Feature article

ARGO in the South Pacific

Dr Philip Sutton, NIWA

The ocean has a remarkable capacity to transport and store heat. With 2.5m depth of water having the same heat capacity as the entire depth of the atmosphere, even small changes in ocean temperature can have large impacts on climate. Observations of the distributions of heat and freshwater (through measuring changes in salinity) are essential for understanding the oceans’ role in climate and for forecasting climate and ocean conditions. A continual problem with understanding the role of the ocean in climate, and including the ocean state in climate predictions, is lack of ocean data. The ocean covers 75% of the surface of the earth and much of the ocean is inaccessible, making observations difficult and expensive.

Argo is an international observing project designed to address the lack of ocean data. Argo uses a global broad-scale array of profiling floats to measure the upper ocean. The eventual aim is have an array of 3000 floats: to date 1270 floats have been deployed by 17 countries (See Figure 1). Each float provides real-time measurements of the temperature and salinity of the upper ocean that will help forecast climate change and events like El Niño and improve the prediction of tropical cyclones. The data are freely available to anyone interested over the internet. The floats are somewhat ironically named because the first thing they do is sink! Each float sinks to a pre-determined depth of between 1000 and 2000 metres, where it is carried by the currents for nine days. It then sinks to 2000 metres before rising to the surface, measuring the temperature and salinity of the water as it ascends. Once on the surface, it transmits the profile data and its position via satellite before sinking and beginning the next cycle (See Figure 2). The floats have a design life of five years, over which time they should collect about 180 profiles of the upper ocean as well as provide information about the deep flow fields through their drift. Each float costs about $NZ 20,000.

So far, the South Pacific is relatively poorly populated with floats. The situation was vastly improved by a voyage by NIWA’s research vessel R/V Kaharoa in March/April 2004 which deployed 61 floats between New Zealand and Chile. These floats were deployed as part of a collaboration between the University of Washington, Scripps Institution of Oceanography (San Diego) and NIWA. Kaharoa is about to embark on a second collaborative voyage, this time from New Zealand, north through the Tasman Sea, then across to Tahiti for a port call. From Tahiti, Kaharoa will sail most of the way to Peru before returning to Tahiti and then finally, back to Wellington. Along the way 80 floats will be deployed, largely filling the gap in the tropical South Pacific.

Figure 1. Global status of Argo as of 6/7/2004. 1270 Floats.

Figure 2. Diagram of a float cycle.