Atmospheric and Oceanic Variability associated with Growing Season Droughts and Pluvials on the Canadian Prairies

This study documents and assesses the atmospheric and oceanic variability associated with growing season (May to August) droughts over the Canadian Prairies. For comparison, extreme wet seasons or pluvials are also examined. Using the Palmer Z-Index as a drought indicator, extreme dry and wet seasons are firstly identified for the period 1950 to 2007. Inter-relationships among several atmospheric parameters including large to synoptic-scale circulation patterns, low-level moisture transport, moisture convergence, precipitable water content, and cyclone frequency are then assessed during extreme drought and pluvial periods. In addition, links to the previous winter's global sea-surface temperature (SST) patterns are identified using the multivariate technique of singular value decomposition. Results show that moisture from the Gulf of Mexico is notably decreased during the identified drought seasons. Stronger than normal subsidence associated with anomalously high pressure over north-western North America also leads to weakened moisture transport from the Pacific Ocean. Conversely, during pluvial seasons, low-level flow aided by the circulation associated with increased cyclone frequency over western North America brings abundant moisture northward into the southern Prairie region. These circulation patterns over western North America and their associated moisture transport anomalies into the Prairies show some linkages to previous winter SST patterns both globally, and in the Pacific Ocean where the SSTs are similar to those associated with inter-annual El Niño/Southern Oscillation (ENSO) events and ENSO-like inter-decadal North Pacific variability. This is the first study to examine several inter-connected atmospheric and oceanic processes at various scales as they relate to the occurrence of growing season extreme climate over the Canadian Prairies. Results provide a better understanding into the physical mechanisms responsible for the initiation of and perpetuation of these extremes.