eurasian snow cover and seasonal forecast o. s. r. u. bhanu...

Hydrologtcal Sciences - Journal - des Sciences Hydrologiques, 33, 5, 10/1988

Eurasian snow cover and seasonal forecast of Indian summer monsoon rainfall

O. S. R. U. BHANU KUMAR Department of Meteorology, University of Nairobi, PO Box 30197, Nairobi, Kenya

Abstract This paper presents the relationship between Indian summer monsoon total rainfall and two parameters from Eurasian snow cover, one being the winter snow cover extent and the other the area of spring snowmelt. Satellite-derived Eurasian snow cover extent and Indian monsoon rainfall data were obtained from the NOAA/NESDIS and the India Meteorological Department (IMD) for the period 1966-1985. Seasonal cyclic variations of snow cover showed a higher swing in both the winter and the spring seasons of the cycle as compared to the remaining seasons of the year in the lower region of the cycle. The established inverse relation between winter snow cover and monsoon rainfall during June to September is further extended. Winter snow cover is very strongly correlated with spring snowmelt over Eurasia. Spring snowmelt area is obtained by subtracting the May snow cover extent from that of the previous February. The variations of spring snowmelt were also compared with Indian total monsoon rainfall. The detected correlation is stronger between snowmelt and monsoon rainfall than between the winter snow cover and the monsoon rainfall. There is also a significant multiple correlation among winter snow cover, spring snowmelt and monsoon rainfall. Lastly, a significant multiple correlation suggested a multiple regression equation which might improve the climatic prediction of monsoon rainfall over India.

Couverture neigeuse sur l'Eurasie et prevision saisonnière des pluies de la mousson d'été aux Indes

Résumé Cet article présente la relation entre la hauteur totale de précipitations de la mousson d'été aux Indes et deux paramètres de la couverture neigeuse sur l'Eurasie; l'une étant l'étendue de la couverture neigeuse en hiver et l'autre la surface affectée par la fonte des neiges au printemps, l'étendue de la couverture neigeuse sur l'Eurasie et les données sur les précipitations de la mousson ont été fournies par la NOAA/NESDIS et le Service Météorologique de l'Inde (IMD) pour la période 1966-1985. Les variations saisonnières cycliques montrent des variations plus importantes sur les

Open for discussion until I April 1989 515

0. S. R. U. Bhanu Kumar 516

parties du cycle concernant l'hiver et le printemps que pour le reste de l'année dans la partie basse du cycle. La relation inverse entre couverture neigeuse hivernale et pluie de mousson de juin à septembre est étudiée plus à fond. La couverture neigeuse hivernale sur l'Eurasie est très étroitement corrélée avec la fonte des neiges de printemps. La surface affectée par la fonte des neiges est obtenue en soustrayant la surface de la couverture neigeuse de mai de celle du mois de février précédent. Les variations de cette surface affectée par la fonte des neiges ont aussi été comparées avec celles de la hauteur totale de précipitation de mousson aux Indes. La corrélation ainsi mise en évidence est plus forte entre surface de fonte des neiges et pluie de mousson qu'entre cette même pluie et la couverture neigeuse hivernale. Il y a aussi une corrélation multiple significative entre couverture neigeuse d'hiver, fonte des neiges de printemps et pluie de mousson. Enfin une corrélation multiple pourrait améliorer la prévision climatique des hauteurs de pluie de mousson d'été sur l'Inde.

INTRODUCnON

Snow cover plays a very crucial role in modulating the seasonal and inter-annual variations of climate because the variations of time scale of the snow cover extent are considerably larger than those of the atmosphere. Recently meteorologists and climatologists, having satellite-derived data on snow cover, have shown that snow cover has a profound influence on surface and air temperatures, storm tracks, global circulation patterns, radiation balance, precipitation etc. (Kukla, 1981; Walsh & Ross, 1986; Namias, 1985). Most of these studies were related to the North American continent but a few have been made on the influence of Eurasian snow cover upon the summer monsoon. The snow cover extent over Eurasia forms over 60% of the Northern Hemisphere snow cover. Hahn & Shukla (1976) observed an apparent relationship between the satellite-derived Eurasian snow cover in winter and the Indian monsoon rainfall, based on the WMO (1975) recommendations. Later, Dickson (1984) extended the above study and found the same inverse relationship. Dey & Bhanu Kumar (1983) also correlated the Himalayan winter snow cover with the Indian monsoon rainfall and found a significant negative correlation. This latter study supported the work of both Blanford (1884) and Walker (1910) who used point-source snow data. All these investigations showed a consistent inverse correlation between the amount of Eurasian/Himalayan snow cover in winter and the ensuing summer monsoon rainfall. That is, the snow cover of the previous season may be related to the current synoptic flow patterns through time-lag mechanisms. Generally, such mechanisms provide an opportunity for seasonal forecasts. The World Climate Programme and the US National Climate Programme have laid great stress on the climatic disasters in tropical regions and have declared climate prediction to be a central objective.

517 Eurasian snow cover and seasonal forecast

All the above studies related to Eurasian/Himalayan snow cover examined the effect of fresh winter snow cover on monsoon rainfall, but the influence of the important snowmelt phenomenon in the spring season has not yet been widely investigated. In this paper, an attempt has been made to investigate the effects of both winter snow cover and the area of spring snowmelt in Eurasia on the Indian total monsoon rainfall. A multiple regression equation has been developed with the aim of providing a possible seasonal forecast of monsoon rainfall. A better seasonal forecast of Indian monsoon rainfall is of paramount importance because the monsoon gives 75-90% of the yearly total rainfall over India. This study is not only useful to the seasonal forecast of monsoon rainfall but also to subsequent hydrological studies.

DATA AND RESEARCH METHODOLOGY

NOAA/NESDIS Northern Hemisphere snow and ice charts from 1966 to 1985 were used for snow cover data. The quality of the data is not the same throughout the period, with more accuracy being achieved in the more recent charts. Monthly mean snow cover data were measured by using the technique developed by Dewey & Heim (1981). These values of snow cover extent are coincident with those of Matson et al. (1986). Indian summer monsoon total rainfall data for the above period are available from the Monsoon Rainfall Summary Reports, a supplement of the IMD and from the study of Mooley & Parthasarathy (1984). The details of rainfall series and statistics are discussed by the above authors.

The winter season snow cover extent is taken as the mean of the monthly December to February mean snow covers. The departures from the mean of 19 years were calculated and are presented in Table 1. The spring snowmelt area is derived by subtracting the May snow cover extent from that of February. The deviations from the average for the study period were obtained and are shown in the same table. The Indian total summer monsoon rainfall is the sum of total rainfalls during June through to September. Monsoon rainfall is an indication of monsoon strength. The departures from the mean for the study period were also evaluated and are presented in Table 1. Synoptic correlations and lag correlations were computed for the above parameters. Finally a multiple regression equation was developed for use as a possible method of estimating a seasonal forecast of monsoon rainfall over India.

EURASIAN SNOW COVER

The monthly mean snow cover extent over Eurasia during the study period varied very drastically from February to August, with a yearly mean of about 15 x 106 km2. Seasonal snow cover variations with respect to annual variations are presented in Fig. 1. That diagram shows a marked cyclic pattern and it is notable that there is a higher variability in the upper part of

O. S. R. U. Bhanu Kumar 518

the cycle (winter and spring seasons) as compared to that of the lower part of the cycle (summer and autumn seasons). Because of the higher swing in the upper part of the cycle, the winter and spring seasons may have a significant interaction with the ensuing summer monsoon circulation. The

Table 1 Departures from the mean for 1967-1985 for Eurasian winter snow cover (December-February), spring snowmelt (February-May) and Indian summer monsoon rainfall (June-September)

Year Departures from 19-year mean: Eurasian winter Spring snowmelt Total monsoon snow cover extent rainfall (106km2) (106km2) (cm)

1967 1968 1969 1970 1071 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985

-1.87 2.03

-1.87 -3.57 -0.77

2.53 1.43 0.83

-1.27 0.23 0.73 3.53 2.43

-0.67 -2.77 -0.27

0.13 -1.07

0.32

-0.07 5.83

-3.77 -2.77 -0.57

4.93 0.73

-0.67 -0.07 -3.37 -2.37

2.33 0.53

-0.17 -2.47

0.63 0.33 0.63 0.33

1.8 - 8.7 - 1.2

9.8 4.5

-18.8 7.1

- 9.4 11.9

1.4 4.0 6.7

- 9.5 4.0 0.4

-10.4 12.3

- 0.6 - 6.2

Sources: Northern Hemisphere snow cover charts, NOAA, and monsoon rainfall summary (a supplement oflMD and Climatic Change vol. 6 (1984)).

Eurasian monthly snow cover area is smoothed by taking a 12-month running mean (Fig. 2). Snow cover was very much below the long-term yearly mean during the period 1967-1971, while it was above that mean from 1972 to 1979 except in 1975. In the later years, snow cover variations were close to the long-term mean. It is interesting to note that the snow cover area was above the yearly mean in the years of persistent drought (1972, 1974 and 1979), while it was below the yearly mean in the years of excessive rainfall and floods (1970, 1975 and 1983) over India. But the deficient rainfall year 1982 was not above the yearly mean. This may be due to the quality of data which is not the same all through the period.


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EURASIAN WINTER SNOW COVER AND MONSOON RAM OVER INDIA

Both winter snow cover and total rainfall showed an increasing trend with time but they were not significant. The winter snow cover extent fluctuated from 3.6 million km2 below the long-term mean in 1970 to 2.4 million km2 above in 1979. The standard deviation about the long-term mean of the snow cover during the study period is 1.9 million km2. Summer monsoon rainfall over India, which is variable in space and time, has a standard deviation of 8.4 cm. Departures of total monsoon rainfall from the mean of the study period were maximum in 1975 and minimum in 1972 (Table 1). Deviations of snow cover extent with departures from the long-term mean of total monsoon rainfall were compared and are presented in Fig. 3 (note the reversed scale for the rainfall data). There is a striking inverse relationship between the two quantities which implies that greater-than-mean Eurasian winter snow cover was associated with less-than-mean monthly rainfall. The negative correlation coefficient (r) between them is 0.4 which is significant at the 10% level of confidence. This result agrees with the works of both Dickson (1984) and Dey & Bhanu Kumar (1983). A large snow cover anomaly over Eurasia/Himalayas apparently produces colder tropospheric temperatures in the ensuing spring. This leads to a delay in the onset of the monsoon and, in general, a rainfall deficiency over India (Bhanu Kumar, 1987).

° I i i i i i i i i i i i i • i » i i g 1967 1959 1971 1973 1975 1977 1979 1981 1983 1985

Pig. 3 Variations of Eurasian winter snow cover area and the corresponding summer monsoon rainfall over India.

EURASIAN SPRING SNOWMELT AND MONSOON RAINFALL

The extent of Eurasian snowmelt, which is a dominant phenomenon in the


spring, is very closely correlated with the previous winter snow cover (r = 0.6). This correlation between them means that there would be more snowmelt during March through to May in the years of excessive winter snow cover. The maximum snowmelt was in 1972 and the minimum in 1976 (Table 1). The standard deviation of snowmelt area is 2.4 million km2. Individual Eurasian spring snowmelt departures were compared with summer monsoon

T i i i 1 r~ 1 r

I 1 1 1 s 1 i , i i 1967 1969 197) 1973 1975 1977 1979 1981 1983 1985

Fig. 4 Variations of Eurasian spring snowmelt area and the corresponding summer monsoon rainfall over India.

rainfall and the relationship is presented in Fig. 4. There is a conspicuous negative relationship between these two parameters as there is between winter snow cover and monsoon rainfall. The coefficient of correlation (r) between them is -0.44 which is stronger than that of winter snow cover and monsoon rain. However, there were five out of 13 cases which showed a positive relationship. Walker (1910) also found 13 out of 34 cases without any relationship. The feedback mechanism between the above parameters may be useful for seasonal forecasts of monsoon rainfall. This relationship is not yet fully examined and further attention is needed by snow cover climatologists and meteorologists.

RELATIONSHIP AMONG WINTER SNOW COVER, SPRING SNOWMELT AND MONSOON RAMS

Figures 3 and 4 clearly show that Indian total monsoon rainfall is negatively correlated with the areas both of winter snow cover and spring snowmelt over Eurasia. A significant multiple correlation coefficient (r = 0.6) among them shows that they are quite closely related and


accounts for 36% of the rainfall variance. Multiple correlation analysis suggests the multiple regression equation for estimating seasonal rainfall:

Y =127.7 - 0.48 Xx - 1.1 X2

where Y is the estimated total rainfall (cm) while X1 and X2 are the areas (106km2) of winter snow cover and spring snowmelt over Eurasia. On comparing the estimated rainfall with the observed rainfall (Fig. 5), the

1301

120-

110-

50-

01 I I I 1 1 I I 1 1967 1969 1971 1973 1975 1977 1979 1981 1983 1985

Fig. 5 Variations of total Indian summer monsoon rainfall and the estimated total rainfall.

former are fairly closely related with the latter. However, the difference between them is about 10 cm in four out of 19 years. This probably arises because the Indian summer monsoon activity is the result of a series of process mechanisms in which snow cover is but one significant parameter (Walker, 1910). The data set used is limited to 19 seasons only. This type of climatological study could be more promising with more data.

The India Meteorological Department has been using a multiple regression equation for total monsoon rainfall using South American pressure departures, mean position of the axis of an upper-air ridge at 500 hPa (hPa = hecta Pascals = millibars) in April along 75°E longitude and mean minimum temperature in March for three Indian stations. This equation explains 55-60% of the observed variance of rainfall (Das, 1986). The multiple correlation coefficient of 0.6 accounts for 36% of rainfall variance. Using this synoptic-statistical information, a prediction equation can be evaluated which is a useful empirical approach for long-range forecasting of the climate. It would be interesting to compare this type of study with the different seasonal forecasts using single parameters such as those by Verma (1980), Joseph et al. (1981) and Shukla & Paolino (1983). The promising result of the present study suggests that Eurasian winter snow cover can be included as one of the significant parameters along with other regional and global


atmospheric parameters. Recently, Hastenrath (1986), Thapliyal (1987) and Parthasarathy & Singh (1987) critically reviewed past studies on long-range forecasting over India with different regional/global oceanic and atmospheric circulation patterns, but snow cover was not included. It would seem worthwhile to include Eurasian snow cover along with other parameters in the long-range multiple regression equation.

CONCLUSIONS

This study shows that digital satellite-derived snow cover data can be used for the study of the intensity of the Indian monsoon and long-range forecasting of total monsoon rainfall. Both winter snow cover and spring snowmelt over Eurasia were negatively correlated with monsoon rainfall over India, while spring snowmelt had a positive relationship with the winter snow cover. The multiple correlation among them accounts for 36% of the variance of monsoon rainfall and provides a multiple regression equation. In the present analysis, it was not possible to clarify exactly the actual physical processes of interaction between Eurasian snow cover and monsoon rainfall. However, the author proposes the following probable mechanism for the interaction.

The increase or decrease of snow cover over Eurasia affects the radiation balance of the Earth-atmosphere system. A larger Eurasian winter fresh snow cover extent enhances the surface albedo, which may lower atmospheric incoming solar radiation and increase sea level pressure over the continents, thereby weakening monsoon circulation (Manabe & Hahn 1977; Winston & Krueger, 1977). A smaller winter snow cover over Eurasia decreases surface albedo, which may be associated with increased sensible heat in the atmosphere. The increased sensible heat coupled with the increased latent heat as derived from the pre-monsoon rainfall over the Himalayas may cause an increase in air temperature in the middle to upper troposphere north of 20°N latitude. The higher middle to upper tropospheric air temperatures over the Tibetan plateau have been associated with the triggering of the easterly jet stream south of the Himalayas and strong monsoon activity (Florin, 1968). Further, accumulated winter snow cover area starts melting in the spring by absorbing the heat in the adjacent air mass and hence cooling it. Thus both the winter snow cover and spring snowmelt areas exercise a profound influence on the Earth-atmospheric radiation budget which in turn affects the summer monsoon.

REFERENCES

Bhanu Kumar, O. S. R. U. (1987) Seasonal variation of Eurasian snow cover and its impact on the Indian summer monsoon. In: Large Scale Effects of Seasonal Snow Cover (Proc. Vancouver Symp., August 1987), 51-60. IAHS Publ. no. 166.

Blanford, H. F. (1884) On the connection of the Himalayan snow with dry winds and seasons of drought in India Proc. Roy. Soc. Lond. 37, 3-22.

Das, P. K. (1986) Monsoons, fifth IMO lecture. WMO no. 613, Geneva, 155, 126-30. Dewey, K. F. & Heim, R., Jr (1981) Satellite observations of variations in Northern


Hemisphere seasonal snow cover. NOAA Tech. Report NESS 87, 1-85. Dey, B. & Bhanu Kumar, O. S. R. U. (1983) Himalayan winter snow cover area and summer

monsoon rainfall over India. /. Geophys. Res. 88 (9), 5471-5474. Dickson, R. R. (1984) Eurasian snow cover versus Indian monsoon rainfall - An extension

of the Hahn-Shukla results. /. Ctim. Appl. Met. 23, 171-173. Flôhn, H. (1968) Contributions to meteorology of the Tibetan Highlands. Atmos. Sci. Pap.

130, Colorado State University, Fort Collins, 18-40. Hahn, D. G. & Shukla, J. (1976) An apparent relationship between Eurasian snow cover and

Indian monsoon rainfall. /. Atmos. Sci. 33, 2461-2462. Hastenrath, S. (1986) On climate prediction in the tropics. Bull. Am. Met. Soc. 67, 697-702. Joseph, P. V., Mukhapadhyaya, R. K., Dixit, W. V. & Vaidya, D. V. (1981) Meriodinal wind

index for long range forecasting of Indian summer monsoon rainfall. Mausam 32, 31-34.

Kukla, G. J. (1981) Snow covers and climate. In: Glaciological Data. Snow Watch 1980, 15-20. Report GD-11 World Data Centre-A for Glaciology, Boulder, Colorado, USA.

Manabe, S. & Hahn, D. G. (1977) Simulation of tropical climate of an ice age. /. Geophys. Res. 82, 3889-3911.

Matson, M., Ropelewski, C. F. & Varnadore, M. S. (1986) An Atlas of Satellite-Derived Northern Hemispheric Snow Cover Frequency, 3-5. US Department of Commerce, Washington, DC.

Mooley, D. A. & Parthasarathy, B. (1984) Fluctuations in all-India summer monsoon rainfall during 1971-1978. Climat. Change 6, 287-301.

Namias, J. (1985) Some empirical evidence for the influence of snow cover on temperature and precipitation. Mon. Weath. Rev. 113, 1542-1553.

Parthasarathy, B. & Singh, S. V. (1987) Prediction of all-India summer monsoon rainfall with regression model. In: Long-range Forecasting Research Report. Tech. Document WMO/TD, no. 87, Geneva, series 6, vol. II, 713-722.

Shukla, J. & Paolino, D. A. (1983) The southern oscillation and long-range forecasting of the summer monsoon rainfall over India. Mon. Weath. Rev. I l l , 1830-1837.

Thapliyal, V. (1987) Prediction of Indian monsoon variability: evaluation and prospects including development of a new model. In: The Climate of China and Global Climate (ed. by D. Ye, C. Fu, J. Chao & M. Yashino), 397-416. China Ocean Press, Beijing.

Verma, R. K. (1980) Importance of upper tropospheric thermal anomalies for long-range forecasting of Indian summer monsoon activity. Mon. Weath. Rev. 108, 1072-1075.

Walker, G. R. (1910) Correlations in seasonal variations of weather. Mem. India Met. Department 21, 22-45.

Walsh, J. E. & Ross, B. (1986) Snow cover, cyclogenesis and cyclone trajectories. In: Glaciological Data. Snow Watch 55 (ed, by G. Kukla, R. G. Barry, A. Hecht & D. Wiesnet), 22-35.

Winston, J. S. & Krueger, A. F. (1977) Diagnosis of satellite observed radiative heating in relation to the summer monsoon. Pure. Appl. Geophys. 115, 1131-1144.

WMO (1975) The Physical Basis of Climate and Climate Modelling 5-10. NSF-GARP Publ. 16.

Received 6 October 1987; accepted 16 May 1988.

eurasian snow cover and seasonal forecast o. s. r. u. bhanu...

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