The effect of North Pacific decadal variability, expansion of the tropical belt, and the North Atlantic Subtropical High on the climate of the South-Central United States

Beginning in the late 1990's, the South-Central United States has experienced a climate shift from historically wet conditions to the drier conditions in the 21st century. Much like the 1950's, the climate of the South-Central region has recently been punctuated by an increase in drought severity and occurrence, and therefore it is important to understand the atmospheric characteristics that contribute to these lengthy dry periods. Changes in atmospheric and oceanic circulation over the North Pacific Ocean have been shown to play a substantial role in the climate regimes of the South-Central US, and the Pacific Decadal Oscillation (PDO) is thought to be the major contributor at the multidecadal time scale. Through the analysis of the general atmospheric circulation we demonstrate a link between low frequency Pacific variability and a multidecadal drift in the intensity of the Pacific-North American (PNA) teleconnection pattern during the boreal fall, winter, and spring months, which is coincident with expansion and contraction of the Northern Hemisphere Hadley Cell boundary. In the summer months, we show a teleconnection bridge between the tropical Pacific and the North Atlantic Subtropical High (NASH), also initiated through expansion and contraction of the global tropical belt. Does the PDO induce a preferred phase of the PNA during fall, winter, and spring? Can we identify the mechanisms that trigger the tropical belt expansion and contraction that are associated with the PDO and variability of the NASH? The analyses that we use to intimate these questions include: a) patterns of 500 hPa geopotential height to determine the progression of the PNA teleconnection associated with the PDO, b) patterns of winds throughout the atmospheric column to elucidate changes in the general circulation, including tropical belt expansion and stratospheric variability, and c) patterns of 850 hPa geopotential height to determine how multidecadal changes in the southern NASH boundary relate to expansion of the tropical belt. Understanding the timing and teleconnection relationships between the PDO, the PNA, the NASH, and tropical belt expansion may uncover a mechanism that can provide advanced warning of multidecadal changes in the general circulation, which will help to increase the skill of decadal climate predictions for the South-Central US.