Semi-daily fluctuations in geopotential off the Washington coast

Fluctuations in geopotential anomaly at eight 12-hr time-series stations off Washington appeared to result from semi-daily internal tides propagating seaward from the shelf break. Comparison of the observations with computations suggests the existence of a first mode wave. The oscillations did not appreciably decay offshore, perhaps as a result of bathymetric focusing

Free oscillations in a beta-plane ocean

This paper is concerned with the free oscillations in a rectangular equatorial beta-plane model of large oceans with a free surface placed symmetrically on the equator. With the depth constant and the stratification horizontally uniform, the motion may be separated into vertical modes, each having free oscillations...

MODE Bottom Experiment

Pressure fluctuations on the deep seafloor at frequencies below inertial and tidal have been measured. Between 0.1 and 1 cycle per day the variance is about 2 mb2, spectra diminish with increasing frequency as ω−n, n=1.5 to 2, and a signal-to-instrument noise ratio of 10 dB is achieved. Fluctuations are in phase and highly coherent within the MODE area (\u3e0.95 at 200 km) and even with inferred (atmosphere plus sea level) Bermuda subsurface pressures (0.8 at 700 km). Station differences (to which MODE-sized eddies would make the principal contribution) are relatively small. The large horizontal scale of the recorded bottom pressure fluctuations resembles that of atmospheric pressure, yet the coherence locally between atmospheric and bottom pressure is slight; the recorded fluctuations may be related to a barotropic ocean response to a variable wind stress on the subtropical gyre. Bottom temperature records show “sudden” (1 day) changes of order 30 millidegrees Celcius separated by long intervals (20 days) of uniform temperatures. The changes are much larger than have been observed in the Pacific. They are correlated at horizontal separations of 2 km, but uncorrelated to bottom pressure and to temperatures 1 km above the seafloor

MODE Tides

IGPP and AOML bottom pressure measurements at four MODE stations constitute a unique set of deep-sea tidal measurements (although deployed for other purposes). A response analysis relative to a Bermuda reference has been optimized with regard to the number of complex weights and the makeup of gravitational and radiational inputs. Duplicate instrumentation on EDIE capsule gave 32.067, 2.5° 32.074, 2.6° for M2 amplitude (cm) and Greenwich epoch, thus attesting the reality of measured small station differences (order 1 cm, 1°). M2 tidal currents (calculated from the M2 surface and bottom slopes) have u and v speeds of 0.5 and 0.8 em s−1, respectively, in rough agreement (both amplitude and phase) with preliminary estimates from current measurements. M2 and K1 tides are in accord with sonic existing cotidal and co-range charts. M2 tides are a fraction of equilibrium magnitude, whereas M4, M5 and M6 (typically 0.07, 0.05, O.03 cm) vastly exceed equilibrium values. Presumably these overtides are generated by nonlinear coupling in the world\u27s shallow basins, from where they radiate into the global oceans to attain a level where radiative and dissipative processes are somehow balanced

Interpretation of North Pacific Variability as a Short- and Long-Memory Process*

A major difficulty in investigating the nature of interdecadal variability of climatic time series is their shortness. An approach to this problem is through comparison of models. In this paper we contrast a first order autoregressive (AR(1)) model with a fractionally differenced (FD) model as applied to the winter averaged sea level pressure time series for the Aleutian low (the North Pacific (NP) index), and the Sitka winter air temperature record. Both models fit the same number of parameters. The AR(1) model is a ‘short memory ’ model in that it has a rapidly decaying autocovariance sequence, whereas an FD model exhibits ‘long memory ’ because its autocovariance sequence decays more slowly. Statistical tests cannot distinguish the superiority of one model over the other when fit with 100 NP or 146 Sitka data points. The FD model does equally well for short term prediction and has potentially important implications for long term behavior. In particular, the zero crossings of the FD model tend to be further apart, so they have more of a ‘regime’-like character; a quarter century interval between zero crossings is four times more likely with the FD than the AR(1) model. The long memory parameter δ for the FD model can be used as a characterization of regime-like behavior. The estimated δs for the NP index (spanning 100 years) and the Sitka time series (168 years) are virtually identical, and their size implies moderate long memory behavior. Although the NP index and the Sitka series have broadband low frequency variability and modest long memory behavior, temporal irregularities in their zero crossings are still prevalent. Comparison of the FD and AR(1) models indicates that regime-like behavior cannot be ruled out for North Pacific processes. 2 1

Influence of topography on tide propagation and amplification in semi-enclosed basins

An idealized model for tide propagation and amplification in semi-enclosed rectangular basins is presented, accounting for depth differences by a combination of longitudinal and lateral topographic steps. The basin geometry is formed by several adjacent compartments of identical width, each having either a uniform depth or two depths separated by a transverse topographic step. The problem is forced by an incoming Kelvin wave at the open end, while allowing waves to radiate outward. The solution in each compartment is written as the superposition of (semi)-analytical wave solutions in an infinite channel, individually satisfying the depth-averaged linear shallow water equations on the f plane, including bottom friction. A collocation technique is employed to satisfy continuity of elevation and flux across the longitudinal topographic steps between the compartments. The model results show that the tidal wave in shallow parts displays slower propagation, enhanced dissipation and amplified amplitudes. This reveals a resonance mechanism, occurring when\ud the length of the shallow end is roughly an odd multiple of the quarter Kelvin wavelength. Alternatively, for sufficiently wide basins, also Poincaré waves may become resonant. A transverse step implies different wavelengths of the incoming and reflected Kelvin wave, leading to increased amplitudes in shallow regions and a shift of amphidromic points in the direction of the deeper part. Including the shallow parts near the basin’s closed end (thus capturing the Kelvin resonance mechanism) is essential to reproduce semi-diurnal and diurnal\ud tide observations in the Gulf of California, the Adriatic Sea and the Persian Gulf