Abstract
The role played by enhanced upper-tropospheric baroclinicity in the poleward shift of the jet streams in global warming scenarios is investigated. Major differences between the twentieth- ...and twenty-first-century simulations are first detailed using two coupled climate model outputs. There is a poleward shift of the eddy-driven jets, an increase in intensity and poleward shift of the storm tracks, a strengthening of the upper-tropospheric baroclinicity, and an increase in the eddy length scale. These properties are more obvious in the Southern Hemisphere. A strengthening of the poleward eddy momentum fluxes and a relative decrease in frequency of cyclonic wave breaking compared to anticyclonic wave breaking events is also observed.
Then, baroclinic instability in the three-level quasigeostrophic model is studied analytically and offers a simple explanation for the increased eddy spatial scale. It is shown that if the potential vorticity gradient changes its sign below the midlevel (i.e., if the critical level is located in the lower troposphere as in the real atmosphere), long and short wavelengths become respectively more and less unstable when the upper-tropospheric baroclinicity is increased.
Finally, a simple dry atmospheric general circulation model (GCM) is used to confirm the key role played by the upper-level baroclinicity by employing a normal-mode approach and long-term simulations forced by a temperature relaxation. The eddy length scale is shown to largely determine the nature of the breaking: long (short) wavelengths break more anticyclonically (cyclonically). When the upper-tropospheric baroclinicity is reinforced, long wavelengths become more unstable, break more strongly anticyclonically, and push the jet more poleward. Short wavelengths being less unstable, they are less efficient in pushing the jet equatorward. This provides an interpretation for the increased poleward eddy momentum fluxes and thus the poleward shift of the eddy-driven jets.
This study investigates the efficiency of baroclinic eddy growth in an effort to better understand the suppression of the North Pacific storm-track intensity in winter. The efficiency of baroclinic ...eddy growth depends on the magnitude and orientation of the vertical tilt of the eddy geopotential isolines. The eddy efficiency is maximized if the orientation of the vertical tilt creates an eddy heat flux that aligns with the mean baroclinicity (defined as minus the temperature gradient divided by a stratification parameter) and if the magnitude of the vertical tilt is neither too strong nor too weak. The eddy efficiency is, in contrast to most other eddy measures, independent of the eddy amplitude and thus useful for improving our mechanistic understanding of the effective eddy growth. During the midwinter suppression, the eddy efficiency is reduced north of 40°N over a region upstream of the main storm track, and baroclinic growth is reduced despite a maximumin baroclinicity. Eulerian diagnostics and feature tracking suggest that the reduction in eddy efficiency is due to a stronger poleward tilt with height of eddies entering the Pacific through the northern seeding branch, which results in a more eastward-oriented eddy heat flux and a reduced alignment with the baroclinicity. The stronger poleward tilt with height is constrained by the eddy propagation direction, which is more equatorward when the subtropical jet moves equatorward in winter. In addition, the westward tilt with height is too strong. South of 40°N, the eddy efficiency increases during midwinter but in a region far away from the main storm track.
An analysis of the potential vorticity gradient and the refractive index in quasigeostrophic (QG) flows on the sphere reveals that the absolute vorticity and the stretching parts have two ...contradictory effects on the horizontal shape of the baroclinic waves when the full variations of the Coriolis parameter are taken into account in each term. The absolute vorticity effect favors the anticyclonic (southwest-northeast) tilt and anticyclonic wave breaking (AWB) and is stronger in the upper troposphere. In contrast, the stretching effect promotes the cyclonic (northwest-southeast) tilt and cyclonic wave breaking (CWB) and is more efficient at lower levels. A positive eddy feedback acting on the latitudinal variations of the zonal winds is deduced. Because the absolute vorticity and the stretching effects are respectively more and less efficient with increasing latitude, a more northward (southward) jet renders AWB more (less) probable and CWB less (more) probable; the jet is pushed or maintained more northward (southward) by the eddy feedback. Idealized numerical experiments using two aquaplanet models on the sphere, a three-level QG model, and a 10-level primitive equation (PE) model, confirm our analysis. Two strategies are employed: first, a normal-mode approach for jets centered at different latitudes; second, an analysis of long-term integrations of the models where the temperature is relaxed toward zonally as well as nonzonally uniform restoration-temperature profiles located at different latitudes. The positive eddy feedback is much less visible in the QG model and CWB is very rare because it does not contain the stretching effect (because of the constant Coriolis parameter in the stretching term).
The North Atlantic Oscillation (NAO) response to the northeast Pacific climate variability is examined using the ERA-40 dataset. The main objective is to validate a mechanism involving downstream ...wave propagation processes proposed in a recent idealized companion study: a low-frequency planetary-scale ridge (trough) anomaly located in the eastern Pacific–North American sector induces more equatorward (pole-ward) propagation of synoptic-scale wave packets on its downstream side, which favors the occurrence of anticyclonic (cyclonic) wave breakings in the Atlantic sector and the positive (negative) NAO phase.
The mechanism first provides an interpretation of the canonical impact of the El Niño–Southern Oscillation on the NAO in late winter. The wintertime relationship between the Pacific–North American oscillation (PNA) and the NAO is also investigated. For out-of-phase fluctuations between the PNA and NAO indices (i.e., the most recurrent situation in late winter), the eastern Pacific PNA ridge (trough) anomaly modifies the direction of downstream wave propagation, triggering more anticyclonic (cyclonic) wave breakings over the North Atlantic. For in-phase fluctuations, the effect of the eastern Pacific PNA anomalies is cancelled out by the North American PNA anomalies. The latter anomalies being deeper and more centered in the latitudinal band of downstream wave propagation, they are able to reverse the direction of wave propagation just before waves enter the Atlantic domain. The contrasting relationship between the PNA and NAO is similar to what occurs for the two leading hemispheric EOFs of geopotential height: the northern annular mode (NAM) and the cold ocean–warm land (COWL) pattern. The proposed mechanism provides a physical meaning for the NAM and COWL patterns.
Abstract
The link between Rossby wave breaking (RWB) and the four wintertime weather regimes over the North Atlantic domain is studied in this paper. Using the 40-yr ECMWF Re-Analysis (ERA-40) data, ...frequencies of occurrence of anticyclonic and cyclonic wave-breaking (AWB and CWB, respectively) events are computed. Each weather regime has its own characteristic pattern of RWB frequencies. CWB events are found to be most frequent for the Greenland anticyclone weather regime whereas AWB events occur more for the Atlantic ridge and the zonal regimes. Time-lagged composites show that the RWB events characterizing each weather regime occur more often during the formation of the regime rather than during its decay. This suggests a reinforcement of the weather regime by RWB. An exception is the blocking weather regime, which is destroyed by an increase of CWB events south of Greenland.
Weather regime transitions are then studied using the low-frequency streamfunction tendency budget. Two types of precursors for the transitions have been identified. One is related to linear propagation of low-frequency transient eddies and the other to nonlinear interactions among the low- and high-frequency transient eddies. The latter has been related to the anomalous frequencies of occurrence of RWB. Two transitions are more precisely analyzed. The transition from blocking to Greenland anticyclone is triggered by a decrease of AWB events over Europe as well as a strong CWB event south of Greenland. The zonal to blocking transition presents evidence of two distinct precursors: one is a low-frequency wave train coming from the subtropical western Atlantic and the other, which occurs later, is characterized by a decrease of AWB and CWB events over western Europe that cannot continue to maintain the westerlies in that region.
Abstract
The aim of the paper is to investigate the influence of the Madden–Julian oscillation (MJO) on the North Atlantic Oscillation (NAO) using a quasigeostrophic model on the sphere. A simplified ...forcing based on potential vorticity anomalies in the tropics is used to mimic the MJO. The idealized nature of our setup allows us to determine the distinct roles played by stationary and synoptic waves. This is done by means of several series of almost 10 000 short runs of 30 days. Ensemble averages and a streamfunction budget analysis are used to study the modifications of the flow induced by the MJO. We find that a stationary Rossby wave is excited in the tropics during MJO phase 3. The western part of the Pacific jet is displaced poleward, which modifies the transient eddy activity in that basin. These changes create a ridge south of Alaska, which favors equatorward propagation of synoptic waves and larger poleward eddy momentum fluxes from the eastern Pacific toward the Atlantic, increasing the frequency of occurrence of the positive NAO events. The situation is essentially reversed following phase 6 of the MJO and conducive to the negative phase of the NAO. For a realistic MJO forcing amplitude, we find increases in both NAO phases to be around 30%, in reasonable agreement with the observations given the model simplicity. Finally, we present a series of experiments to assess the relative importance of linear versus nonlinear effects.
Rapid onsets of positive and negative tropospheric northern annular mode (NAM) events during boreal winters are studied using ERA-Interim datasets. The NAM anomalies first appear in the North Pacific ...from low-frequency Rossby wave propagation initiated by anomalous convection in the western tropical Pacific around 2 weeks before the peak of the events. For negative NAM, the enhanced convection leads to a zonal acceleration of the Pacific jet, while for positive NAM, the reduced convection leads to a poleward-deviated jet in its exit region. The North Atlantic anomalies, which correspond to North Atlantic Oscillation (NAO) anomalies, are formed in close connection with the North Pacific anomalies via downstream propagation of low-frequency planetary-scale and high-frequency synoptic waves, the latter playing a major role during the last onset week. Prior to positive NAM, the generation of synoptic waves in the North Pacific and their downstream propagation is strong. The poleward-deviated Pacific jet favors a southeastward propagation of the waves across North America and anticyclonic breaking in the North Atlantic. The associated strong poleward eddy momentum fluxes push the Atlantic jet poleward and form the positive NAO phase. Conversely, prior to negative NAM, synoptic wave propagation across North America is significantly reduced and more zonal because of the more zonally oriented Pacific jet. This, together with a strong eddy generation in the North Atlantic, leads to equatorward eddy momentum fluxes, cyclonic wave breaking, and the formation of the negative NAO phase. Even though the stratosphere may play a role in some individual cases, it is not the main driver of the composited tropospheric NAM events.
The present study investigates the two contrasting winters of 2010 and 2014 during which the North Atlantic Oscillation (NAO) was mainly negative and positive, respectively. In the North Pacific, ...contrasting anomalies were also present, with a straight zonal Pacific jet in 2010 and a strong poleward deviation of the Pacific jet in its exit region in 2014. Using reanalysis data sets and adopting a nonlinear initial‐value approach with a quasi‐geostrophic model, we show that the Pacific‐North American anomalies are responsible for shaping synoptic wave trains propagating across North America. This in turn largely determines the nature of wave breaking and the synoptic eddy feedback onto the mean flow in the North Atlantic and finally the NAO phase. In such a proposed mechanism, synoptic wave activity forms the cornerstone of the dynamical relationship between the North Pacific and North Atlantic large‐scale anomalies during the contrasting winters of 2010 and 2014.
Key Points
The North Atlantic Oscillation phases of 2010 and 2014 come from North Pacific anomalies
Synoptic waves form the cornerstone of the link between North Pacific and North Atlantic anomalies
Synoptic Rossby waves are shown to trigger new low‐frequency atmospheric anomalies
The North Atlantic storminess of Last Glacial Maximum (LGM) fully coupled climate simulations is generally less intense than that of their preindustrial (PI) counterparts, despite having stronger ...baroclinicity. An explanation for this counterintuitive result is presented by comparing two simulations of the IPSL full climate model forced by Paleoclimate Modelling Intercomparison Project Phase 3 (PMIP3) LGM and PI conditions. Two additional numerical experiments using a simplified dry general circulation model forced by idealized topography and a relaxation in temperature provide guidance for the dynamical interpretation. The forced experiment with idealized Rockies and an idealized Laurentide Ice Sheet has a less intense North Atlantic storm-track activity than the forced experiment with idealized Rockies only, despite similar baroclinicity. Both the climate and idealized runs satisfy or support the following statements. The reduced storm-track intensity can be explained by a reduced baroclinic conversion, which itself comes from a loss in eddy efficiency to tap the available potential energy as shown by energetic budgets. The eddy heat fluxes are northeastward oriented in the western Atlantic in LGM and are less well aligned with the mean temperature gradient than in PI. The southern slope of the Laurentide Ice Sheet topography forces the eddy geopotential isolines to be zonally oriented at low levels in its proximity. This distorts the tubes of constant eddy geopotential in such a way that they tilt northwestward with height during baroclinic growth in LGM while they are more optimally westward tilted in PI.
It has been hypothesized that enhanced Arctic warming with respect to midlatitudes, known as Arctic amplification, had led to a deceleration of eastward propagating Rossby waves, more frequent ...atmospheric blocking, and extreme weather in recent decades. We employ a novel, daily climatology of Rossby wave phase speed between March 1979 and November 2018, based on upper‐level wind data, to test this hypothesis and describe phase speed variability. The diagnostic distinguishes between periods of enhanced or reduced eastward wave propagation and is related to the occurrence of blocking and extreme temperatures over midlatitudes. While remaining tied to the upper‐level geopotential gradient, decadal trends in phase speed did not accompany the observed reduction in the low‐level temperature gradient. These results confirm the link between low phase speeds and extreme temperature events, but indicate that Arctic amplification did not play a decisive role in modulating phase speed variability in recent decades.
Plain Language Summary
The Arctic is warming more rapidly than midlatitudes and the temperature difference between those regions is being reduced. As a result, it has been hypothesized that the jet stream will decrease in intensity and its meanders will move more slowly eastward, leading to more persistent or even extreme weather conditions. As the persistence of weather can substantially vary within and between seasons, assessing long‐term changes is not trivial. To tackle this problem, we develop a “weather speedometer” and quantify the west‐east displacements of jet meanders over Northern Hemisphere midlatitudes. This metric diagnoses whether jet meanders are on average propagating eastward (positive values), stagnating, or even retrogressing westward (negative values) on each day between March 1979 and November 2018. Using this metric, we confirm that low‐speed periods are related to temperature extremes over northern midlatitudes. We also assess that there has not been an overall decrease in the propagation of jet meanders despite the significant reduction of the meridional temperature difference observed in recent decades. Results suggest the need of an improved understanding of the factors determining the persistence of weather conditions and remind caution is needed when attributing recent extreme weather to an increased stagnation of jet stream meanders.
Key Points
A diagnostic of the daily evolution of Rossby wave phase speed was developed using time‐space spectral analysis of upper‐level wind data
Occurrence of low phase speeds is related to enhanced atmospheric blocking activity and extreme temperatures over midlatitudes
Phase speed trends do not necessarily follow trends in Arctic‐to‐midlatitude temperature gradient
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