Relationship between Tropical Easterly Waves and Precipitation during the North American Monsoon

Abstract

Relationships between tropical easterly waves (TEWs) and precipitation over Mexico and the United States are examined during the North American monsoon (NAM). The National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis data are used to identify 137 TEWs that cross Mexico north of 208N after monsoon onset over a 31-yr period from 1975 to 2005. Mean precipitation anomalies over two-day periods both before and after TEW passage are determined using Climate Prediction Center daily precipitation analyses. Results indicate that positive precipitation anomalies occur along the west coast of Mexico and extending into the west-central United States in association with TEW passage. Negative precipitation anomalies are found in the south-central United States. These precipitation anomaly patterns share many similarities to precipitation anomaly patterns previously defined in association with gulf surge events. On longer time scales, correlations between the total number of these northern TEWs crossing Mexico and 90-day monsoon period precipitation anomalies are also examined. An out-of-phase relationship is found between monsoon period precipitation anomalies in the southwestern and south-central United States, suggesting that increasing the number of northern TEWs crossing Mexico leads to enhanced monsoon period rainfall in Arizona and New Mexico and reduced monsoon period rainfall in Texas and Oklahoma. Thus, these northern TEWs likely play an important role in producing the distribution of precipitation throughout the NAM region and the south-central United States during the monsoon season, and extendedrange predictions of northern TEW frequency may lead to improved seasonal rainfall anomaly forecasts in these regions.

An anomaly in the occurrence of the month of maximum precipitation distribution in northwest Mexico

“…This kind of mechanism is not uncommon in latitudes of Nayarit (Zuidema et al, 2007). The propagation of easterly waves, that could be associated with strong vertical coupling from diabatic heating, from across the tropical Atlantic to the eastern Pacific also affect precipitation patterns over parts of western Mexico (Giovannettone and Barros, 2008;Ladwig and Stensrud, 2009). Interactions between terrain heating and synoptic-scale disturbances not necessarily associated with NAM could also play an important role in explaining the temporal variability of precipitation in central and southern Mexico.…”

Flow, Moisture, and Thermodynamic Variability Associated with Gulf of California Surges within the North American Monsoon

“…The positive anomaly associated with the surge then moves north along the western Mexican coastline as the surge progresses, reaching northwestern Mexico by the time the surge arrives in Yuma and the southwestern United States in the subsequent days. Despite the long-held belief that surges are associated with enhanced regional mean NAM precipitation, recent work by Ladwig and Stensrud (2009) showed that surge events might redistribute NAM precipitation within the domain rather than enhance it.…”

Historical and Projected Eastern Pacific and Intra-Americas Sea TD-Wave Activity in a Selection of IPCC AR5 Models

“…Here, we examine the environmental factors contributing to the southward shift in TD wave tracks, with a focus on the eastern Pacific, where shifts in the wave tracks have implications with respect to cyclogenesis through interactions of the flow with the topography of Central America (Zehnder et al 1999;Molinari et al 2000). A southward shift in these waves could also have implications for seasonal rainfall patterns over Central America, Mexico, and the southwestern United States (Amador et al 2006;Adams and Stensrud 2007;Ladwig and Stensrud 2009;Seastrand et al 2015).…”