Geo News

Taking the ocean's temperature

It is now well established that the Earth's climate and weather are controlled by the interplay between the atmosphere and oceans. We know, for example, that a redistribution of heat in the oceans of the tropical Pacific during an El Nifio Southern Oscillation (or ENSO) event—or its opposite twin, La Nifia—affects the weather and climate of the entire planet. Much of our improved understanding of ocean-atmosphere interactions results from new observation systems, especially those provided by satellites. To this end, the present generation of NOAA (National Oceanic and Atmospheric Administration) polar orbiting satellites has been continuously monitor­ing our atmospheric and ocean environments since 1979. These satellites orbit 850 km above the Earth, measuring the radiation emitted by atmospheric gases such as carbon dioxide, oxygen, ozone, and water vapour, as well as reflections of sunlight and emissions of heat from cloud, land and ocean. It is these measure­ments that are leading to advances in today's, and tomorrow's, weather and climate forecasting systems. Scientists at the National Institute of Water and Atmospheric Research (NIWA) are using data from these same satellites to measure the characteristics of New Zealand's ocean areas. Each day about 1.6 gigabytes of data (sufficient to fill about 1200 floppy discs) from three NOAA satellites flow into NIWA computers. These data are then used to derive analyses of cloud cover and sea surface temperature (SST) over our region. For the first time, the sea surface temperature of every square kilometre in an area that stretches from Fiji to 60 degrees south and from Australia to a third of the way across the Pacific is being monitored up to six times daily. These data have revealed that the ocean environment around the New Zealand region during the El Nino summer of 1997/ 98 was very different from that observed for any previous El Nino. Direct local measure­ments of SST have also played a part in solving the El Nifio puzzle. Such readings have been taken at the Leigh Marine Laboratory since 1967, and have shown that sea surface temperatures in the New Zealand region have always been colder than normal (by about 1 degree) during El Ninos, and, conversely, warmer than normal during La Ninas. In 1998 that pattern changed, as can be seen from Figure 1, which shows the mean sea surface tempera­ture in the New Zealand region during February. Although the satellite measures the radiation emitted from just the top few microns (1000 microns equals one mm) of the ocean, these measurements reveal much about the oceanography of our region. The southward flow of warm water down the east coast of Australia, the East Australian Current, is clearly visible as are the warm southward-flowing currents (the East Auckland and East Cape currents) east of the North Island, and the warm northward-flowing Southland Current just east of the South Island. The eastward flowing Antarctic Circumpolar Current is just visible at the southern edge of the map. The difference (or anomaly) between the February 1998 sea surface temperatures and the long­term mean for February was as much as +4° C around much of the North Island, which is nearly half the normal annual difference between winter (August) and summer (February) sea surface temperatures. In a normal year, the 20° isotherm east of New Zealand is found near East Cape in February. In 1998, 20° waters reached as far south as Cape Palliser. However, despite this very large positive anomaly, east of the North Island and shoreward of the 1000 m colder than normal. This was because the East Auckland Current was flowing strongly during the summer months, leading to a nearshore upwelling of cold, nutrient-rich bottom waters. Upon meeting sunlight in the euphotic zone, this water generated nearshore algal blooms, thereby supporting an extraordinarily productive marine environment. The spectacular warm anomaly had other effects too: warm-water fish were carried much further south than normal—marlin were even caught off the Wairarapa coast—while subtropical species were found off Northland. Also, as a result of this large anomaly, summer air temperatures were around 3° warmer than normal in Auckland. In a normal El Nino, air tempera­tures in the New Zealand region tend to be near normal, or a little cooler than normal, but apart from the West Coast of the South Island, air temperatures were warmer than normal over much of the country, and in February the anomaly was of the order of 2 to 3° every­where. So why were the SSTs around New Zealand so warm in 1998? It is not possible to give a definitive answer, but we do know that the East Australian Current, which is fed from the South Pacific Gyre, a flow that encircles the whole South Pacific, has been transporting more warm tropical water than normal into the Tasman Sea since late 1996. This probably explains why the whole Tasman Sea was anomalously warm in 1998. And the prospects for the 1998/99 summer? Figure 2 shows the mean SST anomalies over a region that includes the Tasman Sea and New Zealand waters as far east as the Chatham Islands. While the very strong positive anomaly that was present in February 1998 decayed rapidly, it has not returned to zero. Instead, it is clear that the region is again warming. This is typical of La Nina conditions, which have always been associated with positive anomalies in the New Zealand region. So it looks as though it is going to be another summer of warm sea temperatures, although they are unlikely to reach the extremes of last February. We may also expect more harmful algal blooms, as in the last La Nina summer of 1995.

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