Long-term (2004–2015) tendencies and variabilities of tropical UTLS water vapor mixing ratio and temperature observed by AURA/MLS using multivariate regression analysis
Introduction
Though the water vapor content in stratosphere is small, it plays a major role in determining the radiative energy balance (Forster and Shine, 1999, Foster et al., 2002), in acting as a source of hydroxyl radicals and in the destruction of polar ozone by involving in the activation of chlorine on polar stratospheric clouds (Solomon, 1999). Water vapor is important in heterogeneous ozone loss in the polar regions because it lowers the threshold for formation of polar stratospheric clouds. More important perhaps, it also increases the heterogeneous reactivity of key reactions in polar ozone loss (Drdla and Muller, 2012). Brewer (1949) suggested that the water vapor in the lower stratosphere must have passed through the cold tropopause region over the tropics, slowly ascending within the circulation that later became known as Brewer–Dobson circulation (BDC). Methane oxidation is an important source of water vapor in the middle stratosphere (Jones and Pyle, 1984, Rohs et al., 2006). The tropical tropopause temperature controls the water vapor entering into the stratosphere (Fueglistaler et al., 2009). The annual variation in stratospheric water vapor is a response to the annual cycle in tropopause temperatures (Mote et al., 1996, Holton et al., 1995). Increase of tropospheric temperature leads to high stratospheric water vapor and through feedback process to further warming of troposphere (Dessler et al., 2013). The large positive anomaly of stratospheric WVMR during 1997–98 was attributed to the El-Niño event, which warmed the tropical tropospheric temperature by 2 K (Randel et al., 2004). The interannual variability of stratospheric water vapor is less, when compared to its annual changes and is mainly governed by quasi-biennial oscillation (QBO) (e.g. Giorgetta and Bengtsson, 1999), El-Niño Southern Oscillation (ENSO), and Brewer–Dobson circulation (BDC) (Randel et al., 2006, Dhomse et al., 2008). Thermal-wind relationship of QBO suggests warmer tropopause temperature during the eastward shear of QBO winds leading to more stratospheric water vapor, when compared to the westward shear of QBO (Baldwin et al., 2001). During winter, water vapor along with other chemical constituents get transported from the upper troposphere to extra-tropical stratosphere by BDC, which is forced mainly by the breaking of planetary-scale Rossby waves mostly at mid-latitudes. Besides seasonal and interannual variabilities, it is important to investigate the long-term tendencies in water vapor quantitatively. All climate models predict that there is an increase the stratospheric water vapor trends (Gettelman, 2010). Multi-year observations over mid-latitudes show that there is a positive trend in water vapor data considered for the years 1954–2000 (Rosenlof et al., 2001) and 1980–2010 (Hurst et al., 2011). However, there are only a few studies, which showed altitude dependence with positive trends in upper stratosphere and negative trends in lower stratosphere for the water vapor data extending back to 1986 (Scherer et al., 2008, Hegglin et al., 2014). Over the tropics, stratospheric water vapor for the years shows no significant long-term trend just above the tropopause (82 hPa) (Dessler et al., 2014).
In this paper, long-term trends and variabilities of upper tropospheric and lower stratospheric (UTLS) water vapor mixing ratio (WVMR) over the tropics (30°S–30°N) obtained by the Microwave Limb Sounder (MLS) instrument on board Aura Earth Observing System satellite (EOS) for the period October 2004–September 2015 are studied using multivariate regression analysis.
Section snippets
Water vapor mixing ratio (WVMR) data
The MLS instrument on board Aura EOS satellite was launched on 15 July 2004. It uses a sun-synchronous orbit at an altitude of 705 km and with 98° inclination. The standard water vapor product of v4.2 is taken from the 190 GHz channel. The horizontal grid is every 1.5° or ~165 km along the orbit track. The version 4.2 (v4.2) stratospheric water vapor data used in this study are the update to the v3.3 and v2.2 versions, which are validated and described by Hurst et al. (2014) and Lambert et al.,
WVMR trend
Fig. 4a shows the height profile of trend shown by the WVMR for the pressure levels 178-14.7 hPa obtained from the regression analysis described in Section 2. There is not much difference in the trends with and without the SC term. It is found that there is a decreasing trend of 0.02–0.1 ppmv/year in the WVMR below 100 hPa while the trend is positive (0.02–0.035±0.005 ppmv/year) above 100 hPa. There is no significant trend at 121 hPa. The maximum positive trend of 0.04±0.008 ppmv/year is observed at 68
Discussion and conclusion
In this study, long-term variabilities and tendencies in tropical UTLS WVMR and temperature obtained from the MLS instrument on board EOS satellite are studied using multivariate regression analysis. There is a decreasing trend of 0.02–0.1 ppmv/year in WVMR below 100 hPa while the trend is positive above 100 hPa with the maximum of 0.04 ppmv/year at 68 hPa. Almost all climate models predict that stratospheric WVMR will continue to increase in future (Gettelman et al., 2009). Oltmans and Hofmann
Summary
In this study, long-term variabilities and tendencies of tropical UTLS WVMR and temperature have been reported and discussed considering most of the important parameters, which could influence the WVMR variations. The WVMR shows an increasing (decreasing) trend in WVMR (temperature) above 100 hPa (56 hPa). There is no significant trend in WVMR and temperature near tropopause (100 hPa). The responses of WVMR and temperature to different parameters are discussed. The interannual variability of
Acknowledgments
The MLS team (NASA JPL) is gratefully acknowledged for their data. ERA-interim data used this study were provided by ECMWF and downloaded from their servers. The QBO winds, solar flux and MEI are downloaded from the websites http://www.geo.fu-berlin.de/met/ag/strat/produkte/qbo, ftp://ftp.geolab.nrcan.gc.ca/data/; http://www.esrl.noaa.gov/psd/enso/mei/table.html. The authors thank the Editor and the two Reviewers for their comments and suggestions, which greatly helped them to improve the
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