**1. Introduction**

The Upper Troposphere and Lower Stratosphere (UTLS) is one of the important regions of the Earth's atmosphere and crucial for the Earth's energy balance [1]. The redistribution of the water vapour (WV), ozone (O3) and other chemical species in the UTLS region has a direct impact on the Earth's radiation budget. WV acts as a major source of cooling in the upper troposphere whereas O3 is the main source for the warming in the lower stratosphere (LS). These WV and O3 changes in the UTLS region can strongly influence the temperature structure of the atmosphere and hence atmospheric transport [2–6]. The variability in the UTLS trace gases is strongly influenced by several dominant atmospheric oscillations such as quasi-biennial oscillation (QBO) and El Niño Southern Oscillation (ENSO). ENSO is a dominant mode of the inter-annual variability of the tropical troposphere climate. El Niño is a warm phase of the ENSO, which is having a strong impact on the global atmosphere. In general, during El Niño, enhancement of atmospheric convection and increase of sea surface temperatures (SSTs) in the tropical Eastern Pacific (EP) or Central Pacific (CP) generally observed [7]. The opposite signatures are noticed during the La Niña periods. Due to increase of the convection, lots of large scale atmospheric waves released into the atmosphere vertically. These waves indirectly strengthen the Brewer Dobson Circulation (BDC), increasing the upward motion in the tropics and downward motion in poles in the stratosphere [8]. Due to these circulation changes in the El Niño period, tropical troposphere generally warms while the tropical lower stratosphere (LS) cools [8,9]. The warm troposphere allows more WV into the LS in El Niño events. The negative zonal mean O3 and temperature anomalies in the LS are also evident during the El Niño period. These anomalies are attributed to the strengthening tropical upwelling of the BDC in El Niño period [10,11]. Due to the warm, high troposphere temperatures in El Niño period, drought conditions generally observed over South East Asia (SEA) region with lot of biomass burning and forest fires [12–14]. In addition, more carbon emissions observed in El Nino period over SEA region, which causes high concentrations of CO in the troposphere [12–14]. Several previous observational and modelling studies are reported on the atmospheric composition (mainly on O3, WV and CO) changes during the El Niño periods [12–16].

The recent El Niño event in 2015–16 was one of the strongest El Niño events in the 21st century satellite era [7]. It is also only one strongest boreal summer El Nino event in the Microwave Limb Sounder (MLS) record [17]. During this event, several unusual atmospheric changes happened and were reported well. For example, strong drought conditions were observed over SEA especially over the Indonesia region and caused a record amount of forest fires and biomass burning in September and October [18,19]. The carbon emissions that occurred over SEA in 2015 are the largest one since 1997 [18]. Due to these forest fires, a huge amount of carbon was released into the atmosphere in the form of CO2 and CO [18–20]. These trace gases have strong influence on global atmospheric chemical budget. By using Greenhouse gases Observing SATellite (GOSAT) data, Parker et al. [21] reported the strong enhancement of CO2 and CH4 over the Indonesian region. Whitburn et al. [22] observed 3 times higher amounts of ammonia during this period than in the previous seven years based on satellite measurements acquired in 2008–2015. It is also well reported that the second half of 2015 witnessed massive propagation of Rossby waves into the tropics [23,24].

It was also reported that during the 2015–16 El Niño event, the large anomalies of WV and ice in the LS were observed in December 2015 over SEA and WP regions [7,25]. This enhancement in WV was either due to the warm tropopause temperatures [7] or due to the 2015–16 QBO disruption [25]. However, very recently, Diallo et al. [9] argued that the interplay between the El Nino event and the QBO disruption made an important contribution to the change in the LS WV anomalies during this event. Another study by Garfinkel et al. [26] discussed the impact of ENSO on the LS temperature and WV and suggested that the impact is nonlinear in boreal spring whereas linear in boreal winter. It is well known that most of the WV enters the LS through the tropical tropopause layer (TTL) and the temperature in the TTL controls the WV entering the LS [27–30]. However, the trace gases such as CO and O3 variability in the UTLS region during the 2015–16 El Niño event have not been well investigated. O3 is a strong radiative gas and plays a significant role on the radiative forcing of the atmosphere along with CO2 [31]. The inter-annual variability of O3 at tropopause level is strongly dominated by ENSO [32]. The strength of the BDC and SSTs in the equatorial pacific has strong impact on the O3 anomalies in ENSO events. CO is important for global warming and it acts as a precursor for CO2 and tropospheric O3 and a major sink for OH radicals [33]. It is well established that the strong enhancement of the CO concentrations during the El Niño events [7–9]. In the present study, we present the observed large anomalies of these trace gases in the tropical UTLS region during the recent strong El Nino event of 2015–16.

#### **2. Data base and Methodology**

In the present study, we used version 4.2 level 2 profiles of O3, CO and WV data from Aura MLS provided by the Jet Propulsion Laboratory along with CO data from Atmosphere Infrared Radio Souder (AIRS) in order to see the vertical changes in CO during the year 2015. The high resolution temperature profiles were obtained from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) Global Position System (GPS) Radio Occultation (RO) are used for tropopause changes. Multivariate ENSO Index (MEI) data obtained from http://www.esrl.noaa.gov/psd/enso/ mei were used as an ENSO index [34,35].
