An Overview of the Oceanography and Meteorology of the Falkland Islands

Home , Falkland Current, Patagonian Shelf, Sea state

AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS Aquatic Conserv: Mar. Freshw. Ecosyst. 12: 15–25 (2002) Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/aqc.496

An overview of the oceanography and Meteorology of the Falkland Islands

J. UPTONa* and C.J. SHAWb a Fugro GEOS, Hargreaves Road, Swindon, UK b Shell International Exploration and Production, b.v. Volmerlaan 8, PO Box 60, 2280 AB Rijswijk, Netherlands

ABSTRACT 1. This paper describes an overview of the oceanography and meteorology of the Patagonian Shelf and Falkland Islands, derived from reference material and a 16 month measurement program carried out on behalf of the Falklands Operators Sharing Agreement (FOSA) between June 1997 and October 1998. It was not intended that the referenced information in this presentation be exhaustive, the bias has been placed on describing the measurement survey. 2. Due to commercial confidentiality, presentations of data measured during the survey have been limited to time series and statistics. However, the confidentiality of the data does not necessarily preclude use of the data for future research purposes. 3. The analysis of current, tide, wave, wind and meteorological data is described and the nature of mechanisms driving the current and wave regime are suggested. 4. Offshore oceanographic measurements were carried out at three locations prior to a drilling programme by the semi-submersible drilling rig Borgny Dolphin and on board whilst drilling. Meteorological measurements were performed at two locations on the Islands and on the Borgny Dolphin. Drifting buoys were deployed and conductivity, temperature and depth (CTD) measurements were carried out to determine the position and intra-annual variation of the Falklands Current in the North Falklands Basin. 5. The primary oceanographic current flow features of this area are the Brazil and Falklands Currents, which meet at a confluence off the coasts of Argentina and Uruguay. The position of the front varies between approximately 30 and 408S, depending on changes in the relative strength of the two current systems. Current flows are stronger on the slope than on the Continental Shelf and are less tidally dominated. Although it is thought that the FOSA mooring locations were influenced by the margins of the Falklands Current, stronger flows have been reported to the north of the measurement location. Typical maximum recorded near-surface currents reached 0.8 msÀ1, with near-bed maxima reaching 0.55 msÀ1. 6. The wave climate was primarily driven by locally derived storms, resulting in events of short duration. Typical maximum significant wave heights of 8.9 and 9.4 m were recorded to the north and south of the Islands, respectively. Copyright # 2002 John Wiley & Sons, Ltd.

KEY WORDS: oceanography; meteorology; currents; waves; temperature; Falklands Current; Brazil Current; conductivity; forecast

*Correspondence to: J. Upton. Fugro Geos, Hargreaves Road, Swindon, SN25 5AL, UK.

INTRODUCTION

This paper describes an overview of the oceanography and meteorology of the Patagonian Shelf and Falkland Islands. It is considered that the scope of the paper should not include a detailed interpretation of the results or forcing mechanisms, which may be more appropriate for an oceanographic journal. Information has been derived from reference material and a 16 month measurement program carried out in the North Falklands Basin on behalf of the Falklands Operators Sharing Agreement (FOSA) between June 1997 and October 1998. Due to commercial confidentiality, presentations of data measured during the survey have been limited to time series and statistics. However, the confidentiality of the data does not necessarily preclude use of the data for future research purposes. Offshore oceanographic measurements were carried out at three locations prior to a drilling programme by the semi-submersible drilling rig Borgny Dolphin and on board whilst drilling. Meteorological data were recorded at three locations on the Islands and on the Borgny Dolphin. Drifting buoys were deployed and conductivity, temperature and depth (CTD) measurements were carried out to determine the position and intra-annual variation of the Falklands Current in the North Falklands Basin. Prior to this survey, measured data in the Patagonian Shelf area were concentrated to the north and south of the Falkland Islands, including studies such as the Deep Basin Experiment between 1993 and 1995 (Harkema and Weatherly, 1996) and bottom pressure recording experiments in Drake Passage (Vassie et al., 1998). Measurements of current and water properties have also been taken during the regular transit of research vessels to and from Antarctica. Widely available satellite data have been utilized in more recent years by researchers such as de Souza and Robinson (1998) for observation of Southwestern Atlantic oceanography. The FOSA measurement survey has provided a significant contribution to oceanographic information in the area.

BACKGROUND OCEANOGRAPHY OF THE PATAGONIAN SHELF AND FALKLANDS BASIN

Historically, there has been limited investigation of the oceanography of the Patagonian Shelf around the Falklands Basin. Measurements within the Falkland’s Current have been limited, although Saunders and King (1995) have presented results of two measurement transects across the Falklands Current. Primary topics of research in the area have concentrated on the large scale water mass movements and their interaction with bottom topography, coastal morphology and eachother, as well as bottom flow patterns concerning the fate of cooler water masses produced in the Southern Ocean. Within the area of interest, there are two distinct regions of bathymetry affecting large-scale water mass movements. The first is the Patagonian Shelf, extending over 300 km from the South American coastline, with the Falkland Islands situated on an eastward extension at the southern extent of the shelf. The second region is the Falklands Basin, comprising a steep-sided shelf break to the east and south of the Islands (Plate 1), combined with bottom features associated with energetic currents such as steep sided troughs, transient mud waves and scouring at the seabed. Two large-scale water masses affect the area; the first being a branch of the Atlantic South Equatorial Current that forms the Brazil Current (BC). The BC is a western boundary current flowing southwards from near the equator from Cape St. Roque, Brazil, to about latitude 30–408 S, and extending over the region of the shelf plateau. This current is warm and saline, being made up of subtropical water, with typical temperature and salinity values of around 198C and 36PSU, respectively (Piccolo, 1998). The second major water mass to affect the region is the Falklands Current (FC), which comprises cooler Sub-Antarctic Water and has a typical temperature of 78C and relatively low salinity of 33.5PSU. This current originates as a branch of the Antarctic Circumpolar Current (ACC) and flows eastward through

Copyright # 2002 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 12: 15–25 (2002) FALKLAND ISLANDS OCEANOGRAPHY AND METEOROLOGY 17

Noaa Satellite

UK Datawell Directional Waverider Buoy

SEA SURFACE

120m 25m 2 Rubber Cords

Sub-Surface Buoy 300 kHz WH ADCP with Flotation Collar and Argos Beacon Sub-Surface Buoyancy 490m

Aanderaa RCM7 Current Meter 200m

Aanderaa RCM7 Current Meter 100m 465m

Aanderaa RCM7 Sonardyne Acoustic Current Meter Release 10m Sonardyne Acoustic Release and Aanderaa WLR7 Tide Gauge

Ballast Weight Ballast Weight

SEABED NOT TO SCALE Figure 1. Mooring deployment diagrams.

Drake Passage before flowing round the shelf break to the east of the Falklands and northwards along the shelf edge off South America. These two water masses meet at a confluence region situated off the Argentina and Uruguay coast, extending between 30 and 508S, and as far offshore as 458W (Provost et al., 1992; Gordon, 1989; Garzoli

Copyright # 2002 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 12: 15–25 (2002) 18 J. UPTON AND C. J. SHAW and Giulivi, 1994). The actual location of the confluence is dependent upon annual variability in the BC, which results from variability in the wind forcing of the South Atlantic gyre system, and to a lesser extent, semi-annual variability in the FC, which is forced by Southern Ocean flow changes. During the Austral summer, the confluence region is situated at its southern extreme and in winter (June–August) is situated at its northern extreme. Geostrophic currents in the region of the confluence are of the order of 1 msÀ1, with energetic shear and mixing in the frontal zone (Piccolo, 1998). The primary forcing mechanisms in the region are attributable to tide, wind and balance between Coriolis and pressure gradient forces, with density driven mechanisms having a lesser influence. Depth- averaged density driven currents have been reported to be of the order of 0.02–0.07 msÀ1 (Forbes and Garraffo, 1988). Geostrophic currents (primarily the Brazil and Falklands Currents) contribute the most significant forcing mechanism in terms of current speed and mass water transport. The presence of the FC raises the sea level over the whole Patagonian Shelf by around 0.65 m, forming the pressure gradient that maintains the FC (Glorioso and Flather, 1995). Flow velocities of the FC north of the Islands, obtained by model output (Glorioso and Flather, 1995), give maximum values due to geostrophic balance of 0.27 msÀ1, although surface currents have been shown to be up to 0.40 msÀ1 (Peterson, 1992). In the northern extent of the FC, where flow begins to move onto the shelf (around 35–408S), maximum current speeds are around 1.5 msÀ1 (Glorioso and Flather, 1995). South of the Falklands, the bathymetry of the seabed, particularly the Burdwood Bank and Falklands Trough, have significant effects on current flows. Gloriso and Flather’s model (1995) generated the strongest flows in the FC to the south of the Islands, around the Burdwood Bank. The most influential region affecting current flow is the Falklands Trough, where the Falklands Current splits off from the ACC and turns northward to pass east of the Falklands. At this location, there is evidence of westward flow into the Falklands Trough, both through model predictions (Glorioso and Flather, 1995) and direct observation. Wind forcing is a major driving factor of the Brazil and Falklands Currents, both currents resulting from wind stress in the Atlantic and Southern Oceans, respectively. Seasonal changes in current strength and the position of confluence between both currents have been attributed to annual and semi–annual variations in winds driving the currents. The Falklands region is affected mainly by active weather systems in the form of depressions moving eastward through Drake Passage, bringing predominantly westerly winds. On the Patagonian shelf, the main forcing mechanism is the tide, which is semi-diurnal with typical ranges of over 3 m at the south-east coast of Argentina and reducing offshore to around 1 m near the Falklands. Tidal ranges at certain points along the Argentinean coast are among the largest in the world, reaching over 12 m in places.

AIMS AND OBJECTIVES

The purpose of the FOSA Measurement Program was to: * Provide suﬃcient data to verify the preliminary estimates of the extremes, which had been used for the selection of the drilling rigs and equipment (in particular the drilling risers). * Provide real time information, which would be used in preparing the weather and sea-state forecasts during drilling operations (the land-based station at Cape Dolphin was speciﬁcally installed to provide weather information for the intermediate helicopter re-fuelling stop between Mount Pleasant Airport (MPA) and the drilling rig) (Plate 1). * Provide ground-truth data for environmental impact assessments (in particular for studies of oil-spill scenarios). * Provide statistics, which would be used for planning future operations and design of facilities should the drilling campaign prove successful.

Copyright # 2002 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 12: 15–25 (2002) Plate 1. Mooring and conductivity/temperature/depth deployment locations and meteorological data measurement locations.

Copyright # 2002 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 12 (1) (2002) Plate 2. Argos drifter tracks, showing six individual tracks.

Copyright # 2002 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 12 (1) (2002) Plate 3. Temperature data derived from CTD transect, also showing mooring locations.

Copyright # 2002 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 12 (1) (2002) FALKLAND ISLANDS OCEANOGRAPHY AND METEOROLOGY 19

The mechanism for deriving most of the above information would have been to use the ﬁeld measurements to calibrate numerical wind, wave and current models of the region. Such models will be needed for future design and operational planning and would be run in ‘hindcast’ mode to produce 20–30 year historical databases of the necessary metocean parameters, such as signiﬁcant wave height and wind speed. In the event of course, the results of the measurement survey were not required for design or operational statistics, due to the disappointing results from the drilling campaign. Nevertheless the data were extremely valuable in verifying preliminary estimates of environmental conditions and in the generation of weather forecasts. In addition the data will be used the Falkland Islands Government (FIG) Fisheries Department in their ongoing research program.

METHODS

In order to determine operational and design statistics, measurements of currents, waves, tides and winds were carried out before and during the drilling campaign undertaken by the semi-submersible drilling rig Borgny Dolphin (Fugro GEOS, 1999). Moorings were deployed at two locations for 12 months to the north of the Falklands and for 3 months at one location to the south (Plate 1). Directional Waverider Buoys logging at 30 min intervals and current metres logging at 10 min intervals, were deployed at all three sites (Figure 1). In addition, CTD measurements were carried out at the deployment locations and along a transect traversing the Falklands Current (Plate 1). Drifting drogues were also deployed on service visits to the moorings (Plate 2) to provide information on Lagrangian current flows and sea surface temperature. During the drilling programme, the Borgny Dolphin was equipped with an Acoustic Doppler Current Profiler (ADCP), meteorological and wind sensors, and a heave compensated wave height and period radar. Relevant met, wind and wave data were transmitted to the United Kingdom Meteorological Office (UKMO) via the Global Tele-communications System (GTS) in near real time, in order that on-site measurements could be used to improve meteorological forecasts. A comparison of the UKMO Global Wave Model and measured wave data is presented in Hopkins (1999, personal communication). Statistical, exceedence and persistence analyses of time series data were carried out and harmonic analysis of current data was used to identify tidal and non-tidal current flow components. Extreme value analysis was used to derive design values for facilities should exploration continue.

RESULTS

The following sections describe the main results derived from the 16-month measurement survey carried out for FOSA.

Current ﬂows Within the measurement area, current speeds were generally higher and more tidally driven further up the slope (Tables 1 and 2). Current speeds at Locations A and B to the north of the Islands were relatively low, and reduced through depth, with all year maximum near-surface current speeds (approx 20 m below mean sea level (MSL)) not exceeding 0.8 msÀ1 and maximum near-bed current speeds of 0.55 msÀ1. Figure 2 shows an all year time series of observed current recorded 50 m below MSL at Location A. Annual progressive vector plots of currents showed a mean drift towards west–north-west at Location A and north-west at Location B, further up the slope. In addition, currents showed a greater degree of uniformity through depth at Location B.

Copyright # 2002 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 12: 15–25 (2002) 20 J. UPTON AND C. J. SHAW

Figure 2. Observed current speed (msÀ1), 50 m below surface, Location A.

Table 1. Monthly maximum current speed statistics, Location A, 490 m water depth Height Depth Current speed (msÀ1) (m asb) (m bmsl) July Aug. Sept. Oct. Nov. Dec. Jan. Feb. March April May. June All Year 440 50 0.37 0.34 0.43 0.55 0.44 0.39 0.56 0.48 0.48 0.47 0.44 0.43 0.56 200 290 0.27 0.26 0.25 0.32 0.26 0.29 0.30 0.26 0.26 0.37 0.26 0.30 0.37 10 480 0.29 0.27 0.27 0.28 0.26 0.26 0.27 0.27 0.26 0.23 0.22 0.26 0.29

Note: asb is above sea bed and bmsl below mean sea level.

Table 2. Monthly maximum current speed statistics, Location B, 200 m water depth Height Depth Current speed (msÀ1) (m asb) (m bmsl) July Aug. Sept. Oct. Nov. Dec. Jan. Feb. March April May June All year 150 50 0.48 0.51 0.55 0.51 0.65 0.54 0.63 0.63 0.72 0.55 0.56 0.45 0.72 100 100 0.51 0.51 0.54 0.53 0.49 0.49 0.60 0.62 0.64 0.52 0.56 0.49 0.64 10 190 0.44 0.44 0.48 0.48 0.48 0.46 0.47 0.55 0.47 0.45 0.52 0.44 0.55

Note: asb is above sea bed and bmsl below mean sea level.

One of the drifter tracks in Plate 2 shows an anti-cyclonic loop in the current to the north–north-east of the Falklands, which has been shown to be an area of marine life accumulation. It is features of the data such as this that show the potential for use in fisheries and other marine life research. Periods of weak inertial flow of the order of 0.10 msÀ1 enhanced the current speeds at all locations during the measurement period. The inertial flow period at the latitude 508S is 15.6 h. The current regime at Location C was notably different from that north of the Islands (Location A). The sites A and C are of similar water depth, but current flows are evidently more energetic at the southern site (Table 3). Interestingly, the maximum current speed recorded at the southern location was measured near- bed, and was associated with a marked drop in water temperature, suggesting a surge of ACC water. During this period, the current speed reached 1.01 msÀ1, five times that at the surface during the same event. At the southern location, current direction was occasionally different from that anticipated, with periods of flow towards the south-westerly sector, although there was a pre-dominance of north-westward residual flow and weak tidal current flow.

Copyright # 2002 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 12: 15–25 (2002) FALKLAND ISLANDS OCEANOGRAPHY AND METEOROLOGY 21

Table 3. Maximum monthly current speed statistics, Location C, 480 m water depth Height Depth Current speed (msÀ1) (m above bed) (m below msl) July Aug. Sept. Total period 430 50 0.62 0.48 0.66 0.66 100 380 0.62 0.44 0.62 0.71 10 470 0.65 0.54 0.66 1.01

Figure 3. Signiﬁcant wave height (m) and mean zero up-crossing wave period (s).Location B.

Plate 3 shows water temperature derived from the CTD transect carried out during the October service visit, and clearly shows the presence of the Falklands Current as a cooler body of water bounded by CTD Dip sites 5 and 12, with warmer water either side. Also evident from this is the fact that the mooring locations (A and B on Plate 3) were not fully within the extent of the Falklands Current.

Wave climate Due to the predominant direction of wave energy approach in this area (south-west to north-west), increases in sea-state were typically generated by local storms tracking through Drake Passage and over South America. The passage of storms from the west generated very rapid increases in sea-state that did not persist for more than 2 or 3 days unless consecutive storms came through in quick succession. The maximum significant wave heights (Hs) were in the order of 9.0 m to the north of the Islands, these events being driven by south-westerly or north-westerly winds (Figure 3). Mean wave zero up-crossing periods (Tz) typically ranged between 4 and 8 s, although more persistent events with Tz values of between 8 and 10 s were recorded during May 1998 (Figure 3). Seasonality in wave height was evident, with monthly wave statistics and monthly variation in Hs clearly showing a more energetic wave environment between the months of July and September (Table 4 and Figure 3). This correlated reasonably well with increased monthly mean wind speed (Figure 4). Few events were recorded when waves approached from the north-east (Figure 5). However, on a limited number of occasions, significant long-period swell energy was recorded from the north-east, with waves of peak periods between 14 and 16 s being measured. Despite the shorter measurement duration, a maximum Hs of 9.4 m was measured to the south of the Islands, demonstrating the significantly more energetic environment. Direction of wave approach at the southerly measurement location was more predominantly from the south-west, with fewer waves recorded from the direction sector west to north and virtually no waves from the quadrant north-east to south-east.

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Table 4. Signiﬁcant wave height statistics, Location A Signiﬁcant wave height (m) Percentage Exceedence Minimum 50% 10% Maximum January 1.1 2.1 3.2 5.6 February 0.9 1.7 3.2 5.5 March 0.8 1.8 3.4 6.3 April 0.8 1.5 2.5 3.4 May 0.8 1.6 3.0 5.4 June 0.9 1.9 3.0 5.5 July 0.9 2.7 5.0 8.9 August 1.4 2.6 4.6 8.5 September 0.9 2.0 3.7 6.3 October 0.6 1.8 3.1 5.4 November 1.0 1.8 3.7 6.9 December 0.9 1.9 3.0 5.1

Total 0.8 2.0 3.6 8.9

9.0 45.0 8.0 40.0 7.0 35.0 6.0 30.0 5.0 25.0

Hs (m) 4.0 20.0

3.0 15.0 U 10min (kt) 2.0 10.0 1.0 5.0 0.0 0.0

July April May June March August January February October September NovemberDecember

max Hs (m) Wind Speed (kt)

Figure 4. Monthly mean wind speed and monthly maximum signiﬁcant wave height, Location A.

Meteorology Meteorological data were taken from 2 UKMO locations, Pebble Island and MPA (Plate 1), and meteorological stations were installed at Cape Dolphin and on the Borgny Dolphin. Maximum all year 10 min mean wind speeds from all sites were slightly in excess of 40 knots. Hourly mean wind speeds were persistently high, with between 65 and 80% of winds exceeding 10 knots at all sites. The predominant wind directions were from the south-west and north-west, which originated as a result of storms passing through Drake Passage and across South America. The number of wind records from the south-east was very low (Figure 6), with only 10% of records in the sector 090 to 1808.

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Figure 5. All year signiﬁcant wave height rose plot, Location A.

Mean air temperatures were similar at all locations, but the daily variation onshore was greater than that oﬀshore. The maximum recorded temperatures during concurrent measurement periods were 14.98C on the Borgny Dolphin and 21.18C at MPA, with minimum values of À0.7 and À0.48C, respectively. There was a clear seasonal variation in the air temperatures measured at the Pebble Island/MPA sites, with an increase from August to November, a decrease from March through to May and stable transitional periods in between. Daily variation in air temperatures was more marked on the Islands during the summer than the rest of the year. There was no clear seasonal variation in the atmospheric pressure, (a 14 mbar diﬀerence between monthly maximum values was recorded throughout the measurement period), with maximum values ranging between 1003 and 1035 mbar.

CONCLUSIONS

A large volume of high quality wave, current and meteorological data was collected during the FOSA metocean survey. The data proved valuable in verifying preliminary estimates of extreme conditions in the area and also in enhancing the safety of operations through improved weather and sea-state forecasts during the drilling campaign. Should the industry return to the area in the future, the data will no doubt prove to be of great value in the calibration of the numerical wind, wave and current models for the region which will be needed for future engineering design and operational planning. The data measured during the measurement survey will

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Figure 6. All year wind speed rose plot, Mount Pleasant.

also be useful for correlation with ﬁsheries research and may provide valuable insights into spatial variations of ﬁsh stocks, bird populations, and marine mammals. Comparison of these data to historical datasets will also provide indications of inter-annual variations in the Falklands and Brazil Current systems, which may also prove useful to FIG Fisheries Department.

REFERENCES Forbes MC, Garraffo ZD. 1988. A note on the mean seasonal transport on the Argentine Shelf. Journal of Geophysical Research 93: 2311–2319. Fugro GEOS. (June 1999). North Falklands metocean survey. Final Report. Garzoli SL, Giulivi C. 1994. What forces the variability of the south-western Atlantic boundary currents? Deep Sea Research 41(10): 1527–1550. Glorioso PD, Flather RA. 1995. A barotropic model of the current off SE South America. Journal of Geophysical Research, 100: 13427–13440. Gordon AL. 1989. Brazil-Malvinas Confluence-1984. Deep Sea Research 36(3): 359–384. Harkema R, Weatherly GL. 1996. A compilation of moored current meter data from the southern boundary of the Brazil Basin for the deep basin experiment, September 1993–March 1995. Technical Report for National Science foundation, Department of Oceanography, Florida State University. Hopkins J. 1999. Use of an operational Global Model to define wave climate at a South Atlantic location. Conference Paper for ‘Climar99’ (personal communication). Peterson RG. 1992. The boundary currents in the western Argentine Basin. Deep Sea Research 39: 623–644. Piccolo MC. 1998. Oceanography of the western South Atlantic Continental Shelf from 33 to 558S Coastal Segment (5,W). In The Sea, vol. II, Robinson AR, Brink KH. (eds). Wiley: New York; 253–269. Provost C, Garcia O, Garcon V. 1992. Analysis of satellite sea surface temperature time series in the Brazil–Malvinas current confluence region: dominance of the annual and semi-annual periods. Journal of Geophysical Research 97: 17841–17858.

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Saunders PM, King BA. 1995. Oceanic ﬂuxes on the WOCE A11 section. Journal of Geophysical Research 25: 1942– 1957. de Souza RB, Robinson IS. 1998. Lagrangian and infrared observations of surface currents in the Brazil–Malvinas conﬂuence zone, 1993–1994. International WOCE Newsletter 31: 32–35. Vassie JM, Spencer R, Foden PR. 1998. Bottom Pressure Measurements across the Drake Passage Choke Point. WOCE International Newsletter 30: 14–16.