2 25 Abstract 26 27 The Agulhas Current plays a crucial role in the thermohaline circulation through its 28 leakage into the South Atlantic. Under both past and present climates, the trade winds and 29 westerlies could have the ability to modulate the amount of Indian-Atlantic inflow. 30Compelling arguments have been put forward suggesting that trade winds alone have 31 little impact on the magnitude of Agulhas leakage. Here, employing three ocean models 32 for robust analysis -a global coarse resolution, a regional eddy-permitting and a nested 33 high-resolution eddy-resolving configuration -and systematically altering the position 34 and intensity of the westerly wind belt in a series of sensitivity experiments, it is shown 35 that the westerlies, in particular their intensity, control the leakage. Leakage responds 36proportionally to the westerlies intensity up to a certain point. Beyond this, through the 37 adjustment of the large-scale circulation, energetic interactions occur between the 38 Agulhas Return Current and the Antarctic Circumpolar Current that result in a state 39 where leakage no longer increases. This adjustment takes place within 1 to 2 decades. 40Contrary to previous assertions, our results further show that an equatorward (poleward) 41 shift in westerlies increases (decreases) leakage. This occurs due to the redistribution of 42 momentum input by the winds. It is concluded that the reported present-day leakage 43 increase could therefore reflect an unadjusted oceanic response mainly to the 44 strengthening westerlies over the last few decades. 45 46 3

The relationship between the Agulhas Current and the Agulhas leakage is not well understood. Here, this is investigated using two basin-scale and two global ocean models of incrementally increasing resolution. The response of the Agulhas Current is evaluated under a series of sensitivity experiments, in which idealized anomalies, designed to geometrically modulate zonal trade wind stress, are applied across the Indian Ocean Basin. The imposed wind stress changes exceed plus or minus two standard deviations from the annual-mean trade winds and, in the case of intensification, are partially representative of recently observed trends. The Agulhas leakage is quantified using complimentary techniques based on Lagrangian virtual floats and Eulerian passive tracer flux. As resolution increases, model behavior converges and the sensitivity of the leakage to Agulhas Current transport anomalies is reduced. In the two eddy-resolving configurations tested, the leakage is insensitive to changes in Agulhas Current transport at 328S, though substantial eddy kinetic energy anomalies are evident. Consistent with observations, the position of the retroflection remains stable. The decoupling of Agulhas Current variability from the Agulhas leakage suggests that while correlations between the two may exist, they may not have a clear dynamical basis. It is suggested that present and future Agulhas leakage proxies should be considered in the context of potentially transient forcing regimes.

No abstract

The Agulhas Current (AC) is the strongest western boundary current in the Southern Hemisphere and is key for weather and climate patterns, both regionally and globally. Its heat transfer into both the midlatitude South Indian Ocean and South Atlantic is of global significance. A new composite coral record (Ifaty and Tulear massive Porites corals), is linked to historical AC sea surface temperature (SST) instrumental data, showing robust correlations. The composite coral SST data start in 1660 and comprise 200 years more than the AC instrumental record. Numerical modelling exhibits that this new coral derived SST record is representative for the wider core region of the AC. AC SSTs variabilities show distinct cooling through the Little Ice Age and warming during the late 18th, 19th and 20th century, with significant decadal variability superimposed. Furthermore, the AC SSTs are teleconnected with the broad southern Indian and Atlantic Oceans, showing that the AC system is pivotal for inter-ocean heat exchange south of Africa.

Over the last decade, West African coastal countries, including Ghana, have experienced extensive economic damage due to illegal, unreported and unregulated (IUU) fishing activity, estimated at about USD 100 million in losses each year. Illegal, unreported and unregulated fishing poses an enormous threat to the conservation and management of the dwindling fish stocks, causing multiple adverse consequences for fisheries, coastal and marine ecosystems and for the people who depend on these resources. The Integrated System for Surveillance of Illegal, Unlicensed and Unreported Fishing (INSURE) is an efficient and inexpensive system that has been developed for the monitoring of IUU fishing in Ghanaian waters. It makes use of fast-delivery Earth observation data from the synthetic aperture radar instrument on Sentinel-1 and the Multi Spectral Imager on Sentinel-2, detecting objects that differ markedly from their immediate background using a constant false alarm rate test. Detections are matched to, and verified by, Automatic Identification System (AIS) data, which provide the location and dimensions of ships that are legally operating in the region. Matched and unmatched data are then displayed on a web portal for use by coastal management authorities in Ghana. The system has a detection success rate of 91% for AIS-registered vessels, and a fast throughput, processing and delivering information within 2 h of acquiring the satellite overpass. However, over the 17-month analysis period, 75% of SAR detections have no equivalent in the AIS record, suggesting significant unregulated marine activity, including vessels potentially involved in IUU. The INSURE system demonstrated its efficiency in Ghana’s exclusive economic zone and it can be extended to the neighbouring states in the Gulf of Guinea, or other geographical regions that need to improve fisheries surveillance.