Search Results

You are looking at 31 - 40 of 63 items for

  • Author or Editor: Steven B. Feldstein x
  • Refine by Access: All Content x
Clear All Modify Search
Joseph P. Clark and Steven B. Feldstein

Abstract

Composite analysis is used to examine the physical processes that drive the growth and decay of the surface air temperature anomaly pattern associated with the North Atlantic Oscillation (NAO). Using the thermodynamic energy equation that the European Centre for Medium-Range Weather Forecasts implements in their reanalysis model, we show that advection of the climatological temperature field by the anomalous wind drives the surface air temperature anomaly pattern for both NAO phases. Diabatic processes exist in strong opposition to this temperature advection and eventually cause the surface air temperature anomalies to return to their climatological values. Specifically, over Greenland, Europe, and the United States, longwave heating/cooling opposes horizontal temperature advection while over northern Africa vertical mixing opposes horizontal temperature advection. Despite the pronounced spatial correspondence between the skin temperature and surface air temperature anomaly patterns, the physical processes that drive these two temperature anomalies associated with the NAO are found to be distinct. The skin temperature anomaly pattern is driven by downward longwave radiation whereas stated above, the surface air temperature anomaly pattern is driven by horizontal temperature advection. This implies that the surface energy budget, although a useful diagnostic tool for understanding skin temperature changes, should not be used to understand surface air temperature changes.

Free access
Dehai Luo, Jing Cha, and Steven B. Feldstein

Abstract

In this study, attention is focused on identifying the dynamical processes that contribute to the negative North Atlantic Oscillation (NAO) to positive NAO (NAO+) and NAO+ to NAO transitions that occur during 1978–90 (P1) and 1991–2008 (P2). By constructing Atlantic ridge (AR) and Scandinavian blocking (SBL) indices, the composite analysis demonstrates that in a stronger AR (SBL) winter NAO (NAO+) event can more easily transition into an NAO+ (NAO) event. Composites of 300-hPa geopotential height anomalies for the NAO to NAO+ and NAO+ to NAO transition events during P1 and P2 are calculated. It is shown for P2 (P1) that the NAO+ to SBL to NAO (NAO to AR to NAO+) transition results from the retrograde drift of an enhanced high-latitude, large-scale, positive (negative) anomaly over northern Europe during the decay of the previous NAO+ (NAO) event. This finding cannot be detected for NAO events without transition.

Moreover, it is found that the amplification of retrograding wavenumber 1 is more important for the NAO to NAO+ transition during P1, but the marked reintensification and retrograde movement of both wavenumbers 1 and 2 after the NAO+ event decays is crucial for the NAO+ to NAO transition during P2. It is further shown that destructive (constructive) interference between wavenumbers 1 and 2 over the North Atlantic during P1 (P2) is responsible for the subsequent weak NAO+ (strong NAO) anomaly associated with the NAO to NAO+ (NAO+ to NAO) transition. Also, the weakening (strengthening) of the vertically integrated zonal wind (upstream Atlantic storm track) is found to play an important role in the NAO regime transition.

Full access
Steven B. Feldstein and Isaac M. Held

Abstract

A two-layer quasi-geostrophic model is used to study the effects of a meridionally sheared zonal flow on the life cycle of a weakly unstable baroclinic wave. In most of the cases analyzed, the fluid is inviscid with the exception of scale-selective fourth-order horizontal diffusion. The initial zonal flow is identically zero in the lower layer. The character of the eddy life cycle in the limit of weak supercritically is shown to depend on whether or not the meridional shell in the upper layer is strong enough to produce a critical latitude for the wave.

If the shear is sufficiently weak, the wave undergoes periodic amplitude vacillation characterized by symmetric growth and baroclinic decay. However, when the meridional shear is strong enough to allow for the existence of a critical layer, the flow undergoes an asymmetric life cycle which resembles that found by Simmons and Hoskins in a primitive equation model on the sphere: the wave grows baroclinically but decays barotropically toward a wave-free state. Throughout the barotropic decay stage, the wave is breaking and being absorbed either at or before the critical layer. As the supercriticality is increased, strong reflection begins to occur at the location of the wave breaking, resulting in irregular amplitude vacillation. Consistent with critical layer theory, when a reflecting state is created the solution is sensitive to the inclusion of higher zonal harmonies of the fundamental wave.

By relaxing the potential vorticity distribution back to an unstable state, periodic solutions are obtained in which each episode of growth and decay is similar to that found in these nearly inviscid solutions.

Full access
Christian Franzke, Sukyoung Lee, and Steven B. Feldstein

Abstract

Given the recent observational evidence that the positive (negative) phase of the North Atlantic Oscillation (NAO) is the remnant of anticyclonic (cyclonic) wave breaking, this study uses a multilevel primitive equation model to investigate important dynamical attributes of the above wave breaking behavior. For this purpose, a hierarchy of different basic states (two- and three-dimensional) and initial perturbations are used.

With the three-dimensional climatological flow as the basic state, it is found that initial perturbations located equatorward (poleward) and upstream of the climatological Atlantic jet lead to wave breaking similar to that of the positive (negative) NAO phase. Consistently, analysis of observational data indeed shows that the Pacific storm track is displaced equatorward (poleward) prior the onset of the positive (negative) NAO phase. This result suggests that the latitudinal position of the Pacific storm track plays an important role for determining the phase of the NAO. Sensitivity experiments show that individual life cycles resemble each other only within the NAO region, but have large case-to-case variability outside of the NAO region.

Calculations with zonally symmetric basic states fail to produce wave breaking of the correct spatial and temporal scale, underscoring the dynamical significance of the three-dimensional climatological flow.

Full access
Dehai Luo, Yao Yao, and Steven B. Feldstein

Abstract

In this paper, large-scale aspects for the onset of the extreme cold European weather event in January–February 2012 are investigated. It is shown that the outbreak of this extreme cold weather event may be attributed to the transition from a positive North Atlantic Oscillation (NAO+) event to a long-lasting blocking event over the eastern Atlantic and western Europe (hereafter ENAO). A persistent decline of the surface air temperature (SAT) is seen over all of Europe during the long-lived ENAO event, while the main region of enhanced precipitation is located over southern Europe and part of central Europe, in association with the presence of a persistent double storm track: one over the Norwegian and Barents Seas and the other over southern Europe.

The NAO+ to NAO transition events are divided into NAO+ to ENAO and NAO+ to WNAO transition events [ENAO (WNAO) events correspond to eastward- (westward-) displaced NAO events whose positive center is defined to be located to the east (west) of 10°W], and a statistical analysis of the NAO+ to ENAO transition events during 1978–2012 is performed. It is found that there has been a marked increase in the frequency of the NAO+ to ENAO transition events during the period 2005–12. Composites of SAT anomalies indicate that the marked decline of the SAT observed over much of Europe is primarily associated with NAO+ to ENAO transition events. Thus, NAO+ to ENAO transition events may be more favorable for the extreme cold events over Europe observed in recent winters than other types of NAO events.

Full access
Changhyun Yoo, Sukyoung Lee, and Steven B. Feldstein

Abstract

Using lagged composites and projections with the thermodynamic energy equation, in this study the mechanisms that drive the boreal winter Arctic surface air temperature (SAT) change associated with the Madden–Julian oscillation (MJO) are investigated. The Wheeler and Hendon MJO index, which divides the MJO into 8 phases, where phase 1 (phase 5) corresponds to reduced (enhanced) convection over the Maritime Continent and western Pacific Ocean, is used. It is shown that the more zonally localized (uniform) tropical convective heating associated with MJO phase 5 (phase 1) leads to enhanced (reduced) excitation of poleward-propagating Rossby waves, which contribute to Arctic warming (cooling). Adiabatic warming/cooling, eddy heat flux, and the subsequent change in downward infrared radiation (IR) flux are found to be important for the Arctic SAT change. The adiabatic warming/cooling initiates the Arctic SAT change, however, subsequent eddy heat flux makes a greater contribution. The resulting SAT change is further amplified by alteration in downward IR. It is shown that changes in surface sensible and latent heat fluxes oppose the contribution by the above processes.

Full access
Dehai Luo, Yina Diao, and Steven B. Feldstein

Abstract

The winter-mean North Atlantic Oscillation (NAO) index has been mostly positive since the 1980s, with a linear upward trend during the period from 1978 to 1990 (P1) and a linear downward trend during the period from 1991 to 2009 (P2). Further calculations show that the Atlantic storm-track eddy activity is more intense during P2 than during P1, which is statistically significant at the 90% confidence level for a t test. This study proposes a hypothesis that the change in the trend of the positive NAO index from P1 to P2 may be associated with the marked intensification of the Atlantic storm track during P2.

A generalized nonlinear NAO model is used to explain the observed trend of the positive NAO index within P2. It is found that even when the Atlantic storm-track eddies are less intense, a positive-phase NAO event can form under the eddy forcing if the planetary-scale wave has an initial value with a low-over-high dipole structure during P1 and P2. A blocking flow can occur in the downstream side (over Europe) of the Atlantic basin as a result of the energy dispersion of Rossby waves during the decay of the positive-phase NAO event. This blocking flow does not strictly correspond to a negative-phase NAO event because the blocking stays mainly over the European continent. However, when the Atlantic storm-track eddies are rather strong, the blocking flow occurring over the European continent is enhanced and can retrograde into the Atlantic region and finally become a long-lived negative-phase NAO event. In this case, the NAO event can transit from the positive phase to the negative phase. Thus, the winter-mean NAO index during P2 will inevitably decline because of the increase in days of negative-phase NAO events in winter because the Atlantic storm track exhibits a marked intensification in the time interval. The transition of the NAO event from the positive phase to the negative phase can also be observed only when the downstream development of the Atlantic storm-track eddy activity is rather prominent.

Thus, it appears that there is a physical link between intraseasonal and interannual time scales of the NAO when the Atlantic storm track exhibits an interannual variability.

Full access
Tingting Gong, Steven B. Feldstein, and Sukyoung Lee

Abstract

The relationship between latent heating over the Greenland, Barents, and Kara Seas (GBKS hereafter) and Rossby wave propagation between the Arctic and midlatitudes is investigated using global reanalysis data. Latent heating is the focus because it is the most likely source of Rossby wave activity over the Arctic Ocean. Given that the Rossby wave time scale is on the order of several days, the analysis is carried out using a daily latent heating index that resembles the interdecadal latent heating trend during the winter season. The results from regression calculations find a trans-Arctic Rossby wave train that propagates from the subtropics, through the midlatitudes, into the Arctic, and then back into midlatitudes over a period of about 10 days. Upon entering the GBKS, this wave train transports moisture into the region, resulting in anomalous latent heat release. At high latitudes, the overlapping of a negative latent heating anomaly with an anomalous high is consistent with anomalous latent heat release fueling the Rossby wave train before it propagates back into the midlatitudes. This implies that the Rossby wave propagation from the Arctic into the midlatitudes arises from trans-Arctic wave propagation rather than from in situ generation. The method used indicates the variance of the trans-Arctic wave train, but not in situ generation, and implies that the variance of the former is greater than that of latter. Furthermore, GBKS sea ice concentration regression against the latent heating index shows the largest negative value six days afterward, indicating that sea ice loss contributes little to the latent heating.

Open access
Nathaniel C. Johnson, Steven B. Feldstein, and Bruno Tremblay

Abstract

In this study, the method of self-organizing maps (SOMs) is used with NCEP–NCAR reanalysis data to advance the continuum perspective of Northern Hemisphere teleconnection patterns and to shed light on the secular eastward shift of the North Atlantic Oscillation (NAO) that began in the late 1970s. A 20-pattern SOM analysis of daily, wintertime, Northern Hemisphere sea level pressure reveals a continuum of patterns that correspond closely with well-known teleconnection patterns. This analysis also reveals that interdecadal variability of the hemispheric sea level pressure field may be understood in terms of changes in the frequency distribution within the continuum of sea level pressure patterns described by the SOM. Based on the continuum perspective illustrated with the SOM, the above secular shift of the NAO may be understood as a change in dominance from westward-displaced, negative NAO-like patterns to eastward-displaced, positive NAO-like patterns, though westward- and eastward-displaced NAO-like patterns existed during all time periods and for both phases.

Full access
Seok-Woo Son, Sukyoung Lee, and Steven B. Feldstein

Abstract

The physical processes that drive the fluctuations of the extratropical tropopause height are examined with the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data.

A composite zonal-mean heat budget analysis for the Northern Hemisphere winter shows that fluctuations in the extratropical tropopause height result not only from a warming of the troposphere but also from an even stronger cooling of the lower stratosphere. While the tropospheric warming is caused by a poleward eddy heat transport associated with baroclinic eddies, the stratospheric cooling is driven primarily by planetary-scale waves. The results from analyses of synoptic- and planetary-scale eddy kinetic energy and Eliassen–Palm fluxes are consistent with the planetary waves first gaining their energy within the troposphere, and then propagating vertically into the stratosphere.

For the Southern Hemisphere, while lower-stratospheric temperature anomalies still play an important role for the fluctuations in the tropopause height, the temperature anomalies are accounted for primarily by a poleward eddy heat transport associated with synoptic-scale eddies, and by diabatic heating.

These results indicate that, although the height of the extratropical tropopause is modulated by baroclinic eddies, which is consistent with existing theories, the amount of the modulation is highly influenced by stratospheric processes.

Full access