Sustained Decadal Warming Phase in the Southwestern Indian Ocean since the Mid-1990s

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  • Corresponding author: Jingzhi SU, sujz@cma.gov.cn
  • Funds:

    Supported by the National Key Research and Development Program of China (2016YFA0600602) and National Natural Science Foundation of China (41776039)

  • doi: 10.1007/s13351-021-0112-4

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  • Regardless of the slowdown in global warming during the hiatus period, sea surface temperatures (SSTs) in the southwestern Indian Ocean (SWIO) have experienced sustained decadal warming for more than two decades since the mid-1990s. The SWIO SSTs warmed steadily during 1996–2016, causing a warming hot spot of 0.4 K decade−1 in a large region east of Madagascar. An upper-layer heat budget analysis indicated that heat advection by ocean currents was the greatest contributor to the warming of the SWIO SSTs. The existence of an anticyclonic geostrophic current along the western boundary of the SWIO tended to maintain such warming by transporting warmer water from the west into the SWIO region. In addition, net positive heat transport by ocean currents also occurred at the southern boundary of the SWIO as the climatological northward transport of cold water from the Southern Ocean weakened. This reduction in northward ocean currents at the surface was caused by local wind stress changes, leading to a southward Ekman current. Below the surface, an anticyclonic geostrophic current pattern existed around the warming center near the southeastern SWIO, which reduced the transport of cold waters from the Southern Ocean and warmed the SWIO. These processes near the two boundaries formed a self-sustaining positive feedback mechanism and favored the maintenance of sustained warming in the SWIO. More attention is needed to analyze the sustained long-lasting warming in the SWIO, as it is a unique phenomenon occurring under the background of the ongoing global warming.
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  • Fig. 1.  Spatial distributions of the linear trend in four SST datasets in the Indian Ocean region during 1996–2016 (shading; K decade−1). (a) HadISST, (b) OISST, (c) Kaplan SST, and (d) ERSST.v5. The robustness of the trend is indicated by a green contour line of “8” for the ratio of the mean value to the standard deviation of 36 trends. The 36 trends are calculated based on the six starting years (1995, 1996, 1997, 1998, 1999, and 2000) and the six ending years (2014, 2015, 2016, 2017, 2018, and 2019). The red contour is the area that passes the 95% confidence level for the warming trend. The SWIO basin is indicated by the blue box (34°–13°S, 54°–92°E).

    Fig. 2.  Time series of annual SSTAs (K) in HadISST (green), OISST (red), Kaplan SST (yellow), and ERSST.v5 (black), averaged over the SWIO basin. A three-point moving average was applied.

    Fig. 3.  (a) Diagnostic items for the three-dimensional spatially averaged ocean advection terms for the upper 100 m in the SWIO, where the average values are the climatological annual cycle variables, and the anomaly variables are the difference between the mean values in the last 11 years (2006–2016) and those in the previous 11 years (1996–2006). (b) Diagnostic items in the heat budget Eq. (1) (K yr−1). The vertical current $ w $ is corrected by the OSCAR dataset, and the air–sea heat flux is the sum of the shortwave radiation flux and longwave radiation flux of the JRA dataset as well as the latent heat flux and sensible heat flux of the OAFlux dataset.

    Fig. 4.  As in Fig. 3, but for the upper 50 m in the SWIO.

    Fig. 5.  The spatial distributions of sea temperature trend (shading; K decade−1) and climatic current (vector; m s−1) averaged over (a) 0–50 and (b) 0–100 m in the Indian Ocean. The purple dots indicate the region with positive $ -\overline{{u}}\dfrac{\partial {{T}}^{{{'}}}}{\partial {x}} $ (K yr−1). The red contour is the area that passes the 95% confidence level. The SWIO is enclosed by the blue lines over 34°−13°S, 54°−92°E.

    Fig. 6.  (a) The sea temperature (T) trend (shading; with intervals of 0.05 K decade−1) during 1996–2016 and climatic zonal (u) and vertical (w) currents (vector; m s−1). All the values are averaged within 34°–13°S. The area between the blue dotted lines indicates the location of the SWIO (54°–92°E). (b) The sea temperature (T) trend (shading; with intervals of 0.05 K decade−1) and climatic meridional (v) and vertical (w) currents (vector; m s−1). All the values are averaged within 54°–92°E. The purple dots indicate the region with positive $ -\overline{{w}}\dfrac{\partial {{T}}^{{{'}}}}{\partial {z}} $. The red contour is the area that passes the 95% confidence level. The area with the blue dashed lines indicates the location of the SWIO (34°–13°S).

    Fig. 7.  (a) The sea temperature trend (shading; with intervals of 0.05 K decade−1), ocean zonal and vertical current trends (vector; m s−1 decade−1) during 1996–2016, and climatic sea temperature (contour; with intervals of 2 K). All the values are averaged within 34°–13°S. The area between the blue dotted lines indicates the location of the SWIO (54°–92°E). (b) The sea temperature trend (shading; with intervals of 0.05 K decade−1), ocean meridional and vertical current trends (vector; m s−1 decade−1) during 1996–2016, and climatic sea temperature (contour; with intervals of 2 K). All the values are averaged within 54°–92°E. The red contour is the area that passes the 95% confidence level, and only current vectors significant at the 95% confidence level are plotted in black; the rest are gray vectors. The area between the blue dashed lines indicates the location of the SWIO (34°–13°S).

    Fig. 8.  (a) The spatial distribution of climatic sea temperature (shading; K) and current trend (vector; m s−1 decade−1) averaged over 0–100 m in the Indian Ocean. Only current vectors significant at the 95% confidence level are plotted in black, and the rest are gray vectors. (b) The spatial distribution of climatic sea temperature (shading; K) and climatic current (vector; m s−1) averaged over 0–100 m in the Indian Ocean. The SWIO is enclosed by the blue lines over 34°–13°S, 54°–92°E.

    Fig. 9.  (a) The climatological heat flux (PW; 1 PW = 1015 W) along all the four vertical sections, sea surface section, and bottom section at 100 m for the SWIO during 1996–2016. (b) As in (a), but for the anomalous heat flux (PW). The red and blue arrows indicate that the SWIO gains and loses heat, respectively.

    Fig. 10.  As in Fig. 9, but for at 50 m for the SWIO during 1996–2016.

    Fig. 11.  The spatial patterns of the trend in sea surface heat flux (W m−2 decade−1; downward positive) in the Indian Ocean region: (a) net shortwave radiation flux of the JRA dataset, (b) net longwave radiation flux of the JRA dataset, (c) net latent heat flux based on the mean value of the JRA and OAFlux datasets, and (d) net sensible heat flux based on the mean value of the JRA and OAFlux datasets. The red contour is the area that passes the 95% confidence level. The red (blue) shading indicates that there is heat absorption (loss) in the ocean, and the blue lines indicate the SWIO (34°–13°S, 54°–92°E).

    Fig. 12.  The spatial distributions of (a) the SSH trend (shading; m decade−1) and sea surface geostrophic current trend (vector; m s−1 decade−1) as well as (b) the wind stress trend (blue vector; N m−2 decade−1) and Ekman current trend (black vector; m s−1 decade−1) in the Ekman layer (set as 50 m over the Indian Ocean region during 1996–2016. In (b), the light blue arrow is the regression of the wind stress distribution. The red contour is the area that passes the 95% confidence level. The current vectors significant at the 95% confidence level are plotted in black, and the rest are gray vectors. Only wind stress vectors significant at the 95% confidence level are plotted in blue. The SWIO region is indicated by the blue lines (34°–13°S, 54°–92°E). The SSH data are obtained from GODAS.

    Fig. 13.  The spatial distribution of the sea temperature trend (shading; K decade−1) and current trend (vector; m s−1 decade−1) at 120 m in the Indian Ocean. The red contour is the area that passes the 95% confidence level, and only current vectors significant at the 95% confidence level are plotted in black; the rest are gray vectors. The SWIO is enclosed by the blue lines (34°–13°S, 54°–92°E).

    Fig. 14.  (a) Time series of the sea temperature anomaly (K yr−1) in the GODAS dataset, averaged over the upper 100 m in the SWIO basin. (b) Time series of the sea surface zonal wind stress (N m−2) in the GODAS dataset, averaged over the region of the southern boundary of the SWIO (34°–28°S, 70°–92°E). (c) As in (b), but for the meridional ocean current (m s−1) in the upper 100 m. A three-point moving average has been applied.

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Sustained Decadal Warming Phase in the Southwestern Indian Ocean since the Mid-1990s

    Corresponding author: Jingzhi SU, sujz@cma.gov.cn
  • Institute of Climate System, Chinese Academy of Meteorological Sciences, China Meteorological Administration, Beijing 100081
Funds: Supported by the National Key Research and Development Program of China (2016YFA0600602) and National Natural Science Foundation of China (41776039)

Abstract: Regardless of the slowdown in global warming during the hiatus period, sea surface temperatures (SSTs) in the southwestern Indian Ocean (SWIO) have experienced sustained decadal warming for more than two decades since the mid-1990s. The SWIO SSTs warmed steadily during 1996–2016, causing a warming hot spot of 0.4 K decade−1 in a large region east of Madagascar. An upper-layer heat budget analysis indicated that heat advection by ocean currents was the greatest contributor to the warming of the SWIO SSTs. The existence of an anticyclonic geostrophic current along the western boundary of the SWIO tended to maintain such warming by transporting warmer water from the west into the SWIO region. In addition, net positive heat transport by ocean currents also occurred at the southern boundary of the SWIO as the climatological northward transport of cold water from the Southern Ocean weakened. This reduction in northward ocean currents at the surface was caused by local wind stress changes, leading to a southward Ekman current. Below the surface, an anticyclonic geostrophic current pattern existed around the warming center near the southeastern SWIO, which reduced the transport of cold waters from the Southern Ocean and warmed the SWIO. These processes near the two boundaries formed a self-sustaining positive feedback mechanism and favored the maintenance of sustained warming in the SWIO. More attention is needed to analyze the sustained long-lasting warming in the SWIO, as it is a unique phenomenon occurring under the background of the ongoing global warming.

    • Since the end of the 20th century, the warming of global mean sea surface temperatures (SSTs) has slowed, and this change is referred to as the hiatus period (Kaufmann et al., 2011; Allan et al., 2014). As a special case, the entire Indian Ocean has been continuously warming in the upper 700 m since the 1950s (Yamagata et al., 2004; Han et al., 2014). The basin-wide warming in the tropical Indian Ocean has occurred at a faster rate than that in other tropical oceans since the 1950s (Du and Xie, 2008). In particular, the tropical western Indian Ocean has experienced significant warming for more than a century (Roxy et al., 2014), which has contributed to an increase in the global average SST.

      Indian Ocean warming has received much attention because of its global atmospheric impacts. One famous example is its potential impact on the positive phase shift of the North Atlantic Oscillation (Hoerling et al., 2001). Additionally, Indian Ocean warming is found to influence the Walker Circulation in the tropical Pacific (Luo et al., 2012). More recently, Indian Ocean warming has been proposed to play a role in sustaining the Atlantic meridional overturning circulation (Hu and Fedorov, 2019).

      At the same time, the causes of changes in the upper Indian Ocean over the past two decades have attracted considerable attention. Generally, it is hypothesized that the accumulated heat in the Pacific Ocean can be transported into the Indian Ocean by the Indonesian through flow (ITF; Song et al., 2004), resulting in a sudden increase in heat content in the Indian Ocean (Lee et al., 2015; Liu et al., 2016). Several regions of the Indian Ocean have undergone significant warming during the last two decades. Li et al. (2017) illustrated that the upper 400-m heat in the Indian Ocean was concentrated in the southeastern Indian Ocean, forming a warming hot spot of 0.8–1.2 K decade−1 during the hiatus period of 2000–2012. Zhang et al. (2018) emphasized the significant warming in the upper 700 m in the southern Indian Ocean (32°–5°S, around 40°–115°E) during 1998–2015.

      In addition to the above-mentioned warming regions in the Indian Ocean, the SST in the southwestern Indian Ocean (SWIO; 34°–13°S, 54°–92°E) has experienced decadal warming for more than two decades since the mid-1990s (Fig. 1), which has rarely been mentioned in previous studies. Although there have been some studies on the phenomena and causes of warming in the Indian Ocean, most of these efforts have focused on the warming of the Indian Ocean as a whole (Han et al., 2014; Li et al., 2017; Zhang et al., 2018) or on the seasonal scale of sea surface warming affected by El Niño (Xie et al., 2002, 2009; Rao and Behera, 2005; Chowdary et al., 2009; Chen et al., 2019). The longer-term warming trend since the 1950s does not show the strongest warming in this SWIO region. Hence, this paper aims to investigate the processes and formation mechanisms causing sustained decadal warming in the SWIO. Such an investigation may help us better understand the characteristics and mechanisms of the SST changes in the Indian Ocean, broaden our understanding of the internal mechanisms of the climate system, and improve climate prediction skills.

      Figure 1.  Spatial distributions of the linear trend in four SST datasets in the Indian Ocean region during 1996–2016 (shading; K decade−1). (a) HadISST, (b) OISST, (c) Kaplan SST, and (d) ERSST.v5. The robustness of the trend is indicated by a green contour line of “8” for the ratio of the mean value to the standard deviation of 36 trends. The 36 trends are calculated based on the six starting years (1995, 1996, 1997, 1998, 1999, and 2000) and the six ending years (2014, 2015, 2016, 2017, 2018, and 2019). The red contour is the area that passes the 95% confidence level for the warming trend. The SWIO basin is indicated by the blue box (34°–13°S, 54°–92°E).

    2.   Data and methods
    • The National Oceanic and Atmospheric Administration Extended Reconstructed SST version 5 (NOAA ERSST.v5) dataset provides monthly data on SST, with a resolution of 2.0° × 2.0° (Huang et al., 2017). Three ot-her monthly SST datasets are obtained: the Hadley Centre Global Sea Ice and SST (HadISST) with a resolution of 1.0° × 1.0° (Rayner et al., 2003), NOAA Opti-mum Interpolation SST version 2 (NOAA OISST.v2) with a resolution of 1.0° × 1.0° (Reynolds and Smith, 1994), and Kaplan Extended SST version 2 with a resolution of 5.0° × 5.0° (Kaplan et al., 1998). The Global Ocean Data Assimilation System (GODAS) provides the monthly mean reanalysis dataset of sea temperature (40 levels in the upper 4500 m), currents (40 levels in the upper 4500 m), sea surface height (SSH), and wind stress with a resolution of 1.0° × 0.33° (Saha et al., 2006). Another monthly mean sea surface current data set is the Ocean Surface Current Analysis (OSCAR) dataset with a resolution of 1.0° × 1.0° (Johnson et al., 2007). The Japanese Meteorological Agency-Tokyo (JRA) provides monthly surface heat flux data with a resolution of 1.25° × 1.25° (Onogi et al., 2007). The Objectively Analyzed air–sea Fluxes (OAFlux) of the Woods Hole Oceanographic Institution (WHOI) provides monthly surface heat flux data with a resolution of 1.0° × 1.0° (Yu et al., 2008). Unless noted otherwise, all anomalies are defined as the departure from the 1981–2010 climatological values for each month. The SWIO is defined over 34°–13°S, 54°–92°E.

      To better understand the mechanism that causes the change in SSTAs, an upper-layer heat budget analysis method (Su et al., 2010) was used in this study. The heat budget is formulated as:

      $$ \dfrac{\partial T’}{\partial t}=-\left({V}^{{'}}\cdot\triangledown \overline{T}+\overline{V}\cdot\triangledown {T}^{{'}}\right)-({V}^{{'}}\cdot\triangledown {T}^{{'}})+\dfrac{Q{’}_{\rm{net}}}{ {\textit{ρ}}{{c}}_{p} H}+R{,} $$ (1)

      where $ T $ is the upper-layer temperature; $ V=(u,v,w) $ represents the three-dimensional ocean current; $ \triangledown =\left(\dfrac{\partial }{\partial x}, \dfrac{\partial }{\partial y}, \dfrac{\partial }{\partial z}\right) $ represents the three-dimensional gradient operator; $ \left( \right){'} $ represents the anomaly of a variable, which is the difference between the mean values in the last 11 years (2006–2016) and the previous 11 years (1996–2006); $ \overline{\left( \right)} $ represents the climatological annual cycle variables; $ -\left({V}^{{{'}}}\cdot\triangledown \overline{T}+\overline{V}\cdot\triangledown {T}^{{{'}}}\right) $ represents a three-dimensional linearized advection term; $ -({{{V}}}^{{{'}}}\cdot\triangledown {T}^{{{'}}}) $ represents a three-dimensional nonlinear advection term; $ Q{’}_{\rm {net}} $ is the net air–sea heat flux anomaly, including the solar radiation flux, longwave radiation flux, sensible heat flux, and latent heat flux; ρ (assumed constant at 1026 kg m−3) and $ c_{p} $ [assumed constant at 3850 J (kg K)−1] represent the density and heat capacity of seawater, respectively; $ H $ is the depth of the mixed layer (50 m) or of the upper layer (100 m); and $ R $ is the residual.

      The heat flux formula is defined as:

      $$ F=\sum {\textit{ρ}} {c}_{ p}uTS_{ij}{,} $$ (2)

      where i and j are the grid index along each section, $ u $ is the current velocity, $ T $ is the sea temperature, and $ S $ is the area of a single grid in different sections of the SWIO. Then, the heat flux $ F $ of the total area of the boundary cross sections in the six sections of the SWIO is obtained. The mean value of temperature averaged within the SWIO three-dimensional box is first removed from the sea temperature along each section before these heat flux calculations.

      To analyze the causes of the changes in ocean currents, geostrophic currents and Ekman currents are calculated. The formula for geostrophic currents is as below:

      $$ \hspace{-8pt} u_{\rm g}=-\dfrac{\rm g}{f}\dfrac{\partial h}{\partial y},\quad v_{\rm g}=\dfrac{\rm g}{f}\dfrac{\partial h}{\partial x}{.} $$ (3)

      The formula for Ekman currents is as follows:

      $$ {u}_{\rm E}=\dfrac{{\tau }_{y}}{f {\textit{ρ}} d_{\rm E}},\quad {v}_{\rm E}=-\dfrac{{\tau }_{x}}{f {\textit{ρ}} d_{\rm E}}{,} $$ (4)

      where gravitational acceleration g is 9.8 m s−2, the Coriolis parameter $ f=2\varOmega {\rm{sin}}\varphi $ rad s−1 ($ {\rm{\varphi }} $ is latitude), $ h $ is the SSH, $ \tau $ is wind stress, and $ {{d}}_{\rm{E}} $ is the depth of the Ekman layer of the SWIO (50 m).

    3.   Results
    • The four observed SST datasets (HadISST, OISST, Kaplan SST, and ERSST.v5) have shown general warming across the entire Indian Ocean in recent decades, but the most significant warming trend was centered over the SWIO, with a warming hot spot of approximately 0.4 K decade−1 (Fig. 1a). The SWIO warming pattern obtained here is different from the generally accepted SST modes in the Indian Ocean proposed by previous studies, e.g., the Indian Ocean basin mode (IOBM; Nigam and Shen, 1993; Tourre and White, 1995; Chambers et al., 1999), Indian Ocean dipole (IOD; Saji et al., 1999; Webster et al., 1999), subtropical IOD mode (SIOD mode; Behera and Yamagata, 2001), and subtropical IOD-like mode (SIOD-like mode; Cao et al., 2014). The spatial pattern of sustained warming is not sensitive to the choice of the beginning/ending year when calculating the trend. The time series of SST anomalies (SSTAs) in SWIO (Fig. 2) revealed that the four datasets are consistent. The SSTAs from 1996 to 2016, with no strong interannual variability signal, are in a sustained and decadal warming phase, even during the hiatus period. As the SSTA in 1996 is the lowest value of these neighboring years, the year 1996 is chosen as the beginning of the decadal turning point of the SWIO’s warming. Therefore, a period of 21 years from 1996 to 2016 was selected to study the sustained and decadal phases in the SWIO.

      Figure 2.  Time series of annual SSTAs (K) in HadISST (green), OISST (red), Kaplan SST (yellow), and ERSST.v5 (black), averaged over the SWIO basin. A three-point moving average was applied.

    • The warming trend in the SWIO is mainly confined within the upper 100 m, with a mean value of 0.040 K yr−1, and the maximum warming area of the SWIO was found to be approximately 50–70 m. The three-dimensional oceanic advection terms in the SWIO region are calculated by Eq. (1), where the average values are the climatological annual cycle variables, and the anomaly variables are the difference between the mean values in the last 11 years (2006–2016) and the previous 11 years (1996–2006). According to the heat budget analysis of three-dimensional ocean advection terms in the SWIO (Figs. 3, 4), including linear and nonlinear advection terms of ocean currents, the results show that the major terms that affect sea temperature warming in the upper 100 m are $ -\overline{{u}}\dfrac{\partial {{T}}^{{{'}}}}{\partial {x}},-\overline{w}\dfrac{\partial {{T}}^{{{'}}}}{\partial {z}}, -{u}{{'}}\dfrac{\partial \overline{{T}}}{\partial {x}} $, and $ -{v}{{'}}\dfrac{\partial \overline{{T}}}{\partial {y}} $, and those in the upper 50 m are $ -\overline{{u}}\dfrac{\partial {{T}}^{{{'}}}}{\partial {x}}, -{u}{{'}}\dfrac{\partial \overline{{T}}}{\partial {x}} $, and $ -{v}{{'}}\dfrac{\partial \overline{{T}}}{\partial {y}} $, where $ -{u}{{'}}\dfrac{\partial \overline{{T}}}{\partial {x}} $ and $ -{v}{{'}}\dfrac{\partial \overline{{T}}}{\partial {y}} $ are two key active contributions for local warming in the SWIO, indicating that the mechanism of SWIO warming in these two levels is consistent (Figs. 3b, 4b).

      Figure 3.  (a) Diagnostic items for the three-dimensional spatially averaged ocean advection terms for the upper 100 m in the SWIO, where the average values are the climatological annual cycle variables, and the anomaly variables are the difference between the mean values in the last 11 years (2006–2016) and those in the previous 11 years (1996–2006). (b) Diagnostic items in the heat budget Eq. (1) (K yr−1). The vertical current $ w $ is corrected by the OSCAR dataset, and the air–sea heat flux is the sum of the shortwave radiation flux and longwave radiation flux of the JRA dataset as well as the latent heat flux and sensible heat flux of the OAFlux dataset.

      Figure 4.  As in Fig. 3, but for the upper 50 m in the SWIO.

      It should be noted that the selection of the ocean current dataset and heat flux dataset can have a great influence on the heat budget results. The values of the OGDAS current are compared with those of OSCAR, which is an independent frequently used dataset. The zonal and meridional current trends of OSCAR $ (u, v) $ are the same as those of GODAS $ (u, v) $ (figures omitted), indicating that the horizontal currents of OSCAR data are consistent with those of GODAS. Furthermore, the vertical current $ w $ at 15 m was determined by calculating the horizontal currents of OSCAR $ (u, v) $, and the $ w $ trend of OSCAR was found to be 1.8 times smaller than that of GODAS at 15 m, indicating that the $ w $ trend of GODAS is larger. Therefore, the $ w $ trend of GODAS at all depths was revised by multiplying by 0.56. In addition, the shortwave and longwave radiation fluxes of the OAFlux dataset are only available until 2009. Therefore, the heat flux of the sum of the JRA dataset (shortwave and longwave radiation fluxes) and OAFlux dataset (latent heat flux and sensible heat flux) was used to calculate the air–sea heat budget. In this way, the heat budget analysis is almost in balance. The calculated result for the upper 50 m is affected by the depth level, since the negative value of $ -\overline{{w}}\dfrac{\partial {{T}}^{{{'}}}}{\partial {z}} $ in the upper 50 m is due to the maxi-mum warming center being below 50 m and the climatic vertical current being downward ($ \overline{w} $ < 0), which cannot directly heat the upper surface seawater (Fig. 4a).

    • According to the diagnosis results, the major positive terms for the SWIO warming are $ -\overline{{u}}\dfrac{\partial {{T}}^{{{'}}}}{\partial {x}}, -{u}{{'}}\dfrac{\partial \overline{{T}}}{\partial {x}} $, and $ -{v}{{'}}\dfrac{\partial \overline{{T}}}{\partial {y}} $ for 0–50 m and $ -\overline{{u}}\dfrac{\partial {{T}}^{{{'}}}}{\partial {x}},-\overline{{w}}\dfrac{\partial {{T}}^{{{'}}}}{\partial {z}}, -{u}{{'}}\dfrac{\partial \overline{{T}}}{\partial {x}} $, and $ -{v}{{'}}\dfrac{\partial \overline{{T}}}{\partial {y}} $ for 0–100 m. The physical meaning for those terms is explained as follows.

      The distribution of advection term $ -\overline{{u}}\dfrac{\partial {{T}}^{{{'}}}}{\partial {x}} $ at 0–50 m showed that the positive value center of $ -\overline{{u}}\dfrac{\partial {{T}}^{{{'}}}}{\partial {x}} $ is mainly distributed in two regions (Fig. 5a): (1) In the northwestern sector of the SWIO (north of 20°S), where the climatic zonal current flows westward ($ \overline{u} $ < 0), and the anomaly of sea temperature increases with longitude $\Bigg( \dfrac{\partial {{T}}^{{{'}}}}{\partial {x}} > 0 \Bigg) $; therefore, $ -\overline{{u}}\dfrac{\partial {{T}}^{{{'}}}}{\partial {x}} $ > 0 indicates that the heat anomalies are transmitted from the northeast of the SWIO region via the westward current; (2) in the east-central area (20°–30°S) of the SWIO, where the climatic current flows eastward ($ \overline{u} $ > 0), the SWIO center is the most significant area of SST warming, and its SST anomaly decreases with the increasing longitude $\Bigg( \dfrac{\partial {{T}}^{{{'}}}}{\partial {x}} $ < 0; hence, $ -\overline{{u}}\dfrac{\partial {{T}}^{{{'}}}}{\partial {x}} > 0 \Bigg)$, indicating that the heat anomalies are transmitted from the central sector of the SWIO to the east, which leads to warming in the eastern region. The distribution of $ -\overline{{u}}\dfrac{\partial {{T}}^{{{'}}}}{\partial {x}} $ in the upper 100 m is similar to that in the upper 50 m (Fig. 5b).

      Figure 5.  The spatial distributions of sea temperature trend (shading; K decade−1) and climatic current (vector; m s−1) averaged over (a) 0–50 and (b) 0–100 m in the Indian Ocean. The purple dots indicate the region with positive $ -\overline{{u}}\dfrac{\partial {{T}}^{{{'}}}}{\partial {x}} $ (K yr−1). The red contour is the area that passes the 95% confidence level. The SWIO is enclosed by the blue lines over 34°−13°S, 54°−92°E.

      The profile of the sea temperature and current trend (Fig. 6) indicates that the advection term $ -\overline{{w}}\dfrac{\partial {{T}}^{{{'}}}}{\partial {z}} $ is negative from 0–50 m and positive from 50–100 m. Since the maximum warming center is at approximately 60 m, the climatic downward vertical current ($ \overline{w} $ < 0) can carry warmer water below around 60 m, thus heating the seawater at 60–100 m. The negative value of $ -\overline{{w}}\dfrac{\partial {{T}}^{{{'}}}}{\partial {\rm{z}}} $ in the upper 50 m is due to the maximum warming center being below the level of 50 m; thus, it cannot directly heat the upper surface seawater. Due to the warming trend and the climatic downward vertical current ($ \overline{w} $ < 0) in the upper 50 m, there will be a positive contribution to the 60–100-m warming. In addition, the warming of the subsurface layer comes from the upper layer. The warming of 0–100 m will further deepen the accumulation of heat in the subsurface layer below 100 m, but as the depth increases, the warming trend gradually weakens.

      Figure 6.  (a) The sea temperature (T) trend (shading; with intervals of 0.05 K decade−1) during 1996–2016 and climatic zonal (u) and vertical (w) currents (vector; m s−1). All the values are averaged within 34°–13°S. The area between the blue dotted lines indicates the location of the SWIO (54°–92°E). (b) The sea temperature (T) trend (shading; with intervals of 0.05 K decade−1) and climatic meridional (v) and vertical (w) currents (vector; m s−1). All the values are averaged within 54°–92°E. The purple dots indicate the region with positive $ -\overline{{w}}\dfrac{\partial {{T}}^{{{'}}}}{\partial {z}} $. The red contour is the area that passes the 95% confidence level. The area with the blue dashed lines indicates the location of the SWIO (34°–13°S).

      Climatologically, the South Equatorial Current (SEC) is mainly driven by the southeast trade winds (Wyrtki, 1965). Relatively warm waters are transported by the SEC from the equatorial region to the SWIO, and the zonal temperature gradient decreases from west to east in the SWIO region. In the linear trend pattern, the anomalous meridional currents in the SWIO are directed southward (Fig. 7b), reducing the climatic northward transport of cold water from the Southern Ocean into the SWIO region and leads to a net warming effect on the temperature trend in the SWIO (Fig. 7b). The anomalous meridional advection term associated with the mean meridional temperature gradient became positive $\Bigg( -{v}{{'}}\dfrac{\partial \overline{{T}}}{\partial {y}}>0 \Bigg)$, which was a major contributor to the temperature warming in the SWIO. In addition, there is a counterclockwise subtropical gyre (STG) in the SWIO region (Fig. 8b). During 1996–2016, the trend of ocean currents changed, forming a counterclockwise circulation in the western SWIO, and the STG strength was enhanced in the western subtropical Indian Ocean. For the linear trend pattern, there were anomalous eastward currents near the western boundary of the SWIO (Fig. 8a), and the average zonal current anomaly along the western boundary was positive (approximately 4 × 10−4 m s−1), which indicated that the transport of the eastward current was stronger than that of the westward current. These anomalous currents transported the climatological warm water from the west into the SWIO region, warming the upper-layer temperature in this region (Fig. 8a). Hence, the anomalous zonal advection term associated with the climatological zonal temperature gradient became positive $\Bigg( -{u}{{'}}\dfrac{\partial \overline{{T}}}{\partial {x}}>0 \Bigg)$. A similar case also existed at the southern boundary of the SWIO. Climatologically, colder water is located in the Southern Ocean, and this colder water can be transported northward by the climatological ocean current (Fig. 8b). In summary, the major contributors to the upper 50/100-m ocean warming in the SWIO were the two anomalous positive heat advection terms: meridional heat transport at the southern boundary $\Bigg( -{v}{{'}}\dfrac{\partial \overline{{T}}}{\partial {y}}>0 \Bigg)$ and zonal heat transport at the western boundary $ \Bigg( -{u}{{'}}\dfrac{\partial \overline{{T}}}{\partial {x}}>0 \Bigg)$.

      Figure 7.  (a) The sea temperature trend (shading; with intervals of 0.05 K decade−1), ocean zonal and vertical current trends (vector; m s−1 decade−1) during 1996–2016, and climatic sea temperature (contour; with intervals of 2 K). All the values are averaged within 34°–13°S. The area between the blue dotted lines indicates the location of the SWIO (54°–92°E). (b) The sea temperature trend (shading; with intervals of 0.05 K decade−1), ocean meridional and vertical current trends (vector; m s−1 decade−1) during 1996–2016, and climatic sea temperature (contour; with intervals of 2 K). All the values are averaged within 54°–92°E. The red contour is the area that passes the 95% confidence level, and only current vectors significant at the 95% confidence level are plotted in black; the rest are gray vectors. The area between the blue dashed lines indicates the location of the SWIO (34°–13°S).

      Figure 8.  (a) The spatial distribution of climatic sea temperature (shading; K) and current trend (vector; m s−1 decade−1) averaged over 0–100 m in the Indian Ocean. Only current vectors significant at the 95% confidence level are plotted in black, and the rest are gray vectors. (b) The spatial distribution of climatic sea temperature (shading; K) and climatic current (vector; m s−1) averaged over 0–100 m in the Indian Ocean. The SWIO is enclosed by the blue lines over 34°–13°S, 54°–92°E.

      For each section along the four vertical sections, sea surface section, and bottom section at 100 m for the SWIO region, the time-averaged heat flux is calculated by Eq. (2) to quantify the heat budgets. The results indicate that the SWIO heat flux in the climatic state (Fig. 9a) is mainly balanced by warmer water from the equator at the northern boundary and the loss of heat at all other boundaries. For the anomalous heat flux terms (Fig. 9b), a strong eastward current anomaly at the western boundary can enhance the positive heat transfer of warmer water from the west throughout the entire SWIO. The positive anomalous heat flux along the southern boundary also contributes to a net heat gain for the SWIO by reducing the input of cold water from the Southern Ocean into the SWIO. The heat flux values at each boundary of 0–50 m (Fig. 10) show that the magnitude distribution is similar to that of 0–100 m.

      Figure 9.  (a) The climatological heat flux (PW; 1 PW = 1015 W) along all the four vertical sections, sea surface section, and bottom section at 100 m for the SWIO during 1996–2016. (b) As in (a), but for the anomalous heat flux (PW). The red and blue arrows indicate that the SWIO gains and loses heat, respectively.

      Figure 10.  As in Fig. 9, but for at 50 m for the SWIO during 1996–2016.

      The trend distribution of the four heat fluxes [net shortwave radiation flux of the JRA dataset, net longwave radiation flux of the JRA dataset, latent heat flux (the mean value of the JRA and OAFlux datasets), and sensible heat flux (the mean value of the JRA and OAFlux datasets)] in the SWIO (Fig. 11) revealed that the trend of the shortwave radiation flux is negative, which indicates that the ocean loses heat. However, the net longwave radiation, latent heat flux, and sensible heat flux showed a positive trend, indicating that the ocean absorbs heat. It is calculated that the net surface radiation heat flux would cause a warming of 0.0071 K yr−1 in the SWIO (Fig. 3b).

      Figure 11.  The spatial patterns of the trend in sea surface heat flux (W m−2 decade−1; downward positive) in the Indian Ocean region: (a) net shortwave radiation flux of the JRA dataset, (b) net longwave radiation flux of the JRA dataset, (c) net latent heat flux based on the mean value of the JRA and OAFlux datasets, and (d) net sensible heat flux based on the mean value of the JRA and OAFlux datasets. The red contour is the area that passes the 95% confidence level. The red (blue) shading indicates that there is heat absorption (loss) in the ocean, and the blue lines indicate the SWIO (34°–13°S, 54°–92°E).

    • The reasons for the ocean current changes can be theoretically determined by calculating the geostrophic current and Ekman current. At the western boundary of the SWIO, the spatial distribution of ocean currents can be well explained by the distribution of the geostrophic current (Fig. 12a). The SSH trend in the western region of the SWIO (20°–26°S, 55°–76°E) is positive. Around the center of the positive SSH trend, there is an anomalous westward geostrophic flow on the north side and an anomalous eastward geostrophic current on the south side, forming an anticyclonic (Southern Hemisphere) flow field in this area. As the eastward geostrophic current can transport warmer waters from the west into the SWIO, more net heat is retained in the SWIO, leading to an enhanced SSH pattern there. This is a positive feedback process of self-sustainment and self-reinforcement. The southeast trade winds prevail in the southern Indian Ocean throughout the whole year. During 1996–2016, the trade winds weakened in the subtropical southern Indian Ocean, and the trend of wind stress showed a clockwise circulation in the SWIO (Fig. 12b). The surface ocean current changes at the southern boundary can be attributed to the surface Ekman current induced by atmospheric circulation variabilities, which formed an anticyclonic pattern in the southeastern section of the SWIO. Such anomalous wind patterns can lead to southward Ekman current anomalies. This anomalous southward transport of the Ekman current at the southern boundary of the SWIO reduced the transport of cold water from the Southern Ocean into the SWIO. Below the surface, a positive feedback can also be seen near the southeastern SWIO, with a warming center and geostrophic currents around it (Fig. 13). This self-sustainment pattern favored southward transport near the southern SWIO boundary, which can reduce the climatic northward transport of cold water from the Southern Ocean and warm the SWIO region.

      Figure 12.  The spatial distributions of (a) the SSH trend (shading; m decade−1) and sea surface geostrophic current trend (vector; m s−1 decade−1) as well as (b) the wind stress trend (blue vector; N m−2 decade−1) and Ekman current trend (black vector; m s−1 decade−1) in the Ekman layer (set as 50 m over the Indian Ocean region during 1996–2016. In (b), the light blue arrow is the regression of the wind stress distribution. The red contour is the area that passes the 95% confidence level. The current vectors significant at the 95% confidence level are plotted in black, and the rest are gray vectors. Only wind stress vectors significant at the 95% confidence level are plotted in blue. The SWIO region is indicated by the blue lines (34°–13°S, 54°–92°E). The SSH data are obtained from GODAS.

      The processes of the two boundaries formed a self-sustaining positive feedback mechanism that occurred on each level above at least 120 m (Fig. 13) and favored the maintenance of a sustained warming trend in the SWIO region. The distribution of sea temperature at each depth layer (figures omitted) shows that apparent warming signals existed from the surface to a depth of 120 m, with a maximum value concentrated at 50–70 m. At each level, the most significant areas of warming were located on the western and southern boundaries of the SWIO. In addition, around each warming center, there were geostrophic-like patterns of ocean currents, indicating positive feedback between warming and geostrophic currents. As the depth increases, the warming phenomenon in the SWIO area gradually disappears below 120 m, and the trend of current circulation also gradually weakens (figure omitted).

      Figure 13.  The spatial distribution of the sea temperature trend (shading; K decade−1) and current trend (vector; m s−1 decade−1) at 120 m in the Indian Ocean. The red contour is the area that passes the 95% confidence level, and only current vectors significant at the 95% confidence level are plotted in black; the rest are gray vectors. The SWIO is enclosed by the blue lines (34°–13°S, 54°–92°E).

      The trend of SST, as well as the 0–100-m temperature, over the SWIO is almost constant during 1996–2006. There are obvious interannual changes in wind field changes, indicating that the surface dynamical forcing is not stable, nor is it the key reason for the stable warming of the 100-m sea temperature. The interannual changes in ocean currents are not large, and the ocean current terms mainly show interdecadal trends, indicating that the heat transported by ocean currents should be the key reason for the stable warming phenomenon. In particular, the meridional/zonal ocean currents near the southern/western boundary of the upper 100 m are stable, which is the fundamental reason for maintaining the sustained warming of the SWIO (Fig. 14).

      Figure 14.  (a) Time series of the sea temperature anomaly (K yr−1) in the GODAS dataset, averaged over the upper 100 m in the SWIO basin. (b) Time series of the sea surface zonal wind stress (N m−2) in the GODAS dataset, averaged over the region of the southern boundary of the SWIO (34°–28°S, 70°–92°E). (c) As in (b), but for the meridional ocean current (m s−1) in the upper 100 m. A three-point moving average has been applied.

    4.   Summary and discussion
    • Since the mid-1990s, the SSTs in the SWIO have experienced sustained decadal warming for more than two decades, even during the hiatus period, during which a slowdown in the global warming speed occurred. The linear advection of heat by ocean surface currents, including anomalous eastward heat transport at the western boundary and anomalous northward heat transport at the southern boundary, has made a notable contribution to warming in this region. Corresponding to the local warming in the SWIO, a self-maintaining anticyclonic geostrophic current pattern was formed in the western part of the SWIO region, which could further enhance the eastward heat transport at the western boundary of the SWIO. However, at the southern boundary of the SWIO, the anomalous wind stress drives a southward Ekman flow, which reduces the climatic input of cold water from the Southern Ocean into the SWIO. Below the surface, a self-maintaining anticyclonic geostrophic current pattern also favored local warming near the southeastern SWIO by reducing cold water transport from the Southern Ocean. These processes near the two boundaries formed a self-sustaining positive feedback mechanism and led to a sustained warming trend in the SWIO for more than two decades.

      Although there have been some studies on the mechanism and effects of SST warming in the Indian Ocean, most of these efforts have focused on the overall warming of the Indian Ocean (Han et al., 2014; Li et al., 2017; Zhang et al., 2018) or seasonally scaled sea surface warming affected by El Niño (Xie et al., 2002, 2009; Rao and Behera, 2005; Chowdary et al., 2009; Chen et al., 2019). This study reveals that the sustained decadal warming in the SWIO is mainly affected by heat advection from ocean currents. With significant increases in greenhouse gas emissions, the rate of global warming will continue to increase after a brief slowdown (Su et al., 2017). Over the past century, the warming trend in the Indian Ocean has become the most prominent among the world’s oceans, and the SWIO’s warming trend over the past century is also significantly positive. However, this paper highlights that the SWIO has maintained sustained and steady warming over the two decades since the mid-1990s, which is a unique phenomenon of global warming. It should be said that the cause of the SWIO’s sustained and steady warming is a special aspect of glo-bal warming. In other words, the warming of the SWIO was bound to occur sooner or later. The fundamental reason lies in the driving force of the global warming background, which is irreversible in the near future. Even if the warming is temporarily slowed on a decadal scale, the temperature will rise definitely again later. On the other hand, this unique phenomenon of the sustained and decadal warming in the SWIO should be subject to local oceanic/atmospheric dynamics. In this paper, the dynamic effect of the atmosphere on the ocean is emphasized unilaterally. The unique phenomenon should be analyzed from an integrated perspective of ocean–atmospheric coupling in the future to obtain a more comprehensive understanding.

      Acknowledgments. The authors wish to thank two anonymous reviewers for their constructive and helpful comments.

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