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  • 1
    Electronic Resource
    Electronic Resource
    Rossby wave propagation into the tropics in two GFDL general circulation models (1994)
    Springer
    Climate dynamics 9 (1994), S. 245-252 
    Details
    ISSN: 1432-0894
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract During the northern winter the eastern Pacific is characterized by upper level westerly flow extending from the equator into the midlatitudes of both hemispheres. Theoretical and simple modeling studies suggest that such a region should favor the penetration of Rossby waves into the tropics from higher latitudes. Observational results by Kiladis and Weickmann using 200 mb data indicate that Rossby waves do indeed propagate freely into the tropical eastern Pacific during the northern winter from the Asian jet exit region. They also confirmed that cross-equatorial dispersion of energy from the Northern into the Southern Hemisphere occurs frequently. The present study examines these interactions in climatological runs of two GFDL GCMs. The northern wintertime mean states of these models are characterized by a rather realistically simulated upper level westerly regime in the tropical Pacific. Despite the relative weakness of the Asian jet and wave activity with respect to observations, propagation of Rossby waves into the tropics is present in both models, and these waves are strongly positively tilted as seen in the observations. A momentum budget of the zonal wind and E vector diagnostics over the tropical Pacific indicate that these transients are an important component of the momentum balance of the equatorial westerlies in both the observations and in the models.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00208256
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Rossby wave propagation into the tropics in two GFDL general circulation models (1994)
    Springer
    Climate dynamics 9 (1994), S. 245-252 
    Details
    ISSN: 1432-0894
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract. During the northern winter the eastern Pacific is characterized by upper level westerly flow extending from the equator into the midlatitudes of both hemispheres. Theoretical and simple modeling studies suggest that such a region should favor the penetration of Rossby waves into the tropics from higher latitudes. Observational results by Kiladis and Weickmann using 200 mb data indicate that Rossby waves do indeed propagate freely into the tropical eastern Pacific during the northern winter from the Asian jet exit region. They also confirmed that cross-equatorial dispersion of energy from the Northern into the Southern Hemisphere occurs frequently. The present study examines these interactions in climatological runs of two GFDL GCMs. The northern wintertime mean states of these models are characterized by a rather realistically simulated upper level westerly regime in the tropical Pacific. Despite the relative weakness of the Asian jet and wave activity with respect to observations, propagation of Rossby waves into the tropics is present in both models, and these waves are strongly positively tilted as seen in the observations. A momentum budget of the zonal wind and E vector diagnostics over the tropical Pacific indicate that these transients are an important component of the momentum balance of the equatorial westerlies in both the observations and in the models.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003820050023
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    Paper (German National Licenses)
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  • 3
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    Corrigendum (2016)
    American Meteorological Society
    Details
    Publication Date: 2016-10-21
    Print ISSN: 0894-8755
    Electronic ISSN: 1520-0442
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 4
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    Linear and Nonlinear Dynamics of North Atlantic Oscillations: A New Thinking of Symmetry Breaking (2018)
    American Meteorological Society
    Details
    Publication Date: 2018-05-24
    Description: Observations reveal that the North Atlantic Oscillation (NAO) exhibits a strong asymmetry: large amplitude, long persistence, and westward movement in its negative phase (NAO−) and conversely in its positive phase (NAO+). Further calculations show that blocking days occur frequently over the North Atlantic (Eurasia) after the NAO− (NAO+) peaks, thus indicating that North Atlantic blocking occurs because of the retrogression of the NAO−, whereas blocking occurs over Eurasia because of enhanced downstream energy dispersion of the NAO+. Motivated by a unified nonlinear multiscale interaction (UNMI) model, the authors define dispersion, nonlinearity, and movement indices to describe the basic characteristics of the NAO. On this basis, the physical cause of the strong asymmetry or symmetry breaking of the NAO is examined. It is revealed that the strong asymmetry between the NAO+ and NAO− may be associated with the large difference of the North Atlantic jet in intensity and latitude between both phases. When the NAO+ grows, the North Atlantic jet is intensified and shifts northward and corresponds to reduced nonlinearity and enhanced energy dispersion because of an increased difference between its group velocity and phase speed related to enhanced meridional potential vorticity gradient. Thus, the NAO+ has smaller amplitude, eastward movement, and less persistence. Opposite behavior is seen for the NAO− because of the opposite variation of the North Atlantic jet during its life cycle. Thus, the above results suggest that the NAO+ (NAO−) tends to be a linear (nonlinear) process as a natural consequence of the NAO evolution because of different changes in the North Atlantic jet between both phases.
    Print ISSN: 0022-4928
    Electronic ISSN: 1520-0469
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 5
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    Impact of Winter Ural Blocking on Arctic Sea Ice: Short-Time Variability (2018)
    American Meteorological Society
    Details
    Publication Date: 2018-03-01
    Print ISSN: 0894-8755
    Electronic ISSN: 1520-0442
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 6
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    Why Do Similar Patterns of Tropical Convection Yield Extratropical Circulation Anomalies of Opposite Sign? (2017)
    American Meteorological Society
    Details
    Publication Date: 2017-01-25
    Description: Tropical precipitation anomalies associated with El Niño and Madden–Julian oscillation (MJO) phase 1 (La Niña and MJO phase 5) are characterized by a tripole, with positive (negative) centers over the Indian Ocean and central Pacific and a negative (positive) center over the warm pool region. However, their midlatitude circulation responses over the North Pacific and North America tend to be of opposite sign. To investigate these differences in the extratropical response to tropical convection, the dynamical core of a climate model is used, with boreal winter climatology as the initial flow. The model is run using the full heating field for the above four cases, and with heating restricted to each of seven small domains located near or over the equator, to investigate which convective anomalies may be responsible for the different extratropical responses. An analogous observational study is also performed. For both studies, it is found that, despite having a similar tropical convective anomaly spatial pattern, the extratropical response to El Niño and MJO phase 1 (La Niña and MJO phase 5) is quite different. Most notably, responses with opposite-signed upper-tropospheric geopotential height anomalies are found over the eastern North Pacific, northwestern North America, and the southeastern United States. The extratropical response for each convective case most closely resembles that for the domain associated with the largest-amplitude precipitation anomaly: the central equatorial Pacific for El Niño and La Niña and the warm pool region for MJO phases 1 and 5.
    Print ISSN: 0022-4928
    Electronic ISSN: 1520-0469
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 7
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    An Investigation of the Presence of Atmospheric Rivers over the North Pacific during Planetary-Scale Wave Life Cycles and Their Role in Arctic Warming (2016)
    American Meteorological Society
    Details
    Publication Date: 2016-10-10
    Description: Heretofore, the tropically excited Arctic warming (TEAM) mechanism put forward that localized tropical convection amplifies planetary-scale waves, which transport sensible and latent heat into the Arctic, leading to an enhancement of downward infrared radiation and Arctic surface warming. In this study, an investigation is made into the previously unexplored contribution of the synoptic-scale waves and their attendant atmospheric rivers to the TEAM mechanism. Reanalysis data are used to conduct a suite of observational analyses, trajectory calculations, and idealized model simulations. It is shown that localized tropical convection over the Maritime Continent precedes the peak of the planetary-scale wave life cycle by ~10–14 days. The Rossby wave source induced by the tropical convection excites a Rossby wave train over the North Pacific that amplifies the climatological December–March stationary waves. These amplified planetary-scale waves are baroclinic and transport sensible and latent heat poleward. During the planetary-scale wave life cycle, synoptic-scale waves are diverted northward over the central North Pacific. The warm conveyor belts associated with the synoptic-scale waves channel moisture from the subtropics into atmospheric rivers that ascend as they move poleward and penetrate into the Arctic near the Bering Strait. At this time, the synoptic-scale waves undergo cyclonic Rossby wave breaking, which further amplifies the planetary-scale waves. The planetary-scale wave life cycle ceases as ridging over Alaska retrogrades westward. The ridging blocks additional moisture transport into the Arctic. However, sensible and latent heat amounts remain elevated over the Arctic, which enhances downward infrared radiation and maintains warm surface temperatures.
    Print ISSN: 0022-4928
    Electronic ISSN: 1520-0469
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 8
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    Exploring the Tropically Excited Arctic Warming Mechanism with Station Data: Links between Tropical Convection and Arctic Downward Infrared Radiation (2016)
    American Meteorological Society
    Details
    Publication Date: 2016-02-11
    Description: The Tropically Excited Arctic Warming (TEAM) mechanism ascribes warming of the Arctic surface to tropical convection, which excites poleward-propagating Rossby wave trains that transport water vapor and heat into the Arctic. A crucial component of the TEAM mechanism is the increase in downward infrared radiation (IR) that precedes the Arctic warming. Previous studies have examined the downward IR associated with the TEAM mechanism using reanalysis data. To corroborate previous findings, this study examines the linkage between tropical convection, Rossby wave trains, and downward IR with Baseline Surface Radiation Network (BSRN) downward IR station data. The physical processes that drive changes in the downward IR are also investigated by regressing 300-hPa geopotential height, outgoing longwave radiation, water vapor flux, ERA-Interim downward IR, and other key variables against the BSRN downward IR at Barrow, Alaska, and Ny-Ålesund, Spitsbergen. Both the Barrow and the Ny-Ålesund station downward IR anomalies are preceded by anomalous tropical convection and poleward-propagating Rossby wave trains. The wave train associated with Barrow resembles the Pacific–North America teleconnection pattern, and that for Ny-Ålesund corresponds to a northwestern Atlantic wave train. It is found that both wave trains promote warm and moist advection from the midlatitudes into the Arctic. The resulting water vapor flux convergence, multiplied by the latent heat of vaporization, closely resembles the regressed ERA-Interim downward IR. These results suggest that the combination of warm advection, latent heat release, and increased cloudiness all contribute toward an increase in downward IR.
    Print ISSN: 0022-4928
    Electronic ISSN: 1520-0469
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 9
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    Stationary Wave Interference and Its Relation to Tropical Convection and Arctic Warming (2016)
    American Meteorological Society
    Details
    Publication Date: 2016-02-10
    Description: The interference between transient eddies and climatological stationary eddies in the Northern Hemisphere is investigated. The amplitude and sign of the interference is represented by the stationary wave index (SWI), which is calculated by projecting the daily 300-hPa streamfunction anomaly field onto the 300-hPa climatological stationary wave. ERA-Interim data for the years 1979 to 2013 are used. The amplitude of the interference peaks during boreal winter. The evolution of outgoing longwave radiation, Arctic temperature, 300-hPa streamfunction, 10-hPa zonal wind, Arctic sea ice concentration, and the Arctic Oscillation (AO) index are examined for days of large SWI values during the winter. Constructive interference during winter tends to occur about one week after enhanced warm pool convection and is followed by an increase in Arctic surface air temperature along with a reduction of sea ice in the Barents and Kara Seas. The warming of the Arctic does occur without prior warm pool convection, but it is enhanced and prolonged when constructive interference occurs in concert with enhanced warm pool convection. This is followed two weeks later by a weakening of the stratospheric polar vortex and a decline of the AO. All of these associations are reversed in the case of destructive interference. Potential climate change implications are briefly discussed.
    Print ISSN: 0894-8755
    Electronic ISSN: 1520-0442
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 10
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    The Positive North Atlantic Oscillation with Downstream Blocking and Middle East Snowstorms: Impacts of the North Atlantic Jet (2016)
    American Meteorological Society
    Details
    Publication Date: 2016-03-01
    Description: A recent study revealed that cold winter outbreaks over the Middle East and southeastern Europe are caused mainly by the northeast–southwest (NE–SW) tilting of European blocking (EB) associated with the positive-phase North Atlantic Oscillation (NAO+). Here, the North Atlantic conditions are examined that determine the EB tilting direction, defined as being perpendicular to the dipole anomaly orientation. Using daily reanalysis data, the NAO+ events are classified into strong (SJN) and weak (WJN) North Atlantic jet types. A composite analysis shows that the EB is generally stronger and located more westward and southward during SJN events than during WJN events. During SJN events, the NAO+ and EB dipoles exhibit NE–SW tilting, which leads to strong cold advection and large negative temperature anomalies over the Middle East and southeastern Europe. In contrast, northwest–southeast (NW–SE) tilting without strong negative temperature anomalies over the Middle East is seen during WJN events. A nonlinear multiscale interaction model is modified to investigate the physical mechanism through which the North Atlantic jet (NAJ) affects EB with the NAO+ event. It is shown that, when the NAJ is stronger, an amplified EB event forms because of enhanced NAO+ energy dispersion. For a strong (weak) NAJ, the EB tends to occur in a relatively low-latitude (high latitude) region because of the suppressive (favorable) role of intensified (reduced) zonal wind in high latitudes. It exhibits NE–SW (NW–SE) tilting because the blocking region corresponds to negative-over-positive (opposite) zonal wind anomalies. The results suggest that the NAJ can modulate the tilting direction of EB, leading to different effects over the Middle East.
    Print ISSN: 0894-8755
    Electronic ISSN: 1520-0442
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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