Midwinter Suppression of Baroclinic Wave Activity in the Pacific

Seasonal variations in baroclinic wave activity and jet stream structure in the Northern Hemisphere are investigated based upon over 20 years of daily data. Baroclinic wave activity at each grid point is represented for each day by an envelope function, the lowpass-filtered time series of the squared highpass-filtered geopotential height. Baroclinic wave activity over the Atlantic exhibits a single maximum in January, whereas in the Pacific it exhibits peaks in late autumn and in early spring and a significant weakening in midwinter, which is more evident at the tropopause level than near the surface. This suppression occurs despite the fact that the low-level baroclinity and the intensity of the jet stream are strongest in midwinter.Based on the analysis of 31-day running mean fields for individual winters, it is shown that over both the oceans baroclinic wave activity is positively correlated with the strength of the upper-tropospheric jet for wind speeds up to ∼45 m s−1. When the strength of the westerlies exceeds this optimal value, as it usually does over the western Pacific during midwinter, the correlation is negative: wave amplitude and the meridional fluxes of heat and zonal momentum all decrease with increasing wind speed. The phase speed of the waves increases with wind speed, while the steering level drops, which is indicative of the increasing effects of the mean flow advection and the trapping of the waves near the surface.