scholarly journals The Annual Cycle of Northern Hemisphere Storm Tracks. Part I: Seasons

2019 ◽  
Vol 32 (6) ◽  
pp. 1743-1760 ◽  
Author(s):  
B. J. Hoskins ◽  
K. I. Hodges
Keyword(s):  
Northern Hemisphere ◽  
Annual Cycle ◽  
Storm Track ◽  
Lower Troposphere ◽  
Meridional Wind ◽  
Diagnostic Techniques ◽  
Storm Tracks ◽  
The Pacific ◽  
Wide Range ◽  
Close Relationship

Abstract In this paper and Part II a comprehensive picture of the annual cycle of the Northern Hemisphere storm tracks is presented and discussed for the first time. It is based on both feature tracking and Eulerian-based diagnostics, applied to vorticity and meridional wind in the upper and lower troposphere. Here, the storm tracks, as diagnosed using both variables and both diagnostic techniques, are presented for the four seasons for each of the two levels. The oceanic storm tracks retain much of their winter mean intensity in spring with only a small change in their latitude. In the summer they are much weaker, particularly in the Pacific and are generally farther poleward. In autumn the intensities are larger again, comparable with those in spring, but the latitude is still nearer to that of summer. However, in the lower troposphere in the eastern ocean basins the tracking metrics show northern and southern tracks that change little with latitude through the year. The Pacific midwinter minimum is seen in upper-troposphere standard deviation diagnostics, but a richer picture is obtained using tracking. In winter there are high intensities over a wide range of latitudes in the central and eastern Pacific, and the western Pacific has high track density but weak intensity. In the lower troposphere all the diagnostics show that the strength of the Pacific and Atlantic storm tracks are generally quite uniform over the autumn–winter–spring period. There is a close relationship between the upper-tropospheric storm track, particularly that based on vorticity, and tropopause-level winds and temperature gradients. In the lower troposphere, in winter the oceanic storm tracks are in the region of the strong meridional SST gradients, but in summer they are located in regions of small or even reversed SST gradients. However, over North America the lower-tropospheric baroclinicity and the upstream portion of the Atlantic storm track stay together throughout the year.

Observed Patterns of Month-to-Month Storm-Track Variability and Their Relationship to the Background Flow*

2010 ◽  
Vol 67 (5) ◽  
pp. 1420-1437 ◽  
Author(s):  
Justin J. Wettstein ◽  
John M. Wallace
Keyword(s):  
Northern Hemisphere ◽  
Geopotential Height ◽  
Storm Track ◽  
Meridional Wind ◽  
Storm Tracks ◽  
Mean Flow ◽  
Annular Mode ◽  
The North ◽  
Jet Variability ◽  
The North Atlantic Oscillation

Abstract Month-to-month storm-track variability is investigated via EOF analyses performed on ERA-40 monthly-averaged high-pass filtered daily 850-hPa meridional heat flux and the variances of 300-hPa meridional wind and 500-hPa height. The analysis is performed both in hemispheric and sectoral domains of the Northern and Southern Hemispheres. Patterns characterized as “pulsing” and “latitudinal shifting” of the climatological-mean storm tracks emerge as the leading sectoral patterns of variability. Based on the analysis presented, storm-track variability on the spatial scale of the two Northern Hemisphere sectors appears to be largely, but perhaps not completely, independent. Pulsing and latitudinally shifting storm tracks are accompanied by zonal wind anomalies consistent with eddy-forced accelerations and geopotential height anomalies that project strongly on the dominant patterns of geopotential height variability. The North Atlantic Oscillation (NAO)–Northern Hemisphere annular mode (NAM) is associated with a pulsing of the Atlantic storm track and a meridional displacement of the upper-tropospheric jet exit region, whereas the eastern Atlantic (EA) pattern is associated with a latitudinally shifting storm track and an extension or retraction of the upper-tropospheric jet. Analogous patterns of storm-track and upper-tropospheric jet variability are associated with the western Pacific (WP) and Pacific–North America (PNA) patterns. Wave–mean flow relationships shown here are more clearly defined than in previous studies and are shown to extend through the depth of the troposphere. The Southern Hemisphere annular mode (SAM) is associated with a latitudinally shifting storm track over the South Atlantic and Indian Oceans and a pulsing South Pacific storm track. The patterns of storm-track variability are shown to be related to simple distortions of the climatological-mean upper-tropospheric jet.

scholarly journals Are the Northern Hemisphere Winter Storm Tracks Significantly Correlated?

2004 ◽  
Vol 17 (21) ◽  
pp. 4230-4244 ◽  
Author(s):  
Edmund K. M. Chang
Keyword(s):  
Northern Hemisphere ◽  
Storm Track ◽  
Reanalysis Data ◽  
Storm Tracks ◽  
The Pacific ◽  
Aircraft Observations ◽  
Highly Correlated ◽  
Hemisphere Winter ◽  
Aircraft Data ◽  
Northern Hemisphere Winter

Abstract In this study, the correlation between the Northern Hemisphere winter Pacific and Atlantic storm tracks is examined using the NCEP–NCAR reanalysis and the 40-yr ECMWF Re-Analysis (ERA-40), as well as unassimilated aircraft observations. By examining month-to-month variability in the 250-hPa meridional velocity variance, the correlation between the two storm track peaks is found to be as high as 0.5 during the winters between 1975/76 and 1998/99. Here, it is shown that the correlation between the two storm tracks can be clearly detected from the aircraft data. Further analyses of the reanalysis data show that the correlation can also be seen in other eddy variance and covariance statistics, including the poleward heat flux at the 700-hPa level. The correlation between the two storm tracks, as seen in both reanalysis datasets, is shown to be much weaker during the period 1957/58–1971/72, suggesting a possible regime transition from largely uncorrelated storm tracks to highly correlated storm tracks during the 1970s. However, during this earlier period, the number of aircraft observations is insufficient to verify the low correlation seen in the reanalyses. Thus, low biases in the reanalyses during the earlier period cannot be ruled out. An ensemble of four GCM simulations performed using the GFDL GCM forced by global observed SST variations between 1950 and 1995 has also been examined. The correlation between the two storm tracks in the GCM simulations is much lower (0.18) than that observed, even if the analysis is restricted to the GCM simulations from the period 1975/76–1994/95. A Monte Carlo test shows that the observed correlation and the GCM correlation are statistically distinct at the 1% level. Correlations between the Southern Hemisphere summer Pacific and Atlantic storm tracks have also been examined based on the reanalyses datasets. The results suggest that the amplitude of the SH summer Pacific and Atlantic storm tracks are not significantly correlated, showing that seeding of the Atlantic storm track by the Pacific storm track does not necessarily lead to significant correlations between the two storm tracks.

scholarly journals Diabatic and Orographic Forcing of Northern Winter Stationary Waves and Storm Tracks

2009 ◽  
Vol 22 (3) ◽  
pp. 670-688 ◽  
Author(s):  
Edmund K. M. Chang
Keyword(s):  
Storm Track ◽  
Lower Troposphere ◽  
Circulation Model ◽  
Storm Tracks ◽  
Minor Role ◽  
Stationary Waves ◽  
Global Circulation Model ◽  
Mean Flow ◽  
The Pacific ◽  
Eddy Fluxes

Abstract In this study, a dry global circulation model is used to examine the contributions made by orographic and diabatic forcings in shaping the zonal asymmetries in the earth’s Northern Hemisphere (NH) winter climate. By design, the model mean flow is forced to bear a close resemblance to the observed zonal mean and stationary waves. The model also provides a decent simulation of the storm tracks. In particular, the maxima over the Pacific and Atlantic, and minima over Asia and North America, are fairly well simulated. The model also successfully simulates the observation that the Atlantic storm track is stronger than the Pacific storm track, despite stronger baroclinicity over the Pacific. Sensitivity experiments are performed by imposing and removing various parts of the total forcings. In terms of the NH winter stationary waves in the upper troposphere, results of this study are largely consistent with previous studies. Diabatic forcings explain most of the modeled stationary waves, with orographic forcings playing only a secondary role, and feedbacks due to eddy fluxes probably play only minor roles in most cases. Nevertheless, results of this study suggest that eddy fluxes may be important in modifying the response to orographic forcings in the absence of zonal asymmetries in diabatic heating. On the other hand, unlike the conclusion reached by previous studies, it is argued that the convergence of eddy momentum fluxes is important in forcing the oceanic lows in the lower troposphere, in agreement with one’s synoptic intuition. Regarding the NH winter storm-track distribution, results of this study suggest that NH extratropical heating is the most important forcing. Zonal asymmetries in NH extratropical heating act to force the Pacific storm track to shift equatorward and the Atlantic storm track to shift poleward, attain a southwest–northeast tilt, and intensify. It appears to be the main forcing responsible for explaining why the Atlantic storm track is stronger than the Pacific storm track. Tibet and the Rockies are also important, mainly in suppressing the storm tracks over the continents, forcing a clearer separation between the two storm tracks. In contrast, asymmetries in tropical heating appear to play only a minor role in forcing the model storm-track distribution.

scholarly journals The Annual Cycle of Northern Hemisphere Storm Tracks. Part II: Regional Detail

2019 ◽  
Vol 32 (6) ◽  
pp. 1761-1775 ◽  
Author(s):  
B. J. Hoskins ◽  
K. I. Hodges
Keyword(s):  
North Atlantic ◽  
Annual Cycle ◽  
North Pacific ◽  
East Asian ◽  
Storm Track ◽  
Meridional Wind ◽  
Storm Tracks ◽  
Annual Cycles ◽  
The North ◽  
The North Pacific

Abstract In Part I of this study, the annual cycle of the Northern Hemisphere storm tracks was investigated using feature tracking and Eulerian variance-based diagnostics applied to both the vorticity and meridional wind fields. Results were presented and discussed for the four seasons at both upper- (250 hPa) and lower- (850 hPa) tropospheric levels. Here, using the meridional wind diagnostics, the annual cycles of the North Pacific and North Atlantic storm tracks are examined in detail. This is done using monthly and 20° longitudinal sector averages. Many sectors have been considered, but the focus is on sectors equally spaced in the two main oceanic storm tracks situated at their western, central, and eastern regions, with the western ones being mainly over the upstream continents. The annual cycles of the upper- and lower-tropospheric storm tracks in the central and eastern Pacific, as well as in the western and central Atlantic sectors, all have rather similar structures. In amplitude, each sector at both levels has a summer minimum and a relatively uniform strength from October to April, despite the strong winter maxima in the westerly jets. However, high-intensity storms occur over a much wider latitudinal band in winter. The storm track in each sector moves poleward from May to August and returns equatorward from October to December, and there is a marked asymmetry between spring and autumn. There are many differences between the North Pacific and North Atlantic storm tracks, and some of these seem to have their origin in the behavior over the upstream East Asian and North American continents, suggesting the importance of seeding from these regions. The East Asian storm track near 48°N has marked spring and autumn maxima and weak amplitude in winter and summer. The 33°N track is strong only in the first half of the year. In contrast, the eastern North American storm track is well organized throughout the year, around the baroclinicity that moves latitudinally with the seasons. The signatures associated with these features are found to gradually decrease downstream in each case. In particular, there is very little latitudinal movement in the storm track in the eastern Atlantic.

scholarly journals The Annual Cycle of the Axial Angular Momentum of the Atmosphere

2005 ◽  
Vol 18 (6) ◽  
pp. 757-771 ◽  
Author(s):  
Joseph Egger ◽  
Klaus-Peter Hoinka
Keyword(s):  
Angular Momentum ◽  
Northern Hemisphere ◽  
Annual Cycle ◽  
Lower Troposphere ◽  
Mass Term ◽  
Friction Torque ◽  
Hadley Cell ◽  
Trade Winds ◽  
Latitude Belt ◽  
The Tropics

Abstract Earlier analyses of the annual cycle of the axial angular momentum (AAM) are extended to include mass flows and vertical transports as observed, and to establish angular momentum budgets for various control volumes, using the European Centre for Medium-Range Forecasts (ECMWF) Re-Analyses (ERA) for the years 1979–92, transformed to height coordinates. In particular, the role of the torques is examined. The annual cycle of the zonally averaged angular momentum is large in the latitude belt 20° ⩽ |ϕ| ⩽ 45°, with little attenuation in the vertical up to a height of ∼12 km. The oscillation of the mass term (AAM due to the earth’s rotation) dominates in the lower troposphere, but that of the wind term (relative AAM) is more important elsewhere. The cycle of the friction torque as related to the trade winds prevails in the Tropics. Mountain torque and friction torque are equally important in the extratropical latitudes of the Northern Hemisphere. The annual and the semiannual cycle of the global angular momentum are in good balance with the global mountain and friction torques. The addition of the global gravity wave torque destroys this agreement. The transports must be adjusted if budgets of domains of less than global extent are to be considered. Both a streamfunction, representing the nondivergent part of the fluxes, and a flux potential, describing the divergences/convergences, are determined. The streamfunction pattern mainly reflects the seasonal shift of the Hadley cell. The flux potential links the annual oscillations of the angular momentum with the torques. It is concluded that the interaction of the torques with the angular momentum is restricted to the lower troposphere, in particular, in the Tropics. The range of influence is deeper in the Northern Hemisphere than in the Southern Hemisphere, presumably because of the mountains. The angular momentum cycle in the upper troposphere and stratosphere is not affected by the torques and reflects interhemispheric flux patterns. Budgets for the polar as well as for the midlatitude domains show that fluxes in the stratosphere are important.

scholarly journals Northern hemisphere winter storm tracks of the Eemian interglacial and the last glacial inception

2007 ◽  
Vol 3 (2) ◽  
pp. 181-192 ◽  
Author(s):  
F. Kaspar ◽  
T. Spangehl ◽  
U. Cubasch
Keyword(s):  
Northern Hemisphere ◽  
North Atlantic ◽  
Storm Track ◽  
Storm Tracks ◽  
Winter Storm ◽  
The North ◽  
The North Atlantic ◽  
Hemisphere Winter ◽  
Eemian Interglacial ◽  
The Last Glacial

Abstract. Climate simulations of the Eemian interglacial and the last glacial inception have been performed by forcing a coupled ocean-atmosphere general circulation model with insolation patterns of these periods. The parameters of the Earth's orbit have been set to conditions of 125 000 and 115 000 years before present (yr BP). Compared to today, these dates represent periods with enhanced and weakened seasonality of insolation in the northern hemisphere. Here we analyse the simulated change in northern hemisphere winter storm tracks. The change in the orbital configuration has a strong impact on the meridional temperature gradients and therefore on strength and location of the storm tracks. The North Atlantic storm track is strengthened, shifted northward and extends further to the east in the simulation for the Eemian at 125 kyr BP. As one consequence, the northern parts of Europe experience an increase in winter precipitation. The frequency of winter storm days increases over large parts of the North Atlantic including the British Isles and the coastal zones of north-western Europe. Opposite but weaker changes in storm track activity are simulated for 115 kyr BP.

Variations of Northern Hemisphere Storm Track and Extratropical Cyclone Activity Associated with the Madden–Julian Oscillation

2017 ◽  
Vol 30 (13) ◽  
pp. 4799-4818 ◽  
Author(s):  
Yanjuan Guo ◽  
Toshiaki Shinoda ◽  
Jialin Lin ◽  
Edmund K. M. Chang
Keyword(s):  
Storm Track ◽  
Meridional Wind ◽  
Reanalysis Data ◽  
Boreal Winter ◽  
Madden Julian Oscillation ◽  
Mean Flow ◽  
Cyclone Activity ◽  
Intraseasonal Variations ◽  
The Pacific ◽  
Cyclone Frequency

This study investigates the intraseasonal variations of the Northern Hemispheric storm track associated with the Madden–Julian oscillation (MJO) during the extended boreal winter (November–April) using 36 yr (1979–2014) of reanalysis data from ERA-Interim. Two methods have been used to diagnose storm-track variations. In the first method, the storm track is quantified by the temporal-filtered variance of 250-hPa meridional wind (vv250) or mean sea level pressure (pp). The intraseasonal anomalies of vv250 composited for eight MJO phases are characterized by a zonal band of strong positive (or negative) anomalies meandering from the Pacific all the way across North America and the Atlantic into northern Europe, with weaker anomalies of opposite sign at one or both flanks. The results based on pp are consistent with those based on vv250 except for larger zonal variations, which may be induced by surface topography. In the second method, an objective cyclone-tracking scheme has been used to track the extratropical cyclones that compose the storm track. The MJO-composite anomalies of the “accumulated” cyclone activity, a quantity that includes contributions from both the cyclone frequency and cyclone mean intensity, are very similar to those based on pp. Further analysis demonstrates that major contribution comes from variations in the cyclone frequency. Further analysis suggests that the intraseasonal variations of the storm track can be primarily attributed to the variations of the mean flow that responds to the anomalous MJO convections in the tropics, with possible contribution also from the moisture variations.

scholarly journals Eddy-Induced Growth Rate of Low-Frequency Variability and Its Mid- to Late Winter Suppression in the Northern Hemisphere

2014 ◽  
Vol 71 (7) ◽  
pp. 2281-2298 ◽  
Author(s):  
Hong-Li Ren ◽  
Fei-Fei Jin ◽  
Jong-Seong Kug
Keyword(s):  
Northern Hemisphere ◽  
Storm Track ◽  
Low Frequency ◽  
Lower Troposphere ◽  
Surface Friction ◽  
Late Winter ◽  
Wind Maximum ◽  
Low Frequency Variability ◽  
Storm Track Activity ◽  
Effect Of Surface

Abstract Synoptic eddy and low-frequency flow (SELF) feedback plays an important role in reinforcing low-frequency variability (LFV). Recent studies showed that an eddy-induced growth (EIG) or instability makes a fundamental contribution to the maintenance of LFV. To quantify the efficiency of the SELF feedback, this study examines the spatiotemporal features of the empirical diagnostics of EIG and its associations with LFV. The results show that, in terms of eddy vorticity forcing, the EIG rate of LFV is generally larger (smaller) in the upper (lower) troposphere, whereas, in terms of eddy potential vorticity forcing, it is larger in the lower troposphere to partly balance the damping effect of surface friction. The local EIG rate shows a horizontal spatial distribution that corresponds to storm-track activity, which tends to be responsible for maintaining LFV amplitudes and patterns as well as sustaining eddy-driven jets. In fact, the EIG rate has a well-defined seasonality, being generally larger in cold seasons and smaller in the warmest season, and this seasonality is stronger in the Northern Hemisphere than in the Southern Hemisphere. This study also reveals a mid- to late winter (January–March) suppression of the EIG rate in the Northern Hemisphere, which indicates a reduced eddy feedback efficiency and may be largely attributed to the eddy kinetic energy suppression and the midlatitude zonal wind maximum in the midwinter of the Northern Hemisphere.

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>

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