Stratospheric polar cap mean height and temperature as extended-range weather predictors

The skill of stratospheric and tropospheric predictors in predicting near-surface quantities at the extended range (~10 days-2 months) has been investigated, using 40 years of reanalysis data from the European Centre for Medium-Range Weather Forecasts. The predictors are 1) the geopotential height (Z) at various levels, 2) the difference between Z and the 1000 hPa-geopotential (Z-Z(1000)), and 3) the temperature at various levels. The predictors are averages over the area north of 65ºN. The predictands are Z(1000) averaged over the same area, and geographical fields of several near-surface quantities. The predictive skill has been investigated for different lead-times between predictor and predictand, and different averaging periods of the predictor and the predictand.

The results show that the predictive skill of Z in the troposphere is mainly due to the predictive skill of sea-level pressure, whereas the predictive skill of Z in the stratosphere is mainly due to the predictive skill of stratospheric temperature. The predictive skill is largest in the end of December, for the predictor Z at 50 hPa and the temperature between 250 and 50 hPa. The temperature has also significant predictive skill in the upper stratosphere in the summer. In winter, for lead-times larger than 5 days the stratospheric Z is a better predictor of the daily Z(1000) than Z(1000) itself. Whereas the predictive skill of the stratospheric Z is largest for zero lead-time, the predictive skill of the stratospheric Z-Z(1000) and temperature are largest for lead-times of about ten days, evidencing the finite propagation time of geopotential anomalies from the stratosphere to the surface. The skill of the stratospheric height and temperature in predicting the wintertime monthly-mean field of Z(1000), is mainly limited to the region north of 60ºN. The stratospheric predictive skill for the monthly-mean fields of the zonal wind at 850 hPa and the near-surface temperature is particularly large around 60ºN. The correlation pattern of the near-surface temperature field and the stratospheric temperature is qualitatively similar to the corresponding pattern for the Arctic Oscillation index, except at middle latitudes over Eurasia and over the subtropical Pacific.