Mapping Tropical Cyclone Disturbance of the Great Barrier Reef

University of Wollongong Research Online

Faculty of Science, Medicine and Health - Papers: part A Faculty of Science, Medicine and Health

2004

Mapping tropical cyclone disturbance of the Great Barrier Reef

Marji Puotinen University of Wollongong, [email protected]

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Recommended Citation Puotinen, Marji, "Mapping tropical cyclone disturbance of the Great Barrier Reef" (2004). Faculty of Science, Medicine and Health - Papers: part A. 2025. https://ro.uow.edu.au/smhpapers/2025

Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: [email protected] Mapping tropical cyclone disturbance of the Great Barrier Reef

Abstract Tropical cyclones periodically cross the Great Barrier Reef (GBR). The large waves they generate can damage coral reefs. This paper will describe the challenges of mapping cyclone disturbance across such a vast geographic region, and how cartographic visualisation can help us understand the implications of these challenges on the quality of the results.

Keywords reef, great, cyclone, tropical, barrier, disturbance, mapping

Disciplines Medicine and Health Sciences | Social and Behavioral Sciences

Publication Details Puotinen, M. (2004). Mapping tropical cyclone disturbance of the Great Barrier Reef. The Globe, 55 31-41.

This journal article is available at Research Online: https://ro.uow.edu.au/smhpapers/2025 Mapping Tropical Cyclone Disturbance of the Great Barrier Reef 31

MAPPING TROPICAL CYCLONE DISTURBANCE OF THE GREAT BARRIER REEF Marji Puotinen

Tropical cyclones periodically cross the Great Barrier Reef (GBR). The large waves they generate can damage coral reefs. This paper will describe the challenges of mapping cyclone disturbance across such a vast geographic region, and how cartographic visualisation can help us understand the implications of these challenges on the quality of the results.

ustralia’s Great Barrier Reef (GBR) destinations or fisheries, or as habitat for other forms the largest coral reef system in resident species. In combination with other A the world, including nearly 3000 disturbances, this can cause permanent individual reefs that stretch for more than 2000 changes. For example, in Jamaica, hard corals km along the Queensland coast and cover an now struggle to compete with algae after area of over 348 000 km2 (CRC Reef Research damage from two hurricanes was exacerbated Centre, 2002). In recognition of its outstanding by overfishing (Hughes, 1994). Thus, to natural beauty and bio-diversity, the GBR has provide for “the protection, wise use, been protected within a Marine Park since understanding and enjoyment of the Great 1975 and a World Heritage Area since 1981. Barrier Reef in perpetuity”, the Great Barrier Further, the GBR region supports a billion- Reef Marine Park Authority needs to dollar tourism industry as well as several understand which reefs are likely to be affected economically important fisheries. However, a by cyclones and how often. range of natural and human-caused disturbances threaten the region, including Mapping the risk of cyclone disturbance across tropical cyclones, coral bleaching, crown-of- the GBR over time poses many challenges. thorns starfish and reduced water quality. While the region is vast and cyclones affect large areas at a time, damage surveys and reef Over time, tropical cyclones track throughout vulnerability to impacts take place over short the entire GBR region (Figure 1). The large time periods and across small areas (square waves they generate break along shallow reef metres) – creating a mismatch of scales. areas, resulting in impacts ranging from Further, the positional and attribute uncertainty broken corals to removal of entire sections of inherent in key data sets, such as cyclone paths reef structure. Severe disturbance, where long- and reef polygons, is unquantifiable but lived, large coral colonies are destroyed or estimated to be significant. major damage is widespread (Figure 2), can require centuries or longer for reefs to fully Given these difficulties, is it really necessary recover. Over time, repeated widespread to map cyclone disturbance across the entire impacts have the potential to significantly alter GBR? While local-scale studies of cyclone coral reef community structure, which may disturbance of particular reefs or groups of reduce the value of particular reefs as tourist reefs are valuable, it is not possible to build a regional picture from these studies alone. Such Marji Puotinen, Lecturer in GIS, School of Tropical studies presently cover only a limited extent of Environmental Studies and Geography, James Cook the GBR and a short time span of the cyclone University, Townsville, QLD, 4811. [email protected] history (Table 1).

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8 A 6

2 # of Cyclones 0 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 Year (19xx)

1000 km

Figure 1. Number (A) and paths (B) of tropical cyclones that tracked near Queensland, Australia from 1910 to 1999. Tracks were generated from the tropical cyclone database (Australian Bureau of Meteorology 2000).

Figure 2. Dislodgment, breakage, and burial of massive coral heads during Cyclone Ivor, 1990. The white bar equals ~1 metre. Photograph by Dr. Terry Done (from Puotinen et al. 1997).

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Year Cyclone # Sites # Reefs Region Source 1971 Althea 27 10 Swains Hartcher 2001 Cairns - 1990 Ivor 63 33 Cooktown Done 1992 Cairns - 1990 Joy 186 33 Cooktown Ayling 1991 1971- 1992 Various 6 1 Heron Island Connell et al 1997

1996 Celeste 7 6 Whitsundays DPI 1996 Townsville, 1997 Justin 54 13 Whitsundays Puotinen 1997 Swains - 1997 Justin 55 35 Cooktown Puotinen 1997

TOTAL 398 131

Table 1. Cyclone damage observations available for the Great Barrier Reef.

Latitude (S) Latitude 20

24 0102030405060

Maximum Distance from Reefs to Nearest Cyclone (km)

Figure 3. Maximum distance of reefs (measured from the geographic centre) to the nearest cyclone path in the GBR by latitude, 1969-1997.

Moreover, although cyclones track through the cyclone’s path, every reef in the GBR has been far north (between 10-110 S) of the GBR less located within striking distance of at least one frequently than elsewhere, there are no cyclone over the past 30 years. Further, sections of the GBR that are completely free of researchers have found that the coral larvae cyclones (Puotinen et al 1997) and that could produced on one reef can be transported to be eliminated from consideration. In fact, the other reefs by ocean currents, though the maximum distance of most reefs in the region spatial extent of this connectivity has yet to be to the nearest cyclone from 1969-1997 was established. This means that the ability of coral within 30 km (Figure 3). species to recolonise a particular reef after a major cyclone disturbance could depend on the Given that studies indicate cyclone damage to extent and severity of disturbance sustained by reefs is possible up to 100 to 200 km (Done nearby reefs throughout the GBR. 1992, Connell et al 1997) away from a

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Issues Explanation Cyclone Eye Estimated positional error +20-200 km. Positions Largely unreliable before 1969. Cyclone movement between eye Cyclone positions unknown and presumed Paths linear. Cyclone Estimated intensity error +30 hPa. Intensity Largely unreliable before 1969. Often difficult to measure, often not Cyclone Eye recorded, very important to cyclone Width energy models. Mesoscale in nature (10s of km) - don't Cyclone incorporate local scale effects such as Models rain bands Very limited number available. Biased Reef damage towards damaged sites. Measured at a observations very local scale (meters). Very little data exists. Highly variable Reef over short time periods and small vulnerability distances.

Table 2. Challenges associated with mapping cyclone disturbance of reefs.

MAPPING CYCLONE DISTURBANCE Issues and Challenges The ability to map cyclone disturbance is Very few direct observations of cyclone wind limited by a general lack of key data sets, as and wave speeds exist in Australia, as well as high levels of uncertainty in the data instruments are expensive and tend to fail that is available (Table 2). during the extreme conditions typically experienced. Also, because cyclone paths are Eye Positions virtually impossible to predict, it is difficult to Obviously it is vital to know where a particular know when and where to deploy instruments to cyclone tracked in order to map the measure cyclone energy. However, basic disturbance it caused. Cyclones are tracked cyclone characteristics (the location of the using a combination of satellite and radar cyclone along its path, its intensity and its speed imagery and land and ship-based observations. and direction of forward motion) are typically The timing of the observation varies depending recorded by the Australian Bureau of on the proximity of the cyclone to land-based Meteorology in their tropical cyclone database. communities. The quality of the observation Meteorological models can be used to depends on the combination of methods used. reconstruct the distribution of cyclone energy Radar produces the most accurate from these basic characteristics (Holland 1980). observations, though many cyclones track The degree to which a particular reef is affected outside radar range. by a cyclone, however, depends just as much on that reef’s vulnerability to impact as it does on While cyclone eye positions have been the intensity of the cyclone. Thus, a recorded in Australia since about 1910, the combination of hindcast cyclone energy and reliability of these positions is largely reef vulnerability measures was used with actual unacceptable before the advent of widely field observations of cyclone damage to build a available satellite imagery in 1969 (Holland predictive model of the spatial distribution of 1981). Researchers at the Australian Bureau of cyclone disturbance across the GBR. This Meteorology Research Centre estimate that model was then used to map cyclone errors in initial cyclone positions can exceed disturbance for each cyclone passing near the 400 km in any direction (Figure 4A), though GBR from 1969 to 2003. most are within 100 km (Woodcock 1995).

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(A) 35 Errors exceeding 400km 664 30 year month day error 610 cases out of 2276 Pancho 1983 11 30 430

25 Quenton 1986 01 21 430 Sharon 1994 03 17 420 20

332 15

234

Percentage 10

126 113 5 62 41 31 20 10 10 5 3 0001 2 1 1 3 0 10 50 90 130 170 210 250 290 330 370 410 30 70 110 150 190 230 270 310 350 390 430 Great circle error (km) (B) Distribution of initial central pressure errors

45 907 cases 40 Overestimate Underestimate 35

25 419 20

15 279 222 10 124 112

Percentage cases of 5 53 64 22 1 3 12 21 8 2 2 0 -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40

Error range (hPa)

Figure 4. Estimated errors in initial measurements of tropical cyclone: A) eye positions and B) intensity (adapted from Woodcock 1995) based on subsequent data.

While this does not reveal the level of error and is typically estimated using satellite remaining in the revised cyclone tracks, it imagery (Dvorak 1975). Initial central pressure suggests a high level of positional uncertainty. measurements have also been found to be in error (Figure 4B), suggesting considerable Intensity uncertainty in cyclone intensity estimates. The magnitude and extent of the winds and waves generated by a particular cyclone Eye Width depends largely on its intensity. The central air Every cyclone contains a relatively calm region at pressure, in hPa, of a cyclone is commonly the centre of the storm circulation called the eye. used as a measure of intensity. As central Wind speeds are near zero within the eye, while pressure falls, the difference in air pressure they are at their maximum around the boundary, between the cyclone and the external called the eye wall. Defining the diameter of the environment increases, powering the storm. eye is thus important to accurately reconstructing Central pressure is rarely measured directly the cyclone’s energy. However, cyclone eyes can

Journal of The Australian Map Circle Inc. 36 The Globe, Number 55, 2004 be very dynamic over short time periods, % Surveyed % GBR Sites sometimes with a double eye structure Year Cyclone Sites Potentially (Willoughby 1990). In addition, the presence of Damaged Damaged upper atmospheric cloud can make it difficult to 1990 Ivor 75 74 measure eye width from satellite imagery 1990 Joy 63 12 (Figure 5). Subsequently, eye width is not 1996 Celeste 63 8 Justin - field always recorded in the tropical cyclone database. 1997 56 7 survey Justin - 1997 60 7 questionnaire

Table 3. Percentage of field survey observations found to be damaged versus the number of reefs potentially damaged by cyclone waves. A B Reef Vulnerability

Figure 5. Satellite images of tropical cyclone eyes. As previously mentioned, the likelihood of Though the cyclones are of similar intensity, in B cyclone damage to a reef depends just as much the eye is clearly visible, while in A it is obscured on the vulnerability of that reef to impact as it by upper atmospheric clouds. does on the intensity of cyclone energy. Vulnerability is controlled by a range of highly Model Scale variable factors, such as the history of Another problem is that the meteorological disturbance at a site and the predominant size models used to estimate cyclone energy are and types of corals growing there. This designed to operate across broad spatial scales information is poorly known for much of the with a resolution of 1-10 km. The processes that GBR, most of which has never been directly control the finer scale dynamics of cyclones have surveyed (Figure 6). Vulnerability also depends not yet been adequately modelled. on the relative exposure of sites to waves during a cyclone versus ambient conditions, which can Damage Observations be modelled based on the relative positions of While basic data about the cyclones that have reefs and other wave blocking obstacles such as tracked through the GBR during the last three the coastline and islands. Although the decades is available, observations of reef boundaries of the reefs have been mapped as damage from these cyclones are very limited in vector polygons, this was done using satellite both spatial and temporal distribution (Table 1). imagery and was verified by field surveys for Broad scale field surveys were conducted only only a small fraction of the reefs. Further, the for cyclones Ivor and Joy in 1990 and Justin in definition of reef that is used, as well as the 1997. Some data can be gleaned from crown-of- scale and timing of the observations, can make a thorns starfish survey records (Althea in 1971) big difference to the resultant reef polygons and from incidental reports (Celeste in 1996). (Spalding and Grenfell, 1997). The only long term data set available is a study of reefs near Heron Island over three decades In summary, much of the data needed to map (Connell et al 1997). Field surveys in the marine cyclone disturbance of the GBR is missing. The environment are expensive and time data that is available contains unknown, but consuming. It is simply not feasible to visit potentially large, uncertainties in both position every reef that could have been affected by a and attribute. Further, there is a mismatch of cyclone across 100s of square kilometres. Thus, spatial scales between the cyclone data from 1 researchers typically target surveys in areas km to 10 or more and the reef vulnerability data where cyclone damage seems most likely in square metres. Finally, the study area is vast (Table 3). However, this creates a bias in the but must be modelled at a high resolution (500 data, which reduces the potential strength of m pixel) in order to adequately depict the basic predictive models that are developed. outlines of most of the reefs.

Journal of The Australian Map Circle Inc. Mapping Tropical Cyclone Disturbance of the Great Barrier Reef 37

Number of Reefs per 10 of Latitude

110S 7 (0) 120S 9 (2) 130S 11 (4) 140S 30 (17) 150S 26 (6) 160S 31 (18) 170S 15 (4) 180S 15 (12) 190S 30 (14) 200S 12 (7) 210S 19 (5) 220S 7 (3) 230S 6 (4)

Figure 6: Location of reefs where at least one (black squares) or at least three (grey squares) surveys were conducted from 1992 to 1999 by the Australian Institute of Marine Science. The grey polygon outlines the GBR Marine Park. The number of reefs that were monitored for each 10 of latitude is listed (number of reefs surveyed at least three times are in brackets).

CREATIVE SOLUTIONS To visualise this potential error, I constructed uncertainty circles around each eye position for Although the major uncertainties inherent in this cyclones Ivor and Justin, with the radius of project are unavoidable, the work is still each circle equal to the likely maximum worthwhile as long as some estimate of the positional error (Figure 7). Thus, each cyclone confidence of the results, and thus how they eye position could be located anywhere within should be appropriately used, can be made. For its uncertainty circle. Interestingly, eye example, cartographic visualisation can be used positions for cyclone Ivor were more uncertain to assess the implications of large uncertainties in when the cyclone was closer to land. This the position of the cyclone eye to the modelling occurred because it was relatively weak at that based on those positions. The cyclone database stage, and thus was tracked less diligently. In records the methods used to estimate each contrast, cyclone Justin’s eye positions were cyclone eye position. They involve direct, radar most uncertain when it was located far out to and satellite observations, or a combination of sea. This occurred because it was out of radar these. As previously mentioned, Holland (1981) range and posed no immediate threat to the and Woodcock (1995) have estimated the level Queensland coast at that time. of potential uncertainty associated with using each of these methods. Although this is not an actual measure of error, it does provide a ‘worst case scenario’ of the maximum error likely in each eye position.

Journal of The Australian Map Circle Inc. 38 The Globe, Number 55, 2004

200 km A

200 km B

Figure 7. Uncertainty circles (grey) for each recorded eye position (black dots) of cyclones. A) Ivor (1990) and B) Justin (1997).

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Cyclone Eye Position Reef

Cyclone Track Uncertainty Circle

Figure 8. Uncertainty zones for the tracks of cyclones: A) Ivor (1990) and B) Justin (1997). Shading indicates the number of times each position was located within an uncertainty circle during the cyclone.

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((Blank page for back of single-sided colour plate, not printed in hard copy))

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To get an idea of how this uncertainty varied REFERENCES throughout each entire cyclone, I counted the number of times each position across the GBR Ayling, T (1991). Unpublished data. was located inside an uncertainty circle, and Connell, JH, Hughes, TP and Wallace, CC (1997). thus could have a cyclone eye passing directly ‘A 30-year study of coral abundance, through it (Figure 8). Where the colours are recruitment and disturbance at several scales in more pink than red, the uncertainty of the space and time’. Ecological Monographs, 67(4): 461-488. cyclone eye positions is low, such as when CRC Reef Research Centre (2002). The Great cyclone Justin tracked southward down the Barrier Reef Fact Sheet. Queensland coast. Where the colours are more http://www.reef.crc.org.au red than pink, the uncertainty is high, such as Done, TJ (1992). ‘Effects of tropical cyclone where cyclone Ivor tracked down the coast or waves on ecological and geomorphological where cyclone Justin was located far out to structures on the Great Barrier Reef’. sea. Reefs of interest, such as those for which Continental Shelf Research 12, 859-872. damage data is available, can be overlaid with Department of Primary Industries (1996). the uncertainty zones to assess the confidence Unpublished data. with which the relevant cyclone energy Dvorak, V F (1975). ‘Tropical cyclone intensity measures should be used. For example, analysis and forecasting from satellite imagery’. Monthly Weather Review, 103, 420- uncertainty in the location of cyclone Justin’s 430. eye was highest while the cyclone was located Hartcher, M (2001). Unpublished data. in the middle of the Coral Sea (Figure 8), Holland, GJ (1981). ‘On the quality of the which is when most of the reef sites surveyed Australian tropical cyclone database’. Austl. after the cyclone were probably damaged. This Met. Mag. 29, 169-181. suggests that reconstructed cyclone conditions Holland, G J (1980). ‘An Analytic Model of the for this period are likely to be in error and that Wind and Pressure Profiles in Hurricanes’. the predictive model for cyclone Justin is Monthly Weather Review 108 (August), 1212- likely to be poor. In addition, animated movies 1218. of this uncertainty could be used to show how Hughes, TP (1994). ‘Catastrophes, phase shifts, and large scale degradation of a Caribbean it changes over time. coral reef’. Science 265: 1547-1551. Puotinen, ML, Done, TJ and Skelly, WC, (1997): ‘An atlas of tropical cyclones in the Great CONCLUSION Barrier Reef Region, 1969-1997’. CRC Reef Research Centre, Technical Report No. 19. Mapping cyclone disturbance of the GBR is Townsville; CRC Reef Research Centre, 203 pp. difficult due to a general lack of necessary data Puotinen, M (1997). Unpublished data. and potentially large uncertainties in both the Spalding, MD and Grenfell, AM (1997). ‘New location and attributes of the data that is estimates of global and regional coral reef available. However, it is possible to build a areas’. Coral Reefs, 16:225-230. predictive model of cyclone disturbance by Willoughby, H E, (1990), ‘Temporal changes of the primary circulation in tropical cyclones’. linking field observations of reef damage from Journal of the Atmospheric Sciences 47(15 cyclones to hindcast cyclone energy and reef January), 242-264. vulnerability measures. Creative use of Woodcock, F (1995). Australian Bureau of cartographic visualisation, as demonstrated in Meteorology Research Centre. Unpublished the previous section, is essential for estimating data. the implications of data uncertainty on the model results and the confidence with which they should be used.

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Journal of The Australian Map Circle Inc.