ARCTIC CLIMATE OBSERVATIONS USING UNDERWATER SOUND (ACOUS)
Peter N. Mikhalevsky (Ocean Sciences Division, Science Applications International Corporatlon, 1710 Goodridge Drive, McLean, Virginia, USA)
e-mail: peter@osg.saic.com
ABSTACT
In October 1998 a 20Hz acoustic source was deployed from the Russian icebreaker, Akademic Fedorov in the Franz Victoria Trough between Spitzbergen and Franz Josef Land. At the same time an acoustic receive array was deployed in the Lincoln Sea approximately 1000kmsaway. These are the first installations of an acoustic thermometry grid for the Arctic Ocean as part of the Arctic Climate Opservations using Underwater Sound (ACOUS from the Greek "ακονζ" meaning "listen!"). The source is autonomous and is designed for a three-year life. The array is also autonomous and designed for an 18 month life. Travel time measurements are being used to measure the average Arctic Ocean temperature along the propagation path. Research is underway to use the acoustic attenuation changes to measure changes in average sea ice roughness and thickness, and the use of multiple frequencies to measure the depth of the thermocline and thus the thickness and stratification of the upper mixed layer in the Arctic Ocean. An acoustic network that is beginning under ACOUS will provide real-time, year-round, synoptic Arctic Ocean temperature, ice, and water column stratification data on spatial and temporal scales that are simply not possible by submarine, ice breaker or ice camp methods.
INTRODUCTION
Since the early 1990's inflow of warmer Atlantic Water into the Arctic Ocean has resulted in temperature increases in the Atlantic Layer that are continuing until today. Point measurements from icebreakers in 1991 and 1993 showed temperature increases of several tenths of a degree C over historical climatologies. In 1994 acoustic transmissions were made from a site north of the Svalbard Archipelago across the entire Arctic Ocean to receiver arrays located in the Lincoln Sea and the Beaufort Sea as part of the Transarctic Acoustic Propagation (TAP) Experiment [Mikhalevsky, et al., 1995ab, Mikhalevsky, et al., 1999] (Fig. 1). These travel time measurements revealed an average of. 4℃ increase in the Atlantic Layer. This was the first basin scale measurement of this large scale warming. The Arctic Ocean Section of the USCGS Polar Sea and the CCGS Louis S. St Laurent [Carmack, et al., 1995] conducted in Aug. 1994, and transects performed by the U.S. Navy SCICEX (Submarine Science Expedition) submarines (Fig. 1) confirmed these results. Whether these results are a manifestation of a secular global climate change trend with an anthropogenic fingerprint [Overpeck, et al., 1997], or a “natural” oscillation [Grotefendt et al 1998 Johnson et al., 1999] is an area of active research. Recent modeling has suggested that major shifts in the Arctic Ocean circulation occur on a decadel time scale [Proshutinsky and Johnson, 1997, Johnson, et al., 1999] between two dominant circulation regimes. They postulate that the recent warming is a manefestation of a shifi which is characterized by a weakening and contraction of the
