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 ϳ20 m s Ϫ1 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 ϳ17-25 m s Ϫ1 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.

The Impact of Horizontal Resolution on North American Monsoon Gulf of California Moisture Surges in a Suite of Coupled Global Climate Models

“…All models, with the exception of CM2.1, fairly realistically reproduce the strong southeasterly wind anomalies along the GoC seen in ERA-I (Anderson et al 2000;Berbery and Fox-Rabinovitz 2003;Gochis et al 2004;Rogers and Johnson 2007;Douglas and Leal 2003). Also evident are a concomitant progression of a TEW, or the passage of a TC (Higgins and Shi 2005;Corbosiero et al 2009;Wood and Ritchie 2013), to the south of the GoC and a midlatitude westerly disturbance over the central United States (e.g., Pascale and Bordoni 2016).…”

“…A large humidity anomaly, located right over the GoC, typically precedes the arrival of the moisture surge over land (e.g., Rogers and Johnson 2007). Thereafter the anomalous moisture spreads into the southwestern United States.…”

“…Synoptic-scale convective activity over AZWNM tends to be preceded by Gulf of California (GoC) moisture surges (or simply GoC surges). These are coastally trapped, northward-propagating disturbances, characterized by anomalous southerly flow and moisture transport along the GoC (e.g., Hales 1972;Brenner 1974;Douglas et al 1993;Adams and Comrie 1997;Stensrud et al 1997;Bordoni et al 2004;Zehnder 2004;Higgins et al 2004;Rogers and Johnson 2007;Svoma 2010;Newman andJohnson 2012b, 2013). GoC surges modulate, and are responsible for, the establishment of the summertime GoC southerly low-level jet (Douglas 1995;Berbery 2001;Bordoni et al 2004).…”

“…GoC surges modulate, and are responsible for, the establishment of the summertime GoC southerly low-level jet (Douglas 1995;Berbery 2001;Bordoni et al 2004). While GoC surges are often initiated by outflows of mesoscale convective systems in the lower GoC region (Rogers and Johnson 2007;Mejia et al 2010Mejia et al , 2015, associated precipitation over the southwestern United States is strongly linked to largescale tropical and extratropical waves on synoptic (2-8 days; Stensrud et al 1997;Higgins et al 2004;Adams and Stensrud 2007;Ladwig and Stensrud 2009;Bieda et al 2009;Bordoni and Stevens 2006;Schiffer and Nesbitt 2012;Seastrand et al 2015), quasi-biweekly (10-20 days; Kiladis and Hall-McKim 2004;Jiang and Lau 2008;Kikuchi and Wang 2009), and subseasonal (25-90 days; Lorenz and Hartmann 2006;Jiang and Waliser 2009;Wu et al 2009;Pascale and Bordoni 2016) time scales. In particular, Pascale and Bordoni (2016) show that strong precipitation events in this region are associated with time scales longer than synoptic, with the quasi-biweekly and subseasonal modes playing a dominant role in the occurrence of these more intense precipitative events.…”

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

“…Although studies have observed the presence of diurnal low-level jets over the Gulf of California (Douglas 1995;Douglas et al 1998;Bordoni et al 2004), the most influential transport mechanism for the core monsoon region is the gulf surge (Hales 1972(Hales , 1974Brenner 1974;McCollum et al 1995;Stensrud et al 1995Stensrud et al , 1997Fuller and Stensrud 2000;Douglas and Leal 2003;Rogers and Johnson 2007;Bordoni and Stevens 2006). A gulf surge is a coastally trapped wavelike disturbance that travels northward over the Gulf of California.…”