Indian Ocean sources of Agulhas leakage

Abstract

We examine the mean pathways, transit timescales, and transformation of waters flowing from the Pacific and the marginal seas through the Indian Ocean (IO) on their way toward the South Atlantic within a high‐resolution ocean/sea‐ice model. The model fields are analyzed from a Lagrangian perspective where water volumes are tracked as they enter the IO. The IO contributes 12.6 Sv to Agulhas leakage, which within the model is 14.1 ± 2.2 Sv, the rest originates from the South Atlantic. The Indonesian Through‐flow constitutes about half of the IO contribution, is surface bound, cools and salinificates as it leaves the basin within 10–30 years. Waters entering the IO south of Australia are at intermediate depths and maintain their temperature‐salinity properties as they exit the basin within 15–35 years. Of these waters, the contribution from Tasman leakage is 1.4 Sv. The rest stem from recirculation from the frontal regions of the Southern Ocean. The marginal seas export 1.0 Sv into the Atlantic within 15–40 years, and the waters cool and freshen on‐route. However, the model's simulation of waters from the Gulfs of Aden and Oman are too light and hence overly influenced by upper ocean circulations. In the Cape Basin, Agulhas leakage is well mixed. On‐route, temperature‐salinity transformations occur predominantly in the Arabian Sea and within the greater Agulhas Current region. Overall, the IO exports at least 7.9 Sv from the Pacific to the Atlantic, thereby quantifying the strength of the upper cell of the global conveyor belt.

Role of El Niño Southern Oscillation (ENSO) Events on Temperature and Salinity Variability in the Agulhas Leakage Region

“…In other words, it takes approximately 2 years for tropical warm anomalies formed from El Niño-associated wind anomalies to reach the Agulhas leakage region. This is relatively consistent with the earlier mentioned time scales of ocean circulation, where it takes a little more than~1.3 years for waters to cross the Indian Ocean basin [6] and theñ 6 months for eddies from EMC and MC to interact with Agulhas leakage [11]. Our study aims to define the relationship between ENSO events and SST and SSS variability in the Agulhas leakage region.…”

“…This relationship cannot be fully understood without first connecting the influence of the ENSO signal across the three ocean basins involved: the Pacific Ocean, the Indian Ocean, and the Atlantic Ocean. In terms of global circulation, the Indian Ocean acts as the link between the Pacific and the Atlantic Ocean, contributing nearly 12.6 Sv to Agulhas leakage, of which, about 7.9 Sv originates from the Pacific moving into the Atlantic [6]. Nearly half of the Indian Ocean contribution to Agulhas leakage comes from the Indonesian Throughflow (ITF) with a smaller portion originating south of Australia by Tasman leakage [6].…”

“…In terms of global circulation, the Indian Ocean acts as the link between the Pacific and the Atlantic Ocean, contributing nearly 12.6 Sv to Agulhas leakage, of which, about 7.9 Sv originates from the Pacific moving into the Atlantic [6]. Nearly half of the Indian Ocean contribution to Agulhas leakage comes from the Indonesian Throughflow (ITF) with a smaller portion originating south of Australia by Tasman leakage [6]. The ITF has been found to increase during La Niña and decrease during El Niño [7].…”

“…Within the Indian Ocean basin, the westward flowing South Equatorial Current (SEC) circulates water from the Indian Ocean subtropical gyre and ITF to the Madagascar coast. This westward transport of water between 60 • E and 100 • E is modeled at mean speeds of~0.1 m·s −1 , taking~1.3 years for waters from the ITF to reach 77 • E [6]. Upon reaching the Madagascar coast, the SEC splits at 17 • S into a northern and southern branch.…”

Glacial–interglacial changes and Holocene variations in Arabian Sea denitrification

“…Upper water masses (< 1500 m of water depth) in the Indian Ocean have a net westward circulation, while deep waters follow an eastward route as part of the Great Ocean Conveyer Belt connecting the Pacific and Atlantic Ocean (Broecker, 1991). However, this general direction comprises rather complicated routes and pathways (Durgadoo et al, 2017). Four water masses contribute to the subsurface waters of the OMZ: Arabian Sea High-Salinity Water (ASHSW), Persian Gulf Water (PGW), Red Sea Water (RSW), and Indian Ocean Central Water (ICW).…”