Analysis of the 13–14 July Gulf Surge Event during the 2004 North American Monsoon Experiment

Abstract

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 because of the lack of reliable high-resolution data across the NAM region. High temporal and spatial observations collected during the 2004 North American Monsoon Experiment are used to investigate the structure and dynamical mechanisms of a significant gulf surge on 13?14 July 2004. Integrated Sounding Systems deployed along the east coast of the GOC and an enhanced network of rawinsonde sites across the NAM region are used in this study. Observations show that the 13?14 July gulf surge occurred in two primary stages. The first stage was preceded by anomalous low-level warming along the northern GOC on 13 July. Sharp cooling, moistening, and increased low-level south-southeasterly flow followed over a 12?18-h period. Over the northern gulf, the wind reached ?20 m s?1 at 750 m AGL. Then there was a brief respite followed by the second stage?a similar, but deeper acceleration of the southerly flow associated with the passage of Tropical Storm (TS) Blas on 14 July. The initial surge disturbance traversed the GOC at a speed of ?17?25 m s?1 and resulted in a deepening of the mixed layer along the northern gulf. Dramatic surface pressure rises also accompanied the surge. The weight of the evidence suggests that the first stage of the overall surge itself consisted of two parts. The initial part resembled borelike disturbances initiated by convective downdrafts impinging on the low-level stable layer over the region. The secondary part was characteristic of a Kelvin wave?type disturbance, as evident in the deeper layer of sharp cooling and strong wind that ensued. Another possible explanation for the first part is that the leading edge of this Kelvin wave steepened nonlinearly into a borelike disturbance. The second stage of the surge was associated with the increased circulation around TS Blas.