Northeast United States growing season moisture conditions: Associations with a North American mid‐tropospheric wave train

Abstract

The Palmer Drought Severity Index is examined in the Northeast United States during the May–September growing seasons of 1950–2016, seeking to better understand the role of atmospheric circulation in moisture variability. Moist conditions for the Northeast are associated with the development of a mid‐tropospheric wave train over the United States, with enhanced ridging over the west, a trough over the Ohio Valley, and ridging over the Atlantic Ocean east of Newfoundland that favours enhanced uplift, precipitation, and southeasterly flow at the surface in the Northeast. A scalar representation of the wave train, a Summer Atmospheric Drought Index (SADI), is developed and robustly validated as a strong indicator of the Northeast moisture conditions. The SADI mid‐tropospheric wave train is the second largest mode of 500 hPa height variability over the North American sector from 1950–2016, and has an increasing trend such that the ridge‐trough‐ridge pattern is more prevalent and/or has become more enhanced with time. The wave train pattern has a significant and positive relationship to the phases of the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO). This indicates that when the AO and NAO indices are positive, the wave train pattern favours the ridge‐trough‐ridge configuration over North America, and can lead to moist conditions in the Northeast. The opposite pathway is also valid and can lead to dry conditions in the Northeast. This pathway provides a physical link between Northeast moisture conditions and global‐scale variability, improving the understanding of atmospheric influences on regional drought and excessive moisture conditions.