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    Three-Dimensional Linear Instability of Persistent Anomalous Large-Scale Flows

    Source: Journal of the Atmospheric Sciences:;1988:;Volume( 046 ):;issue: 018::page 2783
    Author:
    Robertson, Andrew W.
    ,
    Metz, Werner
    DOI: 10.1175/1520-0469(1989)046<2783:TDLIOP>2.0.CO;2
    Publisher: American Meteorological Society
    Abstract: The linear baroclinic instability of three-dimensional basic flows on the Northern Hemisphere is examined in terms of a simple two-level, quasi-geostrophic model. The basic flows considered comprise an observed six-winter mean flow, as well as anomalous flows which represent episodes where large-scale persistent flow anomalies, such as the Pacific/North American (PNA), East Atlantic (EA), or North Atlantic Oscillation (NAO) patterns exhibit large amplitudes. For the climatological basic state, the fastest-growing normal modes with periods of around 4 days consist of regionally confined, synoptic-scale, baroclinic wave trains. These are considered as cyclogenesis modes, characterizing the linear synoptic-scale eddy activity associated with a given basic flow. This eddy activity has a pronounced maximum over the Pacific, close to the position of the observed Pacific storm track, but the second maximum over the Atlantic, corresponding to the Atlantic storm track, is considerably underestimated. Nevertheless, comparing the structure of the cyclogenesis modes with that of the leading complex EOFs of the observed bandpass-filtered flow, a pattern correlation squared of up to 0.4 is obtained. Truncating the basic state to comprise only the ultralong waves (zonal wavenumber m ≤ 4) results in rather little change in the cyclogenesis modes obtained. Finally, the sensitivity of the cyclogenesis modes to the anomalous basic flows is investigated, using persistent anomaly patterns (PNA, EA, NAO) obtained from a rotated principal component analysis of the observed lowpass-filtered flow. The anomalous basic states are evaluated by adding or subtracting these patterns to/from the winter climatological mean flow. It turns out that the normal-mode wave trains are significantly deflected from their climatological positions, particularly in the EA and NAO cases. This model response is verified against composite maps of observed bandpass variance, obtained for episodes of strong PNA, EA or NAO anomalies respectively. It is found that, although the normal-mode wave trains are still relatively too weak over the Atlantic (compared to the Pacific), the structural differences in the observed bandpass eddy activity between positive and negative anomaly cases are captured quite well by the normal modes.
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      Three-Dimensional Linear Instability of Persistent Anomalous Large-Scale Flows

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    contributor authorRobertson, Andrew W.
    contributor authorMetz, Werner
    date accessioned2017-06-09T14:29:17Z
    date available2017-06-09T14:29:17Z
    date copyright1989/09/01
    date issued1988
    identifier issn0022-4928
    identifier otherams-20183.pdf
    identifier urihttp://onlinelibrary.yabesh.ir/handle/yetl/4156383
    description abstractThe linear baroclinic instability of three-dimensional basic flows on the Northern Hemisphere is examined in terms of a simple two-level, quasi-geostrophic model. The basic flows considered comprise an observed six-winter mean flow, as well as anomalous flows which represent episodes where large-scale persistent flow anomalies, such as the Pacific/North American (PNA), East Atlantic (EA), or North Atlantic Oscillation (NAO) patterns exhibit large amplitudes. For the climatological basic state, the fastest-growing normal modes with periods of around 4 days consist of regionally confined, synoptic-scale, baroclinic wave trains. These are considered as cyclogenesis modes, characterizing the linear synoptic-scale eddy activity associated with a given basic flow. This eddy activity has a pronounced maximum over the Pacific, close to the position of the observed Pacific storm track, but the second maximum over the Atlantic, corresponding to the Atlantic storm track, is considerably underestimated. Nevertheless, comparing the structure of the cyclogenesis modes with that of the leading complex EOFs of the observed bandpass-filtered flow, a pattern correlation squared of up to 0.4 is obtained. Truncating the basic state to comprise only the ultralong waves (zonal wavenumber m ≤ 4) results in rather little change in the cyclogenesis modes obtained. Finally, the sensitivity of the cyclogenesis modes to the anomalous basic flows is investigated, using persistent anomaly patterns (PNA, EA, NAO) obtained from a rotated principal component analysis of the observed lowpass-filtered flow. The anomalous basic states are evaluated by adding or subtracting these patterns to/from the winter climatological mean flow. It turns out that the normal-mode wave trains are significantly deflected from their climatological positions, particularly in the EA and NAO cases. This model response is verified against composite maps of observed bandpass variance, obtained for episodes of strong PNA, EA or NAO anomalies respectively. It is found that, although the normal-mode wave trains are still relatively too weak over the Atlantic (compared to the Pacific), the structural differences in the observed bandpass eddy activity between positive and negative anomaly cases are captured quite well by the normal modes.
    publisherAmerican Meteorological Society
    titleThree-Dimensional Linear Instability of Persistent Anomalous Large-Scale Flows
    typeJournal Paper
    journal volume46
    journal issue18
    journal titleJournal of the Atmospheric Sciences
    identifier doi10.1175/1520-0469(1989)046<2783:TDLIOP>2.0.CO;2
    journal fristpage2783
    journal lastpage2801
    treeJournal of the Atmospheric Sciences:;1988:;Volume( 046 ):;issue: 018
    contenttypeFulltext
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