Effect of Gulf of California Sea Surface Temperature on North American monsoon
Abstract
The North American Monsoon (NAM) is a seasonal shift in the large-scale circulation that provides the majority of annual rainfall in northwestern Mexico and US southwest. Although regional climate models have succeeded in reproducing some characteristics of the NAM, its onset, strength and evolution are not well predicted. A physical understanding of key processes governing its life-cycle is needed to guide improvements in regional and global climate modeling of the NAM and its remote impacts on the summer circulation and precipitation patterns over North America. In this study, we propose a partial mechanistic understanding of the NAM incorporating local- and large-scale processes. The local scale mechanism explains the effect of marine boundary layer (MBL) over the northern Gulf of California (GC) using satellite observations and ship soundings launched over the GC. The strong low-level inversion capping the top of shallow MBL weakens with increasing SSTs and generally disappears once SSTs exceed 29°C, allowing the trapped MBL moisture to mix with free tropospheric air. This leads to a deep, moist, well-mixed layer that can be transported inland by favorable low-level jets to form thunderstorms in Arizona and elsewhere. The large-scale mechanism relates tropical surface water, tropospheric moisture and the NAM anticyclone by means of climatologies of satellite SST, outgoing longwave radiation (OLR) and NCEP/NCAR reanalysis of 500 hPa geopotential height from 1983 to 2010. As warm Pacific SSTs propagate northwards up the Mexican coastline, deep convection follows this northward advance, with associated descending air north/northeast of the convection region possibly advancing the position of the anticyclone. This evolution brings mid-level tropical moisture into the NAM region. A set of carefully designed simulations of WRF is used to investigate the dependence of NAM precipitation, onset and circulation on SSTs along the Mexican coastline and in the GC. North American Monsoon Experiment (NAME) field campaign in summer 2004 provides unique enhanced observations to evaluate the modeling results. Black solid lines depict the vertical profile of temperature and relative humidity (RH) for 10% of soundings having strongest inversion cap and 10% having weakest inversion cap during June and August 2004, based on RV balloon soundings in GC. Purple lines show the standard deviations.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.A53N..06E
- Keywords:
-
- 3335 ATMOSPHERIC PROCESSES North American Monsoon;
- 3355 ATMOSPHERIC PROCESSES Regional modeling;
- 3339 ATMOSPHERIC PROCESSES Ocean/atmosphere interactions