Journal of Atmospheric & Earth Sciences Category: Agriculture Type: Research Article
Observational Studies of Monsoonal Droughts and Large Scale Traits
- Kumar V1*, Kulkarni JR2
- 1 Department Of Earth And Ocean And Atmospheric Sciences, Florida State University, Tallahassee, United States
- 2 Indian Institute Of Tropical Meteorology, Pune, India
*Corresponding Author:Kumar V
Department Of Earth And Ocean And Atmospheric Sciences, Florida State University, Tallahassee, United States
Received Date: Aug 10, 2017 Accepted Date: Sep 28, 2017 Published Date: Oct 12, 2017
Rainfall variability of the Indian summer (JJAS) monsoon droughts are linked to regional and global parameters. Here, with the use of 30 years of various observational datasets of rainfall, winds, temperature and other major indices, the variability of Indian summer monsoon is explored for drought years. In recent, due to increase in ocean surface temperature, meridional temperature gradient over Indian land and nearby sea is decreased. On the scale of months to season the rainfall and circulation features grow in and out of phase over Indian, Pacific Ocean and African regions. Additionally, droughts over Indian regions are manifestation of the ITCZ’s sway in meridional direction influenced by circulation and convective activities over Pacific, Indian and African regions. Ultimately, ENSO, warming in the regional oceans and internal dynamics remain the potential drivers of the ISMR droughts. In recent decades, Indian monsoon is showing favoritism towards drought.
ocean surface temperature; meridional temperature; Monsoonal Droughts; Large Scale Traits
Indian monsoonal drought in recent years (e.g., 2002, 2009, 2014 and 2015) drew our attention to revisit this problem. Since 1871 Indian region confronted total 25 droughts, while there were as many 32 years, when rainfall was below normal on all India basis . Every summer (June-September) the distribution of rainfall is such that India is bound to faces drought over a smaller or bigger region. The severity of drought may vary amazingly from place to place and time to time. Once a wave of drought is blown out, the life of a common-man and farmers start to experience hardship in their daily activities . Climatologically western part of India is virtually known a drought prone region, and rest of the India experiences wet and dry spells contemporary during June to September. In the literature of Indian monsoon, various drought indices and methods are defined to measure the intensity of monsoon droughts severity. The droughts during the period 1871-1995 are given in [14,12]. Even the hiatus in the summer monsoon rainfall for a large number days over India may lead to drought like conditions [15,16]. Long break of 20-25 days in the month July in 2002, made 2002 as the one of the worst drought years in the Indian history .
In last 30 years 6 major India monsoon droughts (1982, 1986, 1987, 2002, 2009 and 2015) were associated with ENSO event [2,18-22,12). These drought years, were linked with above than normal winter snow cover over Eurasian region [23,24,5,6]. The role of dry air intrusion during monsoonal break also linked with monsoonal droughts [17,25,10]. Thus, internal dynamics, Eurasian snow cover and dry air intrusion are some of the bigger players in producing droughts over the Indian region. The main characteristics of ISMR are 1) Almost no rainfall over central India and heavy rainfall over foothills of Himalaya and east equatorial Indian ocean. 2) High pressure over central India. 3) Shift of monsoonal trough northern region of India.
The motivation of the research is inspired from the recent warming of the Indian Ocean and changes in various large scale circulation patterns related to Indian monsoon. Understanding the large-scale traits of the Indian monsoon droughts is the main goal of the study. In this work, droughts are defined based on rainfall variability only. A drought year is defined when monsoon seasonal rainfall (June-September) is less than 10% of the climatological normal. All such drought years are considered in this study. Authors believe, simply understanding the synoptic and dynamical features of various atmospheric-oceanic systems during a monsoonal drought may help readers immensely. Thus, one of the main issue addressed in this manuscript is the representation of all the large-scale factors those affects ISMR’s rainfall. Previous studies and reports considered one aspect of large scale features associated with drought, one at time. This observational study raise questions, what are the impact of 1) the weakening of meridional gradient of surface temperature, 2) impact of ENSO on monsoon, 3) role of internal dynamics in the changing climate, on monsoonal droughts?
MATERIAL AND METHODS
All the datasets are freely available online on their respective websites for research purposes. Large scale circulation features like Walker and Hadley circulations are calculated based on the zonal and meridional component of the wind field. Rainfall anomalies are calculated based on the 30 years (1981-2010) of rainfall climatology. The study area is limited to the Indian landmass and Indian summer (JJAS) monsoonal droughts for the composite plots years 1982, 1985, 1986, 1987, 2002, 2004 and 2009 are considered throughout the manuscript.
Climatological features of monsoonal droughts over Indian subcontinent
Figure 2, shows climatological characteristics of rainfall distribution over Indian subcontinent region. Here maximum rainfall is usually received by Western Ghats, the Northeastern India, Bangladesh and Myanmar. Furthermore, two extensions of maximum rainfall (ITCZ) are located one over central Africa and another over western Pacific region. One more distinguishable patch of rainfall is seen over east equatorial Indian region, which is known as double ITCZ over Indian region during boreal monsoon. In most of the drought cases northeastern Indian region and western Indian shows opposite rainfall variability. As a matter of fact, during droughts most of the India experiences above normal atmospheric pressures and subsidence of colder and drier air-mass. It is shown that Indian summer monsoon rainfall does not show any trend .
Figure 2: Climatological variations of summer monsoon rainfall over South Asian and African region. The climatology is based 30 years of datasets (1982-2011) from CMAP.
COMPOSITE MAPS SEVERAL VARIABLES FOR INDIAN SUMMER MONSOON DROUGHTS
Figure 4: Rainfall anomalies for the composite of drought years.
Figure 5a: Rainfall anomalies for the composite of drought years.
Figure 6: OLR anomalies for the composite of drought years.
One of the fundamental reasons of the existence of monsoonal flow is the meridional gradient of lower tropospheric temperature (Ocean vs Land). How, this temperature gradient swings from pre-monsoon month of May to monsoonal summer months is very exciting (Figures 8a&8b). There is barely any warming in the total column of atmosphere, which can pull the monsoon over Indian landmass in May (Figure 8a). Contrasting to the old fact of surface temperature gradient across Indian subcontinent and Indian Ocean, which drives the monsoon, latter on heating of the lower tropospheric column found an important factor for monsoon . In figure 8b, the vertical gradient of the temperature in the lower tropospheric column changes a lot than figure 8a. If this warming is extended up to 600-500mb level, then it is expected that the monsoon may perform better over Indian region.
Figure 8a: Vertical profile of temperature averaged over lon=72 to lon=85 for May.
Figure 9a: Zonal Walker circulation (averaged over lat=-10 to lat=20).
The variability of rainfall brings natural calamities to the society, disaster to the environment, food scarcity, and imbalance to ecology. Among all the possibilities of the advanced forecasts we should facilitate the society to prepare for the natural disasters. We should educate and prepare society for a judicial use of natural resources, manage water problems, landslides, floods and droughts because the oddity of the monsoon is difficult to dodge. Further, there modeling of droughts may verify the weakening of meridional gradient of surface temperature, impact of ENSO on monsoon, role of internal dynamics and for other influences in the changing climate for monsoonal droughts?
Some of the important points as the discussion are raised here in the time of global warming how the monsoon will behave: -
a. On the disappearance of Eurasia and Arctic Ice completely
b. On the shift of the warming in the middle of Pacific Ocean than eastern Pacific Ocean during El-Nino years
c. The land and sea temperature gradient across the Indian continent and Indian ocean is decreasing
d. It has been shown that the depressions are decreasing in Bay of Bengal
e. Local circulation e.g., local Hadley cell, over Indian subcontinent are weakening
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Citation:Vinay Kumar, Kulkarni JR (2017) Observational Studies of Monsoonal Droughts and Large Scale Traits. J Atmos Earth Sci 1: 001.
Copyright: © 2017 Kumar V, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.