Variability of the Indian Ocean Shallow Meridional Overturning Circulation during Boreal Winter and its Impact on Ocean Heat Content
Rahul U Pai*, Anant Parekh, Jasti S Chowdary, C Gnanaseelan
Indian Institute of Tropical Meteorology, Pune
Rahul U Pai, Senior Research Fellow, TS Division, IITM, Pune-411008 Pune, Maharashtra, India Pincode: 411008 Mobile: 7887301890 E-mail: firstname.lastname@example.org
The understanding of the Indian Ocean (IO) Shallow Meridional Overturning Circulation (SMOC) variability and its impact on the IO Ocean Heat content (OHC) remains unexplored. The Century-long ocean reanalysis data provides an unprecedented opportunity to study the variability of SMOC during the last century. The present study makes use of the 20th century ocean reanalysis dataset Simple Ocean Data Assimilation version 2.2.4, available for the period 1871-2010, to study the variability of SMOC. To confirm the robustness in the results the study also uses a set of reanalysis datasets from different agencies, available for different periods. These include Ocean Reanalysis System 4 (1958-2009), Climate Forecast System version 2 Reanalysis (1979-present), German contribution of the Estimating the Circulation and Climate of the Ocean project version2 (1948-2016). The quantification of SMOC strength is estimated using the meridional overturning stream function anomaly integrated from surface to 60m. The standard deviation zonally averaged stream function across the IO basin shows highest variability between the latitudes 8oS and 15oS. Therefore, the meridional overturning stream function anomaly spanning 50°E to 110°E and 8.5°S to 15°S is considered to represent variability in strength of SMOC. The SMOC comprises the transport associated with the shallow Cross Equatorial Cell (CEC) and SubTropical Cell (STC). To quantify the CEC strength, the upper 60m meridional transport between the latitudes 5oN to the equator is considered. The difference between the strength of SMOC to that of CEC is considered the strength of STC. In order to remove the trend, detrending is applied to time series data. The seasonal mean of SMOC stream function indicates that the transport is strongest during the boreal summer (JJA)(~24Sv) and weaker during winter (DJF)(~0.5Sv). However, the season-wise standard deviation of the SMOC stream function shows that the variability is largest during DJF (~3.6 Sv) compared to JJA (~2.8 Sv). The power spectrum and wavelet analysis of the SMOC stream function for annual, summer (June to September) and winter (December to February) is carried out. It is observed that the summer and winter SMOC display variability with periodicity less than 7 years at 95% confidence level, with the peak between 5 to 7 year. The strength of the peak signal during winter is higher than that for the summer. The robustness in the signal from SODA is confirmed in other ocean reanalysis datasets also. This motivated us to study winter SMOC variability in detail. The variability in strength of CEC and STC during DJF is also studied. Apart from summer and winter, the CEC and STC also display similar peaks in spectrum. The strong and weak years are identified from the 5 to 7year bandpass filtered signal of the SMOC. During strong (weak) SMOC years, the composite displayed more (less) southward (northward) transport (~1.2 Sv) and showed excess (less) subduction over the South Indian Ocean. The spectrum analysis of upper 200m Ocean Heat Content (OHC200) over SWIO also displays significant peak with 5 to 7 year periodicity, similar to SMOC during DJF. The analysis reveals that the strong (weak) SMOC (less) leads to reduction (increase) in the OHC200 over the Southwest Indian Ocean (SWIO). Thus, the study concludes that the SMOC displays strong variability with 5 to 7 year periodicity during boreal winter. These variations in transport associated with SMOC are seen to impact OHC200 over SWIO. The mechanisms responsible for this is also explored.