Tropical and Extratropical Controls of Gulf of California Surges and Summertime Precipitation over the Southwestern United States

Abstract

In this study ERA-Interim data are used to study the influence of Gulf of California (GoC) moisture surges on the North American monsoon (NAM) precipitation over Arizona and western New Mexico (AZWNM), as well as the connection with larger-scale tropical and extratropical variability. To identify GoC surges, an improved index based on principal component analyses of the near-surface GoC winds is introduced. It is found that GoC surges explain up to 70% of the summertime rainfall over AZWNM. The number of surges that lead to enhanced rainfall in this region varies from 4 to 18 per year and is positively correlated with annual summertime precipitation. Regression analyses are performed to explore the relationship between GoC surges, AZWNM precipitation, and tropical and extratropical atmospheric variability at the synoptic (2-8 days), quasi-biweekly (10-20 days), and subseasonal (25-90 days) time scales. It is found that tropical and extratropical waves, responsible for intrusions of moist tropical air into midlatitudes, interact on all three time scales, with direct impacts on the development of GoC surges and positive precipitation anomalies over AZWNM. Strong precipitation events in this region are, however, found to be associated with time scales longer than synoptic, with the quasi-biweekly and subseasonal modes playing a dominant role in the occurrence of these more extreme events.

Subseasonal Predictability of Boreal Summer Monsoon Rainfall from Ensemble Forecasts

“…For the AM, additional maximum RPSS during phases 6, 8, 1, and 2 indicate bettter skill for monsoon breaks compared to active episodes most frequent during phase 4 and for which skill is minimum, in agreement with greater spatial coherence and predictability than for the core of the monsoon season (Moron et al, 2017). For the NAM and WAM, skill is minimum during MJO phase 3 when convection is enhanced over the Indian Ocean, in turn increasing the activity of AEWs within the WAM and tropical Atlantic TCs (Zhang, 2013;Klotzbach and Oliver, 2015) linked to Gulf moisture surges feeding in the NAM (Pascale and Bordoni, 2016), which synoptic nature might be less predictible. Contrasting skill relationships to MJO phases 7 and 3-4 suggest potential skill improvement when RMM1 and RMM2 are respectively negative and positive, consistently with mean RPSS and PC1 correlations with MJO RMMs for the AM and WAM in particular.…”

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

“…As in Pascale and Bordoni (2016), the focus of this study is on the influence of GoC surges on precipitation over AZWNM, defined as the area between 1148 and 1088W and between 318 and 368N (Fig. 1).…”

“…While modulated by daytime heating, AZWNM convective activity also features synoptic to submonthly variability (Berbery and Fox-Rabinovitz 2003;Wu et al 2009;Pascale and Bordoni 2016), as revealed by significant spectral peaks in Arizona rainfall in the 5-20-day band or even longer time scales (e.g., Cavazos et al 2002;Nolin and Hall-McKim 2006). Synoptic-scale convective activity over AZWNM tends to be preceded by Gulf of California (GoC) moisture surges (or simply GoC surges).…”

“…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. Thus, capturing GoC surge events, as well as their mesoscale and large-scale tropical and extratropical controls, appears to be essential to have some success in modeling the NAM precipitation.…”

“…This is the first study to provide a detailed analysis of GoC surges in global coupled GCMs of uniform horizontal resolution. Given the large uncertainty of GCM projections of the NAM response in the southwestern United States (e.g., Cook and Seager 2013;Pascale et al 2016), assessing the simulated present-day synoptic-scale variability, such as that associated with GoC surges, is a necessary effort to gain any mechanistic understanding of how the NAM will respond to global warming or to build a basis for dynamical seasonal prediction of the NAM. The paper will be structured as follows.…”