El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) signatures in tropical ozone in the Upper Troposphere Lower Stratosphere (UTLS)

Abstract

This study examines the combined influence of El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) on Upper Troposphere Lower Stratosphere (UTLS) ozone variability. The investigation employs data from the Microwave Limb Sounder (MLS) aboard the Aura Satellite and the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis, spanning the period 2005–2020 across tropical latitudes (20º N–20º S). Three specific events were chosen for analysis: a strong La Niña event in 2010, the co-occurrence of El Niño and moderate IOD in 2015, and a robust IOD event in 2019. During years marked by the simultaneous occurrence of ENSO and IOD events, the UTLS (100 hPa altitude is considered for the present study. 82 hPa is the altitude just above the tropopause, therefore also shown in the results) ozone mixing ratio demonstrates a decline in absolute values. The Quasi-biennial Oscillation (QBO) was also investigated, revealing a synchronized variation with the ozone anomaly in the UTLS region. Furthermore, the calculated eddy heat flux, utilized as a proxy for the Brewer–Dobson Circulation (BDC), aligns with the UTLS ozone anomalies, indicating a positive (negative) anomaly during periods of intense tropical downwelling (upwelling). To quantitatively elucidate the contributions of ENSO, IOD, and QBO to the observed ozone anomaly, a multivariate linear regression analysis was executed utilizing the least square method. The findings underscore that a notable fraction—about one-fourth of the observed UTLS ozone anomaly within the study timeframe (2005–2020) can be attributed collectively to ENSO, IOD, and QBO. This preliminary exploration underscores the substantial role played by large-scale climate drivers emanating from the Pacific and Indian oceans in shaping UTLS ozone distribution. These insights emphasize the significance of considering these climatic influences when examining the intricate dynamics and variability of UTLS ozone patterns.