Analysis of the 13 July gulf surge event during the 2004 North American Monsoon Experiment

Gulf surges are disturbances that move northward along the Gulf of California (GoC) frequently advecting cool, moist air from the GoC or eastern tropical Pacific Ocean into the deserts of the southwest United States and northwest Mexico during the North American Monsoon (NAM). Little attention has been given to the dynamics of these disturbances due to the lack of reliable high-resolution data across the NAM region. High temporal and spatial observations collected during the 2004 North American Monsoon Experiment (NAME) are used to investigate the structure and dynamical mechanisms of a significant gulf surge on 13 July. Integrated Sounding Systems (ISSs) deployed along the east coast of the GoC and an enhanced network of rawinsonde sites across the NAM domain are used in this study.

Observations show that the gulf surge occurs amid complex synoptic, mesoscale, and convective environments. Preceded by anomalous warming and a low-level nocturnal stable layer, the surge propagates rapidly up the length of the GoC (~17-22 m/s) and is characterized by sharp anomalous cooling and strengthening of the wind in the lowest 2 km of the atmosphere. The surge arrives along the northern GoC during the early morning hours and causes a deepening of the well-mixed boundary layer. Surface pressures rapidly rise as well. Surge characteristics are best observed at sites along the northern GoC and become increasingly difficult to discern farther south.

The weight of the evidence presented in this study suggests that the initial surge over the northern gulf may be due to bore-like disturbances that owed their existence to convective downdrafts impinging on the nocturnal inversion over the region. Following these initial pulses, a deeper layer of sharp cooling and strong winds ensued, which likely represents a Kelvin wave-type disturbance. Another possibility is that the leading edge of this Kelvin wave steepened nonlinearly into a bore-like disturbance. The data are not adequate to delineate between these possible mechanisms.