Boreal winter Arctic Oscillation as an indicator of summer SST anomalies over the western tropical Indian Ocean
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The inter-annual relationship between the boreal winter Arctic Oscillation (AO) and summer sea surface temperature (SST) over the western tropical Indian Ocean (TIO) for the period from 1979 to 2015 is investigated. The results show that the January–February–March AO is significantly correlated with the June–July–August SST and SST tendency. When both El Niño/Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) variance are excluded, the winter AO is significantly correlated with the regional mean SST of the western TIO (40\(^\circ\)–\(60^\circ\)E and \(10^\circ\)S–\(10^\circ\)N), \(r=0.71\). The multi-month SST tendency, i.e., the SST difference of June–July–August minus April–May, is correlated with the winter AO at \(r=0.75\). Composite analysis indicates similar warming over the western TIO. Two statistical models are established to predict the subsequent summer’s SST and SST tendency. The models use the winter AO, the winter ENSO and the autumn-winter IOD indexes as predictors and explain 65 and 62 % of the variance of the subsequent summer’s SST and SST tendency, respectively. Investigation of the regional air–sea fluxes and oceanic dynamics reveals that the net surface heat flux cannot account for the warming, whereas the oceanic Rossby wave plays a predominant role. During positive AO winters, the enhanced Arabian High causes stronger northern winds in the northern Indian Ocean and leads to anomalous cross-equatorial air-flow. The Ekman pumping in association with the anomalous wind stress curl in the central TIO generates a significantly deeper thermocline and above-normal sea surface height at 60\(^\circ\)–75\(^\circ\)E and 5\(^\circ\)–\(10^\circ\)S. The winter AO-forced Rossby wave propagates westward and arrives at the western coast in summer, resulting in the significant SST increase. Forced by the observed winter AO-related wind stress anomalies over the Indian Ocean, the ocean model reasonably reproduces the Rossby wave as well as the resulting surface ocean warming over the western TIO in the subsequent summer. Observational analysis and numerical experiments suggest the importance of the oceanic dynamics in connecting the winter AO and summer SST anomalies.
KeywordsWinter Arctic Oscillation Summer SST Tropical Indian Ocean Prediction model
This study was supported by projects of NSFC-41375071, NSFC-41321001, and 2012GB955401. SJ Kim was supported by project PE16010 of the Korea Polar Research Institute. The SODA datasets were obtained from http://soda.tamu.edu. The air–sea flux data were provided by the WHOI OAFlux project (http://oaflux.whoi.edu) which was funded by the NOAA Climate Observations and Monitoring program. The ERSST and OISST data were provided by the NOAA/OAR/ESRL PSD from their Web site at http://www.esrl.noaa.gov/psd/. The comments and suggestions from two anonymous reviewers were helpful in improving the manuscript.
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