Identifying the evolving human imprint on heat wave trends over the United States and Mexico

Abstract

Changes in frequency, duration and intensity of three heat wave (HW) types (compound, daytime, and nighttime) over the United States (U.S.) and Mexico during the second half of the 20th-century are investigated using the Community Earth System Model Large Ensemble (CESM-LE). The individual role of anthropogenic aerosols and greenhouse gases (GHGs), as well as the contribution from internal variability (IV), are identified and contrasted by means of the CESM-LE single forcing experiments during two periods: 1950–1975, when North American aerosol emissions peaked, and 1980–2005, when aerosol emissions declined. HW changes are strongly affected by anthropogenic forcing. During 1950–1975, aerosols, via both aerosol-radiation and aerosol-cloud interactions, dominate the decreasing trends in compound HWs over the central U.S., the daytime HWs in large parts of the domain and the nighttime HWs over Mexico. Conversely, all three HW types are considerably more frequent (2 HWs summer−1 decade−1), longer-lasting (with increases of up to 2 days HW−1 decade−1 in some regions) and more intense (e.g., C HW−1 decade−1 in compound HWs) across large regions of the domain during the 1980–2005 period. The results show that the decline in aerosol emissions and the continuous rise in GHGs lead to widespread warming and subsequent circulation adjustments, contributing to the positive HW trends. The contribution of IV is large during 1950–1975 (over 60% in most areas), and considerably reduced later on. This study provides a comprehensive picture of the role of anthropogenic forcing and IV on the marked HW changes in the recent decades and their underpinning physical mechanisms.