Holocene tropical western Indian Ocean sea surface temperatures in covariation with climatic changes in the Indonesian region

Abstract

The sea surface temperature (SST) of the tropical Indian Ocean is a major component of global climate teleconnections. While the Holocene SST history is documented for regions affected by the Indian and Arabian monsoons, data from the near-equatorial western Indian Ocean are sparse. Reconstructing past zonal and meridional SST gradients requires additional information on past temperatures from the western boundary current region. We present a unique record of Holocene SST and thermocline depth variations in the tropical western Indian Ocean as documented in foraminiferal Mg/Ca ratios and δ 18 O from a sediment core off northern Tanzania. For Mg/Ca and thermocline δ 18 O, most variance is concentrated in the centennial to bicentennial periodicity band. On the millennial time scale, an early to mid-Holocene (~7.8-5.6 ka) warm phase is followed by a temperature drop by up to 2°C, leading to a mid-Holocene cool interval (5.6-4.2 ka). The shift is accompanied by an initial reduction in the difference between surface and thermocline foraminiferal δ 18 O, consistent with the thickening of the mixed layer and suggestions of a strengthened Walker circulation. However, we cannot confirm the expected enhanced zonal SST gradient, as the cooling of similar magnitude had previously been found in SSTs from the upwelling region off Sumatra and in Flores air temperatures. The SST pattern probably reflects the tropical Indian Ocean expression of a large-scale climate anomaly rather than a positive Indian Ocean Dipole-like mean state.

Tropical cyclone genesis potential across palaeoclimates

“…The tropical region of the mid-Holocene period might have encountered slightly elevated SSTs of around 1 • C (Gagan et al, 1998), although recent studies indicate some uncertainty in terms of negative SST anomaly for regions such as the western Indian Ocean (Kuhnert et al, 2014). Despite the limited proxy record agreement on whether tropical oceans may have warmed (Koutavas et al, 2002;Rimbu et al, 2004;Stott et al, 2004), prior PMIP simulations suggest SST in the Northern Hemisphere was generally warmer by less than 1 • C in the midHolocene period compared to the preindustrial era, and the Southern Hemisphere might have been slightly cooler (Braconnot et al, 2007).…”

Holocene variations of thermocline conditions in the eastern tropical Indian Ocean

“…The IOD signal might be masked by the strong monsoon signal in the Holocene climate records from East Africa and the western Indian Ocean (de Boer et al, 2014) which further complicates a straightforward comparison of the eastern and the western Indian Ocean climate archives. This assumption is further supported by coherent changes in reconstructed Holocene SST anomalies off Tanzania (Kuhnert et al, 2014) and rainfall anomalies in Flores, Indonesia (Griffiths et al, 2009), with positive SST anomalies off East Africa corresponding to positive rainfall anomalies over South Indonesia and vice versa. Kuhnert et al (2014) therefore suggest that mid-Holocene climate was anomalous on a global scale and hypothesize that the unusual temperature pattern in the Indian Ocean reflects remote forcing rather than one of the climate modes internal to the Indian Ocean.…”

“…This assumption is further supported by coherent changes in reconstructed Holocene SST anomalies off Tanzania (Kuhnert et al, 2014) and rainfall anomalies in Flores, Indonesia (Griffiths et al, 2009), with positive SST anomalies off East Africa corresponding to positive rainfall anomalies over South Indonesia and vice versa. Kuhnert et al (2014) therefore suggest that mid-Holocene climate was anomalous on a global scale and hypothesize that the unusual temperature pattern in the Indian Ocean reflects remote forcing rather than one of the climate modes internal to the Indian Ocean. Proxy-based reconstructions, including this study, might also incorporate other seasons and years which would hamper a direct comparison additionally.…”

Automated cleaning of foraminifera shells before Mg/Ca analysis using a pipette robot

“…Kuhnert et al . [] rejected samples where Al/Ca was >0.3 mmol/mol. J. Groeneveld (personal communication) rejects Mg/Ca where Al/Ca is more than 2 standard deviations from the mean value.…”