Wintertime forcing of the 1998-2002 Northern Hemisphere drought

During 1998-2002 severe drought extended over much of the Northern Hemisphere mid-latitudes with greatest severity over North America and West Asia during the boreal cold season. Both observational analysis and linear modeling are used to investigate: 1) the role of subsidence in forcing the drought; 2) the evolution of the drought on the subseasonal time scale; and 3) transients and the role of zonally symmetric dynamics.

Within the overall drought period, precipitation patterns and seasonal circulation for individual years differ significantly from the mean 1998-2002 pattern. A linear primitive equation model is used to diagnose key forcing areas for each year, especially with respect to differentiating the roles of transient vorticity forcing and tropical diabatic heating. The interaction of the stationary waves with mean flow is examined in the observed data in terms of changes to the thermodynamic balance that produce rainfall-suppressing subsidence, and perhaps in some cases (i.e. Western United States during 2000/01) increased rainfall and ascent. The observed transient forcing is also analyzed by synoptically filtering the terms of the vorticity equation. Aside from indicating the position of subseasonal storm tracks, this analysis will indicate to what degree transient waves support or oppose the stationary wave response.