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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

Abstract

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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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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Arthur J. Miller, Warren B. White, and Daniel R. Cayan

Abstract

The North Pacific thermocline (250 to 400 m) is studied using XBT observations acquired during the 1970s and 1980s. Interannual variations (3–5 yr timescales) in thermocline temperature, with O(0.1°C) amplitude at 400 m, are found to exhibit westward propagation throughout the extratropical North Pacific up to 45°N. Southward of 30°N, the features propagate intact across the basin from the eastern boundary to the western boundary. Northward of 30°N, the features can be observed to propagate only as far as the date line. The observed midlatitude thermocline anomalies are often related to tropical ENSO events in that they occur most strongly after the development of tropical El Niño or La Niña conditions and propagate westward from near the eastern boundary in the midlatitudes. But it is found that the observed midlatitude thermocline anomalies have larger phase speeds than theoretically predicted free baroclinic Rossby waves. Also, the observed anomalies have larger wavelength and faster propagation speeds than baroclinic Rossby waves that radiate from coastal Kelvin-like waves near the eastern boundary in well-known high-resolution models.

Large-scale thermocline fluctuations that have spatial scale and phase speeds similar to the observations are also found in a coarse-resolution model of the Pacific Ocean forced by observed wind and heat flux anomalies over the 1970–88 period. In the midlatitudes, north of 30°N, large-scale Ekman pumping by interannual wind stress curl variations provides a significant driving mechanism for the modeled large-scale thermocline anomalies. The modeled ocean response is a combination of the static thermocline response to large-scale Ekman pumping plus a train of westward traveling Rossby waves, which accounts for part of the propagating temperature fluctuations. A tropical, remotely forced component is prominant near the eastern boundary, but this only contributes weakly in the model open ocean.

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Niklas Schneider, Arthur J. Miller, and David W. Pierce

Abstract

A systematic analysis of North Pacific decadal variability in a full-physics coupled ocean–atmosphere model is executed. The model is an updated and improved version of the coupled model studied by Latif and Barnett. Evidence is sought for determining the details of the mechanism responsible for the enhanced variance of some variables at 20–30-yr timescales. The possible mechanisms include a midlatitude gyre ocean–atmosphere feedback loop, stochastic forcing, remote forcing, or sampling error.

Decadal variability in the model is expressed most prominently in anomalies of upper-ocean streamfunction, sea surface temperature (SST), and latent surface heat flux in the Kuroshio–Oyashio extension (KOE) region off Japan. The decadal signal off Japan is initiated by changes in strength and position of the Aleutian low. The atmospheric perturbations excite SST anomalies in the central and eastern North Pacific (with opposing signs and canonical structure). The atmospheric perturbations also change the Ekman pumping over the North Pacific, which excites equivalent barotropic Rossby waves that carry thermocline depth perturbations toward the west. This gyre adjustment results in a shift in the border between subtropical and subpolar gyres after about five years. This process consequently excites SST anomalies (bearing the same sign as the central North Pacific) in the KOE region. The SST anomalies are generated by subsurface temperature anomalies that are brought to the surface during winter by deep mixing and are damped by air–sea winter heat exchange (primarily latent heat flux). This forcing of the atmosphere by the ocean in the KOE region is associated with changes of winter precipitation over the northwestern Pacific Ocean. The polarity of SST and Ekman pumping is such that warm central and cool eastern Pacific anomalies are associated with a deep thermocline, a poleward shift of the border between subtropical and subpolar gyres, and warm SST anomalies and an increase of rain in the KOE region.

The preponderance of variance at decadal timescales in the KOE results from the integration of stochastic Ekman pumping along Rossby wave trajectories. The Ekman pumping is primarily due to atmospheric variability that expresses itself worldwide including in the tropical Pacific. A positive feedback between the coupled model KOE SST (driven by the ocean streamfunction) and North Pacific Ekman pumping is consistent with the enhanced variance of the coupled model at 20–30-yr periods. However, the time series are too short to unambiguously distinguish this positive feedback hypothesis from sampling variability. No evidence is found for a midlatitude gyre ocean–atmosphere delayed negative feedback loop.

Comparisons with available observations confirm the seasonality of the forcing, the up to 5-yr time lag between like-signed central North Pacific and KOE SST anomalies, and the associated damping of SST in the KOE region by the latent heat flux. The coupled model results also suggest that observed SST anomalies in the KOE region may be predictable from the history of the wind-stress curl over the North Pacific.

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