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Mukougawa, Hitoshi 1 , Yuhji Kuroda 2 and Toshihiko Hirooka 3

(1/15). 4th SPARC General Assembly, Bologna, Italy Poster Presentation A 00075, September 1, 2008. ID: 00075. Influence of Stratospheric Circulation on the Predictability of the Tropospheric Northern Annular Mode. Mukougawa, Hitoshi 1 , Yuhji Kuroda 2 and Toshihiko Hirooka 3.

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Mukougawa, Hitoshi 1 , Yuhji Kuroda 2 and Toshihiko Hirooka 3

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  1. (1/15) 4th SPARC General Assembly, Bologna, Italy Poster Presentation A 00075, September 1, 2008 ID: 00075 Influence of Stratospheric Circulation on the Predictability of the Tropospheric Northern Annular Mode Mukougawa, Hitoshi1, Yuhji Kuroda2 and Toshihiko Hirooka3 Presenter: Mukougawa, Hitoshi (mukou@dpac.dpri.kyoto-u.ac.jp) 1Disaster Prevention Research Institute, Kyoto University, JAPAN2Meteorological Research Institute, JMA, JAPAN 3Department of Earth and Planetary Sciences, Kyushu University, JAPAN

  2. (2/15) Downward Migration of NAM index Baldwin and Dunkerton(1999,2001)

  3. (3/15) Stratospheric Influence on Tropospheric Forecast Skill Baldwin et al. (2003) Statistical prediction of the monthly-mean AO(t+t1;1000hPa) with a forecast period beginning after 10 days by a linear regression model which uses the present value of the NAM(t) at one level between 1000 and 100 hPa: AO(t+t1) = B0+B1*NAM(t)+ε Winter The 150-hPa NAM predicts the monthly-mean AO better than the AO itself ← Downward migration of the NAM variation

  4. (4/15) Aim of Our Study Examinestratospheric influence on the forecast skill of thetropospheric circulation using operational 1-month ensemble forecast dataset provided by the JMA Influence of stratospheric NAM variation on the forecast skill of tropospheric NAM index 5-winter (2001/02-2005/06; DJF) archive of the JMA 1-month ensemble forecasts Forecast skill of upper tropospheric NAM index for a lead time from 4 to 11 days tends to be better when negative NAManomaly (easterly wind anomaly) is observed in the stratosphere at the initial time of forecast Forecast skill of tropospheric circulation over the North Atlantic region is significantly improved

  5. (5/15) Data 1-month forecast dataset of JMA ensemble prediction system Resolution T106L40 hybrid coordinate Top Boundary 0.4 hPa Ozone specified by zonal-mean climatological value SST constant SST anomaly at the initial time Integration Period 34 days Number of Ensemble 13 members Perturbation Method BGM(Breeding of Growing Mode) Initialization Date Every Wednesday and Thursday Interval of Stored Data Daily (2.5°×2.5°) 5 winter seasons 2001/02 - 2005/06 5 X 26 = 130 forecasts 7-day averaged ensemble-mean forecasts JMA Global Analysis (GANAL) dataset (1.25°×1.25°)

  6. (6/15) Northern Hemisphere Annular Mode(NAM)Baldwin and Dunkerton (2001) 50 hPa 10 hPa NAM index: 100 hPa 300 hPa ERA-40 monthly mean Z Nov.-Apr. 1957-2002 Arctic Oscillation (AO) 1000 hPa 500 hPa

  7. (7/15) 03/04 Winter 04/05 Winter Observed NAM index Observed NAM index Ensemble-Mean Error of NAM Ensemble-Mean Error of NAM 20 30 10 20 10 30 lead time (days) Observed NAM & Ensemble-Mean Forecast Error of NAM Prediction skill of the 7-day averaged NAM index for 03/04 winter is better than that for 04/05 winter

  8. (8/15) 04/05 Winter 03/04 Winter Observed NAM index Observed NAM index Spread of Predicted NAM Spread of Predicted NAM 30 10 20 10 20 30 lead time (days) Observed NAM & Spread of the Predicted NAM Spread of the predicted 7-day averaged NAM index in the troposphere for 03/04 winter is also smaller than that for 04/05 winter

  9. 95% Error (9/15) 1000hPa Dependence of Ensemble-MeanForecast Error of Predicted NAM Indexon the Initial NAM Index at 50 hPa lead time (days) Classified based on NAM index at 50hPa at the initial time of forecast 500hPa p: NAM@50hPa >1: 18 forecasts n: NAM@50hPa<-1 43 forecasts 0: others 69 forecasts ●Predicted NAM index error for negative NAMcase is significantly smaller than positiveNAM case for lead time from 4 to 11 days inthe upper troposphere, which suggests thestratospheric influence on the forecast skill of the prediction of the troposphere ●For extended-range period, the error of predicted NAM for positive case becomes significantly large even in the lower troposphere 250hPa

  10. lead time(days) (10/15) Dependence of Ensemble-Mean Forecast Error of Predicted NAM Index on the Initial NAM Index at 50 hPa Classified based on NAM index at 50hPa at the initial time of forecast Error difference of p-n with significance level 95(90)% Prediction skill of tropospheric NAM anomaly for extended-range forecast would be improved when negative NAM anomaly is observed at 50hPa

  11. (11/15) 1000hPa 95% Dependence of Ensemble-MeanForecast Error of predicted NAM Indexon the Initial NAM Index at 1000hPa Error lead time (days) p: NAM@1000hPa >1: 27 forecasts n: NAM@1000hPa<-1 34 forecasts 0: others 69 forecasts 500hPa ・Improvement of error for groupn is not significant ・Upward influence on the stratosphere error difference p-n with significance 95(90)% 250hPa lead time (days)

  12. (12/15) Correlation between Forecast Error of Predicted 500hPa NAM Index and Initial U Anomaly day-17 fcst day-10 fcst day-7 fcst day-4 fcst Shaded: 99(95)% High correlated region gradually migrates downward from upper to lower stratosphere, but does not penetrate into troposphere

  13. (13/15) Regressed Initial U & WN1 E-P flux w.r.t. Forecast Error of 500-hPa NAM Index day-17 fcst day-10 fcst 3.1X104 5x106(kg/s2) day-7 fcst day-4 fcst Shaded: 99(95)% EP-flux > 90% When the forecst error of 500-hPa day-7 NAM index is large, westerly anomaly prevails in the lower stratosphere associated withequatorward & downward propagation of anomalous WN1 activity

  14. P N (14/15) Dependence of Ensemble-Mean Forecast Error of Predicted Z250 on the Initial NAM Index at 50 hPa Day 7 error (m) Positive Case Negative Case Colored > 100m significance 95(90)% Forecast error of Z250 over the North Atlantic for positive NAM case is significantly larger than that for negative NAM case

  15. (15/15) Summary • Forecast skill of the 7-day averaged ensemble-mean upper tropospheric NAM index for a lead time from 4 to 11 days tends to be better when the negative NAM index is observed in the stratosphere • Influence of tropospheric NAM anomaly at the initial time on the forecast skill of tropospheric NAM index is insignificant • When the extended-range forecast error of the tropospheric NAM index is large, westerly anomaly prevails in the stratosphere associated with suppressed upward and enhanced equatorward propagation of WN1 activity • The impact of the stratospheric state on the forecast error of the troposphere could be well understood in the framework of regional mode (NAO), rather than annular mode variability

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