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Matthew Dzieciuch, Walter Munk, and Andrew Forbes

Abstract

We have recorded time series of ship GPS (global positioning system) positions at about 10-s intervals under favorable satellite conditions. Errors are less than 10 m rms, as demonstrated by comparing the GPS acoustic-source positions with Doppler-inferred positions from a receiver site 9200 km away. Offsets from a steady course held for 20 miles are of the order of 100 m, and could be the result of orbital surface velocities associated with internal waves. We suggest that such GPS measurements, taken routinely while under way, might produce useful data concerning small-scale oceanographic processes.

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Walter Munk, Matthew Dzieciuch, and Steven Jayne

Abstract

Orbital forcing has long been the subject of two quite separate communities: the tide community is concerned with the relatively rapid gravitational forces (periods up to 18.6 yr) and the climate community with the long-period Milankovitch insolation terms (exceeding 20 000 yr). The wide gap notwithstanding, the two subjects have much in common. Keeling and Whorf have proposed that the millennial climate variability is associated with high-frequency tidal forcing extending into the 10-octave gap by some nonlinear process. Here, the authors distinguish between two quite distinct processes for generating low frequencies: (i) the “traditional” analogy with eclipse cycles associated with near coincidence of the appropriate orbital alignment of the Sun, the Moon, and Earth, and (ii) sum and differences of tidal frequencies and their harmonics producing low beat frequencies. The first process is associated with long time intervals between extreme tides, but the events are of short duration and only marginally higher than conventional high tides. With proper nonlinearities, (ii) can lead to low-frequency tidal forcing. A few candidate frequencies in the centurial and millennial band are found, which prominently include the Keeling and Whorf forcing at 1795 yr. This is confirmed by a numerical experiment with a computer-generated tidal time series of 275 000 yr. Tidal forcing is very weak and an unlikely candidate for millennial variability; the Keeling and Whorf proposal is considered as the most likely among unlikely candidates.

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Ganesh Gopalakrishnan, Bruce D. Cornuelle, Matthew R. Mazloff, Peter F. Worcester, and Matthew A. Dzieciuch

Abstract

A strongly nonlinear eddy field is present in and around the Subtropical Countercurrent in the Northern Philippine Sea (NPS). A regional implementation of the Massachusetts Institute of Technology general circulation model–Estimating the Circulation and Climate of the Ocean four-dimensional variational (MITgcm-ECCO 4DVAR) assimilation system is found to be able to produce a series of two-month-long dynamically-consistent optimized state estimates between April 2010 and April 2011 for the eddy-rich NPS region. The assimilation provides a stringent dynamical test of the model, showing that a free run of the model forced using adjusted controls remains consistent with the observations for two months. The 4DVAR iterative optimization reduced the total cost function for the observations and controls by 40–50% from the reference solution, initialized using the Hybrid Coordinate Ocean Model 1/12° global daily analysis, achieving residuals approximately equal to the assumed uncertainties for the assimilated observations. The state estimates are assessed by comparing with assimilated and withheld observations and also by comparing one-month model forecasts with future data. The state estimates and forecasts were more skillful than model persistence and the reference solutions. Finally, the continuous state estimates were used to detect and track the eddies, analyze their structure, and quantify their vertically-integrated meridional heat and salt transports.

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