Suppression of Baroclinic Eddies by Strong Jets

2021 ◽  
Author(s):  
Or Hadas ◽  
Yohai Kaspi
Keyword(s):  
Baroclinic Instability ◽  
Storm Track ◽  
Wave Structure ◽  
Optimal Growth ◽  
Lagrangian Analysis ◽  
Storm Activity ◽  
Instability Theory ◽  
The Pacific ◽  
The Difference ◽  
Upper Level

AbstractThe midlatitude storm tracks are of the most prominent features of extratropical climate. Despite the theoretical expectation, based on baroclinic instability theory, that baroclinic eddies strengthen with jet intensification, there is evidence that this relation breaks when the jet is particularly strong. The most known case is the Pacific midwinter minimum in storm track activity. To isolate the effect of jet strength on storm activity, we conduct a series of GCM experiments systematically varying jet intensity. The simulations are analyzed using Lagrangian tracking to understand the response from a single-eddy perspective. The Lagrangian analysis shows that while the response of upper-level eddies is dominated by a reduction in the amount of tracked features, the lower-level eddies’ response is also affected by a reduction in their lifetime. Analyzing the jet strength effect on the pairing between the upper- and lower-level eddies, we find that the jet intensification increases the relative speed of the upper-level eddies, breaking the baroclinic wave structure and limiting its growth. We show that the Lagrangian response correlates with a shift in the midlatitude spectrum to low wavenumbers. The shift settles these results with linear baroclinic instability theory, as under the stronger jet conditions synoptic-scale eddies are predicted to have a sub-optimal growth rate. These results can potentially explain the midwinter suppression of storm activity over the Pacific and the difference from the Atlantic response.

Suppression of Baroclinic Eddies by Strong Jets

2021 ◽  
Author(s):  
Or Hadas ◽  
Yohai Kaspi
Keyword(s):  
General Circulation ◽  
Baroclinic Instability ◽  
Storm Track ◽  
Wave Structure ◽  
Lagrangian Analysis ◽  
Storm Activity ◽  
The Pacific ◽  
The Difference ◽  
Lagrangian Tracking ◽  
Upper Level

<p>The midlatitude storm tracks are one of the most prominent features of extratropical climate. Despite the theoretical expectation, based on baroclinic instability theory that baroclinic eddy strength correlates with jet intensity, there is a decrease in storm-track activity during midwinter over the Pacific compared to the shoulder seasons. Recent studies suggest this phenomenon is a result of the general circulation effect on the storm-track through interaction with the jet-stream. To isolate the effect of jet strength, we conduct a series of GCM experiments with a systematically varied jet intensity. The simulations are analyzed using Lagrangian tracking to understand the response from a single eddy perspective. The results of the Lagrangian analysis show that while the response of upper-level eddies is dominated by a reduction in the amount of tracked features, the lower-level eddies' response is also affected by a reduction in their lifetime. Analyzing the effect of the jet strength on the pairing between the upper- and lower-level eddies, we show how the jet intensification break the baroclinic wave structure and limits its growth. Furthermore, we show that these results can be settled with linear baroclinic instability models if the eddies' spatial scale is considered. The intensification of the jet and increase in the deformation radius shift the preferred scale for growth from the synoptic-scale toward the planetary-scale, consistent with the reduction in storm activity. This mechanism potentially explains the midwinter suppression of storm activity over the Pacific and the difference from the response over the Atlantic.</p>

The Effect of a Strong Zonal Jet Stream on the Temporal Evolution of Baroclinic Eddies

2020 ◽  
Author(s):  
Or Hadas ◽  
Yohai Kaspi
Keyword(s):  
General Circulation ◽  
Baroclinic Instability ◽  
Storm Track ◽  
Storm Tracks ◽  
Jet Stream ◽  
Similar Response ◽  
Jet Streams ◽  
Eddy Activity ◽  
Zonal Jets ◽  
The Pacific

<p>The midlatitude storm tracks are one of the most prominent features of the extratropical climate. Much of our understanding of what controls the storm tracks comes from linear theory of baroclinic instability, which explains generally most of the observed response of storms to the general circulation. One example to where this approach is lacking is the Pacific midwinter minimum, a decrease in the eddy activity over the Pacific storm track during midwinter when baroclinicity is at its peak due to extremely strong zonal jets. A similar response was found recently for the Atlantic storm track<strong>,</strong> in correlation to periods of strong zonal jets. Following on these findings we study the effect of strong zonal jet streams on eddy activity in the midlatitudes. In order to isolate the effect of the jet strength we used several idealized GCM experiments with different jet strengths, and analyze the formed storm track from a Lagrangian perspective by using a storm tracking algorithm. In both the Eulerian analysis and analysis of the tracks a strong reduction of high level eddy activity is prominent, as well as a modest weakening of the low-level activity. The observed response is then further analyzed by studying the connection between the upper and lower wave and how it changes with jet-stream intensity. </p><p> </p>

The Source of the Midwinter Suppression in Storminess over the North Pacific

2010 ◽  
Vol 23 (3) ◽  
pp. 634-648 ◽  
Author(s):  
Sandra Penny ◽  
Gerard H. Roe ◽  
David S. Battisti
Keyword(s):  
Growth Rates ◽  
Feature Tracking ◽  
Storm Track ◽  
Static Stability ◽  
The Tibetan Plateau ◽  
The North ◽  
The Pacific ◽  
Upper Level ◽  
The North Pacific ◽  
Midwinter Suppression

Abstract Feature-tracking techniques are employed to investigate why there is a relative minimum in storminess during winter within the Pacific storm track (the midwinter suppression). It is found that the frequency and amplitude of disturbances entering the Pacific storm track from midlatitude Asia are substantially reduced during winter relative to fall and spring and that the magnitude of this reduction is more than sufficient to account for the midwinter supression. Growth rates of individual disturbances are calculated and compared to expectations from linear theory for several regions of interest. Although there are discrepancies between linear expectations and observed growth rates over the Pacific, the growth of disturbances within the Pacific storm track cannot explain why the midwinter suppression exists. Furthermore, it is determined that the development of a wintertime reduction in storminess over midlatitude Asia is consistent with linear expectations, which predict a wintertime minimum in Eady growth rates in this region, mainly because of increased static stability. Several other mechanisms that may contribute to the initiation of the midwinter suppression over midlatitude Asia are discussed, including the interaction between upper-level waves and topography, the behavior of waves upwind of the Tibetan Plateau, and the initiation of lee cyclones.

An Idealized Model Study Relevant to the Dynamics of the Midwinter Minimum of the Pacific Storm Track

2005 ◽  
Vol 62 (4) ◽  
pp. 1209-1225 ◽  
Author(s):  
Yi Deng ◽  
Mankin Mak
Keyword(s):  
Nonlinear Model ◽  
Baroclinic Instability ◽  
Storm Track ◽  
Simultaneous Increase ◽  
Synoptic Variability ◽  
Model Study ◽  
The Pacific ◽  
Reference Potential ◽  
Dynamical Nature ◽  
Instability Growth

Abstract The synoptic variability of a two-level quasigeostrophic flow in response to plausible changes in the forcing of a localized baroclinic jet is investigated in the context of the midwinter minimum of the Pacific storm track (MWM). The changes in the model forcing are introduced in terms of a reference potential vorticity field that is associated with plausible changes in the global baroclinicity, zonal variation of the baroclinicity, and horizontal deformation over the Pacific from early winter to midwinter conditions. It is found that the modal instability growth rate of perturbation in such a localized jet is significantly reduced in spite of an increase in the local baroclinicity. The dynamical nature of such an effect can be interpreted as a generalized barotropic governor effect on localized baroclinic instability. The existence of three instability regimes is established on the basis of energetics characteristics. The intensity of the nonlinear model storm track is reduced by about 30% in response to a change in the forcing condition from early to midwinter. The characteristics of the linear model storm track and nonlinear model storm track are compared. The overall results support a hypothesis that MWM could stem from a sufficiently large increase in the stabilizing influence of the local barotropic process in spite of a simultaneous increase in its local baroclinicity in the Pacific jet from early to midwinter.

scholarly journals Connecting flow-topography interactions, vorticity balance, baroclinic instability and transport in the Southern Ocean: the case of an idealized storm track

2020 ◽  
Author(s):  
Julien Jouanno ◽  
Xavier Capet
Keyword(s):  
Southern Ocean ◽  
Baroclinic Instability ◽  
World Ocean ◽  
Storm Track ◽  
Comprehensive Treatment ◽  
Global Circulation Models ◽  
The World ◽  
The Difference ◽  
Circumpolar Current ◽  
Baroclinic Modes

Abstract. The dynamical balance of the Antarctic circumpolar current and their implications on the functioning of the world ocean are not fully understood and poorly represented in global circulation models. In this study, the sensitivities of an idealized Southern Ocean (SO) storm track are explored with a set of eddy-rich numerical simulations. The classical partition between barotropic and baroclinic modes is sensitive to current-topography interactions in the mesoscale range 10–100 km, as comparisons between simulations with rough or smooth bathymetry reveal. Configurations with a rough bottom have weak barotropic motions, no wind-driven gyre in the lee of topographic ridges, less efficient baroclinic turbulence, and thus larger circumpolar transport rates. The difference in circumpolar transport depends on the strength with which (external) thermohaline forcings by the rest of the world ocean constrain the stratification at the northern edge of the SO. The study highlights the need for a comprehensive treatment of the Antartic Circumpolar Current (ACC) interactions with the ocean floor. It also sheds some light on the behavior of idealized storm tracks recently modelled: i) the saturation mechanism, whereby the circumpolar transport does not depend on wind intensity, is a robust and generic attribute of ACC-like circumpolar flows ii) the adjustment toward saturation can take place over widely different time scales (from months to years) depending on the possibility (or not) for barotropic Rossby waves to propagate signals of wind change and accelerate/decelerate SO wind-driven gyres. The real SO with a typical ACC saturation time scale of 2–3 years seems to lie in the “rough bottom/no wind-driven gyre” regime.

scholarly journals Characteristics of Extratropical Cyclones That Cause Tornadoes in Italy: A Preliminary Study

Atmosphere ◽  
2021 ◽  
Vol 12 (2) ◽  
pp. 180
Author(s):  
Eigo Tochimoto ◽  
Mario Marcello Miglietta ◽  
Leonardo Bagaglini ◽  
Roberto Ingrosso ◽  
Hiroshi Niino
Keyword(s):  
Environmental Conditions ◽  
Convective Available Potential Energy ◽  
Static Stability ◽  
Extratropical Cyclones ◽  
Cool Season ◽  
Southeastern Us ◽  
Different Seasons ◽  
The Difference ◽  
Upper Level ◽  
Strong Evaporation

Characteristics of extratropical cyclones that cause tornadoes in Italy are investigated. Tornadoes between 2007 and 2016 are analyzed, and statistical analysis of the associated cyclone structures and environments is performed using the JRA-55 reanalysis. Tornadoes are distributed sporadically around the cyclone location within a window of 10° × 10°. The difference in the cyclone tracks partially explains the seasonal variability in the distribution of tornadoes. The highest number of tornadoes occur south of the cyclone centers, mainly in the warm sector, while a few are observed along the cold front. Composite mesoscale parameters are examined to identify the environmental conditions associated with tornadoes in different seasons. Potential instability is favorable to tornado development in autumn. The highest convective available potential energy (CAPE) in this season is associated with relatively high-temperature and humidity at low-levels, mainly due to the strong evaporation over the warm Mediterranean Sea. Upper-level potential vorticity (PV) anomalies and the associated cold air reduce the static stability above the cyclone center, mainly in spring and winter. On average, the values of CAPE are lower than for US tornadoes and comparable with those occurring in Japan, while storm relative helicity (SREH) is comparable with US tornadoes and higher than Japanese tornadoes, indicating that the environmental conditions for Italian tornadoes have peculiar characteristics. Overall, the conditions emerging in this study are close to the high-shear, low-CAPE environments typical of cool-season tornadoes in the Southeastern US.

scholarly journals The Annular Response to Tropical Pacific SST Forcing

2006 ◽  
Vol 19 (9) ◽  
pp. 1802-1819 ◽  
Author(s):  
Shuanglin Li ◽  
Martin P. Hoerling ◽  
Shiling Peng ◽  
Klaus M. Weickmann
Keyword(s):  
General Circulation ◽  
Storm Track ◽  
Circulation Model ◽  
Tropical Pacific ◽  
Transient Eddy ◽  
West Pacific ◽  
Core Energy ◽  
The Pacific ◽  
Sst Forcing ◽  
Sst Anomaly

Abstract The leading pattern of Northern Hemisphere winter height variability exhibits an annular structure, one related to tropical west Pacific heating. To explore whether this pattern can be excited by tropical Pacific SST variations, an atmospheric general circulation model coupled to a slab mixed layer ocean is employed. Ensemble experiments with an idealized SST anomaly centered at different longitudes on the equator are conducted. The results reveal two different response patterns—a hemispheric pattern projecting on the annular mode and a meridionally arched pattern confined to the Pacific–North American sector, induced by the SST anomaly in the west and the east Pacific, respectively. Extratropical air–sea coupling enhances the annular component of response to the tropical west Pacific SST anomalies. A diagnosis based on linear dynamical models suggests that the two responses are primarily maintained by transient eddy forcing. In both cases, the model transient eddy forcing response has a maximum near the exit of the Pacific jet, but with a different meridional position relative to the upper-level jet. The emergence of an annular response is found to be very sensitive to whether transient eddy forcing anomalies occur within the axis of the jet core. For forcing within the jet core, energy propagates poleward and downstream, inducing an annular response. For forcing away from the jet core, energy propagates equatorward and downstream, inducing a trapped regional response. The selection of an annular versus a regionally confined tropospheric response is thus postulated to depend on how the storm tracks respond. Tropical west Pacific SST forcing is particularly effective in exciting the required storm-track response from which a hemisphere-wide teleconnection structure emerges.

scholarly journals Subantarctic Mode Water Formation, Destruction, and Export in the Eddy-Permitting Southern Ocean State Estimate

2013 ◽  
Vol 43 (7) ◽  
pp. 1485-1511 ◽  
Author(s):  
Ivana Cerovečki ◽  
Lynne D. Talley ◽  
Matthew R. Mazloff ◽  
Guillaume Maze
Keyword(s):  
Southern Ocean ◽  
Buoyancy Flux ◽  
Differential Heat ◽  
Diapycnal Mixing ◽  
Mode Water ◽  
State Estimate ◽  
Surface Formation ◽  
Subantarctic Mode Water ◽  
The Pacific ◽  
The Difference

Abstract Subantarctic Mode Water (SAMW) is examined using the data-assimilating, eddy-permitting Southern Ocean State Estimate, for 2005 and 2006. Surface formation due to air–sea buoyancy flux is estimated using Walin analysis, and diapycnal mixing is diagnosed as the difference between surface formation and transport across 30°S, accounting for volume change with time. Water in the density range 26.5 < σθ < 27.1 kg m−3 that includes SAMW is exported northward in all three ocean sectors, with a net transport of (18.2, 17.1) Sv (1 Sv ≡ 106 m3 s−1; for years 2005, 2006); air–sea buoyancy fluxes form (13.2, 6.8) Sv, diapycnal mixing removes (−14.5, −12.6) Sv, and there is a volume loss of (−19.3, −22.9) Sv mostly occurring in the strongest SAMW formation locations. The most vigorous SAMW formation is in the Indian Ocean by air–sea buoyancy flux (9.4, 10.9) Sv, where it is partially destroyed by diapycnal mixing (−6.6, −3.1) Sv. There is strong export to the Pacific, where SAMW is destroyed both by air–sea buoyancy flux (−1.1, −4.6) Sv and diapycnal mixing (−5.6, −8.4) Sv. In the South Atlantic, SAMW is formed by air–sea buoyancy flux (5.0, 0.5) Sv and is destroyed by diapycnal mixing (−2.3, −1.1) Sv. Peaks in air–sea flux formation occur at the Southeast Indian and Southeast Pacific SAMWs (SEISAMWs, SEPSAMWs) densities. Formation over the broad SAMW circumpolar outcrop windows is largely from denser water, driven by differential freshwater gain, augmented or decreased by heating or cooling. In the SEISAMW and SEPSAMW source regions, however, formation is from lighter water, driven by differential heat loss.

scholarly journals Barotropic Regeneration of Upper-Level Synoptic Disturbances in Different Configurations of the Zonal Weather Regime

2008 ◽  
Vol 65 (10) ◽  
pp. 3159-3178 ◽  
Author(s):  
Gwendal Rivière
Keyword(s):  
Large Scale ◽  
Deformation Field ◽  
Regeneration Process ◽  
Barotropic Model ◽  
Weather Regime ◽  
The North ◽  
The Difference ◽  
Upper Level ◽  
Composite Maps ◽  
Large Scale Flow

Barotropic dynamics of upper-tropospheric midlatitude disturbances evolving in different configurations of the zonal weather regime (i.e., in different zonal-like large-scale flows) were studied using observational analyses and barotropic model experiments. The contraction stage of upper-level disturbances that follows their elongation stage leads to an increase of eddy kinetic energy that is called the barotropic regeneration process in this text. This barotropic mechanism is studied through notions of barotropic critical regions (BtCRs) and effective deformation that have been introduced in a previous paper. The effective deformation field is equal to the difference between the square of the large-scale deformation magnitude and the square of the large-scale vorticity. Regions where the effective deformation is positive correspond to regions where the large-scale flow tends to strongly stretch synoptic disturbances. A BtCR is an area separating two large-scale regions of positive effective deformation, one located upstream and on the south side of the jet and the other downstream and on the north side. Such a region presents a discontinuity in the orientation of the dilatation axes and is a potential area where the barotropic regeneration process may occur. Winter days presenting a zonal weather regime in the 40-yr ECMWF Re-Analysis dataset are decomposed, via a partitioning algorithm, into different configurations of the effective deformation field at 300 hPa. A six-cluster partition is obtained. Composite maps of the barotropic generation rate for each cluster exhibit a succession of negative and positive values on both sides of the BtCRs. It confirms statistically that the barotropic regeneration mechanism occurs preferentially about BtCRs. Numerical experiments using a forced barotropic model on the sphere are performed. Each experiment consists of adding a synoptic-scale perturbation to one of the zonal-like jet configurations found in observations, which is kept fixed with time. The combined effects of the effective deformation and nonlinearities are shown to be crucial to reproduce the barotropic regeneration process about BtCRs.

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