The land‐sea thermal contrast (LSTC) that is present at the initial stage of the North American Monsoon is analyzed as a principal driving mechanism for monsoon onset interannual variability. The vertically integrated moisture flux convergence (MFC) averaged over the core monsoon during June 16–30, derived from North American Regional Reanalysis daily fields for the period 1979–2006, is proposed as an index for initial monsoon intensity. We quantify the LSTC associated to the monsoon and propose a dynamic connection between the thermal contrast and the initial monsoon intensity. It consists of a directly proportional relation between the LSTC, the surface pressure gradient along the Gulf of California, and the ensuing low level (below 850 mb) moisture transport (from the southern Gulf of California and eastern tropical Pacific) and precipitation in the core region.

The hypothesis that global warming during the twenty-first century will increase the land–sea thermal contrast (LSTC) and therefore the intensity of early season precipitation of the North American monsoon (NAM) is examined. To test this hypothesis, future changes (2075–99 minus 1979–2004 means) in LSTC, moisture flux convergence (MFC), vertical velocity, and precipitation in the region are analyzed using six global climate models (GCMs) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) under the representative concentration pathway 8.5 (RCP8.5) emission scenario. A surface LSTC index shows that the continent becomes warmer than the ocean in May in the North American Regional Reanalysis (NARR) and ECMWF Interim Re-Analysis (ERA-Interim) and in June in the mean ensemble of the GCMs (ens_GCMs), and the magnitude of the positive LSTC is greater in the reanalyses than in the ens_GCMs during the historic period. However, the reanalyses underestimate July–August precipitation in the NAM region, while the ens_GCMs reproduces the peak season surprisingly well but overestimates it the rest of the year. The future ens_GCMs projects a doubling of the magnitude of the positive surface LSTC and an earlier start of the continental summer warming in mid-May. Contrary to the stated hypothesis, however, the mean projection suggests a slight decrease of monsoon coastal precipitation during June–August (JJA), which is attributed to increased midtropospheric subsidence, a reduced midtropospheric LSTC, and reduced MFC in the NAM coastal region. In contrast, the future ens_GCMs produces increased MFC and precipitation over the adjacent mountains during JJA and significantly more rainfall over the entire NAM region during September–October, weakening the monsoon retreat.

We estimate trends of extreme daily precipitation (P95 > 95th percentile) events in the core of the North American monsoon region in Northwest Mexico during JJAS of 1961–1998. The intensity and seasonal contribution of P95 show significant upward linear trends in the mountain sites, which appear to be related to an increased contribution from heavy precipitation derived from tropical cyclones (TCs). Frequency of P95, total monsoon precipitation, and P95 in coastal stations did not change significantly. TC‐derived P95 events are associated with SST anomalies similar to weak La Niña conditions in the eastern Equatorial Pacific, SSTs > 28.5°C in the Caribbean Sea, and strong land‐sea thermal contrast over Northwest Mexico and the U.S. Southwest two weeks prior to their onset.

In this study the results of two regional fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5) simulations forced at their boundaries with low-pass-filtered North American Regional Reanalysis (NARR) composite fields from which synoptic-scale variability was removed are presented. The filtered NARR data are also assimilated into the inner domain through the use of field nudging. The purpose of this research is to investigate wet and dry onset modes in the core region of the North American monsoon (NAM). Key features of the NAM that are present in the NARR fields and assimilated into the regional simulations include the position of the midlevel anticyclone, low-level circulation over the Gulf of California, and moisture flux patterns into the core monsoon region, for which the eastern Pacific is the likely primary source of moisture. The model develops a robust diurnal cycle of deep convection over the peaks of the Sierra Madre Occidental (SMO) that results solely from its radiation scheme and internal dynamics, in spite of the field nudging. The wet onset mode is related to a regional land-sea thermal contrast (LSTC) that is ;28C higher than in the dry mode, and is further characterized by a northward-displaced midlevel anticyclone, a stronger surface pressure gradient along the Gulf of California, larger mean moisture fluxes into the core region from the eastern Pacific, a stronger diurnal cycle of deep convection, and the more northward distribution of precipitation along the axis of the SMO. A proposed regional LSTC mechanism for NAM onset interannual variability is consistent with the differences between both onset modes.

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