Using Observational and Reanalysis Data to Explore the Southern Gulf of California Boundary Layer During the North American Monsoon Onset

Abstract

This paper uses rawinsondes and pilot balloon data from the 2004 North American Monsoon (NAM) Experiment, as well as satellite‐based products and reanalysis datasets that span 1982–2018, to analyze the mixing mechanisms responsible for the temporal and spatial variations of the Gulf of California (GoC) boundary layer during the NAM onset in the core monsoon region. We show that the regional diurnal cycle is strongly affected by low‐level convergence and divergence associated with local land‐sea breezes and by the presence of a thermal inversion over the gulf. Earlier starting monsoons have less moisture available for precipitation than those starting later in the calendar year. Therefore, early onset monsoons have less rainfall during their first month, which is a result that is in contrast with previous studies that have analyzed the timing of the NAM but only reported seasonal precipitation totals. The GoC boundary layer height at the time of monsoon onset, found to be controlled by the gulf’s surface temperature, has a significant impact on the precipitation over Sonora, Sinaloa, and southern Arizona. After the erosion of the thermal inversion over the GoC that coincides with the NAM onset, wind shear produced by the region’s unique geographic and topographic features is the largest source of turbulence for the mixing of the boundary layer. Our results suggest that a numerical model used to forecast or analyze NAM precipitation must have enough spatial resolution to adequately reproduce the effects that the GoC’s features have on its complex diurnal circulation systems.

The Sensitivity of the North American Monsoon to Gulf of California Sea Surface Temperatures

“…This deep moist layer, when accompanied by southeasterly flow along the GoC, can advect into the southwest US and western Mexico and enhance precipitation. These conclusions agreed well with subsequent work from Fonseca-Hernandez et al (2021) who used observations to study the boundary layer over the GoC and how it correlates to precipitation in the NAM region. They found that GoC SSTs were highly correlated with boundary layer height over the GoC and that GoC boundary layer heights were moderately correlated with the magnitude of monsoon precipitation in western coastal Mexico and portions of southwestern Arizona.…”

“…However, recent work using a set of convection-permitting models with embedded water vapor tracers indicate that the moisture contributions of the GoC may be overestimated in past work utilizing convection-parameterizing models forced by North American Regional Reanalysis data (Wallace, 2022). Despite this, observational evidence and numerical sensitivity studies indicate that NAM rainfall demonstrates a sensitivity to SSTs in the GoC (Mitchell et al, 2002;Kim et al, 2005;Erfani and Mitchell, 2014;Fonseca-Hernandez et al, 2021;Johnson and Delworth, 2023). This calls into question the specific role the GoC plays in modulating NAM rainfall and whether its moisture contributions to local rainfall are more important than its effects on local stability.…”

Expansion and Intensification of the North American Monsoon During the Pliocene

“…In the present climatology, the region around DSDP 475 receives less than 1 mm/day of rain on average, and in many years receives no monsoon rainfall (Fonseca‐Hernandez et al., 2021) (Figure S3 in Supporting Information ). A positive Pliocene leaf wax δ D p signature at DSDP 475 likely reflects intensification of the monsoon in its core region, on the slopes of the Sierra Madre Occidental, as well as its expansion into Baja.…”

“…Statistically significant rainfall changes occur in the core NAM domain, but also in peripheral regions like the Baja California Peninsula, which normally experiences atmospheric subsidence and receives little monsoon rain (Figure S6 in Supporting Information S1) (Fonseca-Hernandez et al, 2021). This is illustrated by a shift in daily summertime OLR over Baja, which shows an increase to near 240-250 W/m 2 .…”

Expansion and intensification of the North American Monsoon during the Pliocene

“…Statistically significant rainfall changes occur in the core NAM domain, but also in peripheral regions like the Baja California Peninsula, which normally experiences atmospheric subsidence and receives little monsoon rain (Figure S6) (Fonseca-Hernandez et al, 2021). This is illustrated by a shift in daily summertime outgoing longwave radiation (OLR) values over Baja, which shows an increase in values near 240-250 W/m 2 .…”