Synoptic moisture pathways associated with mean and extreme precipitation over Canada for summer and fall

Abstract

Dominant synoptic moisture pathway patterns of vertically integrated water vapor transport (IVT) in winter and spring over Canada West and East were identified using the self-organizing map method. Large-scale meteorological patterns (LSMPs), together with synoptic moisture pathway patterns, were related to the variability in seasonal precipitation totals and occurrences of extreme precipitation events. Changes in both occurrences of LSMPs and seasonal precipitation occurred under those LSMPs were analyzed to explain observed changes in seasonal precipitation totals and occurrences of extreme precipitation events. The effects of large-scale climate anomalies on occurrences of LSMPs were also examined by composite analyses. Results show that synoptic moisture pathways and LSMPs exhibit the propagation of jet streams resulting from the Rossby wave resonance, as the location and direction of ridges and troughs, and the strength and center of pressure lows and highs varied considerably. Even though LSMPs resulting in positive precipitation anomalies are associated with more frequent occurrences of extreme precipitation events than those resulting in negative precipitation anomalies, the patterns featured with anomalously low IVT are sometimes associated with more frequent occurrences of extreme precipitation events. Significant decreases in occurrences of synoptic moisture pathway patterns that are favorable with positive precipitation anomalies and more precipitation extremes in winter over Canada West, and significant decreases in seasonal precipitation and occurrences of precipitation extremes under most synoptic moisture patterns resulted in decreases in seasonal precipitation and the occurrence of extreme precipitation events. LSMPs resulting in a hot and dry (cold and wet) climate and less (more) frequent extreme precipitation events over the Canadian Prairies in winter and northwestern Canada in spring are more likely to occur in years with a negative (positive) phase of PNA. Occurrences of LSMPs for a wet climate and frequent occurrences of extreme precipitation events over southeastern Canada are associated with a positive phase of NAO. In El Nino years or negative PDO years, occurrences of LSMPs tend to associate with a dry climate and less frequent precipitation extremes over western Canada.