Langzeitdiagnose des Einflusses der tropischen 30-60-taegigen Madden-Julian-Oszillation auf die mittleren Breiten der Nordhemisphaere

The impact of tropical intraseasonal convection and circulation anomalies associated with the Madden-Julian-Oscillation on the low-frequency winter circulation of the Northern Hemisphere is examined using 7 years (1985-1991) of daily ECMWF analyses and NOAA outgoing longwave radiation data (OLR). Episodes with a strong tropical forcing of the east Asian Jet are composited to identify consistent relationships between anomalous tropcial heating and the middle latitude circulation pattern. Enhanced convection in the tropical western Pacific is found to be associated with an amplifying of the anticyclone north of the heating region. Based on streamfunction anomalies a strengthening and eastward expansion of the East Asian Jet into the central Pacific is correlated with the active phase of the Madden-Julian-Oscillation over the western Pacific. The increase of upper-level northward divergent wind suggests that the Coriolis force applied to the diabatically driven meridional wind plays a major role with regard to the maintenance of the subtropical jet wind maxima during Northern Hemisphere winter. Thus, it is concluded that the tropical forcing contributes considerably to the significant strengthened East-Asian-Jet. Additionally, the thermodynamic forcing coincides with an enhanced local Hadley type circulation to the south of the jet while eddy momentum transport becomes stronger to the north. The increased meridional gradient of westerly wind is responsible for an enhanced Rossbywave source. Employing time lags of 200 hPa streamfunction anomalies for different pentades, it becomes evident that, simultaneously to enhanced tropical convection within the western Pacific, wave-trains emanate from the region of this strong Rossby-wave source into higher latitudes of the Northern Hemisphere. By separating the anomalies into 30-60 day and 10-30 day time scales, the existence of two different wave-trains become apparent. The weak 30-60 day pattern over the North-Pacific-American region indicates that only a small part of the PNA-variability is directly forced through the Madden-Julian-Oscillation. However, the wave pattern in the 10-30 day time scale, which show similarities with Rossby-waves, is considerably stronger and is directed into the western Pacific and towards North America on a more southerly propagation path. The oscillation within this intermediate time scale gives an important contribution to the low-frequency variability of the extratropical circulation. However, it is speculated whether the observed extratropical variability can be attributed indirectly to a forcing by the tropical 30-60 day Madden-Julian-Oscillation through nonlinear interactions. (orig.)SIGLEAvailable from TIB Hannover: RN 1818(94) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman