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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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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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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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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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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
,
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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Hyodae Seo
,
Arthur J. Miller
, and
John O. Roads

Abstract

A regional coupled ocean–atmosphere model is introduced. It is designed to admit the air–sea feedbacks arising in the presence of an oceanic mesoscale eddy field. It consists of the Regional Ocean Modeling System (ROMS) and the Regional Spectral Model (RSM). Large-scale forcing is provided by NCEP/DOE reanalysis fields, which have physics consistent with the RSM. Coupling allows the sea surface temperature (SST) to influence the stability of the atmospheric boundary layer and, hence, the surface wind stress and heat flux fields. The system is denominated the Scripps Coupled Ocean–Atmosphere Regional (SCOAR) Model.

The model is tested in three scenarios in the eastern Pacific Ocean sector: tropical instability waves of the eastern tropical Pacific, mesoscale eddies and fronts of the California Current System, and gap winds of the Central American coast. Recent observational evidence suggests air–sea interactions involving the oceanic mesoscale in these three regions. Evolving SST fronts are shown to drive an unambiguous response of the atmospheric boundary layer in the coupled model. This results in significant model anomalies of wind stress curl, wind stress divergence, surface heat flux, and precipitation that resemble the observations and substantiate the importance of ocean–atmosphere feedbacks involving the oceanic mesoscale.

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

Abstract

From the early 1970s to the mid-1980s, the main thermocline of the subarctic gyre of the North Pacific Ocean shoaled with temperatures at 200–400-m depth cooling by 1°–4°C over the region. The gyre-scale structure of the shoaling is quasi-stationary and intensified in the western part of the basin north of 30°N, suggesting concurrent changes in gyre-scale transport. A similar quasi-stationary cooling in the subtropical gyre south of 25°N is also observed but lags the subpolar change by several years. To explore the physics of these changes, the authors examine an ocean model forced by observed wind stress and heat flux anomalies from 1970–88 in which they find similar changes in gyre-scale thermocline structure. The model current fields reveal that the North Pacific subpolar and subtropical gyres strengthened by roughly 10% from the 1970s to the 1980s. The bulk of the eastward flow of the model Kuroshio–Oyashio Extension returned westward via the subpolar gyre circuit, while the subtropical gyre return flow along 20°N lags the subpolar changes by several years. The authors demonstrate that the model thermocline cooling and increased transport occurred in response to decadal-scale changes in basin-scale wind stress curl with the quasi-stationary oceanic response being in a time-dependent quasi-Sverdrup balance over much of the basin east of the date line. This wind stress curl driven response is quasi-stationary but occurs in conjunction with a propagating temperature anomaly associated with subduction in the central North Pacific that links the subpolar and subtropical gyre stationary changes and gives the appearance of circumgyre propagation. Different physics evidently controls the decadal subsurface temperature signal in different parts of the extratropical North Pacific.

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Matthew J. Bunkers
,
James R. Miller Jr.
, and
Arthur T. DeGaetano

Abstract

Spatially homogeneous climate regions were developed from long-term monthly temperature and precipitation data for a subset of the U.S. Northern Plains. Climate regions were initially defined using the “best” of three agglomerative and hierarchical clustering methodologies, then the clusters were objectively modified using a “pseudohierarchical” iterative improvement technique. Under the premise of hierarchical cluster analysis, once an object has been assigned to a cluster, it cannot later he reassigned to a different cluster, even if it is statistically desirable. The objective modification technique used herein is employed to compensate for this problem.

Principal component analysis (PCA) was used to reduce a 147-station dataset, consisting of 24 climatic variables averaged over the 1931–1990 period, to three orthogonal components. The new standardized mars, which explain 93% of the original dataset variance, were then subjected to the Ward's, average linkage, and complete linkage clustering methods. The average linkage method produced the most representative statistical results in identifying the climate regions. An iterative improvement technique was then utilized to test “border station” membership and to modify the climate region houses. Fifteen climate regions resulted from the clustering (with two single-station clusters in the Black Hills alone), although they age just one possible partitioning of the data. The within-cluster variability is generally the same for the 15 climate regions and the corresponding 21 National Climatic Data Center (NCM) climate divisions. However, since data within-cluster variability tends to decrease with increasing cluster number, this result favors the new climate regions. Additionally, the new climate regions am shown to be superior to the NCDC climate, divisions in wont of between-cluster variability. These results suggest that the NCDC climate divisions could be redefined, improving their climatic homogeneity.

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