A partial mechanistic understanding of the North American monsoon

Abstract

An understanding of the major governing processes of North American monsoon (NAM) is necessary to guide improvement in global and regional climate modeling of the NAM, as well as NAM's impacts on the summer circulation, precipitation, and drought over North America. A mechanistic understanding of the NAM is suggested by incorporating local-and synoptic-scale processes. The local-scale mechanism describes the effect of the temperature inversion over the Gulf of California (GC) on controlling low-level moisture during the 2004 NAM. The strong low-level inversion inhibits the exchange between the moist air in the marine boundary layer (MBL) and the overlying dry air. This inversion weakens with increasing sea surface temperatures (SSTs) in GC and generally disappears once SSTs exceed 29.5°C, allowing the moist air, trapped in the MBL, to mix with free tropospheric air. This leads to a deep, moist layer that can be transported by across-gulf (along-gulf) flow toward the NAM core region (southwestern U.S.) to form thunderstorms. On the synoptic scale, climatologies from 1983 to 2010 exhibit a temporal correspondence between coastal warm tropical surface water, NAM deep convection, NAM anticyclone center, and NAM-induced strong descent. A hypothesis is proposed to explain this correspondence, based on limited soundings at the GC entrance (suggesting this local mechanism may also be active in that region), the climatologies, and the relevant literature. The warmest SSTs moving up the coast may initiate NAM convection and atmospheric heating, advancing the position of the anticyclone and the region of descent northward.

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

“…Since the seminal studies by Stensrud et al (1997) and Fuller and Stensrud (2000), it has been well known that at synoptic time scales (2-8 days) the initiation of a GoC surge is linked to the passage of a tropical easterly wave (TEW) trough or a tropical cyclone (TC) across southwestern Mexico (at around 208N). The relationship between gulf surges and TEWs/TCs has been confirmed by subsequent studies (Anderson et al 2000;Higgins et al 2004;Higgins and Shi 2005;Bordoni and Stevens 2006;Schiffer and Nesbit 2012;Favors and Abatzoglou 2013;Seastrand et al 2015). Among these, Higgins et al (2004) emphasized how the extent to which a surge is followed by positive (wet surge) or negative (dry surge) precipitation anomalies over the southwestern United States cannot be discriminated based on the presence of TEWs.…”

“…The seasonal cycle of rainfall over northwestern Mexico and the southwestern United States is dominated by the North American monsoon (NAM), a distinctive summertime circulation characterized by the following features: a sharp rainfall increase in early July after a very dry June (Higgins et al 1997), the establishment of a midto upper-level monsoon anticyclone centered over New Mexico (Adams and Comrie 1997), a reversal in lowerlevel winds from northwesterly to southeasterly (Douglas 1995;Bordoni et al 2004), and a marked warming of Gulf of California (GoC) waters, with sea surface temperatures (SSTs) reaching or even exceeding everywhere 308C (Erfani and Mitchell 2014). The NAM precipitation accounts for as much as approximately 70% of the total annual rainfall in northwestern Mexico, the core monsoon region, and for approximately 40%-50% in the southwestern United States, its northernmost extremity (Douglas et al 1993;Anderson et al 2000) Hence, this system plays an important role in sustaining water resources and ecosystems in these regions.…”

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

“…This allows us to conclude that from an oceanic perspective the GoC impacts the NAM primarily by acting as a low-level moisture source, owing to its very warm SSTs (*288C; e.g., Erfani and Mitchell 2014). A more realistic representation of the GoC oceanic circulation, through improved resolution of mesoscale eddies, does not seem to be playing any significant role in NAM simulations.…”

“…7), which is in turn due to the strong reduction of the warm SST bias. Furthermore, the inaccurate representation of the northern GoC (with land surface rather than ocean) may also contribute to decreased precipitation in this region since the northern GoC is an important moisture source for Arizona's summertime rainfall (Mullen et al 1998;Schmitz and Mullen 1996;Berbery 2001;Erfani and Mitchell 2014). We also note that models miss the southward extent of the rainfall anomaly over Mexico seen in ERA-I at day 11.…”

Comparisons between marine productivity and terrestrial input records in the Gulf of California over the last 28 ka

“…Its climate is classified as BWh – hot desert or dry arid, with dry winters and relatively humid summers (García & CONABIO, 2001; INEGI, 2018). Seasonal warming links the summertime circulation to the North American Monsoon (NAM) intensification, causing intense precipitation (Erfani & Mitchell, 2014). Besides tropical storms and hurricanes, the NAM provides ~50% of the annual rainfall to this arid region, but studies show that it varies over time (Bhattacharya et al ., 2018; Erfani & Mitchell, 2014; Metcalfe et al ., 2015; Mitchell et al ., 2002).…”