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Dehai Luo

Abstract

In this paper, a basic-state flow that has a linear, weak meridional shear is used as a mean flow condition to see if the blocking activity is related to the state of the zonal mean flow based upon the envelope soliton theory proposed in Part I. It is found that an isolated coherent structure similar to a dipole block can arise from the resonant interaction between preexisting planetary- and synoptic-scale waves, but its asymmetry, intensity, and persistence depend strongly upon the horizontal shear of the basic-state flow prior to block onset. The cyclonic shear of the basic-state flow not only plays an important role in preventing the eastward spread of block energy, but also provides a strong diffluent flow as a precursor of block flow. In such an environment, a high amplitude dipole anomaly is maintained and the deformed synoptic-scale eddies tend to deflect northward. But the anticyclonic shear of a basic-state flow plays a reverse role, which causes deformed eddies to deflect southward.

It is also found that positive westerly wind anomalies at both high and low latitudes and negative anomalies at middle latitudes are maintained by synoptic-scale eddies through the self-interaction of block. In this case, the zonal mean westerly wind is accelerated at high and low latitudes, and decelerated at middle latitudes. Only for a basic state with cyclonic shear is the meridional profile of the zonal mean wind during the onset of a dipole block in agreement with observations.

In addition, it can be shown by computing scatter diagrams of potential vorticity against streamfunction in different sheared environments that an eddy-induced isolated block can exhibit a local free-mode characteristic. Two approximately linear functional relationships between potential vorticity and streamfunction are found to be probably attributed to synoptic eddies.

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Dehai Luo

Abstract

The role of westward-traveling planetary waves in the block onset and the deformation of eddies during the interaction between synoptic-scale eddies and an incipient block is first examined by constructing an incipient block that consists of a stationary dipole wave for zonal wavenumber 2 and a westward-traveling monopole wave with constant amplitude (C wave) for zonal wavenumber 1 or 2. It is shown that the C-wave can affect the onset and strength of blocking through influencing the preblock (diffluent) flow even though it does not affect the amplification of the dipole wave associated with the synoptic-scale eddies. Whether the storm tracks organized by the deformed eddies deflect northward depends upon the zonal wavenumber, amplitude, and phase of the C wave relative to the stationary dipole wave. A typical retrograde blocking anticyclone can arise through the interaction of an incipient block with synoptic-scale perturbations when the C-wave ridge with zonal wavenumber 1 shifts westward from the east of the dipole wave in an incipient block. In this process, a slight northward deflection of organized storm tracks is also observed, particularly under the condition of a large-amplitude C wave.

In addition, the interaction between a diffluent flow, consisting of a coupled dipole and monopole waves, and upstream synoptic-scale eddies is investigated. It is found that the eddy forcing tends to not only periodically amplify the dipole soliton and to retard its eastward movement, but to make the monopole wave break up. The breaking of the traveling monopole wave will suppress the eddy-induced blocking ridge that exhibits a surf zone structure where the negative meridional gradient of planetary-scale potential vorticity exists and cause the planetary-scale blocking field to lose its closed circulation compared to that without coupling.

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Dehai Luo

Abstract

A new forced envelope Rossby soliton model in an equivalent barotropic beta-plane channel is proposed to describe the interaction between an incipient block (planetary scale) and short synoptic-scale eddies. This model is based on two assumptions, motivated by observations that (i) there exists a zonal scale separation between the planetary-scale and synoptic-scale waves and (ii) that the range of synoptic-scale zonal wavenumber is comparable to the planetary-scale zonal wavenumber. These assumptions allow an analytical treatment. The evolution of the planetary-scale block under the influence of synoptic-scale eddies is described by a forced nonlinear Schrödinger equation that is solved numerically, while the feedback of block development on the preexisting synoptic-scale eddies is derived analytically. It is shown that the planetary-scale projection of the nonlinear interaction between synoptic-scale eddies is the most important contributor to the amplification and decay of the planetary-scale blocking dipole or anticyclone, while the synoptic–planetary-scale interaction contributes significantly to the downstream development of preexisting synoptic-scale eddies. Large-scale topography plays a secondary role compared to the synoptic-scale eddies in exciting the block. However, it plays a role in inducing a standing planetary-scale ridge prior to block onset, which fixes the geographical location of the block and induces meridional asymmetry in the flow. In particular, the topographically induced planetary-scale ridge that is almost in phase with a dipole component of blocking flow is found to be a controlling factor for the northward deflection of storm tracks associated with blocking anticyclones.

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Dehai Luo

Abstract

In a series of previous papers, an envelope Rossby soliton theory was formulated to investigate the interaction between a preexisting planetary wave and synoptic-scale eddies leading to a typical blocking flow. In this paper, numerical and analytical studies are presented in order to examine the interactive relationship between an isolated vortex pair block and deformed synoptic-scale eddies during their interaction. The deformed blocked flow and eddies are found to satisfy the wavenumber conservation theorem. It is shown that the feedback by a blocked flow on the preexisting synoptic eddies gives rise to two types of eddies: one is the Z-type eddies with a meridional monopole structure that appears at the middle of the channel and the other is the M-type eddies with a meridional tripole structure that have long wavelength and large amplitude. Both the total wavenumber of the blocked flow and M-type eddies and the total wavenumber of the Z- and M-type eddies are conserved. The M- and Z-type eddies are compressed and elongated, respectively, as the blocked flow is elongated zonally during its onset phase, but the reverse is observed during the decay phase. The zonally elongated Z-type eddies are found to counteract the compressed M-type eddies in the blocking region, but strengthen the M-type eddies upstream, causing the split of eddies around the blocking region.

In addition, it is also verified theoretically that the blocked flow and synoptic-eddy activity are symbiotically dependent upon one another. The deformed (Z and M type) eddies also display a low-frequency oscillation in amplitude, wavenumber, group velocity, and phase speed, consistent with the blocked flow by the eddy forcing. Thus, it appears that the low-frequency eddy forcing is responsible for the low-frequency variability of the blocked flow and synoptic-eddy activity.

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Dehai Luo and Yan Lu

Abstract

In this paper, a new barotropic wind-driven circulation model is proposed to explain the enhanced transport of the western boundary current and the establishment of the recirculation gyre. In this model the harmonic Laplacian viscosity including the second-order term (the negative viscosity term) in the Prandtl mixing length theory is regarded as a tentative subgrid-scale parameterization of the boundary layer near the wall. First, for the linear Munk model with weak negative viscosity its analytical solution can be obtained with the help of a perturbation expansion method. It can be shown that the negative viscosity can result in the intensification of the western boundary current. Second, the fully nonlinear model is solved numerically in some parameter space. It is found that for the finite Reynolds numbers the negative viscosity can strengthen both the western boundary current and the recirculation gyre in the northwest corner of the basin. The drastic increase of the mean and eddy kinetic energies can be observed in this case. In addition, the analysis of the time-mean potential vorticity indicates that the negative relative vorticity advection, the negative planetary vorticity advection, and the negative viscosity term become rather important in the western boundary layer in the presence of the negative viscosity. Further, the influence of the resolution and the negative viscosity intensity on the solutions are also examined.

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Wenqi Zhang and Dehai Luo

Abstract

In this paper, the impact of winter Arctic sea ice concentration (SIC) decline over Baffin Bay, Davis Strait, and Labrador Sea (BDL) on Greenland blocking (GB) is first examined. It is found that the GB has a longer duration, a more notable westward movement, and a larger zonal scale in the low SIC winter than in the high SIC winter. In particular, the decay of GB may become slower than its growth in the low SIC winter, but the reverse is seen in the high SIC winter. The GB in the low SIC winter can have a more important impact on cold anomalies over North American midlatitudes than in the high SIC winter because of its slower decay and stronger retrogression. The influence of large BDL SIC loss on the GB mainly through reduced meridional potential vorticity gradient (PVy) related to reduced zonal winds over the North Atlantic mid- to high latitudes (NAMH) due to BDL warming is further examined by using the nonlinear phase speed and energy dispersion speed formula of blocking based on a nonlinear wave packet theory of atmospheric blocking. In this theory, the preexisting synoptic-scale eddies rather than the eddy straining or deformation is important for the blocking intensification and maintenance, which contradicts the eddy straining theory of Shutts. It is revealed from this theoretical model that under weaker NAMH zonal wind conditions the energy dispersion speed of GB may become smaller due to weaker PVy during its decaying phase than during the blocking growing phase, in addition to the GB having larger negative phase speed and stronger nonlinearity. The opposite is true when the PVy is larger. Thus, under a large SIC loss condition the GB shows notable retrogression, large zonal scales, and a long lifetime, which has a slower decay than its growth.

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Dehai Luo and Zhe Chen

Abstract

This paper is an extension of a theoretical study by Luo on the effect of large-scale land–sea contrast (LSC) topography on the formation of an eddy-driven blocking. It is found that the topography term can be included explicitly in the blocking evolution equation because of the inclusion of the higher-order wave–topography interaction. Although the blocking flow cannot be excited purely by the LSC topography, the LSC topography is found to be capable of enhancing the amplification of the dipole component in a blocking flow associated with upstream synoptic-scale eddies. In this case, a strong omega-type blocking high can be driven by the joint action of synoptic-scale eddies and LSC topography. This seems to provide an explanation of a difference in blocking intensity between the Northern and Southern Hemispheres. The most important finding of this paper is that in the presence of LSC topography the double jets that appear during the onset of an eddy-driven dipole block collapse into a strong single westerly jet that is within the south side of an omega-type blocking high, which is different from the result predicted by the theoretical model proposed in Luo’s previous work.

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Dehai Luo and Wenqi Zhang

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In this paper, an extended nonlinear multiscale interaction model is proposed to examine nonlinear behavior of eddy-driven blocking as a Rossby wave packet in a three-dimensional background flow by dividing the background meridional potential vorticity gradient (PVy) into dynamical PVy (PVyD) related to the horizontal (mainly meridional) shear of background westerly wind (BWW) and thermodynamic PVy (PVyT) associated with the meridional temperature gradient (MTG). It is found that eddy-driven baroclinic blocking with large amplitude in the midtroposphere tends to have a longer lifetime (~20 days) in a baroclinic atmosphere with stratification than eddy-driven barotropic blocking without vertical variation (less than 15 days). It is shown that barotropic blocking shows a northwest–southeast orientation and has long lifetime, large retrogression, and slow decay only for weaker barotropic BWW and PVyD in higher latitudes. In a baroclinic atmosphere with stratification, baroclinic blocking shows long lifetime, strong eastward movement, slow decay, weak strength, and less local persistence for large barotropic BWW and PVyD under PVyT=0, but becomes less slow decay, weak retrogression, and large local persistence for small barotropic BWW and PVyD. Such a blocking with a north–south antisymmetric dipole, large amplitude, and long local persistence, characterized by a persistent large meander of westerly jet streams, is easily seen when baroclinic BWW and PVyT are small in the lower to midtroposphere. Comparatively, the magnitude of PVyT plays a larger role in the blocking change than that of PVyD, whereas the vertical variation of MTG is more important for the blocking change than the MTG itself for some cases.

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Dehai Luo and Wenqi Zhang

Abstract

In this paper, a nonlinear multiscale interaction model is used to examine how the planetary waves associated with eddy-driven blocking wave packets propagate through the troposphere in vertically varying weak baroclinic basic westerly winds (BWWs). Using this model, a new one-dimensional finite-amplitude local wave activity flux (WAF) is formulated, which consists of linear WAF related to linear group velocity and local eddy-induced WAF related to the modulus amplitude of blocking envelope amplitude and its zonal nonuniform phase. It is found that the local eddy-induced WAF reduces the divergence (convergence) of linear WAF in the blocking upstream (downstream) side to favor blocking during the blocking growth phase. But during the blocking decay phase, enhanced WAF convergence occurs in the blocking downstream region and in the upper troposphere when BWW is stronger in the upper troposphere than in the lower troposphere, which leads to enhanced upward-propagating tropospheric wave activity, though the linear WAF plays a major role. In contrast, the downward propagation of planetary waves may be seen in the troposphere for vertically decreased BWWs. These are not seen for a zonally uniform eddy forcing. A perturbed inverse scattering transform method is used to solve the blocking envelope amplitude equation. It is found that the finite-amplitude WAF represents a modified group velocity related to the variations of blocking soliton amplitude and zonal wavenumber caused by local eddy forcing. Using this amplitude equation solution, it is revealed that, under local eddy forcing, the blocking wave packet tends to be nearly nondispersive during its growth phase but strongly dispersive during the decay phase for vertically increased BWWs, leading to strong eastward and upward propagation of planetary waves in the downstream troposphere.

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Tingting Gong and Dehai Luo

Abstract

In this paper, the lead–lag relationship between the Arctic sea ice variability over the Barents–Kara Sea (BKS) and Ural blocking (UB) in winter (DJF) ranging from 1979/80 to 2011/12 is examined. It is found that in a regressed DJF-mean field an increased UB frequency (days) corresponds to an enhanced sea ice decline over the BKS, while the high sea surface temperature over the BKS is accompanied by a significant Arctic sea ice reduction. Lagged daily regression and correlation reveal that the growth and maintenance of the UB that is related to the positive North Atlantic Oscillation (NAO+) through the negative east Atlantic/west Russia (EA/WR) wave train is accompanied by an intensified negative BKS sea ice anomaly, and the BKS sea ice reduction lags the UB pattern by about four days. Because the intensified UB pattern occurs together with enhanced downward infrared radiation (IR) associated with the intensified moisture flux convergence and total column water over the BKS, the UB pattern contributes significantly to the BKS sea ice decrease on a time scale of weeks through intensified positive surface air temperature (SAT) anomalies resulting from enhanced downward IR. It is also found that the BKS sea ice decline can persistently maintain even when the UB has disappeared, thus indicating that the UB pattern is an important amplifier of the BKS sea ice reduction. Moreover, it is demonstrated that the EA/WR wave train formed by the combined NAO+ and UB patterns is closely related to the amplified warming over the BKS through the strengthening (weakening) of mid-to-high-latitude westerly wind in the North Atlantic (Eurasia).

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