Pure Appl. Geophys. Ó 2013 Springer Basel DOI 10.1007/s00024-013-0691-9

Pure and Applied Geophysics

Indian Summer Monsoon Rainfall Characteristics During Contrasting Monsoon Years HAMZA VARIKODEN,1 M. R. RAMESH KUMAR,2 and C. A. BABU3 Abstract—The present paper presents a diagnostic study of two recent monsoon years, of which one is dry monsoon year (2009) and the other is wet monsoon year (2010). The study utilized the IMD gridded rainfall data set in addition to the Reynolds SST, NCEP-NCAR reanalysis wind and temperature products, and NOAA OLR. The study revealed that the months July and August are the most crucial months to decide whether the ISMR is wet or dry. However, during July 2009, most of the Indian subcontinent received more than 60 % in the central and western coastal regions. In a wet monsoon year, about 35–45 % of rainfall is contributed during June and July in most parts of India. During these years, the influence of features in the Pacific Ocean played vital role on the Indian summer monsoon rainfall. During 2009, Pacific SST was above normal in nino regions, characteristic of the El Nino structure; however, during 2010, the nino regions were clearly below normal temperature, indicating the La Nina pattern. The associated atmospheric general circulation through equatorial Walker and regional Hadley circulation modulates the tropospheric temperature, and hence the organized convective cloud bands. These cloud bands show different characteristics in northward propagation during dry and wet years of ISMR. During a dry year, the propagation speed and magnitudes are considerably higher than during a wet monsoon year. Key words: Contrasting monsoon, sea surface temperature, monthly contribution, tropospheric temperature.

1. Introduction One of the main characteristics of the Indian summer monsoon rainfall (ISMR) is the year-to-year variation. This variation is subject to many atmospheric and oceanic parameters in fast and slow response. India is an agrarian country, and therefore

1

Indian Institute of Tropical Meteorology, Dr. Homo Bhabha Road, Pashan, Pune 08, India. E-mail: [email protected] 2 National Institute of Oceanography, Dona Paula, Goa, India. 3 Cochin University of Science and Technology, Cochin 16, India.

its economy mainly depends upon the seasonal rainfall from June to September. About 75–90 % of the rainfall is received during these four months, and hence the reduction/excess in rainfall will affect the economy of the country (MOOLEY et al. 1981). This year-to-year variation of the ISMR is a physical manifestation of the interannual variability, and this interannual variability is subject of intense research (MOOLEY and PARTHASARATHY 1984; PARTHASARATHY 1984; MOOLEY and SHUKLA 1987; PANT et al. 1988; PARTHASARATHY et al. 1988, 1991). The variation of the ISMR from year to year is not coherent over the Indian region, both spatially and temporally (GADGIL et al. 2003). In general, some regions experience above-normal rainfall and others below-normal. Thus the anomalies of rainfall from the climatology are positive over some of the meteorological subdivisions and negative in others. In a flood year, the rainfall is above normal in most of the subdivisions, and vice versa during drought years (SHUKLA 1987; GREGORY 1989; GADGIL et al. 2007). RAMESH KUMAR et al. (1986) have looked into the SST variability during the monsoon season over the Indian Ocean and its relation with the summer monsoon rainfall from a predictive point of view, using satellite-derived fields of SST for two contrasting monsoon years (1979 and 1983). Their study showed that the zonal anomaly of SST off the coast of Somalia and the central Indian Ocean are highly correlated with the monsoon rainfall over the western and central parts of India during the same week, based on a 80 years of data; RAO and GOSWAMI (1988) have shown that the pre-monsoon anomalies of SST in the western and southern Arabian Sea have a predictive value for the monsoon rainfall. Most of these studies presumed that higher values of SST lead to higher rates of evaporation, which in turn can contribute to a normal or even excessive monsoon

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rainfallRAMESH KUMAR and SCHLUESSEL (1998) has obtained similar result for two contrasting monsoon years, namely 1987 (deficit) and 1988 (excess). In another study, RAMESH KUMAR et al. (2005) looked at the air–sea interaction over the tropical Indian Ocean during two contrasting monsoon years 2002 (deficit) and 2003 (normal). They found that evaporation rates were lower (higher) over the Arabian Sea during active (weak) monsoon conditions. Water vapor contents decreased substantially prior to the break over the Arabian Sea, and low values prevailed throughout the break period. RAMESH KUMAR (2009), using a suite of data sets, have shown that there is an increased propensity towards breaks in monsoon conditions over the Indian subcontinent during recent decades. They attributed this to largescale changes in the atmospheric circulation and warming of the eastern equatorial Indian Ocean at the rate of 0.015 8C year-1. Their study further showed that this warming in turn has altered the ocean atmospheric processes over the Indian Ocean so as to intensify the Southern Hemisphere equatorial trough, and also led to the weakening of the moisture flow into the Indian subcontinent during recent decades. The Indian summer monsoon is influenced by local and large-scale atmospheric and oceanic features. The Indian Ocean dipole (IOD) is one of the factors which influences the ISMR (SAJI et al. 1999; ASHOK et al. 2004). The influence of El Nino– Southern Oscillation (ENSO) on the Indian monsoon rainfall has been a subject of intense research during the last three decades (SIKKA 1980; RASMUSSEN and CARPENTER 1983; SHUKLA 1987; ASHOK et al. 2001). The Indian summer monsoon is influenced by ENSO through equatorial Walker circulation, and thus through the regional Hadley circulation (WEBSTER et al. 1998; GOSWAMI 1998). ASHOK et al. (2001) further determined that the IOD modulates the ENSO influence on ISMR through regional circulation. The atmospheric part of IOD called EQUINOO (Equatorial Indian Ocean Oscillation) also plays a vital role in modulating the ISMR. It is well known that the ISMR has considerable interannual variability (PARTHASARATHY et al. 1995) with different periodicities, such as biennial, interdecadal, decadal and multidecadal periods. The 2009 and 2010 Indian summer monsoons highly contrast in

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rainfall amounts and associated atmospheric and oceanic features. The 2009 Indian summer monsoon was abnormally below the climatological mean rainfall and was reported as the third major drought year in the recent century (IMD report 2009). During 2009, the ISMR has departed about 23 % from its normal value; however, in certain regions, the ISMR has departed 48 % from its normal value. This massive departure of ISMR leads to a drastic deficit condition in the Indian peninsula. FRANCIS and FRANCIS (2010) reported that the non-cooperative behaviour of Bay of Bengal meridional SST leads to a suppressed condition. It inhibits organized convection over the monsoon regime, and thus the rainfall during the year. In the case of the 2010 ISMR, the rainfall itself is above normal in most parts of the country, and was reported as 103 % of the long-term mean of the rainfall. WEBSTER et al. (2011) studied the Pakistan floods that occurred during the peak month of the 2010 summer monsoon period, and they argued that these deluges were highly predictable, at least 6–8 days in advance. HONG et al. (2011) have pointed out that the anomalous rainfall activity over the Indo-Pak region during 2010 was related to persistent activity of European blocking and the cooccurrence of tropical monsoon surges. All these studies revealed that ISMR is above normal over the Asian monsoon regions in addition to the Indian continent. In the present study, we made an attempt to bring out the contrasting features in the atmospheric and oceanic parameters associated with the contrasting monsoons of 2009 and 2010.

2. Data and Methodology The present study utilizes daily gridded rainfall data set with a 1° latitude 9 1° longitude grid spatial resolution, available from the India Meteorological Department (IMD) for 2009 and 2010. The climatology of the rainfall was made from 1974–2004 as the base period. In this data set, a geographical area from 6.5°N to 37.5°N and 66.5°E to 101.5°E was considered, and the station rainfall data was interpolated to gridded rainfall data. The interpolation method is based on the weights calculated from the distance between the station and the grid point and

Indian Summer Monsoon Rainfall Characteristics

also the directional effects (SHEPARD 1968). Standard quality controls were performed before carrying out the interpolation analysis (RAJEEVAN et al. 2006; RAJEEVAN and BHATE 2009). For the present work, we considered the daily data for ISMR (June–September) and it has been converted to monthly values. Daily Reynolds SST data was also used to understand the ocean temperature features during the contrasting years (2009 and 2010) of ISMR. This data set has a spatial resolution of a 0.258 latitude 9 0.258 longitude grid and has been developed using the optimum interpolation (OI) technique (REYNOLDS et al. 2007). In addition to the Reynolds SST data set, the NCEP-NCAR (National Centre for Environmental Prediction , National Centre for Atmospheric Research) wind and temperature data sets were used at different levels in order to understand the behavior of the atmosphere during contrasting years of ISMR. These data sets have a spatial resolution of a 2.58 9 2.58 latitude–longitude grid with and a temporal resolution of a day. Even though the wind and temperature data sets are a reanalysis data set, it is well related to observed data, and hence it is in the most reliable class of measured wind observations (KALNAY 1996). Tropospheric temperature is the average temperature between 200 and 700 hPa. This tropospheric temperature is also analyzed. Daily NOAA (National Oceanic and Atmospheric Administration) interpolated OLR (Outgoing Longwave Radiation) with the same spatial resolution (LIEBMANN and SMITH 1996) was also used to study the propagation of convection in different time scales over the summer monsoon domain. This interpolated OLR can be used as a proxy for convection and the values of OLR \220 Wm-2 represent to indicate convection and precipitation in the tropics (ARKIN and MEISNER 1987).

3. Results and Discussion 3.1. Mean Rainfall During 2009 and 2010 Indian summer monsoon rainfall (ISMR) on an average during 2009 is below normal and during 2010 is above normal in most parts of the Indian subcontinent. The spatial distribution of the seasonal

rainfall during 2009 and 2010 is given in Fig. 1. During 2009 ISMR, only the western coast gets rainfall above 200 mm, and rest of the country received rainfall below 75 mm. This is less than the normal in the country as a whole. In the northwestern regions, the rainfall is below 25 mm in the southwest monsoon season. However, during the 2010 southwest monsoon, the entire subcontinent received above-normal rainfall, especially in the Konkan coasts and eastern regions. There, the monsoon rainfall is about 300 mm. In central India, the rainfall is above 100 mm/season. 3.2. Monthly Contribution to the Seasonal Rainfall Monthly distributions of rainfall during summer monsoon months are clearly different in the two contrasting years (Fig. 2). In 2009, a deficit year, maximum contribution of rainfall occurred in July in most part of the country. During this month, most part of the western coast and central India received more than 60 % of rainfall. However, in other months the contribution of rainfall is too low, especially during June. In the June, most parts of the peninsular India and central India contributed\15 % of the total seasonal rainfall. During August, the foothills of Himalaya and eastern regions received more rainfall than in the western coastal regions. In the 2010 monsoon, June rainfall contributed a small amount compared with other months in the southwest monsoon period. During the months of July and August, 35–45 % of the rainfall is contributed in most of the Indian regions, indicating that these two months are contributing most of the rainfall to the total seasonal rainfall (70–90 % of annual rainfall). The contribution of rainfall in the month of September is \20 % in all the regions, except some northern areas. FENNESSY and SHUKLA (1994) brought out similar rainfall characteristics for two contrasting monsoon years 1987 (deficit) and 1988 (excess). 3.3. Departures from Normal for Two Contrasting Monsoon Years Monthly departures of ISMR during 2009 and 2010 are presented in Fig. 3. In 2009, as we expected, most part on the Indian subcontinent experienced dry

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Figure 1 Spatial pattern of summer monsoon seasonal rainfall in mm per season (June–September) during 2009 and 2010

Figure 2 Spatial pattern of ISMR departures for each month during June to September for 2009 and 2010

condition in all the months. The departure during June is negative everywhere, except for some parts of the southeastern regions. During the July, August and

September, rainfall departures are greater in the northern and northwestern regions. Only in the Konkan coasts during July and September, and over

Indian Summer Monsoon Rainfall Characteristics

Figure 3 Spatial pattern of ISMR contribution for each month during June to September for 2009 and 2010

the western ghats during August, are rainfall departures above normal. In the case of a wet year (2010 monsoon), we observed above-normal rainfall over the Indian subcontinent in all the summer monsoon months except during June. In the month of June, central India received below-normal rainfall. 3.4. Tropospheric Temperature During 2009 and 2010 In order to understand the reasons for the persistent rainfall in 2010 and decreased rainfall in 2009, we tried to explore the possibility of the tropospheric temperature (TT). The tropospheric temperature is one of the factors governing the southwest monsoon. A warm TT indicates high heating, which ultimately draws the moisture towards its core, and hence produces a good amount of rainfall. LIU and YANAI (2001) stated that the good monsoon over the Asian region is linked with a high

TT over the Asian region. During the 2009 southwest monsoon season, the TT shows low values compared with the 2010 southwest monsoon period (Fig. 4). The abnormally high rainfall during the 2010 monsoon is due to the high TT. The difference of TT between the two contrasting monsoons is given in Fig. 4 (bottom panel). During the 2010 southwest monsoon, the TT is increased everywhere in the Indian subcontinent, and this high TT causes pulling of the atmospheric moisture through the westerly jet, and produces organized convection over the entire Indian region. The TT values are greater during July and August, during which the TT values are more than -5° C, however, during 2009, the TT is\-6° C for the same months. The high values of TT contribute to the organization of high amounts of monsoon clouds over the Indian subcontinent through the mid-tropospheric heat flux. The studies of GOSWAMI and XAVIER (2005) and YU et al. (2004) support the present analysis.

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Figure 4 Spatial pattern of TT during June to September for 2009 and 2010

3.5. Seasonal SST for 2009 and 2010 Sea surface temperatures from the Reynolds data set have been studied for the Indo-Pacific regions for the two contrasting monsoon years (Fig. 5). During the 2009 summer monsoon period, it is clear that a strong El Nin˜o developed in the eastern Pacific Ocean with a considerable magnitude. The SST pattern shows a typical El Nin˜o with a magnitude of more than 1 °C in the equatorial belt up to 150 W, and the magnitude exceeds more than 1.5 °C over the nin˜o 1 and nin˜o 2 regions. This clearly shows the influence of Pacific SST on the ISMR over the Indian subcontinent by governing general atmospheric circulation. This circulation descends over the Indian subcontinent and leads to dryness in the upper atmosphere, as we showed in the TT. Therefore, during 2009 the combined TT and Pacific SST anomaly leads to the abnormally low rainfall over the Indian regions. However, during 2010 ISMR, the SST pattern is different from that of the 2009 SST anomaly pattern. This SST anomaly pattern shows negative, indicating

La Nin˜a with moderate intensity, as reported by MUJUMDAR et al. (2012). The negative SST anomaly spreads all over the nin˜o 4 region in addition to the nin˜o 1 and nin˜o 2 regions, with maximum intensity over nin˜o 3.4 regions. This negative pattern of SST anomaly over the Pacific region leads to an inverse general circulation in the atmosphere from that of the 2009 circulations. This opposite circulation leads to a sinking motion over the equatorial Indian Ocean through Walker circulation, and the descended component rises over the Indian peninsular region through regional Hadley circulations. This upward motion of the regional Hadley circulation helps to maintain the TT warmth, and this maintains the organized monsoon clouds over the region, ultimately resulting in a high amount of rainfall over the monsoon domain. RAMESH KUMAR et al. (1986) and PATTANAIK and PATTANAIK (2000) studied the SST features during different contrasting monsoon years and they reached similar conclusions. Our results were also coherent with the previous findings.

Indian Summer Monsoon Rainfall Characteristics

Figure 5 Spatial pattern of SST for monsoon period for 2009 and 2010

3.6. Anomalies of Wind and OLR Understanding the low-level circulation anomalies during recent contrasting monsoon years is interesting because the circulation pattern is very different from the climatological pattern. Climatology of OLR (shaded) and wind at the 850 hPa level (top panel) and the anomalies of OLR and wind at the 850 hPa are given in Fig. 6. During the dry year, the wind anomaly shows easterlies in the entire LLJ (low-level jet stream) zone, indicating that the LLJ is very much weakened during the year 2009; this is coherent with the results of GADGIL et al. (2003) for the 2002 monsoon. Over the land mass of the Indian Peninsula, we observed that the divergent pattern in the low-level circulation, and therefore the OLR anomaly, is high over the regions (Fig. 6b). The anomaly over the land mass region is more than 15 Wm-2 , indicating the absence of convection due to the divergence in the low-level circulation. However, during 2010, an abnormally high rainfall year, the low-level circulation anomalies are different from

those expected during the wet years (expecting intense LLJ to carry the moisture from the oceanic region to the land mass, Fig. 6c). Even though the strength of the LLJ is weak, the organized convection anomalies (OLR anomaly) are favorable over the peninsular region, Arabian Sea and the Bay of Bengal regions. The OLR anomaly is above -15 Wm-2 in all these regions. This organization of clouds is due to the convergent pattern of the low-level circulation and is clear from Fig. 5c. Due to the convergence in the low-level circulation fields causes rising motion (upward limb of Hadley circulation) and this also favours the increase of the tropospheric temperature. This increased TT maintains the organization of monsoon clouds over the region and thereby occurrence of abnormal rainfall. 3.7. Intraseasonal Oscillation of OLR During Contrasting Monsoons Here, we tried to explore the intraseasonal features of the organized convection (OLR anomaly) during

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Figure 6 Spatial pattern of LLJ and OLR for monsoon period for 2009 and 2010

two contrasting monsoon seasons. In this, we are attempting to the northward propagation of the OLR bands over the area averaged 72.5°E to 85°E. To get the correct signal for both the contrasting years, we filtered the data with a band-pass filter between 30 and 90 days. The oscillation features are clearly different for both the years (Fig. 7). For the 2009 southwest monsoon, the northward propagation of the convection bands are very distinct and clear with high magnitudes. The propagation speed from south to

north is also faster during the 2009 monsoon period. The northward propagation of the cloud bands starts from around the equatorial regions and covers the entire region within 15 days. However, during the wet year, the northward propagation is slower and does not occur in a well-organized manner, even though the propagation is present. The propagation starts from about 10°S and covers the entire region within about 30 days. The slow propagation of the 30–90 day band of organized cloud bands causes the persistent rainfall

Indian Summer Monsoon Rainfall Characteristics

Figure 7 Time latitude variation of OLR during 2009 and 2010 summer monsoon period. The OLR is subjected to 30–90 day band pass filter

over the entire regions of Indian subcontinent during 2010, and fast propagation causes short spells of rainfall, and therefore during 2009 the monsoon is abnormally below normal. It is reported that during weak monsoon periods, the 30–60 day mode of oscillation is strong and it is opposite in the intense monsoon year (KULKARNI et al. 2011). This result matches with the 30–60 day oscillations of summer monsoon years during 2009 and 2010. 3.8. Role of Convective Systems over the North West Pacific (NWP) KANAMITSU and KRISHNAMURTI (1978) found that the excess system days over the NWP was one of the

reasons for the deficient monsoon rainfall for the year 1972. They pointed out that when the NWP becomes convectively active with excess system days, intense atmospheric heating leads to a shift in the Tibetan anticyclone southeastwards and weakens the Indian summer monsoon. VINAY KUMAR and KRISHNAN (2005) showed that there is a greater tendency for the cyclones which from the NWP to recurve and move northwards during the deficit monsoon years. RAMESH KUMAR (2009) studied the characteristics of the convective systems such as frequency, geographical location, duration of the systems and the direction of movement of the systems over the NWP have been examined in relation to breaks in monsoon conditions over the Indian subcontinent during several

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contrasting monsoon years. They found that the lowlevel wind flow at 850 hPa was substantially more (less) and directed towards the Indian subcontinent (equatorial region) during the excess (deficit) monsoon years. Further, it was found that during the deficit years and prolonged breaks in monsoon conditions, more systems (about 69 %) formed further south than in the case of excess monsoon years. In order to look into the relative role of the convective systems over the Bay of Bengal and NWP, we analysed the data from the website (http://www.weather.unisys.com/hurricane) for the contrasting years 2009 and 2010. Our study showed that there were four (three) systems over the Bay of Bengal during 2009 (2010), respectively. Further analysis of the systems over the NWP, revealed that more systems formed over the NWP during a dry monsoon year (2009). Further, there were about 16 convective systems formed over this region, and the majority of them travelled in a northward or northeastward direction, whereas during the wet monsoon year (2010) only 11 convective systems formed over the NWP region, and most of them travelled in a westward or north-westward direction. Our results are thus in good agreement with the previous results of RAMESH KUMAR (2009), who found that convective systems were about 1.83 times greater over the NWP than the Bay of Bengal while analyzing the results for several contrasting monsoon years.

4. Summary and Conclusion Here, we studied the features of contrasting monsoons, of which one is dry (2009) and the other one is wet (2010). The 2009 monsoon is 23 % below normal and 2010 monsoon is 3 % above normal rainfall over the Indian subcontinent. During the dry monsoon, most of the rainfall contributed during the month of July and contribution of rainfall during other months is relatively low. During the wet year the contribution is about 35–45 % in the peak monsoon months (June and July) over the most parts of India. During 2009, the departure of rainfall from climatological pattern is below everywhere in the county except some parts in the western coastal

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belts during the months of July and September. In 2010, almost everywhere receives above-normal rainfall. The SST in the Pacific is crucial to regulating the ISMR during these two years, through the equatorial Walker and regional Hadley circulations. El Nin˜o is associated with dry monsoon (2009) and La Nin˜a is associated with wet monsoon (2010). In the 2009 dry monsoon, the sinking component of regional Hadley circulation is more prominent, and this creates divergence at a low level and thus produces the low values of tropospheric temperature. The low values of TT are unfavourable for organized convection. In the case of the 2010 wet monsoon, the rising limb of regional Hadley circulation is generated as a result of La Nin˜a, which helps the low-level wind to converge and thus results in organized convection through the heat flux from the enhancement of tropospheric temperature. The intraseasonal behavior of these convective cloud bands is also different during the two contrasting years. During the dry year, the northward propagation is too fast and may be due to the frequent evolution and propagation of monsoon surges, leading to the occurrence of active and break phases of the Indian summer monsoon. In contrast to the dry phase, during the wet phase the speed and magnitude of the northward propagating mode is slow and low. This indicates that during the wet phase, the rainy phase is prolonged and stagnant. Therefore, the movement of the northward propagating band is slow. The present studies of contrasting monsoon years are also in good agreement with earlier studies.

Acknowledgments The first author acknowledges the Director, Indian Institute of Tropical Meteorology, Pune, for support. Dr. M. R. Ramesh Kumar acknowledges the Director, National institute of Oceanography for support, and the third author is grateful to the Cochin University of Science and Technology for providing facilities and other required support. The data sets used in this study are properly acknowledged. Comments from the editor and reviewers helped greatly to improve the manuscript.

Indian Summer Monsoon Rainfall Characteristics

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(Received November 15, 2012, revised May 14, 2013, accepted June 12, 2013)