U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications

The Pacific Subsurface Countercurrents and an Inertial Model

Gregory C. Johnson and Dennis W. Moore

National Oceanic and Atmospheric Administration/Pacific Marine Environmental Laboratory, Seattle, Washington

Journal of Physical Oceanography, 27, 2448-2459
Not subject to U.S. copyright. Published in 1997 by the American Meterological Society.

2. Data and data reduction

For this study, CTD stations within different areal bins in the tropical Pacific are averaged to make a mean hydrographic profile for each bin (Fig. 3: the dots indicate the locations of mean hydrographic profiles near the bin centers). In latitude, the bins are centered on every 1 from 20S to 20N. Bin widths are 1 in latitude, and hence include CTD stations halfway between integer latitudes. In longitude, the bins are centered on the TAO array mooring longitudes (McPhaden 1993), with 85W and 126E added as central longitudes for the far eastern and western Pacific. Bin widths ranging from 9 to 15 of longitude result from bin edges located midway between these nominal longitudes. This strategy yields 13 mean meridional hydrographic sections, some better sampled than others, which are used to make maps of properties on and between neutral density anomaly, The CTD station data within each bin, consisting of salinity, S, and temperature, T, as a function of pressure, P, are averaged as a function of n, surfaces, and meridional property sections at 165E, 155W, and 110W, the three best-sampled longitudes. CTD stations include all high-resolution data available from the NODC archives, as well as those in the PMEL database from PMEL and some other cruises not yet available from NODC. Nearly 14,000 individual CTD stations are used, located between 20.5S and 20.5N latitude and between 120E and 80W longitude, taken from 1967 through 1996.

The CTD station data within each bin, consisting of salinity S and temperature T as a function of pressure P are averaged as a function of n to create mean hydrographic profiles. Averaging S, T, and P as functions of n is more involved than the conventional approach of averaging S, T, and n as functions of P, but the technique better preserves water properties in the sharp tropical pycnocline (Gouriou and Toole 1993). First, to reduce small-scale noise, individual CTD station S and T profiles are filtered in P with a 10-dbar half-width Hanning filter and subsampled at 10-dbar intervals. Then n is computed for these subsampled data and averaged as a function of P to obtain a mean n(P) profile at 10-dbar resolution. Following this step, the individually filtered and sampled profiles of S, T, and P are linearly interpolated to each n(P) profile value to allow averaging of the individual CTD station data as a function of n at roughly 10-dbar resolution. Mean profiles for S(n), T(n), and P(n) are then calculated. Finally the mean P(n) profile is used to put the mean profiles of S(n) and T(n) onto an even 10-dbar grid and construct a final mean n profile.

Depth and properties of the surface mixed layer, including n, vary over time. To construct a mixed layer for the mean profiles of S(n), T(n), and n, the following procedure is used. First, the mixed-layer P for each CTD station is defined as the P above which n is less than 0.1 kg m-3 denser than the mean n of the top 10 dbar. Mean S and T from the surface to the mixed-layer P are calculated for each CTD station. Then these S and T values are averaged, weighted by the individual mixed-layer P's so that deeper mixed-layer values get more weight, to find mean mixed-layer values for S and T. These values are used with the mean mixed-layer P to calculate a mean mixed-layer n. In order to avoid n inversions in the mean profiles, the pressure of the mean mixed-layer n in the final mean n profile is used to define a final mean mixed-layer P. To finish, the mean mixed-layer S, T, and n values are substituted into the mean S(n), T(n), and n profiles from the mean mixed-layer P to the surface.

The resulting 10-dbar mean hydrographic profiles are used exclusively in the analysis. The only further smoothing performed for the mean meridional sections presented is on the square of the buoyancy frequency, N2, which is calculated from the mean hydrographic profiles and then smoothed with a 30-dbar half-width Hanning filter before use. Isopycnal maps are made by linearly interpolating the 10-dbar values of each mean hydrographic profile to the appropriate n. These values are then objectively mapped assuming a Gaussian covariance with correlation length scales of 1.5 lat and 18 long and an error energy of 0.04. These correlation length scales are roughly 1.5 times the data separation, with anisotropy appropriate for the interior of the Tropics, where zonal scales greatly exceed meridional scales.

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