Simulation of a North American Monsoon Gulf Surge Event and Comparison to Observations

Gulf surges are transient disturbances that propagate along the Gulf of California (GoC) from south to north, transporting cool moist air toward the deserts of northwest Mexico and the southwest United States during the North American monsoon. They have been shown to modulate precipitation and have been linked to severe weather and flooding in northern Mexico and the southwest United States. The general features and progression of surge events are well studied, but their detailed evolution is still unclear. To address this, several convection-permitting simulations are performed over the core monsoon region for the 12–14 July 2004 gulf surge event. This surge event occurred during the North American Monsoon Experiment, which allows for extensive comparison to field observations. A 60-h reference simulation is able to reproduce the surge event, capturing its main characteristics: speed and direction of motion, thermodynamic changes during its passage, and strong northward moisture flux. While the timing of the simulated surge is accurate to within 1–3 h, it is weaker and shallower than observed. This deficiency is likely due to a combination of weaker convection and lack of stratiform precipitation along the western slopes of the Sierra Madre Occidental than observed, hence, weaker precipitation evaporation to aid the surge. Sensitivity simulations show that convective outflow does modulate the intensity of the simulated surge, in agreement with past studies. The removal of gap flows from the Pacific Ocean across the Baja Peninsula into the GoC shows they also impact surge intensity.

The Influence of Monsoonal Gulf Surges on Precipitation and Diurnal Precipitation Patterns in Central Arizona

North American gulf-surge events, northward low-level influxes of cool and moist air from the Gulf of California, were statistically related to monsoonal precipitation and the associated diurnal cycle for the state of Arizona. Using Dixon’s Assessing Low-Level Atmospheric Moisture using Soundings (ALARMS) method as an indicator for gulf surges, a sequence of surge events was identified for the months of July and August for the period from 1957 to 2008. A network of Arizona precipitation gauges was stratified by the surge events occurring over this period. The findings indicate that gulf surges accounted for a significant majority of rainfall events in Arizona. This signal was most apparent in the drier central and southwestern deserts (including the Phoenix metropolitan area) and least apparent in the wetter eastern and southeastern portions of the state. Diurnal patterns in rainfall were identified for the Phoenix metropolitan area and its surroundings. A strong diurnal cycle was apparent in precipitation associated with both surge and nonsurge periods over the Phoenix area, with a greater tendency toward nocturnal precipitation during gulf-surge events. These findings suggest that dissipating afternoon thunderstorms east and northeast of the Phoenix area act as catalysts for the nocturnal storm development that is prevalent in this area.

Impact of Tropical Easterly Waves on the North American Monsoon

The North American monsoon (NAM) is a prominent summertime feature over northwestern Mexico and the southwestern United States. It is characterized by a distinct shift in midlevel winds from westerly to easterly as well as a sharp, marked increase in rainfall. This maximum in rainfall accounts for 60%–80% of the annual precipitation in northwestern Mexico and nearly 40% of the yearly rainfall over the southwestern United States. Gulf surges, or coastally trapped disturbances that occur over the Gulf of California, are important mechanisms in supplying the necessary moisture for the monsoon and are hypothesized in previous studies to be initiated by the passage of a tropical easterly wave (TEW). Since the actual number of TEWs varies from year to year, it is possible that TEWs are responsible for producing some of the interannual variability in the moisture flux and rainfall seen in the NAM. To explore the impact of TEWs on the NAM, four 1-month periods are chosen for study that represent a reasonable variability in TEW activity. Two continuous month-long simulations are produced for each of the selected months using the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model. One simulation is a control run that uses the complete boundary condition data, whereas a harmonic analysis is used to remove TEWs with periods of approximately 3.5 to 7.5 days from the model boundary conditions in the second simulation. These simulations with and without TEWs in the boundary conditions are compared to determine the impact of the waves on the NAM. Fields such as meridional moisture flux, rainfall totals, and surge occurrences are examined to define similarities and differences between the model runs. Results suggest that the removal of TEWs not only reduces the strength of gulf surges, but also rearranges rainfall over the monsoon region. Results further suggest that TEWs influence rainfall over the Southern Plains of the United States, with TEWs leading to less rainfall in this region. While these results are only suggestive, since rainfall is the most difficult model forecast parameter, it may be that TEWs alone can explain part of the inverse relationship between NAM and Southern Plains rainfall.

Principal Component Analysis of the Summertime Winds over the Gulf of California: A Gulf Surge Index

A principal component analysis of the summertime near-surface Quick Scatterometer (QuikSCAT) winds is used to identify the leading mode of synoptic-scale variability of the low-level flow along the Gulf of California during the North American monsoon season. A gulf surge mode emerges from this analysis as the leading EOF, with the corresponding principal component time series interpretable as an objective index for gulf surge occurrence. This index is used as a reference time series for regression analysis and compositing meteorological fields of interest, to explore the relationship between gulf surges and precipitation over the core and marginal regions of the monsoon, as well as the manifestation of these transient events in the large-scale circulation. It is found that, although seemingly mesoscale features confined over the Gulf of California, gulf surges are intimately linked to patterns of large-scale variability of the eastern Pacific ITCZ and greatly contribute to the definition of the northward extent of the monsoonal rains.

Using Sounding Data to Detect Gulf Surges during the North American Monsoon

Periodic surges of moisture from the Gulf of California are considered to be partly responsible for widespread showers and thunderstorms across the deserts of Arizona during the summer monsoon season. Presently, the primary method for detecting these surges is to look for changes in the surface observations at Yuma, Arizona. Unfortunately, these surface data are easily influenced by gust fronts, marine layers, and the dramatic diurnal patterns of the desert environment. Therefore, a new method for Assessing Low-level Atmospheric Moisture using Soundings (ALARMS) is proposed. Gulf surge–induced moisture increases identified by this new set of criteria are compared to other methods in order to determine the usefulness of each. This study demonstrates that the proposed method performs much better than the others that were tested, with the additional advantage of being relatively easy to apply.