Reduction in Meridional Heat Export Contributes to Recent Indian Ocean Warming

Abstract

Since 2000, the Indian Ocean has warmed more rapidly than the Atlantic or Pacific. Air-sea fluxes alone cannot explain the rapid Indian Ocean warming, which has so far been linked to an increase in temperature transport into the basin through the Indonesian Throughflow (ITF). Here, we investigate the role that the heat transport out of the basin at 36°S plays in the warming. Adding the heat transport out of the basin to the ITF temperature transport into the basin, we calculate the decadal mean Indian Ocean heat budget over the 2010s. We find that heat convergence increased within the Indian Ocean over 2000-2019. The heat convergence over the 2010s is the same order as the warming rate, and thus the net air-sea fluxes are near zero. This is a significant change from previous analyses using trans-basin hydrographic sections from 1987, 2002, and 2009, which all found divergences of heat. A two year time series shows that seasonal aliasing is not responsible for the decadal change. The anomalous ocean heat convergence over the 2010s compared to previous estimates is due to changes in ocean currents at both the southern boundary (33%) and the ITF (67%). We hypothesize that the changes at the southern boundary are linked to an observed broadening of the Agulhas Current, implying that temperature and velocity data at the western boundary are crucial to constrain heat budget changes.

ENSO Explains the Link Between Indian Ocean Dipole and Meridional Ocean Heat Transport

“…The time mean ITF transport is the same in CTRL and NoENSO (11.2 Sv).The time mean MHT IO in CTRL and NoENSO are nearly identical (Figure S2a). There is reasonable qualitative agreement between the mean MHT IO in the models and estimates of MHT IO from reanalyses (Trenberth & Zhang, 2019) and ocean observations (Hernández-guerra & Talley, 2016;McMonigal et al, 2021;Sloyan & Rintoul, 2001;Talley, 2003Talley, , 2008. Both models and observations show a maximum heat transport of about 2 PW southward (1 PW = 10 15 W) at the latitude of the ITF inflow.…”

“…For example, the Indian Ocean is projected to warm at a faster rate than other ocean basins, partially due to a projected decrease in MHT IO (Ma et al., 2020). This decrease in MHT IO may already be evident in the observational record (McMonigal et al., 2021), and could be related to the observed rapid warming of the Indian Ocean since 2000 (Ummenhofer et al., 2021; Volkov et al., 2020; Y. Zhang, Feng, et al., 2018). A relative warming of the Indian Ocean as compared to other ocean basins can mediate climate through changes in the Atlantic meridional overturning circulation and sea surface temperatures (SST; Hu & Fedorov, 2019; Hu & Fedorov, 2020) and through modulation of the Pacific Walker circulation (L. Zhang et al., 2019).…”

“…Anomalous MHT IO can also contribute to the persistence of marine heatwaves, with detrimental ecosystem impacts (Behrens et al., 2019; Trenberth & Zhang, 2019). Recent studies have used observations to infer interannual and longer changes in MHT IO (Hernández‐guerra & Talley, 2016; McMonigal et al., 2021). However, natural variability in MHT IO is not fully understood, making it difficult to contextualize observations and assess whether a trend has emerged (Ummenhofer et al., 2021).…”

“…For example, the Indian Ocean is projected to warm at a faster rate than other ocean basins, partially due to a projected decrease in MHT IO (Ma et al, 2020). This decrease in MHT IO may already be evident in the observational record (McMonigal et al, 2021), and could be related to the observed rapid warming of the…”

Historical Changes in Wind‐Driven Ocean Circulation Can Accelerate Global Warming

“…Externally forced changes to the atmospheric circulation, and the impacts of these changes onto the oceanic circulations, have already begun to occur over the historical record. The Southern Hemisphere midlatitude winds have increased over the past four decades (Thompson et al., 2011; Thompson & Solomon, 2002), altering the wind‐driven circulation in the South Indian and South Pacific subtropical gyres (Beal & Elipot, 2016; Lee et al., 2015; McMonigal et al., 2018, 2022; Palmer et al., 2004; Roemmich et al., 2007, 2016). In the tropical Pacific, the trade winds have increased in strength (M. H. England et al., 2014; Mcgregor et al., 2012; Merrifield et al., 2012; Timmermann et al., 2010), leading to cooling in the equatorial Pacific (Seager et al., 2022).…”