meridional overturning circulation (MOC)1. Comprehensive ocean measurement programmes aiming to monitor MOC variations have been established in the subtropical North Atlantic2, 3 (RAPID, at latitude 26.5° N, and MOVE, at latitude 16° N) and show strong variability on intraseasonal to interannual timescales. Observational evidence of longer-term changes in MOC transport remains scarce, owing to infrequent sampling of transoceanic sections over past decades4, 5. Inferences based on long-term sea surface temperature records, however, supported by model simulations, suggest a variability with an amplitude of plusminus1.5–3 Sv (1 Sv = 106 m3 s-1) on decadal timescales in the subtropics6. Such variability has been attributed to variations of deep water formation in the sub-arctic Atlantic, particularly the renewal rate of Labrador Sea Water7. Here we present results from a model simulation that suggest an additional influence on decadal MOC variability having a Southern Hemisphere origin: dynamic signals originating in the Agulhas leakage region at the southern tip of Africa. These contribute a MOC signal in the tropical and subtropical North Atlantic that is of the same order of magnitude as the northern source. A complete rationalization of observed MOC changes therefore also requires consideration of signals arriving from the south.

Description: 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.