| Drought persistence is extremely important in the western United States, not only in influencing the cooperation between different economic sectors (particularly industry and agriculture) regulated by government, but also for allocating the water supplies and maintenance of a diverse ecosystem handled by water resource planners and managers. The persistence of cold-season severe sustained droughts (CSSD) in northern California and the adjacent areas in Nevada and Oregon is systematically studied here. In this area, several severe sustained droughts have occurred during this century, each lasting several years and having caused immeasurable damage to the natural resources and extensive disruption to industry, agriculture, and all aspects of life.;Through a self-adaptive iterative algorithm and based on the NCDC climatological divisional precipitation data, the optimal drought-study region has been objectively determined. Inside such a region, the CSSD events of this century have been identified based on the water-year precipitation record. The physical causes responsible for the remarkable persistence of these identified CSSD episodes have been assessed through statistical presupposition and dynamical confirmation. The most significant factor responsible for the CSSD persistence has been found to be the synergistic pressure change in the troposphere in both local and certain remote regions. Through contrast analysis, the CSSD persistence has been demonstrated not explainable by random process but controlled by some underlying drought mechanism. Furthermore, the underlying drought mechanism has been found to be completely different from that for any other type of climatological episode (e.g., flood).;The major contribution of this research is the physical understanding of CSSD persistence. Such persistence is a coupling effect of atmospheric baroclinicity, the associated oceanic surface water movement and the regional hydroclimatological processes (especially the topographic distribution and local thermodynamic processes). The physics of the CSSD has been exploited through eigenmode analyses on the dominant factor. Based on such drought eigenmodes, simulation of the CSSD persistence demonstrates a very hopeful future in providing realistic CSSD persistence prediction. Future work includes the incorporation of other drought factors and the probabilistic consideration into the drought persistence models and application of the models to the real drought-persistence prediction. |
