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Glider measurements of the Solomon Sea: Towards monitoring of the equatorial-subtropical exchange

By William S. Kessler (NOAA/PMEL, Seattle, WA), Russ Davis (Scripps Institution of Oceanography, La Jolla, CA) and Lionel Gourdeau (Institut de Recherche pour le Developpement, Noumea, New Caledonia)

Just as wind blowing on the ocean sets the water in motion to produce currents, the currents also feed back to modify the winds. When currents push warmer or colder water from place to place, this heats or cools the overlying atmosphere and thus modifies the conditions that cause particular winds to blow. Though the currents move slowly, the amount of heat they transport can be a major contribution to the year-to-year variations in climate. NOAA therefore studies and monitors the ocean currents as a key piece of the climate system.

The circulation of the Pacific Ocean encompasses a great overturning cell in which cool, salty water sinks in the subtropics, flows at depth towards the equator, and upwells back to the surface in the eastern equatorial Pacific. This circulation pattern is why the equatorial ocean is much colder in the east than in the west.

Bathymetry of the Solomon Sea (depth scale in meters at right), with schematic circulation arrows (red). (larger image)

Variations of the circulation cell produce slow changes in the temperature of equatorial water, which can then influence the strength of El Niño events. Because El Niños, when sea-surface temperatures in the eastern Pacific become much warmer, affect weather of the entire Pacific and beyond, fluctuations of the overturning cell probably play a major role in the year-to-year and decade-to-decade variations in climate.

Observations suggest that perhaps 70% of equatorial upwelling due to the overturning cell originates from the South Pacific, with a large fraction of that arriving via the narrow boundary currents along the coast of New Guinea through the Solomon Sea. (Such powerful western boundary currents are analogous to the Gulf Stream, but flow towards the equator in the tropics).

Measuring the Solomon Sea currents, however, has lagged other elements of the circulation because the region is remote, expensive and difficult to work in, with strong narrow filaments of current flowing among a complex network of islands and reefs. As a result, there have been only sparse measurements that have barely outlined the variability. Producing a time series of this system is recognized as one of the most important challenges in gaining a full picture of the climate of the Pacific.

Diagram of the Spray glider showing its internal machinery, and a typical dive profile. (larger image)

Ocean gliders are newly-developed small autonomous vehicles based on Argo float technology, whose only propulsion is to pump oil in and out of an external bladder. This makes the glider sink and rise in the water, and with its wings it slowly glides forward (at about 1/5th walking speed). A glider typically dives to 600m or so every 3-4 hours, gliding about 25 kilometers per day, while reporting its data and receiving instructions by satellite each time it surfaces.

Although the glider moves very slowly, it stays in the water unattended for 4 to 6 months, so it can cover a substantial distance. It measures profiles of temperature and salinity, and the current velocity is inferred from the glider's motion. The glider has two important advantages over previous technology: it can be deployed and recovered entirely by small boats near shore, making the operations much cheaper and more flexible than a research ship, and it makes dense profiles right up to the coast, necessary for sampling narrow boundary currents.

The Spray glider used in this project is designed and built by the Instrument Development Group at Scripps, funded by NOAA's Climate Program Office, as a coastal and detailed-structure sampler for the climate observing system.

Routes of the two glider missions and measured currents (larger image)

A joint NOAA/Scripps/France program, with the Pacific Marine Environmental Laboratory in Seattle, WA, as the lead for NOAA, is deploying four glider missions in a continuous series to measure the Solomon Sea transport and water properties over one year. The first mission has been completed and the second is under way.

Early results show a powerful northward current close along the coast of Papua New Guinea and intertwined among its extensive reef system. In the eastern Solomon Sea, the glider measured a complex set of eddies that may be permanent or temporary, and the water properties hinted at a southward countercurrent below the surface. The second mission was just begun in November of 2007, and will repeat much of the first track to examine both the main branch's variability and the eddy structure. Readers interested in following the glider's progress can view regular updates of the latest observations.

Beyond the research value of providing the first measures of variability of this current system, the one-year series of regular instrument rotations tests the capability of the glider to be operated in a remote region as part of the routine climate monitoring network.

12/26/07

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