Mid-Holocene expansion of the Indian Ocean warm pool documented in coral Sr/Ca records from Kenya

Abstract

Proxy reconstructions suggest that mid-Holocene East African temperatures were warmer than today between 8 and 5 ka BP, but climate models cannot replicate this warming. Precessional forcing caused a shift of maximum insolation from boreal spring to fall in the mid-Holocene, which may have favored intense warming at the start of the warm season. Here, we use three Porites corals from Kenya that represent time windows from 6.55 to 5.87 ka BP to reconstruct past sea surface temperature (SST) seasonality from coral Sr/Ca ratios in the western Indian Ocean during the mid-Holocene. Although the Indian monsoon was reportedly stronger in the mid-Holocene, which should have amplified the seasonal cycle of SST in the western Indian Ocean, the corals suggest reduced seasonality (mean 3.2 °C) compared to the modern record (mean 4.3 °C). Warming in austral spring is followed by a prolonged period of warm SSTs, suggesting that an upper limit of tropical SSTs under mid-Holocene conditions was reached at the start of the warm season, and SSTs then remained stable. Similar changes are seen at the Seychelles. Bootstrap estimates suggest a reduction in SST seasonality of 1.3 ± 0.22 °C at Kenya and 1.7 ± 0.32 °C at the Seychelles. SST seasonality at Kenya corresponds to present-day SST seasonality at 55° E–60° E, while SST seasonality at the Seychelles corresponds to present day SST seasonality at ~ 65° E. This implies a significant westward expansion of the Indian Ocean warm pool. Furthermore, the coral data suggests that SST seasonality deviates from seasonal changes in orbital insolation due to ocean–atmosphere interactions.

Coral reef potential connectivity in the southwest Indian Ocean

“…S15), it is possible that the South Equatorial Countercurrent previously separated from East Africa at a more southerly latitude during the spawning season. However, given the generally muted response of monsoons in the southwest Indian Ocean to mid-Holocene climate change (Brierley et al, 2020; Leupold et al, 2023), we consider it unlikely that these changes would have substantially changed connectivity networks. Whilst our predictions of coral reef connectivity may therefore be robust with respect to mid-to-late Holocene climate change, western Indian Ocean biogeography has its roots in the Palaeogene, where the tectonic configuration - and therefore surface currents - were considerably different (e.g.…”

Climate Changes and Sustainability

“…These effects, commonly referred to as teleconnections, arise because variations in low-latitude atmospheric circulation patterns in the Pacific area impact atmospheric circulation in upstream and neighboring systems. As a result, storm tracks and the typical locations of ridges and troughs of atmospheric pressure-areas of high and low pressure-are altered [56]. Westerlies and storm tracks are redirected towards the Equator, resulting in stormy, rainy winter weather in California and the dry Southwest of the United States, while the Pacific Northwest's typically wet winter conditions become drier and warmer.…”

Coral reef potential connectivity in the southwest Indian Ocean

“…S15), it is possible that the South Equatorial Countercurrent previously separated from East Africa at a more southerly latitude during the spawning season. However, given the generally muted response of monsoons in the southwest Indian Ocean to mid-Holocene climate change (Brierley et al 2020;Leupold et al 2023), we consider it unlikely that these changes would have substantially changed connectivity networks. Whilst our predictions of coral reef connectivity may therefore be robust with respect to mid-tolate Holocene climate change, western Indian Ocean biogeography has its roots in the Palaeogene, where the tectonic configuration-and therefore surface currents-were considerably different (e.g.…”