Initially Forced Long Planetary Waves in the Presence of Nonzonal Mean Flow

2006 ◽  
Vol 36 (3) ◽  
pp. 507-525 ◽  
Author(s):  
Ivana Cerovečki ◽  
Roland A. de Szoeke
Keyword(s):  
Steady Flow ◽  
Forced Response ◽  
Planetary Waves ◽  
Subtropical Gyre ◽  
Ekman Pumping ◽  
Mean Flow ◽  
Background Flow ◽  
Uppermost Layer ◽  
Lateral Extent ◽  
Long Wave Approximation

Abstract The purpose of this paper is to understand how long planetary waves evolve when propagating in a subtropical gyre. The steady flow of a wind-driven vertically sheared model subtropical gyre is perturbed by Ekman pumping that is localized within a region of finite lateral extent and oscillates periodically at about the annual frequency after sudden initiation. Both the background flow and the infinitesimal perturbations are solutions of a 2½-layer model. The region of forcing is located in the eastern part of the gyre where the steady flow is confined to the uppermost layer (shadow zone). The lateral scales of the forcing and of the response are supposed to be small enough with respect to the overall gyre scale that the background flow may be idealized as horizontally uniform, yet large enough (greater than the baroclinic Rossby radii) that the long-wave approximation may be made. The latter approximation limits the length of time over which the solutions remain valid. The solutions consist of (i) a forced response oscillating at the forcing frequency in which both stable (real) and zonally growing (complex) meridional wavenumbers are excited plus (ii) a localized transient structure that grows as it propagates away from the region of forcing. Application of the method of stationary phase provides analytical solutions that permit clear separation of the directly forced part of the solution and the transient as well as estimation of the temporal growth rate of the transient, which proves to be convectively unstable. The solutions presented here are relevant to understanding the instability of periodic (including annual period) perturbations of oceanic subtropical gyres on scales larger than the baroclinic Rossby radii of deformation.

scholarly journals Understanding of European Cold Extremes, Sudden Stratospheric Warming, and Siberian Snow Accumulation in the Winter of 2017/18

2020 ◽  
Vol 33 (2) ◽  
pp. 527-545 ◽  
Author(s):  
Zhuozhuo Lü ◽  
Fei Li ◽  
Yvan J. Orsolini ◽  
Yongqi Gao ◽  
Shengping He
Keyword(s):  
Snow Depth ◽  
Time Lag ◽  
Snow Accumulation ◽  
Arctic Sea Ice ◽  
Planetary Waves ◽  
Stratospheric Warming ◽  
Mean Flow ◽  
Sudden Stratospheric Warming ◽  
Cold Extremes

AbstractIt is unclear whether the Eurasian snow plays a role in the tropospheric driving of sudden stratospheric warming (SSW). The major SSW event of February 2018 is analyzed using reanalysis datasets. Characterized by predominant planetary waves of zonal wave 2, the SSW developed into a vortex split via wave–mean flow interaction. In the following two weeks, the downward migration of zonal-mean zonal wind anomalies was accompanied by a significant transition to the negative phase of the North Atlantic Oscillation, leading to extensive cold extremes across Europe. Here, we demonstrate that anomalous Siberian snow accumulation could have played an important role in the 2018 SSW occurrence. In the 2017/18 winter, snow depths over Siberia were much higher than normal. A lead–lag correlation analysis shows that the positive fluctuating snow depth anomalies, leading to intensified “cold domes” over eastern Siberia (i.e., in a region where the climatological upward planetary waves maximize), precede enhanced wave-2 pulses of meridional heat fluxes (100 hPa) by 7–8 days. The snow–SSW linkage over 2003–19 is further investigated, and some common traits among three split events are found. These include a time lag of about one week between the maximum anomalies of snow depth and wave-2 pulses (100 hPa), high sea level pressure favored by anomalous snowpack, and a ridge anchoring over Siberia as precursor of the splits. The role of tropospheric ridges over Alaska and the Urals in the wave-2 enhancement and the role of Arctic sea ice loss in Siberian snow accumulation are also discussed.

scholarly journals Climatic variability of the mean flow and stationary planetary waves in the NCEP/NCAR reanalysis data

2008 ◽  
Vol 26 (5) ◽  
pp. 1233-1241 ◽  
Author(s):  
A. Yu. Kanukhina ◽  
E. V. Suvorova ◽  
L. A. Nechaeva ◽  
E. K. Skrygina ◽  
A. I. Pogoreltsev
Keyword(s):  
Lower Stratosphere ◽  
Climatic Variability ◽  
Reanalysis Data ◽  
Planetary Waves ◽  
Lower Atmosphere ◽  
Wave Number ◽  
Mean Flow ◽  
The Mean ◽  
The 1960S ◽  
Environmental Prediction

Abstract. NCEP/NCAR (National Center for Environmental Prediction – National Center for Atmospheric Research) data have been used to estimate the long-term variability of the mean flow, temperature, and Stationary Planetary Waves (SPW) in the troposphere and lower stratosphere. The results obtained show noticeable climatic variabilities in the intensity and position of the tropospheric jets that are caused by temperature changes in the lower atmosphere. As a result, we can expect that this variability of the mean flow will cause the changes in the SPW propagation conditions. The simulation of the SPW with zonal wave number m=1 (SPW1), performed with a linearized model using the mean flow distributions typical for the 1960s and for the beginning of 21st century, supports this assumption and shows that during the last 40 years the amplitude of the SPW1 in the stratosphere and mesosphere increased substantially. The analysis of the SPW amplitudes extracted from the geopotential height and zonal wind NCEP/NCAR data supports the results of simulation and shows that during the last years there exists an increase in the SPW1 activity in the lower stratosphere. These changes in the amplitudes are accompanied by increased interannual variability of the SPW1, as well. Analysis of the SPW2 activity shows that changes in its amplitude have a different sign in the northern winter hemisphere and at low latitudes in the southern summer hemisphere. The value of the SPW2 variability differs latitudinally and can be explained by nonlinear interference of the primary wave propagation from below and from secondary SPW2.

How Purely Wind-Driven Long Planetary Geostrophic Waves May Be Energized in the Western Part of Ocean Subtropical Gyres

2007 ◽  
Vol 37 (1) ◽  
pp. 60-70 ◽  
Author(s):  
Ivana Cerovečki ◽  
Roland de Szoeke
Keyword(s):  
Large Scale ◽  
Satellite Observations ◽  
Subtropical Gyre ◽  
Background Flow ◽  
Flow Perturbation ◽  
Spatial Growth ◽  
Unstable Solutions ◽  
Annual Frequency ◽  
Spatially Varying ◽  
Annual Period

Abstract Satellite observations and idealized numerical studies reveal intensification of long-period (on the order of one cycle per year) waves in the western part of ocean basins. The authors explore the idea that the intensification is associated with the spatial growth of purely time periodic, but baroclinically unstable, motions. The framework is a simple idealized 2½-layer model in which only the upper layer is directly forced by the wind, a setting similar to the shadow zone of the Luyten–Pedlosky–Stommel (LPS) model. The upper two layers participate in the wave motion, which is driven by a large-scale wind stress fluctuating with the annual period, representing the seasonal cycle. Although possibly unstable solutions exist everywhere in the subtropical gyre on account of the nonzero meridional background flow, they are not seen in the eastern part of the basin in satellite observations nor are they excited there by model gyre-scale annual-period winds. Instead, energy injected into the model ocean at a fixed frequency and with zonal and meridional wavenumbers, such that the resulting flow perturbation is locally stable, refracts westward as it propagates through the spatially varying background flow without change of frequency and reaches distant regions where the spatial wavenumber becomes complex so that spatial growth occurs. This process results in spatially growing solutions of annual or near-annual frequency only in the southwestern part of the model subtropical gyre, thus explaining why the intensification is preferentially manifested in the southwestern subtropical gyre in published numerical model results. The paper concludes with a discussion of relevant satellite and in situ observations.

Evolution of solitary marginal disturbances in baroclinic frontal geostrophic dynamics with dissipation and time-varying background flow

2007 ◽  
Vol 463 (2083) ◽  
pp. 1749-1769 ◽  
Author(s):  
Mattea R Turnbull ◽  
Gordon E Swaters
Keyword(s):  
Lower Layer ◽  
Baroclinic Instability ◽  
Variable Coefficients ◽  
Time Variability ◽  
Perturbation Approach ◽  
Dynamical Regime ◽  
Time Varying ◽  
Mean Flow ◽  
Background Flow ◽  
Frontal Geostrophic Dynamics

A two-layer frontal geostrophic flow corresponds to a dynamical regime that describes the low-frequency evolution of baroclinic ocean currents with large amplitude deflections of the interface between the layers on length-scales longer than the internal deformation radius within the context of a thin upper layer overlying a dynamically active lower layer. The finite-amplitude evolution of solitary disturbances in baroclinic frontal geostrophic dynamics in the presence of time-varying background flow and dissipation is shown to be governed by a two-equation extension of the unstable nonlinear Schrödinger (UNS) equation with variable coefficients and forcing. The soliton solution of the unperturbed UNS equation corresponds to a saturated isolated coherent anomaly in the baroclinic instability of surface-intensified oceanographic fronts and currents. The adiabatic evolution of the propagating soliton and the uniformly valid first-order perturbation fields are determined using a direct perturbation approach together with phase-averaged conservation relations when both dissipation and time variability are present. It is shown that the soliton amplitude parameter decays exponentially due to the presence of the dissipation but is unaffected by the time variability in the background flow. On the other hand, the soliton translation velocity is unaffected by the dissipation and evolves only in response to the time variability in the background flow. The adiabatic solution for the induced mean flow exhibits a dissipation-generated ‘shelf region’ in the far field behind the soliton, which is removed by solving the initial-value problem.

Mixing Enhancement in a Chaotic Micromixer Using Pulsating Flow

2014 ◽  
Author(s):  
Mohammad Karami ◽  
Mojtaba Jarrahi ◽  
Ebrahim Shirani ◽  
Hassan Peerhossaini
Keyword(s):  
Reynolds Number ◽  
Lyapunov Exponent ◽  
Steady Flow ◽  
Concentration Distribution ◽  
Pulsating Flow ◽  
Chaotic Advection ◽  
Mixing Enhancement ◽  
Mean Flow ◽  
Number Range ◽  
Velocity Amplitude

This study determines the simultaneous effects of spatial disturbance and flow pulsation on micromixing by using three different metrics: concentration distribution, Lyapunov exponent and axial vorticity. Numerical simulations are performed for both steady and pulsating flows through a microchannel made up of C-curved repeating units. Moreover, a straight microchannel is analyzed to compare the effects of chaotic advection and molecular diffusion, the main mechanisms of transverse mixing in the chaotic and straight mixer respectively. Simulations are carried out in the steady flow for the Reynolds number range 1≤Re≤50 and in the pulsating flow for velocity amplitude ratios 1≤β≤2.5, and the ratio of the peak oscillatory velocity component to the mean flow velocity, Strouhal numbers 0.1≤St≤0.5. It was found that chaotic advection improves mixing without significant increase in pressure drop. The analysis of concentration distribution implied that full mixing occurs after Reynolds number 50 in the steady flow. When the flow is pulsatile, small and moderate values of the Strouhal number (0.1≤St≤0.3) and high values of velocity amplitude ratio (β ≥ 2) are favorable conditions for mixing enhancement. Moreover, mixing has an oscillating trend along the microchannel due to the coexistence of regular and chaotic zones in the fluid. These results correlate closely with those obtained using two other metrics, analysis of the Lyapunov exponent and axial vorticity.

scholarly journals Interannual Variability of the Intraseasonal Oscillation and Its Impact on the Summertime Wave Patterns and Tropical Cyclones over the Western North Pacific

2017 ◽  
Vol 145 (9) ◽  
pp. 3465-3483 ◽  
Author(s):  
Ken-Chung Ko ◽  
Jyun-Hong Liu
Keyword(s):  
Tropical Cyclones ◽  
Large Scale ◽  
Intraseasonal Oscillation ◽  
Low Frequency ◽  
Mean Flow ◽  
Background Flow ◽  
Wave Patterns ◽  
Intraseasonal Oscillations ◽  
The Mean ◽  
Local Weather

In this study, intraseasonal oscillations (ISOs) and submonthly wave patterns were separated into maximal variance (MaxV) and minimal variance (MinV) years on the basis of ISO variance from July to October. The mean-state 850-hPa streamfunction for submonthly cases indicated that, in the MinV years, tropical cyclones (TCs) formed near areas southeast of those in the MaxV years. ISOs propagated northward in the MaxV years, whereas a weaker westward-propagating tendency was observed in the MinV years. Track analysis of the centers of the submonthly cyclonic anomalies suggested that the background flow dictated the propagation routes of the easterly cyclonic anomalies in the MaxV years. However, the propagation routes of the westerly cyclonic anomalies were barely affected by the background flow. Further analysis of the ISO mean flow patterns showed that in the MaxV years, the propagation routes of the westerly cyclonic anomalies were more likely controlled by the anomalous easterly flow generated by the ISO westerly cyclonic anomalies. Moreover, rainfall was heavier in Taiwan in the MaxV years because the background flow in the MinV years caused the submonthly cyclonic anomaly tracks to shift away from Taiwan. Therefore, low-frequency large-scale circulations can affect smaller-scale phenomena and local weather.

scholarly journals On Baroclinic Instability over Continental Shelves: Testing the Utility of Eady-Type Models

2020 ◽  
Vol 50 (1) ◽  
pp. 3-33
Author(s):  
Shih-Nan Chen ◽  
Chiou-Jiu Chen ◽  
James A. Lerczak
Keyword(s):  
Growth Rate ◽  
Phase Locking ◽  
Baroclinic Instability ◽  
Lower Boundary ◽  
Ekman Pumping ◽  
Horizontal Shear ◽  
Bottom Slope ◽  
Mean Flow ◽  
Model Calculations ◽  
The Mean

AbstractThis study examines the utility of Eady-type theories as applied to understanding baroclinic instability in coastal flows where depth variations and bottom drag are important. The focus is on the effects of nongeostrophy, boundary dissipation, and bottom slope. The approach compares theoretically derived instability properties against numerical model calculations, for experiments designed to isolate the individual effects and justified to have Eady-like basic states. For the nongeostrophic effect, the theory of Stone (1966) is shown to give reasonable predictions for the most unstable growth rate and wavelength. It is also shown that the growing instability in a fully nonlinear model can be interpreted as boundary-trapped Rossby wave interactions—that is, wave phase locking and westward phase tilt allow waves to be mutually amplified. The analyses demonstrate that both the boundary dissipative and bottom slope effects can be represented by vertical velocities at the lower boundary of the unstable interior, via inducing Ekman pumping and slope-parallel flow, respectively, as proposed by the theories of Williams and Robinson (1974; referred to as the Eady–Ekman problem) and Blumsack and Gierasch (1972). The vertical velocities, characterized by a friction parameter and a slope ratio, modify the bottom wave and thus the scale selection. However, the theories have inherent quantitative limitations. Eady–Ekman neglects boundary layer responses that limit the increase of bottom stress, thereby overestimating the Ekman pumping and growth rate reduction at large drag. Blumsack and Gierasch’s (1972) model ignores slope-induced horizontal shear in the mean flow that tilts the eddies to favor converting energy back to the mean, thus having limited utility over steep slopes.

scholarly journals Mean Flow in the Gulf of Mexico

2008 ◽  
Vol 38 (7) ◽  
pp. 1501-1514 ◽  
Author(s):  
Wilton Sturges ◽  
Kern E. Kenyon
Keyword(s):  
Gulf Of Mexico ◽  
Data Sources ◽  
Loop Current ◽  
Independent Data ◽  
Ekman Pumping ◽  
Mean Flow ◽  
The West ◽  
Net Flux ◽  
Yucatan Channel ◽  
Current Potential

Abstract Several independent data sources suggest that there is a net upper-layer mass flux O(3 Sv) (Sv ≡ 106 m3 s−1) to the west in the central Gulf of Mexico, even though the western gulf is a closed basin. A plausible explanation is that this net flux is pumped downward by the convergent wind-driven Ekman pumping, as is typical of all midlatitude anticlyclonic gyres. The downward flux can follow isopycnals to depths O(500–600 m) and deeper by eddy mixing; a mechanism for forcing deep water to the south through the Yucatan Channel is provided by the intrusion and ring-shedding cycle of the Loop Current. Potential vorticity maps show that a deep flow from the western gulf back to the Yucatan Channel is likely.

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