scholarly journals Dynamics of Eddy-Driven Low-Frequency Dipole Modes. Part IV: Planetary and Synoptic Wave-Breaking Processes during the NAO Life Cycle

2008 ◽  
Vol 65 (3) ◽  
pp. 737-765 ◽  
Author(s):  
Dehai Luo ◽  
Tingting Gong ◽  
Yina Diao
Keyword(s):  
Life Cycle ◽  
Wave Breaking ◽  
Planetary Wave ◽  
Initial Conditions ◽  
Low Frequency ◽  
Phase Evolution ◽  
Life Cycles ◽  
Necessary Condition ◽  
Mean Flow ◽  
The North

Abstract Based on a highly idealized, analytical solution of the North Atlantic Oscillation (NAO) derived in Part III of this series, it is shown that wave breaking is not a necessary condition for the occurrence of NAO events. The breaking of synoptic waves can arise from the interaction between planetary and synoptic waves that gives rise to NAO events, and the type of wave breaking is dominated by the initial conditions of the two waves that determine the phase of the NAO. The planetary wave breaking (PWB) seems to be attributed to an amplification of the NAO amplitude. It is further found that both the planetary wave breaking and the cyclonic (anticyclonic) breaking of synoptic waves undergo an in-phase (out phase) evolution during the life cycles of negative (positive) phase NAO, or NAO− (NAO+), events. An interesting result found is that for NAO− (NAO+) events the breaking of synoptic waves is enhanced (weakened) during the growing phase, but is weakened (enhanced) during the decaying phase. In the absence of a topographic planetary wave (TPW), PWB occurs mainly in the midlatitude regions of the Atlantic basin for NAO− events, but is concentrated in subtropical and subpolar regions for NAO+ events. However, once the TPW is involved, the reversed planetary-scale potential vorticity (PV) gradient that characterizes the PWB exhibits a southwest–northeast (southeast–northwest) tilted tripole for NAO− (NAO+) events, in agreement with the diagnostic results presented herein. The PWB in the subtropical Atlantic is found to occur more frequently for NAO+ events than for NAO− events because the weaker subtropical mean flow is more likely to emerge during the NAO+ life cycle. In conclusion, the results of the highly idealized model used here appear to show that the PWB, synoptic wave breaking, and meridional shift of the westerly jet may be different descriptions of the NAO phenomenon.

scholarly journals Response of Idealized Baroclinic Wave Life Cycles to Stratospheric Flow Conditions

2009 ◽  
Vol 66 (8) ◽  
pp. 2288-2302 ◽  
Author(s):  
Torben Kunz ◽  
Klaus Fraedrich ◽  
Frank Lunkeit
Keyword(s):  
Refractive Index ◽  
Initial Conditions ◽  
Spherical Geometry ◽  
Zonal Flow ◽  
Equation Model ◽  
Life Cycles ◽  
Baroclinic Wave ◽  
Flow Conditions ◽  
Mean Flow ◽  
The North

Abstract Dynamical stratosphere–troposphere coupling through a response of baroclinic waves to lower stratospheric flow conditions is investigated from an initial value approach. A series of adiabatic and frictionless nonlinear baroclinic wave life cycles in a midlatitude tropospheric jet with different initial zonal flow conditions in the stratosphere is simulated, using a dry primitive equation model with spherical geometry. When a stratospheric jet, located at various latitudes between 35° and 70°, is removed from the initial conditions, the wavenumber-6 life cycle behavior changes from the well-known LC1 to LC2 evolution, characterized by anticyclonic and cyclonic wave breaking, respectively. Linear theory, in terms of refractive index and the structure of the corresponding fastest-growing normal mode, is found to be unable to explain this stratosphere-induced LC1 to LC2 transition. This implies that altered nonlinear wave–mean flow interactions are important. The most significant stratosphere-induced change that extends into the nonlinear baroclinic growth stage is a region of downward wave propagation in the lower stratosphere associated with positive values of the squared refractive index near 20 km. Furthermore, it is demonstrated that the difference between the response of the tropospheric circulation to LC1 and LC2 life cycles closely resembles the meridional and vertical structure of the North Atlantic Oscillation (NAO), with positive (negative) NAO-like anomalies being driven by LC1 (LC2). Thus, a weakened stratospheric jet induces the generation of negative NAO-like anomalies in the troposphere, consistent with the observed stratosphere–NAO connection.

Kinematics of Eddy–Mean Flow Interaction in an Idealized Atmospheric Model

2013 ◽  
Vol 70 (8) ◽  
pp. 2574-2595 ◽  
Author(s):  
Sergey Kravtsov ◽  
Sergey K. Gulev
Keyword(s):  
Time Series ◽  
Storm Track ◽  
Low Frequency ◽  
Atmospheric Model ◽  
Life Cycles ◽  
Mean Flow ◽  
Atmospheric Variability ◽  
Low Frequencies ◽  
The North ◽  
Position Time Series

Abstract The authors analyze atmospheric variability simulated in a two-layer baroclinic β-channel quasigeostrophic model by combining Eulerian and feature-tracking analysis approaches. The leading mode of the model's low-frequency variability (LFV) is associated with the irregular shifts of the zonal-mean jet to the north and south of its climatological position accompanied by simultaneous intensification of the jet, while the deviations from the zonal-mean fields are dominated by propagating anomalies with wavenumbers 3–5. The model's variability is shown to stem from the life cycles of cyclones and anticyclones. In particular, synthetic streamfunction fields constructed by launching idealized composite-mean eddies along the actual full-model-simulated cyclone/anticyclone tracks reproduce nearly perfectly not only the dominant propagating waves, but also the jet-shifting LFV. The composite eddy tracks conditioned on the phase of the jet-shifting variability migrate north or south along with the zonal-mean jet. The synoptic-eddy life cycles in the states with poleward (equatorward) zonal-jet shift exhibit longer-than-climatological lifetimes; this is caused, arguably, by a barotropic feedback associated with preferred anticyclonic (cyclonic) wave breaking in these respective states. Lagged correlation and cross-spectrum analyses of zonal-mean jet position time series and the time series representing mean latitudinal location of the eddies at a given time demonstrate that jet latitude leads the storm-track latitude at low frequencies. This indicates that the LFV associated with the jet-shifting mode here is more dynamically involved than being a mere consequence of the random variations in the distribution of the synoptic systems.

scholarly journals Dynamics of Eddy-Driven Low-Frequency Dipole Modes. Part I: A Simple Model of North Atlantic Oscillations

2007 ◽  
Vol 64 (1) ◽  
pp. 3-28 ◽  
Author(s):  
Dehai Luo ◽  
Anthony R. Lupo ◽  
Han Wan
Keyword(s):  
Life Cycle ◽  
Theoretical Model ◽  
North Atlantic ◽  
Low Frequency ◽  
Positive Phase ◽  
Negative Phase ◽  
Synoptic Scale ◽  
Planetary Scale ◽  
The North ◽  
The North Atlantic Oscillation

Abstract A simple theoretical model is proposed to clarify how synoptic-scale waves drive the life cycle of the North Atlantic Oscillation (NAO) with a period of nearly two weeks. This model is able to elucidate what determines the phase of the NAO and an analytical solution is presented to indicate a high similarity between the dynamical processes of the NAO and zonal index, which is not derived analytically in previous theoretical studies. It is suggested theoretically that the NAO is indeed a nonlinear initial-value problem, which is forced by both preexisting planetary-scale and synoptic-scale waves. The eddy forcing arising from the preexisting synoptic-scale waves is shown to be crucial for the growth and decay of the NAO, but the preexisting low-over-high (high-over-low) dipole planetary-scale wave must be required to match the preexisting positive-over-negative (negative-over-positive) dipole eddy forcing so as to excite a positive (negative) phase NAO event. The positive and negative feedbacks of the preexisting dipole eddy forcing depending upon the background westerly wind seem to dominate the life cycle of the NAO and its life period. An important finding in the theoretical model is that negative-phase NAO events could be excited repeatedly after the first event has decayed, but for the positive phase downstream isolated dipole blocks could be produced after the first event has decayed. This is supported by observed cases of the NAO events presented in this paper. In addition, a statistical study of the relationship between the phase of the NAO and blocking activity over Europe in terms of the seasonal mean NAO index shows that blocking events over Europe are more frequent and long-lived for strong positive-phase NAO years, indicating that the positive-phase NAO favors the occurrence of European blocking events.

The Role of Extratropical Air–Sea Interaction in the Autumn Subseasonal Variability of the North Atlantic Oscillation

2019 ◽  
Vol 32 (22) ◽  
pp. 7697-7712 ◽  
Author(s):  
Yu Nie ◽  
Hong-Li Ren ◽  
Yang Zhang
Keyword(s):  
North Atlantic Oscillation ◽  
North Atlantic ◽  
Gulf Stream ◽  
Low Frequency ◽  
Mean Flow ◽  
The North ◽  
Subseasonal Variability ◽  
The North Atlantic ◽  
The North Atlantic Oscillation

Abstract Considerable progress has been made in understanding the internal eddy–mean flow feedback in the subseasonal variability of the North Atlantic Oscillation (NAO) during winter. Using daily atmospheric and oceanic reanalysis data, this study highlights the role of extratropical air–sea interaction in the NAO variability during autumn when the daily sea surface temperature (SST) variability is more active and eddy–mean flow interactions are still relevant. Our analysis shows that a horseshoe-like SST tripolar pattern in the North Atlantic Ocean, marked by a cold anomaly in the Gulf Stream and two warm anomalies to the south of the Gulf Stream and off the western coast of northern Europe, can induce a quasi-barotropic NAO-like atmospheric response through eddy-mediated processes. An initial southwest–northeast tripolar geopotential anomaly in the North Atlantic forces this horseshoe-like SST anomaly tripole. Then the SST anomalies, through surface heat flux exchange, alter the spatial patterns of the lower-tropospheric temperature and thus baroclinicity anomalies, which are manifested as the midlatitude baroclinicity shifted poleward and reduced baroclinicity poleward of 70°N. In response to such changes of the lower-level baroclinicity, anomalous synoptic eddy generation, eddy kinetic energy, and eddy momentum forcing in the midlatitudes all shift poleward. Meanwhile, the 10–30-day low-frequency anticyclonic wave activities in the high latitudes decrease significantly. We illustrate that both the latitudinal displacement of midlatitude synoptic eddy activities and intensity variation of high-latitude low-frequency wave activities contribute to inducing the NAO-like anomalies.

Stratospheric influence on idealised baroclinic life cycles

2020 ◽  
Author(s):  
Philip Rupp ◽  
Thomas Birner
Keyword(s):  
Life Cycle ◽  
Initial Conditions ◽  
Polar Vortex ◽  
Life Cycles ◽  
Stratospheric Polar Vortex ◽  
Shift Response ◽  
Tropospheric Circulation ◽  
Wave Development ◽  
Final Steady State ◽  
Numerical Model Experiments

<p>The importance of understanding the dynamical coupling of troposphere and stratosphere to make accurate weather and climate predictions is well-known. Over the past years and decades various signatures of such a<br>coupling have been discovered. A very robust result, for example, seems to be an equatorward shift of the tropospheric eddy driven jet following sudden stratospheric warming events, where the westerly winds of the stratospheric polar vortex weaken or even reverse. However, many aspects of this fundamental coupling are still not fully understood and research on how the state of the stratosphere can influence the tropospheric circulation and what dynamical processes are involved is still ongoing.</p><p><br>An important such process arises due to the interaction of a sharp, localised maximum in potential vorticity gradient near the tropopause with baroclinic eddies in the troposphere. Here, we analyse the sensitivity of baroclinic wave development and evolution to changes of various basic state characteristics, by performing a series of idealised baroclinic eddy life cycle experiments. Special attention is paid to sensitivities associated with the dynamical state of the stratosphere. We find that the final (steady) state of the life cycle simulations corresponds to an equatorward shift of the tropospheric jet in cases where the initial conditions do not include a stratospheric polar vortex (such as following sudden warming events) compared to those that do. These results further support the idea that the stratospheric state can strongly influence tropospheric dynamics and, in particular, highlight the robustness of the jet shift response following sudden warmings, that can be seen in a range of observations and numerical model experiments.</p>

An Eddy-Feedback Model for Propagating Annular Modes

2020 ◽  
Author(s):  
Sandro Lubis ◽  
Pedram Hassanzadeh
Keyword(s):  
Low Frequency ◽  
Wave Activity ◽  
Mean Flow ◽  
Wave Source ◽  
Annular Mode ◽  
Annular Modes ◽  
The North ◽  
Feedback Model ◽  
Low Frequency Variability ◽  
Extratropical Circulation

<p>Some types of extreme events<span> in the extratropics are often associated with anomalous jet behaviors. A well-known example is the annular mode, wherein its variation e.g., the meandering in the north-south direction of the jet, disrupts the normal eastward migration of troughs and ridges.</span> <span>Since the seminal works of Lorenz and Hartmann, the annular mode has been mostly analyzed based on single EOF mode. However, a recent study showed that the first and second leading EOFs are strongly correlated at long lags and are manifestations of a single oscillatory decaying-mode. This means that the first and second leading EOF modes interact and exert feedbacks on each other. The purpose of this study is to develop an eddy-feedback model for the extratropical low-frequency variability that includes these cross-EOF feedbacks to better isolate the eddy momentum/heat flux changes with time- and/or zonal-mean flow. Our results show that, in the presence of the poleward-propagation regime, the first and second leading EOF modes interact and exert positive feedbacks at lags ~10 (~20) days about ~0.07 (~0.16) day</span><span><sup>-1</sup></span><span> in the reanalysis (idealized GCM). This feedback is often ignored in the previous studies, and in fact, the magnitude is nearly double the feedback exerted by the single EOF mode. We found that this apparent positive eddy feedback is a result of the effect of jet pulsation (strengthening and weakening) in zonal flow variability (z</span><span><sub>2</sub></span><span>) on the eddy momentum flux due to the meandering in the north-south direction of the jet (m</span><span><sub>1</sub></span><span>). A finite-amplitude eddy-mean flow interaction diagnostic has been performed to demonstrate the dynamics governing the positive feedback in the propagating regime of the annular modes. It is shown that the poleward propagation is caused by an orchestrated combination of equatorward propagation of wave activity (baroclinic process), nonlinear wave breaking (barotropic processes), and radiative relaxation. The latter two processes follow the first one, and as such, the meridional propagation of Rossby wave activity (likely generated by an enhanced baroclinic wave source at a low level) is the central mechanism. Finally, our model calculations suggest the rule of thumb that the propagating annular modes (i.e., when EOF1 and EOF2 together represent quasi-periodic poleward propagation of zonal-mean flow anomalies) exist if the ratio of the fractional variance and decorrelation time-scale of EOF2 to that of EOF1 exceeds 0.5 or the two leading PCs showing maximum correlations at larger lags. These criteria can be used to assess the predictability of preferred modes of extratropical circulation in GCMs. The present study advances and potentially transforms the state of our understanding of the low-frequency variability of the extratropical circulation.</span></p>

scholarly journals Stationary Wave Reflection as a Mechanism for Zonalizing the Atlantic Winter Jet at the LGM

2016 ◽  
Vol 73 (8) ◽  
pp. 3329-3342 ◽  
Author(s):  
Marcus Löfverström ◽  
Rodrigo Caballero ◽  
Johan Nilsson ◽  
Gabriele Messori
Keyword(s):  
North Atlantic ◽  
Wave Breaking ◽  
Planetary Wave ◽  
Wave Reflection ◽  
Stationary Wave ◽  
Laurentide Ice Sheet ◽  
The North ◽  
Nonlinear Reflection ◽  
The Last Glacial Maximum ◽  
The Last Glacial

Abstract Current estimates of the height of the Laurentide Ice Sheet (LIS) at the Last Glacial Maximum (LGM) range from around 3000 to 4500 m. Modeling studies of the LGM, using low-end estimates of the LIS height, show a relatively weak and northeastward-tilted winter jet in the North Atlantic, similar to the modern jet, while simulations with high-end LIS elevations show a much more intense and zonally oriented jet. Here, an explanation for this response of the Atlantic circulation is sought using a sequence of LGM simulations spanning a broad range of LIS elevations. It is found that increasing LIS height favors planetary wave breaking and nonlinear reflection in the subtropical North Atlantic. For high LIS elevations, planetary wave reflection becomes sufficiently prevalent that a poleward-directed flux of wave activity appears in the climatology over the midlatitude North Atlantic. This entails a zonalization of the stationary wave phase lines and thus of the midlatitude jet.

Latitudinal variation in the life cycle of allodapine bees (Hymenoptera; Apidae)

1999 ◽  
Vol 77 (6) ◽  
pp. 857-864 ◽  
Author(s):  
Adam L Cronin ◽  
Michael P Schwarz
Keyword(s):  
Life Cycle ◽  
Life Cycles ◽  
Alloparental Care ◽  
The North ◽  
Brood Development ◽  
Weak Evidence ◽  
Indirect Fitness ◽  
Fitness Benefits ◽  
Latitudinal Range ◽  
Eastern Seaboard

The life cycles of Exoneura robusta and Exoneura angophorae are examined in four populations along the eastern seaboard of Australia, where climate ranged from temperate in the south to subtropical in the north over a latitudinal range of approximately 10°. These species were univoltine throughout the range examined and most colonies produced a single brood. Timing and duration of brood development in E. robusta varied between sites, with brood development being more rapid in northern populations; there was only weak evidence of any effect of latitude in E. angophorae. All populations of E. angophorae exhibited a small proportion of doubly brooded colonies, but doubly brooded colonies were found only in northernmost populations of E. robusta. Two-brooded colonies can give rise to opportunities for sib rearing, which can alter the indirect fitness benefits for alloparental care. Our results indicate that there is an effect of climate on sociality in E. robusta but no, or very little, such effect in E. angophorae.

Influence of energetic particle precipitation on polar vortex mediated by planetary wave activity

2021 ◽  
Author(s):  
Timo Asikainen ◽  
Antti Salminen ◽  
Ville Maliniemi ◽  
Kalevi Mursula
Keyword(s):  
Planetary Wave ◽  
Energetic Electron ◽  
Polar Vortex ◽  
Analysis Data ◽  
Wave Activity ◽  
Necessary Condition ◽  
Atmospheric Conditions ◽  
Mean Flow ◽  
Ozone Loss ◽  
Near Earth Space

<p>The northern polar vortex experiences considerable inter-annual variability, which is also reflected to tropospheric weather. Recent research has established a link between polar vortex variations and energetic electron precipitation (EEP) from the near-Earth space into the polar atmosphere, which is mediated by EEP-induced chemical changes causing ozone loss in the mesosphere and stratosphere. However, the most dramatic changes in the polar vortex are due to strong enhancements of planetary wave activity, which typically result in a sudden stratospheric warming (SSW), a momentary breakdown of the polar vortex. Here we use the SSWs as an indicator of high planetary wave activity and consider their influence of SSWs on the atmospheric response to EEP in 1957-2017 using combined ERA-40 and ERA-Interim re-analysis data and geomagnetic activity as a proxy of EEP. We find that the EEP-related enhancement of the polar vortex and other associated dynamical responses are seen only during winters when a SSW occurs, and that the EEP-related changes take place slightly before the SSW onset. We show that the atmospheric conditions preceding SSWs favor enhanced wave-mean-flow interaction, which can dynamically amplify the initial polar vortex enhancement caused by ozone loss. These results highlight the importance of considering SSWs and sufficient level of planetary wave activity as a necessary condition for observing the effects of EEP on the polar vortex dynamics.</p>

Finite-Amplitude Lagrangian-Mean Wave Activity Diagnostics Applied to the Baroclinic Eddy Life Cycle

2012 ◽  
Vol 69 (10) ◽  
pp. 3013-3027 ◽  
Author(s):  
Abraham Solomon ◽  
Gang Chen ◽  
Jian Lu
Keyword(s):  
Life Cycle ◽  
Second Life ◽  
Wave Breaking ◽  
General Circulation ◽  
Early Stage ◽  
Potential Temperature ◽  
Circulation Model ◽  
Finite Amplitude ◽  
Wave Activity ◽  
Life Cycles

Abstract Lagrangian-mean wave activity diagnostics are applied to the nonlinear baroclinic eddy life cycle in a simple general circulation model of the atmosphere. The growth of these instabilities through baroclinic conversion of potential temperature gradients and their subsequent barotropic decay can exhibit two distinct life cycles. One life cycle results in equatorward propagation of the growing eddy, anticyclonic wave breaking, and a poleward shift of the mean jet. The second life cycle is distinguished by limited equatorward propagation and cyclonic wave breaking on the poleward flank of the jet. Using a conservative finite-amplitude, Lagrangian-mean wave activity (negative pseudomomentum) to quantify wave growth and propagation reveals more details about the life cycles than could be discerned from eddy kinetic energy (EKE) or other Eulerian metrics. It is shown that the distribution of pseudomomentum relative to the latitude of the axis of the jet can be used to provide a clear distinction between the two life cycles at an early stage in their development and, hence, a prediction for the subsequent shift of the jet. This suggests that the distribution of pseudomomentum may provide some predictability for the atmospheric annular modes.

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