Large-scale Aspects of the 1978-79 Winter Circulation over the Greater WMONEX Region

Part II: Long-period Perturbations

Abstract

Some of the characteristic features of large-scale disturbances over the greater WMONEX region were investigated by using 12-30 days filtered wind, vorticity and divergence data at 850 and 200mb for the 1978-79 winter (1 December 1978 to 28 February 1979). Composites of these meteorological variables at each grid point and level were made with respect to changes in the first eigenvector (empirical orthogonal function) coefficients for 12-30 days filtered vorticity data at 200mb.
Over South and Southeast Asia, the 200mb circulation undergoes massive changes in association with the eastward passage of upper-tropospheric troughs and ridges. The mechanisms that are responsible for the development, slow eastward propagation (-4ms^-1), and dissipation of these disturbances are identical to those described by Murakami (1981c). Pronounced long-period (-20 days) disturbances at 200mb can also be traced from Japan, across the central North Pacific, to the equatorial Pacific. These upper-tropospheric disturbances are primarily maintained through barotropic nonlinear interactions with the winter mean flow and disturbances of all period ranges. Over the western South Pacific, 200mb long-period disturbances are most intense off the east coast of Australia. In this area, vertical vorticity transports due to convective motions appear to be significant.
At 850mb, northerly (anomaly) surges are predominant over the Afghanistan-Pakistan, Japan-Philippines and central North Pacific regions. Along the western periphery of the Tibetan Plateau, low-level southwesterlies ahead of an eastward moving trough flow up the extremely steep topographic gradient. This induces strong low-level convergnece, which results in increased westerly divergent winds near the trough and contributes to the enhancement of the local northerlies. Orographic enhancement is also responsible for the acceleration of the northerlies over the Japan-Philippines region. Converesly, over the central North Pacific along about 170°W, intensification of the northerlies is largely associated with transient disturbance activity poleward of 30°N.
Along the equator, zonal wind and divergence fields at 850mb exhibit marked intraseasonal (-20 day) changes. Here, anomalous westerlies (easterlies) and convergence (divergence) over the central Pacific coincide with easterlies (westerlies) and divergence (convergence) over the Indian Ocean. The east-west pressure gradient appears to be the primary factor for initiating changes in the equatorial 850mb zonal winds.