Icebergs often take quite eccentric paths, which can be two or three times the distance of their straight line movement. In addition to the physical features of icebergs (shape, depth, density) a number of other factors are responsible for the speed and direction of iceberg drift. Among the most important of these factors are the variable pressure of seawater, the intensity and orientation of ocean currents and surface winds, the orientation of the Coriolis force at the sea surface, the viscosity of water, and their state (Chapman, n.d.).
A maritime current is characterised by horizontal motions of water at the ocean's surface. Two forces driving ocean currents are the sun and the rotation of the Earth. According to Bezryadina (200649 p.1): “the Sun heats the atmosphere, creating winds which [are] moving the sea surface through friction.” Its influence does not extend much below about 100 m in depth. Bezryadina also explains that:
[t]he Sun is to alter the density of the ocean surface water directly by changing its temperature and/or its salinity. If water is cooled or becomes saltier through evaporation, it becomes denser. This can result in the water column becoming unstable, setting up density-dependent currents, also known as the thermohaline circulation.
Scientists can use the density structure of the ocean to calculate the pressure field and gradients. The variability of the height of the sea surface changes according to pressure levels. Surface ocean currents maps can be designed (Figure 3.25).
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Figure 3.25 Global Surface Current System under Average Conditions for North Hemisphere Winter.
Red arrows represent warm currents and blue arrows cold currents (credit Chapman50,
n.d.n.p.)
Compared to surface flows subsurface currents or gyres are low speed currents (Chapman, n.d.; Pidwirny, 2006). They are formed by the seawater density gradients. Five main gyres exist (North Pacific, South Pacific, North Atlantic, South Atlantic and Indian Ocean). In Antarctica, several currents influence the routes that icebergs take in the Southern Ocean.
Perhaps the most critical current to affect iceberg movement is the Antarctic Convergence that links all the oceans of the world together in a clockwise flow around the South Pole (Schmitz, 1996).
Westerly winds blow permanently in the Southern Ocean as there are no land masses; a continuous circum-global current can form (Barker, 2007). The Antarctic Circumpolar Current (ACC) flows eastward around the Antarctic through South Atlantic, Indian and Pacific Oceans (Klinck and Nowlin, 2001). The ACC has depths from 2,000 m to 4,000 m and width up to 2,000 km. The vertically integrated flow of the ACC consists of interactions between the density fields and the bottom topography and is concentrated in the upper 1,000m of the
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ocean (Hibler and Zhang, 1994 and 1995). The region within which the ACC operates experiences significant water mixing from several ocean basins. The ACC volume flux is 120 Sv (Whitworth, 1983; Peterson, 1988).It increases the transport south of Tasmania where the ACC becomes one of the largest volume currents on the planet with a volume flux of 147 Sv (Knauss, 1996). According to Smith et al., (2001-2008 n.p.) the ACC limits change because of:
[t]ides (5-10 cm/s), mesoscale eddies (1 cm/s), near-inertial motion (10 cm/s), and large- scale wind stress (25 cm/s) (Sarukhanyan, 1985) … The ACC‟s boundaries are generally
defined by zonal variations in specific water properties of the Southern Ocean (Gordon et
al., 1977).
Smith et al., (2001-2008) situate the Subtropical Convergence or Subtropical Front (STF), north of the ACC (between 35° S and 45° S). Its average Sea Surface Temperature (SST) decreases from 12°C to between 7 and 8°C. According to Smith et al., (2001-200851 n.p.): “[T]he eastward flow of the Subantarctic Front (SAF), found between 48° S and 58° S in the Indian and Pacific Ocean and between 42° S and 48° S in the Atlantic Ocean, defines the ACC's northern boundary”.
South of the STF and north of the SAF, the Subantarctic Zone (SAZ) has an average Sea Surface Temperature (SST) of greater than 4°C. Further south, the SAF and the Polar Front (PF) have an average SST of less than 2°C (Smith et al., 2001-2008). The bulk of the transport is carried in these middle two fronts. The Antarctic Convergence is located around 200 km south of the Polar Front. According to Smith et al. (2001-200852 n.p.): “[I]n the Antarctic Convergence, summer SST varies between 3°C to 5°C, while winter SST varies between 1°C to 2°C” (Figure 3.26).
51 http://oceancurrents.rsmas.miami.edu/southern/antarctic-cp.html 52 http://oceancurrents.rsmas.miami.edu/southern/antarctic-cp.html
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Figure 3.26 Antarctic Frontal Systems (credit Grobe, Alfred Wegener Institute, 200753 n.p.)
Further south still is the southern boundary front. The westward flowing Antarctic Polar Current is located pole ward of 65° S, between the southern front and the Antarctic continent (Rintoul et al., 200154). The SST of this region is about -10°C in winter (Deacon, 1984). According to Smith et al. (2001-200855 n.p.):
The southern boundary of the ACC is approximately at 65° S in most of the Indian and Pacific Ocean, from 50° E to the dateline; moves northward to 60° S, east of the dateline to 140° W; is near 70° S by 120° W and moves northward to 60° S, east of the Drake Passage.
The surface current is powerful northward, and induced by westerlies.
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According to Smith et al. (2001-200856 n.p.):
[Strong pressure] gradients give rise to stronger flows, and the majority of the ACC transport is associated with fronts within the current. Gille (1994) analysed GEOSAT altimeter data and found two well-defined jets in the ACC, at the PF and the SAF, with widths between 35 and 50 km and a dominant meander wavelength of 150 km.
At the southern boundary front, very dense abyssal waters up-well within a few hundred meters of the surface in the Continental Zone (Klinck and Nowlin, 1986 in Smith et al., 2001- 2008). The southern limit of the ACC could be located further south. According to Orsi et al. (1995) and Park (1997) the ACC limit could be assimilated with the Upper Circumpolar Deep Water (T > 1.8°C).
According to Pidwirny (2006 n.p.) the deep currents are made from: “[C]old and dense seawater which comes from the equator, travels eastward joining another deep current created by evaporation occurring between Antarctica and the southern tip of South America”.
These deep currents do not directly affect iceberg routes. Figure 3.27 shows the different currents involved in the Southern Ocean.
Figure 3.27 Patterns of Different Currents Involved in the Southern Ocean (credit Grobe, Alfred Wegener Institute57 2000 n.p.)
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Currents of the Southern Ocean have persistent spatial and seasonal patterns even though these patterns can change subject to weather conditions, which are related to hydrological and wind cycle variability. The ACC‟s eastward flow is relatively slow in regions between the fronts: less than 20 cm/s at the STF, 40 cm/s in the SAF and PF (Hoffmann, 1985) and 15 cm/s in the PFZ (Zambianchi et al., 1999). According to Smith et al. (2001-2008 n.p.):
[The ACC‟s] eastward flow is driven by strong westerly winds (average wind speed between 40° S and 60° S is 15 to 24 knots with strongest winds typically between 45° S and 55° S).Surface winds blow towards areas of low pressure and upper level winds would blow away from the corresponding area of high air pressure above.
A polar pressure cell is best developed over Antarctica because of a relatively uniform ice covered continent. Mid latitude circulation is driven by interactions between polar and subtropical air. Near the fronts, powerful eastward jets flow (Klinck and Nowlin, 2001).The Coriolis force is created by the rotation of the Earth and the friction of the sea with the Earth at the seafloor. The eastward linear velocity is strongest at the equator and smallest at the poles. When it moves north water moves eastward to keep the same momentum according to its mass and velocity. Consequently the Coriolis force increases while it moves from the equator. The surface of the ocean has then a limit of horizontal pressure gradient. According to Chapman58 (n.d. n.p.):
The combination of the Coriolis force and the horizontal pressure gradient produces a current that flows at right angles to the pressure gradient; when the two forces are equal the current is geostrophic. All major ocean current systems can essentially be considered geostrophic.
The atmosphere has a complex impact on the calving, drifting, and climatic environment within which icebergs are formed and degrade. The stress state of iceberg has been discussed by Diemand (1986). The circumpolar tracking of icebergs provided indication of the general drift directions of icebergs (Swithinbank et al., 1977; Budd, 1980; Stuart et al., 2007; Scambos, 2008). Schodlok et al. (2005a and 2005b) also studied iceberg paths in Antarctica. Fifty-nine icebergs were tagged with buoys in the Weddell Sea by AWI in 1996 (Schodlok, 2004; Schodlok et al., 2005a and 2005b). Figure 3.27 shows the drift path of the tabular iceberg A-38 from the Ronne Ice Shelf to the Weddell Sea, the Scotia Sea and South
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Georgia, from October 1998 to March 2004 (Jansen, Sandhager and Rack, 2005; Jansen Schodlock and Rack, 2007).
Figure 3.28 Drift Path of the Tabular Iceberg A-38B (credit Jansen, 2008 QuikScat data from the Antarctic Iceberg Data Base59 BYU p. 86)
The GPS position of the selected tagged icebergs was transmitted daily over a two year research period. For buoy deployment, icebergs with edges smaller than 2 km long are preferred, however, some larger icebergs (among them A-43B, 40 km by 7 km) were also tagged. According to Schodlok et al. (200560 p.1) the iceberg: “A-43B was grounded southwest of South Georgia for about a year, before it started to break up and move north again in early 2004. Part of the iceberg, containing the buoy, broke off about half a year earlier”.
The buoy survived the calving and continued to transmit for six more months. The majority of icebergs buoy systems transmits between one and two years. The study drew several conclusions, including:
59 epic.awi.de/Publications/Jan2009d.pdf 60 http://folk.uib.no/ngfso/FRISP/Rep16/schodlok.pdf
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- the existence of a coastal current indicated the dependence of northward movement towards the inner Weddell Sea. Icebergs size was small and varies with season;
- the Weddell Scotia Confluence featured different iceberg sea-ice behaviour, with large icebergs moving north into the Scotia Sea, before being trapped in the Convergence front. The eastward drift followed the Weddell Gyre toward the east (Thompson et al., 2009); - the Eastern Weddell Gyre part indicated a recirculation around the continent.
Iceberg drift determines where fresh water from the Antarctic continent is supplied to the world‟s oceans. Iceberg drift speed is influenced by iceberg size and shape, ocean currents, sea temperature, and the force of waves and winds. Icebergs drift more when the atmospheric pressure is low and the currents are stronger. Once released, icebergs are carried northward and westward with an average drift speed for tabular icebergs of 1 m/s, 3 km/day and up to 8 km/day in the easterly wind and currents around the continent (Schodlock, 2005). Icebergs eventually reach the Convergence or the Polar Front of the Antarctic Circumpolar where they drift and melt (Figure 3.29).
Figure 3.29 Iceberg Dispersion around Antarctica (see white spots) (credit adapted from Vendée
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Tabular icebergs tracked by satellites have shown maximum long-term drift rates of up to 4 m/s or 12 km/day (Marko et al., 1983). At the northern limit of the circumpolar waters, icebergs meet warmer water and they rapidly melt. The furthest north an iceberg has been sighted is at 26°30 S in the South Atlantic, almost in the tropics (Marko et al., 1983). This gives indications about the potential sites where icebergs could be harvested for freshwater utilisation.