Rapid Shrinking of the Warming Hole Over the United States in ERA5 and SPEAR
Bibliographic record
Abstract
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.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.002 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".