The daily level magnitude of regional heat wave activity as defined in Table 1: (red ovals) and (blue ovals). The x axis corresponds to each year on record, while the y axis corresponds to each summer date. (a) Regional magnitude for unspecified local duration n = 1, and (b) local duration of at least 3 consecutive dates. The larger the oval, the greater the magnitude. The scale is given by the maximum magnitude recorded each summer and shown at the top of each panel and again in Figs. 5a,b. The overall magnitude for each summer is shown in Figs. 2a,b.
Seasonal maxima of regional heat wave components: total magnitude on the peak (a) day and (b) night of the greatest events, maximum spatial extent in % of stations by (c) day and (d) night, and maximum continuous regional duration of (e) daytime and (f) nighttime heat waves. All variables were computed for each summer on record from data presented in Fig. 3. Components were computed given local durations of at least 1, 2, and 3 consecutive days/nights (n = 1, 2, 3) and delineated in progressively darker shades of gray. Correlations between these indices and trends are given in Table 3.
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A more precise and useful description of heat wave activity should include an explicit and separate quantification of daily and nightly temperature extremes. During a persistent daytime heat wave, cool nights provide respite from the stressful effects of heat on the health and general well-being of plants and animals, as well as for the energy sector, and prepare society and nature to face another day of scorching heat. Heat waves strongly manifested at night eliminate this badly needed opportunity for rejuvenation and increase the chances for catastrophic failure in human and natural systems. Extreme daytime heat is known to endanger health most directly via heat stroke but health dangers are exacerbated by associated air pollution including near-surface ozone formation (e.g., Fischer et al. 2004; Stedman 2004; Gosling et al. 2008). Health impacts of nighttime heat are less well known, but there are indications that high minimum temperatures during heat waves enhance morbidity and mortality (Hemon and Jougla 2003; Grize et al. 2005; Gosling et al. 2008). Excess mortality across Switzerland due to the June and August 2003 European heat waves has been attributed to the compounding effect of elevated nighttime temperatures (Grize et al. 2005). During the July 2006 California event, a significant number of victims, most of whom were elderly and living alone, had not used their functioning air conditioning (Margolis et al. 2008). Perhaps they had turned off air conditioning in the evening expecting the strong nighttime cooling characteristic for this region, which did not materialize.
Physical mechanisms causing daytime and nighttime heat waves may differ. Observed warming trends are known to have been stronger at night than during the day (e.g., Easterling et al. 1997, 2000b) resulting in a decreased diurnal temperature range. Stronger nighttime heating trends have been observed at many locations around the globe and, in spite of modeling inconsistencies (Lobell et al. 2007) and recent observations that trends in diurnal temperature range may have ceased globally (Trenberth et al. 2007) or may be increasing over some regions (e.g., southern Mexico; Peralta-Hernandez et al. 2008), warmer nights are among the most widespread expectations from anthropogenic global climate change (e.g., Tebaldi et al. 2006). In this regard, the California region has been meeting expectations. The observed summertime average warming here has been largely due to minimum temperatures (not shown). In this topographically, environmentally, economically, and climatically complex region, global, regional, and local natural and anthropogenic effects abound (e.g., Duffy et al. 2007; Bonfils et al. 2008).
The purpose of this work is to describe the climatic behavior and regional causes of great heat waves over California and Nevada. Using this foundation, we investigate whether and to what extent the 2006 event may be considered an aberration or a manifestation of a long-term change. After describing the data, our approach to quantifying heat waves, and their general behavior (section 2), we illustrate the observed variability of regional daytime and nighttime heat waves (section 3), describe the synoptic characteristics of the greatest observed events in recent history (section 4), and explain the 2006 event in that context (section 5) as well as in the context of trends in daytime and nighttime heat waves (section 6).
To describe the spatial extent of heat waves affecting the California region, their duration, and differential symptoms during day and night, we start with day- and nighttime temperatures (Tmax and Tmin, respectively) recorded at 95 stations distributed more or less uniformly over the adjacent states of California and Nevada. All station data were selected from the updated National Climatic Data Center (NCDC) first-order and cooperative observer summary of the day dataset, known as DSI-3200 (NCDC 2003). The original set of 141 stations with daily Tmin and Tmax records going back to at least 1 January 1948 and running through August 2006 was selected for having no more than 15% of missing data at each station per summer. The choice of 1948 as the starting point was a reasonable compromise between record length and spatial completeness. All stations were purged of unnatural outliers. This original set of stations was characterized by a spatial sampling bias toward most populated areas. The 95 core stations were selected from this original set as representative of the region by keeping one best quality station (i.e., station with the least missing data) per locale of 30-km radius and thereby removing the urban density bias. Stations with the most complete records are typically found at lower elevations. To retain the effects of mountain climate diversity important in this topographically complex region, the highest elevation station, in addition to the best quality station, was retained wherever the elevation range exceeded 300 m per locale. The sparsely populated and observed areas of the southeastern California and Nevada deserts are, by necessity, underrepresented and downplayed by subsequent analyses. We computed local linear summertime Tmax and Tmin trends at all stations and visually examined trend maps for spatial outliers. Time series at several stations exhibiting conspicuous trends were examined for obvious discontinuities and outliers possibly exerting undue influence on the linear trends, but none were found. Although no formal homogenization procedure was performed, the use of many stations to characterize a region strongly reduces possible biases arising from occasional spurious local glitches.
To define regional heat wave activity, we first compute the 99th percentiles at all stations (Figs. 1c,d). This result indicates that the highest temperature extremes during both day and night typically occur in the southeastern low deserts and interior valley regions, while the lowest hot extremes occur in the high Sierra Nevada and along the coast and coastal ranges. Extremes of both Tmax and Tmin display a very similar spatial distribution with few local exceptions, for example, the Southern California coast exhibits relatively hot extremes at night while the northern coastal hills are relatively more prone to intense daytime heat.
The 99th percentile temperatures are used to quantify regional heat wave activity simply by summing threshold exceedances (departures over these local thresholds) over each summer and all stations given three minimum local durations (Fig. 2). These indices, and , reflect region-wide summertime heat wave activity, that is, intensity, frequency, duration, and spatial extent of individual heat waves aggregated over each summer (Figs. 2a,b). The significant trend in daytime heat wave activity for local durations of at least 3 days (n = 3) is mostly due to enhancement toward the end of the record. In contrast, the increasing trend in nighttime heat wave activity is a feature of the entire record that holds regardless of local duration, although it is accentuated by the most recent summers 2003 and 2006, each unprecedented (Fig. 2b). The broad coherent patterns of entirely positive correlations of regional and with local values (a median correlation of 0.50/0.48 for for n = 1) indicate the widespread nature of intense heat waves (Figs. 2c,d).
We next examine the frequency and magnitude (i.e., duration, intensity, and spatial extent) of regional heat waves more closely at daily and nightly resolution. Figure 3a documents the magnitude of extreme heat waves of unspecified local duration (n = 1) for each day and night on our 59-year record. Figure 3b shows the same information for heat waves of 3 or more days/nights local duration (n = 3). Timing and duration of strong heat waves can be visually identified on these plots.2 Regionally, 2ff7e9595c
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