Dr Phil Sutton of NIWA explains how New Zealand sits at the boundary between warm, salty subtropical water and cool, less salty subantarctic water. Argo and Jason have shown that changes in ocean heat content are focused in this region. Dr Sutton also describes how Argo can be used in local studies, such as fish distribution.
DR PHIL SUTTON
New Zealand is in a very interesting part of the world as far as most oceanography is concerned, but we span two ocean current regimes. The north of New Zealand, north of Chatham Rise, basically, or north of a line between Banks Peninsula and Chatham Islands, is all in subtropical water, which is relatively warm and salty water, and then south of Chatham Rise, you get into subantarctic water, which is relatively cold and fresh.
Subtropical water is connected to a big current system that flows anticlockwise around the South Pacific all the way across to South America and then westward south of the equator, southward along the coast of Australia, and then ultimately along the north-east coast of New Zealand and then a general eastward trend at about 40° south.
The subantarctic system is associated with the Antarctic Circumpolar Current, which is a strong current that flows eastward all the way around Antarctica, driven by the strong winds at those latitudes.
The thing that we have learnt since the Argo deployment and the Jason satellites is that the changes in the world’s oceans in the last decade or so have been focused really on the 40° south latitude – so the South Pacific, South Indian Ocean and South Atlantic – and the largest changes in heat content have been seen centred on about 40° south, in other words, right around the New Zealand region. We think that is because of the changes in the large-scale wind field in the Southern Hemisphere, and we are just keeping an eye on what is happening exactly and still trying to work out what it all means. But in terms of heat content, changes globally, the Southern Hemisphere is dominating the current picture.
There have been a number of detailed smaller process studies using Argo data. Argo is relatively young and so I think people are still learning what they can do with Argo data. A couple of the examples around New Zealand are looking at what is happening at the parking – the depth where the float parks at about 1,000 metres – where it’s freely drifting.
And the reason that has been interesting is because a number of our fish stock hangs out at about those depths – so orange roughy and hoki are a bit shallower than that. And we have used float trajectory data from around Chatham Rise to look at what is happening with the water mass movements near the orange roughy breeding grounds.
It turns out that, where the orange roughy – their spawning ground is really in a region where the warm water from the subtropical side, the north side of Chatham Rise is coming around a little bit, around the southern side of Chatham Rise, and meeting the cold water coming north in the subantarctic system – and so the next question is whether the orange roughy really care about that or not, and we still have to work on that.
NASA/Goddard Space Flight Centre Scientific Visualization Studio/courtesy of nasaimages.org.
Argo data in this video clip was collected and made freely available by the International Argo Project and the national programmes that contribute to it. (www.argo.ucsd.edu http://argo.jcommops.org ). Argo is a pilot programme of the Global Ocean Observing System.
Scripps Institution of Oceanography, UCSD
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