The southwestern United States (US) and Northern Mexico experienced their longest stretch of record-breaking heat wave in the summer of 2023 from mid-June to early August, affecting over 100 million people, causing over 200 deaths, and $14.5 billion in economic loss. In Phoenix, Arizona, for example, the heat wave lasted for 31 days from 30 June to 30 July with the daily maximum temperature exceeding 43.3 °C (110 °F) and with the warmest nighttime minimum temperature on record (36.1 °C). A new study published in Nature Communications showed that the 2023 heat wave event was linked to a strong anticyclonic blocking pattern that persisted for more than six weeks across the western US. Further data analysis and atmospheric model simulations suggested that the anticyclonic pattern was largely forced by the extremely warm sea surface temperature in the Atlantic, and added by a developing Pacific El Niño.
Interestingly, the synergic effect of a warm Atlantic and a developing Pacific El Niño significantly amplified regional heat waves, doubling their number, tripling their days, and increasing their duration by about 50%. More specifically, the study found that the developing Pacific El Niño alone had very little impact on the 2023 heat wave. However, when it was combined with the record warm Atlantic, it significantly boosted the heat wave. Thus, the study concluded that the synergy effect of Pacific-Atlantic forcing was central in further exacerbating the likelihood of heat waves in western North American, including extending their duration and enhancing the warm temperature anomalies. Further study is needed to better understand the physical processes involving the synergic effect of Pacific-Atlantic forcing.
Figure 6 from Lopez et al. (2025): Anomalous 200 hPa temperature (color) and streamfunction (black contour, 106 s−1) for June–August of 2023. b same as (a) but for 2-m maximum air temperature anomaly. c Regression coefficient of Niño3 plus tropical North Atlantic (TNA) SST and 200 hPa temperature (color) and 200 hPa streamfunction (black contour, 106 s−1). d Same as (c) but for 2-m maximum air temperatures. The regression coefficients are computed for June–August for the 1979–2023 period through partial regression, and the units are per standard deviation of the SST anomalies. Panels (e, f) are the same as (b, c) but for the regression coefficients with TNA SSTs only and holding Niño3 SSTs constant. Similarly, (g, h) show the regression coefficients with respect to Niño3 SSTs, holding TNA SSTs constant.
Lopez, H., Lee, SK., West, R. Kim, D. & Jia, L. The longest-lasting 2023 western North American heat wave was fueled by the record-warm Atlantic Ocean. Nature Communications, 16, 6544 (2025). https://doi.org/10.1038/s41467-025-61859-y
Ocean to Climate 
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