Physical oceanography

Super spin in the southern seas

The southern oceans are generally considered as isolated systems, much like their northern counterparts. But a combination of historical data and new density profiles suggests that they may be connected on a global scale.

All Earth's major oceans contain a subtropical gyre, a vast circulation system spanning the entire ocean basin at mid-latitudes. These gyres have the crucial role in the climate system of exporting excess tropical heat polewards1. Writing in Geophysical Research Letters, Ridgway and Dunn2 analyse new and historical data from the Southern Hemisphere oceans and confirm a previous idea3,4 of an intermediate-depth connection linking the subtropical gyre circulations of the southern oceans. This opposes the conventional picture5 that the Southern Hemisphere mid-latitude circulation consists of distinct separated gyres within the Indian, Pacific and Atlantic oceans.

Ocean gyres are driven by the combination of low-latitude easterly trade winds and high-latitude westerlies, and so rotate clockwise in the Northern Hemisphere and anticlockwise in the Southern. The western boundary currents of these gyres, such as the Gulf Stream in the North Atlantic, are among the strongest ocean currents in the world. Despite having the same basic form, there are pronounced differences between the northern and southern subtropical gyres. The North Atlantic and Pacific oceans are completely separated by the continents on their eastern and western sides, whereas the Southern Hemisphere is truly the 'ocean hemisphere'. The surface area between the Equator and 60° S is 84% ocean, and land masses form only partial barriers to the ocean currents.

The geography of the southern oceans made connections between them seem entirely plausible. Nevertheless, the confirmation of the existence of a 'supergyre' (an outer gyre surrounding the individual ocean gyres) connecting the Southern Hemisphere circulations represents a new understanding of the southern oceans. The centres of the South Pacific and South Indian gyres are located at 35° S–45° S at a depth of 1,000 m (Fig. 1a), with the South Indian gyre extending far eastward south of Australia. New Zealand is effectively the western boundary of the South Pacific gyre between 35° and 45° S; without the islands there would be much more extensive connectivity between the Southern Pacific and Indian oceans. Despite this barrier, the relatively small amount of land mass in the Southern Hemisphere allows global winds to drive Southern Ocean circulation and hydrographic variability6,7 with minimal interruption.

Figure 1: Southern supergyre.
figure1

D. ROEMMICH

a, The black arrows show ocean circulation and heat exchange between the southern Pacific and Indian oceans at a depth of 1,000 m. The orange band represents the area of supergyre circulation, and the pink and red bands show the distinct inner gyres. These gyres operate north of the Antarctic Circumpolar Current (blue band). The contour lines are dynamic height, which is generally derived from temperature and salinity profiles. b, The dark-blue dots represent the locations of Argo profiles used to generate a

The presence of the supergyre creates pathways for ocean-to-ocean mixing and exchange of water properties. The near-surface portion of the supergyre controls a significant amount of the planetary heat balance. For ocean surface waters, the westward limb of the South Pacific gyre (the South Equatorial Current) bifurcates at the coast of Australia, where part of the current turns northward across the Equator and feeds the Indonesian Throughflow (the flow from the Pacific to the Indian Ocean through the Indonesian archipelago). This large transport of warm water into the Indian Ocean, balanced by much colder eastward flow south of Australia, represents an immense heat exchange of about 4 × 1014 joules per second from the Pacific8,9.

At intermediate depth, the existence of the southern supergyre has been suggested by simple models of wind-driven ocean circulation7. An agreement between models and data increases confidence in modelled climate trends in the Southern Hemisphere. These include a strengthening of the supergyre caused by increases in the subpolar westerly wind, a result of greenhouse warming and/or ozone depletion7,10. Strengthened gyre and supergyre circulations may result in southward displacement of species' boundaries owing to warming of the Tasman Sea7 south of Australia, or a widespread decrease in primary productivity caused by a downward displacement of nutrient-rich, near-surface layers6.

Beyond illustrating a new model of southern circulation, the work of Ridgway and Dunn2 highlights a revolution in ocean-observing capability over the past decade that is providing unprecedented views of the mean and time-varying ocean circulation. Inter-ocean connections are relatively subtle features, less obvious in the eddy-rich oceans than are the stronger circulations of individual ocean gyres. Observing such features, and their variability, requires data sets of geographical scope and quantity that did not exist until now. A search in the World Ocean Database11 shows that from 1950 to 2000, a total of 22,809 temperature and salinity profiles to depths of at least 1,000 m were collected in the region in Fig. 1 (15° E to 70° W, 60° S to 15° S). The limited number of profiles and the temporal and geographical unevenness of those profiles make the historical data set grossly inadequate for detailed and accurate mapping of large-scale Southern Ocean circulation.

Since January 2004, however, the Argo global profiling float project12 has collected more than 66,000 temperature and salinity profiles in the same region (Fig. 1b), including over 26,000 in the past 12 months. The deployment of the nearly 3,000 free-floating Argo instruments across the globe allows an unprecedented level of year-round data collection throughout the open ocean. Over the coming decade, our view of global ocean circulation and its variability will improve enormously as the new data set brings previously unknown features into sharp focus.

By combining the World Ocean Database and the new Argo profiles, Ridgway and Dunn2 not only confirm that the southern gyres are connected, exchanging heat and water masses on a global scale, but also show how much the new observational capabilities contribute to our understanding of the role of oceans in the climate system. The Southern Ocean, a critical region for climate research and one of the most hostile and inaccessible regions on Earth, now can be seen and studied by anyone with an Internet connection.

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Roemmich, D. Super spin in the southern seas. Nature 449, 34–35 (2007). https://doi.org/10.1038/449034a

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