Abstract
Abstract. A weak El Niño during 2014–2015 boreal winter developed as a strong boreal summer event in 2015 which continued and even enhanced during the following winter. In this work, the detailed changes in the structure, dynamics, and trace gases within the Asian summer monsoon anticyclone (ASMA) during the extreme El Niño of 2015–2016 is delineated by using Aura Microwave Limb Sounder (MLS) measurements, COSMIC radio occultation (RO) temperature, and National Centers for Environmental Prediction (NCEP) reanalysis products. Our analysis concentrates only on the summer months of July and August 2015 when the Niño 3.4 index started to exceed values of 1.5. The results show that the ASMA structure was quite different in summer 2015 as compared to the long-term (2005–2014) mean. In July, the spatial extension of the ASMA is greater than the long-term mean in all the regions except over northeastern Asia, where it exhibits a strong southward shift in its position. The ASMA splits into two, and the western Pacific mode is evident in August. Interestingly, the subtropical westerly jet (STJ) shifted southward from its normal position over northeastern Asia, and as a result midlatitude air moved southward in 2015. Intense Rossby wave breaking events along with STJ are also found in July 2015. Due to these dynamical changes in the ASMA, pronounced changes in the ASMA tracers are noticed in 2015 compared to the long-term mean. A 30 % (20 %) decrease in carbon monoxide (water vapor) at 100 hPa is observed in July over most of the ASMA region, whereas in August the drop is strongly concentrated at the edges of the ASMA. A prominent increase in O3 (> 40 %) at 100 hPa is clearly evident within the ASMA in July, whereas in August the increase is strongly located (even at 121 hPa) over the western edges of the ASMA. Further, the temperature around the tropopause shows significant positive anomalies (∼ 5 K) within the ASMA in 2015. The present results clearly reveal the El-Niño-induced dynamical changes caused significant changes in the trace gases within the ASMA in summer 2015.
Highlights
The Asian summer monsoon anticyclone (ASMA) is a distinct circulation system in the upper troposphere and lower stratosphere (UTLS) during Northern Hemisphere boreal summer centered at ∼ 25◦ N and extending roughly between 15 to 40◦ N (Park et al, 2004; Randel et al, 2010)
The intensity and spatial extension of the ASMA are prominent in July and August when the monsoon was in the mature phase (Santee et al, 2017; Basha et al, 2020)
We investigated the detailed changes observed in the structure, dynamics, and trace gas variability within the ASMA in 2015
Summary
The Asian summer monsoon anticyclone (ASMA) is a distinct circulation system in the upper troposphere and lower stratosphere (UTLS) during Northern Hemisphere boreal summer centered at ∼ 25◦ N and extending roughly between 15 to 40◦ N (Park et al, 2004; Randel et al, 2010). Previous studies have concluded that deep convection during the summer monsoon can effectively transport the pollutants, aerosols, and tropospheric tracers from the boundary layer into the UTLS region (Vogel et al, 2016; Santee et al, 2017). These transported pollutants, tracers and aerosols become confined in the ASMA and, affect the trace gas composition in the UTLS region (Randel et al, 2010; Hossaini et al, 2015).
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