The UK has had some notable exceptionally warm summers in recent years prompting the question: Are seasonal summer temperatures in this region predictable? At UKMO empirical techniques using global scale patterns of historical sea surface temperature anomalies (SSTA) have been used successfully to make experimental predictions of seasonal rainfall in NE Brazil, tropical NW Africa and E Africa (Ward et al. 1993; see also recent March and June issues of this Long­Lead Bulletin). The problem of predicting UK climate is more difficult than that of predicting rainfall in these tropical regions: however there is some evidence that UK climate may be predictable from SST (e.g. Ratcliffe and Murray 1970, Palmer and Sun 1985).

This year an experimental prediction of July­August average Central England Temperature (CET) has been made at UKMO using SSTA data. CET is an homogenized temperature series from 1659 to now, first put together by Manley (1974) and updated by Parker et al. (1992). From 1877 on, CET is based on daily maximum and minimum temperatures at selected stations in the region. July and August are usually the warmest months in the UK, and persistence in CET between these months is high. Correlating July and August monthly average CET for the period 1896­1995 gives a temporal anomaly correlation r=0.53, whereas June and July give r=0.17, and August and September give r=0.35 for the same period.

THE SST PREDICTOR

By correlating July­August CET with monthly and bi­monthly 10x10E latitude­longitude area averages of SST, significant correlations were found between January­February SST anomalies in the extratropical north Atlantic and July­August CET. An eigenvector analysis of North Atlantic winter (January­February) SSTA was also carried out. The pattern (Fig. 1) which explained the most variability in the SSTA data over the period 1901­1990 is similar to the correlation pattern between SST and CET. The time series of this eigenvector which indicates the strength and sign of the eigenvector pattern was calculated for each winter (January­February average SSTA), and used to develop a simple linear regression prediction scheme for CET.

PREDICTION SKILL ESTIMATE

The temporal anomaly correlation between the predictor time series and July­August CET over the period 1871­1995 is 0.40, which is significant (different from zero) at the 99.9% level. Over the shorter period 1971­1995 the correlation is 0.51 (99% significant).

Fig. 2 shows the inflated regression predictions for 1971­1996, made using independent data for 1871­1970 to train the prediction model. (In inflated regression, a scaling factor is obtained in the training period from the variance of the predictions and of the observed predictand. This rescaling does not affect the correlation.)

This inflated regression prediction scheme was also assessed over a longer period (1946­95) using a jack­knife technique. The jack­knife technique involves forecasts being made for each year using a regression equation obtained from a dataset that excludes the year being forecast. The subsequent two years are also excluded, as these may be related to the forecast year through persistence. Fig. 3 shows the jack­knife forecasts plotted against observed CET. The correlation of 0.47 is less than for the 25­year period but is still significant at the 99.9% level. This level of skill is promising, considering the 6­month lead time, and compares well with other long lead extratropical statistical prediction skills (e.g. Barnston 1994).

FORECAST FOR 1996

The regression scheme gives a forecast for July­August 1996 CET of 15.4EC (Fig. 2). The root mean square error (RMSE) for the 1971­1995 forecasts shown in Fig. 2 is 1.3EC, which provides an indication of the reliability of the current forecast. The vertical bars attached to the 1996 forecast in Fig. 2 show this RMSE range. The 1996 forecast value is 0.6EC cooler than the 1951­80 average, and 3.5EC cooler than the 1995 value.

Note: this forecast for 1996 is experimental, with substantial uncertainty. The method should be regarded as developmental, and the results should be used with caution.

Barnston, A.G., 1994: Linear statistical short­term climate predictive skill in the northern hemisphere. J. Climate, 7, 1513­1564.

Manley, G., 1974: Central England temperatures, monthly means 1659 to 1973. Q. J. Roy. Met. Soc., 79, 242­261.

Palmer, T. and Z. Sun, 1985: A modeling and observational study of the relationship between sea surface temperature in the north west Atlantic and atmospheric general circulation. Q. J. Roy. Met. Soc., 111, 947­975.

Parker, D.E., T.P. Legg and C.K. Folland, 1992: A new daily Central England Temperature series, 1772­1991. Int. J. Climatol., 12, 317­342.

Ratcliffe, R.A.S. and R. Murray, 1970: New lag associations between north Atlantic sea temperatures and European pressure, applied to long­range weather forecasting. Q. J .Roy. Met. Soc., 96, 226­246.

Ward, M.N., C.K. Folland, K. Maskell, A.W. Colman, D.P. Rowell and K.P. Lane, 1993: Experimental seasonal forecasting of tropical rainfall at the UK Meteorological Office. In: Prediction of interannual climate variations, NATO ASI vol. 16, pp 197­216.

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