Decadal Variation of Rainfall Seasonality in the North American Monsoon Region and Its Potential Causes

Abstract

This study shows that the North American monsoon system's (NAMS) strength, onset, and retreat over northwestern Mexico exhibit multidecadal variations during the period 1948-2009. Two dry regimes, associated with late onsets, early retreats, and weaker rainfall rates, occurred in 1948-70 and 1991-2005, whereas a strong regime, associated with early onsets, late retreats, and stronger rainfall rates, occurred in 1971-90. A recovery of the monsoon strength was observed after 2005. This multidecadal variation is linked to the sea surface temperature anomalies' (SSTAs) variability, which is a combination of the Atlantic multidecadal oscillation (AMO) and the warming SST trends. These SST modes appear to cause an anomalous cyclonic circulation and enhanced rainfall over the southeastern United States and the Gulf of Mexico, which in turn increases the atmospheric stability over the monsoon region. However, these SST modes cannot fully explain the circulation and rainfall anomalies observed during the early-retreat monsoons. An expansion of the North Atlantic surface high (NASH) in recent decades also contributes to the anomalous circulation associated with the early retreats of the NAMS. A northwestward expansion of the NASH further enhances the anomalous cyclonic circulation and rainfall over the southeastern United States and the Gulf of Mexico. Its associated northwestward shift of the subtropical jets over the western United States enhances subsidence over the NAMS region. The combined effects of the AMO, the warming trends, and the NASH expansion on atmospheric circulation contribute to a stronger and more persistent earlier retreat during the recent dry regime (1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005), while the earlier dry regime appears to be only influenced by the positive phase of the AMO.

The Holocene history of the North American Monsoon: ‘known knowns’ and ‘known unknowns’ in understanding its spatial and temporal complexity

“…The links between the amount of summer precipitation in the NAM region and sea surface temperatures (SSTs) in both the Atlantic warm pool region of the Caribbean and Gulf of Mexico (GoM) and in the Gulf of California (GoC) have been well established (Higgins and Shi, 2000;Mitchell et al, 2002;Hu and Feng, 2002;Wang et al, 2008;Arias et al, 2012) and as a result, are our focus here. In both regions, increased SSTs and increased surges of monsoon precipitation toward the NAM region are tied to the northward expansion of the ITCZ, although the drivers vary over different timescales.…”

Assessment of CMIP5 Model Simulations of the North American Monsoon System

“…Interannual variability of the NAMS impacts local ecosystems, agriculture, and the general public; therefore, there is considerable interest in determining the NAMS annual onset and retreat dates (e.g., Higgins et al 1997;Zeng and Lu 2004;Ellis et al 2004;Arias et al 2012). Identification of model annual onset and retreat dates using daily precipitation provides a higher time resolution evaluation of monsoon season phasing than the monthly phase lag analysis in section 3.…”

Long-term seasonality of rainfall in the southwest Florida Gulf coastal zone

“…Within the region, the amount of rainfall falling during a year is affected by climate phenomena such as the El Niño−Southern Oscillation (ENSO), which primarily affects winter rainfall (NOAA 2015), and the Atlantic Multidecadal Oscillation, which affects total rainfall amounts and the oc currence of hurricanes (NOAA 2005). Winters in the region are generally cool and wet during El Niño years and warm and Similarly, effects on rainfall amounts and seasonality are noted by Poveda et al (2001) due to ENSO, and by Luo and Zhang (2015) and Arias et al (2012) due to different climate modulators. These pheno mena can also experience shifts, thereby also im pacting seasonality (e.g.…”