A glacial warm water anomaly in the subantarctic Atlantic Ocean, near the Agulhas Retroflection

Abstract

ODP Site 1089 is optimally located in order to monitor the occurrence of maxima in Agulhas heat and salt spillage from the Indian to the Atlantic Ocean. Radiolarian-based paleotemperature transfer functions allowed to reconstruct the climatic history for the last 450 kyr at this location. A warm sea surface temperature anomaly during Marine Isotope Stage (MIS) 10 was recognized and traced to other oceanic records along the surface branch of the global thermohaline (THC) circulation system, and is particularly marked at locations where a strong interaction between oceanic and atmospheric overturning cells and fronts occurs. This anomaly is absent in the Vostok ice core deuterium, and in oceanic records from the Antarctic Zone. However, it is present in the deuterium excess record from the Vostok ice core, interpreted as reflecting the temperature at the moisture source site for the snow precipitated at Vostok Station. As atmospheric models predict a subtropical Indian source for such moisture, this provides the necessary teleconnection between East Antarctica and ODP Site 1089, as the subtropical Indian is also the source area of the Agulhas Current, the main climate agent at our study location.The presence of the MIS 10 anomaly in the d 13 C foraminiferal records from the same core supports its connection to oceanic mechanisms, linking stronger Agulhas spillover intensity to increased productivity in the study area. We suggest, in analogy to modern oceanographic observations, this to be a consequence of a shallow nutricline, induced by eddy mixing and baroclinic tide generation, which are in turn connected to the flow geometry, and intensity, of the Agulhas Current as it flows past the Agulhas Bank.We interpret the intensified inflow of Agulhas Current to the South Atlantic as responding to the switch between lower and higher amplitude in the insolation forcing in the Agulhas Current source area. This would result in higher SSTs in the Cape Basin during the glacial MIS 10, due to the release into the South Atlantic of the heat previously accumulating in the subtropical and equatorial Indian and Pacific Ocean. If our explanation for the MIS 10 anomaly in terms of an insolation variability switch is correct, we might expect that a future Agulhas SSST anomaly event will further delay the onset of next glacial age. In fact, the insolation forcing conditions for the Holocene (the current interglacial) are very similar to those present during MIS 11 (the interglacial preceding MIS 10), as both periods are characterized by a low insolation variability for the Agulhas Current source area. Natural climatic variability will force the Earth system in the same direction as the anthropogenic global warming trend, and will thus lead to even warmer than expected global temperatures in the near future. D

Effects of midlatitude westerlies on the paleoproductivity at the Agulhas Bank slope during the penultimate glacial cycle: Evidence from coccolith Sr/Ca ratios

“…One explanation is that ALF and salinity in the Agulhas Corridor are responding to the same changes in westerly and front position that are driving salinity changes in the AC source region and therefore correlate with productivity. However, if the higher ALF and salinity in the Agulhas Corridor are also driven in part by increased Agulhas leakage, then the productivity‐promoting eddy activity associated with Agulhas leakage [ Cortese et al ., ] might have also contributed to enhanced productivity, in addition to westerly‐modulated productivity variations.…”

“…The generation of mesoscale eddies, which are related to the west flowing AC and are more frequent at increased leakage, may also influence productivity in the Agulhas Corridor. Although the AC water itself is poor in nutrients, periods of intense Agulhas leakage may promote a stronger mixing between deep and surface waters between the STF and the Cape of Good Hope as a consequence of eddy and baroclinic tide formation when the AC shoals against the Agulhas Bank during austral summers [ Cortese et al ., ], which enhance nutrient entrainment in the thermocline. Moreover, the formation of Agulhas rings in the retroflection area forms cold‐core cyclonic eddies from the northward protrusion of the STF [ Lutjeharms and van Ballegooyen , ], which locally stimulate nutrient entrainment to the surface and enhanced production.…”

The last five glacial‐interglacial transitions: A high‐resolution 450,000‐year record from the subantarctic Atlantic

“…[33] We moreover report elsewhere [Cortese et al, 2004] the presence of an episode of warmer than expected SSTs at Site 1089 during the MIS 10 glacial stage. During this interval, glacial boundary conditions (stronger trade winds and upwelling, driven by intensified atmospheric and oceanic gradients) correspond to a stronger than usual Agulhas Current and a more southern position of the STC (Figure 7).…”

“…The SSTs at Site 1089 are only indirectly influenced, via the mediation of the Agulhas Spillage, by temperature changes occurring in the midlatitude Indian Ocean. The very high and prolonged SST maxima during glacial stages 8 and 10 may represent an indication of a different functioning of the Agulhas spillage, with a more direct connection between the prevalent moisture sources to Antarctica (i.e., the midlatitude Indian Ocean) and the location of Site 1089 (as discussed for MIS 10 by Cortese et al [2004]). …”

“…These similarities can be explained by the strong influence of the STF at all these locations, while the remaining differences are due to a variety of climatic mechanisms having local importance for some of them (see above and also detailed discussions by both Schäfer et al [2005] and Rau et al [40] The SST record [McManus et al, 1999] from Site 980 (North Atlantic, Feni Drift) also displays several features in common with the SST record of Site 1089. This concerns, in particular, the presence of very warm events (higher than half full interglacial SST values) and sustained intervals of warm temperatures during full glacial periods, as well as an interval of extremely high SSTs centered around $350 ka, comparable to the Site 1089 MIS 10 warm anomaly [Cortese et al, 2004].…”