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On Climate Prediction in the Tropics Department of Meteorology University of Wisconsin-Madison

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On Climate Prediction in the Tropics Department of Meteorology University of Wisconsin-Madison Stefan Hastenrath On Climate Prediction in the Tropics Department of Meteorology University of Wisconsin-Madison Abstract 2. Concepts and approaches Climatic disasters are common in many tropical regions, and rainfall The term prediction should refer to a statement about an anomalies in particular have a severe human impact. Accordingly, event from information on antecedent conditions. Climate both the World Climate Programme and the U.S. National Climate prediction is a statement on the quality of a (rainy) season or Program have identified climate prediction as a major objective. Approaches can be grouped into five major categories! (i) the ex- year as a whole, based on conditions more than a month pre- trapolation of empirically or theoretically deduced periodicities; (ii) ceding. Conversely, where a statement is made from concur- the assessment of statistical relationships between rainfall and var- rent conditions, terms such as specify, determine, deduce, or ious meteorological elements; (iii) the relation between rainfall in the infer, rather than predict, should be used. preseason and at the height of the rainy season; (iv) comprehensive diagnostic studies of climate and circulation anomalies combined Approaches can be grouped into five major categories: (i) with statistical methods; and (v) numerical modeling. the extrapolation of empirically or theoretically deduced pe- Methods pertaining to (iv) indicate the feasibility of empirically riodicities; (ii) the assessment of statistical relationships be- based climate prediction for certain tropical regions. For the tween rainfall and various meteorological elements; (iii) the drought-prone region of Northeast Brazil and Indonesia, in particu- relation between rainfall in the preseason and at the height of lar, it has been demonstrated on independent data sets that almost half of the interannual rainfall variability can be explained from an- the rainy season; (iv) comprehensive diagnostic studies of tecedent departures in the large-scale circulation. Application of climate and circulation anomalies combined with statistical these methods on an operational basis involves two simultaneous methods; and (v) numerical modeling. This classification is input data requirements: 1) they must be available in quasi real time; intended to bring some order into the diversity and is not 2) long (>10 years) homogeneous reference series of internally con- sistent parameters are needed, while absolute calibration is not es- meant to be dogmatic. sential. The practical benefit of climate forecasts appears to hinge on The following regional-reviews emphasize climate predic- societal and economic factors. tion proper and are limited to published works. 1. Introduction 3. Indian monsoon Large interannual variations of rainfall are an intrinsic part of tropical climate and can have a severe impact on human The work in India is among the earliest climate-prediction ef- activities. It is therefore not surprising that attempts to fore- forts in the tropics (Banerji, 1950; Normand, 1953; Rao and cast anomalous rainy seasons well in advance extend over a Ramamoorthy, 1960; Jaganriathan, 1960; Rao, 1965; Rao, century and that both the World Climate Programme (World 1976; Das, 1984,1986). H. F. Blanford issued monsoon fore- Meteorological Organization (WMO), 1980) and the U.S. casts from 1882 to 1885, calling attention in particular to the National Climate Program (National Climate Program Of- negative relation between southwest monsoon rainfall and fice, NOAA, 1980) declare climate prediction as a central the preceding winter's snowfall in the Himalayas. It was then objective. decided to issue monsoon forecasts annually, but they were Long-range forecasting for the midlatitudes has been re- held confidential in 1902-1906, during World War II, and viewed by Livezey and Jamison (1977), Nicholls (1980), and later (Jagannathan, 1969). Seasonal monsoon forecasting Preisendorfer and Mobley (1982). Charney and Shukla was the motivation for Sir Gilbert Walker's pioneering work (1981) suggested that climatic variability in the tropics on the Southern Oscillation. In 1910 he published a predic- should be more predictable because it was in large part due to tion formula for Indian monsoon forecasting containing slowly varying anomalies of the lower boundary of the at- four predictors (Walker, 1910). By 1924 he differentiated his mosphere. Despite these expectations, Nicholls (1980), in his work for two large regions of India, using two formulae with comprehensive review, was still unable to find climate-pre- a total of five predictors (Walker, 1924). Finally, Walker used diction work proper for the tropics. It is only in the 1980s that 28 predictors in six forecast formulae, differentiated re- major breakthroughs have been achieved. The purpose of the gionally (Banerji, 1950). At the time of Banerji's writing, the present note, based in part on Hastenrath (1985), is to sketch India Meteorological Department still used part of the afore- the history of climate prediction endeavors in the tropics; to mentioned predictors in five formulae, none of which con- summarize the progress made in the 1980s; and to discuss the tained upper-air information. Rao and Ramamoorthy (1960) tasks that lie ahead. After a summary of terminology and report that at the time of their work the India Meteorological methods, climate prediction endeavors are reviewed for var- Department issued three forecasts annually, using upper-air ious tropical regions and finally a synthesis and outlook are information in addition to predictors familiar from Walker's presented. work. Endeavors to forecast the onset of the southwest mon- soon are more recent. Reddy (1977) proposes as predictor the © 1986 American Meteorological Society May 50-mb zonal wind component over Singapore. 696 Vol. 67, No. 6, June 1986 Unauthenticated | Downloaded 10/08/21 12:45 AM UTC Bulletin American Meteorological Society 697 FIG. 1. Monsoon rainfall in central India 1958-77. Dots denote the observed totals and open circles the amounts obtained from Kung and Sharif's (1982) regression equations based, successively, on all years except the particular year plotted. From Kung and Sharif (1982). On the part of the India Meteorological Department (Ja- gannathan, 1960; Rao, 1965; Das, 1984, 1986), the desired standard of performance for forecasting monsoon rainfall has been variously expressed as being an 80 percent chance of success in a three-category scheme in accordance with the de- sired success rate stated by Sir Gilbert Walker (1928) for a two-category scheme. In appraising forecast power, it is es- FIG. 2. Scatter diagram of all-India rainfall index, period 1951— 70, with numbers indicating the years. Forecast values are obtained sential to distinguish between dependent and independent from regression equations based, successively, on all years except the data sets, but it is not clear whether this distinction has al- particular year plotted. Broken line denotes 45-degree angle. The ways been made. correlation coefficient of +0.92 between forecast and observed Banerjee et al. (1978) present a regression formula for pre- values is significant at the 0.1 percent level. From Wu (1984). dicting the ratio of the number of subdivisions with normal or above-normal rainfall in June-September to the total number of subdivisions (31). The latitude position of the 500- Kung and Sharif (1980, 1982) developed regression meth- mb ridge along 75°E in April serves as sole predictor, and the ods for forecasting both the Indian southwest monsoon rain- performance tested on 16 years of independent data appears fall and on the onset date over Kerala, South India, based on remarkable. April upper-air patterns in the India-Australia region and The important role of antecedent upper-air conditions for sea-surface temperature around India in the preseason. In Indian monsoon rainfall, as recognized in various earlier the regression models (Kung and Sharif, 1982), five predic- studies referred to above, is emphasized in the recent work of tors are retrieved for monsoon onset and six for rainfall. Thapliyal (1981,1982). He used the April latitude position of Kung and Sharif (1982) offer a measure of forecast perform- the 500-mb ridge over India as sole input to an ARIMA ance by comparing the observed rainfall amounts with the (auto-regressive integrated moving average) scheme and values calculated from regression equations based succes- applied the model thus obtained to predict the rainfall over sively on all years except the particular year calculated (Fig. peninsular India during the years 1977-80; the success for 1). It must be recognized that such specification of onset the four years being high. dates and rainfall totals within the 1958-78 time span may It appears that from Walker's time to the 1970s climate provide no fair measure of performance for forecasts proper prediction in India evolved from a primarily statistical basis of conditions beyond the base interval. As in Wu's (1984) (category ii) towards reliance on circulation diagnostics study, Kung and Sharif's (1982) approach relies on diagnos- (category iv). Reddy's (1977) paper is considered statistical tics on the large-scale circulation (category iv). (category ii), while the studies using the latitude position of In Wu's study, observations of important elements were the 500-mb ridge (Banerjee et al., 1978; Thapliyal, 1981, likewise limited to about two decades, namely the period 1982) imply some general circulation understanding
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