Glider observations of the North Equatorial Current in the western tropical Pacific

• Published in: Journal of geophysical research. Oceans 2015-05, Vol.120 (5), p.3586-3605
• Main Authors: Schönau, Martha C., Rudnick, Daniel L.
• Format: Article
• Language: English
• Subjects: Density; Eddies; Geophysics; Gliders; Horizontal; Intermediate water; North Equatorial Current; Ocean circulation; Ocean currents; Salinity; spice variance; Spices; Transport; underwater glider; Variance
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1002/2014JC010595

The North Equatorial Current (NEC) of the Pacific Ocean advects subtropical, subpolar, and tropical water masses. Repeat underwater glider observations of the NEC from June 2009 to January 2014 along 134.3°E provide absolute zonal geostrophic velocity, transport, and water mass structure at length scales of 10–1000 km. The NEC is strongest near the surface and persistent eastward undercurrents are identified deeper than potential density surface 26 kg m−3 at 9.6°N and 13.1°N. Mean transport from the surface to 27.3 kg m−3 and 8.5°N–16.5°N is 37.6 Sv (106 m3 s−1), with a standard deviation of 15.6 Sv. The transport variability is greatest deeper than 26 kg m−3 due to undercurrent variability. Wavelet analysis at scales of 10–80 km reveals extrema of fine‐scale salinity variance along isopycnals (spice variance). High spice variance is found in the North Pacific Tropical Water (NPTW) and the North Pacific Intermediate Water (NPIW), with a spice variance minimum between water masses at 25.5 kg m−3. A horizontal Cox number, CH, relates salinity variance at fine scales (10–80 km) to that at greater length scales (120–200 km). As a function of density, CH is nearly vertically uniform, indicating that the stirring of mean salinity gradients enhances fine‐scale salinity variance. NPTW, with an estimated horizontal eddy diffusivity of order 104 m s−2, is a useful tracer for the region and may be used to relate the fine‐scale salinity variance to an eddy diffusivity. Key Points: Persistent eastward undercurrents affect the transport variability of the NEC Subducted NPIW and NPTW have high fine‐scale spice variance The ratio of fine‐scale to large‐scale spice variance indicates horizontal stirring