A STUDY ON THE EFFECT OF EURASIAN SNOW ON THE SUMMER MONSOON CIRCULATION AND RAINFALL USING A SPECTRAL GCM

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 26: 17 (26) Published online January 26 in Wiley InterScience ( DOI:.2/joc.1299 A STUDY ON THE EFFECT OF EURASIAN SNOW ON THE SUMMER MONSOON CIRCULATION AND RAINFALL USING A SPECTRAL GCM S. K. DASH, P. PARTH SARTHI* and S. K. PANDA Centre for Atmospheric Sciences, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 1 16, India Received 21 March Revised 29 September Accepted 11 November ABSTRACT Many studies based on observed data indicate the inverse relationship between the Eurasian snow cover/depth and the Indian summer monsoon rainfall (ISMR). The purpose of this study is to confirm the inverse snow ISMR relationship by using the observed snow depth data as boundary conditions in the spectral general circulation model (GCM) of Indian Institute of Technology, Delhi (IITD), and to examine the influence of Eurasian snow depth on the monsoon circulation. The original model belonging to the European Centre for Medium range Weather Forecasts (ECMWF) at resolution T21 has been modified extensively to a higher resolution of T8L18 at IITD. A two-dimensional Lanczos digital filter has been used to represent the orography realistically. The Historical Soviet Daily Snow Depth (HSDSD) version II data set has been used for conducting sensitivity experiments using the above model. Two sensitivity experiments have been designed, corresponding to two contrasting cases: one with high Eurasian snow depth in spring followed by deficient ISMR and the second with low snow depth followed by excess ISMR. The difference fields of mean monsoon circulation simulated in the above two experiments are examined in detail in order to confirm the influence of Eurasian snow depth on ISMR and to examine the Asian summer monsoon circulation and rainfall. Copyright 26 Royal Meteorological Society. KEY WORDS: spectral model; digital filter; snow depth; Indian summer monsoon rainfall; inverse relationship; sensitivity experiments 1. INTRODUCTION Earlier studies by several authors (Hahn and Shukla, 1976; Dickson, 1984; Sankar-Rao et al., 1996; Bamzai and Shukla 1999; Kripalani and Kulkarni, 1999), based on observed data, show the inverse relationship between the Indian summer monsoon and the Eurasian snow cover/depth in the preceding season. Bamzai and Shukla (1999) studied snowfall frequency data and confirmed that the correlation between the winter/spring snow cover anomaly and subsequent monsoon rains is statistically significant for the western Eurasia region only. Kripalani and Kulkarni (1999) did extensive work on the HSDSD version I data set for the period and showed that winter snow depth over western Eurasia has a significant negative relationship with subsequent monsoon rain, whereas the snow depth over eastern Eurasia has a positive relationship with monsoon rainfall. Kripalani et al. (1996) examined the relationship of Nimbus-7 snow mass over two locations of former USSR and ISMR for the period They also suggested the use of these connections in long-range forecasting of ISMR. Kripalani et al. (22) showed that winter/spring time snow depth over western Eurasia is negatively related, whereas the snow depth over eastern Eurasia (over Manchuria eastern Siberia) is positively related with Korean monsoon rainfall, and the low-level jet over the east Asian sector could be considerably influenced by the snow distribution over Eurasia. Sankar-Rao et al. (1996) using NOAA NESDIS data for the period concluded that, following the winters of more snow, stationary * Correspondence to: P. Parth Sarthi, Centre for Atmospheric Sciences, 6 th Block, IIT Delhi, Hauz Khas, New Delhi 1 16, India; pps@cas.iitd.ernet.in Copyright 26 Royal Meteorological Society

2 18 S. K. DASH, P. PARTH SARTHI AND S. K. PANDA perturbations with higher pressure over central Asia, north of India, are produced in the lower atmosphere, which weakens the following Asian summer monsoon. Simultaneously, in the upper atmosphere, a lowerpressure anomaly during summer weakens the upper-level monsoon high. The upper tropospheric anomaly low-pressure system covers a large area, extending from middle latitude to India. Liu and Yanai (22) used the Eurasian spring snow cover (ESSC) (March April) time series for the period of and snow cover ( ) and snow depth ( ) data from satellite observations. In their study, the influence of ESSC on the all-india monsoon (June September) rainfall and the summer rainfall over all parts of Asia are examined in detail. Comparative analyses for contrasting years with excessive and deficient snow cover show that the anomalies of ESSC occur mainly in northwestern Eurasia. In the years of excessive ESSC anomalies, cooling and a cyclonic circulation anomaly in the lower troposphere appeared over the northern part of Eurasia, which weakened east Asia summer monsoon and caused deficient rainfall. By observational studies, Ueda et al. (23) confirmed the earlier findings that the surface temperature decreased in May (usually the seasonal snow-disappearance period) and that there is a weak dynamical linkage between the east European plain snow cover from 3 to 6 E and Asian summer monsoon. By using General Circulation Models (GCMs), sensitivity experiments were conducted by several authors such as Barnett et al. (1989), Vernekar et al. (1995), etc. Their results clearly indicate that positive snow anomalies over Eurasia in winter/spring affect the monsoon circulation in the following summer in terms of weaker monsoon. However, GCM experiments conducted so far to examine the snow Indian summer monsoon rainfall (lsmr) relationship are idealistic in the sense that the actual observed snow depth/cover values were not used in the model as surface boundary conditions. Either some realistic snow depths were used uniformly over the entire region of experiment or the maximum observed values were prescribed in GCM experiments. On the basis of the observed data of Soviet snow depth data and Indian rainfall data for the period , Dash et al. () have studied the snow monsoon relationship and its links with midlatitude circulations. They have shown that, because of the west Eurasian snow depth anomalies, the midlatitude circulations in winter through spring undergo significant changes in the upper- and lower-level winds, geopotential height, velocity potential and stream function fields. Such changes in the large-scale circulation pattern may be interpreted as precursors to weak/strong monsoon circulation and deficient/excess ISMR. The objective of the present paper is to confirm the inverse snow monsoon relationship by using the actual observed values of snow depth in the IITD global spectral model. The characteristics of upper- and lower-level mean monsoon circulation and midlatitude circulations simulated under the influence of prescribed contrasting high/low snow depths over Eurasia will be discussed. 2. THE MODEL AND INITIAL DATA The original version of the model belonged to the ECMWF at horizontal resolution T21L5. Here, the letter T followed by the numeral stands for the maximum truncation limit in the triangular truncation scheme in the horizontal direction and the letter L followed by the number represents the number of sigma levels in the vertical. Earlier, the T21 model was successfully used for simulating circulation patterns over India (Dash and Chakrapani, 1989) and its higher resolution version T42L9 was used successfully for several sensitivity studies (Dash, 1995, 1999). The earlier model has currently been modified to higher resolution T8L18. The model is based on the spectral representations of nonlinear-coupled equations for momentum, thermodynamics, moisture, continuity and the hydrostatic relation. The basic prognostic variables of the model are wind components in two horizontal directions, vorticity, divergence, temperature, moisture and logarithm of surface pressure. Orography is also presented as a truncated series of spherical harmonics in the horizontal direction. The conventional finite difference scheme in sigma coordinates is used for the vertical descretization. The semi-implicit scheme is used for time integration. The model has equivalent grids in the horizontal, where climatological values of surface fields, such as, orography, surface pressure, sea-surface temperature (SST), soil temperature, soil moisture, albedo, roughness parameter and snow depth obtained from NCMRWF, are provided as input data for the model. Surface pressure is a prognostic parameter, Copyright 26 Royal Meteorological Society Int. J. Climatol. 26: 17 (26)

3 A STUDY ON THE EFFECT OF EURASIAN SNOW USING A SPECTRAL GCM 19 which changes with every time step along with other atmospheric parameters. Daily soil moisture interpolated from the monthly mean climatological values is prescribed to the model. The snow depth is interactive with surface energy balance. The climatological values of carbon dioxide and ozone are also used in the model. The details of the physics package are given in Dash and Chakrapani (1989). The data set of 5th April 1996, analyzed by the National Centre for Medium Range Weather Forecasting (NCMRWF), has been used as the initial data for model integration. It contains the spherical harmonics of vorticity, divergence, moisture and temperature at 18 vertical sigma levels and logarithm of surface pressure. The daily-observed SST data of 1996 obtained from the National Center for Environmental Prediction (NCEP)/National Centre for Atmospheric Research (NCAR) reanalysis have been used in one set of experiments and the climatological SSTs have been used in another. The HSDSD version II data over central Eurasia region for high and low snow depth years 1979 and 1975 respectively have been used in the sensitivity experiments. The mean orography, based on US Navy data, has been prescribed in the model along with the two-dimensional Lanczos filter (Navarra et al., 1994) in order to take care of the sharp discontinuities of the mountains. 3. EXPERIMENTAL DESIGN It is well known that the variation of surface boundary conditions, such as SST, snow depth, sea ice and soil moisture, plays a very important role in the seasonal simulations. In the present paper, climatological values of sea ice and soil moisture and daily-observed values of SST and snow depth are used for conducting sensitivity experiments. Since the objective of this paper is to examine the influence of annual variation of Eurasian snow depth on ISMR, in the control run, climatological values of global snow depth are used as surface boundary conditions in addition to observed SSTs. In the control run, the model integration started from 5th April 1996 using the NCMRWF analysis of atmospheric and surface fields as initial conditions. The model is integrated up to the end of September with daily-observed SST of NCEP/NCAR. Apart from the control run, two other model runs exp1 and exp2 have also been conducted. In both these experiments, daily climatological values of SSTs are used with a view to establish that the changes in the simulations are due to the variations in Eurasian snow depth prescribed to the model in the two years concerned. The HSDSD version II data set and rainfall data of IMD have been used to select two cases: a high snow depth year (exp1) followed by deficient ISMR and a low snow depth year (exp2) followed by excess ISMR. As described in the following paragraph, the years 1979 and 1975 suit the above cases, respectively. Using the April snow depth data for the years , the mean of the series and the standard deviations are calculated over Eurasia. The snow depth of each year for the period is expressed as a standardized snow depth anomaly by dividing the departure of each year from normal by standard deviation. The standardized snow depth anomaly considered for the Eurasian region for the period is shown in Figure 1 by dotted lines. The years having snow depth anomaly between ±1 standard deviation are considered as normal snow depth years. Similarly, the years having snow depth anomaly equal to or above +1 standard deviation are taken as high snow depth years and those having equal to or less than standard deviation snow depth anomaly are identified as low snow depth years. On the basis of this criterion, it is found that 1979 is the high snow depth year and 1975 is the low snow depth year. Similarly, the ISMR anomaly for each year has been computed and plotted in Figure 1 by solid lines. The years having ISMR anomaly more than or equal to +1 standard deviation are termed as excess monsoon years and those less than or equal to standard deviation are considered as deficient monsoon years. The rest of the years are categorized as normal monsoon years. Figure 1 confirms the inverse relationship between the Eurasian snow depth anomaly and the ISMR deviation. The HSDSD version II snow depth data prescribed in the model experiments pertain to the domain N and E. Beyond this domain, the observed snow depth values are very small and hence are not included in the model experiments. The difference in snow depth between the high snow depth year (1979) and the low snow depth year (1975) over Eurasian region is shown in Figure 2. The central part of the region is showing the large difference of snow depth in two years. It is assumed that the interannual variability in the Copyright 26 Royal Meteorological Society Int. J. Climatol. 26: 17 (26)

4 2 S. K. DASH, P. PARTH SARTHI AND S. K. PANDA Standardised Anomaly ISMR Apr Snow Years Figure 1. Standardized snow depth anomalies over Eurasia in the month of April (Apr snow) and Indian summer monsoon rainfall (ISMR) for the period N 64N 62N 6N 58N 56N 54N 52N 52E E 56E E E E 64E 66E Figure 2. Difference (exp1 exp2) of depth (cm) between the high snow depth year 1979 and low snow depth year 1975, based on HSDSD version II data set ( ) prescribed to the model as surface boundary conditions global circulation and rainfall simulated by the model will be the result of this difference in the snow depth over Eurasia prescribed in the model since observed SSTs are used in the control run as well as in exp1 and exp2. The model integration of six months, starting from 5th April up to 3th September, resulted in the output fields, such as wind, temperature, moisture and geopotential, at 18 vertical sigma levels. Surface pressure and accumulated rainfall are also obtained. The seasonal mean of June, July, August and September (JJAS) of the above fields at 18 pressure levels is computed. The differences of (exp1 exp2) of seasonal mean JJAS fields are examined to study the impact of high/low Eurasian snow depth on monsoon circulation. In order to save space, only those fields are depicted in the diagrams, which are relevant to the present discussion. 4. RESULTS AND DISCUSSION Bamzai and Shukla (1999) emphasized that the inverse snow monsoon relationship holds especially in those years when snow is anomalously high or low for both the winter as well as the consecutive spring season. Copyright 26 Royal Meteorological Society Int. J. Climatol. 26: 17 (26)

5 A STUDY ON THE EFFECT OF EURASIAN SNOW USING A SPECTRAL GCM 21 If there is heavy snow cover in winter, it is likely that it will affect the snow cover in spring. Heavy snow cover in midwinter usually does not easily melt because of low-level solar insolation. The difference in the circulation characteristics in high snow depth (1979) and low snow depth (1975) years is studied in detail by analyzing wind in the mean of June, July, August and September (JJAS) winds separately. Figure 3(a) shows the difference (exp1 exp2) in the temperature at 85 hpa as simulated by the model. For comparison, the corresponding difference field of NCEP/NCAR reanalysis is given in Figure 3(b). The model simulation (Figure 3(a)) shows that Eurasia is cooler by a maximum of 1 C in the high snow depth year compared to the low snow depth year, whereas east Asia is cooler by a maximum of 2 C. A comparison of Figure 3(a) and (b) shows that a similar cooling anomaly with little higher intensity and more spread is seen in the NCEP/NCAR reanalysis (Figure 3(b)). 7N.5 6N 5N 4N 3N 2N N (a).5.5 E 2E 3E.5 4E 5E 6E 7E 8E E E 1E 12E 13E 14E 15E 7N 6N N 4N 3N N N (b) E 2E 3E 4E 5E 6E 7E 8E 9E E 1E 12E 13E 14E 15E Figure 3. Difference in the seasonal (JJAS) mean temperature ( C) at 85 hpa (a) simulated (exp1 exp2) by the model in response to snow depth anomaly as given in Figure 2 and (b) based on NCEP/NCAR reanalysis ( ). The superposed rectangular box at the top of the figure (a) depicts the region over which snow depth is prescribed in the model. The outermost contour of snow depth of cm is also shown inside the box Copyright 26 Royal Meteorological Society Int. J. Climatol. 26: 17 (26)

6 22 S. K. DASH, P. PARTH SARTHI AND S. K. PANDA Figure 4(a) shows the difference (exp1 exp2) in the seasonal wind field at 85 hpa as simulated in the model in response to the snow depth anomaly. For comparison, the corresponding difference field of NCEP/NCAR reanalysis is given in Figure 4(b). A well-organized anomalous cyclonic circulation over central Eurasia is noticed by the model simulation shown in Figure 4(a). The model simulates the anomalous westerly to the south of Caspian Sea, which implies that the westerly is stronger in the high snow depth case than in the low snow depth case. A similar anomalous circulation spread over a larger area persists in the midlatitude belt between 4 and 7 N in the NCEP/NCAR reanalysis shown in Figure 4(b). Both figures show an anomalous anticyclonic circulation to the north of Arabian Sea, which leads to anomalous easterlies over the Arabian Sea during monsoon season. This implies that Asian summer monsoon lower-level westerlies are weaker in exp1 in response to high Eurasian snow depth than in exp2, where there is low snow depth. Figure 4(b) indicates a similar response of the low-level monsoon wind to the Eurasian snow depth by the model. 7N 6N 5N 4N 3N 2N N (a) E 2E 3E 4E 5E 6E 7E 8E 9E E 1E 12E 13E 14E 15E 5 7N 6N 5N 4N 3N 2N N (b) E 2E 3E 4E 5E 6E 7E 8E 9E E 1E 12E 13E 14E 15E 5 Figure 4. Same as in Figure 3, except for wind (m/s) at 85 hpa Copyright 26 Royal Meteorological Society Int. J. Climatol. 26: 17 (26)

7 A STUDY ON THE EFFECT OF EURASIAN SNOW USING A SPECTRAL GCM 23 Figure 5(a) and (b) show the difference (exp1 exp2) in upper-level monsoon mean wind fields simulated by the model and by NCEP/NCAR reanalysis respectively. In Figure 5(a), anomalous cyclonic circulation is simulated by the model over central Eurasia and an anomalous weak anticyclonic circulation is simulated over Bay of Bengal and south east Asia. The NCEP/NCAR reanalysis shows that the anomalous cyclonic circulation over central Eurasia in Figure 5(b) is spread over a larger area compared to that in Figure 5(a). The anomalous westerlies over Arabian Sea are similar in both NCEP/NCAR reanalysis and model simulation. Both from model results and analyzed fields, it seems that such anomalous features suppress the development of upper tropospheric anticyclones and hence the monsoon circulations over India. In Figure 5(a) and (b), the upper-level easterlies over the Arabian Sea and Indian peninsula are similar. These results indicate that Asian summer monsoon easterlies are weaker in exp1 in response to high snow depth than in exp2, where the snow depths are less. 7N 6N 5N 4N 3N 2N N (a) E 2E 3E 4E 5E 6E 7E 8E 9E E 1E 12E 13E 14E 15E 7N 6N 5N 4N 3N 2N N (b) E 2E 3E 4E 5E 6E 7E 8E 9E E 1E 12E 13E 14E 15E Figure 5. Same as in Figure 3, except for wind (m/s) at 2 hpa Copyright 26 Royal Meteorological Society Int. J. Climatol. 26: 17 (26)

8 24 S. K. DASH, P. PARTH SARTHI AND S. K. PANDA 7N 6N 5N 4N 3N 2N N E 2E 3E 4E 5E 6E 7E 8E 9E E 1E 12E 13E 14E 15E Figure 6. Differences (exp2 exp1) in the seasonal (JJAS) mean rainfall (cm) as simulated by the model in response to the snow depth anomaly. The superposed rectangular box at the top of the figure depicts the region over which snow depth is prescribed in the model. The outermost contour of snow depth of cm is also shown inside the box Figure 6 shows the difference (exp2 exp1) in Asian summer monsoon rainfall simulated by the model in response to the snow depth anomaly, as indicated in Figure 2. The model-simulated difference in rainfall shows a positive anomaly, meaning that there is an increase in rainfall in response to less snow depth. The anomalies in the Asian monsoon circulation may be responsible for the variation in associated monsoon rainfall. A zone of rainfall difference extends from the west coast of India to northeast India and up to central China with the maximum value ranging between 15 and 2 cm. It appears that the anticyclonic circulation at 85 hpa over Tibet may be responsible for the deficit of rainfall over Asia in the high snow depth year. The accurate magnitude of the rainfall difference and its spatial distribution is beyond the scope of the present paper. 5. CONCLUSIONS In this paper, a modest attempt has been made to use the actual observed snow depth values over Eurasia in April as boundary conditions in the spectral GCM of IITD at resolution T8L18 and integrate the model for six months in order to examine its influence on the monsoon wind and associated rainfall. Contrasting years of snow depth over Eurasia followed by contrasting ISMR are selected to design the model experiments. The model-simulated mean monsoon circulation features for high and low snow depth years have been compared with the corresponding years of NCEP/NCAR reanalysis. Model simulation as well as NCEP/NCAR reanalysis indicate the evolution of weak/strong monsoon circulation from the midlatitude circulation in response to high/low Eurasian snow depth in April. Cooling over the Caspian Sea by about 1 C due to high snow depth might be responsible for the weak Asian monsoon circulation and deficient rainfall. At 85 hpa, the difference in wind fields in high and low snow depth years shows an anomalous anticyclonic circulation to the north of Arabian Sea and the western sector of India, which is responsible for weak easterlies over the Indian subcontinent. The difference in wind fields at 2 hpa shows anomalous cyclonic circulation over south of the Caspian Sea, which might have contributed to the anomalous easterlies over the Arabian Sea and the Indian subcontinent. Results show that excess of summer monsoon rainfall over the Asian region corresponds to low April snow depth over the Eurasia region. The reverse happens in case of high Eurasian snow depth. More sensitivity experiments are being designed in order to understand the details of the physical processes Copyright 26 Royal Meteorological Society Int. J. Climatol. 26: 17 (26)

9 A STUDY ON THE EFFECT OF EURASIAN SNOW USING A SPECTRAL GCM through which the Eurasian snow depth influences the midlatitude circulation features and subsequently the monsoon circulation over the Asian region. ACKNOWLEDGEMENTS The results reported in this paper are the outcome of a project sponsored by the Department of Science and Technology, Government of India, to Prof. S.K. Dash as Principal Investigator. The snow depth data are obtained from HSDSD CD of National Snow and Ice Data Center (NSIDC) and the observed rainfall data are obtained from the India Meteorological Department. REFERENCES Bamzai AS, Shukla J Relation between Eurasian snow cover, SD and the Indian summer monsoon: An observational study. Journal of Climate 12: Barnett TP, Dumenil L, Schlese U, Roeckner E, Latif M The effect of Eurasian snow over on regional and global climate variations. Journal of the Atmospheric Sciences 46: Dash SK Impact of orographic representation on Indian summer monsoon simulation. Published in TMRP Report No. 52, WMO/TD-No., 698. IMD; New Delhi; , 3 Jan-3 Feb 1995, Proceedings of the Fifth WMO/IMD Regional workshop on Asian/African monsoon. Dash SK Realistic representation of the Himalayas in numerical models, Published in the book The Himalayan Environment, Dash SK, Bahadur J (eds). New Age International: IIT Delhi; 79 91, Proceedings of the DST sponsored brain storming seminar on Himalayan experiment, Dec Dash SK, Chakrapani B Simulation of a winter circulation over India using a global spectral model. Proceedings of the Indian Academy of Sciences Section 98: 189, (Earth Planet Science). Dash SK, Singh GP, Shekhar MS, Vernekar AD.. Response of the Indian summer monsoon circulation and rainfall to seasonal snow depth anomaly over Eurasia. Climate Dynamics 24: 1. Dickson RR Eurasian snow cover versus Indian monsoon rainfall An extension of the Hahn-Shukla results. Journal of Climate and Applied Meteorology 23: Hahn DJ, Shukla J An apparent relation between Eurasian snow cover and Indian monsoon rainfall. Journal of the Atmospheric Sciences 33: Kripalani RH, Kulkarni A Climatology and variability of historical Soviet snow depth data: some new perspectives in snow Indian monsoon tele-connection. Journal of the Atmospheric Sciences 15: Kripalani RH, Singh SV, Vernekar AD, Thapliyal V Empirical study on Nimbus-7 snow mass and Indian summer monsoon rainfall. International Journal of Climatology 16: Kripalani RH, Singh SV, Vernekar AD, Thapliyal V. 22. Relationship between Soviet snow and Korean rainfall. International Journal of Climatology 22: Liu X, Yanai M. 22. Influence of Eurasian spring snow cover on Asian summer rainfall. International Journal of Climatology 22: Navarra N, Stern WF, Miyakoda K Reduction of Gibbs oscillation in Spectral model simulations. Journal of Climate 7: Sankar-Rao M, Lau MK, Yang S On the relationship between Eurasian snow cover and the Asian summer monsoon. International Journal of Climatology 16: Ueda H, Shinoda M, Kamahori H. 23. Spring northward retreat of Eurasian snow cover relevant to seasonal and interannual variations of atmospheric circulation. International Journal of Climatology 23: Vernekar AD, Zhou J, Shukla J The effect of Eurasian snow cover on the Indian monsoon. Journal of Climate 8: Copyright 26 Royal Meteorological Society Int. J. Climatol. 26: 17 (26)