Rapid Shrinking of the Warming Hole Over the United States in ERA5 and SPEAR

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Abstract
The “warming hole” over the United States refers to a long‐term summer cooling anomaly in near‐surface temperatures across the central and northern U.S. since the midtwentieth century. It is most pronounced in maximum temperature ( T max ), contrasting with widespread warming elsewhere. Using the European Centre for Medium‐Range Weather Forecasts ERA5 reanalysis and a 30‐member large ensemble of the Seamless System for Prediction and Earth System Research (SPEAR) model, we show that the warming hole has shrunk and weakened in recent decades, with contraction accelerating during 2021–2024. The changes are seasonally asymmetric: June exhibits the strongest weakening, whereas May retains negative anomalies. Composite analyses confirm a well‐known mechanism in which a strengthened southerly low‐level jet enhances precipitation and suppresses T max . We also identify an additional process: enhanced northerly winds from Canada that advect cooler air and increase convergence and rainfall, reinforcing the cooling. In many years, opposite circulation and precipitation anomalies reduced these cooling effects; recently, such anomalies have strengthened in step with the warming trend, contributing to rapid shrinkage. SPEAR reasonably captures the detrended variability and spatial pattern of anomalies but overestimates the warming trend relative to ERA5. Combining SPEAR's detrended variability with the ERA5 trend, we project that the warming hole would vanish around 2050 under a strong external forcing. This result contrasts with recent studies emphasizing seasonal persistence and southerly winds alone, underscoring the importance of both externally forced and internal variability associated with hydrologic cycle changes in shaping the future trajectory of the U.S. warming hole.
Funding Information
  • National Oceanic and Atmospheric Administration (NA23OAR4310603)

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