Ph.D. Seminar, Risoe 4.-5.10.2006

The influence of atmospheric circulation

patterns on surface winds above North Sea

Kay Sušelj, Abha Sood, Detlev Heinemann

Ph.D seminar Risoe 4.-5.10.2006

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 4.-5.10.2006

Reanalysis data used

ECMWF ERA-40

Time span: Sep. 1957- Dec. 2001

Spatial resolution: 1°×1°

NCEP/NCAR Reanalysis 2

Time span: Jan. 1948 - present

Spatial resolution: 1.9°×1.85° (wind) and 2.5°×2.5° (pressure)

Spatial resolution of both datasets: 6 hoursArea under investigation

Ph.D seminar Risoe 4.-5.10.2006

Comparison of ERA-40 and NCEP/NCAR

data (1958-2001)ERA 40 NCEP/NCARMean wind speed

Time series of spatially averagedWS (3 yrs running average filter)

Whole year Winter (DJFM) Summer (JJAS)

Ph.D seminar Risoe 4.-5.10.2006

ERA 40

Comparison (cont.)NCEP/NCAR

Probability density of mean monthly wind speed

Probability density of mean monthly wind angle

Ph.D seminar Risoe 4.-5.10.2006

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 4.-5.10.2006

Large scale circulation patterns derived from

SLPA and WSA

Separate cases (days) for low (<-1), medium (btw. -1 and 1) and high (>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

WSASummer NAOI and PC 1 of WSAWinter NAOI and PC 1 of WSA

Correlation btw. winter NAOI and

WSA

Can we find better predictors for WSA?

Ph.D seminar Risoe 4.-5.10.2006

PCs of WSA and its relation to

circulation patterns ... 1st PC of WSA

SLPA for low PC of WSA SLPA 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 4.-5.10.2006

... and 2nd & 3rd PCs of WSA

Low values High values

2nd P

C3rd

PC

Ph.D seminar Risoe 4.-5.10.2006

Correlation btw. WSA and WSA predicted

from three circulation patterns

Whole year

Summer

Winter

Ph.D seminar Risoe 4.-5.10.2006

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 4.-5.10.2006

Acknowledment

The work is founded by the EU Commission within

the ModObs project (MRTN-CT-2005-019369).

ECMWF and NCEP/NCAR gratefully provided the

reanalysis data.