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Arthur J. Miller

Abstract

Free oscillations in square, midlatitude basins with continental shelves and planetary vorticity gradients are numerically computed using the nondivergent shallow-water equations. The topography may rend a planetary mode into a family of basinwide modes, each comparable to the flat-bottom counterpart in frequency and midbasin structure. This phenomenon can be interpreted in terms of coupled planetary wave-shelf wave oscillations. The mechanism provides an alternative to strong dissipation in explaining broadbanded planetary-wave signals signals observed in tide guage records.

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Arthur J. Miller

Abstract

Forced, nonresonant barotropic response at low frequencies (ω ≪ f) and large scales (Lf/β) can be written in terms of a streamfunction, which is similar to the quasigeostrophically derived streamfunction. However, the “nearly equilibrium” forced vorticity equation is valid on the planetary length scale and is influenced not only by the vortex stretching induced by the driving mechanism (tides, atmospheric pressure, or Ekman-pumping displacement) but also by β coupling to the divergent velocity field of the nearly equilibrium response. A similar result follows for topographic coupling, albeit on the topographic length scale.

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Arthur J. Miller

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Midlatitude ocean-atmosphere interactions are studied in simulations from a simplified coupled model that includes synoptic-scale atmospheric variability, ocean current advection of sea surface temperature (SST), and air-sea heat exchange. Although theoretical dynamical (“identical twin”) predictions using this model have shown that the SST anomalies in this model indeed influence the atmosphere, we find here that standard cross-correlation and empirical orthogonal function analyses of monthly mean model output yield the standard result, familiar from observational studies, that the atmosphere forces the ocean with little or no feedback. Therefore, these analyses are inconclusive and leave open the question of whether anomalous SST is influencing the atmosphere. In contrast, we find that compositing strong warm events of model SST is a useful indicator of ocean forcing the atmosphere. We present additional evidence for oceanic influence on the atmosphere, namely, that ocean current advection appears to enhance the persistence of model SST anomalies through a feedback effect that is absent when only heat flux is allowed to influence SST anomaly evolution. Models with more complete physics must ultimately be used to conclusively demonstrate these results.

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Niklas Schneider and Arthur J. Miller

Abstract

It is shown that wintertime sea surface temperature anomalies in the confluence region of the Kuroshio–Oyashio Currents in the western North Pacific can be skillfully predicted at lead times of up to 3 yr. The predictions are based on the history of the wind stress over the North Pacific and oceanic Rossby wave dynamics. The predictions may be exploitable in fisheries research and other ecological applications.

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Hey-Jin Kim and Arthur J. Miller

Abstract

The 55-yr California Cooperative Oceanic Fisheries Investigations (CalCOFI) dataset in the southern California Current reveals a significant surface-intensified warming and stratification (buoyancy frequency) change across the 1976/77 climate regime shift. However, the average depth of the thermocline, defined as the maximum gradient of temperature, did not change significantly across the regime shift. The maximum-gradient criterion for thermocline depth may be more appropriate than following an isotherm because the isotherm necessarily deepens in the presence of surface-intensified warming. As the surface heating changed the strength of stratification, it also changed the slope of the nitrate–temperature relation for the middepth waters (roughly 30–200 m). Thus, the quality of upwelled water may have been fundamentally altered after the shift.

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Arthur J. Miller and John O. Roads

Abstract

A simplified coupled atmosphere-ocean model is used to explore the influence of evolving midlatitude sea surface temperature (SST) anomalies on the theoretical extended-range predictability of the atmospheric wintertime circulation in the Northern Hemisphere. After approximately two weeks, SST anomalies begin to significantly influence the overlying atmospheric flow, compared to flow over the climatological SST field. If the evolving sea surface temperature field is specified from model “observed” flows, then predictions of atmospheric time-averaged flow, for one month and longer averages, are significantly enhanced over predictions based on the atmospheric model with climatological SST. Predictions using the coupled model, however, are not significantly different from predictions using the atmospheric model with persistent SST anomalies, because SST anomalies are forced increasingly erroneously by atmospheric variables that rapidly lose their predictability.

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Arthur J. Miller, Pierre F. J. Lermusiaux, and Pierre-Marie Poulain

Abstract

An array of current meter moorings along 12°W on the southern side of the lceland-Faeroe Ridge reveals a narrowband barotropic oscillation with period 1.8 days in spectra of velocity. The signal is coherent over at least 55-km scales and propagates phase with shallow water on the right (toward the northwest). Velocity ellipses tend to be elongated (crossing contours of f/H) and rotate anticyclonically. Solutions of the rigid-lid barotropic shallow-water equations predict the occurrence of a topographic-Rossby normal mode on the south side of the ridge with spatial scales exceeding 250 km and with intrinsic period near 1.84 days. This fundamental mode of the south side of the ridge has predicted spatial structure, phase propagation, and velocity ellipses consistent with the observed oscillation. The frictional amplitude e-folding decay time for this normal mode is estimated from the observations to be 13 days. The observed ocean currents are significantly coherent with zonal wind stress fluctuations (but not with wind stress curl) in the relevant period band, which indicates the oscillation is wind forced. This appears to be the first clear evidence of a stochastically forced resonant barotropic topographic-Rossby normal mode in the ocean.

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Antonietta Capotondi, Michael A. Alexander, Clara Deser, and Arthur J. Miller

Abstract

The output from an ocean general circulation model (OGCM) driven by observed surface forcing is used in conjunction with simpler dynamical models to examine the physical mechanisms responsible for interannual to interdecadal pycnocline variability in the northeast Pacific Ocean during 1958–97, a period that includes the 1976–77 climate shift. After 1977 the pycnocline deepened in a broad band along the coast and shoaled in the central part of the Gulf of Alaska. The changes in pycnocline depth diagnosed from the model are in agreement with the pycnocline depth changes observed at two ocean stations in different areas of the Gulf of Alaska. A simple Ekman pumping model with linear damping explains a large fraction of pycnocline variability in the OGCM. The fit of the simple model to the OGCM is maximized in the central part of the Gulf of Alaska, where the pycnocline variability produced by the simple model can account for ∼70%–90% of the pycnocline depth variance in the OGCM. Evidence of westward-propagating Rossby waves is found in the OGCM, but they are not the dominant signal. On the contrary, large-scale pycnocline depth anomalies have primarily a standing character, thus explaining the success of the local Ekman pumping model. The agreement between the Ekman pumping model and OGCM deteriorates in a large band along the coast, where propagating disturbances within the pycnocline, due to either mean flow advection or boundary waves, appear to play an important role in pycnocline variability. Coastal propagation of pycnocline depth anomalies is especially relevant in the western part of the Gulf of Alaska, where local Ekman pumping-induced changes are anticorrelated with the OGCM pycnocline depth variations. The pycnocline depth changes associated with the 1976–77 climate regime shift do not seem to be consistent with Sverdrup dynamics, raising questions about the nature of the adjustment of the Alaska Gyre to low-frequency wind stress variability.

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Jon M. Nese, Arthur J. Miller, and John A. Dutton

Abstract

A low-order moist general circulation model of the coupled ocean-atmosphere system is reexamined to determine the source of short-term predictability enhancement that occurs when an oceanic circulation is activated. The predictability enhancement is found to originate predominantly in thermodynamic processes involving changes in the mean hydrologic cycle of the model, which arise because the mean sea surface temperature is altered by the oceanic circulation. Thus, time-dependent sea surface temperature anomalies forced by anomalous geostrophic currents in the altered mean conditions do not contribute to the dominant ocean-atmosphere feed-back mechanism that causes the predictability enhancement in the model.

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Niklas Schneider, Arthur J. Miller, Michael A. Alexander, and Clara Deser

Abstract

Observations of oceanic temperature in the upper 400 m reveal decadal signals that propagate in the thermocline along lines of constant potential vorticity from the ventilation region in the central North Pacific to approximately 18°N in the western Pacific. The propagation path and speed are well described by the geostrophic mean circulation and by a model of the ventilated thermocline. The approximate southward speed of the thermal signal of 7 mm s−1 yields a transit time of approximately eight years. The thermal anomalies appear to be forced by perturbations of the mixed layer heat budget in the subduction region of the central North Pacific east of the date line. A warm pulse was generated in the central North Pacific by a series of mild winters from 1973 to 1976 and reached 18°N around 1982. After 1978 a succession of colder winters initiated a cold anomaly in the central North Pacific that propagated along a similar path and with a similar speed as the warm anomaly, then arrived in the western tropical Pacific at 18°N around 1991. Tropical Ekman pumping, rather than further propagation of the midlatitude signal, caused the subsequent spread into the equatorial western Pacific and an increase in amplitude. Historical data show that anomalous sea surface temperature in the equatorial central Pacific is correlated with tropical Ekman pumping while the correlation with thermal anomalies in the North Pacific eight years earlier is not significant. These results indicate no significant coupling in the Pacific of Northern Hemisphere midlatitudes and the equatorial region via advection of thermal anomalies along the oceanic thermocline.

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