The annual cycle of sea surface temperature in the Great Australian Bight

Prog. Oceanog.Vol.39, pp. 1-27, 1997 Pergamon

PII: S0079-6611 (97)00010-4

© 1997 ElsevierScienceLtd Printed in Great Britain. All rightsreserved 0079-6611/97 $32.00

The annual cycle of sea surface temperature in the Great Australian Bight MICHAEL HERZFELD

Centre for Water Research, University of W.A., Nedlands, Perth, WA 6907, Australia Abstract - Analysis of a time series of southern Australian sea surface temperature satellite images for the period March 1990 to September 1991 revealed the existence of a warm water mass in the north western Great Australian Bight (GAB). This water mass developed in the summer months, had a temperature 2-3°C above that of surrounding waters and spread in a southeastward direction to about 137 ° to 138°E during late summer and early autumn. Significantly colder water was consistently observed in the eastern GAB during autumn. During late autumn and winter, the Leeuwin Current was seen to intrude into the GAB region and interact with the GAB water, producing a continuous band of warm water stretching across the southern Australian coast. The development of the warm GAB water was observed to be independent of any influence of the Leeuwin Current, and it is proposed that the source of the warm GAB water is from processes local to the region. A calculated heat budget applicable to the northwestern GAB coastal region indicated that a positive net heat flux exists during spring, summer and early autumn. Large air-sea temperature differences (up to 20°C) during periods of offshore wind create a stable atmospheric boundary layer which inhibits sensible and latent heat losses. The net heat flux is almost solely attributable to the large incoming short wave radiation component under these conditions, which may be significant in maintaining the observed positive net heat flux. The region of greatest warming in the northwestern GAB is associated with a large expanse of shallow water ( < 30 m), and a larger temperature change in the water column over this region than over surrounding deeper regions results when a uniform positive heat flux is applied over the GAB. This is the proposed mechanism responsible for the production of the observed warm GAB water. © 1997 Elsevier Science Ltd. All rights reserved.

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CONTENTS 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Introduction Satellite image sources Spring Summer Autumn Winter Diurnal warming Comparison with previous studies Heat budget analysis Sea surface temperatures Hot events Differential beating Conclusions Acknowledgements References

2 5 6 6 10 12 14 16 18 19 20 22 24 26 26

1. INTRODUCTION

The Great Australian Bight (GAB) consists of the southern stretch of Australian coastline from the Recherche Archipelago at 124°E to the South Australian gulfs at 136°E. The continental shelf throughout the region is wide, reaching over 200 km at 131 ° 12'E, its broadest point. The bottom topography and geography of the GAB region is displayed in Fig. 1. The GAB forms the southern boundary of the Nullabor Plain, which is itself the southern limit of the Great Victoria Desert in Western and South Australia. Huge limestone cliffs dominate the coastline 31

II

•I

Nullab°rPlaln.HeadoftheBight

Esperance

34 [U ~

]

J ,----,,--/

/

.................. ~"-.v-1-'~..

.:'J'~-": .........

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'-\ X

35

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South Indian Ocean

38-'

120

i

122

124

126

128

130

132

Longitude East Fig. 1. Geography and bathymetry of the Great Australian Bight

134

136

The annual cycle of sea surface temperature in the Great Australian Bight

3

within the GAB, some reaching up to 100 m in height. The coastal regions bordering the GAB are inhospitable and isolated, and consequently the population along this stretch of Australian coastline is extremely sparse. Access to the waters within the GAB is difficult, as the majority of the coastline has no beach access the nearest coastal locations with a harbour being Ceduna or Esperance. The acquisition of oceanographic data throughout a substantial area of the GAB remains relatively scarce. The GAB attracted the interest of oceanographers through observations of the warm, low salinity Leeuwin Current. This current flows along the western coast of Australia from the North West cape in the form of a narrow (~30 km width) poleward flow on the shelf edge, turns along the southern Australian coast at Cape Leeuwin and enters the GAB region in the winter months. Maximum current speeds exceed 1 ms -I and the total distance of the flow is approximately 2000 km. The I.eeuwin current is characterised by high temperature, low salinity water, and has been the subject of numerous studies (GODFREY and RaDGWAV, 1985; CRESSWELL and GOLDING, 1980; THOMPSON, 1984; MCCREARY et al., 1986). The study of the propagation of surface elevation signals and coastally trapped waves around the Australian coast has also led to the investigation of the GAB region (PRovis and RADOK, 1979). Recently, the GAB has become a popular location to observe the congregation of Southern Right whales, which frequent the waters near the Head of the Bight in winter. For reasons unknown, these mammals choose this location as a calving ground. It has been proposed that the coastal waters surrounding the Head of the Bight be preserved as a marine conservation area. However, few studies of the GAB as a separate entity, rather than a participant in some other process, have been undertaken, and the oceanography of the region is still relatively unknown. ROCnFORD (1962) first directed attention to the water properties within the GAB. Analysis of surface and 50 m depth cruise data revealed that a salinity maximum of greater than 36.3 existed in the GAB. Surface inorganic phosphate levels measured in the area were the lowest (0.02-0.06 mg-atom/1) measured in the Indian and South Indian oceans. A salinity maximum (greater than 35.9) and an inorganic phosphate minimum (0.04 mg-atom/1) was also found in the GAB at a depth of 50 m. ROCrIFORD (1962) concluded that there must exist little mixing of the water in the GAB with surrounding water masses for long periods of time for the high salinity and low phosphate levels to be maintained. Using an evaporation rate of at least 50 inches (1.27 m) per year, ROCnFORD (1962) suggested a residence time for the GAB waters of 1 to 2 years. ROCHFOgD (1962) alSO proposed that the high salinity GAB water probably had an origin within the Bight itself. The possibility of GAB water having connections to the Leeuwin Current was explored by LEGECKIS and CRESSWELL (1981). A satellite image from 14 July 1979, showed a continuous band of warm surface water extending around Cape Leeuwin and across the GAB on the shelf and slope, from about 122 to 136°E. LECECKISand CRESSWELL (1981) concluded that this water was an eastward extension of the Leeuwin Current, "predominantly on the shelf". The warmest water in this band was bounded by the 15°C temperature contour. The salinity at Albany (35°08'S, 118°02'E) at 50 m depth was 35.5 on 19 July, indicating that the Leeuwin Current was the dominant water mass at that location. In October 1979, LEGECKIS and CRESSWELL (1981) also observed the existence of a band of eastward flowing warm water occupying the entire shelf region and extending to about 124°E. Salinity at Albany on 12 October was 35.63, and the warm flow was again presumed to be Leeuwin Current water. ROCHFORD (1986) offers a comprehensive analysis of the GAB waters, and possible connections of water within this region to that found in the Leeuwin Current. From analysis of data

4

M. HERZFELD

obtained over seven cruises spanning the period 1961 to 1982, and one satellite SST image, ROCHFOI~ (1986) found that three water masses coexist in the shelf and slope regions of the GAB throughout, or for only part, of the year. The Leeuwin Current was found to advect warm, low salinity water along the shelf break into the GAB region. This water was found to enter the western shelf region of the GAB around May, reach its furthest eastern limit of approximately 130°E in July and remain in the western region until September-October. ROCHFORD'S (1986) analysis revealed that a second mass of warm water existed in the central GAB for most of the year. This water could be distinguished from the Leeuwin Current water by its very high salinity (greater than 36.0). A third water mass with low temperature and salinity was found across the southern region of the GAB. This water is attributed to a West Wind Drift cold water mass and is not of concern in this study. The very high salinity GAB water was found to form a tongue originating within the central shelf region of the GAB, extending in a southeastward direction. This tongue, also characterised by its high temperature, occupied roughly the same position on the shelf from March until June. Surface salinities in the tongue were greater than 36.6 in May 1980. ROCHFORD(1986) suggested the tongue comprised the core of a thermohaline current, originating in the central GAB in the summer months and whose waters drift southeastward on the shelf and slope, possibly beyond southwestern South Australia. ROCHFORD'S (1986) analysis revealed that in June 1971, the warm, low salinity (35.7 to 35.8) l_eeuwin Current water followed the shelfbreak to about 130°E. Further eastward, the warmest GAB water was found on the shelf region, had high salinity (36.4) and was I°C warmer than the Leeuwin Current water at 130°E. The warmest water in the region around 125°E in May 1980 was also found to have very high salinities, i.e., at certain times of the year ROCHFORD (1986) found the warmest water in the GAB region had the high salinities associated with water of GAB origin. ROCHFOgD (1986) performed a brief analysis of the July 1979 SST satellite image presented by LEGECKISand CRESSWELL (1981) and discovered that part of the warm surface water in the central Bight region was associated with high salinity water. This water again formed a tongue extending southeastward and also contained a SST maximum. Analysis of satellite SST images in relation to the mass pilchard mortality along southern Australia during autumn 1995 by GRIFFIN et al. (1997) reveal that intense local upwellings off the west coast of Eyre Peninsula during February and March are common. It was noted that the presence of strong upwelling favourable winds within this region (i.e. southeasterly) did not always lead to strong upwelling events and upwelling was sometimes observed in the absence of upwelling favourable winds (G~FFIN, personal communication). The circulation within the GAB was observed by MARSHALLSAYand RADOK (1972) via the analysis of drift card releases, which indicated that a westward flow is present in the coastal eastern Bight. Cards released in December 1969 at 35°S, 132°E were found at points along the western coast of Eyre Peninsula, and within the GAB west of 132°E. MARSHALLSAYand RADOK (1972) suggest that the cards released at this location were carded inshore by an "anti-clockwise circulation within the Bight" (p 8), which flows up the west coast of Eyre Peninsula and then westwards across the GAB (e.g. Fig. 9, MARSHALLSAYand RADOK, 1972). GODFREY et al. (1986) found that buoy tracks, ship's drift vectors, and SST along the south coast of Australia in June to July 1982 indicated that a narrow shelf edge current flows eastward along the entire region. Progressively larger salinities were found on the shelf from 124 to 135°E, but not found beyond the shelf west of 135°E. GODFREY et al. (1986) also observed a near zero longshore steric sea level east of 118°E, but inshore levels were consistently higher

The annual cycle of sea surface temperature in the Great Australian Bight

5

than offshore levels, "implying a continuous eastward current everywhere along the southern Australian shelf edge." He suggested the observed longshelf currents in the region were a result of prevailing west winds, or a thermohaline current resulting from different cooling rates found on and off the shelf, or a combination of both. The results of these previous studies seem to agree that there exists a band of water off the southern coast of Australia, including the GAB, which is characterised by water with higher temperature than surrounding waters, and which flows in an eastward direction. The satellite image of LEGECKISand CRESSWELL (1981) showed that this water can extend across the entire GAB. RocrivoaD (1986) found that a portion of this water was associated with salinities much higher than those found in the Leeuwin Current water and constitutes a separate water mass. The fact that several different water masses have been identified in the GAB, and uncertainty exists as to whether warm water found in the region can be attributed to water originating from the Leeuwin Current or warming produced by processes occurring within the GAB itself, has prompted the need to undertake a more detailed analysis of the region. This project aims to gain insight into the processes occurring within the GAB by analysing the annual cycle of SST in the GAB region on a monthly basis, making use of time series of satellite images of the region obtained from several different sources, and a heat budget calculated from meteorological data obtained from the Bureau of Meteorology and acquired in the field.

2. SATELLITE IMAGE SOURCES For the period 2/3/90 to 10/8/91, 91 images were obtained from the CSIRO (Commonwealth Scientific and Industrial Research Organisation) Division of Atmospheric Research, Aspendale reception facility (courtesy of Dr P. Petrusevics, Dept of Fisheries, S.A.), giving about one and a half years' worth of data. These images are supplied calibrated, corrected for atmospheric attenuation, and remapped to a standard map projection. The geographical extent of these images varies. From 3/90 to 28/5/90 the extent represented in the images is 128 to 133°E, after which this changes to 128 to 131°E. As from 12/1/91 the extent changes yet again from 124 to 131°E. Eighteen images have an extent of 115 to 131°E. Some of the images with small longitudinal extent cannot be representative of processes occurring throughout the GAB, and are only of limited use. However, the satellite archives at CSIRO Division of Physical Oceanography, Hobart remote sensing facility, became available for use (courtesy of Dr G. Cresswell), and the Aspendale images made a good base from which to "fill in the gaps" in the data series, both spatially and temporally. The CSIRO images are supplied calibrated, corrected for atmospheric attenuation via the split window technique of MCMILLIN and CROSBV (1984), and remapped to a Lambert conformal map base. The spatial extent of all images is 115 to 139°E, and since all images are registered to the same map base, direct comparison between all images is possible. Images recorded at night have been acquired as often as possible. These images are better suited for analysis purposes, as they do not contain the SST signatures which often result from large heating during the day. The CSIRO images, supplemented by the Aspendale images, provide an excellent datum from which to analyse the seasonal variation of SST in the GAB. The Flinders Institute for Atmospheric and Marine Science (HAMS) initiated the development of their own unique reception facility during 1985. The system became operational around 1988, and NOAA images were received and processed on a regular basis. Images of the GAB region were acquired during 1993 and 1994 from the FIAMS system and were used to sup-

6

M. HERZFELD

plement the CSIRO derived image time series, and confirm the trends observed in the images obtained from these other sources. The ground coverage and frequency of good image reception of these images varied. Although interpretation of the images is greatly hindered by often infrequent acquisition of the images, and often large areas of obscuring cloud in the acquired images, analysis of the satellite images from all these sources revealed the emergence of a basic pattern for the behaviour of the waters within the GAB. Acquisition frequency varies from month-to-month, generally the summer months yield more images than the winter months; a result of less cloud cover in the summer. Acquisition of the Aspendale and Hobart CSIRO images for 1990 and 1991 is summarised in Table 1. Images representing the area of interest which were of sufficient (cloud free) quality to be used for qualitative analysis derived from the FIAMS system for 1993 and 1994 are also included in Table 1.

3. SPRING In the late spring and summer months (October to February) strong heating is evident in the shelf regions, especially where the water is shallow, creating strong SST fronts in some cases. The areas of warm water generally extend all the way to the coast. The onset of this general shelf heating appears to be sometime in early October. For example, note the difference in temperature between water on the shelf in Plate 1., 28 Sep 90 and Plate 2., 22 Oct 90. Some time during this period the onset of heating of the shelf waters must have occurred in order to raise the temperature on the shelf to that observed in the October image. The warmest water seems to accumulate along the sections of the shelf associated with very shallow water (less than 30m depth), the most notable being that stretching from about 126 to 129°E (e.g., Plate 3., 19 Nov 90). In November, some images show evidence of a warm subtropical Leeuwin Current, described by CRESSWELL and PETERSON (1993) on the southwest shelf (Plate 4., 25 Nov 90). A discontinuity (meaning colder water exists between the warm water in the GAB and warm water further west; i.e. a break in the spatial continuity of warm water) exists between the warmest water of the section 126 to 129°E within the GAB and this warm water flowing around Cape Leeuwin.

4. SUMMER The heating of the shallow shelf water (especially from 126 to 129°E) in the summer months creates regions of warm water lying immediately adjacent to the coast. These warmer regions, or warm strips, are characterised by considerably higher SST than is found elsewhere around the coast or further offshore. Often warm plumes of water originating from these strips stretch out eastward. These plumes occasionally become cut off from the coast to create warm pools, as is observed in Plate 5 , 8 Feb 91, near 131°E and 133°E. Cold water also occurs in the eastern GAB, possibly the result of upwelling as observed by GRIF~N et al. (1997). The warm strip occupying the coastal waters from 126 to 129°E, and its associated plume, becomes a permanent feature in the images from late January. The temperature of the warmest water in the strip increased from ~21°C at the beginning of January 1991 to ~23°C at the end of January. The warmest water in the strip was 2 to 3°C warmer than water found on the shelfbreak.

The annual cycle of sea surface temperature in the Great Australian Bight

7

Table 1. Acquisition of Satellite Images from all Sources Month Day

Jan

Feb

Mar

Apr

I 2 3 4 5 6 7 8 9

hA A

hHA hA

A

HA

A AA hAA hhHA AA

h HA HhAA

A

F

F

10

II 12 13 14

Ha

May

Jun

hA a A

A A

H A

A

A

hAAF

hH A HA A

A

A

ha

Sep

HA A

HhaA hA

a

F F

A

A

HA A

hA

A

F F

F AA

aA HhAA H Ha

A

A

A

HA A

A

HA H

h HhAF ha

HA

A

hA

H hHA hHA hHA

F A hA

A

H hA

A h

H

hA HA h

H

ha ha A h

HA A

HA h

h

F

Dec

ha

A

18

HA

Nov

Ha

16 17 19 20 21 22 23 24 25 26 27 28 29 30 31

Oct

A

hH

15

h

Aug

A hA F

H

Jul

A h

h

H hHA HAF

H

A

F

hA HA

hAA

F

A: Aspendale CSIRO daytime acquired image, 1990 a: Aspendale CSIRO nighttime acquired image, 1990 A: Aspendale CSIRO daytime acquired image, 1991 _a: • Aspendale CSIRO nighttime acquired image, 1991 H: Hobart CSIRO daytime acquired image, 1990 h: Hobart CSIRO nighttime acquired image, 1990 H: Hobart CSIRO daytime acquired image, 1991 _h: Hobart CSIRO nighttime acquired image, 1991 F: FIAMS acquired image, 1993 F: FIAMS acquired image, 1994 ~,, a, A, a: Bold Aspendale CSIRO images have extent 115 to 131bE

M a n y i m a g e s e x i s t w h i c h s h o w a definite d i s c o n t i n u i t y b e t w e e n the w a r m s t r i p / p l u m e a n d w a t e r f u r t h e r west, s u g g e s t i n g that the a d v e c t i o n o f w a t e r f r o m c u r r e n t s y s t e m s s u c h as the L e e u w i n C u r r e n t c a n n o t be the s o u r c e r e s p o n s i b l e for the w a r m w a t e r f o u n d in the strip (see Plate 6., 29 J a n 91 ). D u r i n g the s e q u e n c e 29 J a n u a r y 91 to 2 F e b r u a r y 91, i m a g e s w e r e a c q u i r e d

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M. HERZFELD

Plate 1

Plate 2

Plate 3

Plate 4 Plate 1. Satellite SST image; 28 September 1990. Plate 2. Satellite SST image; 22 October 1990. Plate 3. Satellite SST image; 19 November 1990. Plate 4. Satellite SST image; 25 November 1990.

The annual cycle of sea surface temperature in the Great Australian Bight

Plate 5

iii ¸¸ i ¸¸

iii!!ii ~i'i~!i~i~,~

~i~iii~!i~~!ii!

Plate 6

Plate 7

Plate 8 Plate 5. Satellite SST image; 29 January 1991 (day). Plate 6. Satellite SST image; 8 February 1991. Plate 7. Satellite SST image; 16 February 1991. Plate 8. Satellite SST image; 2 to 4 April 1991.

9

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twice daily, and a definite discontinuity at about 125°E was observed throughout the entire sequence. During this time period the warm coastal strip underwent change, specifically the intensification of the strip temperature. Therefore, during this time period, all changes of the strip must have been a result of some mechanism acting locally within the GAB, rather than the advection of warm water into the region by the Leeuwin Current. The images suggest there is no connection between the warmest core of the plume associated with the 126 to 129°E strip, and water to the west of the strip. The fact that the development of the warm coastal strips and plumes begins sometime in January, when the Leeuwin Current is supposed to be weakest, supports that the warm water found in the GAB is created locally and is independent of the Leeuwin Current. During February the plumes originating from the 126 to 129°E strip seem to grow in spatial extent and intensity, as does the cold water in the eastem GAB (Plate 7., 16 Feb 91). The coldest water on the entire southern shelf (-16°C) is found in the eastern region of the GAB at this time, and is -4.5°C colder than that observed in the western GAB. In February 1991 the warmest water in the strip was in excess of 23.5°C and was still 2 to 3°C warmer than water found at the shelfbreak. Temperature increased by 0.5 to I°C in the first half of February 1991 and then stabilised. The warmest water in the strip was still centered on the shallow water at 126 to 129°E in February, and was still isolated from water further to the west. The eastern extent of the plume was about 133°E at this stage. Although the core of the plume was still centered on the region 126 to 129°E, warm water associated with the plume could be found to the west around 124°E.

5. AUTUMN In March the first signs of cooling in the plume became evident. In the beginning of March 1990, the temperature of the warmest section of the strip at 126 to 129°E was approximately 22.5°C, and by the end of March was 22°C. For 1991, the warmest strip water at the beginning of March had a temperature of -22°C and at the end was -21.5°C. The temperature difference between water at the shelfbreak and water in the warm strip was -2.5 to 3°C. By the end of March, the warm water associated with the plume had spread further eastward to a maximum extent of about 134°E, and the core of the plume was still located in the coastal strip but occupied the region 124 to 129°E. This situation continued into April, where there appears to be no further coastal heating to create the warm coastal strips. The warm plume was still in evidence, having a core originating from where the warm coastal strip would be in the summer, and covering the area 124 to 129°E. The plume extended eastward to about 135°E, and the plume water remained isolated from water to the west, which is colder; e.g., Plate 8., 2 to 4 Apr 91. Plate 8 shows the first signs of the warm Leeuwin Current on the southwest shelf. Several images are available at this time (e.g., Plate 9., 29 Apr 90) which show the warm Leeuwin Current water advected along the southwest slope to about 122°E. The core of the plume and the water associated with the plume in the GAB were still completely isolated from the warm water advected onto the southwestern shelf by the Leeuwin Current. The core of the warm strip had cooled by I°C during April 91 (from 21.5 to 20.5°C) and April 90 (from 22 to 21°C). In late summer and early autumn, cold water was still consistently observed near the coast at Ceduna in the eastern GAB. There appears to be little thermal activity in the GAB region in May, apart from general

The annual cycle of sea surface temperature in the Great Australian Bight

Plate 9

Plate 10

Plate 11

Plate 12 Plate9. Satellite SST image; 9 April 1990. Plate 10. Satellite SST image; 31 May 1991. Plate 1 I. Satellite SST image; 20 June 1991. Plate 12. Satellite SST image; 13 to 15 July 1990.

II

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M. HERZFELD

cooling of the plume with time. There is no evidence of heating in the shallow waters. The plume now stretched almost to 136°E, and it has grown to this extent via processes occurring within the GAB, i.e. with no linkage to the Leeuwin Current. The Leeuwin Current continued to flow along the shelf edge in May. Some time in mid May, the Leeuwin Current entered the GAB region and interacted with the warm plume (Plate 10., 31 May 91). These interactions sometimes seem to be comprised of pulses of warm water injected into the GAB region by the Leeuwin Current, since in some instances a discontinuity can be seen to exist in the flow of the Leeuwin Current. Warm offshoots and cool intrusions can be often observed in the eastern GAB near the tip of Eyre Peninsula, possibly in relation to the Flinders Current as reported by BYE (1971, 1972, 1983). This current consists of a northward baroclinic transport towards the coast of South Australia east of 135°E, and an offshore transport west of 135°E. Transport in this system was estimated at 15 Sv relative to 2000 db, and was referred to as the Flinders Current by ByE (1971). 6. WINTER Around mid June, cooling occurred in the shallow coastal strips in the GAB which underwent the heating in the summer, creating cool strips of coastal water. The plume appeared to become disengaged from the coast and drift slowly eastwards into the Bight, gradually cooling as it progresses. The Leeuwin Current flows into the GAB region and connects with the warm plume (or warm tongue now), resulting in an apparent continuous strip of warm water on the southern shelf (Plate 11., 20 Jun 91). The warm GAB plume had been established in the region first, independently of the Leeuwin Current, and the warm band observable across the southern coast of Australia is a combination of the two water masses. The warmest waters within the tongue were still isolated from the warm water advected into the region by the Leeuwin Current. The eastern extent of the tongue was greater than 136°E, and the western extent of the warmest waters within the tongue is -132°30'E on 15 Jul 90. In July, the Leeuwin Current flow into the region intensified, and a general warm band still extended entirely across the southern shelf (Plate 12., 13 to 15 Jui 90), although upon careful analysis the two water masses (Leeuwin Current and GAB) can be distinguished. The Leeuwin Current is narrow and appears to follow the shelfbreak, whereas the GAB water resides on the shelf itself and is broader. The warmest core of the GAB tongue was still not directly connected to the Leeuwin Current water, and occupies the region 133 to 134°E. The warmest core of the Leeuwin Current was advected to about 128°E (see Plate 12). Onshore and offshore flows possibly related to the Flinders Current are again observed at the shelf edge in the eastern GAB. The total eastern extent of the warm water was now beyond the limit of images from Hobart (i.e. beyond the South Australian Gulfs), but the core of the tongue extended to just beyond 136°E. Cooling is still occurring in the shallow regions of the GAB. August had a similar scenario to July, apart from a general cooling of the warmer water across the southern shelf. The narrow band of warm Leeuwin Current water following the shelf edge intruded into the GAB to around 128 to 130°E, where the warm band (probably comprised of GAB water at this point) again broadened and resided on the shelf. Warmest water within the GAB tongue was still isolated from the warmest water in the Leeuwin Current, and cooling in the shallows still occurred (Plate 15., 10 Aug 91). Images from 1 Aug 90 and 91 were available (Plates 13 and 14 respectively), allowing direct interannual comparisons to be made, which reveal the situation to be quite similar from year-to-year.

The annual cycle of sea surface temperature in the Great Australian Bight

Plate 13

Plate 14

Plate 15

Plate 16 Plate 13. Satellite SST image; 1 August 1990. Plate 14. Satellite SST image; 1 August 1991. Plate 15. Satellite SST image; 10 August 1991. Plate 16. Satellite SST image; 29 January 1991 (night).

13

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M. HERZFELD

It appears that the maximum flow of the Leeuwin Current on the west coast of Australia in May is only realized (with small magnitude) in the central Bight in late July or early August. A maximum eastward extent of about 130°E was achieved where it encountered GAB water from the warm tongue, which had roughly the same temperature. This time of the year seems to be the only time that a possibility exists for the Leeuwin Current to flow east of 128°E. September was the end of the cycle, exhibiting a combination of the decay of the warm band across southern Australia, local coastal heating or cooling, and a weaker intrusion of the Leeuwin Current into the GAB. Toward the end of September, the warm intrusion from the Leeuwin Current disappeared (Plate 1., 28 Sep 90). The eastern extent of the warm band was between 135 to 136°E at the end of September. These observations are based on the analysis of the satellite image sequence covering a one and a half years. The NOAA SST images obtained from the FIAMS reception facility confirmed these trends for the summer of 1993/1994; hence the above analysis appears to be representative of the annual cycle of SST within the GAB. However, the satellite image time series is not of sufficient length to gauge the effect of interannual variability of the GAB warm tongue, and the SST signal within the GAB may well be modulated by interannual variability. Any interannual signal is unlikely to have a magnitude as large as the seasonal SST change, which justifies the analysis of the seasonal signal based on 1 to 2 years of data. An interannual signal would impose some uncertainty on the quantitative aspects of the seasonal signal, but cannot alter the essence of the findings outlined above.

7. DIURNAL WARMING Under low wind conditions (e.g., at the center of an anticyclonic pressure system) and high insolation (around midday under cloud free conditions), it is possible for a shallow layer of a few decimetres depth to increase in temperature by several degrees more than that of the main mixed layer. STOMMEL et al. (1969) reported diurnal temperature changes of 0.1 to I°C in the upper 60 m of waters south of Bermuda in March 1968. These temperature changes exhibited large variations over a period of nine days, and were dependent on wind variations and insolation. HALPERN and REED (1976) investigated a 2 km 2 region near the coast of northwest Africa over a threeday period in March 1974. The windspeed was low ( - 2 ms-l), and diurnal surface heating resulted in temperature changes of 0.9 to 1.4°C. In August and September 1974, KAISER (1978) found that under calm wind conditions where over 50% of the sea was glassy, around midday a layer of 3.9 m thickness was formed with a temperature I°C warmer than underlying waters. These diurnal heating events are inevitably evident in satellite SST images. DESCHAMPS and FROUIN (1984) observed diurnal heating of more than I°C during the summer in the Mediterranean Sea under low wind conditions. The diurnal heating was correlated to the wind speed, retrieved from measurements of glitter reflectance using a satellite derived short wave image, and good agreement was found. Diurnal heating of 0.8°C was found for wind speeds of approximately 2 ms -~. CORNILLON and STRAMMA (1985) found that in the western North Atlantic (30°N in the summer, 25°N in the spring and autumn) the magnitude of diurnal warming effects was greater than l°C for 30% of the time. They concluded that the warming was related to the westward extent of a high pressure ridge associated with the Azores-Bermuda high, which is a region generally characterised by low wind speeds.

The annual cycle of sea surface temperature in the Great Australian Bight

15

STRAMMAet al. (1986) discovered a diurnal SST signal of up to 3.5°C was evident in satellite data of the Sargasso Sea (~34°N, 70°W) in the summer. This temperature signal correlated well with a two-year record of temperature obtained from a thermistor at 0.6 m depth. A 20% probability was found that the diurnal warming would exceed 0.5°C. The wanning events covered large areas; there were instances where warming of more than 1°C covered 300,000 kmL The warming correlated well with corresponding atmospheric pressure patterns, the largest warming events could also be associated with the westward extension of the ridge associated with the Bermuda high. The diurnal warming was simulated by STRAMMA et al. (1986) using a one-dimensional numerical model, which revealed that the conditions necessary for large diurnal SST signals are high insolation and low wind speeds. The mixed layer depth during these events was found to be reduced to the convection depth (the depth of convective mixing due to heat loss from the surface) and wind mixing became insignificant. More recently, FAIRALL et al. (1996) investigated diurnal warming using the model of PRICE et al. (1986). Peak afternoon warming of 3.8°C was observed when a wind speed of 1 ms -1 (at 10 m height) was specified. The warm layer depth was 0.7 m in this instance, and increased to 19 m with an associated temperature of 0.2°C when the wind was increased to 7 ms -I. A comparison of model results to data collected during the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (COARE) revealed that the model accurately described the diurnal warming cycle. STRAMMA et al. (1986) concluded that diurnal wanning is expected over large areas of the world ocean. Diurnal warming events were probably not observed by direct field measurements in the past, since they are limited to only 1 to 2 m depth and ship measurements are usually taken from depths greater than 2 m. Warm pools of water attributable to this diurnal warming effect are occasionally observed within the GAB region. The images of 29 January 1991 display conclusively that diurnal warming is indeed present in the central GAB. Plate 6 (29 Jan 91 daytime image) exhibits a large expanse of warm water situated in the central GAB. The surface isobaric chart for 29 January 1991 (00 GMT, taken from BUREAU OF METEOROLOGY(1990), Fig. 2), indicates that a high pressure system is centered over the central Bight where the warm pool resides (Fig. 2). Wind speeds within the center of this pressure system would be expected to be low. There were cloud free conditions over the central Bight (Plate 6) and hence insolation would be high. Plate 16 (29 January 91 night image) represents the night pass for 29 January 1991, and, as expected

Fig. 2. Surface isobaric chart; 29 January 1991

16

M. HERZFELD

since insolation is zero, the warm pool is absent. The warm pool generated during the daytime on 29 Jan 91 is almost certainly attributable to the diurnal warming skin effect observed by CORNILLON and STRAMMA(1985). Warm pools evident in other the daytime images within the GAB (e.g., 25 Nov 90) can also be attributed to the diurnal warming skin effect.

8. COMPARISONWITH PREVIOUS STUDIES The above scenario compares quite favourably to the results given by ROCHFORD(1986). The warm (high salinity) water found by ROCHFORD(1986) in the central and eastern GAB for most of the year, and the formation of the warm (high salinity) tongue is also evident in the satellite images. The water comprising the tongue was also found to have an apparent motion towards the southeast, consistent with the southeastward drift of GAB waters observed by ROCHFORD (1986), who concluded the warm tongue forms the core of a thermohaline current, beginning to flow in January in the central GAB. At all times of the year, cloud cover permitting, onshore and offshore flows are observed at the shelf edge in the eastern GAB off Lyre Peninsula, consistent with previous reports of the Hinders Current (BYE, 1971BYE, 1972BYE, 1983). The satellite images show the warm plume (originating from the coastal strip 126 to 129°E) is detectable as a permanent feature from January, again consistent with the findings of ROCHFORD (1986). ROCHFORD (1986) suggested that the Leeuwin Current advects low salinity, warm water to a maximum eastward extent of 130°E in July. The satellite images show the maximum extent of the Leeuwin Current compares well with the 130°E limit proposed by ROCHFORD (1986), although the time of maximum eastern extent should be some time in August. In late July/early August, water intruding into the GAB from the west and the warm tongue in the central and eastern GAB have roughly the same temperature (see the August images), and it is difficult to differentiate between the two water masses from satellite images alone. The GAB warm tongue is certainly established first. Although the July 1979 image of LEGECKISand CRESSWELL (1981) indicated warm water had been advected to a maximum eastern extent of 136°E, it is still possible that water with a temperature indistinguishable from that found in the Leeuwin Current had resided on the shelf east of 130°E prior to the intrusion of the Leeuwin Current. The July 1979 image could therefore exhibit a continuous band of warm water consisting of two different water masses. On the basis of data available so far, the most reasonable hypothesis is that the Leeuwin Current advects water to about 130°E in late July/August, where it joins with the warm tongue that extends further east than 136°E. The total eastern extent of the warm band then becomes greater than 136°E. ROCHFORD (1986) found low salinity, warm Leeuwin Current water at the shelfbreak (the Leeuwin Current being a predominantly shelfbreak phenomenon), whereas LEGECKISand CRESSWELL (1981) observed warm water within the GAB was "predominantly on the shelf". Based on the satellite images examined here, the GAB water resided on the shelf, and when the Leeuwin Current intrusion existed, the warm band broadened in the central GAB. These features of the warm band are consistent with the hypothesis that Leeuwin Current water is advected along the shelfbreak to the central Bight were it meets the GAB water residing on the shelf, producing a broadening of the warm band at this point. The effect of the warm plume detaching from the coast around June and drifting eastwards may be due to the formation of a thermohaline current in the winter, driven by a cross-shelf pressure gradient, or may only appear to be advected due to progressive cooling originating at the coast. Since cooling is initially evident at the locations where heating was observed in the

The annual cycle of sea surface temperature in the Great Australian Bight

17

summer, i.e., in the shallow regions at the coast where differential heating/cooling results in maximum SST change, the latter is probably the correct presumption. However, GODFREY et al. (1986) found the inshore steric height levels were higher than the offshore levels in June to July 1982, and concluded that a longshore current found in the region may be due to thermohaline effects, wind or both. The different SST observed on and off the shelf in the satellite images could contribute towards maintaining the cross-shore steric height gradient observed by GOOFREY et al. (1986). Finally, HERZFELD and TOMCZAK (1997) simulated the SST distribution in the GAB using a numerical model with idealised topography for the GAB region, and concluded that it is indeed possible for a warm plume to be generated within the GAB as a result of local forcing conditions alone. The satellite images provided evidence for the formation of a warm water mass in the GAB, which formed a tongue originating from the coastal region 124 to 129°E and spread southeastwards to a maximum eastern extent of more than 136°E. This water mass is proposed to be created by processes local to the GAB. Its formation and behaviour are independent of any influence of the Leeuwin Current for the following reasons: 1. The warm GAB water forms and increases in spatial extent when there is no evidence of Leeuwin Current flow into the region (in January to March), i.e., the GAB water is there first. 2. The GAB water temperature increases and undergoes change (plumes develop, increase in size and spread southeastward in the late summer and autumn) when no connection between the Leeuwin Current and the GAB water exists, i.e., the warm Leeuwin Current water may be present in the SST image but remains unconnected to the GAB warm water, especially at times when the Leeuwin Current flow is supposed to be minimum. 3. The warmest GAB water is consistently isolated from the warmest water on the southwest shelf when the Leeuwin Current intrudes. Hence if the warm Leeuwin Current water never connects with the wannest GAB water, it cannot be the responsible source for the warmest GAB water. Also ROCHFORD (1986) found instances when the GAB water was warmer (by I°C) than the Leeuwin Current water. 4. The GAB water extends all the way to the coast with the warmest water occurring at the coast (incidentally where the water depth is shallow, less than 30 m, and heating throughout the water column would be the greatest) and covers a broad area while the Leeuwin Current consistently followed the shelfbreak as a narrow stream on the southwest coast. The broadening of the warm SST signal in the central GAB is inconsistent with the spatial characteristics of the Leeuwin Current found further west. No explanation has been given in the literature as to why the Leeuwin Current should broaden in such a fashion. 5. Salinities found in the GAB water are considerably higher than those found in the Leeuwin Current (RocHFORD, 1986). These factors suggest that the warm GAB water cannot be maintained or produced from flow of the Leeuwin Current into the GAB area. If the GAB water was the remnant of Leeuwin Current flow into the Bight, then the GAB water would not be expected to relocate to the coast when the general flow is eastward in the region (GODFREY et al., 1986), would not intensify and spread throughout summer and autumn months, and would be expected to have a salinity similar to that found in the Leeuwin Current. For these reasons it is proposed that the GAB water is produced by mechanisms local to the GAB region.

18

M. HERZFELD

9. HEAT BUDGET ANALYSIS

To give support to the above arguments and verify the possibility of local water mass formation, a quantitative heat budget analysis for the shallow northwestern region of the GAB was attempted. Data collected at Eucla (31°43'S, 128°54'E) by the Bureau of Meteorology was made available for this purpose. The data consisted of measurements of wet and dry bulb temperature, wind speed and direction, pressure, and cloud amount. These variables were recorded dally every three hours from 6am to 6pm, and the data set covers the period 1 Oct 1990 to 31 July 1991 inclusive. To fill the data gaps during night, FIAMS installed a weather station at the same location during the summer of 1993. This record allowed a relationship between the nighttime temperatures and daytime parameters to be established so that nighttime temperatures could be estimated for the Bureau of Meteorology data and thus generate a diurnal cycle of air temperature. Other variables were linearly interpolated to yield a record with a three-hour sample interval. Clear-sky, instantaneous, global short-wave radiation was calculated via Eq. 2.32 from ZILLSIAN (1972) based on values computed using the tables of SCHUEPP (1966). The clear sky global short wave radiation was corrected for cloud cover after REED (1977) and was then corrected for the amount reflected at the earth's surface. Albedo was calculated from an interpolation of values presented in Fig. 5.1 of ZILLMAN (1972), which was based on data from TER-MARKARIANTZ (1959) and GVaSHCHENKO (1959) of albedo as a function of solar elevation and cloud cover. Net long wave radiation at the earth's surface was calculated for clear skies via Eq. 3.4(a) from ZILLMAN (1972). For cloudy skies the net longwave input was approximated using the correction from BUDYKO (1963). The sensible and latent heat fluxes were calculated using the bulk scheme of KONDO (1975). This scheme can be applied for air-sea temperature differences of _ 20°C and windspeeds from 0.3 to 50 ms -I, which adequately covers the expected conditions in the central GAB. The bulk parameters are corrected for stability via a stability parameter which is a function of windspeed, temperature difference, and height. Correction for stability is important in the central GAB region, where the air temperature can be up to 20°C warmer than the SST, resulting in very stable conditions and, hence, little transfer of heat and water vapour through turbulent processes. When the wind blew from the land over the ocean, turbulent airsea exchanges of temperature and humidity were corrected by solving the one-dimensional steady state advection-diffusion equation of motion, using the solution given by SUTTON (1953). The net flux of heat into the water surface is represented by the sum of the short and long wave radiation components, and latent and sensible heat fluxes. The monthly mean heat budget components resulting from the heat budget calculations are displayed in Table 2 for all months included in the '90/'91 data set. Included in Table 2 are the components of the heat budgets based on the "COADS" data set (OBERHUBER,1988) and that of ESBENSEN and KUSHNIR (1981). In many instances insufficient measurements existed in the GAB region for values to be given, and a large amount of visual interpolation needed to be applied to obtain values representative of the inshore GAB. The values obtained from OBERHUaER (1988) and ESBENSEN and KUSHNIR (1981) display reasonable agreement between each other, as do the calculated components, except for the latent heat flux, especially in winter. This discrepancy in the latent heat flux is probably due to error associated with the interpolation of the latent heat flux into the coastal GAB. Also, the effect of atmospheric advection causing specific humidity to converge to the sea surface values may excessively suppress latent heat loss during winter.

The annual cycle of sea surface temperature in the Great Australian Bight

19

Table 2. Calculated monthly mean heat budget components. (All values in WM -2) Month Jan 1991 Feb 1991 Mar 1991 Apr 1991 May 1991 June 1991 July 1991 Oct 1991 Nov 1990 Dec 1990

Method

SW

LW

Sensible

Latent

Net

Calculated Esbensen Oberhuber Calculated Esbensen Oberhuber Calculated Esbensen Oberhuber Calculated Esbensen Oberhuber Calculated Esbensen Oberhuber Calculated Esbensen Oberhuber Calculated Esbensen Oberhuber Calculated Esbensen Oberhuber Calculated Esbensen Oberhuber Calculated Esbensen Oberhuber

257 240 to 260 225 229 220 200 173 180 150 to 175 148 140 to 160 100 to 125 94 100 to 120 75 to 100 91 80 to 100 75 111 100 75 to 100 198 200 to 220 175 to 200 247 260 225 246 280 225 to 250

-61 -50 -60 -53 -60 -50 to - 6 0 -61 -60 - 5 0 to - 6 0 -68 -60 -50 to - 6 0 -61 - 6 0 to -70 -60 -71 - 6 0 to - 7 0 -60 -71 -70 - 6 0 to -70 -57 -60 - 6 0 to - 7 0 -59 -60 -60 -54 -60 -50 to - 6 0

-7 0 0 to - 1 0 - 10 0 0 to - 1 0 -9 0 to-l(1 -10 -8 -10 -10 - 10 -10 - 1 0 to - 2 0 -8 -20 -20 -8 -20 -20 -3 0 to - 1 0 -10 -5 0 0 to -10 -3 0 0 to - 1 0

-76 -100 -75 -80 -100 -75 to -100 --48 -100 to -120 -75 to -100 -39 -100 to -120 -100 -42 -120 to -140 -100 to -125 -26 -120 to -140 - 100 -65 -100 to -120 -75 to -100 -45 - 8 0 to -100 -100 -60 - 8 0 to - t 0 0 -75 -56 - 8 0 to -100 -75

114 80 75 to 100 86 40 50 57 0 0 37 -40 -50 - 14 - 4 0 to -80 -75 to -100 - 11 -80 - 125 -34 - 8 0 to -120 -125 94 0 to 40 25 to 50 123 40 to 80 75 to 100 133 80 100

10. SEA SURFACE TEMPERATURES T h e h e a t b u d g e t c a l c u l a t i o n s i n d i c a t e t h a t d u r i n g the s u m m e r t h e w a t e r s at E u c l a are r e c e i v i n g heat; a r e s u l t c o n s i s t e n t w i t h the i n f e r e n c e r e s u l t i n g f r o m the a n a l y s i s o f the satellite i m a g e series t h a t w a r m w a t e r in the area is p r o d u c e d locally. T h e t e m p e r a t u r e c h a n g e in the coastal G A B w a t e r s d u e to t h e p o s i t i v e n e t h e a t i n p u t c a n b e e s t i m a t e d . N e g l e c t i n g all a d v e c t i v e effects a n d a s s u m i n g t h a t t h e s u r f a c e e l e v a t i o n a n d a d e p t h h t h r o u g h w h i c h t h e r e is n o flux o f h e a t d o n o t v a r y w i t h time, the t e m p e r a t u r e c h a n g e A T o v e r a t i m e p e r i o d At, t h r o u g h o u t a m i x e d l a y e r o f t h i c k n e s s h, ( a s s u m i n g T is i n d e p e n d e n t o f depth, i.e. h e a t is well m i x e d t h r o u g h o u t h), d u e to t h e h e a t flux AT=

QsAt hpCv

Qs at

the surface is g i v e n by:

(1)

w h e r e p = 1028 k g m -3 is the w a t e r d e n s i t y a n d Cv = 4 × 103 J K g - ~ K - ' is t h e specific h e a t o f w a t e r at c o n s t a n t v o l u m e . U s i n g the c a l c u l a t e d m e a n daily n e t h e a t flux v a l u e s f r o m T a b l e 2

20

M. HERZFELD

Table 3. Calculated and observed temperature changes Month

Satellite (°C)

Nov Dec Jan Feb Mar Apr

1.5 0.8 2.6 0.4 -0.7 -0.8

Calculated (°C) 2.6 2.9 2.5 1.7 1.2 0.8

and a mixed layer depth of 30 m, which is typical of the mixed layer depth found in the northwestern GAB during March (CSIRO, 1966), the temperature change throughout the water column is given in Table 3. Included in Table 3 is the temperature change observed from the satellite images for those months (derived from the Aspendale images). The calculated temperature changes are expected to be larger than the observed, since the heat loss due to the observed southeastward advection revealed in the satellite images is not taken into accounted. The calculated change in temperature for January is similar to the change observed in the satellite images at Eucla. The remaining months exhibit calculated changes significantly larger than those observed, especially during March and April when cooling is observed in the satellite images but the calculated temperature change is positive. This indicates that ample heat is available to increase SST within the GAB during spring, summer, and autumn, but advective and horizontal diffusive effects must remove a large proportion of this heat from the area to provide a significantly lower SST than is potentially possible. The effect of advective heat loss is investigated through the application of a numerical model by HERZFELD and TOMCZAK (1997).

11. HOT EVENTS The most significant feature evident in the records acquired at Eucla were the extremely hot air temperature bursts observed when the wind shifts to southward and blows from the land across the sea (Fig. 3). Air temperature can approach 40°C during these "hot events", and can be elevated to as much as 20°C above the SST. These "hot events" occur four or five times per

50i 40

30

~

20

]

0

5

10

15 Time (days)

20

Fig. 3. Air temperature at Eucla; February 1991

25

30

The annual cycle of sea surface temperature in the Great Australian Bight

21

month, and last for one-to-two days. The wind direction is southward for the majority of these events, with the remaining events having a northwestward direction. The wind speed is also generally low during these events. Based on the heat budget components, net heat flux, and occurrences of "hot events" for February (Fig. 4), the times of maximum net heat flux input to the waters at Eucla correlate very well with the occurrences of the "hot events". The components of the net heat flux are considered to determine why this is so. Clearly, the greatest contribution to the net heat flux is due to the short wave radiation component. The net heat flux maxima coincide with the "hot events", but these are not necessarily the times of maximum short wave radiation input (e.g., 10, 16 and 20 February have short wave inputs much larger than those found during many of the "hot events", but the net heat flux is considerably less than that encountered during the "hot events"). "Hot events" (Fig. 4) are generally associated with minimum heat loss, and hence the smallest differences between the short wave component and the net heat flux, (e.g., on 1, 2, 17, 23, and 24 February), resulting in maximum net heat input under these conditions. During the "hot o

Net Heat Flux (Win -2)

o

Short Wave Radiation (Win -2)

0

Long Wave Radiation (Win -2) Sensible Heat Flux (Win -2/

O

Latent Heat Flux (Wm -2)

300

_= 200-

Z

100-

0-

X -lO0' . . . 0 H H

.

5'

.

.

.

.

' 10

H

'

15

i t

"

' 20

.

.

.

. H H25'

T i m e (days) Fig. 4. Daily mean heat budget components at Eucla; February 1991

"

22

M. HERZFELD

events", when temperature and humidity gradients were maximum across the ocean-atmosphere interface, the sensible heat and latent heat fluxes were not necessarily maximum. Latent heat losses are generally small during "hot events", and the sensible heat flux component provides a small positive contribution, or is very close to zero. This situation is due to the fact that the atmospheric boundary layer at the sea surface is very stable under these conditions, causing the turbulent momentum fluxes to be reduced, and suppressing the amount of sensible and latent heat transferred to the atmosphere via turbulent processes. Since the sensible heat flux can be positive (heat is input) and the latent heat flux is negative during a "hot event", these two components cancel each other to a certain degree (e.g., on 21 January at 1200 during a "hot event" the Bowen Ratio was -1.03). Since the heat input derives predominantly from the shortwave radiation component and heat losses are small during the "hot events", to maintain large net heat flux inputs the short wave radiation component must be large, which requires clear, dry atmospheric conditions. Cloud free conditions also correlate very well with the times of maximum net heat fluxes, and during "hot events", conditions are very suitable for maintaining a cloud-free, dry atmosphere, resulting in minimum short wave radiation attenuation. Therefore, the "hot events" can produce a net heat flux of greater magnitude than that encountered during average conditions, primarily because cloud free, dry conditions are maintained during these events, giving rise to maximum short wave radiation input, and secondly because minimum heat loss exists from the remaining heat budget components. The latter situation is a result of stability reducing the magnitude of the latent heat loss, the sensible heat flux being positive, and diffuse sky long wave input contributing to the reduction of the long wave radiation loss. However, the "hot events" are not mandatory in producing an above average net heat flux; if cloud free conditions are encountered outside a "hot event" and all other heat loss components are not excessively large, then the short wave radiation component can dominate and the resulting net heat flux can also be large (e.g., see Fig. 4 for 20, 21 and 22 February). The "hot events" may indeed be important in providing heat input large enough to raise SST to that observed in the GAB, and may explain the differences observed against climatology (OBERHUBER, 1988 or ESSENSEN and KUSHNm, 1981), which does not account for the "hot events". The maximum short wave radiation input occurs throughout the latitudes 25 to 30°S in December and January, and about 15 to 20°S in November and February (OaERHt;BER, 1988). The warm strips are therefore located at a latitude where they can benefit from maximum short wave radiation input during high summer.

12. DIFFERENTIAL HEATING If the net heat input into the waters local to Eucla during a "hot event" is predominantly due to the short wave radiation input, which is basically a function of cloud cover, then under homogeneous cloud conditions the coastal waters at Eucla should receive the same amount of heat. This situation would create a uniform SST distribution in the coastal waters, providing that the heat is distributed throughout the same depth at each point along the coast. Since some coastal sections exhibit higher temperatures than others, then under conditions of uniform heating the wanner sections must be associated with shallower water depths so that heat input into a unit surface area is distributed throughout a smaller volume, thus raising the temperature. The warm strips in the satellite images suggest that all the warm areas are associated with water depths less than 30 m, e.g., the 126 to 129°E strip, a patch at 124°E, warm water at

The annual cycle of sea surface temperature in the Great Australian Bight

23

Ceduna and in the S.A. Gulfs, and, to a lesser extent, the strip 120 to 122°E (cf. images from 19 Nov 90, 23 Dec 90 and 29 Jan 91). Also the warm strips were the areas which first lost heat and cooled in the winter (20 to 22 Jun 90, 1 Aug 90), suggesting that the water depth also regulates areas of maximum cooling. Finally, cruise data from CSIRO (1966) indicated mixed layer depths in the northwest GAB region in March were 30 m to 40 m. The warm coastal areas all are approximately bounded by the mixed layer depth contour. Since the warm strips are associated with large spatial areas having a depth less than the mixed layer depth, the temperature change in these areas from a constant net heat flux input would be larger than areas where heat can be mixed through the water column down to the mixed layer depth. This situation is investigated further. Consider the average temperature change over shelf sections of fixed width, w, with varying bottom topographies. Assume the water depth at the coast (x -- 0) is do, and at the shelfbreak (x = w) the depth is the mixed layer depth, dw. Assume a section of shelf exists having a linear bottom topography from the coast to the shelfbreak, where the depth, h, at any point, x, is given by h = x(dw - do)/W + do. Now assume that another section of shelf exists with a linear bottom topography from the coast to distance x = w~ from the shore, where. For x = w~ to x -- w the depth is that of the mixed layer, dw. These bottom topographies are illustrated (Fig. 5). The difference in average temperature change between the cases of Fig. 5 (a) and (b) can be shown to be:

.,..._.Qs.,t[ w pCv In

w w.] dw

dw-do

(2)

The average temperature change in any given section of coast normal to the shore due to a fixed heat flux input is dependent on the fraction of the section that has a depth greater than the mixed layer depth.

Qs+

(a)

Surface

do1 W

(b)

I

dw

D

IQsl

doI

dw w 1

~, w

Fig. 5. Linear and variable bottom topography

Surface

24

M. HERZFELD

As an example, consider a heat input of 50 Wm -2 for a period of one month over two sections of shelf. One has a linear slope from 1 m at the coast to 30 m at 15 km offshore (similar to the 126 to 129°E warm strip in the satellite images) and the other has a constant depth of 30 m from the coast to 15 km offshore. In this case, w = 15,000 m, wj = 0, do = l m and dw = 30 m. Usin__g these values in Eq. (2), the average temperature difference between the two sections is A(AT) = 2.65°C. The mean water temperature in the shallow region is ~2.7°C warmer than water with a depth greater than the mixed layer depth when both masses receive the same amount of heat for the same time period. This temperature difference is comparable to that observed between the warm strips and water associated with greater depths at the coast in the satellite images. Hence, the above mechanism is proposed to be responsible for the creation of the warm strips.

13. CONCLUSIONS The results of the analysis of the satellite SST image series conclusively demonstrate that warm SST develops and intensifies in the GAB in summer and autumn independent of the Leeuwin Current influence. The warm GAB water possesses a SST 2 to 3°C higher than surrounding water, and spreads eastward across the GAB in the form of a warm plume, or tongue, to a maximum eastward extent reaching further than 136°E. The origin of the warmer water within the GAB, and the core of this warm tongue is the shallow strip of coastal water located 124 to 129°E in the western Bight. The initiation of this warm plume formation appeared to be some time in early October. The greatest SST within the Bight is observed in the core region throughout the summer and autumn months. Cool water in the eastern Bight is consistently observed in late summer and early autunm, and is attributed to upwelling effects. The nature of this upwelling has been studied by HEgZFELD (personal communication). When wind speeds are low and solar insolation is high, e.g., in the center of anticyclonic pressure systems on clear days, an increase in SST with diurnal oscillation is observed in the Bight, which is attributed to the diurnal warming (skin effect) reported by CORNILLON and STRAMMA (1985). There is no evidence of Leeuwin Current water in the GAB during summer and early autumn. During late autumn the Leeuwin Current is observed to intrude around Cape Leeuwin in the far west, and progress into the GAB some time in May. The SST of the Leeuwin Current water is similar to that of the GAB water, but possesses a considerably lower salinity (ROCHFORD, 1986). During some time in May, the Leeuwin Current connects with the GAB water to produce a continuous band of warm water across the entire southwestern shelf region of Australia, which remains in existence during most of the winter. At the time of this connection, the warm GAB tongue is already well established, with an origin between 124 to129°E, eastward extent further than 136°E, and SST 2 to 3°C warmer than surrounding water. Thus, the warm strip across southern Australia observed in the winter is proposed to be the combination of two water masses: the Leeuwin Current flows along the shelfbreak in a narrow band to approximately 130°E, where the GAB water exists in a broad region covering the shelf. The warmest water within the GAB tongue is isolated from the Leeuwin Current intrusion throughout the winter. Cooling during the winter isolates the warm water in the GAB from the coast, and slowly destroys the band of warm water. By late September/early October, the GAB has again attained uniform temperature throughout, and the onset of heating in the spring initiates the cycle again. Based on a heat budget calculated from data measured at Eucla for (31°43'S, 128°54'E), for the period October 1990 to July 1991, the net heat flux was representative of conditions in the

The annual cycle of sea surface temperature in the Great Australian Bight

25

northern GAB, and revealed that heat is input into the region throughout October to April. This positive net heat flux is proposed to be responsible for the increase in SST in the northwestern GAB region during this time period. Estimated temperature changes in a 30 m deep water column subjected to the calculated net surface heat flux were at least as large as those observed in the satellite images. The time series of air temperature used in the heat budget calculations exhibited occurrences three or four times a month where air temperature increased up to 20°C above that of the SST observed in the satellite images. Typically during these "hot event" conditions, or heat waves, the wind direction was southward, and cloud cover was low so that solar insolation became large. The large air-sea temperature differences during "hot events" established a stable atmospheric boundary layer which suppressed turbulent exchanges of heat, and consequently maintained small sensible and latent heat losses. Since the short wave input remained large as a result of an essentially cloud-free, dry atmosphere, the result was that the largest positive net heat fluxes occurred during the "hot events". These heat waves may be significant in contributing to a larger monthly mean positive net heat flux than would occur if they were absent. A uniform heat flux applied over the GAB produces the largest SST increase in those regions where a large area of water exists with depths less than observed mid-shelf mixed layer depths. From the limited cruise data available in the summer applicable to the northwestern GAB, it appears that the mixed layer depth is greater than 30 m. The section of coast from 124 to 129°E in the western GAB which initially exhibited large SST increases, and subsequently maintained the highest SST in the GAB, was associated with water depths less than 30 m and is the largest expanse of shallow water within the GAB. The occurrence of high SST in the northwestern GAB is attributed to the greater change in temperature within the water column when heat is distributed vertically throughout a shallow depth. The bulk of the GAB coastline consists of cliffs dropping vertically into the Southern Ocean; there is essentially no beach present, except along the shallow strip west of Eucla bounded by the 30 m depth contour, and the scenario considered in Section l 1 is a realistic approximation to the GAB region. A 2.7°C temperature difference occurred in this example when the heating was applied for the same time period over two different bottom topography situations, which agrees very well with the SST difference observed in the satellite images between the 124 to 129°E warm strip, and water residing elsewhere within the GAB. Therefore, assuming that the heat fluxes calculated at Eucla are representative of the entire coastal northern GAB, it is very possible for the warm water observed in the satellite images to be produced by local heating, with the largest SST response to this heating occurring where the water is shallowest. HERZFELD and TOMCZAK (1997) used a three-dimensional coastal ocean model to replicate the situation observed in the satellite images, and concluded that the necessary conditions for the generation of a warm tongue are (a) a positive heat flux over the region with a negative gradient towards the south, (b) the west-to-east passage of anticyclonic pressure systems over the region, where the mean axis of propagation is near to or within the mouth of the GAB, and (c) a bottom depth within the GAB which increases towards the pole. These conditions were found to be satisfied within the GAB, and, hence, the situation observed in the satellite images was explained. The results of this study have conclusively dispelled the notion that the Leeuwin Current is solely responsible for the existence of warm water off the southwestern Australian shelf in the winter, and it has been demonstrated that an independently produced warm water mass exists in the GAB well before the expected annual arrival of the Leeuwin Current. The GAB shelf water mass is hypothesized to be produced by processes local to the region, in agreement with the findings of ROCIJFORI9 (1962) and ROCHFORD (1986).

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M. HERZFELD

14. ACKNOWLEDGEMENTS I am grateful to Dr G. Cresswell for supporting my visit to CSIRO Laboratories, Hobart, where access to the satellite SST image archives was made available. Thanks also to the remote sensing staff who aided my progress in obtaining the dataset, and Prof. M. Tomczak for his guidance and comments throughout the compilation of this paper. Comments provided by two anonymous referees are also appreciated. This research was done as part of the author's PhD thesis at the Hinders Institute for Atmospheric and Marine Sciences.

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