Role of Mid-Latitude Westerly Trough Index at 500 h Pa and its Association with Rainfall in Summer Monsoon over Indian Region

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1 Role of Mid-Latitude Westerly Trough Index at 500 h Pa and its Association with Rainfall in Summer Monsoon over Indian Region S.S. Dugam and S. D. Bansod Indian Institute of Tropical Meteorology, Pune , India dugam@tropmet.res.in Abstract In this paper, quantification of Mid- Latitude Westerly Trough Index called as (MWTI) is derived by using the Principal Component Analysis (PCA) technique on daily basis, over the Indo-Pak region (20 N-45 N; E) and over North West Indian region (15 N-25 N; E) for the period The gradient between (PC1+PC2) time series of daily geo potential field of the first region and second region is calculated during the Indian summer monsoon period (June- September) and these index referred as Mid- Latitude Westerly Trough Index (MWTI).The relationship between the daily MWTI and daily rainfall anomalies in active and break Indian summer monsoon have been studied by the correlation analysis. From the analysis, it is observed that variation of MWTI during the break and active monsoon period shows an inverse relation with rainfall activity. Study was further carried out the relationship between daily North Atlantic Oscillation Index (NAOI) and MWTI in same period. The analysis suggested that during the weak phase and active phase of monsoon the relationship between NAOI and MWTI is an inverse and statistically significant. This relationship is concurrent and may be useful for explaining the role of NAO activity and intrusion of mid-latitude troughs over the Indian region, which affects the rainfall variability during the monsoon period. Key words: Mid- Latitude Westerly Trough, summer monsoon rainfall, North Atlantic Oscillation Introduction Anomalous southward intrusion of troughs in the mid-latitude westerlies into the Indo-Pak region in the middle and upper troposphere plays an important role summer monsoon activity over India. Ramaswamy (1956) from the analysis of synoptic charts established that intrusion of large-amplitude trough from middle-latitudes in to North India was a definite feature in large-scale break monsoon conditions over the Indian sub-continent. Synoptic charts illustrating these types of flow patterns were also published by Ramaswamy (1958). Mid latitude interaction and monsoon variability is studied by number of researchers viz. Ramaswamy (1962); Ramamurthy (1969); Raghavan (1973); Keshavamurty and Awade (1974); Asnani and Awade (1978); Ramaswamy and Pareekh (1978); Banerjee et al. (1978); Sikka (1980); Raman and Rao (1981); Charney and Shukla (1981); Yasunari (1986); Chattopadhyay and Bhatla (1994a,1994b); Joseph and Srinivasan (1999); Kripalani et al. (1997); Annamalai and Slingo (2001); Goswami and Prince Xavier (2003); Annamalai and Sperber ( 2005); Bhat ( 2006); Ding and Wang ( 2007); Rajeevan et al. (2008); Krishnan et al. ( 2009). 49

2 Real time monitoring of rainfall distribution on daily basis is required to evaluate the progress and status of monsoon and to initiate necessary action to control drought/flood situations. Long intense breaks are often associated with poor monsoon seasons, and they have a large impact on rain fed agriculture (Gadgil and Joseph, 2003). The active and break rainfall activity over the Indian region is very important from the agriculture point of view. In the recent years of 2002 and 2004, July rainfall was largely deficient rainfall departure being -49% and -19% respectively due to prolonged break in July. However, no dynamical and statistical models have estimated such a large deficient rainfall anomaly. Therefore, it is necessary to search the atmospheric signal to give some idea about the sub-seasonal rainfall activity over Indian region well in advance. Inter-annual variability of monsoon rainfall is related with the intra-seasonal variability of rainfall (Sperber et al., 2000; Goswami and Mohan, 2001). Even though monsoon is normal, excess and deficient still it have a break or active phases of rainfall activity. But during deficient or excess monsoon year it is not clear that, when these break and active conditions prevails during the monsoon period. Therefore, considering this point the composite analysis of MWTI and rainfall for flood and drought monsoon years have been also carried out. In this paper an attempt is made to quantify the mid-latitude trough index to explain variability of summer monsoon rainfall over India. Even though it was established the role of intrusion of mid-latitude trough and Indian summer monsoon variability by earlier studies. Data and Method The daily grid point ( 2.5 by 2.5 ) geo potential height data over the Indo-Pak region, (20-45 N; E) and North West Indian region (15 N-25 N; E) at 500 h Pa for the period have been taken from ER40 reanalysis from the website nomad3.ncep.noaa.gov. The daily North Atlantic index (NAOI) is taken from website Daily All India Monsoon Rainfall (DAIMR) are taken from daily gridded rainfall data published by Indian Metrological Department (IMD) based on 1803 stations (Rajeevan et al., 2006) for the same period. The simple moving average and correlation technique is used for the analysis. The MWTI Index is developed using the daily grid point (2.5 by 2.5 ) data of geo potential height at 500hPa taken from ERA40 analysis over the region mentioned above using the Principal Component Analysis (PCA). The daily time series of index have been constructed by (combining the PC1+PC2). The variance explained by PC1 plus PC2 is 95.15%. The daily gradient anomalies time series of geo potential height at 500 h Pa are constructed, from the difference between the regions mentioned above for the period

3 (a) (b) (c) Fig.1: (a) Daily variation of mid-latitued Westerly trough Index and rainfall anomalies for 1972; (b) Daily variation of mid-latitued Westerly trough Index and rainfall anomalies for 1979; (c) Daily variation of mid-latitued westerly trough index and rainfall anomalies for

4 (a) (b) Fig.2: (a) Composite of daily Mid-latitude trough index and rainfall for flood monsoon years; (b) Composite of daily Mid-latitude trough index and rainfall for drought monsoon years. Discussion Relationship between MWTI and daily rainfall anomalies: The daily MWTI and rainfall anomalies over the Indian region are plotted together during the monsoon period for drought years 1972, 1979 and normal monsoon year 1997 (Fig. 1a, b and c). From this analysis it is observed that the variation in MWTI and rainfall anomalies are in phase i.e. when gradient between these two region is negative indicating the intrusion of mid-latitude trough, then the rainfall anomalies are negative and vice versa. When compared with this variation in active and break monsoon period it shows the prominent and significant variation and the correlation coefficient between rainfall during break monsoon period and MWTI is (significant at 0.1% level). The composite analysis of daily variation of MWTI and rainfall anomalies is carried out for the extreme drought (1972, 79 and 87) and 52

5 flood (1983, 88 and 94), monsoons (i.e. when the rainfall anomalies less than one stander deviation called it as drought and vice versa). Fig 2a and b shows the variation of MWTI and rainfall in drought and flood monsoon years. It is seen that variations in MWTI and rainfall anomalies are in same phase. North Atlantic Oscillation and MWTI relationship: The term North Atlantic Oscillation (NAO) was originated by Sir Gilbert Walker in the 1920's (Walker, 1924). The NAO index is actually defined by the anomalies in pressure differences recorded between Ponta Delgada (Azores) and Stykkishólmur (Iceland), or, with a slight modification, between Lisbon (Portugal) and Stykkishólmur (Iceland). The NAO, is the name given to the anomalies occurring in the average pressure difference between these two areas (Hurrell, 1995). During a positive NAO there is a strengthening of the Icelandic low and Azores high. This strengthening results in an increased pressure gradient over the North Atlantic, which cause the westerlies to increase in strength. During a negative NAO there is a weakening of both the Icelandic low and Azores high, which act to decrease the pressure gradient across the North Atlantic. This decreased pressure gradient results in a slackening of the westerlies. When the difference is greater than normal, the index is positive and the winds from the west are more intense. These sea level pressure changes are a reflection of what is going on in the mid and upper levels of the atmosphere. The NAO has been found to work on a variety of time scales from weekly to monthly all the way to decadal (Baldwin and Dunkerton, 2001). To see the effect of daily variation of NAO activity on MWTI index, the simple correlation between daily NAO index (NAOI) and MWTI during monsoon period is carried out. It is seen that, the relationship between them is an inverse (direct) in break monsoon condition and in active condition respectively. This suggested that during the low index phase (i.e. when the NAOI anomalies negative in general called as low index phase) the intrusions of mid-latitude troughs are more frequent in break monsoon situation and vice versa (Fig. 3). Probable physical explanation may be given on the basis of earlier study by Krishnan et al. (2009) according to them the intrusions of mid-latitude troughs bring the cold air mass from mid-latitude and decrease of meridional thermal gradient over west-central Asia, Indo- Pak region and northern India subsequently affects the monsoon flow and subdue the rainfall activity over the Indian region (Fig. 4). Conslusions In this study the daily variation of MWTI and rainfall anomalies over India has been examined. Based on this study the following conclusions are drawn: The variations of MWTI and rainfall anomalies over India is in phase in active and break monsoon conditions, irrespective of whether the monsoon is drought or flood. Daily variability of North Atlantic Oscillation index and daily MWTI is showing the inverse relationship in drought and flood monsoon years. 53

6 Fig.3: Daily variation of NAO and mid-latitude westerly trough Index for drought years. Fig.4: Temperature anomalies over west-central Asia, Indo-Pak region and northern India in the middle and upper troposphere during the break monsoon condition. Acknowledgement: Authors are grateful to Prof. B. N. Goswami, Director, IITM, Dr. P.N.Mahajan, Head, Forecasting Research Division for encouragement and guidance and Department of Science and Technology (DST), Government of India for providing necessary facilities. 54

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8 Raman, C. R. V. and Rao, Y. P. (1981) Blocking highs over Asia and monsoon droughts over India. Nature, v. 289, pp Ramaswamy, C. (1962) Breaks in the Indian summer monsoon as a phenomenon of interaction between the easterly and the subtropical westerly jet streams. Tellus, v. 14A, pp Ramaswamy, C. and Pareek, R. S. (1978) The southwest monsoon over India and its teleconnections with the middle and upper tropospheric flow patterns over the Southern Hemisphere. Tellus, v. 30, Sperber, K. R., Slingo, J. M. and Annamalai, H. (2000) Predictability and the relationship between sub-seasonal and interannual variability during the Asian summer monsoon. Quart. J. Roy. Meteor. Soc., v. 126, pp Sikka, D. R. (1980) Some aspects of the large-scale fluctuations of summer monsoon rainfall over India in relation to fluctuations in the planetary and regional scale circulation parameters. Proc. Indian Acad. Sci. (Earth and planetary Science), v. 89, pp Yasunari, T. (1986) Low-frequency interactions between the summer monsoon and the northern hemisphere westerlies. J. Meteorol. Soc. Jn., v. 64, pp Walker, G.T. (1924) Correlation in seasonal variation of weather. IX. Mem. Indian Meteor. Dep., v. 24, pp

Use of Interactions between NAO and MJO for the Prediction of Dry and Wet Spell in Monsoon Season

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