The Relationship of the North American Monsoon to Tropical and North Pacific Sea Surface Temperatures as Revealed by Observational Analyses

Abstract

The North American monsoon is a seasonal shift of upper-and low-level pressure and wind patterns that brings summertime moisture into the southwest United States and ends the late spring wet period in the Great Plains. The interannual variability of the North American monsoon is examined using the NCEP-NCAR reanalysis (1948-98). The diurnal and seasonal evolution of 500-mb geopotential height, integrated moisture flux, and integrated moisture flux convergence are constructed using a 5-day running mean for the months May through September. All of the years are used to calculate an average daily Z score that removes the diurnal, seasonal, and intraseasonal variability. The 30-day average Z score centered about the date is correlated with Pacific sea surface temperature anomaly (SSTA) indices associated with the El Niño-Southern Oscillation (ENSO) and the North Pacific oscillation (NPO). These indices are Niño-3, a North Pacific index, and a Pacific index that combines the previous two. Regional time-evolving precipitation indices for the Southwest and Great Plains, which consider the total number of wet or dry stations in a region, are also correlated with the SSTA indices. The use of nonnormally distributed point source precipitation data is avoided. Teleconnections are computed relative to the climatological evolution of the North American monsoon, rather than to calendar months, thus more accurately accounting for the climatological changes in the large-scale circulation. Tropical and North Pacific SSTs are related to the occurrence of the Pacific Transition and East Pacific teleconnection patterns, respectively, in June and July. A high (low) NPO phase and El Niño (La Niña) conditions favor a weaker (stronger) and southward (northward) displaced monsoon ridge. These teleconnection patterns affect the timing and large-scale distribution of monsoon moisture. In the Great Plains, the spring wet season is lengthened (shortened) and early summer rainfall and integrated moisture flux convergence are above (below) average. In the Southwest, monsoon onset is late (early) and early summer rainfall and integrated moisture flux convergence are below (above) average. Relationships with Pacific SSTA indices decay in the later part of the monsoon coincident with weakening of the jet stream across the Pacific and strengthening of the monsoon ridge over North America. The most coherent summer climate patterns occur over the entire western United States when the Pacific index is substantially high or low, such as during the Midwest flood of 1993 and drought of 1988. The Pacific index in spring is a good predictor of early summer height anomalies over the western United States when the time evolution of the North Pacific SST dipole is considered.

Correlations between sea‐surface temperatures and warm season streamflow in northwest Mexico

“…First, presumably in response to a diminished GPLLJ, there is an appreciable reduction in PW over the central Great Plains region during high SOI phases (La Niña) compared with low SOI phases (El Niño). This continental-scale PW pattern qualitatively resembles the out-of-phase relationship between warm season precipitation in the NAM region and that in the Great Plains, documented by Higgins et al (1997) and Castro et al (2001) among others. There is also comparatively enhanced PW over the southeastern US during high SOI phases.…”

“…Negative correlations between EOF1 and EOF3 streamflow and tropical Pacific SST increase in absolute value during MAM and MJJ but remain relatively modest overall. The combination of cool tropical conditions in, roughly, the N1 + 2 region and warm North Pacific conditions suggests constructive phasing of tropical-extra-tropical SST patterns, as was found by Brito-Castillo et al (2003), and is somewhat similar to the finding by Castro et al (2001) for Arizona and New Mexico. The combined patterns of cool eastern Pacific tropical SSTs and higher eastern Pacific SLPs (not shown) are also suggestive of La Niña conditions (Trenberth, 1997).…”

“…Many recent works have attempted to elucidate the role of ENSO and/or North Pacific SST variability in modulating precipitation associated with the NAM (e.g. Cavazos and Hastenrath, 1990;Reyes et al, 1994;Higgins et al, 1998;Mo and Peagle, 2000;Yu and Wallace, 2000;Castro et al, 2001;Englehart and Douglas, 2002;Higgins and Shi, 2001;Hu and Feng, 2002;Bosilovich et al, 2003;Gutzler, 2004). However, a clear diagnosis of ENSO-precipitation relationships on interannual timescales is complicated by selection from multiple ENSO indices, lag-correlation effects across multiple seasons (Grantz et al, 2006), substantial spatial variability in precipitation across the NAM domain (Gochis et al, , 2006b, and an apparent nonstationarity of ENSOprecipitation relationships on decadal timescales (BritoCastillo et al, 2002;Gutzler, 2004).…”

Shifting covariability of North American summer monsoon precipitation with antecedent winter precipitation

“…The PDO is an index of decadal-scale SST variability in the North Pacific Ocean (Mantua et al, 1997) and has important effects on the hydro-climate of the western US (Mantua et al, 1997;McCabe and Dettinger, 1999;McCabe and Dettinger, 2002;Gershunov and Barnett, 1998;Hidalgo and Dracup, 2003). A comparison of Castro et al (2001) indicate that central North Pacific SSTs have an influence on the location of monsoon precipitation by influencing atmospheric circulation and moisture transport into the southwestern US. Castro et al (2001) indicate that when central North Pacific SSTs are warmer than average, the transport of moisture is focused into the western margins of the southwestern US.…”

“…A comparison of Castro et al (2001) indicate that central North Pacific SSTs have an influence on the location of monsoon precipitation by influencing atmospheric circulation and moisture transport into the southwestern US. Castro et al (2001) indicate that when central North Pacific SSTs are warmer than average, the transport of moisture is focused into the western margins of the southwestern US. In contrast, when central North Pacific SSTs are colder than average, the transport of moisture is focused more to the eastern margins of the southwestern US.…”