Abstract
This article reviews the progress in research on North Pacific mode waters of the past decade from the physical oceanographic perspective. The accumulation of satellite altimeter sea surface height data, the rapid growth of the Argo profiling float array, and the advancement in eddy-resolving ocean general circulation models have greatly improved the traditional views on the mode waters that were formed prior to the 1990s based on the analyses of historical temperature/salinity data. Areas where significant progress was made include: (1) descriptions of the mode waters’ distributions and properties with fine spatial scales, particularly in their formation regions in winter where observational data had been insufficient; (2) clarifications of the mode waters’ formation and subduction processes relating to the large-scale mean circulation, as well as to the time-varying mesoscale eddy field; (3) impacts of the mode waters’ circulation and dissipation processes on the climate and biogeochemical processes; and (4) dynamic versus thermodynamic causes underlying the mode waters’ decadal changes. In addition to the review, future directions for mode water research are also presented.
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Notes
The term “mode water,” introduced by Masuzawa (1969), was subsequently applied to any thick, broadly distributed, near-surface layer characterized by low PV (Hanawa and Talley 2001). Therefore, mode waters nowadays are not necessarily characterized by uniformity in terms of temperature and salinity. In other words, temperature and salinity might be stratified in a compensating way within a pycnostad of mode waters.
In this review, the term “subarctic front” is used for a density-compensating front between the warmer, saltier water in the subtropics and the colder, fresher water in the subarctics, characterized by the outcrop of the 33.0–33.8 isohalines (Roden 1970, 1972; Zhang and Hanawa 1993; Yuan and Talley 1996), as in the literature of physical oceanography. This front often separates into two or more fronts, particularly in the eastern North Pacific, and is also called the subarctic frontal zone. In the literature of fisheries oceanography (e.g., Favorite et al. 1976; Yasuda 2003), the term “subarctic front” is referred to the front at the southern boundary of subsurface temperature inversions characterizing the subarctics, represented by the 4°C isotherm standing almost vertically below the 100-m depth (Uda 1963; Favorite et al. 1976). This front is called the “polar front” in this review, as in the literature of physical oceanography.
An intense hydrographic survey conducted in spring 2003 indicates that the subarctic front and the polar front coincide with each other in this longitude range (Eitarou Oka, personal communication 2011).
In Yasuda (2003), TRMW is referred to as “Dense Central Mode Water.”
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Acknowledgments
The authors thank Atsushi Kubokawa for the opportunity to write this review and two anonymous reviewers for helpful comments on the manuscript. This review is based on discussions by the authors in summer 2010 when EO visited the University of Hawaii at Manoa for 1 month under the Overseas Internship Program for Outstanding Young Earth and Planetary Researchers provided by the Department of Earth and Planetary Science, the University of Tokyo. EO is also supported by the Japan Society for Promotion of Science [KAKENHI, Grant-in-Aid for Scientific Research (B), no. 21340133] and the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT; Grant-in-Aid for Scientific Research on Innovative Areas under grant no. 22106007). BQ is supported by NSF through grant OCE-0926594 and NASA through contract 1207881 as part of NASA’s Ocean Surface Topography Mission.
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Oka, E., Qiu, B. Progress of North Pacific mode water research in the past decade. J Oceanogr 68, 5–20 (2012). https://doi.org/10.1007/s10872-011-0032-5
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DOI: https://doi.org/10.1007/s10872-011-0032-5