Description: The temporal variability of the greater Agulhas Current system has important climatological consequences. Some recent results have suggested that this variability contains a large seasonal component, due to changes in the circulation at latitudes poleward of Madagascar only. A model simulation shows that the contribution of Tropical Surface Water to Agulhas Current waters, via the Mozambique Channel, also has a distinct seasonal characteristic that is brought about by the seasonal wind stress over the tropical Indian Ocean. This simulated flow through the Channel contributes substantially to the seasonality of the Agulhas Current. This model result is shown to be not inconsistent with available hydrographic observations.

Description: Interocean exchange of heat and salt around South Africa is thought to be a key link in the maintenance of the global overturning circulation of the ocean. It takes place at the Agulhas Retroflection, largely by the intermittent shedding of enormous rings that penetrate into the South Atlantic Ocean. This makes it extremely hard to estimate the inter ocean fluxes. Estimates of direct Agulhas leakage from hydrographic and tracer data range between 2 and 10 Sv (1 Sv = 106 m3 s−1). The average ring shedding frequency, determined from satellite information, is approximately six rings per year. Their associated interocean volume transport is between 0.5 and 1.5 Sv per ring. A number of Agulhas rings have been observed to cross the South Atlantic. They decay exponentially to less than half their initial size (measured by their available potential energy) within 1000 km from the shedding region. Consequently, most of their properties mix into the surroundings of the Benguela region, probably feeding directly into the upper (warm) limb of the global thermohaline circulation. The most recent observations suggest that in the present situation Agulhas water and Antarctic Intermediate Water are about equally important sources for the Benguela Current. Variations in the strength of these may lead to anomalous stratification and stability of the Atlantic at decadal and longer timescales. Modeling studies suggest that the Indian-Atlantic interocean exchange is strongly related to the structure of the wind field over the South Indian Ocean. This leads in the mean to a subtropical supergyre wrapping around the subtropical gyres of the South Indian and Atlantic Oceans. However, local dynamical processes in the highly nonlinear regime around South Africa play a crucial role in inhibiting the connection between the two oceans. The regional bottom topography also seems to play an important role in locking the Agulhas Currents' retroflection. State-of-the-art global and regional “eddy-permitting” models show a reasonably realistic representation of the mean Agulhas system; but the mesoscale variability and the local geometrical and topographic features that determine largely the interocean fluxes still need considerable improvement. In this article we present a review of the above mentioned aspects of the interocean exchange around South Africa: the estimation of the fluxes into the South Atlantic from different types of observations, our present level of understanding of the exchanges dynamics and forcing, its representation in state-of-the-art models, and, finally, the impact of the Indian-Atlantic fluxes on regional and global scale both within the Atlantic Ocean and in interaction with the overlying atmosphere.

Description: Rapid increases in upper 700‐m Indian Ocean heat content (IOHC) since the 2000s have focused attention on its role during the recent global surface warming hiatus. Here, we use ocean model simulations to assess distinct multidecadal IOHC variations since the 1960s and explore the relative contributions from wind stress and buoyancy forcing regionally and with depth. Multidecadal wind forcing counteracted IOHC increases due to buoyancy forcing from the 1960s to the 1990s. Wind and buoyancy forcing contribute positively since the mid‐2000s, accounting for the drastic IOHC change. Distinct timing and structure of upper ocean temperature changes in the eastern and western Indian Ocean are linked to the pathway how multidecadal wind forcing associated with the Interdecadal Pacific Oscillation is transmitted and affects IOHC through local and remote winds. Progressive shoaling of the equatorial thermocline—of importance for low‐frequency variations in Indian Ocean Dipole occurrence—appears to be dominated by multidecadal variations in wind forcing.

Description: The Atlantic meridional overturning circulation (AMOC) represents the zonally integrated stream function of meridional volume transport in the Atlantic Basin. The AMOC plays an important role in transporting heat meridionally in the climate system. Observations suggest a heat transport by the AMOC of 1.3 PW at 26°N—a latitude which is close to where the Atlantic northward heat transport is thought to reach its maximum. This shapes the climate of the North Atlantic region as we know it today. In recent years there has been significant progress both in our ability to observe the AMOC in nature and to simulate it in numerical models. Most previous modeling investigations of the AMOC and its impact on climate have relied on models with horizontal resolution that does not resolve ocean mesoscale eddies and the dynamics of the Gulf Stream/North Atlantic Current system. As a result of recent increases in computing power, models are now being run that are able to represent mesoscale ocean dynamics and the circulation features that rely on them. The aim of this review is to describe new insights into the AMOC provided by high-resolution models. Furthermore, we will describe how high-resolution model simulations can help resolve outstanding challenges in our understanding of the AMOC.

Description: Warming of the North Atlantic region in climate history often was associated with massive melting of the Greenland Ice Sheet. To identify the meltwater’s impacts and isolate these from internal variability and other global warming factors, we run single-forcing simulations including small ensembles using three complex climate models differing only in their ocean components. In 200-year long pre-industrial climate simulations, we identify robust consequences of abruptly increasing Greenland runoff by 0.05 Sv: sea-level rise of 44±10 cm, subpolar North Atlantic surface cooling of 0.7˚C and a moderate AMOC decline of 1.1–2.0 Sv. The latter two emerge in under three decades—and reverse on the same timescale after the perturbation ends in year 100. The ocean translates the step-change perturbation into a multi-decadal to centennial signature in the deep overturning circulation. In all simulations, internal variability creates notable uncertainty in estimating trends, time of emergence and duration of the response.

Description: The Makassar Strait, the main passageway of the Indonesian Throughflow (ITF), is an important component of Indo-Pacific climate through its inter-basin redistribution of heat and freshwater. Observational studies suggest that wind-driven freshwater advection from the marginal seas into the Makassar Strait modulates the strait's surface transport. However, direct observations are too short (〈15 years) to resolve variability on decadal timescales. Here we use a series of global ocean simulations to assess the advected freshwater contributions to ITF transport across a range of timescales. The simulated seasonal and interannual freshwater dynamics are consistent with previous studies. On decadal timescales, we find that wind-driven advection of South China Sea (SCS) waters into the Makassar Strait modulates upper-ocean ITF transport. Atmospheric circulation changes associated with Pacific decadal variability appear to drive this mechanism via Pacific lower-latitude western boundary current interactions that affect the SCS circulation. Key Points: - A global ocean model is used to show how freshwater impacts the decadal variability of transport through the main Indonesian Throughflow pathway - Wind-driven advection of South China Sea freshwater induces an upstream pressure gradient that reduces transport - Freshwater input is modulated by atmospheric circulation changes associated with Pacific decadal variability

Description: Deep convection and associated deep water formation are key processes for climate variability, since they impact the oceanic uptake of heat and trace gases and alter the structure and strength of the global overturning circulation. For long, deep convection in the subpolar North Atlantic was thought to be confined to the central Labrador Sea in the western subpolar gyre (SPG). However, there is increasing observational evidence that deep convection also has occurred in the eastern SPG south of Cape Farewell and in the Irminger Sea, in particular, in 2015–2018. Here we assess this recent event in the context of the temporal evolution of spatial deep convection patterns in the SPG since the mid-twentieth century, using realistic eddy-rich ocean model simulations. These reveal a large interannual variability with changing contributions of the eastern SPG to the total deep convection volume. Notably, in the late 1980s to early 1990s, the period with highest deep convection intensity in the Labrador Sea related to a persistent positive phase of the North Atlantic Oscillation, the relative contribution of the eastern SPG was small. In contrast, in 2015–2018, deep convection occurred with an unprecedented large relative contribution of the eastern SPG. This is partly linked to a smaller north-westward extent of deep convection in the Labrador Sea compared to previous periods of intensified deep convection, and may be a first fingerprint of freshening trends in the Labrador Sea potentially associated with enhanced Greenland melting and the oceanic advection of the 2012–2016 eastern North Atlantic fresh anomaly.

Description: While forced ocean hindcast simulations are useful for a wide range of applications, a key limitation is their inability to simulate ocean-atmosphere feedbacks. As a consequence, they need to rely on artificial choices such as sea surface salinity restoring and other corrections affecting the surface freshwater fluxes. Fully coupled models overcome these limitations, but lack the correct timing of variability due to weaker observational constraints. This leads to a mismatch between forced and coupled models on interannual to decadal timescales. A possibility to combine the advantages of both modelling strategies is to apply a partial coupling (PCPL), i.e. replacing the surface winds stress in the ocean component by wind stress derived from reanalysis. To identify the capabilities, limitations and possible use cases of partial coupling, we perform a fully coupled, two partially coupled and an ocean-only experiment using an all-Atlantic nested ocean configuration at eddying resolution in a global climate model. We show that the correct timing of Atlantic Meridional Overturning Circulation (AMOC) variability in PCPL experiments is robust on timescales below 5 years. Mid-latitude wind stress curl changes contribute to decadal AMOC variability, but North Atlantic buoyancy fluxes are not significantly altered by incorporating reanalysed wind stress anomalies, limiting the success of PCPL on this timescale. Long term trends of the AMOC in PCPL mode are consistent with fully coupled model experiments under historic atmospheric boundary conditions, suggesting that a partially coupled model is still able to simulate the important ocean-atmosphere feedbacks necessary to maintain a stable AMOC.

Description: Because new observations have revealed that the Labrador Sea is not the primary source for waters in the lower limb of the Atlantic Meridional Overturning Circulation (AMOC) during the Overturning in the Subpolar North Atlantic Programme (OSNAP) period, it seems timely to re-examine the traditional interpretation of pathways and property variability for the AMOC lower limb from the subpolar gyre to 26.5 degrees N. In order to better understand these connections, Lagrangian experiments were conducted within an eddy-rich ocean model to track upper North Atlantic Deep Water (uNADW), defined by density, between the OSNAP line and 26.5 degrees N as well as within the Labrador Sea. The experiments reveal that 77% of uNADW at 26.5 degrees N is directly advected from the OSNAP West section along the boundary current and interior pathways west of the Mid-Atlantic Ridge. More precisely, the Labrador Sea is a main gateway for uNADW sourced from the Irminger Sea, while particles connecting OSNAP East to 26.5 degrees N are exclusively advected from the Iceland Basin and Rockall Trough along the eastern flank of the Mid-Atlantic Ridge. Although the pathways between OSNAP West and 26.5 degrees N are only associated with a net formation of 1.1 Sv into the uNADW layer, they show large density changes within the layer. Similarly, as the particles transit through the Labrador Sea, they undergo substantial freshening and cooling that contributes to further densification within the uNADW layer. Key Points: - The large majority of upper North Atlantic Deep Water (uNADW) sourced from the Irminger Sea transits through the Labrador Sea before reaching 26.5°N - Interior pathways along the eastern flank of the Mid-Atlantic Ridge connect the Iceland Basin and Rockall Trough to 26.5°N - Though uNADW is mainly sourced in the eastern subpolar gyre, its transit in the Labrador Sea is associated with further property changes

Description: The southward flow of North Atlantic Deep Water makes up the major component of the deepwater limb of the Atlantic Meridional Overturning Circulation (AMOC). In the subtropical North Atlantic, it's flow is concentrated along the continental slope, forming a coherent Deep Western Boundary Current (DWBC). Both, observations and models show a high variability of the flow in this region. Here we use an eddy-rich ocean model to show that this variability is mainly caused by eddies and meanders. Their formation process involves an important contribution from energy transfer by barotropic instability. They occur along the entire DWBC pathway and introduce several recirculation gyres that result in a decorrelation of the DWBC transport at 26.5°N and 16°N, despite the fact that a considerable mean transport of 20 Sv connects the two latitudes. Water in the DWBC at 26.5°N is partly returned northward. Because the amount of water returned depends on the DWBC transport itself, a stronger DWBC does not necessarily lead to an increased amount of water that reaches 16°N. Along the pathway to 16°N, the transport signal is altered by a broad and temporally variable transit time distribution. Thus, advection in the DWBC cannot account for coherent AMOC changes on interannual timescales seen in the model.