Ph.D. Seminar, Risoe The influence of atmospheric circulation patterns on surface winds above North Sea Kay Sušelj, Abha Sood, Detlev Heinemann

Ph.D seminar Risoe Goals of the study Investigation of surface wind (at 10 m height) variability above North Sea:  Comparison of the reanalysis data (ECMWF ERA-40 and NCEP/NCAR)  Statistics of wind speed and wind time series Understanding the relationship between wind and circulation patterns: Wind Speed and North Atlantic Oscillation (NAO) Empirically derived circulation patterns from Sea Level Pressure (SLP)

Ph.D seminar Risoe Reanalysis data used  ECMWF ERA-40  Time span: Sep Dec  Spatial resolution: 1°×1°  NCEP/NCAR Reanalysis 2  Time span: Jan present  Spatial resolution: 1.9°×1.85° (wind) and 2.5°×2.5° (pressure) Spatial resolution of both datasets: 6 hours Area under investigation

Ph.D seminar Risoe Comparison of ERA-40 and NCEP/NCAR data ( ) ERA 40 NCEP/NCAR Mean wind speed Time series of spatially averagedWS (3 yrs running average filter) Whole year Winter (DJFM) Summer (JJAS)

Ph.D seminar Risoe ERA 40 Comparison (cont.) NCEP/NCAR Probability density of mean monthly wind speed Probability density of mean monthly wind angle

Ph.D seminar Risoe EOF analysis (dominant modes)  WSA - wind speed anomaly from the annual cycle  Empirical Orthogonal Funcitions (EOF) - isolate main field variability into spatial EOF patterns and time series (principal components, PCs) EOF 1 (41.5%) EOF 2 (13.5%) EOF 3 (12%) EOF 4 (6%)

Ph.D seminar Risoe Large scale circulation patterns derived from SLPA and WSA  Separate cases (days) for low ( 1) PCs  Plot SLPA for all three cases  Derive circulation patterns from plots  Use indices (time series of circulation patterns) to predict WSA  Compare predicted and 'real' WSA  How good is North Atlantic Oscilation Index (NAOI) in predicting NS WSA Summer NAOI and PC 1 of WSA Winter NAOI and PC 1 of WSA Correlation btw. winter NAOI and WSA Can we find better predictors for WSA?

Ph.D seminar Risoe PCs of WSA and its relation to circulation patterns... 1 st PC of WSA SLPA for low PC of WSASLPA for medium PC of WSA SLPA for high PC of WSA First PC, measured & estimated from circulation pattern Correlation btw. WSA and WSA estimated from circ. pat. Correlation btw. winter WSA and WSA estimated from circ. pat.

Ph.D seminar Risoe and 2 nd & 3 rd PCs of WSA Low values High values 2 nd PC 3 rd PC

Ph.D seminar Risoe Correlation btw. WSA and WSA predicted from three circulation patterns Whole year Summer Winter

Ph.D seminar Risoe The WS above NW Europe has a pronounced annual cycle, with the highest values in the cold part of the year Greater part of the WSA varies coherently over studied domain The winter WSA shows the pronounce increase over the last half of the century, slight decrease over the last decade The WSA over the NW Europe is parly determined by circulation indices as SLPA difference between:  West of Iberia and west of Iceland  Scandinavia and Iceland  Above Great Britain Conclusions and plans for further work  Improvement of the wind surface data base (downscaling reanalysis data)  Special effort toward reliable boundary layer parameterizations  Detailed estimation of the past and future climate and its relation to the wind resources (variability, trends)

Ph.D seminar Risoe Acknowledment The work is founded by the EU Commission within the ModObs project (MRTN-CT ). ECMWF and NCEP/NCAR gratefully provided the reanalysis data.