The 200-mb Climatological Vorticity Budget during 1986–1989 as Revealed by NMC Analyses

Abstract

The primary objectives of this study are 1) a description of the solstice season 200-mb vorticity budgets during 1986?89 and 2) an evaluation of the usefulness of operational analysis products for diagnosing the seasonally averaged 200-mb vorticity balance, particularly the distribution and magnitude of the effective vorticity source term, that is, the Rossby wave source. With the use of the one-level vorticity balance diagnostic approach of Sardeshmukh and Hoskins, the January-February-March (JFM) and June-July-August (JJA) seasonal-mean 200-mb vorticity balances were analyzed. The NMC analyses of 1987 were compared with analyses from ECMWF. Overall agreement was generally satisfactory, in terms of providing consistent answers to the type of diagnostic questions being asked. Of particular importance was the reasonable good agreement between the effective vorticity source fields. The NMC 200-mb divergence fields over the deep tropics were also compared with the seasonal-mean outgoing longwave radiation (OLR) distribution. Overall correlations between OLR and directly computed divergence were around 0.8 for seasonal averages, but the correlation for regions of convergence was relatively insignificant. The NMC three-year seasonal-mean 200-rnb vorticity budget for JFM and JJA were examined. The following conclusions seem warranted. 1. The analyses provide no support for the assumption of a second equivalent-barotropie level in the extratropies around 150 mb. 2. The magnitude of the seasonally averaged Rossby wave source is generally small in the deep tropics (10°S to 10°N), when compared to the subtropics and midlatitudes. 3. Direct computation of the vorticity source indicates a significant contribution from the advection of absolute vorticity by the divergent Hadley flow into the winter hemisphere from three major summer hemisphere regimes of convection (Africa, Central-South America, and the Indian west Pacific monsoon region). However, the indirect evaluation of the divergent flow using the chi technique showed a generally weaker 200-mb Hadley circulation and a smaller contribution to the effective vorticity source by the advection term. With regard to the use of current 200-mb operational analyses for one-level vorticity budget diagnostics, the overall results of this study were viewed as quite positive. However, the inconsistency between the directly and indirectly computed mean meridional circulations during the period studied indicates the limitations of the current analyses and the necessity of performing the chi problem diagnostics before drawing conclusions.