Decadal Shift in the Relationship between Winter Arctic Oscillation and Central Indian Ocean Precipitation during the Early 2000s

The present study investigated the long-term change in the interannual relationship between the boreal winter Arctic Oscillation (AO) and tropical Indian Ocean (TIO) climate during 1979-2019 and found that their linkage experienced a decadal change in 2001/2002. The 19-yr sliding correlation coefficient between the January-February-March (JFM) AO index and central TIO (0-10 degrees S, 65 degrees-80 degrees E) precipitation was significant, with values of approximately 0.50-0.75 during 1979-2001, but abruptly decreased to 0.35 in 2002 and 0.10 in 2010. Meanwhile, the spatial patterns of the AO-related atmospheric circulation anomalies also displayed different features before and after 2001. During 2002-2019, the anomalous anticyclone in the middle troposphere over the Arabian Sea moved northwestward and strengthened, and the JFM AO was more closely correlated to the anticyclone, with correlation coefficient changed from -0.38 before 2001 to -0.63 after 2001; correspondingly, strong cross-equator air flows were observed over the western TIO (40 degrees-50 degrees E), but no significant anomalies of precipitation in the central TIO were observed. During 1979-2001, however, significant southward cross-equator air flows appeared over the central TIO (65 degrees-75 degrees E), which enhanced the intertropical convergence zone and upward air motions, leading to more precipitation in central TIO. An analysis shows that the AO may modulate the Arabian anticyclone through two Rossby wave paths in the upper troposphere: a midlatitude (50 degrees-60 degrees N) path during 1979-2001 from North Atlantic southeastward to the Middle East and the neighboring Arabian Sea; and a subtropical (20 degrees-30 degrees N) path during 2002-2019 from North Atlantic eastward to the Middle East and Arabian Sea. Large wave activity fluxes induced by AO were concentrated along the two paths before and after 2001, and the location of the cross-equator flows depends on the location of the anticyclone. Causes of the decadal changes in the AO-associated wave trains need further investigation.