Extratropical Influence of Upper Tropospheric Equatorial Zonal Wind

• Main Authors: Gollan, Gereon, Greatbatch, Richard John
• Format: Conference Object
• Language: English
• Published: 2015
• Subjects: Pacific; aleutian low; North Atlantic
• Online Access: https://oceanrep.geomar.de/id/eprint/28584/; https://oceanrep.geomar.de/id/eprint/28584/1/poster_zonmeanwind.pdf

Variations in the global tropospheric zonal mean zonal wind ([U]) during boreal winter are investigated using Rotated Empirical Orthogonal Functions applied to monthly means. The first two modes correspond to the Northern and Southern Annular Mode and modes 3 and 4 represent variability in the tropics. One is related to El Nino Southern Oscillation and the other has variability that is highly correlated with the time series of [U] at 150 hPa between 5 ◦ N and 5 ◦ S ([U150]e) and is related to activity of the Madden-Julian Oscillation (MJO). High amplitude of the MJO with strong precipitation anomalies over the western tropical Pacific (late MJO phases) are associated with the westerly phase of [U150]e (and vice versa). The extratropical response to [U150]e is investigated using linear regressions of 500 hPa geopotential height onto the [U150]e time series. Use is made of reanalysis data and of the ensemble mean output from a relaxation experiment using the European Center for Medium Range Weather Forecasts (ECMWF) model in which the tropical atmosphere is relaxed towards reanalysis data. Both the 45- year ECMWF reanalysis (ERA-40) and the ERA-Interim reanalysis data sets are used for the relaxation experiment as well as for the regression analysis. Therefore the analysis is covering 52 boreal winters from 1960/61 to 2012/13. The regression analysis reveals a robust shift of the Aleutian low and a wave train across the North Atlantic associated with [U150]e. It is found that the subtropical Rossby waveguides and the link between the North Pacific and North Atlantic are stronger during the easterly phase of [U150]e. The wave train over the North Atlantic is associated with Rossby wave sources over the subtropical North Pacific and North America.