Abstract
The tropical Indian Ocean (TIO) basin-wide warming occurred in 2020, following an extreme positive Indian Ocean Dipole (IOD) event instead of an El Niño event, which is the first record since the 1960s. The extreme 2019 IOD induced the oceanic downwelling Rossby waves and thermocline warming in the southwest TIO, leading to sea surface warming via thermocline-SST feedback during late 2019 to early 2020. The southwest TIO warming triggered equatorially antisymmetric SST, precipitation, and surface wind patterns from spring to early summer. Subsequently, the cross-equatorial “C-shaped” wind anomaly, with northeasterly–northwesterly wind anomaly north–south of the equator, led to basin-wide warming through wind-evaporation-SST feedback in summer. This study reveals the important role of air–sea coupling processes associated with the independent and extreme IOD in the TIO basin-warming mode, which allows us to rethink the dynamic connections between the Indo-Pacific climate modes.
Introduction
The Indian Ocean Basin (IOB) mode is the first mode of the interannual variability of sea surface temperature in the tropical Indian Ocean (TIO), which is characterized by basin-warming/cooling (Klein et al. 1999; Yang et al. 3b). The downwelling Rossby waves deepened the thermocline, leading to thermocline warming in the south TIO. Meanwhile, the mixed layer is deepened by the Ekman pum**. During the mature phase (SON) of IOD, the positive wind stress curl anomaly became stronger and covered a larger area over the south TIO, forcing and strengthening the westward-propagating downwelling Rossby waves (Fig. 3b). Thus, the thermocline became deeper, and the resultant warming of the thermocline became more significant. Subsequently, the downwelling Rossby wave propagated westward to the thermocline dome in the SWTIO, which deepened the thermocline and caused the SST warming in the SWTIO (Fig. 3c; Chowdary et al. 2009). In DJF, the downwelling Rossby waves and associated thermocline fluctuation reached their peak (Fig. 3d). The SST warming in the SWTIO was maintained by the slow-propagating downwelling Rossby waves and local wind forcing until the 2020 summer (Fig. 3e). This breaks through our previous understanding that SWTIO SST warming cannot persist until the following summer during the pure IOD events.
The SST warming in the SWTIO-induced heavy precipitation by strengthening local deep convection during spring to summer (Fig. 4). Therefore, equatorially antisymmetric SST and convection patterns appeared in the TIO (Fig. 4). Consistently, the surface wind anomalies feature an antisymmetric circulation structure over the TIO (Fig. 4a; Kawamura et al. 2001; Wu et al. 2008; Wu and Yeh 2010). A cross-equatorial “C-shaped” wind anomaly over the TIO, with northeasterlies north and northwesterlies south of the equator, was forced by the north–south temperature gradient and Coriolis effect. In turn, the cross-equatorial “C-shaped” wind anomaly also favored the north–south SST gradient, because surface wind caused less latent heat flux loss over the southern TIO. Thus, in spring, positive WES feedback works in the southern TIO, supporting the maintenance of the antisymmetric mode (Wu et al. 2008; Wu and Yeh 2010). In summer, the anomalous northeasterlies weakened the climatological monsoon southwesterlies over the northern TIO, leading to an increase in SST due to the positive WES feedback mechanism (Fig. 4; **e and Philander 1994; Du et al. 2009; Chen et al. 2019). Meanwhile, the enhanced precipitation extended to the northern TIO, with the development of SST warming and summer monsoon in the TIO (Fig. 4; Annamalai et al. 2005; Izumo et al. 2008).
In addition, the westward-propagating downwelling Rossby waves in the south TIO transformed into the equatorward-propagating coastal-trapped waves after reaching the western boundary and then reflected as the equatorial Kelvin waves that propagate eastward along the equator (Le Blanc and Boulanger 2001; McPhaden and Nagura 2014; Wang et al. 2016; Chen et al. 2019). Thus, the downwelling Rossby waves and reflected-equatorial Kelvin waves sustained the warming of the western TIO and west–east temperature gradient from 2019 winter to the following spring (Fig. 5a, b; Jury and Huang 2004). In early spring, the west–east temperature gradient forced the equatorial easterly wind anomaly (Figs. 4a, 5b; Du et al. 2020), which favored the generation of downwelling Rossby waves and westward heat advection, further maintaining the warming in the western TIO.
In the eastern TIO, the SST warming was affected by the air–sea interface exchanges associated with cloud-radiation-SST feedback and WES feedback, resulting from the weakening of anomalous subsidence of Walker Circulation over the east TIO. During the boreal winter of 2019, a reduction of cloud cover, induced by the cooling pole of IOD off the Sumatran southwest, favored an increase in the shortwave radiation (Figure not shown; Cai and Qiu 2013; Liu et al. 2014). In addition, the strong southeasterly wind anomaly weakened during the decay phase of IOD, resulting in the SST warming due to a decrease in the heat latent flux loss of the ocean (Figs. 4b, 5c; Tokinaga and Tanimoto 2004). In later spring, the reflected-equatorial Kelvin reached the Sumatra-Java coasts, sustaining SST warming in the eastern TIO (Fig. 5a, b). An increase in precipitation followed the SST warming in the eastern TIO (Figs. 2c, 5a).
Summary and discussion
The TIO experienced a basin-wide warming in 2020, following an extreme and prolonged positive IOD event instead of an El Niño event. This is the first record since the 1960s. Persistent warming occurred in the SWTIO from late 2019 to early 2020, sustained by oceanic downwelling Rossby waves associated with the extreme 2019 IOD via thermocline-SST feedback. During 2020 spring to early summer, the SWTIO warming triggered the equatorially antisymmetric SST, precipitation and surface wind patterns over the TIO. The cross-equatorial “C-shaped” wind anomaly, with northeasterly–northwesterly wind anomaly north–south of the equator, weakened the climatological surface winds then led to basin-wide warming via reducing surface evaporation. Moreover, the westward-propagating Rossby waves reflected as the eastward-propagating equatorial Kelvin waves, which favored the persistence of warming in the western TIO in 2020 spring. In the eastern TIO, air–sea interface exchanges play an important role in SST warming. The importance of air–sea coupling processes associated with the independent and extreme IOD in the TIO basin-warming mode has been clarified in this study, which allows us to rethink the relationship between the Indo-Pacific climate modes.
In addition to the local forcing of the TIO, the remote forcing from the Pacific might also impact the development of IOB warming in 2020. A weak El Niño Modoki developed in the tropical Pacific during 2019. Zhou et al. (2021) suggested that the warming in the central-western tropical Pacific during the weak El Niño Modoki contributed to the sustained anticyclone wind curls over the south TIO in early 2020 according to the results of the atmospheric model experiments. Zhang L et al. (2021) further indicated that the warming of the tropical Pacific contributes to the tropical Indian Ocean wind anomalies, regardless of whether the warming is in the western or eastern tropical Pacific. The tropical Pacific warming causes the Pacific convection center to shift eastward, and then triggers atmospheric Rossby waves over the Indian Ocean, resulting in a pair of low-level anomalous anticyclones occurring on both sides of the equator. Nevertheless, our results in this study challenge the perception that only ENSO triggers the IOB. The relationship between ENSO and IOB would change with ocean mean state and ENSO activity on long timescales (**e et al. 2010; Zheng et al. 2011, 2016; Hu et al. 2013; Tao et al. 2015; Liu et al. 2021). In a warming climate, both extreme positive IOD events and El Niño events are projected to become more frequent (Cai et al. 2014; Freund et al. 2020). Thus, it is worth paying attention to changes in the relationship between the Indo-Pacific climate modes, and their relative contributions to climate variability in the Indo-Pacific.
Availability of data and materials
The OISST monthly fields are provided by https://psl.noaa.gov/data/gridded/data.noaa.oisst.v2.html. Argo data were collected and made freely available by the International Argo Program and the national programs that contribute to it (http://www.argo.ucsd.edu, http://argo.jcommops.org). The SSH data set is available at https://resources.marine.copernicus.eu/?option=com_csw&view=details&product_id=SEALEVEL_GLO_PHY_L4_REP_OBSERVATIONS_008_047 and https://resources.marine.copernicus.eu/?option=com_csw&view=details&product_id=SEALEVEL_GLO_PHY_L4_NRT_OBSERVATIONS_008_046. The ERA5 data set is provided by CMEMS at https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-single-levels-monthly-means?tab=form. The GPCP precipitation is obtained from NASA/GSFC (http://precip.gsfc.nasa.gov). The CCMP surface wind data set is available at https://data.remss.com/ccmp/.
Abbreviations
- IOB:
-
Indian Ocean Basin
- IOD:
-
Indian Ocean Dipole
- ENSO:
-
El Niño-Southern Oscillation
- SST:
-
Sea surface temperature
- SSH:
-
Sea surface height
- SSTa:
-
Sea surface temperature anomaly
- SSHa:
-
Sea surface height anomaly
- TIO:
-
Tropical Indian Ocean
- SWTIO:
-
Southwest tropical Indian Ocean
- MAM:
-
March, April and May
- SON:
-
September, October and November
- DJF:
-
December, January and February
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Acknowledgements
We thank to two anonymous reviewers for their constructive comments and suggestions. This study is supported by the National Natural Science Foundation of China (41830538, 42090042, 42006026), the Natural Science Foundation of Guangdong Province, China (2020A1515010361), the Chinese Academy of Sciences (XDA15020901, 133244KYSB20190031, ZDRW-XH-2019-2, XDB42010304, LTOZZ2005, LTOZZ2012), and the Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) (GML2019ZD0303, 2019BT02H594).
Funding
National Natural Science Foundation of China (41830538, 42090042, 42006026); Natural Science Foundation of Guangdong Province, China (2020A1515010361); Chinese Academy of Sciences (XDA15020901, 133244KYSB20190031, ZDRW-XH-2019-2, XDB42010304, LTOZZ2005, LTOZZ2012); Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) (GML2019ZD0303, 2019BT02H594).
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YD conceived the study. YZ performed the analysis, generated the figures, and wrote the manuscript. Both authors read and approved the final manuscript.
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Zhang, Y., Du, Y. Extreme IOD induced tropical Indian Ocean warming in 2020. Geosci. Lett. 8, 37 (2021). https://doi.org/10.1186/s40562-021-00207-6
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DOI: https://doi.org/10.1186/s40562-021-00207-6