Jim L. Mitchell

Inference of subsurface thermohaline structure from fields measurable by satellite

Abstract Satellites now provide global measurements of the oceans surface height and temperature. Ocean climatologies for the northwest Pacific and northwest Atlantic Oceans that relate sea surface height, sea surface temperature, day of the year, latitude, and longitude to temperature and salinity profiles were produced using least-squares regression. These analyses use over 33 000 profiles of historical temperature and salinity data and are considerably streamlined and compacted by expressing each profile in terms of empirical orthogonal functions. Evaluations and error analyses of the climatologies, including a comparison to the navys Generalized Digital Environmental Model, were performed and differences between the regions are discussed. Two sample vertical sections are shown to be closely reproduced with the climatologies. Climatologies based on surface height and temperature are found to offer considerable improvement over climatologies based only on position.

Kuroshio Extension dynamics from satellite altimetry and a model simulation

Altimeter data from the Geosat Exact Repeat Mission (ERM) are analyzed with the aid of a simulation from an eddy-resolving primitive equation model of the North Pacific basin in the region of the Kuroshio and Kuroshio Extension. The model domain covers the Pacific Ocean north of 20°S and has a resolution of 0.125° latitude and 0.176° longitude. The model is synoptically driven by daily 1000-mbar winds from the European Centre for Medium-Range Weather Forecasts (ECMWF) which encompass the Geosat time period. Model output is sampled along Geosat ground tracks for the period of the ERM. Additionally, the model and the Geosat data are compared with climatological hydrography and satellite IR frontal position analyses. Analyses compared include maps of sea surface height (SSH) mean and variability, eddy kinetic energy (EKE), seasonal transport anomaly, and time-longitude plots of SSH anomaly. The model simulation provides annual mean SSH fields for 1987 and 1988 which reproduce the four quasi-permanent meanders seen in hydrographic climatology (cyclonic at 138°E and anticyclonic at 144°E, 150°E, and 160°E). These are linked to the bottom topography. In the model simulation, Geosat altimeter data, and climatology, we observe four peaks in SSH variability associated with meander activity and two peaks in EKE, with the strongest about 3200 cm2 s−2 along the mean Kuroshio path in the Geosat data. The local maxima in SSH variability tend to occur where relatively strong, topographically steered meridional abyssal currents intersect the zonally oriented Kuroshio Extension. Westward propagation of SSH anomalies at phase speeds of 2 to 3 cm s−1 in the region east of 155°E is observed in the model simulation and Geosat observations. A late summer maximum in the upper ocean transport anomaly of the Kuroshio Extension is inferred from changes in the cross-stream differential in SSH from the simulation and Geosat observations.

An examination of the North Pacific Ocean in the spectral domain using Geosat altimeter data and a numerical ocean model

The sea surface height (SSH) variations of the North Pacific ocean and the Kuroshio Extension region, in particular, are examined by frequency and wavenumber decompositions of a 1/8°, six-layer primitive equation Pacific Ocean model and of the Geosat Exact Repeat Mission (ERM) data. Both data sets exhibit peaks in variability at 1 and 2 cycles per year over much of the Kuroshio Extension region. This study is restricted to these two frequencies. Annual variations of equatorial currents in both data sets are similar in both space and time, with the variations in the South Equatorial Current appearing as annual westward propagations. Annual variations in the strength of the Kuroshio Extension are manifested mainly through changes in the strength of the recirculation gyres on the southern side of the current. Annual transport maxima for the Kuroshio Extension occur around late October for both the model and Geosat. Large-scale variations (length scales greater than 1000 km) of the model and Geosat have comparable amplitudes. The main differences between the model SSH and the Geosat ERM data occur over regions where seasonal steric variations are significant (from 20°N to 30°N). Wavenumber spectra over the Kuroshio Extension region reveal similar dynamics in both data sets. Much of the energy in wavenumber spectra appears as westward propagating SSH anomalies near the theoretical Rossby wave dispersion relations. As the Rossby wave dispersion relation changes with latitude (shifting to shorter wavelengths with higher latitudes), the peaks in the wavenumber decompositions follow. Thus the dynamics are generally consistent with quasi-geostrophic dynamics in both the model and altimeter data. Wavelengths of propagating SSH anomalies which have spectral peaks near the Rossby dispersion curve are longer in the Geosat and model than wavelengths indicated by theory. In the semiannual frequency below 35°N, westward propagation dominates over eastward propagation in both Geosat and the model. Most differences in the dynamics of the model and Geosat occur at shorter length and timescales, with Geosat showing higher amplitudes at the shorter scales than the model.

Kuroshio sea surface height fluctuations observed simultaneously with inverted echo sounders and TOPEX/POSEIDON

Sea surface height anomalies measured by inverted echo sounders with pressure gauges (IES/PGs) and TOPEX/POSEIDON are compared at six locations in the northwest Pacific near the Kuroshio where it separates from the Japanese coast. The instruments are positioned along a TOPEX/POSEIDON descending ground track near 35°N, 143°E. Good correlation is found between measurements, with best agreement near the Kuroshio, where the range of sea surface height change is largest. Correlations between IES/PG and TOPEX/POSEIDON sea surface height variations range from 0.70 to 0.97. Tides modeled and removed from the TOPEX/POSEIDON data at the inverted echo sounder sites compare well with tides measured by the IES/PGs. TOPEX/POSEIDON and IES/PG data are complementary in describing mesoscale variability in the Kuroshio region.

A survey of oceanographic satellite altimetric missions

Abstract This paper is a brief survey of past and future satellite altimetric missions. Several summary tables which quantify, in a relative sense, the demonstrated scientific return of past missions and the potential of future missions for studies using altimeter‐derived sea surface topography are presented.