Description: Exchanges of water south of Africa between the South Indian Ocean and the South Atlantic Ocean are an important component of the global thermohaline circulation. Evidence exists that the variability in these exchanges, on both meso- and longer time scales, may significantly influence weather and climate patterns in the southern African region and the significance of these regional ocean–atmosphere interactions is discussed. Observations of the inter-ocean exchange are limited and it is necessary to augment these with estimates derived from models. As a first step in this direction, this study uses an eddy-permitting model to investigate the heat and volume transport in the oceanic region south of Africa and its variability on meso, seasonal and inter-annual time scales. On the annual mean, about Full-size image (〈1 K) (standard deviation Full-size image (〈1 K)) of heat flows west into the South Atlantic across 20°E (longitude of Cape Agulhas, the southernmost point of Africa), with just over Full-size image (〈1 K) (standard deviation Full-size image (〈1 K)) flowing north into the South Atlantic across 35°S. The seasonal variations in this transport are about 10% at 35°S in the South Atlantic and around 20% through 20°E; the model value of Full-size image (〈1 K) for summer (standard deviation ranging from Full-size image (〈1 K) in January to Full-size image (〈1 K) in March) appears consistent with respective estimates of 0.51 and Full-size image (〈1 K) derived from two WOCE summer cruises southwest of Cape Town to 45°S in 1990 and 1993. Volume transports of the Agulhas Current section through 35°S in the SW Indian Ocean range from 58 to Full-size image (〈1 K) in summer/autumn to 64–Full-size image (〈1 K) in winter/spring. The model results suggest that the inter-ocean exchange south of Africa is highly variable on seasonal through to interannual scales. If this variability is also the case in the real ocean (and the limited observations suggest that this is so), then there are likely to be significant implications for climate.

Description: Processes that influence the volume and heat transport across the Greenland–Scotland Ridge system are investigated in a numerical model with ° horizontal resolution. The focus is on the sensitivity of cross-ridge transports and the reaction of the subpolar North Atlantic Ocean circulation to changes in wind stress and buoyancy forcing on seasonal to interannual timescales. A general relation between changes in wind stress or cross-ridge density contrasts and the overturning transport of Greenland–Iceland–Norwegian Seas source water is established from a series of idealized experiments. The relation is used subsequently to interpret changes in an experiment over the years 1992–97 with realistic forcing. On seasonal and interannual timescales there is a clear correlation between heat flux and wind stress curl variability. The realistic model suggests a steady decrease in the strength of the cyclonic subpolar gyre of the North Atlantic with a corresponding decrease in heat transport during the 1990s

Description: Western Boundary Currents, such as the Gulf Stream, are regions of vivid air-sea interaction. Mesoscale features of these currents play a fundamental role in global ocean heat transport and exchange with the atmosphere. Related processes and their interactions across scales have gained increasing attention in the last years, since high-resolution, mesoscale-resolving modeling became computationally feasible on climate time scales. Here, we show the impact of explicitly resolving the oceanic mesoscale in the coupled global climate model FOCI on North Atlantic and European climate. For this purpose, we use the ocean nesting capability in FOCI, which facilitates regional ocean grid refinement. We explore and compare pre-industrial simulations each extending over at least 150 years: a reference run without any grid refinement and an experiment with a nest in the North Atlantic. Technically, the regional ocean nest maintains frequent two-way exchange with the global host grid, which in turn is fully coupled to the atmosphere model. The ocean model NEMO has a global resolution of 1/2˚ model with 46 vertical levels and 1/10˚ refinement in the nest region, while the atmosphere model ECHAM6 has a 1.8˚ horizontal resolution (T63) and 95 vertical levels, including the strato- and mesosphere. Within the nest region, the increased resolution leads to a more eddy-rich simulation and an improved mean state. The North Atlantic Current is considerably better represented, which reduces the typical North Atlantic cold bias from -8˚C in the reference run without nest to -2˚C. Beyond local bias correction of the mean state, we will also discuss the impact of explicitly modeling ocean mesoscale dynamics on atmospheric variability on different time scales, such as the North Atlantic Oscillation or the Atlantic Multidecadal Variability.

Description: In the framework of FLAME (Family of Linked Atlantic Model Experiments) an eddy-permitting model of the Atlantic Ocean was used to hindcast the uptake and spreading of anthropogenic trace gases, CO2 and CFC, during the last century. The code is based on the public domain software MOM (Modular Ocean Model) Version 2.1. Towards a parallel version the code was extended for shmem and MPI message passing to achieve portability to Cray-T3E and NEC SX systems. The performance of this production code on Cray-T3E as well as NEC-SX5 and SX6 systems is discussed. To underline the need for high-resolution modeling some physical model results are presented.

Description: We investigate the dependence of surface fresh water fluxes in the Gulf Stream and North Atlantic Current (NAC) area on the position of the stream axis which is not well represented in most ocean models. To correct this shortcoming, strong unrealistic surface fresh water fluxes have to be applied that lead to an incorrect salt balance of the current system. The unrealistic surface fluxes required by the oceanic component may force flux adjustments and may cause fictitious long-term variability in coupled climate models. To identify the important points in the correct representation of the salt balance of the Gulf Stream a regional model of the northwestern part of the subtropical gyre has been set up. Sensitivity studies are made where the westward flow north of the Gulf Stream and its properties are varied. Increasing westward volume transport leads to a southward migration of the Gulf Stream separation point along the American coast. The salinity of the inflow is essential for realistic surface fresh water fluxes and the water mass distribution. The subpolar-subtropical connection is important in two ways: The deep dense flow from the deep water mass formation areas sets up the cyclonic circulation cell north of the Gulf Stream. The surface and mid depth flow of fresh water collected at high northern latitudes is mixed into the Gulf Stream and compensates for the net evaporation at the surface.

Description: A numerical circulation model with 1/6° resolution and an accurate topography formulation explains details of the observed circulation in the Irminger and Labrador Seas that were recently revealed by Lavender et al. [2000]. We show that the recirculation pattern is established through a locally wind induced flow controlled by the bottom topography and enhanced through remote baroclinic forcing by the dense plume of Denmark Strait overflow water. The basic circulation is a robust feature in a hierarchy of model setups. It exists in the purely barotropic case driven by steady winds and is even maintained when realistic daily forcing is added. The narrow recirculation zone is manifested by a sea level depression spanning from the Denmark Strait across the Irminger into the Labrador Sea.

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.