Long-term variability of Sea Surface Temperature in the Tropical Indian Ocean in relation to climate change and variability

Highlights

• The robust basin-wide warming of the TIO is largely in phase with the warming of the western Pacific and Atlantic Oceans.

• The cyclonic circulation of the Arabian Sea basin resulted in equatorial westerlies and enhanced latent heat loss.

• The oceanic processes reinforce the warming tendency of TIO, which overcomes the cooling by net loss of surface heat flux.

Abstract

The long-term change in the sea surface temperature (SST) of the Tropical Indian Ocean (TIO) and the underlying mechanisms are examined in detail by analysing multiple reanalysis datasets and historical simulations of a global climate model. A robust basin-wide warming signal was found in SST, with the highest long-term warming trend (~0.09 ± 0.015 °C decade⁻¹) seen over the north-western and equatorial Indian Ocean regions. The robust warming trend of TIO SST is a dominant warming signal of the global tropical oceans and is largely in phase with similar warming in western Pacific and Atlantic Oceans. The central equatorial Indian Ocean and south TIO regions went through significant warming during recent decades. The weakened cross-equatorial flow and the associated cyclonic winds over the Arabian Sea region have resulted in equatorial westerlies with an enhanced loss of latent heat over the central and eastern equatorial Indian Ocean. The anomalous cyclonic wind stress curl, in turn, caused mixed layer stratifications in the western Indian Ocean. The advective heat transfer from the south-eastern Indian Ocean to the western Indian Ocean and reduced oceanic cooling by vertical processes, overcome the cooling by the net loss of surface heat fluxes, and favor surface warming the TIO.

Introduction

The Indian ocean warming and its teleconnections play a crucial role in the modulation of the global climate system (e.g., Luo et al., 2012; Chen et al., 2019). Since the 1950s, the tropical Indian Ocean (TIO; 30^oN-30^oS and 30°E-120°E) has warmed faster than elsewhere in the tropical ocean basins (Han et al., 2014) consistent with the global sea surface temperature (SST) trend. The Indian Ocean SST variability has implications on environmental and associated socioeconomic status of the Indian Ocean rim countries. The intensity and distribution of Indian summer monsoon has changed and is projected to change substantially in the future (Sharmila et al., 2015; Sahany et al., 2019; Bhowmick et al., 2019; Mishra et al., 2019). Considering the regional and remote impacts of TIO warming, it is highly desirable to understand the SST variability and change to improve the monsoon predictability (Kucharski and Abid, 2017).

Long-term variations of SST anomaly patterns resulted from a combination of intrinsic modes of atmospheric and oceanic circulation variabilities and coupled ocean-atmosphere interactions (Deser et al., 2010). The SST variability of TIO shows a centennial warming trend superimposed on interannual and decadal variations as well as a rapid warming trend during 2003–2013 (Li et al., 2017). Previous studies (e.g., Li et al., 2016; Han et al., 2014) speculated that the remote forcing from Atlantic and Pacific basins contributes to upper-ocean warming of TIO. However, based on observation and climate model-based analysis, Li et al. (2017) suggested that the tropical ocean basins are tightly connected through multiple atmospheric teleconnections besides the inter-oceanic connection. Atlantic Ocean regional effect can feedback to the Pacific (Kucharski et al., 2015), which influences the Indian Ocean through atmospheric teleconnections (Li et al., 2016).

Previous studies (e.g., Deser et al., 2010; Dong and Zhou, 2014) linked the warming of Indian Ocean, especially since the 1950s, to El Niño driven air-sea interactions and greenhouse gas forced radiative and heat flux contributions. Dong and Zhou (2014) showed that anthropogenic forcing contributes to basin-wide warming pattern causing frequent Indian Ocean Dipole (IOD) events. Enhanced wind stress curl in the south Indian Ocean also contributed to the basin-wide warming of the Indian Ocean (Cai et al., 2007). Anthropogenic activities and warming in the Indian Ocean have influenced the Indian summer monsoon (e.g., Singh et al., 2019; Roxy et al., 2014). Hence, a rigorous analysis of SST variability of TIO and its quantification has scientific and societal significance. Our understanding regarding the persistent long-term warming revealed by observations, reanalysis and climate models is less sufficient for reliable future projection and impact assessments. The previous study by Dong and McPhaden (2016) suggested weaker warming over the north tropical Indian Ocean compared to the enhanced warming of southern Indian Ocean (south of 10^oS) under the influence of Pacific heat transport through an inter-oceanic connection. We limit our study domain to TIO to highlight its warming trend and coherency with global tropical oceans. Also, the tropical Indian Ocean warming/cooling is sensitive to Indian summer monsoon variability and is highly significant to the livelihood of the population across the Indian subcontinent.

In this paper, we examine three gridded reanalysis SST data sets available for the period of 1900–2017 to examine the consistency and causes of TIO warming. The study is aimed at addressing the following questions: Is the previously reported Indian Ocean warming trend also prominent over global tropical oceans and is it robust across different SST products? Is the imprint of natural and anthropogenic warming localized in a specific region across different SST products? The study also attempts to understand how the air-sea fluxes and mixed layer heat content of the tropical Indian Ocean are adjusting in a warming climate. A brief description of datasets and methodologies used in the study is given in Section 2. The results and discussions are presented in Section 3, and the results have been summarised in Section 4.