**4. Summary and Conclusions**

During an El Niño event, warm waters over the Western Pacific (WP) and Indonesian region shift towards the central to EP regions accompanied by shifting of the convection towards central and EP regions. With a shift in the convection pattern and changes in the Walker circulation, the El Niño events strongly alter the precipitation pattern, which leads to strong regional moisture variability, drought conditions and forest fires along with biomass burning especially over Indonesia region [12]. The effects of El Niño events are found to be strong over the tropical WP and most severe over Indonesia, leading to large scale changes in the atmospheric chemical composition [13–16]. Cooling of the tropical LS, strengthening of the BDC, negative O3 and temperature anomalies in tropical LS are observed changes in the El Niño events [10,32]. These changes due to El Niño have a significant impact on the distribution of WV, O3 and other trace gases in the UTLS region over entire tropical region. Recent 2015–16 El Niño was one of the strongest and long lasted events in the 21st century. Lots of unusual changes happened in the atmosphere in 2015–16 El Niño event and were well reported by several studies [7–9,11,25,26].

In the present study, we quantified the observed changes in the trace gases, O3, CO and WV in the UTLS region (146, 100 and 82/68 hPa) along with CPT over the tropics (20◦N–20◦S) from July to December 2015 using Aura MLS/AIRS/COSMIC GPS RO satellite measurements. The background climatology was calculated from 2006 to 2014 period. Before reaching its peak intensity during winter 2015–16, a remarkable change in the UTLS trace gases (O3 and CO) concentrations over the tropics took place. Due to the fact that strong 2015–16 El Niño induced biomass burning and forest fires, huge amount of carbon emissions were released into the atmosphere in September and October [18–20]. Due to large carbon emissions, the CO was released into the atmosphere and transported to the UTLS region and recorded very high values in October and November 2015. The high resolution GPS RO observations clearly show the strong positive cold point tropopause temperatures over the SEA and WP regions in November and December. The variability of these trace gases shows some delay in the time period between them as depicted in Figure 11. The percentage change in the trace gases concentrations over equatorial region with respect to the background climatology clearly shows strong increase/decrease in trace gases concentrations in 2015–16 El Nino event. In October 2015, the CO shows 40% increase at 215 hPa whereas similar increase in CO shows at 146 hPa in November 2015. At 100 hPa, the increase of CO is ~25% in November 2015. Interestingly, the CO change at the 68 hPa shows continues increase from November 2015 to November 2016. This clearly shows the tropical tape recorder signal in CO. In O3, the change is insignificant at 261 hPa and even at 146 hPa also. However, a significant decrease of O3 at 100 hPa and 82 hPa is clearly noticed. Compared to the 82 hPa, the decrease in O3 is quite high at 100 hPa in July and August 2015. At 100 hPa, maximum decrease in O3 is observed in July 2015 whereas at 82 hPa the maximum decrease is observed in August 2015. WV also shows the maximum change (increase) in December 2015 at 82 hPa. However, the change in the WV was started from the October and reached maximum in December at 82 hPa. The strong decrease of ozone in the LS and at the tropopause level is observed in July and August 2015 and this loss in the O3 is the record amount during the MLS data period. The observed strong decrease in LS O3 is mainly due to the 2015–16 El Niño induced changes in the BDC in the LS.

**Figure 11.** Percentage change with respect to the background climatology (2006–2014) in (**a**) carbon monoxide, (**b**) ozone and (**c**) water vapour observed over equatorial region (averaged over 10◦S–10◦N). Different colours represent different pressure levels.

The major findings from the present study are summarized below:


**Author Contributions:** S.R. conceived the project, conducted research, performed initial analyses and wrote the first manuscript draft. M.V.R. and G.B. provided helpful discussions during conception of the project. N.N.R. assisted in processing and analyses of data. Y.-A.L. edited the first manuscript and finalized it for the first communication with the journal.

**Funding:** S. Ravindra Babu and Y.A. Liou appreciate the financial support of Taiwan's Ministry of Science and Technology through grants No. 105-2221-E-008-056-MY3 and 107-2111-M-008-036.

**Acknowledgments:** We thank the MLS and AIRS team for providing data which is used in the present study through their ftp sites. We also thank the NOAA for providing the MEI index data and COSMIC Data Analysis and Archive Centre (CDAAC) for providing GPS RO data used in the present study through their FTP site (http://cdaac-www.cosmic.ucar.edu/cdaac/products.html). Data used in the present study can be obtained freely from respective websites. We also thank Masatomo Fujiwara for his suggestions to improve the manuscript. Thanks are also given to the Ministry of Science and Technology (MOST) of Taiwan for financial support through grants No. 105-2221-E-008-056-MY3 and 107-2111-M-008-036.

**Conflicts of Interest:** The authors declare no conflict of interest.
