Abstract

The Agulhas Current (AC) off the southern tip of Africa is one of the strongest western boundary currents and a crucial chokepoint of inter-ocean heat and salt exchange between the Indian and the South Atlantic Ocean. However, large uncertainties remain concerning the sea surface temperature and salinity variability in the AC region and their driving mechanisms over longer time scales, due to short observational datasets and the highly dynamic nature of the region. Here, we present an annual coral skeletal Sr/Ca composite record paired with an established composite oxygen isotope record from Ifaty and Tulear reefs in southwestern Madagascar to obtain a 334 year-long (1661–1995) reconstruction of δ18Oseawater changes related to surface salinity variability in the wider Agulhas Current region. Our new annual δ18Oseawater composite record from Ifaty traces surface salinity of the southern Mozambique Channel and AC core region from the SODA reanalysis since 1958. δ18Oseawater appears mainly driven by large-scale wind forcing in the southern Indian Ocean on interannual to decadal time scales. The δ18Oseawater and SST at Ifaty show characteristic interannual variability of between 2 to 4 years, typical for ENSO. Lagged correlations with the Multivariate ENSO index reveals a 1–2 year lag of δ18Oseawater and salinity at Ifaty and the AC region, suggesting that propagation of anomalies by ocean Rossby waves may contribute to salinity changes in the wider southwestern Indian Ocean. The δ18Oseawater and SST reconstructions at Ifaty reveal the highest interannual variability during the Little Ice Age, especially around 1700, which is in agreement with other Indo-Pacific coral studies. Our study demonstrates the huge potential to unlock past interannual and decadal changes in surface ocean hydrology and ocean transport dynamics from coral δ18Oseawater beyond the short instrumental record.

Highlights

  • The greater Agulhas Current (AC) system off the southern tip of Africa is a crucial chokepoint of the global thermohaline circulation through inter-ocean heat and salt exchange between the Indian and the South Atlantic Ocean via the 35 so-called Agulhas Leakage (AL), thereby influencing the variability of the Atlantic meridional overturning circulation

  • The new d18Oseawater reconstruction is based on three Porites paired Sr/Ca, and coral oxygen isotope (d18O) records at annual resolution from Ifaty and Tulear coral reefs off southwestern Madagascar (43°E, 23°S), covering the past 334 years (Fig. 2; Zinke et al, 2014)

  • Between 1661 and 1995, Sr/Ca-sea surface temperature (SST) records an increase of 0.94±0.26 oC, while d18O-SST indicates an increase of 0.83±0.21 oC, both statistically significant at p

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Summary

Introduction

The greater Agulhas Current (AC) system off the southern tip of Africa is a crucial chokepoint of the global thermohaline circulation through inter-ocean heat and salt exchange between the Indian and the South Atlantic Ocean via the 35 so-called Agulhas Leakage (AL), thereby influencing the variability of the Atlantic meridional overturning circulation (hereafter AMOC; Peeters et al, 2004; Beal et al, 2011; Biastoch et al, 2009, 2015). The sea surface temperature (SST) and salinity (SSS) variability in the AC, which feeds the AL, are suggested to be related to upstream wind and current variability in the southern Indian Ocean (Backeberg et al, 2010; Biastoch et al, 2008, 2009, 2015; Rouault et al, 2009). There is evidence from satellite altimetry observations that mesoscale variability upstream of the AC has strengthened since 1993, resulting in accelerated eddy propagation into the AC and AR regions (Backeberg, 2012). This is related to enhanced ocean current 45 transport in response to an increase in wind stress curl in the southern Indian Ocean trade winds (Backeberg et al, 2012). Large uncertainties remain concerning the role of large-scale atmospheric circulation (wind and sea level 55 pressure) on SST and SSS over longer time scales and their influence on AC SST and SSS variability due to short observational datasets and the highly dynamic nature of the region (Rouault et al, 2009; Backeberg et al, 2010)

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