Structure, dynamics, and trace gas variability within the Asian summer monsoon anticyclone in the extreme El Niño of 2015–2016

Abstract

Abstract. A weak El Nino during 2014–2015 boreal winter developed as a strong\nboreal summer event in 2015 which continued and even enhanced during the\nfollowing winter. In this work, the detailed changes in the structure,\ndynamics, and trace gases within the Asian summer monsoon anticyclone (ASMA)\nduring the extreme El Nino of 2015–2016 is delineated by using Aura Microwave\nLimb Sounder (MLS) measurements, COSMIC radio occultation (RO) temperature,\nand National Centers for Environmental Prediction (NCEP) reanalysis products. Our analysis concentrates only on the summer\nmonths of July and August\xa02015 when the Nino\xa03.4 index started to exceed values of 1.5. The results show that the ASMA structure was quite different in\nsummer 2015 as compared to the long-term (2005–2014) mean. In July, the\nspatial extension of the ASMA is greater than the long-term mean in all\nthe regions except over northeastern Asia, where it exhibits a strong\nsouthward shift in its position. The ASMA splits into two, and the western\nPacific mode is evident in August. Interestingly, the subtropical westerly\njet (STJ) shifted southward from its normal position over northeastern Asia,\nand as a result midlatitude air moved southward in 2015. Intense Rossby wave\nbreaking events along with STJ are also found in July\xa02015. Due to these\ndynamical changes in the ASMA, pronounced changes in the ASMA tracers are\nnoticed in 2015 compared to the long-term mean. A 30\u2009% (20\u2009%) decrease\nin carbon monoxide (water vapor) at 100\u2009hPa is observed in July over most of\nthe ASMA region, whereas in August the drop is strongly concentrated at the\nedges of the ASMA. A prominent increase in O 3 ( > \u200940\u2009%) at\n100\u2009hPa is clearly evident within the ASMA in July, whereas in August the\nincrease is strongly located (even at 121\u2009hPa) over the western edges of the\nASMA. Further, the temperature around the tropopause shows significant\npositive anomalies ( ∼ \u20095\u2009K) within the ASMA in 2015. The present\nresults clearly reveal the El-Nino-induced dynamical changes caused\nsignificant changes in the trace gases within the ASMA in summer 2015.