A Climatology of Heat Waves from a Multimillennial Simulation

3802 JOURNAL OF CLIMATE VOLUME 20

B. G. HUNT CSIRO Marine and Atmospheric Research, Aspendale, Victoria, Australia

(Manuscript received 24 March 2006, in final form 27 November 2006)

ABSTRACT

A 10 000-yr unforced simulation with the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Mark 2 coupled global climatic model has been used to investigate the occurrence of heat waves over the globe. Results are presented for both seasonal (summer mean) and daily heat waves. Geographical distributions of the occurrence rates of these heat waves are shown for various magnitudes of surface temperature anomalies. The heat waves have specific geographical preferences with regions where relatively frequent, intense, and long-lasting heat waves occur. Time series over all 10 000 yr of the heat waves for the selected model grid boxes illustrate the differing temporal variabilities at these locations, as well as identifying the occurrences of extreme heat waves. To this end, the observed European heat wave of 2003 was simulated remarkably well in its overall characteristics; it occurs once in this simulation. Heat waves for various continental locations are shown to occur as isolated spatial and temporal events, and not as part of larger-scale systems over continental-size domains, suggesting stochastic forcing as a contributor to the initiation of the heat waves. Regional plots of selected heat waves at monthly intervals illustrated the considerable spatial variability, progression, and variation in the intensity of the heat waves. Comparison of year-long daily surface temperature anomalies for heat-wave years for simulated and observed conditions at individual model grid boxes indicated substantial agreement, while spatial plots permitted the progress of a short-term heat wave over the United States to be followed. Multidecadal time series plots of intense heat waves also showed basic similarities between the simulation and observations, despite the brevity of the latter. The simulated time series suggest that more extreme heat waves than currently are observed, owing to the brevity of the observations, may be a possibility as a consequence solely of natural variability. An examination of the physical processes associated with a heat wave showed mutually consistent climatic relationships, such that a heat wave was associated with reduced rainfall and consequently reduced soil moisture content, evaporation, and cloud cover, and increased insolation at the surface. These combined changes created the surface temperature increase intrinsic to the heat wave. All heat waves examined for different regions experienced negative rainfall anomalies prior to a heat wave. The cause of these rainfall anomalies was not readily apparent. While an ENSO influence on heat waves is shown to exist in the simulation, not all ENSO events produce heat waves, suggesting that stochastic influences may determine when a major heat wave occurs in conjunction with these events. The limitations of the adequacy of the model ENSO may, however, have had an influence in this regard.

1. Introduction or more with temperatures sufficiently enough above normal to cause stress resulting in increased human Heat waves are a fact of life. They are also a factor in mortality and damage to vegetation. death, with some thousands of fatalities attributed to The impact of a heat wave varies considerably from the European heat wave in 2003 (WMO 2003), and region to region. For example, a multiday temperature over a thousand deaths associated with the U.S. heat anomaly of 5 K in summer in central Europe would be wave of 1995 (Palecki et al. 2001). expected to create more problems and loss of life than Although there is no agreed definition of a heat a similar heat wave in Iceland, simply because the latter wave, such an event consists of a period of several days has much lower mean temperatures. The characteristics of heat waves also vary considerably. The European Corresponding author address: B. G. Hunt, CSIRO Marine and heat wave of 2003 was sustained over June, July, and Atmospheric Research, PMB1, Aspendale VIC 3195, Australia. August (Schär et al. 2004; Fink et al. 2004), while the E-mail: [email protected] U.S. heat waves of 1995 and 1999 were confined to a

DOI: 10.1175/JCLI4224.1

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JCLI4224 1AUGUST 2007 HUNT 3803 few extreme days in July (Kunkel et al. 1996; Palecki et lar, the U.S. heat waves of 1995 and 1999 and the Eu- al. 2001). Most heat waves seem to be in the latter ropean heat wave of 2003, being very recent, need to be category; see, for example, Burt (2004), Khaliq et al. placed in an appropriate historical perspective. (2005), and Founda et al. (2004). Unfortunately, the lack of surface temperature ob- Because of the loss of life, damage to crops and veg- servations extending back over many centuries pre- etation in general and the impact on water supplies, vents such a perspective from being obtained. How- these recent heat waves have stimulated much interest ever, some insight into the location, duration, intensity, in their climatological features, recurrence times, and, etc. of heat waves can be deduced from multimillennial especially, whether they are a portent of greenhouse- simulations with coupled global climatic models. While induced climatic change. For example, Trigo et al. these models have limitations, they do provide consis- (2005) state that the hot summer of 2003 in Europe tent and comprehensive datasets for all climatic vari- exceeded any over the past 500 yr, and Schäretal. ables of interest, and can be applied to a wide range of (2004) claim that this event was statistically very un- issues. likely, and was also consistent with results from climatic To this end, a 10 000-yr simulation with the Com- change simulations. Stott et al. (2004) estimate that past monwealth Scientific and Industrial Research Organi- anthropogenic influences doubled the probability of the sation (CSIRO) Mark 2 coupled global climatic model occurrence of the 2003 heat wave. More intense and has been analyzed to investigate the characteristics of frequent heat waves are also predicted by Meehl and heat waves across the globe. The simulation was not Tebaldi (2004) and Beniston (2004) on the basis of undertaken simply to examine heat waves, but also to greenhouse simulations. consider a wide range of climatic features generated by The difficulty of estimating the return periods for natural climatic variability. The analysis includes the current observations is well illustrated by Karl and identification of the spatial patterns at seasonal and Knight (1997), who obtained a value of about 100 yr for monthly intervals; time series illustrating the temporal the so-called Chicago heat wave of 1995. Four years variability of heat waves for specific locations; the in- later in 1999 Chicago again experienced a heat wave tensity and duration of heat waves, including daily vari- very similar to that of 1995 (Palecki et al. 2001). Benes- ability; and related hydrological, radiative, and dynamic tad (2003) has also discussed the problem of estimating processes. return periods for rare climatological events. Previous analyses from this simulation have demon- Regional and global estimates of observed heat wave strated the ability of the model to replicate many ob- characteristics are limited by the availability of suitable served climatic features. These include a demonstration data. Frich et al. (2002) have produced a heat wave of the stationarity of the global mean climate over all duration index that was able to cover only North 10 000 yr of the simulation (Hunt 2004), a necessary America, Europe, parts of Asia, and Australia. This outcome given the fixed boundary conditions used in index is defined as the maximum period with more than the simulation, and the lack of any climatic drift in the 5 days with a maximum temperature anomaly 5 K simulation. Hunt (2004) also provides a selected com- above the 1961–90 daily temperature maximum norm. parison of surface temperature and rainfall variability For the second half of the twentieth century they with observations. The simulation has also been used showed increases in the index values, except for eastern to demonstrate that critical observed drought events North America and parts of southern Asia. Thus, their could be reproduced by the model, attributable solely analysis suggests an increase in heat-wave frequency to natural variability (Hunt and Elliott 2002, 2005). It over the past few decades, which is presumably consis- has also been shown that the present simulation can tent with a greenhouse influence. Choi and Meente- reproduce many features of the Medieval Warm Period meyer (2002) have devised a climatology of persistent and Little Ice Age (Hunt 2006c), as well as the charac- positive temperature anomalies for the United States teristics of extreme rainfall events (Hunt 2006b) and for 1850–1995. They present a variety of outcomes climatic outliers (Hunt 2006a). An earlier simulation showing spatial patterns of such anomalies for various was used to demonstrate that many aspects of ENSO durations and intensities. events were reproduced by the model (Hunt and Elliott The potential (or actual?) impact of the greenhouse 2003), while the model has been shown to perform well effect creates problems in determining the characteris- in international comparisons of these events (Achuta- tics of severe climatic events, such as heat waves, owing Rao and Sperber 2000). to the nonstationarity of the climate. There is obviously Previous studies of heat waves using models have a need to quantify pregreenhouse climate in order to been rather restricted and have been primarily con- differentiate it from greenhouse influences. In particu- cerned with future, greenhouse-induced impacts (Schär

Unauthenticated | Downloaded 09/23/21 09:26 PM UTC 3804 JOURNAL OF CLIMATE VOLUME 20 et al. 2004; Meehl and Tebaldi 2004). Huth et al. (2000) initially balanced. The model was set up to simulate investigated the occurrence of heat waves and dry spells “present” climatic conditions; once initiated no changes over the Czech Republic for control and greenhouse were permitted to forcing agents such as CO2 content, conditions. They noted a number of discrepancies with volcanic eruptions, or solar variability. Thus, all of the observations for the control, including too high peak climatic fluctuations generated in the simulation arose temperatures and too long duration heat waves. from naturally occurring climatic variability. This simulation does not therefore represent a pro- 2. Model description gression through the Holocene, as the latter experienced numerous changes to boundary conditions via The Mark 2 version of the CSIRO coupled global external forcings such as volcanic eruptions, composi- climatic model was used in this study. The model has tional changes, etc. A fixed atmospheric CO2 concen- been described in detail by Gordon and O’Farrell tration of 330 ppm was used in the simulation. (1997), who also give a description of some aspects of As mentioned above, the simulation was time invari- the model’s performance. ant at the global mean level. Thus, time series of basic The model consists of atmospheric, oceanic, bio- variables such as surface temperature, cloud amount, spheric, and dynamic sea ice components. A flux ad- and rainfall were constant within 1%–2% over the justment scheme is used to couple the atmospheric and 10 000 yr of the simulation, as are the limited observa- oceanic components. The flux adjustments vary tions (see Hunt 2004). At the regional or local level, monthly but are invariant from year to year. The atmo- climatic time series were also stationary, while exhibit- spheric model has nine vertical levels and R21 spectral ing considerable interannual variability. Marked re- resolution (3.25° latitude ϫ 5.625° longitude) giving gional climatic anomalies, such as those associated with 3584 model grid boxes per vertical level. Diurnal and the Medieval Warm Period or Little Ice Age, occur in seasonal variabilities are included, along with a mass this simulation within the constraint of this global mean flux convection scheme, a cloud parameterization time invariance (Hunt 2006c). As a consequence of this based on relative humidity, gravity wave drag, and invariance, no detrending of climatic variables was nec- semi-Lagrangian water vapor transport. The oceanic essary. component is based on version 2 of the Modular Ocean Model, of the Geophysical Fluid Dynamics Laboratory. 3. Seasonal heat waves This component has 21 vertical levels and realistic bottom topography. The eddy-induced advection scheme Although most heat waves are defined in terms of of Gent and McWilliams (1990) was implemented, per- days, they can have extended durations. The European mitting the horizontal background diffusivity to be set heat wave of 2003 is such an example, while Chang to zero. The land surface scheme consisted of separate and Wallace (1987) list heat waves with durations up to soil moisture and temperature formulations. A two- 3 months for the United States. layer representation was used for soil moisture based An indication of the frequency of occurrence of sum- on Deardorff (1977). Three soil types were specified as mer heat waves [June–August (JJA) for the Northern well as 11 plant types, with the latter having monthly Hemisphere; January–March (JFM) for the Southern varying characteristics. A three-layer scheme was used Hemisphere] over all 10 000 yr of the simulation is pre- for soil temperature, with the bottom of the lowest sented in Fig. 1. Results are given for three ranges of layer being assumed to be insulated. Full technical intensity of the heat waves—those having average specifications are available in the report by McGregor anomalies of 3, 4, or 5 K sustained over these summer et al. (1993). periods. In each set of panels in Fig. 1, results are also The limited resolution of the model inevitably means shown for the corresponding winter hemisphere. The that small-scale features, a few hundred kilometers in latter represents positive temperature anomalies occur- extent, cannot be adequately resolved in this simula- ring within winter conditions and, thus, are indicative of tion. There is also the question of the representative- mild winter states rather than heat wave conditions. ness of results at individual model grid boxes compared Dealing with the Northern Hemisphere first in Fig. 1, to observed station data. These issues arise in all simu- it can be seen that apart from a few restricted locations lations, but the case studies presented here suggest that in this hemisphere heat waves are not experienced over for these specific examples credible outcomes have the oceans. This is a consequence of the large heat been achieved. capacity of the mixed layer ocean. Even over the tropi- The present simulation was commenced from a pre- cal Pacific Ocean no heat waves are recorded in the vious 1000-yr simulation; hence, all climatic fields were region where El Niño events occur. This is because the

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FIG. 1. Occurrence rates of summer heat waves for the (left) Northern Hemisphere (JJA) and (right) Southern Hemisphere (JFM). For each hemisphere the corresponding winter “heat waves” are also shown. The heat waves have mean intensities over the whole summer period of (top) 3, (middle) 4, and (bottom) 5 K. The color bars give the occurrence rates for the 10 000 yr of the simulation.

model produces El Niño warmings of only about one- waves occur, highlighting the susceptibility of these re- half to two-thirds of the observed value (Hunt and gions to these extreme climatic conditions. Elliott 2003), but also because even the lowest range of All of these regions have high standard deviations of intensity in Fig. 1 would appear to be observationally summer temperature (not shown). Over high northern exceptional [see sea surface temperature anomaly maps land areas any heat wave conditions would, of course, in Allan et al. (1996)]. Summer heat waves of these be based on relatively low surface temperatures. magnitudes also do not occur in southeast Asia, parts of The occurrence rates of mean summer anomalies of 4 North America, and over the Arctic Ocean; see Fig. 1. and5KintheNorthern Hemisphere in Fig. 1 decline At other regions in the Northern Hemisphere, summer very rapidly compared to those for 3 K. For 5-K anoma- heat waves with a magnitude of 3 K are restricted to lies they are restricted to small regions in the central about 20 occurrences over the 10 000 yr of the simula- United States, India, and the Middle East, together tion, except for high northern latitudes, the south- with large areas of northern Asia. Given the extreme central United States, and parts of the Middle East and nature of such heat waves, this outcome is to be ex- India. In these latter regions some hundreds of heat pected.

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FIG. 2. The frequency of occurrence of mean summer temperature anomalies above specified thresholds for selected model grid boxes over the 10 000 yr of the simulation. The grid boxes are located as follows: United States (35°N, 91°W), India (20°N, 80°E), Europe (50°N, 10.5°E), and Australia (25°S, 150°E).

The occurrence rates of 3-K summer heat waves for summer, as Fig. 1 confirms. These outcomes are also the Southern Hemisphere in Fig. 1 are noticeably lower consistent with the view that it is easier to obtain posi- than those for the Northern Hemisphere, and are re- tive temperature anomalies from a low temperature stricted to regions in southeast Africa, Australia, and base (i.e., winter) than a high temperature base (i.e., South America and most of Antarctica. These rates summer), and that the greater atmospheric stability in decline very rapidly for the higher-intensity cases, so winter means any thermal anomaly is restricted to the that for 5-K heat waves occurrences are restricted to lower troposphere, whereas in summer anomalies at- northeast Australia and sea ice regions. tain much higher levels owing to convection. In summary, for middle- to high-latitude land areas Figure 2 shows time series of mean summer tempera- summer heat waves with an average intensity of 3 K ture anomalies for selected grid boxes in the model have a probability of occurrence of about 1 in 500 yr. where high occurrence rates are identified in Fig. 1. In The results for the winter hemisphere in Fig. 1 reveal this and a number of subsequent figures, results are contrasting outcomes for the Southern and Northern presented for selected case studies. Although, unlike Hemispheres. In the Southern Hemisphere the rela- Fig. 1, they are not necessarily representative of global tively small area of sea ice and the limited land areas conditions or long-term statistics, such case studies pro- means that it does not experience the climatic impacts vide a perspective of outcomes that potentially can be associated with continentality so apparent in the North- experienced by individuals. Different thresholds are ern Hemisphere. Given the greater climatic variability used in the various panels in Fig. 2 in order to have occurring in winter, there is also an expectation that roughly the same number of events for each panel. For thresholds would be exceeded more frequently than in the selected thresholds, it can be seen that century-long

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FIG. 3. Global distributions of summer mean surface temperature anomalies (K) for selected years. Individual years were selected to highlight heat waves in four separate regions.

periods, and occasionally millennial periods, exist comes suggest that the heat waves were stochastically where no heat waves of the specified intensity occur. In generated. addition, there are periods of relatively frequent heat To determine the representativeness of the heat waves, interspersed with much lower occurrence rates; wave characteristics in the individual panels in Fig. 3, see especially the U.S. panel in Fig. 2. The India panel composite plots were made for the years with the larg- in Fig. 2 had the largest number of extreme heat waves est values in Fig. 2. A minimum of 10 yr was used for (Ͼ6 K) and Europe the lowest. In all four panels out- each composite. Results (not shown) for the heat wave liers can be identified. regions of the United States, India, and Europe were An interesting feature of the summer heat waves is very similar to those displayed in Fig. 3, both in mag- illustrated in Fig. 3, where global distributions of sum- nitude and spatial extent. Thus, the individual heat mer surface temperature anomalies are displayed for waves shown in Fig. 3 are characteristic of those to be individual years selected from each of the panels in expected in other years. As can be seen from Fig. 1, Fig. 2. The most extreme years in Fig. 2 were not used; heat waves are endemic to these regions; hence, the hence, the results in Fig. 3 are more representative of occurrence of systematic patterns. For these three case “typical” heat wave situations. With the exception of studies the composites did not contain any other areas the panel for Europe in Fig. 3, these individual summer with heat waves, indicating that any other substantial heat waves occurred as isolated, localized events, and positive temperature anomalies in the individual panels not as “hotspots” associated with much larger-scale sys- in Fig. 3 were transient events, and not systematic fea- tems. In particular, for the U.S. and Australian ex- tures associated with the heat waves. In the case of amples, the heat waves were restricted to modest areas Australia the composite heat wave pattern was more of their respective continents, with opposite-signed extensive than that shown in Fig. 3, extending over temperature anomalies in adjacent regions. These out- about 75% of the continent but with the maximum am-

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FIG. 4. Probability density functions of summer surface temperature anomalies for all 10 000 yr of the simulation are shown for the same four grid boxes as used in Fig. 2. Superimposed on each pdf is the corresponding Gaussian distribution. plitude still located on the east coast. A number of The pdf distributions for individual grid boxes varied regions in the Northern (winter) Hemisphere had posi- considerably with location. For example, a grid box to tive or negative anomalies of up to3KintheAustra- the north and west of the Australian grid box in Fig. 4d lian composite. Note that since we experience the im- produced an almost Gaussian outcome, as did a grid pact of individual, rather than composite, heat waves, it box over the south-central Pacific Ocean. When a spa- is considered that the results displayed in Fig. 3 are the tial average of the surface temperature anomalies was more relevant outcomes. made for the region of the Australian grid box in Fig. The statistical distribution of the surface temperature 4d, the resulting pdf retained the basic shape of the pdf anomalies for summer conditions for the four grid in that figure, but the “noise” apparent in that distri- boxes in Fig. 2 is shown in the form of probability den- bution was removed. The pdf distribution for a single sity functions (pdf’s) in Fig. 4. Superimposed on each of grid box is preferred in the present analysis, even al- the individual pdf’s is a Gaussian distribution. Consid- lowing for the implied spatial smoothing compared to erable differences are apparent between the grid boxes. observed station data, as it is again considered to be In the case of the European panel in Fig. 4c, the tem- more representative of outcomes experienced by indi- perature pdf corresponds very closely to the Gaussian viduals. distribution, indicating a normal distribution of anoma- Returning to Fig. 2, the most exceptional outlier, in lies. For the other panels in Fig. 4, there was a notable terms of relative anomalies, is that for Europe, where a departure from normality in the form of an extended sustained summer temperature anomaly of4Koc- positive tail to the pdf’s, as might be expected from the curred for year 5190. According to Fink et al. (2004) a time series plots in Fig. 2, and a lower count of events surface temperature anomaly Ͼ5 K occurred across for small positive anomalies. The U.S. and India grid large parts of Europe in June and August of 2003, while boxes also had fewer extreme cold anomalies than ex- small regions with anomalies Ͼ3 K existed during July. pected. Thus, the simulated outcome is plausible. Nevertheless,

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Ϫ FIG. 5. Surface temperature (K) and rainfall (mm day 1) anomalies over Europe for the extreme heat wave year 5190 of the simulation (See Fig. 2). (left) The temperature anomalies and (right) the rainfall anomalies. Results for (top) June, (middle) July, and (bottom) August are given. there was only one such event in the simulation, high- intense and longer lasting but also more frequent, thus lighting the extreme rarity of this heat wave. This out- distinguishing them from the naturally occurring heat come reinforces the conclusions of Trigo et al. (2005) wave simulated here. and Schär et al. (2004) concerning the infrequency of The spatial patterns of surface temperature and rain- the observed 2003 European heat wave. A recurrence fall anomalies over the European region for June, July, of heat wave conditions similar to those of 2003 in the and August individually are illustrated in Fig. 5 for the next few decades would support the contention of Stott extreme year 5190. The heat wave in the model com- et al. (2004) that the 2003 event was partially anthro- menced in June, as in May there were regional positive pogenically influenced, but in the meantime this event surface temperature anomalies only over parts of the may be attributable to naturally occurring climatic vari- United Kingdom, Poland, and eastern Russia. The heat ability. Meehl and Tebaldi (2004) have suggested that wave contracted in magnitude and intensity in July, ex- greenhouse-induced heat waves will not only be more panded again in August, and was reduced to anomalies

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Fig 5 live 4/C 3810 JOURNAL OF CLIMATE VOLUME 20 of ϩ1toϩ2 K over the United Kingdom, France, and the northeast and southeast, but subsequently it con- Germany in September (not shown). The June and tracted throughout June, July, and August, while re- August surface temperature anomalies in Fig. 5 are maining fairly stationary over the south-central United very similar to the observed values; see Fink et al. States; see Fig. 6. Given the movement of the heat wave (2004). Large parts of Europe experienced above aver- pattern, no single location experienced the maximum age rainfall in July 2003 (Fink et al. 2004); hence, temperature anomalies of 4 K for several months con- observed surface temperature anomalies were mainly tinuously. Extreme temperature anomalies of7Koc- Ͻ3 K. In the simulation (Fig. 5), below average rainfall curred in a number of months. If such a summer heat was recorded for June, July, and August, amplifying wave is ever experienced over the United States, and and perpetuating the heat wave. The simulated anoma- the results in Fig. 2 suggest that this is not implausible, lous rainfall patterns for June and August were quite then a death rate similar to that of Europe in 2003 similar to the observed values (Fink et al. 2004). Thus, might be expected. As was shown for the European fortuitously the simulation replicated the main charac- heat wave in Fig. 5, below average rainfall was associ- teristics of the European 2003 heat wave—a remark- ated with the summer heat wave conditions in Fig. 6. able outcome given the rarity of this event. The sequence of temperature and rainfall anomalies for Although recent heat waves in the United States the extreme heat wave in model year 1457 is shown in (1995 and 1999) have been restricted to days, as op- Fig. 7 for the U.S. grid box (35°N, 91°W) used in Fig. 2. posed to months as was the case for Europe in 2003, The figure clearly illustrates the intensity of the heat Chang and Wallace (1987) have presented observations wave at this grid box, with temperatures anomalies that indicate longer-duration heat waves do occur fre- above 4 K from April to August and a peak anomaly of quently. Thus, their Table 1 records heat waves for 7.7 K in June. Below average rainfall occurred for most June, July, and August 1934; July and August 1936; and of the previous year (1456), thus preconditioning the month-long heat waves in the summers of some years in surface for a heat wave by reducing the evaporative all decades from 1930 to 1980. While their analysis was cooling. The negative rainfall anomalies peak during based on Kansas City, Missouri, they show that this was the heat wave year as expected. In year 1458 conditions representative of a much larger area. Temperature returned to “normal,” with no residual influences from anomalies ranging from 2.5 to 5 K were associated with the heat wave in the previous year. these heat waves. Examination of Niño-3.4 sea surface temperature The U.S. panel in Fig. 2 is restricted to mean summer anomalies revealed a modest La Niña event in model temperature anomalies over 4 K, but the numerous oc- year 1456 (maximum anomalies of Ϫ0.55 K), thus ac- casions with anomalies below that limit would corre- counting for the below average rainfall in that year. spond more closely to the observations reported by After weakening in late 1456 to mid-1457, the La Niña Chang and Wallace (1987). Individual years taken from event strengthened and retained anomalies less than the U.S. panel in Fig. 2 normally revealed simulated Ϫ0.4 K from mid-1457 to mid-1458. Subsequently, surface temperature anomaly patterns centered over anomalies of about Ϫ0.1 K prevailed for the remainder the south-central United States, with some movement of 1458. Thus, this particularly extreme heat wave was from month to month, and maximum anomalies Ͼ4K associated with a persistent, but moderate intensity, La for at least three summer months. Given this broad Niña event. The relationship between such events and agreement with the observed temperature anomalies U.S. heat waves is examined in more detail below. for 1934 (Chang and Wallace 1987), results for the most An examination of heat waves over India was also extreme U.S. case in model year 1457 (see Fig. 2) will be made; see Figs. 2 and 3 for background details. Spatial presented as a possible indication of an outcome over plots of surface temperature anomalies over India (not the United States that would correspond to the 2003 shown) revealed a fairly constant location of the maxi- European heat wave in relative severity. mum anomaly over the center of India in any given This simulated U.S. heat wave appears to have com- month. The maximum anomaly was usually 6–7K,and menced in Alaska, and northern Canada, where Ͼ6K the heat waves almost invariably commenced in June surface temperature anomalies were simulated in Janu- and were finished by September. Examination of years ary; see Fig. 6. As far as these regions are concerned, adjacent to extreme years revealed no indication of a this would imply a mild winter rather than a heat wave. heat wave, with normal, or in some months below nor- The “heat wave” conditions over North America in mal surface temperatures in such years. As expected, February expanded and intensified, and subsequently the heat wave years had below average rainfall, usually moved southeastward into the United States. In May over all of India, during the summer monsoon season. the heat wave pattern had bifurcated into lobes over Heat waves over Australia exhibited a wide range of

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FIG. 6. Monthly mean surface temperature anomalies over the North American region for January–August for the extreme U.S. heat wave year 1457 of the simulation (see Fig. 2). The color bars below the panels are in K. spatial patterns, for months within a given year, and reached 6–7 K in February or March, and for less ex- also from one event to another. This situation is illus- treme years these were normally 4–5 K. The Australian trated in Fig. 8 for the most extreme case, model year heat waves were also associated with negative rainfall 9618, depicted in the Australia panel in Fig. 2. As anomalies (not shown), although a clear spatial corre- shown in Fig. 8, the heat wave in January was restricted spondence between temperature and rainfall anomalies to central eastern Australia, but expanded rapidly in was not always apparent. February to a bipolar pattern covering the whole coun- try and then weakening noticeably in March. In gen- 4. Daily heat waves eral, most of these summer heat waves commenced in December and had terminated by April. For extreme The most commonly experienced type of heat waves years, the maximum surface temperature anomalies are those that occur over a number of consecutive days;

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FIG. 7. Time series of monthly surface temperature and rainfall anomalies for the U.S. grid box (35°N, 91°W) for years 1456–1458, which encompass the extreme heat wave year of 1457. The rainfall anomalies have been shifted 0.5 months to the right of the surface temperature anomalies for clarity. see for example Kunkel et al. (1996), Huth et al. (2000), example the largest number of successive days meeting Palecki et al. (2001), Burt (2004), and Khaliq et al. the 4-K criterion was 20, which occurred in model year (2005). More than one such heat wave can occur in a 4470 in Fig. 9. In their observational analysis, Choi and given summer (Khaliq et al. 2005) and the intensity of Meentemeyer (2002) note a maximum duration heat the heat waves can be higher, given their short dura- wave occurrence of 41 days in Texas, where values were tion, than that for seasonal heat waves. one standard deviation (ϳ4 K) above normal. What A comprehensive analysis of heat waves in the appear as single lines in Fig. 9 usually involve several United States for the period 1950–95 has been pre- individual days; hence, any implied heat wave condi- sented by Choi and Meentemeyer (2002). They plot the tions may be longer lasting than they seem. The most frequency of occurrence of heat wave characteristics for intense series of events occurred in year 4470 where various durations and intensities, and, in view of this there was a sequence of runs of consecutive days, sepa- documentation, much of the detailed model analysis in rated by days that did not meet the criterion. This situ- this section will be based on events occurring over the ation appears to be comparable to conditions prevailing United States. in the summer of 1934 at Kansas City, Missouri; see For convenience, results will be presented as anoma- Table 1 of Chang and Wallace (1987). Overall, Fig. 9 lies of daily maximum temperature for the simulated indicates a range of situations, from relatively quiescent period of years 4401–4500. These anomalies are depar- states, the first 20 yr, a number of individual years tures from the mean of years 4001–5000 over which where the heat wave criterion was not attained, and daily data were saved. The 100-yr period used here was episodes of marked anomalies occurring over multiple arbitrarily chosen, but other centennial-long periods days; see years 4443, 4470, etc. produced very similar results. The analysis was prima- As a further indication of the model’s performance, rily confined to 100 yr simply because of the volumi- Fig. 10 compares daily temperature anomalies over a nous nature of the daily data. year for the model and reanalysis results [taken from In Fig. 9 anomalies of the daily maximum tempera- the European Centre for Medium-Range Weather ture, restricted to values greater than 4 K, are plotted Forecasts (ECMWF) 40-yr reanalysis (ERA-40) data- for the U.S. grid box, 35°N, 91°W. Results are for the set] for the same U.S. point as was used in Fig. 9. The summer months of June, July, and August only. A con- model anomalies are for the maximum daily tem- siderable range of variability is apparent in Fig. 9, with perature; the observations are the mean of four daily extreme anomalies of up to 12 K. For this particular values for a height of 2 m. Given that both years used

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FIG. 9. Time series of daily maximum surface temperature anomalies for the U.S. grid box (35°N, 91°W) for the 100-yr period of 4401–4500 from the simulation. Values less than 4 K have been omitted. Results shown are for June, July, and August only.

of heat waves with daily anomalies greater than 4 K for three different durations. For each hemisphere the corresponding winter anomalies for the opposite hemisphere are also displayed. Not surprisingly, there are virtually no oceanic grid boxes recording heat-wave conditions by these definitions in Fig. 11. In addition, no heat waves occur over jungle-covered land regions. The most noticeable feature in Fig. 11 is the high occurrence rate of “heat waves” for winter conditions in both hemispheres. This response can be clearly discerned in the ERA-40 reanalysis in Fig. 10 and is indicative of mild spells during winter. These are associated with synoptic conditions producing intrusions of warmer air from oceanic or midlatitude regions. The noticeable drop in the occurrence rate for 10- and 15- day periods again can be attributed to the time scale of the synoptic events. For summer conditions in both hemispheres in Fig. FIG. 8. Monthly surface temperature anomalies (K) for Austra- 11 the occurrence rate of the specified heat waves de- lia for an extreme heat wave situation. Results are shown for clines rapidly as the duration of the heat waves is ex- summer of year 9618 of the simulation, for January, February, and tended (top to bottom panels in Fig. 11). Given the March. vagaries of the weather and the frequency of synoptic systems, the difficulties of maintaining long-duration in Fig. 10 were arbitrarily selected (but were heat wave heat waves can be readily appreciated. In the Northern years), there is no reason why they should closely agree. Hemisphere, the heat waves mainly occur at high lati- Nevertheless, both time series display similar charac- tudes, where they represent enhancements of relatively teristics with both positive and negative anomalies low temperatures, but persistent heat waves are also throughout the year, and with similar ranges of noted over India and the central and southern United anomaly values, although the model had more positive States. For the Southern Hemisphere, the heat waves anomalies. This comparison indicates that the model are principally located over Australia. simulated the basic features of the observations, pro- The high occurrence rates in the two upper panels in viding confidence in the results to be presented below. Fig. 11, that is, those for heat waves with durations of 5 A global perspective of daily heat waves is now illus- successive days, indicate that in some years more than trated in Fig. 11. This figure shows the occurrence rate one event lasting 5 days was identified.

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over Australia. These anomalies were associated with low insolation at the surface, implying high cloud amounts (daily cloud amounts were not saved); in con- trast, high insolation was incident over the U.S. region experiencing the heat wave. The temperature anomaly pattern over the United States for late June in Fig. 12 was similar to a composite observed anomaly pattern presented by Chang and Wallace (1987) for the hottest summer months in the 1930s. The amplitude of their anomalies was smaller than that simulated, but this would be expected owing to the averaging involved in generating the composite. They also reported negative temperature anomalies over the western third of the United States in their composite figure, similar to those shown in Fig. 12. Chang and Wallace (1987) also reproduced the observed composite surface pressure chart corresponding to their temperature anomaly composite. This had high pressure systems situated off the west and east coasts of the United States and a low pressure system over North America. Figure 13 shows the surface pressure distribution for 30 June of year 4498 of the simulation. This reveals a similar situation to that of Chang and Wallace (1987), indicating again that the model replicated the observed synoptic systems. The pressure patterns over the North American region shown in Fig. 13 were representative of other days of the simulated heat wave, but with small sequential changes corresponding to the movement of the heat wave across the United States. FIG. 10. Time series of daily temperature anomalies (K) for the Given the apparent reasonable agreement of the U.S. grid box (35°N, 91°W). Results are shown for arbitrarily simulation with observations over the United States, it selected years. (top) The model maximum temperature anomalies is of interest to explore some of the more extreme simu- are plotted for year 4401 from the simulation; (bottom) the reanalysis results for daily mean surface air temperature anomalies lated heat waves. Because of the very limited duration are plotted for 2000 A.D. of the observational record, it is plausible to assume that only a limited sample of potential heat waves has Finally, in Fig. 12 a daily sequence of anomalies for been experienced to date. Even without any contribu- the daily maximum temperature is shown for a heat tion from a greenhouse warming, more extreme heat wave over the United States encompassing late June– waves than presently recorded should occur, simply as early July of year 4498 of the simulation. This heat wave a consequence of natural variability; see for example commenced and terminated after the days shown in Fig. 2. Fig. 12, but the most intense phase is given in the figure. In Fig. 14 simulated and reanalysis (ERA-40) tem- This heat wave slowly progressed southeastward across perature anomalies for the U.S. point (35°N, 91°W) are the United States, with peak temperature anomalies of shown, for JJA values only, with the objective of com- about 8 K being maintained during this progression. In paring the frequency of occurrence and the magnitude the early stages of the heat wave, temperatures below of extreme daily temperature anomalies at this specific average occurred to the region north and west of the location. The simulated anomalies are for the maxi- heat wave zone; thus, the continent was not under the mum daily surface temperature, for values exceeding influence of a single synoptic system. In fact, the spatial 10 K, and are for the 500-yr period 4001–4500. The scale of this heat wave emphasizes its isolated nature ERA-40 anomalies are for the mean daily temperature and the lack of any connection to large-scale systems. at a height of 2 m (denominated t2), for values exceed- For the duration of the U.S. heat wave, a large negative ing 4 K only, and are for the 44-yr period 1958–2001. temperature anomaly was sustained over northern Thus, there is a tenfold difference in the two time- Asia, together with another sustained negative anomaly frames.

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FIG. 11. Occurrence rates of summer heat waves for the (left) Northern Hemisphere (JJA) and (right) Southern Hemisphere (JFM) are illustrated. Heat waves having daily maximum temperature anomalies greater than 4 K for (top) 5, (middle) 10, and (bottom) 15 successive days. The color bars give the occurrence rates for the 100-yr period of years 4401–4500 from the simulation.

The use of mean daily t2 data from the ERA-40 Fig. 14 normally involved multiple-day events reaching dataset (daily maximum values were not available) acts the set criteria, as well as other adjacent days with to restrict the magnitude of the resulting anomalies; smaller-amplitude temperature anomalies. As such, hence, the ERA-40 magnitudes peak out between 7 and these results are indicative of local heat wave condi- 8 K compared to 12 and 13 K for the simulation. The tions. brevity of the ERA-40 dataset also limits the extreme amplitudes that might be expected. 5. Mechanistic processes Considerable secular variability is apparent in the simulation in Fig. 14, with a 90-yr period between years A related series of physical processes is involved in 4340 and 4430 with no anomalies reaching the set cri- the generation of a heat wave; see, for example, terion, while at other times these events occur at mul- Manabe and Wetherald (1987) and Wetherald and tiannual intervals. The brevity of the observations lim- Manabe (1999). In Fig. 15 monthly anomalies for year its the possibility of secular variability, while illustrating 4103 of the simulation for the U.S. grid box (35°N, the relatively high frequency of the occurrence rate of 91°W) are compared for six climatic variables. As extreme temperature anomalies. Each of the results in shown in Fig. 14, a heat wave occurred at this location,

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FIG. 12. Sequence of daily maximum surface temperature anomalies (K) for 8 days in June and July of year 4498 of the simulation during which a heat wave occurred in the United States.

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FIG. 13. Surface pressure distribution (mb) corresponding to 30 June of year 4498, one of the days used in Fig. 12. and this is also clearly indicated by the monthly surface temperature anomalies in Fig. 15. The precursor to this heat wave was the below average rainfall that commenced in January of year 4103 (Fig. 15b); above average rainfall occurred for most of year 4102. This below average rainfall was accompanied by below average cloud amount (not shown), resulting in an above average net surface short wave flux (Fig. 15f). This enhanced solar heating was the primary generator of the related heat wave. Because of the below average rainfall, negative soil moisture anomalies resulted (Fig. 15d), and this reduced the surface evaporation (Fig. 15c). Since the surface evaporative flux is the primary FIG. 14. Time series of extreme temperature anomalies for the U.S. grid box (35°N, 91°W). (top) The model values are given for cooling agency in determining the surface temperature, the 500-yr period of years 4001–4500 from the simulation. Daily the reduced evaporation rate also contributed to the maximum surface temperature anomalies are shown for June, maintenance of the heat wave. As shown in Fig. 15e, July, and August conditions and for magnitudes above 10 K. the sensible heat flux at the surface increased, to par- (bottom) The ERA-40 t2 anomalies, as the average of four values tially compensate for the reduced evaporation rate, but over each day, are shown for the same months but for magnitudes above 4 K. such an increase usually requires higher surface temperatures. Thus, a very consistent set of physical processes was The central question related to Fig. 15 is how the associated with this heat wave. A similar analysis for an physical processes displayed in this figure were initi- Indian heat wave in year 9661 of the simulation, re- ated, and subsequently maintained, in order to produce sulted in the same related physical processes as are a heat wave. The apparent precursor is the rainfall de- shown in Fig. 15. This, together with the results in Fig. cline, with its associated reduction in cloud amount, 7, indicate that the relationships shown in Fig. 15 are a which then permitted enhanced solar radiation to ini- robust characteristic of heat waves. tiate the surface warming. This warming was then en- The impact of short-term rainfall on a heat wave has hanced by the subsequent changes to the soil moisture been described by Kunkel et al. (1996) and Palecki et content and evaporation rate. Thus, the question be- al. (2001) in relation to the Chicago heat waves of 1995 comes what initiated and maintained the rainfall decline? and 1998, respectively. They relate such rainfall to an Presumably, some change in the atmospheric circula- increase in soil moisture content and subsequently tion is responsible. Choi and Meentemeyer (2002) have evaporation, which then ameliorated (temporarily) the shown that the composite 500-mb geopotential height heat wave conditions. Brabson et al. (2005) have also has an anomaly of ϩ50 gpm for U.S. heat waves, while shown how the lower soil moisture content to be ex- Chang and Wallace (1987) present various composite pected under greenhouse conditions will cause system- charts for climatological, heat wave, and cool condi- atically higher surface temperature values. tions over the United States. The latter authors make

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FIG. 15. Monthly time series for anomalies of selected climatic variables for year 4103 from the simulation for the U.S. grid box (35°N, 91°W). the following fundamental statement: “It is virtually im- ining simulated monthly surface pressure patterns for possible to distinguish this composite SLP field (for heat wave and cold episode conditions over the United heat wave conditions) from the climatological mean States. While year-to-year differences were apparent, pattern.” A similar outcome was obtained from exam- especially external to the United States, the basic sur-

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FIG. 16. Time series for the 25-yr period of years 4100–4124 from the simulation. Shown are monthly anomalies of surface temperature at the U.S. grid box (35°N, 91°W) as well as Niño-3.4 sea surface temperature. face pressure pattern (see Fig. 13 for a daily pattern) shown for this simulation (Hunt 2006a) that Niño-3.4 was essentially robust. outliers are not associated with other climatic outliers A one-point correlation plot of surface temperature outside of the Niño-3.4 region. for the U.S. grid box (35°N, 91°W) and global surface Thus, the occurrence of ENSO events does not pro- pressure, for years 4001–5000 of the simulation, provide a unique predictive capability for heat waves for duced values of less than |0.1|, thus reinforcing the in- the southwest United States, or elsewhere globally, sensitivity of this surface temperature to surface pres- even allowing for the limitations of the simulated sure distribution. ENSO events. This conclusion is reinforced by the U.S. Given the relationship between rainfall variability and Australian heat waves in Fig. 3, where neither over the United States and ENSO (Ropelewski and La NiñaorElNiño events are apparent. Thus, given Halpert 1987), an impact of ENSO on surface tempera- the invariance of the model’s boundary conditions for ture, and thus heat waves, might be expected. La Niña the duration of the simulation, the only cause of heat conditions are associated with droughts over the U.S. waves, whether or not they occur during ENSO years, southeast and pluvials over the northwest United has to be random (stochastic) processes arising from States. Such moisture relationships then give rise to the nonlinear interactions existing within the climatic positive and negative surface temperature anomalies, system. Again, while the intensity of heat waves on respectively, as implied from Fig. 15. In fact, these out- some occasions may have been underestimated owing comes are apparent in the daily surface temperature to the weakness of the simulated ENSOs, the earlier anomalies shown in Fig. 12. comparisons suggest that this does not seem to have In Fig. 16 plots of monthly surface temperature affected the frequency of the heat waves. The essen- anomalies for the U.S. grid box (35°N, 91°W) are con- tially random nature of heat waves for the various trasted with Niño-3.4 sea surface temperature anoma- continents illustrated in Fig. 2, and the lack of any lies for a 25-yr period of the simulation. Over the mil- simultaneity between occurrences at those different lennium of years 4001–5000, the correlation between locations, strongly supports this attribution of stocha- these two variables was only Ϫ0.135, indicating a mod- cism. est interrelationship. While the heat waves at years 4103 and 4117 in Fig. 16 occur during La Niña years, as 6. Conclusions might be expected, every La Niña year does not have a heat wave. From an analysis of a 10 000-yr simulation with the In terms of climatic extremes or outliers, it has been CSIRO Mark 2 coupled global climatic model, a num-

Unauthenticated | Downloaded 09/23/21 09:26 PM UTC 3820 JOURNAL OF CLIMATE VOLUME 20 ber of insights has been obtained regarding the clima- days. Time series for a selected U.S. grid box high- tology of heat waves. lighted the greater magnitude of surface temperature Summer heat waves, having a duration of 3 months anomalies, with values of up to 12 K. The annual cycle and a magnitude of 3 K are almost totally restricted to of temperature anomalies for this grid box agreed rea- the continents, with maximum responses over high sonably well with observations, having marked positive northern latitudes, the south-central United States, In- and negative anomalies in winter and rather smaller dia, the Middle East, and Australia. These heat waves negative anomalies in summer. did not occur over southeast Asia or in South America, An examination of the physical processes associated apart from Argentina. Many parts of the latter regions with heat waves emphasized the consistent relation- are jungle covered or mountainous. ships occurring among the various climatic variables. The frequency of occurrence of these heat waves de- This revealed that reduced rainfall, and thus reduced clined very fast for magnitudes of 4 and 5 K. The oc- cloud cover, permitted increased solar radiation to pen- currence rate of summer heat waves was considerably etrate to the earth’s surface, providing the energy more spatially limited in the Southern Hemisphere, and source for the subsequent heat wave. The associated declined even faster for more intense heat waves. reduced soil moisture content resulted in low surface Time series of summer heat waves for selected model evaporation, and evaporative cooling, thereby reinforc- grid boxes revealed substantial differences between lo- ing the initial solar heating. calities, with some having fairly uniform occurrence The cause of the precursor reduction of rainfall and rates and others with century or longer periods with no cloud cover is presumed to be stochastic. While ENSO heat waves reaching the set criterion. Marked outliers, influences have a role in creating drought conditions in terms of magnitude of the heat waves, were high- conducive to a heat wave, not all ENSO events produce lighted within these time series. a heat wave. It is this difference between outcomes as Global distributions of surface temperature anoma- regards the occurrence of heat waves that is attributed lies for selected years where a summer heat wave was to stochastic forcing. In the case of the exceptional identified at a specific locality revealed the spatially European heat wave in the simulation, this would seem isolated nature of these heat waves. No relationship to to be totally the result of stochastic forcing in the simu- other climatic events over the globe could be discerned, lation. suggesting that the heat waves were the result of local In summary, coupled global climatic models are ca- stochastic influences. pable of simulating most of the observed characteristics Over the 10 000 yr of the simulation, one outstanding of heat waves, and provide a unique tool for obtaining heat wave was identified over Europe. This heat wave both global and temporal perspectives of the climato- had very similar magnitude and spatial occurrence to logical features of heat waves. An important outcome is the observed heat wave of 2003, and, like the observa- that the present simulation suggests that observed heat tions, was associated with below average rainfall. This waves, to date, may have occurred through natural cli- outcome suggests that the 2003 European heat wave matic variability and that recourse to external influ- may have been a consequence of just naturally occur- ences is not required. However, any future positive shift ring climatic variability, rather than being greenhouse in the near-Gaussian temperature distributions, such as influenced, although Stott et al. (2004) suggest a green- might be anticipated under global warming conditions, house-enhanced probability. The occurrence of a simi- would make the occurrence of more extreme heat lar heat wave within the next few decades would, how- waves more likely. ever, imply a greenhouse impact. Monthly plots of the spatial distribution of intense Acknowledgments. The assistance of Mark Collier, heat waves over the United States and Australia re- Martin Dix, and Tracey Elliott in different aspects of vealed considerable spatial variability of the heat the production of this paper is noted with thanks. waves, with that for the United States progressing from Alaska to northwest Canada in January to southeast REFERENCES United States in August. In each case these heat waves were also associated with below average rainfall over AchutaRao, K., and K. R. Sperber, 2000: El Niño Southern Os- the summer months. cillation in coupled GCMs. PCMDI Rep. 61, PCMDI, Lawrence Livermore Laboratory, Livermore, CA, 46 pp. The spatial patterns of daily heat wave occurrence Allan, R., J. Lindesay, and D. 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