Anomalous Momentum Flux Research Articles

The Role of Interactions between Multiscale Circulations on the Observed Zonally Averaged Zonal Wind Variability Associated with the Madden–Julian Oscillation

Abstract The mechanisms driving the upper-tropospheric zonal mean intraseasonal zonal wind associated with the Madden–Julian oscillation are examined through budget analysis during boreal winter. To diagnose the role of nonlinear and cross-scale interactions, the wind fields are decomposed into three temporal bands, including the intraseasonal time scale (30–100 days), and periods shorter and longer than the intraseasonal time scales. The intraseasonal zonal mean circulation and its driving mechanisms are first examined based on the leading EOFs of the intraseasonal zonal wind. Consistent with previous studies of intraseasonal atmospheric angular momentum, the upper-troposphere zonal mean intraseasonal zonal wind anomaly begins in the tropics and propagates poleward. Results show that interaction between the background state and intraseasonal-time-scale zonally symmetric and asymmetric circulation helps drive changes in the tropical intraseasonal zonal wind and its poleward propagation. The intraseasonal anomalous circulation also modulates the characteristics of the transient eddies that induce anomalous momentum flux convergence that then helps to accelerate further the intraseasonal zonal wind in the extratropics. Results also suggest that feedbacks between the anomalous intraseasonal circulation and transient eddies have some sensitivity to event-to-event variability of the MJO.

Stratosphere‐troposphere evolution during polar vortex intensification

Stratosphere‐troposphere evolution associated with polar vortex intensification (VI) events is examined during the Northern Hemisphere winter. The incipient stage of a VI event is marked by anomalously low wave activity and descending westerly anomalies over the depth of the polar stratosphere. Reduced poleward planetary wave heat flux occurs as the circumpolar wind becomes strongest and pressure anomalies penetrate toward the surface. Descending pressure patterns project strongly onto the positive state of the Northern Hemisphere Annular Mode (NAM). Concurrently, anomalous poleward momentum flux develops in the upper troposphere, and the related tropospheric mean meridional circulation maintains the attendant wind and temperature anomalies against surface drag. The gross behavior of the composite VI event is similar in shape but opposite in sign to that associated with sudden stratospheric warming events (SSWs). However, the descent of the wind and temperature anomalies over the VI life cycle is generally weaker and slower than its SSW counterpart preceding the maximum vortex anomaly. Similarly, after the maximum wind event, the weakening of the winds is faster than the strengthening of the winds after a SSW. This is because stratospheric wind reduction anomalies are produced by wave driving, which can be rapid, and increases in wind speed are associated with the radiative cooling of the polar cap, which happens more gradually. While the contributions of the anomalous momentum fluxes by the quasi‐stationary and synoptic eddies are similar to SSWs, the much stronger anomalous momentum flux observed during VI can be attributed to the larger role of eddies with timescales between 15 and 40 days and of wave number 2 scale. Notable differences between VI and SSW appear in the tropical region. In particular, anomalous vortex intensification seems to occur preferentially during La Niña conditions.

The Life Cycle of the Northern Hemisphere Sudden Stratospheric Warmings

Motivated by recent evidence of strong stratospheric‐tropospheric coupling during the Northern Hemisphere winter, this study examines the evolution of the atmospheric flow and wave fluxes at levels throughout the stratosphere and troposphere during the composite life cycle of a sudden stratospheric warming. The composite comprises 39 major and minor warming events using 44 years of NCEP‐NCAR reanalysis data. The incipient stage of the life cycle is characterized by preconditioning of the stratospheric zonal flow and anomalous, quasistationary wavenumber-1 forcing in both the stratosphere and troposphere. As the life cycle intensifies, planetary wave driving gives rise to weakening of the stratospheric polar vortex and downward propagation of the attendant easterly wind and positive temperature anomalies. When these anomalies reach the tropopause, the life cycle is marked by momentum flux and mean meridional circulation anomalies at tropospheric levels that are consistent with the negative phase of the Northern Hemisphere annular mode. The anomalous momentum fluxes are largest over the Atlantic half of the hemisphere and are associated primarily with waves of wavenumber 3 and higher.

Mechanism of Zonal Index Evolution in a Two-Layer Model

A mechanism that drives a zonal jet to meander in the meridional direction is investigated with a two-layer multiwave quasigeostrophic β-plant channel model. This model isolates the zonal index characteristics of a purely eddy-driven jet. Empirical orthogonal function analysis is used to characterize the northward- and southward-shifted states of the jet, and the authors refer to these two states as “high” and “low” zonal indexes, respectively. Composite analysis is used to examine the evolution of the zonal-mean flow, eddy heat and momentum fluxes, storm tracks, and energetics associated with both zonal indexes. It is found that the zonal index is the most prominent form of variability over a broad range of meridional scales for the initially unstable region. As expected, the onset of either index is marked by an anomalous momentum flux convergence-divergence pair on either side of the time-mean jet, and the high and low indexes are dynamically equivalent. This eddy forcing of the zonal-mean flow takes place on a timescale much shorter than that for the persistence of the zonal index itself. During most of the zonal index persistence, the zonal wind anomaly decays slowly. From a qualitative viewpoint, the zonal index can be interpreted as being impulsively forced by the eddies. Both the composite analysis and maps of instantaneous potential vorticity suggest that the zonal index persistence is not maintained by eddy-zonal-mean flow feedback. It is shown that the eddy forcing in normal-mode baroclinic life cycles does not explain the onset to either zonal index; however, its role during the zonal index persistence is inconclusive. When two consecutive persistent zonal index states are of opposite sign, the onset for the latter state is typically characterized by merging of two disturbances along two potential vorticity “fronts.”

Organization of Storm Track Anomalies by Recurring Low-Frequency Circulation Anomalies

Abstract From previous studies it is known that anomalous momentum fluxes by bandpass eddies are important in maintaining long-lasting tropospheric flow anomalies. Evidence is presented that suggests that these anomalous fluxes do not occur at random but happen because the structure of storm track activity is modified by the presence of prominent low-frequency, large-scale circulation anomalies. This behavior is noted in an extended integration of a perpetual January simulation with a general circulation model (GCM). Because nonlinear feedbacks between low- and high-frequency variability make it difficult to establish causal relationships between these two ranges of variability, a model is constructed that approximates the storm track activity that would be expected to accompany a given low frequency without including the feedback of the high frequencies onto the low-frequency state. This model is based on the linearized primitive equations and uses a series of short integrations from random initial condi...

Simulation of Kelvin‐Helmholtz instability at the magnetospheric boundary

A two‐dimensional magnetohydrodynamic simulation of Kelvin‐Helmholtz instability at the terrestrial magnetospheric boundary is performed by including gradients of plasma and magnetic field normal to the dayside low‐latitude magnetospheric boundary. A magnetopause current layer is corrugated highly nonlinearly by the instability, and a plasma blob is formed by an interchange motion associated with the instability. The magnetosheath plasma flow momentum is diffused into the magnetosphere by the anomalous tangential (Reynolds plus Maxwell) stresses associated with the instability, and a wide velocity boundary layer is formed just inside the magnetopause current layer, while the thickness of the magnetopause current layer remains almost constant during the evolution of the instability. The convection voltage drop (integral of the convection electric field) across the velocity shear layer is amplified several times by the anomalous momentum transport associated with the instability. The anomalous momentum flux into the magnetosphere (tangential stress) reaches 0.6 to a few percent of the magnetosheath flow momentum flux, and this anomalous momentum flux into the magnetosphere is sufficient for accounting for the observed tailward momentum flux in the low‐latitude boundary layer. The value of the anomalous viscosity νano depends importantly on the magnetosheath Alfvén Mach number MA, which is defined by a magnetosheath magnetic field component parallel to the magnetosheath flow velocity; for MA = 2.5,νano is equal to ∼ 0.014 × 2aV0, where 2a is the thickness of the initial velocity shear layer and V0 is the total jump of the flow velocity across the magnetospheric boundary, and it increases with MA, and for MA > 5.0 it is equal to ∼0.2 × 2aV0. For a reasonable set of parameters at the dayside magnetospheric boundary the anomalous viscosity obtained is just the right magnitude for driving a magnetospheric convection in the terrestrial magnetosphere.