Seasonal and annual variation of AIRS retrieved CO2 over India during 2003-2011
The present study shows spatio-temporal variability in carbon dioxide (CO2) in the mid-tropospheric region over India (0–32°N, 60–100°E) during 2003–2011. The CO2 data used in the study is retrieved from Atmospheric Infra-Red Sounder (AIRS). Analysis of 9 yrs of data shows that the CO2 exhibits a linear increasing trend of 2.01 ppm/year. Besides displaying the linear increasing trend, data show strong seasonal and annual variability. Concentration of CO2 is observed to be highest around April–May (summer months), which decreases by 4–5 ppm during the monsoon months. Seasonal decrease in CO2 concentration appeared to be influenced by the monsoonal activity. Low OLR (proxy of convection) associated with high rainfall during summer monsoon via increasing vegetation index (NDVI) appears to be the primary cause for the seasonal decrease in CO2 through photosynthesis. Correlation coefficient between CO2 and NDVI is of the order of –0.90 suggesting vegetation as a seasonal sink of CO2. Decrease in CO2 concentration takes place at a delay of 2–3 months of rainfall. However, convection seems to be another component, which causes uplifting of CO2 during dry summer (April and May) making high concentration in the mid-troposphere as shown by increase in the planetary boundary layer (PBL) height in this period. Eastward propagating intra-seasonal oscillations with period 30–40 days in OLR anomalies are found to modulate (with a fluctuation of 1–2 ppm) mid-tropospheric CO2. Analysis of seasonal anomalies in CO2 over four different regions (northern, southern, western and eastern) of India is also being investigated. The regional variability of CO2 in northern region show marginal larger values suggesting more anthropogenic activities especially during late winter.