Spring-to-Summer Transitions of Global Circulations during May–July 1979

Abstract

The seasonal transition from spring to summer in the Northern Hemisphere and from fall to winter in the Southern Hemisphere is studied for 1979 using gridded datasets produced by the Goddard Laboratory for Atmospheres. Winds at 200, 500 and 850 mb are decomposed into their rotational and divergent components. The streamfunction (?), velocity potential (?), and height fields (Z) are projected onto spherical harmonics to quantify the behavior of the lowest-order planetary modes. Comparison of the planetary scales with those at full resolution reveals that the low-order truncation represents over half of the total energy at 200 mb and that zonal deviations dominate the week-to-week time changes. The velocity potential spectrum is dominated by the wave component of largest meridional and zonal scale (?11). This mode exhibits a clear eastward propagation, circling the globe in about six weeks. This propagation is discussed in connection with observed changes of outgoing longwave radiation and related to the 30?60 day oscillation, which exhibited a strong signal during the 1979 summer season. It is found that ?11, ?21 and Z41 propagate with similar phase speeds, linking the planetary scale divergent patterns with those of the rotational flow and height field. The streamfunction is found to contain more of the 30?60 oscillation signal than the height fields for global scales. Analyses of outgoing longwave radiation and velocity potential at 5°N at full resolution are compared with the zonal wind at 34°S. It is found that the divergence pulse and low values of outgoing longwave radiation are observed up to one week before zonal-wind acceleration. These accelerations peak when the divergence pulse is at about 160°E, with high values of the subtropical jet at similar longitudes. Removal of the seasonally averaged circulation from the weekly averages reveals a dominant baroclinic structure, with the 250- to 850-mb wind shear exceeding the vertically averaged wind over extensive areas, which extend for particular weeks to the polar caps. A dominant barotropic structure emerges when the basic sate is included. The seasonal transition is found to occur abruptly during the middle of the 12-week periods considered here, with pronounced rearrangements of global patterns due to excitation of planetary scales.