bluuebunny/crossref_metadata_2025_split huggingface dataset:
Mean Three-Dimensional Circulation in the Northeast Tropical Pacific*
Authors: Kessler William S. 
Published: February 2011

Abstract
Historical XBT data are used to construct a mean climatology of the three-dimensional geostrophic circulation in the northeast tropical Pacific (southwest of Mexico and Central America) and are diagnosed based on linear dynamics forced with satellite scatterometer winds. Unlike the familiar central tropical Pacific, where the zonal scales are very large and the wind forcing nearly a function of latitude alone, the North Pacific east of about 120°W is strongly influenced by wind jets blowing through gaps in the Central American cordillera. The curl imposed by these wind jets imprints on the ocean, producing a distinctive pattern of thermocline topography and geostrophic currents that are consistent with the Sverdrup balance. Notably, the weakening of the North Equatorial Countercurrent near 110°W is due to the wind forcing. Given the observed stratification and wind stress curl, planetary vorticity conservation also determines the distribution of vertical velocity in the region, with about 3.5 Sv (Sv ≡ 106 m3 s−1) of upwelling through the base of the thermocline under the Costa Rica Dome. This upwelling is associated with stretching of the water column under the dome, which thereby causes the northern “Subsurface Counter Current” (SSCC or Tsuchiya Jet) to turn away from the equator; about half the transport of the SSCC upwells through the thermocline via this mechanism. This may be part of the process by which intermediate-depth water, flowing into the Pacific from the south, is brought to the surface and into the Northern Hemisphere.
Balanced Ocean-Data Assimilation near the Equator
Authors: Burgers Gerrit , Balmaseda Magdalena A. , Vossepoel Femke C. , van Oldenborgh Geert Jan , van Leeuwen Peter Jan 
Published: February 2011

Abstract
The question is addressed whether using unbalanced updates in ocean-data assimilation schemes for seasonal forecasting systems can result in a relatively poor simulation of zonal currents. An assimilation scheme, where temperature observations are used for updating only the density field, is compared to a scheme where updates of density field and zonal velocities are related by geostrophic balance. This is done for an equatorial linear shallow-water model. It is found that equatorial zonal velocities can be detoriated if velocity is not updated in the assimilation procedure. Adding balanced updates to the zonal velocity is shown to be a simple remedy for the shallow-water model. Next, optimal interpolation (OI) schemes with balanced updates of the zonal velocity are implemented in two ocean general circulation models. First tests indicate a beneficial impact on equatorial upper-ocean zonal currents.
Thermohaline and Wind Forcing of a Circumpolar Channel withBlocked Geostrophic Contours
Authors: Borowski Daniel , Gerdes Rüdiger , Olbers Dirk 
Published: February 2011

Abstract
The Antarctic Circumpolar Current is governed by unique dynamics. Because the latitude belt of Drake Passage is not zonally bounded by continents, the Sverdrup theory does not apply to the Antarctic Circumpolar Current. However, most of the geostrophic contours are blocked at Drake Passage, which provides an important dynamic constraint for the vorticity equation of the depth averaged flow.
This study addresses the effects of thermohaline and wind forcing on the large-scale transport of a circumpolar current with blocked geostrophic contours. Various numerical experiments with three different idealized model geometries were conducted. Based on the results and theoretical arguments, the authors promote an indirect wind effect on the circumpolar current: while the direct effects of the wind in driving the circumpolar current through a vertical transfer of the applied wind stress are of minor importance, the wind does substantially influence the circumpolar current transport through its effects on the density field.
This indirect wind effect is discussed in two steps. First, at the latitudes of the circumpolar current and longitudes where the geostrophic contours are blocked, the meridional gradient of the mass transport streamfunction is to leading order balanced by the meridional gradient of the baroclinic potential energy. This balance implies that the total transport is to leading order baroclinic and that the deep transport is small. For this statement, some theoretical arguments are offered. Second, a simplified analytical model is used to obtain the distribution of the baroclinic potential energy. Assuming an advective–diffusive balance for the densities in the deep downwelling northern branch of the Deacon cell, this model reproduces the qualitative dependence of the circumpolar current transport on the imposed wind and thermohaline forcing as well as on the turbulent diffusivities.
On Determining the Onset and Strength of Breaking for Deep Water Waves. Part I: Unforced Irrotational Wave Groups
Authors: Song Jin-Bao , Banner Michael L. 
Published: February 2011

Abstract
Finding a robust threshold variable that determines the onset of breaking for deep water waves has been an elusive problem for many decades. Recent numerical studies of the unforced evolution of two-dimensional nonlinear wave trains have highlighted the complex evolution to recurrence or breaking, together with the fundamental role played by nonlinear intrawave group dynamics. In Part I of this paper the scope of two-dimensional nonlinear wave group calculations is extended by using a wave-group-following approach applied to a wider class of initial wave group geometries, with the primary goal of identifying the differences between evolution to recurrence and to breaking onset. Part II examines the additional influences of wind forcing and background shear on these evolution processes.
The present investigation focuses on the long-term evolution of the maximum of the local energy density along wave groups. It contributes a more complete picture, both long-term and short-term, of the approach to breaking and identifies a dimensionless local average growth rate parameter that is associated with the mean convergence of wave-coherent energy at the wave group maximum. This diagnostic growth rate appears to have a common threshold for all routes to breaking in deep water that have been examined and provides an earlier and more decisive indicator for the onset of breaking than previously proposed breaking thresholds. The authors suggest that this growth rate may also provide an indicative measure of the strength of wave breaking events.
Equatorial Solitary Waves. Part V: Initial Value Experiments, Coexisting Branches, and Tilted-Pair Instability
Authors: Boyd John P. 
Published: February 2011

AbstractA series of high-resolution numerical experiments, augmented by theory, to further explore the dynamics of equatorial dipole vortices (Rossby solitary waves) is performed. When the amplitude is sufficiently large so that the vortices trap fluid internally, the solitary waves for a given phase speed are not unique. The potential vorticity–streak function (q–Ψ) relationship is everywhere linear for one branch, but highly nonlinear in the recirculation region for the second branch. Westward-traveling vortex pairs are highly unstable on the midlatitude beta plane, but the equatorial wave guide stabilizes vortex pairs that straddle the equator, even when given a strong initial tilt. As discovered by Williams and Wilson and explained theoretically by Boyd, the author confirms that higher latitudinal mode solitary waves are weakly nonlocal through radiation of sinusoidal Rossby waves of lower latitudinal mode number. The amplitude and wavelength of the radiation are in good agreement with nonlocal soliton theory.It is sometimes said that the great discovery of the nineteenth century was that the equations of nature were linear, and the great discovery of the twentieth century is that they are not.—Thomas Körner, Fourier Analysis (1988, p. 99)
Effects of Wind on Convection in Strongly and Weakly Baroclinic Flows with Application to the Labrador Sea*
Authors: Straneo Fiammetta , Kawase Mitsuhiro , Pickart Robert S. 
Published: February 2011

Abstract
Large buoyancy loss driving deep convection is often associated with a large wind stress that is typically omitted in simulations of convection. Here it is shown that this omission is not justified when overturning occurs in a horizontally inhomogeneous ocean. In strongly baroclinic flows, convective mixing is influenced both by the background horizontal density gradient and by the across-front advection of buoyancy due to wind. The former process—known as slantwise convection—results in deeper convection, while the effect of wind depends on the relative orientation of wind with respect to the baroclinic front. For the case of the Labrador Sea, wintertime winds act to destabilize the baroclinic Labrador Current causing a buoyancy removal roughly one-third as large as the air–sea buoyancy loss. Simulations using a nonhydrostatic numerical model, initialized and forced with observed fields from the Labrador Sea, show how the combination of wind and lateral gradients can result in significant convection within the current, in contrast with previous ideas. Though the advection of buoyancy due to wind in weakly baroclinic flows is negligible compared to the surface buoyancy removal typical of convective conditions, convective plumes are substantially deformed by wind. This deformation, and the associated across-front secondary circulation, are explained in terms of the vertical advection of wind-generated vorticity from the surface boundary layer to deeper depths. This mechanism generates vertical structure within the convective layer, contradicting the historical notion that properties become vertically homogenized during convection. For the interior Labrador Sea, this mechanism may be partly responsible for the vertical variability observed during convection, which modeling studies have until now failed to reproduce.
Internal Waves and Turbulence in the Upper Central Equatorial Pacific: Lagrangian and Eulerian Observations
Authors: Lien R-C. , D'Asaro E. A. , McPhaden M. J. 
Published: February 2011

Abstract
In the shear stratified flow below the surface mixed layer in the central equatorial Pacific, energetic near-N (buoyancy frequency) internal waves and turbulence mixing were observed by the combination of a Lagrangian neutrally buoyant float and Eulerian mooring sensors. The turbulence kinetic energy dissipation rate ε and the thermal variance diffusion rate χ were inferred from Lagrangian frequency spectral levels of vertical acceleration and thermal change rate, respectively, in the turbulence inertial subrange. Variables exhibiting a nighttime enhancement include the vertical velocity variance (dominated by near-N waves), ε, and χ. Observed high levels of turbulence mixing in this low-Ri (Richardson number) layer, the so-called deep-cycle layer, are consistent with previous microstructure measurements. The Lagrangian float encountered a shear instability event. Near-N waves grew exponentially with a 1-h timescale followed by enhanced turbulence kinetic energy and strong dissipation rate. The event supports the scenario that in the deep-cycle layer shear instability may induce growing internal waves that break into turbulence. Superimposed on few large shear-instability events were background westward-propagating near-N waves. The floats' ability to monitor turbulence mixing and internal waves was demonstrated by comparison with previous microstructure measurements and with Eulerian measurements.
Energy Levels, Phase, and Amplitude Modulation of the Baroclinic Tide off Hawaii
Authors: Chiswell Stephen M. 
Published: February 2011

Abstract
Inverted echo sounder data from Station Aloha north of Oahu, Hawaii, are used to determine the absolute energy levels and time-varying nature of the first-mode baroclinic tide north of Hawaii. The semidiurnal tide amplitude and phase are modulated with 60–120 day periodicity, which indicates interaction between the baroclinic tide and intra-annual mesoscale variability seen off Hawaii. Close to the presumed generation site, there is less phase modulation, and amplitude modulation is uncorrelated with the far-field variability. These results are consistent with a model of phase-locked generation, together with phase and amplitude distortion during propagation. The sense of the amplitude modulation is opposite to that previously observed at decadal timescales. Semidiurnal energy flux at Station Aloha ranges from 2.2 to 8.0 kW m−1, with a mean value of 4.4 kW m−1.
Long Rossby Wave Basin-Crossing Time and the Resonance of Low-Frequency Basin Modes
Authors: Primeau François 
Published: February 2011

Abstract
The ability of long-wave low-frequency basin modes to be resonantly excited depends on the efficiency with which energy fluxed onto the western boundary can be transmitted back to the eastern boundary. This efficiency is greatly reduced for basins in which the long Rossby wave basin-crossing time is latitude dependent.
In the singular case where the basin-crossing time is independent of latitude, the amplitude of resonantly excited long-wave basin modes grows without bound except for the effects of friction. The speed of long Rossby waves is independent of latitude for quasigeostrophic dynamics, and the rectangular basin geometry often used for theoretical studies of the wind-driven ocean circulation is such a singular case for quasigeostrophic dynamics.
For more realistic basin geometries, where only a fraction of the energy incident on the western boundary can be transmitted back to the eastern boundary, the modes have a finite decay rate that in the limit of weak friction is independent of the choice of frictional parameters. Explicit eigenmode computations for a basin geometry similar to the North Pacific but closed along the equator yield basin modes sufficiently weakly damped that they could be resonantly excited.
A Non-normal View of the Wind-Driven Ocean Circulation
Authors: Moore Andrew M. , Perez Cristina L. , Zavala-Garay Javier 
Published: February 2011

Abstract
Generalized linear stability theory is applied to the wind-driven ocean circulation in the form of a double gyre described by the barotropic quasigeostrophic vorticity equation. The development of perturbations on this circulation is considered. The circulation fields are inhomogeneous, and regions of straining flow render non-normal the tangent linear operators that describe the time evolution of perturbation energy and enstrophy. When the double-gyre circulation is asymptotically stable, growth of perturbation energy and enstrophy is still possible due to linear interference of its nonorthogonal eigenmodes. The sources and sinks of perturbation energy and enstrophy associated with the interference process are traditionally associated with the interaction of perturbation stresses with the mean flow. These ideas are used to understand the response of an asymptotically stable double-gyre circulation to stochastic wind stress forcing. Calculation of the optimal forcing patterns (stochastic optimals) reveals that much of the stochastically induced variability can be explained by one pattern. Variability induced by this pattern is maintained by long and short Rossby waves that interact with the western boundary currents, and perturbation growth occurs through barotropic processes. The perturbations that maintain the stochastically induced variance in this way have a large projection on some of the most non-normal, least-damped eigenmodes of the double-gyre circulation.
Perturbation growth in nonautonomous and asymptotically unstable systems is also considered in the same framework. The Lyapunov vectors of unstable flows are found to have a large projection on some of the most non-normal, least-damped eigenmodes of the time mean circulation.
The Stabilization of the Thermohaline Circulation by the Temperature–Precipitation Feedback
Authors: Tziperman Eli , Gildor Hezi 
Published: February 2011

Abstract
The meridional freshwater flux in the atmosphere, and hence the strength of the hydrological cycle, undergoes variations on glacial–interglacial as well as on some shorter timescales. A significant portion of these changes to the hydrological cycle are due to the temperature–precipitation feedback according to which there is more precipitation over the higher latitudes during warm periods when the moisture holding capacity of the atmosphere is higher.
It is proposed here that this feedback may play an important role in determining the stability of the thermohaline circulation (THC). The THC stability to different parameterizations of the meridional atmospheric freshwater flux is therefore investigated using a simple box model of the ocean, atmosphere, and sea ice. It is demonstrated that parameterizations that are consistent with the temperature–precipitation feedback, and hence with the observed variations of the hydrological cycle during glacial–interglacial cycles, stabilize the THC for a wide range of forcing parameters.
Performance of Observation-Based Prediction Algorithms for Very Short-Range, Probabilistic Clear-Sky Condition Forecasting
Authors: Hall Timothy J. , Mutchler Carl N. , Bloy Greg J. , Thessin Rachel N. , Gaffney Stephanie K. , Lareau Jonathan J. 
Published: February 2011

AbstractVery short-range sky condition forecasts are produced to support a variety of military, civil, and commercial activities. In this investigation, six advanced, observation (obs)-based prediction algorithms were developed and tested that generated probabilistic sky condition forecasts for 1-, 2-, 3-, 4-, and 5-h forecast intervals, for local and regional target types, in six geographic regions within the continental United States. Three of the methods were based on predictive learning algorithms including neural network, random forest, and regression tree. The other three methods were statistical techniques including a k–nearest neighbor algorithm, a classifier based on the Bayes decision rule, and a multialgorithm ensemble. The performances of these six algorithms were compared with forecasts from three benchmark methods: basic persistence, the climatological-expectancy-of-persistence, and satellite cloud climatology. The obs database for each forecast target was composed of a multiyear, half-hourly time series of atmospheric parameters that included cloud features extracted from weather satellite imagery and meteorological variables extracted or derived from data assimilation–based model analyses generated by NCEP’s Eta Data Assimilation System. The performances of the advanced prediction algorithms exceeded those of the benchmarks at all five forecast intervals for both target types in all regions, on the basis of a group of metrics that included receiver operating characteristic score, sharpness, accuracy, expected best cost, and reliability.
A Quasigeostrophic Model for Moist Storm Tracks
Authors: Laîné Alexandre , Lapeyre Guillaume , Rivière Gwendal 
Published: February 2011

Abstract
The effect of moisture and latent heat release is investigated in the context of a three-level quasigeostrophic model on the sphere. The model is based on an existing dry model that was shown to be able to reproduce the midlatitude synoptic and low-frequency variability of the troposphere. In addition to potential vorticity equations, moisture evolution equations are included with a simple precipitation scheme. The model can be forced using reanalysis datasets to represent the observed climatology.
After the description of the model, the Northern Hemisphere midlatitude climatic characteristics of the moist model are compared to its dry counterpart and to the 40-yr ECMWF Re-Analysis (ERA-40). The jet of the moist model is weakened in its central and northern part and enhanced in its southern part compared to the dry version, which generally decreases the model biases compared to reanalyses. Latent heating processes are mainly responsible for the global decrease in westerlies in the jet-core regions. Precipitation mainly occurs slightly poleward of the jet axes, thereby reducing the meridional temperature gradient and the wind through thermal wind balance. The mean synoptic activity is reduced according to the decrease in baroclinicity, as well as the mean low-frequency variability. A diagnosis of synoptic wave breaking is performed and the characteristics of the moist model are closer to the ones found in reanalyses, especially with more occurrence of cyclonic wave breaking. Weather regimes are slightly better represented in the moist model, although changes are weak compared to the intrinsic model biases. The behavior of the moist model around its climatology indicates that it could be used to run sensitivity experiments.
Dynamical Response of the South Asian Monsoon Trough to Latent Heating from Stratiform and Convective Precipitation
Authors: Choudhury Ayantika Dey , Krishnan R. 
Published: February 2011

AbstractSimulation experiments using a simplified atmospheric GCM and supplementary diagnostic analyses of observations are performed to understand how the South Asian monsoon trough (MT) responds dynamically to latent heating from mesoscale convective systems (MCSs). Observations reveal that the MT during active monsoons is characterized by a deep cyclonic vorticity extending from the surface to 350 hPa and organized MCSs covering over 3500–4000 km along the Indo-Gangetic plains. The MCSs during active monsoons are composed of a relatively higher abundance of stratiform-type precipitation (mostly nimbostratus) as compared to the convective type. The results suggest that a stratiform-type heating profile is very effective in promoting upward development of continental-scale cyclonic circulation well above the midtroposphere over the MT region. The vertical development involves a dynamical uplift of the layer of cyclonic circulation and is induced by midlevel (600–500 hPa) convergence and vorticity stretching above 500 hPa. By varying the population of stratiform and convective rain types in the simulation, the horizontal scale of midlevel vorticity response is shown to increase significantly with stratiform population; in contrast, the midlevel response is more localized when the MCS is dominated by deep convective clouds. For large stratiform populations, the midlevel response is found to extend far westward up to the northern flanks of the African ITCZ, indicative of Rossby wave dispersion of PV anomalies that are generated near the level of maximum heating gradient. From the present findings, one can conclude that the vertical deepening of MT during active monsoons is not merely a localized phenomenon; instead it represents a large-scale dynamical response to organized MCSs that exert pivotal influence on the upward development of cyclonic circulation well above the midtroposphere.