Intra-decadal variability of the Indian Ocean shallow meridional overturning circulation during boreal winter

• Published in: Climate dynamics Vol. 60; no. 9-10; pp. 2803 - 2818
• Main Authors: Pai, Rahul U., Parekh, Anant, Chowdary, Jasti S., Gnanaseelan, C.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.05.2023; Springer; Springer Nature B.V
• Subjects: Analysis; Anomalies; Antarctic Oscillation; Climatology; Confidence intervals; Correlation analysis; Correlation coefficient; Correlation coefficients; Data assimilation; Data collection; Earth and Environmental Science; Earth Sciences; Enthalpy; Environmental aspects; Geophysics/Geodesy; Heat content; Mean sea level; Meridional overturning circulation; Mixed layer depth; Ocean temperature; Oceanic analysis; Oceanography; Oceans; Pressure; Sea level; Sea level pressure; Sea surface; Sea surface temperature; Sea surface temperature anomalies; Spectrum analysis; Surface temperature; Temperature anomalies; Thermal properties; Thermocline; Thermocline depth; Thermodynamic properties; Thermohaline circulation; Tropical environments; Upper ocean; Variability; Wind variations; Winds; Winter; Winter circulation; Zonal winds; Indian Ocean; Madagascar; Intra-decadal variability; Meridional circulation; Southern Indian Ocean; Southern annular mode
• ISSN: 0930-7575; 1432-0894
• DOI: 10.1007/s00382-022-06475-y

The variability of Indian Ocean shallow meridional overturning circulation (SMOC) is studied using the century long ocean reanalysis simple ocean data assimilation (SODA) data. Though SMOC exhibits stronger southward transport during boreal summer, it displays stronger variability during boreal winter. The spectrum analysis of winter SMOC index reveals presence of highest amplitude between 5 to 7 years at 95% confidence level, suggesting the dominance of intra-decadal SMOC variability. The robustness of intra-decadal SMOC variability is also confirmed in different ocean reanalysis data sets. Composite analysis of filtered upper Ocean Heat Content, sea level, thermocline depth and Sea Surface Temperature anomalies for strong (weak) SMOC years show negative (positive) anomaly over north and East of Madagascar. Correlation analysis, of filtered SMOC index and sea level pressure (zonal winds) over the India Ocean, found significant negative (positive) correlation coefficient north of 40 °S (around 10 °S) and significant positive (negative) correlation coefficient over the 45 °S to 70 °S (20 °S to 50 °S and north of 5 °S). This meridional pattern of correlation coefficient for sea level pressure, manifesting the out of phase relationship between sub-tropics and high latitude mean sea level pressure, resembles with Southern Annular Mode (SAM). We conclude that the intra-decadal variability of mean sea level pressure leads to zonal wind variation around 10 °S modulating SMOC, which in turn affects the upper ocean thermal properties in the east and north of Madagascar. This study for the first time brought out coherent intra-decadal evolution of SAM and SMOC during boreal winter.