Ocean Drifter Deployments from the NAI’A

This is blog entry posted from the field during the 2012 Phoenix Islands Marine Protected Area (PIPA) Expedition. The Phoenix Islands are an isolated island chain more than 1,000 miles southwest of Hawaii. They are part of the island nation of Kiribati, which partnered with the New England Aquarium and Conservation International to create PIPA in 2008. Today it is one of the world's largest marine protected areas and a UNESCO world heritage site. This voyage is part of a regular series of scientific expeditions to investigate coral health and study ecosystems and biodiversity.

This post is by Ben Hodges, from WHOI.

Surface currents in the world’s oceans are like highways, transporting nutrients, plankton, and other water properties, and are part of a changing, coupled climate system. In order to understand how different parts of the ocean are connected, to predict the events we might expect in the future, and to monitor the changes our planet is undergoing, we need to map these surface currents.  Drifters are oceanographic instruments designed to accurately measure surface currents by mimicking the motion of a single parcel of seawater. A drifter is composed of two parts: a surface float containing a transmitter, and a drogue, hanging below, which is a hollow tube of canvas 60 cm in diameter and a few meters long. The transmitter regularly relays the position of the drifter to scientists on land via satellite, and the drogue ensures that the drifter accurately follows a water parcel, unaffected by the wind above. A drifter can continuously report positions for approximately two years.

Ben Hodges, researcher at Woods Hole Oceanographic Institution,
describes the the science behind ocean drifters (Photo: K. Ellenbogen)

During the 2012 PIPA cruise aboard the NAI’A, several drifters will be deployed along the eastern rim of the Phoenix Islands archipelago. The prevailing westward current will likely carry these drifters near downstream islands, providing multiple realizations of the paths taken by water (and everything suspended in it) as it flows from island to island. With these current measurements, we hope to better understand how the transport of biota influences the connectivity of the islands’ ecosystems.

Ben Hodges checks drifter function prior to deployment. (Photo: K. Ellenbogen)

The PIPA cruise also provides a unique opportunity to study ocean currents over a broader area as well. Drifters aren’t the only game in town when it comes to measuring surface currents. By observing sea surface height from satellites, scientists can estimate surface currents over most of the globe (a relationship known as geostrophy), without ever leaving their cozy offices. But there’s no substitute for drifters when it comes to accurately following a parcel of near-surface water.

Deploying  a drifter from the NAI’A on 5 June 2012 during the transit from
Apia, Samoa, to the Phoenix Islands, Kirbati (Photo: K. Ellenbogen)

This is particularly true near the equator—geostrophy works better at higher latitudes. Ironically, the equatorial Pacific, one of the places where drifters can contribute the most to our knowledge of ocean currents, is also one of the hardest places to maintain a population of drifters. It is less well travelled than many other parts of the world ocean, and because of the divergence of surface currents near the equator, drifters that are deployed there usually don’t stay long. That’s where we come in. During the transit of the NAI’A from Samoa to the Phoenix Islands a drifter was deployed at each degree of latitude, seeding the southern equatorial Pacific with instruments that will help fill in a perennial gap in the existing array. While waking up at 3:00 a.m. to arm, test, and deploy a drifter in dark, pitching seas can certainly be a hassle, the detailed information about currents in such a remote part of the world more than makes up for the challenges

Map of drifter deployments planned for the 2012 NAI’A PIPA cruise. Also shown are: bathymetry (color); average currents during the month of June, calculated from satellite measurements of sea surface height (arrows);
and the planned arrival date of the NAI’A at each site.

-Ben Hodges

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