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A new mode of decadal variability in the Tropical Indian Ocean subsurface temperature and its association with shallow meridional overturning circulation - ScienceDirect
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## Global and Planetary Change

# A new mode of decadal variability in the Tropical Indian Ocean subsurface temperature and its association with shallow meridional overturning circulation

Author links open overlay panelSandeepMohapatraab, ChellappanGnanaseelana

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## Highlights

- •
  A new decadal subsurface mode of variability in the tropical Indian Ocean is reported.

- •
  The subsurface mode is driven by the equatorial winds and associated wind stress curl.

- •
  The subsurface mode modulates the shallow meridional overturning circulation.

## Abstract

A north-south subsurface dipole mode (SDM) is found to be the leading mode of interannual variability in the thermocline and subsurface temperature over the tropical Indian Ocean. The present study reports the existence of a decadal SDM with a north-south dominant structure. The mechanisms responsible for the formation of the decadal SDM and its close association with the evolution of local atmospheric and ocean circulation over tropical Indian Ocean are examined. Though the interannual SDM also has a north-south dominant dipole structure, it is imprinted with a weak east-west dipole structure as well. Unlike the interannual SDM, decadal SDM is characterized by a pure north-south pattern with the northern mode covering the entire longitudinal extent of equatorial Indian Ocean. The decadal variability in the surface winds along the equatorial Indian Ocean and the associated wind stress curl and their persistence are found to be the forcing mechanisms responsible for the decadal evolution of the north-south mode. Persistent positive wind stress curl anomalies south of 8oS intensify the downwelling Rossby waves in the south during the positive phase of the decadal SDM. On the other hand, the northern cooling is driven mostly by the equatorial upwelling Kelvin waves, the Ekman divergence and the equatorially trapped reflected upwelling Rossby waves. It is found that the phase transition in the subsurface mode is determined by the strength of the surface winds and the associated changes in the Ekman transport. Consistent with SDM, upper 50-300 m and 500 m oceanic heat contents reveal conventional north-south dipole structure highlighting the importance of SDM on the tropical Indian Ocean heat redistribution. The meridional overturning circulation associated with SDM modulates the conventional shallow meridional overturning circulation in the Indian Ocean (amplifying it during the positive phase and weakening it during the negative phase) and the associated meridional heat transport.

## Introduction

The climate modes, such as El Niño Southern Oscillation (ENSO), Interdecadal Pacific Oscillation, Pacific Decadal Oscillation, Atlantic Multidecadal Oscillation etc. influence the Indian Ocean via oceanic and atmospheric pathways both in the interannual and decadal time scales (e.g., Chambers et al., 1999; Shinoda et al., 2004; Gnanaseelan et al., 2012; Han et al., 2014; Dong et al., 2016; Zhou et al., 2017; Huang et al., 2019; Sun et al., 2019). Similar to the Pacific and Atlantic, Indian Ocean has its own internal mode of variabilities such as Indian Ocean Dipole (IOD; Saji et al., 1999; Webster et al., 1999) and subsurface dipole mode (SDM; Sayantani and Gnanaseelan, 2015), which are mostly developed and sustained through ocean-atmosphere interaction over tropical Indian Ocean (e.g. Kakatkar et al., 2020). IOD is linked to sea surface temperature (SST) variability (e.g., Saji et al., 1999; Webster et al., 1999), whereas the SDM is associated with thermocline and subsurface temperature variability (e.g., Sayantani and Gnanaseelan, 2015; Rao et al., 2002; Shinoda et al., 2004). Many studies have discussed the impact of IOD on Indian Summer Monsoon and global climate (e.g. Ashok and Saji, 2007; Saji and Yamagata, 2003) though it is very short-lived. SDM, on the other hand has longer memory than IOD with signals persisting for several seasons (Sayantani and Gnanaseelan, 2015); however, the role of SDM on the regional climate variability is not well established.

A typical positive phase of SDM is characterized by the meridional gradient in subsurface temperature anomalies with cooling in the eastern equatorial Indian Ocean (northward extension up to the head Bay of Bengal) and warming in the southwestern Indian Ocean (extending up to the southern Arabian Sea). Sayantani and Gnanaseelan (2015) attributed such interannual variability in subsurface temperature to meridional shear in winds and the associated wind stress curl anomalies induced Rossby and wind induced Kelvin waves. Rao et al. (2002) and Shinoda et al. (2004) also attributed the generation and development of subsurface east-west dipole to equatorial wind forcing. The structure and evolution of wind circulation patterns in the decadal time scale have not been explored in detail, especially over the tropical Indian Ocean, which is vital for explaining the thermocline and subsurface variability on decadal time scales. Moreover, only few studies have discussed the tropical-extratropical teleconnections (Wang et al., 2012) and their impact on tropical Indian Ocean variability.

Many studies have reported that ENSO and IOD modulate the subsurface temperature over tropical Indian Ocean (e.g., Sayantani and Gnanaseelan, 2015; Kakatkar et al., 2019). Shinoda et al. (2004) have shown through ocean model experiments that the subsurface dipole is mostly associated with IOD events and weakly correlated with ENSO events. Chakravorty et al. (2014) on the other hand, using coupled model experiments, highlighted the role of ENSO in strengthening the IOD generated Rossby waves, which are the mechanisms for the persistence of the subsurface mode for several seasons. Yu et al. (2005) and Rao and Behera (2005) also studied the impact of ENSO events on the thermocline and subsurface temperature variability in the region south of 10oS. However, none of the earlier studies have reported the decadal variation in the subsurface temperature, and the associated triggering mechanisms. On the other hand, several studies have already discussed the decadal IOD and the associated internal local forcing and remote forcing from Pacific (e.g., Ashok et al., 2004; Tozuka et al., 2007; Yuan et al., 2008; Yang et al., 2017). Therefore, in this paper, we have given more emphasis on the decadal subsurface variability and the underlying mechanisms.

Very few studies have discussed the decadal variability in oceanic heat content (OHC), subsurface temperature and thermocline over the tropical Indian Ocean (e.g., Mohapatra et al., 2020; Trenary and Han, 2013; Jin et al., 2018; Zhou et al., 2017). Thermocline variability over southwestern tropical Indian Ocean, which is a part of SDM, is a region of strong SST thermocline coupling as compared to other regions of tropical Indian Ocean (e.g., Xie et al., 2002; Chowdary et al., 2009; Kakatkar et al., 2020). Recent studies attributed the decadal variability over southwestern tropical Indian Ocean to surface wind forcing associated with Pacific decadal variability (e.g., Deepa et al., 2019; Jin et al., 2018; Trenary and Han, 2013). In addition to this the decadal subsurface temperature variability over the eastern Indian Ocean plays a vital role in predicting the IOD events (e.g., Annamalai et al., 2005; Ummenhofer et al., 2017). Mohapatra et al. (2020) attributed the significant decadal variability seen in the OHC of upper 110 m to the OHC of 50-110 m, which further justifies the importance of subsurface variability on decadal time scales. The subsurface temperature and thermocline variability is significant in the surface dynamics due to its possible interaction with the surface (e.g. Kakatkar et al., 2020) and thus modulating the regional climate over tropical Indian Ocean. Therefore, the subsurface temperature and thermocline variability is increasingly being recognised as an essential factor of the climate system.

Further, the relationship between the Indian Ocean and the Pacific Ocean has been weakened from the mid-1980s (e.g., Han et al., 2014; Dong and McPhaden, 2017; Mohapatra et al., 2020). Such weakening relationship observed in SST is also reflected in the Indian Ocean and Pacific Walker Cell, especially during boreal summer on decadal time scales (Han et al., 2017). They further suggested that the Indian Ocean Walker Cell is modulated mainly by the decadal variability in the ENSO, IOD, tropical Indian Ocean SST with maximum impact from local forcing (summer monsoon convection) from the late 1990s. In addition to this, a recent study by Mohapatra et al. (2020) discussed the growing role of local processes and associated ocean dynamics on the decadal variability in the subsurface temperature over equatorial Indian Ocean. The main purpose of this study is to have an improved understanding of the subsurface temperature variability and its association with the wind and ocean circulation in decadal time scale. We also examine how the shallow meridional overturning circulation (SMOC) in the tropical Indian Ocean and the heat redistribution are modulated by the decadal phase transition of SDM. The paper is organised as follows. Section 2 briefly discusses the different data sets used in the study and methodoly adopted for computing OHC, meridional heat transport etc. Section 3.1 discusses the evolution and structure of subsurface dipole. Ekman transport and meridional heat transport across the tropical Indian Ocean and its association with the winds and SMOC are described in Section 3.2. Section 4 summarizes the major findings of the present study.

## Section snippets

## Data sets used and methodology

Reanalysis data sets (globally gridded) are used to analyse the decadal variability in the tropical Indian Ocean subsurface temperature. The subsurface temperature and sea surface height (SSH) data sets are taken from Ocean Reanalysis System 4 (ORAS4) for the period 1958–2017. ORAS4 employs the variational ocean data assimilation system NEMOVAR using Nucleus for European Modelling of the Ocean (NEMO) version 3.0 ocean model. Balmaseda et al. (2013) provide a detailed description of ORAS4

## Structure and evolution of decadal SDM

The decadal variability of tropical Indian Ocean subsurface temperature and thermocline is relatively an unexplored area, which is mainly addressed in this paper. We have first computed the thermocline (the depth of maximum vertical gradient in temperature), pycnocline, D22, D23 and D24 for the well observed period of 2005–2017 from both ARGO and ORAS4. The correlation between thermocline and D22, D23 and D24 reveals that D23 is closely associated with the thermocline depth as compared to D22,

## Discussion and summary

The present study discusses the decadal subsurface mode over the tropical Indian Ocean and its close association with the wind circulation, which has not been reported earlier. It also reports that D23 better represents the thermocline depth over the tropical Indian Ocean compared to D20 or D22 or D24. The leading mode of variability in the decadal subsurface temperature and D23 shows a clear north-south dipole with the northern mode covering the entire equatorial belt. In this study, we have

## Declaration of Competing Interest

There is no conflict of interest

## Acknowledgement

We thank Director of IITM and Ministry of Earth Sciences (MoES), Government of India for the support. The comments from two anonymous reviewers have helped us to improve the manuscript. Sandeep Mohapatra thanks IITM for research fellowship. ARGO, ORAS4 and ORAS3 data are available from Asia-Pacific Data Research Centre (APDRC) ( http://apdrc.soest.hawaii.edu/data/data.php). ERA40 and ERA-Interim surface wind data are downloaded from the ECMWF website ( http://apps.ecmwf.int/datasets/). Figures

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Show abstract

The dominant mode of the Indian Ocean variability can be impacted by changes in the background state (multidecadal timescale) of the subsurface heat content. The multidecadal variability of subsurface ocean heat content (sub-OHC) in the Indian Ocean is examined using four reanalysis products from 1958 to 2017. The analysis reveals a meridional basin-wide dipole mode in the subsurface OHC until the late 1980s, followed by the mode embedded in the uniform basin-wide patterns. These patterns are also observed in the trends of thermocline and sea surface height. The observed patterns in the Indian Ocean are explained by two distinct mechanisms. Firstly, the multidecadal variability of dipole patterns over the Indian Ocean is influenced by local wind forcing. Wind stress trends and Ekman pumping velocity trends favor downwelling (upwelling) in the off-equatorial southern Indian Ocean region, leading to thermocline depth deepening (shallowing) during 1958–1975 and 1976–1987, respectively. Secondly, the combined effect of heat transport from the western Pacific through Indonesian Through Flow and local wind forcing accounts for the basin-wide cooling and warming trends observed during 1988–2000, and 2001–2014, respectively.

2023, Journal of Geophysical Research: Oceans

2023, Climate Dynamics

2023, Journal of Climate

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