Studies of the Motion and Development of Long Waves in the Westerlies

Namias, Jerome; Clapp, Philip F.

doi:10.1175/1520-0469(1944)001<0057:sotmad>2.0.co;2

Cited by 59 publications

(27 citation statements)

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“…In this model, the zonal index is almost periodic, and thus predictable. Therefore, it is possible that the impressive poleward propagation noted by Riehl et al (1950) and the apparent success of the zonal-index predictability shown by Namias (1950) may be linked, particularly because the data periods used by these two studies overlap. Therefore, one may then speculate that the original zonal index cycle, in the sense of Rossby and Namias, may indeed operate during some time periods.…”

Section: Discussionmentioning

confidence: 99%

“…To this end, it is also worthwhile to consider a possible link between the poleward propagation and the zonal index cycle considered by Rossby (1939) and Namias (1950). In Fig.…”

Section: Discussionmentioning

confidence: 99%

“…During the past decade, much progress has been made toward improving our understating of zonalmean flow dynamics, a subject of research pioneered by Rossby (1939) and Namias (1950). However, there is one particularly striking and peculiar feature of zonal mean flow variability that has not been widely recognized by the atmospheric science community.…”

Section: Introductionmentioning

confidence: 99%

“…The conclusions follow in section 7 and the discussion and implications in section 8. 1 The zonal index, defined by Rossby (1939) and Namias (1950), refers to an alternation between two atmospheric states in midlatitudes, where one state is characterized by a strengthening and the other state by a weakening of the midlatitude westerlies. More recent studies find that the zonal index is better understood in terms of a meandering in the latitude of the midlatitude westerly jet (e.g., Robinson 1991;Yu and Hartmann 1993;Feldstein and Lee 1998).…”

Section: Introductionmentioning

confidence: 99%

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A Mechanism for the Poleward Propagation of Zonal Mean Flow Anomalies

Lee

Son

Grise

et al. 2007

Journal of the Atmospheric Sciences

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Observational studies have shown that tropospheric zonal mean flow anomalies frequently undergo quasi-periodic poleward propagation. A set of idealized numerical model runs is examined to investigate the physical mechanism behind this poleward propagation.This study finds that the initiation of the poleward propagation is marked by the formation of negative zonal wind anomalies in the Tropics. These negative anomalies arise from meridional overturning/breaking of waves that originate in midlatitudes. This wave breaking homogenizes the potential vorticity (PV) within the region of negative zonal wind anomalies, and also leads to the formation of positive zonal wind anomalies in the subtropics. Subsequent equatorward radiation of midlatitude waves is halted, which results in wave breaking at the poleward end of the homogenized PV region. This in turn generates new positive and negative zonal wind anomalies, which enables a continuation of the poleward propagation. The shielding of the homogenized PV region from equatorward wave propagation allows the model's radiative relaxation to reestablish undisturbed westerlies in the Tropics, while extratropical westerly anomalies arise from eddy vorticity fluxes.The above process indicates that the poleward zonal mean anomaly propagation is caused by an orchestrated combination of linear Rossby wave propagation, nonlinear wave breaking, and radiative relaxation. The importance of the meridional wave propagation and breaking is consistent with the fact that the poleward propagation occurs only in the parameter space of the model where the PV gradient is of moderate strength. Implications for predictability are briefly discussed.

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Section: Discussionmentioning

confidence: 99%

“…To this end, it is also worthwhile to consider a possible link between the poleward propagation and the zonal index cycle considered by Rossby (1939) and Namias (1950). In Fig.…”

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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A Mechanism for the Poleward Propagation of Zonal Mean Flow Anomalies

Lee

Son

Grise

et al. 2007

Journal of the Atmospheric Sciences

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“…It has long been appreciated that some extratropical troughs depend on an upstream trough or ridge for their movement and intensification (Namias and Clapp, 1944;Hovmöller, 1949). Comprehensive surveys of waves in the upper troposphere suggest that this dynamical dependence, known as downstream development, occurs throughout the extratropics (Lee and Held, 1993;Chang, 1993;Chang and Yu, 1999).…”

Section: Introductionmentioning

confidence: 99%

Downstream baroclinic development among forty‐one cold‐season Eastern North Pacific cyclones

2004

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On the Role of the Tropics in the General Circulation of the Atmosphere

Riehl

1969

Weather

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Present knowledge on the role of the tropics in the general circulation is reviewed in comparison with our knowledge twenty years ago. Increasing amounts of upper‐air data in the tropics since that time plus the advent of rawin measurements have placed many calculations of the mean seasonal exchange of heat, moisture and angular momentum between the tropics and the extratropical regions on a fairly firm basis. The large troughs seen extending from higher latitudes to the heart of the tropics even with late World War II observations have been confirmed in their role as the principal exchange mechanism. On the other hand, the mean meridional circulation can account for the upward transport of angular momentum which is generated with easterlies within the tropics and also for the net generation of kinetic energy south of 30°N. About 50 per cent of the kinetic energy required north of 30°N in winter to maintain the circulation against frictional dissipation, is imported from the tropics by means of the standing waves of the subtropical jet‐stream. Hence it is clear that the tropics help to sustain the kinetic energy of the westerlies and that the tropical circulation is not driven from higher latitudes as often claimed. The sense of total energy transport by the mean meridional circulation inside the tropics is not easily calculated and may even reverse sign. Complications on total energy export also have arisen through the nimbus ii albedo observations which have shown lower albedo (less cloudiness), hence also a larger energy source, for the tropics than previously estimated. It appears certain, merely, that the equatorial trough migrates seasonally so that energy very likely does not pass through it, hence that it constitutes a true “meteorological equator”. No evidence concerning important coupling between the hemispheres across the equator has come to light; however, the new data systems of the next decade may well alter this conclusion. Because of the sensitive computational situation concerning energy transport by the mean meridional circulation, it may be that successful operational application of cloud seeding techniques, such as demonstrated in Project Stormfury in the Caribbean, could exert a subtle influence on the general circulation. This apart, the present status of observational networks is judged to be so far advanced that fluctuations of quantities such as the mean meridional circulation within a season, and in the same month from year to year, can shortly be calculated, possibly shedding light on the variability of the general circulation and precipitation excesses and deficits in tropical regions.

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Studies of the Motion and Development of Long Waves in the Westerlies

Cited by 59 publications

References 0 publications

A Mechanism for the Poleward Propagation of Zonal Mean Flow Anomalies

A Mechanism for the Poleward Propagation of Zonal Mean Flow Anomalies

Downstream baroclinic development among forty‐one cold‐season Eastern North Pacific cyclones

On the Role of the Tropics in the General Circulation of the Atmosphere

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