Invited research ArticleEast-west coastal asymmetry in the summertime near surface wind speed and its projected change in future climate over the Indian region
Introduction
Even though satellite and in-situ observations indicate that annual near-surface wind speed over land has decreased in numerous sites around the globe during the past few decades (Xu et al., 2006, Roderick et al., 2007, McVicar et al., 2008, Pryor et al., 2009, Jiang et al., 2010, Vautard et al., 2010, Guo et al., 2011, McVicar et al., 2012, Padmakumari et al., 2013), the interactions and variabilities of summertime convective mesoscale phenomena associated with surface wind speed have become a subject of contemporary interest in recent period over the tropics (Rajeevan et al., 2010, Midya et al., 2011, Litta et al., 2012, Madhulatha et al., 2013, Saha and Maitra, 2014, Chakraborty et al., 2014, Chakraborty et al., 2015, Chakraborty et al., 2016, Saha et al., 2012, Saha et al., 2014, Saha et al., 2017). A small percentage of these tropical convective storms produce severe weather which comprises of tornadoes, hail with a diameter > 20 mm and the straight line winds exceeding 26 m s− 1 (Barnes, 2010). The development of tropical cyclones is also associated with the outbreak of surface westerly wind on either side of the equator (Jones and Thorncroft, 1998). Wang et al. (2009) demonstrated that storminess in the North Atlantic–European region undergoes considerable long-term temporal fluctuations including seasonality and regionality to them. Likewise, eastern, north-eastern and central part of the Indian sub-continent witness intense convective features during summertime (Das et al., 2014, Saha et al., 2014, Saha et al., 2017). The surface wind is thus frequently subject to rapid fluctuations in speed and direction, resulting in gustiness of the wind.
Vecchi and Soden (2007) indicated that greenhouse effect might have an important role to play in the weakening of atmospheric and tropical circulations in the past few years. Large-scale urbanization induces urban warming as well as PBL heating which intensifies vertical motion and convective activity and generates local circulations (Carlson et al., 1983, Saha et al., 2011, Feng et al., 2013, Chakraborty et al., 2017a). Chan and Lee (2011) has shown that wind speeds are gradually decreasing in about 642 stations all over China and the probable reason for this might be the lower tropospheric pressure gradient force and large scale urbanization effects. The surface wind speed over land is strongly influenced by local-scale circulation and is found to decrease over middle and low latitudes and with increasing trends over high latitudes and some oceanic regions (Zhao et al., 2011). The simulated wind speed observations and anomalies averaged over 30 years over the whole Northern Hemisphere and Arabian Peninsula indicates a wind speed decline by 0.3% and 0.24% respectively (Bichet et al., 2012). According to Vautard et al. (2010), 5–15% of the suppressed surface wind speed over almost all continental areas in the northern mid-latitudes is strongly associated with the increases in biomass and land-use change in Eurasia. As a result, strong winds have slowed faster than weak winds in the past few decades over China, Netherlands, Czech Republic, United States and Australia. The declining near-surface wind speeds and increasing aerosols are found to be closely associated with suppressed convective activities over the tropical and sub-tropical region (Jacobson and Kaufman, 2006, Xu et al., 2006, Zhao et al., 2006, Saha et al., 2014). Other recent studies indicate that in a globally warming climate, observed rates of atmospheric pan evaporation also depend on the trends of terrestrial horizontal wind speeds (McVicar et al., 2008, Padmakumari et al., 2013). The decreasing frequencies of extreme wind events in and around the tropics in relation to global warming has been an important feature in the past few decades (Gastienau and Soden, 2009, Rummukainen, 2013, Mika, 2013). Baule and Shulski (2014) have further indicated sharp distinctions in both seasonal and diurnal variation of wind speeds between the inland and coastal regions. Holley et al. (2014) has recorded and showed annual wind speeds in terms of Convective Available Potential Energy (CAPE) to be decreasing in various parts of Great Britain due to reduced water vapor over urban environments, regardless of urban temperature increments. Shanas and Kumar (2014) has showed that in the central BoB, mean and extreme (90th percentile) wind speeds have shown an overall statistically decreasing trend (0.75 and 1 cm s− 1 yr− 1 respectively).
In an effort to better understand past and future climate change, general circulation models have become the forerunners of attempts to simulate future climate. With the release of new generation climate model simulations, known as Coupled Model Intercomparison Project phase 5 (CMIP5), that represent the latest advancements in general circulation model (GCM) development and are an important component of the Intergovernmental Panel on Climate Change's (IPCC) assessment of climate science (Taylor et al., 2012). The CMIP5 simulations for the 21st Century are driven by the radiative forcing deduced from different scenarios of anthropogenic emissions of GHG and industrial aerosols. In the present study, the long term variations of meteorological and atmospheric instability parameters at different regions of the Indian sub-continent (8°N–37°N, 66°E–98°E) have been investigated during last three decades (1981–2010). The probable causes of such asymmetric variability of summertime near surface wind associated with convective activities are represented in order to investigate any gradual but significant changes in the regional climate scenario since the last three decades. With the aim to contribute in the improvement of understanding of future climatic variability over the coastal boundaries of Indian sub-continent, this work utilizes the IPCC derived 11 GCMs in the CMIP5. Table 1 lists the Indian region-mean fractional changes in near surface wind speed for the GCMs in our ensemble. We have selected 7 GCMs (out of a total of 11 models taken for study) that best capture the observed near surface wind speed from the historical period. These GCMs are represented in our analysis to best assess the future changes in near surface wind speed variability associated with convective features during the late 21st century over this sub-continent.
Section snippets
Geographical and climatic features of the region
India (8°N–37°N, 66°E–98°E) enjoys a subtropical climate in north and a tropical climate in south. This region is bounded by the Himalayan range in the north, Thar Desert in the west and the southern side is bounded by seas (Bay of Bengal in the South-East, Arabian Sea in the South-West and Indian Ocean at the South). The coastal regions of India are also bounded by hilly terrain (Eastern Ghats facing the Bay of Bengal and the Western Ghats adjacent to the Arabian Sea). Other geographical
Datasets used
The long term variation of various atmospheric parameters has been studied for the Indian sub-continent during the past three decades (1981–2010). Meteorological parameters and instability indices have been taken from high resolution Radiosonde measurements in this study. Radiosonde observations made at 00:00 GMT (05:30 IST), obtained by the University of Wyoming (from the website: http://www.uwyo.edu) at sixteen sites for March–May (1981–2010), have been used in the present study. The
Spatio-temporal distribution of summertime surface wind speed climatology during 1981–2010
Fig. 2 shows the spatial distribution of the detailed decadal time-averaged maps of surface wind speed during summertime over the Indian sub-continent. The average summertime wind speed during the first decade (1981–1990) was 3.5–4 m s− 1 in the eastern coast and 4.5 m s− 1 in the western coast (Fig. 2a). This mean summertime wind speed in the eastern coasts found to 3 m s− 1 and 2.5 m s− 1 during the second (1991–2000) and third (2001 − 2010) decade respectively (Fig. 2b and Fig. 2c). Thus the summertime
CMIP5 model and projected change in future regional climate
With the release of new generation climate model simulations, known as CMIP5, it is required to evaluate the model performances before using their projections for future change in climate in terms of thermodynamic and anthropogenic resources. The transient climate experiments in CMIP5 are conducted in three phases. The first phase covers the start of the modern industrial period through to the present day, years 1850–2005. The second phase covers the future, 2006–2100, and is described by a
Conclusions
In order to investigate the cause of declining surface wind speeds over the Indian sub-continent various meteorological parameters are analyzed on a large spatial and temporal scale. Our analysis shows that although most of the meteorological parameters remained almost constant in the past three decades a significant decreasing trend of wind speeds is observed on the eastern coast whereas an insignificant decrease is noted on the western coast. However, the rest of the sub-continent has shown
Acknowledgements
U.S. thankfully acknowledges the financial assistance provided by University Grants Commission (UGC), New Delhi, India, under the scheme of Dr. D.S. Kothari Post-Doctoral Fellowship (No. F.4-2/2006(BSR)/ES/15-16/0022). This work has also been partially supported by the UGC-MRP (No. P-01/717) under A K Singh and collaboration programme between BHU, Varanasi and CU, Kolkata. We acknowledge the World Climate Research Programme's Working Group on Coupled Modeling, which is responsible for CMIP, and
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