Wildfires periodically burn large areas ofchaparral and adjacent woodlands in autumnand winter in southern California.These firesoften occur in conjunction with Santa Anaweather events, which combine high windsand low humidity, and tend to follow a wetwinter rainy season. Because conditions fos-tering large fall and winter wildfires in Californiaare the result of large-scale patterns in atmos-pheric circulation, the same dangerous condi-tions are likely to occur over a wide area atthe same time.

Furthermore, over a century of watershedreserve management and fire suppressionhave promoted fuel accumulations,helping toshape one of the most conflagration-proneenvironments in the world [Pyne,1997].Com-bined with a complex topography and a largehuman population, southern Californian ecologyand climate pose a considerable physical andsocietal challenge to fire management.

October 2003 Wildfires

Both antecedent climate and meteorologyplayed important roles in the recent extremewildfires in southern California.After a multi-year drought contributed to extensive mortalityin western forests and chaparral, late winterprecipitation and a cool spring and early sum-mer fostered the growth of grasses that werecured out during a hot summer and autumnin 2003, producing an extensive fine fuel cov-erage. Fanned by moderate Santa Ana winds,

12 major fires started between 21 Octoberand 27 October in southern California andanother began on 28 October near Ensenadain Baja California,Mexico (see Figure 1).Together, the fires had burned over 300,000hectares by November.All of these human-

induced fires started in chaparral on or belowthe western slopes of California’s coastalmountain ranges and initially burned towardthe Pacific Ocean.Their paths toward the seawere, in many cases, coincident with some ofthe most densely populated urban areas inthe United States.

The Santa Ana winds that fostered the rapidgrowth of these fires were not in themselvesextraordinary, though the hot and dry condi-tions leading up to the fire events were atrecord or near-record levels. Large wildfires inchaparral in the autumn and winter monthsare also not extraordinary events in southernCalifornia.They have occurred frequently

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BY ANTHONY L.WESTERLING, DANIEL R. CAYAN,TIMOTHY J. BROWN, BETH L. HALL, and LAURENCE

G. RIDDLE

Fig.1. Smoke from southern Californiawildfires is shown on 26 October 2003.Activefire perimeters are outlined in red in Ventura,San Bernardino,Los Angeles,and San Diegocounties, and in Baja California,Mexico.Selected city names are in black; fire names inwhite. Source image courtesy of NASA/MODISRapid Response Team.

Climate, Santa Ana Winds and Autumn Wildfires in Southern California

Eos,Vol. 85, No. 31, 3 August 2004

during the last century. Moreover, charcoalrecords from Santa Barbara Channel sedimentsindicate the frequency of wildfires in theregion has not changed significantly in thelast 500 years [Mensing et al.,1999].The severityof the immediate human impact of the October2003 wildfires was exacerbated by the rapid

growth of an extensive wildland-urban inter-face proximate to a population of nearly 20million in southern California, where the pop-ulation has more than doubled since 1950.

The intensity of the fires and the severity of theirecological impact on the region’s forests wereexacerbated by the long-term accumulation

of fuels such as snags, logs, and heavy brushdue to 20th-century fire suppression policiesand watershed preservation efforts since thelate 1800s.

Santa Ana Winds

In southern California, Santa Ana winds area recurrent regional-scale influence in theautumn, winter, and spring months, peaking inDecember. Dry Santa Ana winds promote theignition and rapid spread of wildfires by dry-ing fuels and fanning the flames of fires oncethey are started.Their greatest effect on firetends to be in autumn, when vegetation hasbeen desiccated after a long dry summer andbefore the onset of the winter rainy season.Winter rainfall is highly variable in southernCalifornia, however, and in some years, largefires have occurred during Santa Ana condi-tions as late as February.

The Santa Ana is the name given to foehn-like winds in California, which result when a cool, dry air mass flows downslope fromhigh-elevation basins in the western NorthAmerican interior toward lower atmosphericpressures off the Pacific coast.This flow is fun-neled toward passes in the southern Californiacoastal ranges by the higher Sierra Nevadarange in the west and the Rocky Mountains tothe east.As the air sinks, it is compressed,warming it and reducing its relative humidity.Compression of this air mass through moun-tain passes often produces winds of 40–60km/hr, and in extreme cases, may yield windspeeds in excess of 100 km/hr.These dry,sometimes hot, winds quickly reduce fuelmoistures, greatly enhancing the risk of fire,and fan the flames of any fire once started.

Four national forests in southernCalifornia—the main portion of the LosPadres, and the San Bernardino, Angeles, andCleveland National Forests—cover about700,000 hectares in a broad arc running alongthe coastal mountain ranges from about 35.5°to 32.5°N latitude. Digitized fire histories forthese forests are available from 1970 and pro-vide a representative sample of large wildlandfires in chaparral and forest ecosystems forthe region.

Inspection of weather maps associated withlarge autumn and winter fires reveals that themajority occurred during Santa Ana events.Composites of 700 hPa heights—that is, theaverage height of a 700 hPa atmospheric pres-sure—for each forest for the ignition dates oflarge fires exceeding 400 hectares from Sep-tember to January (1970–2000) indicate a consis-tent pattern of anomalous atmospheric circulationin the mid-troposphere.The top panel in Figure 2is the composite for 29 large fires reportedduring this period in the Cleveland NationalForest near San Diego.This pattern is charac-teristic of Santa Ana conditions, with ananomalously deep Aleutian low and a high-pressure system centered over the northwest.The anti-cyclonic flow around the high-pres-sure feature brings dry air from the interiorbasins toward the southern California coast.

Fire ignitions peaked on 25 October for theOctober 2003 Santa Ana-driven wildfires; fourlarge fires started that subsequently burnednearly 200,000 hectares, including the largestfire in California history—the Cedar fire inCleveland National Forest (CNF) that burned

Fig.2. (top) Composite, 700-hPascal height anomalies are shown for the ignition date for firesburning > 400 hectares (1000 acres) in Cleveland National Forest.Black dots indicate statisti-cally significant anomalies compared to the historical record at the 95% confidence level; (mid-dle) 700-hPascal height anomalies for 25 October 2003, ignition date for Cedar fire. (bottom)daily Burning Index for Cleveland National Forest in October (1992–2000 mean and maximum,and 2003 values).

over 113,000 hectares.Like 97% of large autumnand winter wildfires in southern California’sfour national forests since 1970, these ignitionswere of human origin.The 700-hPa heightmap for 25 October (Figure 2, middle panel)shows a pattern similar to that of the large firecomposites. Observed wind speeds during thefire period were typically 25–40 km/hr,withgusts of around 65 km/hr.While these windswere not exceptional for a Santa Ana event,they were, in combination with homogenousdry fuel conditions, effective in promoting rap-id fire spread. One of the fires—the Cedar firenear San Diego—grew from 200 to 12,500hectares in 4 hours.The combination of warmtemperatures and wind,even though moderatein speed, reduced relative humidity to around10% during most afternoons during the fires.Humidity recovery overnight was also limited,increasing to only 20–30%.The extended periodof low relative humidity was partially respon-sible for the extreme fire behavior observedduring the events.

Fire managers in the United States use firedanger indices from the National Fire DangerRating System (NFDRS) to support decisionsrelated to fire management.The NFDRS BurningIndex (BI) is related to the potential length offlames at the leading edge of a fire and describesthe difficulty of controlling a fire under worst-case conditions (see Schlobohm and Brain[2002]).The BI responds to changes in windspeed and relative humidity on short timescales of hours to days, and thus, it is sensitiveto the onset of Santa Ana conditions.

The BI calculated for CNF in October 2003was at or below daily means for the previous

decade up until 21 October (Figure 2, bottompanel).The period of 25–27 October saw a rapidincrease in the index, with the onset of SantaAna conditions on the night of the 25th.The Cedar fire started the evening of the 25thand grew rapidly overnight and through themorning of the 26th, while the large Paradiseand Otay fires started to the north and southof the Cedar fire on the 26th.Fire fighters did notmake significant progress in containing theCedar and Paradise fires until 30 October,whenthe Santa Ana weakened and relative humidityrose and temperatures fell.The BI for CNF clearlyreflects both the onset and progression ofSanta Ana conditions, as well as their reversal(Figure 2, bottom panel).

The NFDRS Energy Release Component (ERC)is an indicator of heat energy at the head ofthe flaming front calculated in units of J/m2.ERC is also an index of longer-term droughtconditions, and thus provides a general stateof fuel dryness.All NFDRS stations in the regionobserved numerous daily record ERC valuesleading up to and during the main fire period(17–30 October).Many of these extremely highvalues were also near or exceeded all-timeOctober monthly records.

Antecedent Climate

Large fires in autumn and winter in coastalsouthern California chaparral appear to belinked to antecedent climate as well as toconcurrent meteorology.Precipitation tends tobe above normal in the winter or early springprior to the fire season (Figure 3, top panel),suggesting that large fall and winter fires are

preconditioned two or more seasons in advance.Wet winters enhance the growth of grassesand other fine fuels.The 2003 fire season dif-fered somewhat from the composite in thatthe positive precipitation anomalies in latewinter and spring followed an extended 5-yeardrought. However, this precipitation and sub-sequent cooler temperatures were sufficientto promote the growth of fine fuels during thewinter and spring, increasing the continuityand load of fine fuels across the region [Predictive Services, 2003].

Temperatures during years with large autumnand winter fires do not follow a distinct pattern.In composite,monthly mean temperatures areslightly warmer than average in the precedingwinter and spring and during the autumn periodwhen large fires occur, but they contain agreat deal of variability across large fire events.The year 2003 saw a very warm winter followedby a cool spring and then a very warm summerand autumn (Figure 3).Temperatures duringthe first week of the fires were record-settingand averaged 10–20°C above normal.This pat-tern probably accentuated the growth and cur-ing cycle for the grasses and other annualvegetation that burned over much of the area.

Fire Management

Wildfires can become large and costly whenantecedent climate and meteorology facilitatetheir rapid spread.They are also more likely toescape early control under dangerous condi-tions when there are multiple ignitions thatoverwhelm locally available suppressionresources,and when ignition locations are noteasily accessible.Although Santa Ana windsare intensified locally in settings such asmountain passes and canyons, they are drivenby regional and larger-scale atmospheric patterns.Consequently, they usually occur over a broadfootprint so that the same dangerous conditionsare likely to affect the entire 400-km coastalmountain reach of southern California.

This synchrony of risks in inaccessible ter-rain over a large region poses a serious chal-lenge for wildfire management. Judging fromU.S. National Forest Service fire records, theprobability of multiple large fires occurringsimilar to those in 2003 is historically high. Insouthern California, from 1970 through 2003,whenever one or more large fires (greater than400 hectares) burned in a national forest inSeptember through January, there was a nearlyone in four chance that one or more of theother three forests would also experience alarge fire starting within the same 9-day period.

Early control is crucial when conditionsfavor rapid spread. Not only does the rapidgrowth of the fire perimeter greatly complicatecontainment, but the heat of a large fire driesneighboring fuels and generates its own winds,facilitating both the spread of the fire andextreme fire behavior that increases the risk tofire fighters and the public.The 2003 southernCalifornia fires behaved unlike any reportedbefore [Mission-Centered Solutions, 2003].Theextreme nature and severity of the fire behaviorsurprised firefighting crews and subsequentlycaused them to realize that traditional suppression

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Fig.3.Monthly precipitation and temperature anomalies are shown for coastal southern Califor-nia 2003 versus a composite of years,with fires burning more than 400 hectares in ClevelandNational Forest in September through January.Data are for the California “South CoastDrainage”Climate Division, from the National Climate Data Center.

Eos,Vol. 85, No. 31, 3 August 2004

attack tactics were ineffective. Fire whirls weredramatic, as much as 800 m wide, crossingburned areas into unburned areas and caus-ing spot ignitions ahead of the fire by asmuch as 1200 m.Pilots reported burning debrisas high as 450 m above ground. Fire and heatfrom burning structures and ornamental vege-tation were as much of a problem as any ofthe natural fuels.

Many forested areas that burned in October2003 probably burned much more completelythan they would have in the absence of suc-cessful fire suppression over the past century.Dense vegetation fostered the rapid spread ofhigh-intensity fires in the forest canopy.Wide-spread die-offs of this vegetation may help toreduce the risk of canopy fires in the future. Inrecent years, large portions of southern Cali-fornia have experienced substantial vegetation mortality resulting from drought,insects, and disease [Predictive Services,2003].After the fine canopy fuels (needles,leaves, twigs, etc.) fall to the ground, the stand-ing dead trees may be less prone to spreadingfires in the canopy (Craig Allen, pers. comm.).

About 80% of the area burned in the October2003 southern California fire siege, however,

was in chaparral or grassland.Whereas healthyforests in southern California can sustain alow-intensity fire regime with regular surfacefires in the absence of fire suppression, chap-arral always experiences canopy fires thatconsume much of the vegetation. Not onlydoes chaparral regenerate quickly, but a rela-tively small proportion of the areas with homesproximate to wildland vegetation actuallyburned—only about 5% of southern California’swildland-urban interface in total (S. Stewart,pers. comm.). Consequently, the risk of largeregional-scale fires remains high.

Acknowledgments

Support for this research came from the U.S.National Oceanographic and AtmosphericAdministration’s Office of Global Programs.We thank E. Bainto for assistance with thegraphics and C.Allen and M. Moritz for theircomments.

References

Mensing, S.A., J. Michaelsen, and R. Byrne (1999),A560-year record of Santa Ana Fires reconstructed

from charcoal deposited in the Santa BarbaraBasin,California,Quaternary Research 51, 295–305.

Mission-Centered Solutions (2003), Southern Califor-nia Firestorm 2003: Report for the Wildland FireLessons Learned Center, 63 pp., Mission-CenteredSolutions, Inc., Parker, Colo.; http://wildfirelessons.net/.

Predictive Services (2003), Monthly Fire Weather/Fire Danger Outlook, Southern CaliforniaGeographic Area Coordination Center,Vallejo,Calif., November.

Pyne, S. J. (1997), Fire in America, 654 pp., Universityof Washington Press, Seattle.

Schlobohm, P. and J. Brain (2002), Gaining an Under-standing of the National Fire Danger Rating System, National Wildfire Coordinating Group Publication: NFES # 2665, http://www.nwcg.gov.

Author Information

Anthony L.Westerling and Daniel R. Cayan, ScrippsInstitution of Oceanography, La Jolla, Calif.;TimothyJ. Brown and Beth L. Hall, Desert Research Institute,Reno, Nevada; and Laurence G. Riddle, Scripps Insti-tution of Oceanography, La Jolla, Calif.

A number of marine biological, geological,and archaeological applications share the needfor high-resolution optical and acoustic imag-ing of the sea floor [Ballard et al., 2002; Greeneet al., 2000; Shank et al., 2002]. In particular,there is a compelling need to conduct studiesin depths beyond those considered reasonablefor divers (~50 m) down to depths at the shelfedge and continental slope (~1000–2000 m).Some of the constraints associated with suchwork include the requirement to work off ofsmall coastal vessels or fishing boats of oppor-tunity, and the requirement for the vehiclecomponents to be air-shippable to enableinexpensive deployments at far-flung oceano-graphic sites of interest.

Over the last 2 and a half years, the SeabedAutonomous Underwater Vehicle (AUV) hasbeen designed and deployed in support ofsuch tasks off of Puerto Rico, Bermuda,Stellwagen Bank off Massachusetts, and theU.S.Virgin Islands.

Components and Capabilities

In designing the Seabed AUV (see http://www.whoi.edu/DSL/hanu/seabed), a decision wasmade to incorporate standard oceanographic

sensors as far as possible to minimize costs andallow for easy maintenance.It currently supportsa 300-kHz side-scan sonar,a 1200-kHz AcousticDoppler Current Profiler, a Seabird pumped Con-ductivity, Temperature and Depth sensor, a675-kHz mechanically scanned pencil beam sonar,and a 12-bit (high dynamic range) camera sys-tem.A 2-kWhr lithium-ion battery pack provides an 8-hour endurance for Seabed running atspeeds between 0.3 m/s to 1 m/s. Its size—each of its two torpedo-shaped hulls is 1.5 min length—and weight of 250 kg permit easydeployment off small coastal or fishing vesselsof opportunity, and it can be shipped by air ata very reasonable cost to most departure ports.

Optical Imaging and Navigation and Control

Seabed has been designed specifically tofurther the growing interests in the area of seafloor optical imaging; specifically, high-resolu-tion color imaging and the processes of pho-tomosaicking and three-dimensional imagereconstruction. In addition to high-quality sen-sors, this imposes additional constraints onthe ability of the AUV to carry out structuredsurveys, while closely following the sea floor.The distribution of the four thrusters, coupledwith the passive stability inherent in a two-hulledvehicle with a large meta-centric height,allowsthe Seabed AUV to survey close to the seafloor, even in very rugged terrain.

Figure 1 illustrates vehicle performance insuch challenging applications.The vehicle

(top left) can follow steep gradients at constantaltitudes; in this case, on a coral reef off theU.S.Virgin Islands, and follow a greater than45° slope at 3 m (top center) while obtaininghigh-resolution imagery with good colorfidelity.

Figure 2 shows an image obtained from thevehicle using its camera and lighting systemat night in 65 m depth, in the absence ofambient lighting.The nonlinear preferentialattenuation of different parts of the visiblespectrum underwater led to images that arenot color-balanced. However, the 12 bits ofdynamic range provided by the camera allowus to compensate and color balance theimagery as shown in Figure 2 (bottom) byusing methods including frame averaging andparametric surface fitting in the homomorphicdomain.

To accomplish high-resolution, large-areasurveys, the AUV is run under closed loopcontrol [Whitcomb et al., 2000]. Navigation inshallow water (down to a few hundred meters)is accomplished by using a doppler velocitylog in combination with Global PositioningSystem data before and after the dive.Algorithmsfor bottom-following in rugged terrain alsoallow the AUV to carry out autonomous surveysin regions of high relief while running precisetrack lines.The results of such a task are illus-trated in Figure 3. Here, the AUV was imaginga boulder pile with a series of closely spacedtrack lines, while maintaining a constant 3-maltitude off the bottom in an area where therelief was also of that order.

Photomosaicking Underwater Imagery

Photomosaicking underwater is a challeng-ing task [Pizarro et al., 2003].The nonlinearattenuation of color underwater, the absenceof uniform ambient lighting,an unstructured

Seabed AUV Offers New Platformfor High-Resolution Imaging

BY HANUMANT SINGH,ALI CAN, RYAN EUSTICE,STEVE LERNER, NEIL MCPHEE, OSCAR PIZARRO,AND CHRIS ROMAN

PAGES 289, 294–295