Characterizing the regional XCO variability and its association with ENSO over India inferred from GOSAT and OCO-2 satellite observations.

India is primarily concerned with comprehending regional carbon source-sink response in the context of changes in atmospheric CO concentrations or anthropogenic emissions. Recent advancements in high-resolution satellite's fine-scale XCO measurements provide an opportunity to understand unprecedented details of source-sink activity on a regional scale. In this study, we investigated the long-term variations of XCO concentration and growth rates as well as its covarying relationship with ENSO and regional climate parameters (temperature, precipitation, soil moisture, and NDVI) over India from 2010 to 2021 using GOSAT and OCO-2 retrievals. The results show since the launch of OCO-2 in 2014, the number of monthly high-quality XCO soundings over India has grown nearly 100-fold compared to GOSAT, launched in 2009. Also, the discrepancy in XCO increase of 2.54(2.43) ppm/yr was observed in GOSAT (OCO-2) retrieval during an overlapping measurement period (2015-2021). Additionally, wavelet analysis indicated that the OCO-2 retrieval is able to capture a better frequency of local-scale XCO variability compared to GOSAT, owing to its high-resolution cloud-free XCO soundings, providing more well-defined regional-scale source-sink features. Furthermore, dominant spatial pattern of XCO variability observed over south and southeast of India in both satellites, with XCO semi-annual and annual variability more distinctly present in OCO-2 compared to GOSAT. A cross-correlation analysis suggested GOSAT XCO growth rate positively correlates with ENSO in different homogeneous monsoon regions of India, with ENSO leading the GOSAT XCO growth rate in all homogeneous regions by 3-9 months. The South Peninsular region sensitive to ENSO changes, especially during 2015-2016 ENSO event, where a decrease in CO uptake was observed is closely linked with precipitation, soil moisture, and temperature anomalies. However, regional climate parameters show a low correlation with XCO growth since CO is a long-lived well-mixed gas primarily having an imprint of large-scale transport in column CO.