Role of the land‐sea thermal contrast in the interannual modulation of the North American Monsoon

Abstract

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.

An anomaly in the occurrence of the month of maximum precipitation distribution in northwest Mexico

“…Seasonal changes in the thermal contrast between the continent and adjacent oceanic regions are characteristic features of lowlatitude continental regions causing generally favorable conditions for the development of the monsoon (Vera et al, 2006;Turrent and Cavazos, 2009). However, high orography such as the SMO, with maximum elevations in excess of 3000 m, can serve to reduce the moisture transport into continental interior regions from nearby oceanic sources (Manabe and Broccoli, 1990).…”

“…In addition to variations in Pacific sea surface temperature (SST) and circulation patterns (Carleton et al, 1990;Castro et al, 2001;Brito-Castillo et al, 2003;Higgins et al, 2003), the intensification of the North Atlantic Subtropical High (NASH), in response to decadescale variations in western Atlantic and Caribbean sea surface temperatures, has been linked to increased moist instability and NAM rainfall across much of Mexico (Cavazos and Hastenrath, 1990;Gochis et al, 2007a;Wang et al, 2007). Additionally, the thermal contrast between the continent and adjacent oceanic regions in the eastern Pacific also helps support the formation of the NAM circulation system (Douglas et al, 1993;Higgins et al, 2003;Vera et al, 2006;Zhu et al, 2007;Giovannettone andBarros, 2008, Turrent andCavazos, 2009).…”

Seasonality and anomalies of sea surface temperature off the coast of Nayarit, Mexico

“…It is necessary to mention that the maximum SST occurs in September, while maximum volume of rain occurs in August, so the peaks between rainfall and SST are out of phase by 1 month with rainfall peaks occurring earlier. The rainfall peak in Nayarit may not only be associated to the seasonal march of SSTs but also to the differential heating between land and ocean (e.g., Turrent and Cavazos 2009). It is not only the warming of the ocean that counts but also the heating of the land.…”

Regional Pattern of Trends in Long-Term Precipitation and Stream Flow Observations: Singularities in a Changing Climate in Mexico

“…Summer precipitation in Mexico (May through October, see Figure 3 right maps) is, in large part, controlled by the North American Monsoon (NAM) system (Higgins et al, 2003), which is associated with thermal contrasts between the continent and adjacent eastern Pacific Ocean (Giovannettone & Barros, 2008;Higgins et al, 2003;Turrent & Cavazos, 2009). …”