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Benkui Tan

Abstract

A theory is developed here to describe the propagation of nonlinear Rossby wave packets in a barotropic atmospheric model and their interactions by using the multiple-scale method. It is shown that the propagation of a single Rossby wave packet can be described by the nonlinear Schrödinger equation that has envelope soliton solutions. For two interacting packets with slightly different wavenumbers they satisfy a set of two coupled nonlinear Schrödinger equations. These equations are used to study the collision interactions of two envelope Rossby solitons. It is found that despite the complexity of the interaction, the energy of each soliton is conserved, while the shapes and velocities of the two solitons may be altered significantly by the interaction. The action of one soliton on another is realized by providing a field of force or potential for it through the cross-modulation terms.

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Benkui Tan

Abstract

In this paper, the classic Ekman pumping formulas for the vertical flow out of a boundary layer are generalized for both the layer above a rigid surface of variable slope and also for the boundary layer underneath a moving free surface. The assumptions that the density is constant and the geostrophic velocity does not vary with height in the boundary layer are relaxed without compromising the simplicity of the final approximation. Similarly, it proves to be unnecessary to assume a specific eddy viscosity law.

The pumping formulas obtained here are compared with the old ones and significant differences are found.

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Benkui Tan and Shikuo Liu

Abstract

In this paper the interactions between two marginally unstable baroclinic wave packets in the two-layer Phillips model are investigated by using the multiple-scale method. It is shown that the interactions can be described by a set of two coupled nonlinear Schrödinger equations. Except for two special cases the equations have only four invariants of motion and cannot be solved by the inverse scattering method.

The equations are solved numerically to study the collision interactions between two solitons. It is found that though the coefficients in the equations are fixed, the behavior of the two solitons may be quite different, which is closely related to the initial states of the two solitons (the speeds and the amplitudes of the solitons well before the interactions). For some initial conditions the collision interactions may be soliton-like in that the properties of the two solitons change very little, while for other initial conditions some “inelastic” phenomena are observed: one soliton may be destroyed by the other, or two solitons may change their speeds and directions of propagation and fuse into a new bound state.

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Anran Zhuge and Benkui Tan

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Based on daily data from the Japanese 55-year Reanalysis (JRA-55) covering the springs from 1958 to 2018, this study examines the formation mechanisms and climate impacts of springtime western Pacific (WP) pattern as subseasonal climate variability over North Pacific. Results suggest that the springtime WP pattern arises from a weak dipole-like disturbance over North Pacific and disturbances over East Asia. The energetic analysis suggests that the baroclinic energy conversion acts as an important energy source to balance the available potential energy loss caused by transient eddies and diabatic heating and acts as a kinetic energy (KE) source for the WP pattern. For the feedback forcing by total transient eddies, it acts as a major KE source for the WP pattern before day 0 and acts as a strong KE sink after day 0. It turns out that the barotropic energy conversion makes only weak KE contribution to the WP pattern.

Once the WP pattern forms, East Asia and North America experience strong surface air temperature anomalies of opposite signs, while strong sea surface temperature anomalies are found to occur over mid-latitude and tropical North Pacific at the same time. Concurrently, the Pacific jet and the storm track shift north-southward around their climatological position. In addition, a dipole-like shallow convective anomaly appears over mid-latitude North Pacific, and a band of anomalous deep convection tends to occur in the tropics as the energy of the WP pattern propagates into the region.

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Panxi Dai and Benkui Tan

Abstract

Through a cluster analysis of daily NCEP–NCAR reanalysis data, this study demonstrates that the Arctic Oscillation (AO), defined as the leading empirical orthogonal function (EOF) of 250-hPa geopotential height anomalies, is not a unique pattern but a continuum that can be well approximated by five discrete, representative AO-like patterns. These AO-like patterns grow simultaneously from disturbances in the North Pacific, the North Atlantic, and the Arctic, and both the feedback from the high-frequency eddies in the North Pacific and North Atlantic and propagation of the low-frequency wave trains from the North Pacific across North America into the North Atlantic play important roles in the pattern formation. Furthermore, it is shown that the structures and frequencies of occurrence of the five AO-like patterns are significantly modulated by El Niño–Southern Oscillation (ENSO). Warm (cold) ENSO enhances the negative (positive) AO phase, compared with ENSO neutral winters. Finally, the surface weather effects of these AO-like patterns and their implications for the AO-related weather prediction and the AO-North Atlantic Oscillation (NAO) relationship are discussed.

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Mingyuan Wang and Benkui Tan

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On the basis of daily data from the Japanese 55-year Reanalysis (JRA-55) for extended winters (December–March) from 1958/59 to 2014/15, this study examines the formation mechanisms and climate impacts of the subseasonal Scandinavian (SCA) pattern. Results indicate that the SCA pattern manifests itself as Rossby wave trains, arising from the initial height disturbances over the North Atlantic and propagating into the Scandinavian peninsula and central Siberia. One type of SCA may arise from a Rossby wave train over the North Atlantic that is closely coupled to an anomalous convective heating dipole and persists for about 2 weeks (convection-preceded SCAs). Another type of SCA arises from the weak height disturbances over the North Atlantic; the height disturbance over the Arctic also contributes to the SCA formation, with no significant convective heating anomalies being observed in the North Atlantic (convection-free SCAs). The results also indicate that both SCA types may cause strong climate anomalies in the Arctic and Eurasia that persist for about 2 weeks. The surface air temperature (SAT) anomalies assume a dipolar structure with one extremum located over the Greenland Sea through Barents Sea and the other extremum over the Eurasian continent. Associated with the SAT anomalies is a significant increase or decrease of sea ice cover over the Greenland Sea and Barents Sea, while over the Eurasian continent snow depth anomalies are found to occur over eastern Europe, western Asia, and the Russian Far East. Furthermore, as convection-free SCAs propagate vertically into the stratosphere, significant changes of intensity and air temperature of the stratospheric polar vortex are observed.

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Benkui Tan and John P. Boyd

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The evolution, both stable and unstable, of contrarotating vortex pairs (“modons”) perturbed by upper-surface and bottom Ekman pumping is investigated using a homogeneous model with a variable free upper-surface and bottom topography. The Ekman pumping considered here differs from the classical Ekman pumping in that the divergence-vorticity term in the vorticity equation, nonlinear and omitted in previous studies, is explicitly included. Under the influence of both nonlinear Ekman pumping and the beta term, eastward- and westward-moving modons behave very differently.

Eastward-moving modons are stable to the upper-surface perturbation but westward-moving modons are not. The latter move southwestward, triggering the tilt instability: the beta effect deepens the cyclones but weakens the anticyclone, and the vortex pair disperses into wave packets.

Eastward-moving modons are stable to bottom friction in the sense that they diminish in time gradually at a rate independent of the signs of the vortices. Westward-moving modons behave differently depending on the strength of bottom friction. Cyclones decay faster than anticyclones, triggering the tilt instability in westward-moving modons, but only if the bottom friction is very weak. For sufficiently strong bottom friction, in contrast, modons decay monotonically: the cyclones still decay faster than anticyclones, but no wave packets formed before the modons completely dissipate.

Westward-moving modons are always unstable to topographic forcing. Eastward-moving modons have varying behavior controlled by the height and width of the topography. Below a critical height, determined by the width, modons survive the topographic interaction: their trajectory meanders but the two contrarotating vortices always remain bound together after escaping the topography. Above the critical height, modons disassociate: the two vortices separate and disperse into wave packets. When the width of the topography is comparable to modon width, there exists a stable window within the unstable region of the topographic height in which the modons also survive the topographic encounter.

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Ying Dai and Benkui Tan

Abstract

Previous studies have mainly focused on the influence of El Niño–Southern Oscillation (ENSO) on seasonal-mean conditions over East Asia and North America. This study, instead, proposes an ENSO pathway that influences the weather events over East Asia and North America, in which the eastern Pacific teleconnection pattern (EP) plays an important role. On the one hand, the EP pattern can induce significant surface temperature anomalies over East Asia during its development and mature stages, with the positive (negative) EPs causing colder (warmer) than normal weather events. Besides, the frequency of occurrence of EPs is significantly modulated by ENSO, with 50% of the positive EPs occurring in La Niña winters, and 47% of the negative EPs occurring in El Niño winters. As a result, in El Niño winters, more negative and fewer positive EPs tend to occur, and thus more warm and fewer cold weather events are expected in East Asia. For La Niña winters, the reverse is true. On the other hand, for the EP pattern without its canonical convection pattern (referred to as the nonconvective EP), extremely cold anomalies over the northern United States and western Canada are induced in its negative phase. Moreover, when there are positive sea surface temperature anomalies in the central equatorial Pacific, the frequency of occurrence of negative nonconvective EPs is 2.0 times greater than the climatological value, and thus an enhanced likelihood of extremely cold spells over North America may be expected.

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Ying Dai and Benkui Tan

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The western Pacific (WP) pattern is a major teleconnection pattern that influences the wintertime Northern Hemisphere climate variations. Based on daily NCEP–NCAR reanalysis data, this study examines the climate impacts and the El Niño–Southern Oscillation (ENSO) modulation of two types of the WP pattern. The result shows that the WP patterns may arise from precursory disturbances over Asia and the North Pacific or from the Pacific–North American (PNA) pattern of the same polarity as or opposite polarity to that of the WP patterns. Among these WP patterns, the WP patterns that arise from the PNA pattern of the same polarity are most influential on North American near-surface and polar stratospheric air temperatures; furthermore, their frequency of occurrence, amplitude, and duration can be affected by ENSO phases: the positive WP patterns occur more frequently with larger amplitude and longer duration in El Niño than in La Niña; and the negative WP patterns occur less frequently with smaller amplitude and shorter duration in El Niño than in La Niña. The above findings suggest that the PNA pattern plays a crucial role in the climate impacts and the ENSO modulation of the WP patterns.

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Y. Qiang Sun, Yuxin Jiang, Benkui Tan, and Fuqing Zhang

Abstract

Through successful convection-permitting simulations of Typhoon Sinlaku (2008) using a high-resolution nonhydrostatic model, this study examines the role of peripheral convection in the storm's secondary eyewall formation (SEF) and its eyewall replacement cycle (ERC). The study demonstrates that before SEF the simulated storm intensifies via an expansion of the tangential winds and an increase in the boundary layer inflow, which are accompanied by peripheral convective cells outside the primary eyewall. These convective cells, which initially formed in the outer rainbands under favorable environmental conditions and move in an inward spiral, play a crucial role in the formation of the secondary eyewall. It is hypothesized that SEF and ERC ultimately arise from the convective heating released from the inward-moving rainbands, the balanced response in the transverse circulation, and the unbalanced dynamics in the atmospheric boundary layer, along with the positive feedback between these processes.

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