Spatially Distinct Seasonal Patterns and Forcings of the US Warming Hole

We present a novel approach to characterize the spatiotemporal evolution of regional cooling across the eastern United States (commonly called the U.S. warming hole), by defining a spatially explicit boundary around the region of most persistent cooling. The warming hole emerges after a regime shift in 1958 where annual maximum (T-max) and minimum (T-min) temperatures decreased by 0.83 degrees C and 0.46 degrees C, respectively. The annual warming hole consists of two distinct seasonal modes, one located in the southeastern United States during winter and spring and the other in the midwestern United States during summer and autumn. A correlation analysis indicates that the seasonal modes differ in causation. Winter temperatures in the warming hole are significantly correlated with the Meridional Circulation Index, North Atlantic Oscillation, and Pacific Decadal Oscillation. However, the variability of ocean-atmosphere circulation modes is insufficient to explain the summer temperature patterns of the warming hole. Plain Language Summary The U.S. warming hole is a region in the eastern United States that experienced a broad decline in temperatures beginning in the late 1950s. The warming hole is fundamentally different than global temperature trends, which have been rising since 1880. There are several ideas as to why the warming hole exists, but most cannot fully explain the observed temperature patterns. Interestingly, there is also disagreement about the location and timing of the warming hole, which may add to the difficulty in diagnosing its cause. Here we analyze temperature patterns since 1901 and present a new way to define the location of the warming hole, thereby clarifying much of the variance in location described in previous studies. We find that temperatures in the warming hole are associated with changes in climate indices over the Pacific and Atlantic Oceans, which are likely related to changes in the waviness of the jet stream over the eastern United States. We find evidence that the jet stream exhibited a shift in the late 1950s coincident with the start of the warming hole, resulting in a greater tendency of northerly winds to bring cool air to the southern United States.