Tuesday, 11 February 2003
Modes of inter–annual variability of atmospheric moisture flux transport
Identifying the most important mechanisms of moisture transport in the atmosphere is an important step in understanding the interaction between the atmospheric and terrestrial branches of the hydrologic cycle. By analyzing the predominant modes of water vapor flux we will better understand how global patterns as well as local processes affect a region’s climate, weather, precipitation and water resources. The central hypothesis of this study is that large scale atmospheric circulation patterns as well as local recycling are the driving mechanisms for water vapor flux. The goal of this work is to obtain the dominant spatial modes of seasonal and monthly moisture flux in the atmosphere. In order to identify the transport modes, we first perform a vertical integration of the moisture flux in the atmosphere using the 52-year NCEP/NCAR ReanalysisI data. The integration is performed over the total atmospheric column (twenty-eight pressure levels) for seasonal and monthly averages. The deviations from the mean are also calculated and their relative contribution to the total moisture flux is evaluated. Because these eddy fluxes are responsible for 10 to 20% of the total flux, their importance cannot be overlooked. The moisture transported in the boundary layer is calculated separately in order to evaluate its contribution to the total amount of moisture flux. Rotated principal component analysis (RPCA), specifically verimax rotation, is then performed on the integrated average monthly and seasonal datasets. This analysis serves as a tool to identify the principal modes of variability in moisture flux magnitudes. The RPCA method was chosen as opposed to Principal Component Analysis (PCA) in order to obtain a more simple structure, and in this way simplify the interpretation of the modes. Flow patterns for positive (negative) phase of each mode may be obtained by averaging the anomalies for the years above (below) one standard deviation in that mode. In previous work, this methodology has proven to be useful as it highlights the relationships that can be found between the principal modes of moisture flux transport and the inter-annual precipitation and flow field variability. As a result from this study, we expect some of the principal modes of moisture flux to be driven by large-scale circulation patterns. We will be able to relate these modes to patterns of atmospheric variability that have been established in previous studies such as the Artic Oscillation (AO), North Atlantic Oscillation (NAO), Pacific North American Pattern (PNA) etc. Some modes, however, will not be generated by large-scale forcing. Local recycling, and memory from surface and subsurface storage of water, snow or ice are the driving forces behind these water vapor fluxes. The hydrology of a specific region is greatly influenced by the relative contribution of these two mechanisms of moisture transport. This research will provide further insight into the inter-annual variability of the hydrologic cycle, and has the potential to explain phenomena such as inter-annual lake level fluctuations. It will also attempt to determine the role of land surface and subsurface storage in regulating the inter-annual variability of the hydrologic cycle.