The oceanographic measurement system began recording near-surface data on 15 September 2004 aboard the Tustumena.  View a movie of changing salinity and temperature along the ferry route (animated gif and QuickTime format), or scroll down through maps showing data, and Timeseries Plots.  Timeseries data are from Septembr 15, 2004 to July, 2006.

The next two maps show the near-surface temperature along Tustumena's route for a few days in August 2005 and 2006. 2005 is clearly warmer than 2006. This pattern persists over the two summers along the Alaska Coastal Current's (ACC) axis from the Kenai Peninsula ports of Seward and Homer to Dutch Harbor. These results show that near-surface monitoring from an Alaskan ferry can detect a climate signal in the Gulf of Alaska. The ferry has the advantage of covering a large area; therefore the results are truly regional and applicable to the ACC in general, not just to a particular local site.

The following time series of near-surface ocean temperature from 15 September 2004 to 28 September 2006 was measured along Tustumena's route. The high-frequency variations (red line) come from spatial differences along the track as the ship moves between confined and open waters. Local temperature extremes occur in bays such as Kachemak Bay (near Homer), Resurrection Bay (near Seward) and Prince William Sound. Sheltered from winds and subject to freshwater inflow and ice from rivers and glaciers, the confined surface waters cool in winter to form ice and warm in summer in thin, stratified layers. The 30-day-averaged (blue line) annual cycle of cooling and warming is shown over two years. With more than one year of data, it is possible to compare years and detect interannual differences. The observations show that 2005 was a warm year compared to 2006. At its peak, the 30-day-averaged temperature in August was 2.3°C warmer in 2005. Recent work by W. Crawford (Recent trends in waters of the subarctic NE Pacific, PICES Press, vol. 14, no. 2, 24-25, July 2006) using Argo profiling floats shows that 2005 was an anomalously warm year in the central Gulf of Alaska as well. This warming began in 2002 as observed in the Pacific Decadal Oscillation (PDO) climate index. The PDO entered its cold phase in late 2005, and the cooling we observe between 2005 and 2006 is consistent with that.

The time series of near-surface ocean salinity measured along Tustumena's track is dominated by high-frequency variations (blue line) due to spatial differences as the ship moves between bays and the open ocean. Fresher surface water occurs near river mouths and glaciers when temperatures are warm enough for melting. During winter, salinity variability is reduced because freezing binds the fresh water into ice. The 30-day-average observations (red line) show that summer 2005 was fresher than summer 2006. This is consistent with warming and more ice melt in 2005.

Nitrate is an essential nutrient for the growth of phytoplankton in the ocean. A nitrate meter based on optical principles and requiring no chemical reagents was installed aboard Tustumena in 2004. It quickly fouled with an organic-iron compound that blocked the ultraviolet light beam required to make the measurements. After a technique to clean this invisible film was found in July 2005, the nitrate meter began making reliable nitrate measurements. In the following figure, the high-frequency variations of the green curve are due to spatial differences as the ship moves from bays into the open ocean. The smooth red curve represents a monthly running average. Nitrate increases through the winter as nutrient- rich water is mixed up by storms from depth. In summer, storm-mixing abates, and nitrate decreases as it is utilized by phytoplankton for growth. These are preliminary results awaiting laboratory calibration from water samples collected along the ship track.

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