Barotropic Wind Research Articles

Separation of the Icelandic Coastal Current from the Reykjanes Peninsula

The Icelandic Coastal Current (ICC) is a buoyant flow fueled by multiple rivers. Satellite observations during the October–November 2019 study period indicate the ICC often separates from the southwest tip of the Reykjanes Peninsula; it subsequently has large offshore excursion during west-southwestward winds. Results from a high-resolution realistic simulation exhibit this ICC behavior and provide detail on the associated currents and salinities. Sensitivity tests with simplified bathymetry, no winds, or no local buoyancy inputs are compared to the standard run to isolate contributions of those factors to flow separation. Analysis indicates that ICC separation occurs because the coast turns more tightly than the inertial radius; this behavior is consistent with inviscid theory. Partial flow deflection over the shelf expression of the Reykjanes Ridge and the widening shelf both increase ICC offshore excursion. The ICC is strongly influenced by the barotropic wind response to west-southwestward winds that includes a downshelf jet (flowing in the direction of Kelvin wave propagation) along the coast upshelf of the peninsula's tip where winds are downwelling-favorable, the downshelf extension of the jet that bends around the peninsula and progresses farther offshore with isobaths, a band of offshore and upshelf currents along the coast downshelf of the peninsula's tip, and surface flow aligned with Ekman transport located offshore beyond the downshelf jet. Similar wind-influence scenarios are discussed for other plumes. There are likely many such hotspots for offshore freshwater transport around the world.

Planetary geostrophic Boussinesq dynamics: Barotropic flow, baroclinic instability and forced stationary waves

AbstractMotions on planetary spatial scales in the atmosphere are governed by the planetary geostrophic equations. However, little attention has been paid to the interaction between the baroclinic and barotropic flows within the planetary geostrophic scaling. This is the focus of the present study, which utilizes planetary geostrophic equations for a Boussinesq fluid supplemented by a novel evolution equation for the barotropic flow. The latter is affected by meridional momentum flux due to baroclinic flow and drag by the surface wind. The barotropic wind, on the other hand, affects the baroclinic flow through buoyancy advection. Via a relaxation towards a prescribed buoyancy profile the model produces realistic major features of the zonally symmetric wind and temperature fields. We show that there is considerable cancellation between the barotropic and the baroclinic surface zonal mean zonal winds. Linear and nonlinear model responses to steady diabatic zonally asymmetric forcing are investigated, and the arising stationary waves are interpreted in terms of analytical solutions. We also study the problem of baroclinic instability on the sphere within the present model.

Baroclinic-to-Barotropic Pathway in El Niño–Southern Oscillation Teleconnections from the Viewpoint of a Barotropic Rossby Wave Source

Abstract The baroclinic-to-barotropic pathway in ENSO teleconnections is examined from the viewpoint of a barotropic Rossby wave source that results from decomposition into barotropic and baroclinic components. Diagnoses using the NCEP–NCAR reanalysis are supplemented by analysis of the response of a tropical atmospheric model of intermediate complexity to the NCEP–NCAR barotropic Rossby wave source. Among the three barotropic Rossby wave source contributions (shear advection, vertical advection, and surface drag), the leading contribution is from shear advection and, more specifically, the mean baroclinic zonal wind advecting the anomalous baroclinic zonal wind. Vertical advection is the smallest term, while surface drag tends to cancel and reinforce the shear advection in different regions through damping on the baroclinic mode, which spins up a barotropic response. There are also nontrivial impacts of transients in the barotropic wind response to ENSO. Both tropical and subtropical baroclinic vorticity advection contribute to the barotropic component of the Pacific subtropical jet near the coast of North America, where the resulting barotropic wind contribution approximately doubles the zonal jet anomaly at upper levels, relative to the baroclinic anomalies alone. In this view, the barotropic Rossby wave source in the subtropics simply arises from the basic-state baroclinic flow acting on the well-known baroclinic ENSO flow pattern that spreads from the deep tropics into the subtropics over a scale of equatorial radius of deformation. This is inseparably connected to the leading deep tropical Rossby wave source that arises from eastern Pacific climatological baroclinic winds advecting the tropical portion of the same ENSO flow pattern.

Roles of Barotropic Convective Momentum Transport in the Intraseasonal Oscillation*

Abstract Both observational data analysis and model simulations suggest that convective momentum transport (CMT) by cumulus convection may play a significant role in the intraseasonal oscillations (ISO) by redistributing atmospheric momentum vertically through fast convective mixing process. The authors present a simple theoretical model for the ISO by parameterizing the cumulus momentum transport process in which the CMT tends to produce barotropic wind anomalies that will affect the frictional planetary boundary layer (PBL). In the model with equatorial easterly vertical wind shear (VWS), it is found that the barotropic CMT tends to select most unstable planetary-scale waves because CMT suppresses the equatorial Ekman pumping of short waves, which reduces the shortwave instability from the PBL moisture convergence and accelerates the shortwave propagation. The model with subtropical easterly VWS has behavior that can be qualitatively different from the model with equatorial easterly VWS and has robust northward propagation. The basic mechanism of this northward propagation is that the CMT accelerates the barotropic cyclonic wind to the north of ISO, which will enhance the precipitation by PBL Ekman pumping and favor the northward propagation. The simulated northward propagation is sensitive to the strength and location of the seasonal-mean easterly VWS. These results suggest that accurate simulation of the climatological-mean state is critical for reproducing the realistic ISO in general circulation models.

Steady, barotropic wind and boundary-driven circulation on a polar plane

Steady, linear, barotropic wind and boundary forced circulation solutions in the presence of linear bottom friction are analytically derived in a circular basin of uniform depth on a polar tangent plane in which only first order effects of the Earth’s curvature are retained. Approximate solutions are constructed by using the well known method of aggregating the interior inviscid Sverdrup balance solution and the frictional wall boundary layer solution. In contrast to the width of mid-latitude frictional western boundary layers that scale as , the width of the polar frictional boundary layer adjacent to the basin wall is wider, scaling as , where is the bottom friction coefficient, is the coriolis parameter. Solutions are presented for a variety of wind stress curl distributions and for a prescribed inflow/outflow representative of the exchange of water masses between the Arctic and Atlantic basins. Boundary forced solutions are also derived in a basin with a uniform width step shelf. For this basin geometry the flow is mainly confined to the shelf, although a parameter regime is identified that supports significant flow in the deep basin.

Structure and Origin of the Quasi-Biweekly Oscillation over the Tropical Indian Ocean in Boreal Spring

Abstract The structure and evolution features of the quasi-biweekly (10–20 day) oscillation (QBWO) in boreal spring over the tropical Indian Ocean (IO) are investigated using 27-yr daily outgoing longwave radiation (OLR) and the National Centers for Environment Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data. It is found that a convective disturbance is initiated over the western IO and moves slowly eastward. After passing the central IO, it abruptly jumps into the eastern IO. Meanwhile, the preexisting suppressed convective anomaly in the eastern IO moves poleward in the form of double-cell Rossby gyres. The analysis of vertical circulation shows that a few days prior to the onset of local convection in the eastern equatorial IO an ascending motion appears in the boundary layer. Based on the diagnosis of the zonal momentum equation, a possible boundary layer–triggering mechanism over the eastern equatorial IO is proposed. The cause of the boundary layer convergence and vertical motion is attributed to the free-atmospheric divergence in association with the development of the barotropic wind. It is the downward transport of the background mean easterly momentum by perturbation vertical motion during the suppressed convective phase of the QBWO that leads to the generation of a barotropic easterly—the latter of which further causes the free-atmospheric divergence and, thus, the boundary layer convergence. The result suggests that the local process, rather than the eastward propagation of the disturbance from the western IO, is essential for the phase transition of the QBWO convection over the eastern equatorial IO.

Gravity Waves in Shear and Implications for Organized Convection

AbstractIt is known that gravity waves in the troposphere, which are often excited by preexisting convection, can favor or suppress the formation of new convection. Here it is shown that in the presence of wind shear or barotropic wind, the gravity waves can create a more favorable environment on one side of preexisting convection than the other side.Both the nonlinear and linear analytic models developed here show that the greatest difference in favorability between the two sides is created by jet shears, and little or no difference in favorability is created by wind profiles with shear at low levels and no shear in the upper troposphere. A nonzero barotropic wind (or, equivalently, a propagating heat source) is shown to also affect the favorability on each side of the preexisting convection. It is shown that these main features are captured by linear theory, and advection by the background wind is the main physical mechanism at work. These processes should play an important role in the organization of wave trains of convective systems (i.e., convectively coupled waves); if one side of preexisting convection is repeatedly more favorable in a particular background wind shear, then this should determine the preferred propagation direction of convectively coupled waves in this wind shear.In addition, these processes are also relevant to individual convective systems: it is shown that a barotropic wind can lead to near-resonant forcing that amplifies the strength of upstream gravity waves, which are known to trigger new convective cells within a single convective system. The barotropic wind is also important in confining the upstream waves to the vicinity of the source, which can help ensure that any new convective cells triggered by the upstream waves are able to merge with the convective system.All of these effects are captured in a two-dimensional model that is further simplified by including only the first two vertical baroclinic modes. Numerical results are shown with a nonlinear model, and linear theory results are in good agreement with the nonlinear model for most cases.

Northern Hemisphere Stationary Waves in Future Climate Projections

Abstract The response of the atmospheric large-scale circulation to an enhanced greenhouse gas (GHG) forcing varies among coupled global climate model (CGCM) simulations. In this study, 16 CGCM simulations of the response of the climate system to a 1% yr−1 increase in the atmospheric CO2 concentration to quadrupling are analyzed with focus on Northern Hemisphere winter. A common signal in 14 out of the 16 simulations is an increased or unchanged stationary wave amplitude. A majority of the simulations may be categorized into one of three groups based on the GHG-induced changes in the atmospheric stationary waves. The response of the zonal mean barotropic wind is similar within each group. Fifty percent of the simulations belong to the first group, which is categorized by a stationary wave with five waves encompassing the entire NH and a strengthening of the zonal mean barotropic wind. The second and third groups, respectively consisting of three and two simulations, are characterized by a broadening and a northward shift of the zonal mean barotropic wind, respectively. A linear model of barotropic vorticity is employed to study the importance of these mean flow changes to the stationary wave response. The linear calculations indicate that the GHG-induced mean wind changes explain 50%, 4%, and 37% of the stationary wave changes in each group, respectively. Thus, for the majority of simulations the zonal mean wind changes do significantly explain the stationary wave response.

The Role of Stationary Waves in the Maintenance of the Northern Annular Mode as Deduced from Model Experiments

Abstract The influence of stationary waves on the maintenance of the tropospheric annular mode (AM) is examined in a simple global circulation model with perpetual January conditions. The presented model experiments vary in the configurations of stationary wave forcing by orography and land–sea heating contrasts. All simulations display an AM-like pattern in the lower troposphere. The zonal momentum budget shows that the feedback between eddies with periods less than 10 days and the zonal-mean zonal wind is generally the dominating process that maintains the AM. The kinetic energy of the high-frequency eddies depends on the stationary wave forcing, where orographic forcing reduces and thermal forcing enhances it. The AMs in the model experiments differ in the superposed anomalous stationary waves and in the strength of the zonally symmetric component. If only orographic stationary wave forcing is taken into account, the mountain torque decelerates the barotropic wind anomaly, and thus acts to weaken the AM. However, the combined forcing of orography and land–sea heating contrasts produces a feedback between the anomalous stationary waves and the AM that compensates for the mountain torque. The different behavior of the model experiments results from the fact that only the thermal forcing changes the character of the anomalous stationary waves from external Rossby waves for orographic forcing alone to vertically propagating waves that enable the feedback process through wave–mean flow interaction. Only with this feedback, which is shown to be due to linear zonal–eddy coupling, does the model display a strong AM with centers of action over the oceans. The main conclusions are that this process is necessary to simulate a realistic northern AM, and that it distinguishes the northern from the southern AM.

Adjoint goal-based error norms for adaptive mesh ocean modelling

Flow in the world’s oceans occurs at a wide range of spatial scales, from a fraction of a metre up to many thousands of kilometers. In particular, regions of intense flow are often highly localised, for example, western boundary currents, equatorial jets, overflows and convective plumes. Conventional numerical ocean models generally use static meshes. The use of dynamically-adaptive meshes has many potential advantages but needs to be guided by an error measure reflecting the underlying physics. A method of defining an error measure to guide an adaptive meshing algorithm for unstructured tetrahedral finite elements, utilizing an adjoint or goal-based method, is described here. This method is based upon a functional, encompassing important features of the flow structure. The sensitivity of this functional, with respect to the solution variables, is used as the basis from which an error measure is derived. This error measure acts to predict those areas of the domain where resolution should be changed. A barotropic wind driven gyre problem is used to demonstrate the capabilities of the method. The overall objective of this work is to develop robust error measures for use in an oceanographic context which will ensure areas of fine mesh resolution are used only where and when they are required.

Rossby Wave Breaking in the Southern Hemisphere Wintertime Upper Troposphere

Abstract The characteristics of Rossby wave propagation and breaking in the Southern Hemisphere upper troposphere during winter are examined. Although the Southern Hemisphere subtropical jet is more zonally symmetric than that of the Northern Hemisphere, there are still significant zonal variations in the upper-tropospheric flow. In particular, the flow within a given sector (≈120° longitude) can generally be characterized into one of four different configurations: (i) a single jet, (ii) a “broken” subtropical jet, (iii) a polar jet at the upstream end of the subtropical jet, or (iv) a polar jet at the downstream end of the subtropical jet. Using “potential vorticity thinking” and barotropic wind shear arguments, it is argued that the characteristics of the Rossby wave propagation and breaking will differ between each flow configuration. Consistent with these arguments, examination of potential vorticity maps and contour advection calculations show differing wave-breaking characteristics. In particular, t...

Waves and Instabilities for Model Tropical Convective Parameterizations

Models of the tropical atmosphere with crude vertical resolution are important as intermediate models for understanding convectively coupled wave hierarchies and also as simplified models for studying various strategies for parameterizing convection and convectively coupled waves. Simplified models are utilized in a detailed analytical study of the waves and instabilities for model convective parameterizations. Three convection schemes are analyzed: a strict quasi-equilibrium (QE) scheme and two schemes that attempt to model the departures from quasi equilibrium by including the shorter timescale effects of penetrative convection, the Lagrangian parcel adjustment (LPA) scheme and a new instantaneous convective available potential energy (CAPE) adjustment (ICAPE) scheme. Unlike the QE parameterization scheme, both the LPA and ICAPE schemes have scale-selective finite bands of unstable wavelengths centered around typical cluster and supercluster scales with virtually identical growth rates and wave structure. However, the LPA scheme has, in addition, two nonphysical superfast parasitic waves that are artifacts of this parameterization while such waves are completely absent in the new ICAPE parameterization. Another topic studied here is the fashion in which an imposed barotropic mean wind triggers a transition to instability in the Tropics through suitable convectively coupled waves; this is the simplest analytical problem for studying the influence of midlatitudes on convectively coupled waves. For an easterly barotropic mean flow with the effect of rotation included, both supercluster-scale moist Kelvin waves and cluster-scale moist mixed Rossby‐gravity waves participate in the transition to instability. The wave and stability properties of the ICAPE parameterization with rotation are studied through a novel procedure involving complete zonal resolution but low-order meridional truncation. Besides moist Kelvin, mixed Rossby‐gravity, and equatorial Rossby waves, this approximation retains other slowly propagating moist gravity waves in a consistent fashion.

The influence of the 1997–99 El Niňo Southern Oscillation on extratropical baroclinic life cycles over the eastern North Pacific

AbstractThe El Niňo Southern Oscillation (ENSO) strongly influences the interannual and seasonal atmospheric circulation over the North Pacific. The present study shows that the meridional displacement of the time‐mean upper‐level jet associated with ENSO modulates the time‐mean barotropic (meridional) wind shear over the central and eastern North Pacific storm track. Earlier theoretical and observational studies established the influence of barotropic wind shear on the life cycles of extratropical cyclones and their upper‐level potential vorticity (PV) waves. The present study suggests that differences in the time‐mean flow associated with the 1997–99 ENSO cycle had a similar impact on tropopause PV structure, meridional eddy fluxes of momentum and temperature, and predictability.

A simple model for the symmetry properties of nonlinear wind driven ocean circulation

Abstract It is shown that the symmetry properties of barotropic nonlinear wind driven ocean circulation, as presented in the numerical simulations of Veronis (1966a, b), can easily be derived analytically for large Ekman number, the so-called “small beta regime”, and for any strength of the nonlinearity, by a perturbation expansion in the inverse Ekman number. The solution is a superposition of a vorticity monopole, dipole and quadrupole, the symmetry properties of which are qualitatively the same as those of the Veronis' sequence. It is also shown that the Veronis' sequence does not lead to the inertial Fofonoff (1954) mode, but that the latter is the limit of another sequence in the parameter plane spanned by the Rossby and Ekman numbers.

Barotropic and Baroclinic Ageostrophic Winds and Completeness of Solution for the Psi Equations

Abstract This paper examines the completeness of the solutions for three-dimensional ageostrophic circulations determined from the quasigeostrophic psi equations (or from the counterpart semigeostrophic psi equations in geostrophic-coordinate space). It is shown that a complete solution should contain two parts: a baroclinic part and a barotropic part. The solution of the psi equations with homogeneous (zero) upper and lower boundary conditions yields only the baroclinic part of the ageostrophic wind field, while the barotropic part can be recovered from the ageostrophic vorticity equation. The barotropic ageostrophic wind field is two-dimensional and nondivergent. It may be subdivided into rotational and harmonic pans, respectively forced by barotropic ageostrophic vorticity and by nonhomogeneous lateral boundary conditions (with nonzero cross-boundary mass flux). Methods for obtaining the complete solution are proposed and examples are given, showing the possible significance of the barotropic ageostrop...

Wintertime Blocking and Mountain Forcing of the Zonally-Averaged Flow: A Cross-Spectral Time Series Analysis of Observed Data

Abstract Nine winters of Northern Hemisphere observations are used for a statistical analysis of the relation between the mountain forcing of the zonally-averaged barotropic wind and blocking. Thereby, the temporal variability of the phenomenon “blocking” is expressed by a blocking index lime series. It is demonstrated that the zonally averaged wind exhibits statistically significant negative anomalies at midlatitudes (40°N, 53°N) for the early blocking stage. The mountain drag is anomalously negative (corresponding to a slow-down of the zonal mean wind) at high (53°N, 70°N) and at middle (40°N, 53°N) latitudes. These significant negative mountain drag anomalies are found before the onset or at the very first stage of blocking. In terms of a signal-to-noise ratio, the strongest impact of the mountain drag on the zonal mean wind tendency occurs at the latitudes (30°N, 40°N), where the main mountain ranges of the earth are located.

A numerical model for viscous, free-surface, barotropic wind driven ocean circulations

As a first step in investigating the consequences of making the fixed-surface assumption in ocean circulation models, a numerical model for free-surface barotropic flow in a basin on a beta-plane is proposed. Two numerical solutions are presented. The first solution was obtained using a standard linear eddy viscosity (κ = 10 8 cm 2/sec) and it agrees with Gates' solution. The second solution was obtained using a nonlinear eddy viscosity due to Leith and based on two-dimensional turbulence arguments. The two solutions differ in that the nonlinear eddy viscosity permits an intensification of interior flows and a transient modification of the western boundary current.