Acoustic remote sensing of large-scale temperature variability in the North Pacific Ocean

Large-scale, depth-averaged temperatures have been measured by long-range acoustic transmissions in the North Pacific Ocean for the past nine years. Acoustic sources located off central California and north of Kauai transmitted to receivers distributed throughout the North Pacific from 1996 through 1999 during the Acoustic Thermometry of Ocean Climate (ATOC) project. The Kauai transmissions resumed in early 2002 and are now continuing as part of the North Pacific Acoustic Laboratory (NPAL) project; a six-year time series has been obtained so far. Even at long time and large spatial scales the ocean is highly variable. The paths from Kauai to California show a modest cooling trend (longer travel times) until the present time. A path to the northwest showed modest warming and a weak annual cycle from 1999 until early 2003, when a strong annual cycle returned. In retrospect, these changes stemmed from the warming of the central Pacific that occurred in this interval, possibly associated with the Pacific Decadal Oscillation (PDO). Comparisons between measured travel times and those predicted using ocean models, constrained by satellite altimeter and other data, show significant similarities and differences. Comparison between upper-ocean Argo profiling float temperatures and the acoustically measured temperature along one path illustrates the strength of the integral measurements, with substantially lower uncertainty. The acoustic data ultimately need to be combined with sea-surface height Argo float data to determine the complementarity of the various data types. In particular, combining the acoustic and Argo data by inverse techniques will quantify the ability of the float data to resolve large-scale, upper-ocean heat content and the ability of the acoustic data to resolve abyssal temperature changes.