Attribution of trends in rising nighttime temperatures in the Western US

The observed rise in nighttime temperatures constitutes a possible signal of human-influenced changes in global climate. The California and Nevada heat waves of 2003 and 2006 were found to have a significant nighttime component, which made the events both deadly and costly for the local population (Gershunov et al. 2009). The present work focuses on attribution of these events by employing a superensemble of the global Hadley Center Atmospheric General Circulation Model 3P (HadAM3P) and Hadley Centre Regional Model (HadRM) simulations from the climateprediction.net (CPDN) experiment, which allows for robust statistical analysis of climate change impacts. CPDN provides an ensemble size large enough to examine the tails of the distribution of climate variables critical for reducing the uncertainty for rare events. The analysis is divided into two sub-regions: California/Nevada (CA/NV) and the Pacific Northwest (PNW), which includes Oregon, Washington and Idaho. Specifically, the present study compares La Nina years [using Nino3.4 as a proxy for El Nino/Southern Oscillation (ENSO)] during the decade of the 2000s and the 1960s, in which anthropogenic greenhouse gases were not thought to have had as large an influence. La Nina years during the June-July-August (JJA) period are chosen because low temperatures are highly-affected by ENSO in the model runs, with La Nina years being markedly warmer than El Nino years (1°C for CA/NV and 0.5°C for PNW). In contrast, there is no difference in maximum temperatures between ENSO phases for either domain. Also, it is found that the Pacific Decadal Oscillation (PDO) only has a marginal influence in low temperatures for the PNW (0.1°C) between positive and negative phases, and does not affect low temperatures for CA/NV nor high temperatures for either domain. Variables computed for JJA include the lowest and highest daily temperature in a month, monthly low and high average temperatures, the mean monthly temperature, and the highest 5-day mean temperatures and wet bulb globe temperature (WBGT) in each month. The lowest daily temperature in a month is chosen as a proxy for shrinking diurnal range due to the constraint in available variables in the climate model output. The lowest daily temperature in a month for JJA is found to be over 1°C warmer for La Nina years in the 2000s compared to the 1960s over PNW for short return periods (5-10 years) and also 1°C warmer for CA/NV for longer return periods (10-100 years). The 5-day WBGT, a crucial variable for healthcare concerns during heat waves displays a marked rise in magnitude and a strong positive shift in frequency from the 1960s to the 2000s. However, unlike the observed heat waves of the 2000s, the moisture component (vapor pressure) is not as high a contributor. It is argued that the strong rise in nighttime temperatures and shortening of the diurnal range can be attributed to increasing greenhouse gas concentrations.

Gershunov, A., D.R. Cayan and S.F. Iacobellis, 2009: The Great 2006 heat wave over California and Nevada: Signal of an Increasing Trend. J. Climate., 22(23), 6181-6203.