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

• 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/s13351-020-0112-9
• Note: This paper has been peer-reviewed and is just accepted by J. Meteor. Res. Professional editing and proof reading are underway. Please use with caution.
• 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 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 sustained long-lasting warming in the SWIO, as it is a unique phenomenon occurring under the background of ongoing global warming.
• Fig. 1.  Spatial distribution 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. (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% significance test for the warming trend. The SWIO basin is indicated by the blue box (54°-92°E, 34°-13°S).

Fig. 2.  Time series of annual SST anomalies (K) in HadISST (blue), 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 year−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 and the latent heat flux and sensible heat flux of the OAFlux dataset.

Fig. 4.  (a) Diagnostic items for the three-dimensional spatially averaged ocean advection terms for the upper 50 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 year−1). The vertical current $w$ is corrected by the OSCAR dataset, and the air-sea heat flux is the sum of the shortwave and longwave radiation flux of the JRA dataset and the latent heat flux and sensible heat flux of the OAFlux dataset.

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

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

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

Fig. 8.  (a) Spatial distribution of climatic sea temperature (shading; K) and current trend (vectors; 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) Spatial distribution of climatic sea temperature (shading; K) and climatic current (vectors; m s−1) averaged over 0-100 m in the Indian Ocean. The SWIO is enclosed by the blue lines between 54° and 92°E and between 34° and 13°S.

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

Fig. 10.  (a) The climatological heat flux (PW; 1 PW = 1015 W) along all four vertical sections, the sea surface section, and the bottom section at 50 m for the SWIO during 1996-2016. (b) Same as (a) but for the anomalous heat flux (PW). The red arrow indicates that the SWIO gains heat, and the blue arrow indicates that the SWIO loses heat.

Fig. 11.  Spatial pattern 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. (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% significance test. The red (blue) shading indicates that there is heat absorption (loss) in the ocean, and the blue lines indicate the SWIO: 54°-92°E, 34°-13°S.

Fig. 12.  (a) Spatial distribution of the SSH trend (shading; m decade−1) and the sea surface geostrophic current trend (vectors; m s−1 decade−1) in the Indian Ocean region during 1996-2016. (b) Spatial distribution of the wind stress trend (blue vectors; N m−2 decade−1) and Ekman current trend (black vectors; m s−1 decade−1) in the Ekman layer (set as 50 m) over the Indian Ocean region during 1996-2016, where the light blue arrow is the regression of the wind stress distribution. The red contour is the area that passes the 95% significance test. 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 (54°-92°E, 34°-13°S). The SSH data are obtained from GODAS.

Fig. 13.  Spatial distribution of sea temperature trends (shading; K decade−1) and current trends (vectors; m s−1 decade−1) at 120 m in the Indian Ocean. The red contour is the area that passes the 95% significance test, 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 (54°-92°E, 34°-13°S).

Fig. 14.  (a) Time series of sea temperature anomalies (K year−1) in the GODAS dataset, averaged over the upper 100 m in the SWIO basin. (b) Time series of sea surface zonal wind stress (N m−2) in the GODAS dataset, averaged over the region of the southern boundary of the SWIO (70°-92°E, 34°-28°S). (c) is the same as (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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• 1.

沈阳化工大学材料科学与工程学院 沈阳 110142

## 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, Beijing 100081, China
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 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 sustained long-lasting warming in the SWIO, as it is a unique phenomenon occurring under the background of ongoing global warming.

Reference (36)

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