|Title||Investigating biases in the representation of the Pacific sub-tropical jet stream and associated teleconnections (a UGROW sub-project)|
|Publication Type||Technical memorandum|
|Secondary Title||ECMWF Technical Memoranda|
|Authors||Vitart, F, Emerton, R, Rodwell, M, Alonso-Balmaseda, M, Haiden, T, Johnson, S, Magnusson, L, Roberts, CD, Sandu, I|
UGROW is an ECMWF cross-departmental project focused on Understanding systematic error GROWth from hours to seasons ahead. UGROW-JET, one of the three UGROW sub-projects focuses on biases in the representation of the Pacific sub-tropical jet stream, which plays a key role in the tropical-extratropical teleconnections.
In this study the Pacific sub-tropical jet stream is diagnosed using zonal wind at 300 hPa. An analysis of the model climatology from operational reforecasts reveals that a main issue in the representation of the Pacific sub-tropical jet stream is a westward shift of the eastward extension of the jet stream. This issue can be diagnosed as a negative bias in the 300 hPa zonal wind over the central north Pacific. This bias is the main focus of this study. It is likely to affect the representation of teleconnections originating from the Madden Julian Oscillation (MJO).
The bias increases with lead time up to week 4. It has a strong seasonal variability and is largest in January. It is significantly larger in the control ensemble forecast than in the perturbed ensemble forecasts. This difference is due to the impact of the stochastic physics scheme in the Tropics. A series of sensitivity experiments, where part of the atmospheric circulation is nudged towards ERA5, suggests that there is not a unique source for this bias. However, model errors in the high latitudes seem to be a major contributor. The bias has been considerable reduced in the integrated Forecast System (IFS) cycle 47r3.