An Investigation of 40-50 Day Large Scale Divergent Circulations in the Tropical Troposphere

Intraseasonal fluctuations of the large scale divergent circulation of the tropical troposphere during the one year period December 1, 1978 November 30, 1979 are examined using analyses prepared by the European Centre for Medium Range Weather Forecasts (ECMWF). It is demonstrated through a number of empirical orthogonal function (EOF) analyses that highly coherent, equatorially symmetric fluctuations in a narrow band of frequencies centered near 45 days dominate the zonally asymmetric variance in both the 200 and 850 mb velocity potential. This 40-50 day variability appears as an eastward-moving anomaly in the velocity potential at each level which is related to local modulations of the Walker circulation on the order of several meters per second. In addition it is shown that local changes in the divergent meridional wind occur in tandem with the zonal wind anomalies. A primary characteristic of the variability of the divergent circulation is a vertical coherence in which negative anomalies of velocity potential at 200 mb, indicating large scale divergence, tend to be paired with positive anomalies and large scale convergence at 850 mb and vice versa. The role played by convection in the oscillation is examined with an index of highly reflective cloud (HRC). In the Northern Hemisphere, 40-50 day anomalies in this index are seen to propagate eastward and are in phase with the local anomalies of meridional divergent winds reconstructed from the leading modes in the EOF analyses. The intraseasonal variability in the Hadley circulation is also investigated by analysis of both the zonal average meridional winds and the twice-daily values of the streamfunction of the meridional circulation. EOF analysis shows that the streamfunction has a tendency to oscillate at intraseasonal time scales, but in a somewhat wider band of frequencies than is evident in the Walker circulation. Likewise, the zonally symmetric variability of the HRC index shows no clear 40-50 day signal; 40-50 day variability in convection appears to be restricted primarily to the maritime continent region. Certain aspects of the dynamical relationship between the zonally asymmetric Walker circulation variations, the Hadley circulation and the very prominent 40-50 day oscillation in the zonal circulation are examined by twice-daily evaluation of the zonal average zonal momentum budget at 200 mb in the tropics. The forcing terms explicitly calculated are the advection of zonal momentum by both the zonallyaveraged meridional and vertical wind fields, the Coriolis torque and the convergence of the horizontal eddy flux of momentum. Residuals are generally equal in magnitude to the net forcing, but correlations indicate that none of the first three terms above is strongly linked to the zonal acceleration. In particular, at the 40-50 day time scale only the eddy flux convergence shows a significant in-phase relationship with the zonal acceleration. These findings thus do not support a zonally symmetric mechanism in the tropics, for generating the 40-50 day oscillation in the zonal circulation; rather the oscillation appears to be more closely linked to the basic asymmetry of tropical convection which is manifested in the 40-50 day variation of the Walker circulation. Thesis Supervisor: Reginald E. Newell Title: Professor of Meteorology