The Role of Linear Interference in the Annular Mode Response to Extratropical Surface Forcing

2010 ◽  
Vol 23 (22) ◽  
pp. 6036-6050 ◽  
Author(s):  
Karen L. Smith ◽  
Christopher G. Fletcher ◽  
Paul J. Kushner
Keyword(s):  
Rossby Wave ◽  
General Circulation ◽  
Classical Problem ◽  
Circulation Model ◽  
Wave Activity ◽  
Surface Cooling ◽  
Annular Mode ◽  
Positive Tendency ◽  
Phase Wave ◽  
Wave Response

Abstract The classical problem of predicting the atmospheric circulation response to extratropical surface forcing is revisited in the context of the observed connection between autumnal snow cover anomalies over Siberia and wintertime anomalies of the northern annular mode (NAM). Previous work has shown that in general circulation model (GCM) simulations in which autumnal Siberian snow forcing is prescribed, a vertically propagating Rossby wave train is generated that propagates into the stratosphere, drives dynamical stratospheric warming, and induces a negative NAM response that couples to the troposphere. Important questions remain regarding the dynamics of the response to this surface cooling. It is shown that previously unexplained aspects of the evolution of the response in a comprehensive GCM can be explained by examining the time evolution of the phasing, and hence the linear interference, between the Rossby wave response and the background climatological stationary wave. When the wave response and background wave are in phase, wave activity into the stratosphere is amplified and the zonal-mean stratosphere–troposphere NAM response displays a negative tendency; when they are out of phase, wave activity into the stratosphere is reduced and the NAM response displays a positive tendency. The effects of linear interference are probed further in a simplified GCM, where an imposed lower tropospheric cooling is varied in position, strength, and sign. As in the comprehensive GCM, linear interference strongly influences the response over a realistic range of forcing strengths. The transition from linear to nonlinear behavior is shown to depend simply on forcing strength.

The Dynamical Response to Snow Cover Perturbations in a Large Ensemble of Atmospheric GCM Integrations

2009 ◽  
Vol 22 (5) ◽  
pp. 1208-1222 ◽  
Author(s):  
Christopher G. Fletcher ◽  
Steven C. Hardiman ◽  
Paul J. Kushner ◽  
Judah Cohen
Keyword(s):  
Snow Cover ◽  
General Circulation ◽  
Circulation Model ◽  
Wave Activity ◽  
Dynamical Response ◽  
Mean Flow ◽  
Surface Cooling ◽  
Fall Season ◽  
Annular Mode ◽  
Northern Annular Mode

Abstract Variability in the extent of fall season snow cover over the Eurasian sector has been linked in observations to a teleconnection with the winter northern annular mode pattern. Here, the dynamics of this teleconnection are investigated using a 100-member ensemble of transient integrations of the GFDL atmospheric general circulation model (AM2). The model is perturbed with a simple persisted snow anomaly over Siberia and is integrated from October through December. Strong surface cooling occurs above the anomalous Siberian snow cover, which produces a tropospheric form stress anomaly associated with the vertical propagation of wave activity. This wave activity response drives wave–mean flow interaction in the lower stratosphere and subsequent downward propagation of a negative-phase northern annular mode response back into the troposphere. A wintertime coupled stratosphere–troposphere response to fall season snow forcing is also found to occur even when the snow forcing itself does not persist into winter. Finally, the response to snow forcing is compared in versions of the same model with and without a well-resolved stratosphere. The version with the well-resolved stratosphere exhibits a faster and weaker response to snow forcing, and this difference is tied to the unrealistic representation of the unforced lower-stratospheric circulation in that model.

The Role of Linear Interference in the Annular Mode Response to Tropical SST Forcing

2011 ◽  
Vol 24 (3) ◽  
pp. 778-794 ◽  
Author(s):  
Christopher G. Fletcher ◽  
Paul J. Kushner
Keyword(s):  
Rossby Wave ◽  
General Circulation ◽  
Opposite Sign ◽  
Southern Oscillation ◽  
Circulation Model ◽  
Tropical Indian Ocean ◽  
Stationary Wave ◽  
Annular Mode ◽  
Wave Response ◽  
Mode Response

Abstract Recent observational and modeling studies have demonstrated a link between eastern tropical Pacific Ocean (TPO) warming associated with the El Niño–Southern Oscillation (ENSO) and the negative phase of the wintertime northern annular mode (NAM). The TPO–NAM link involves a Rossby wave teleconnection from the tropics to the extratropics, and an increase in polar stratospheric wave driving that in turn induces a negative NAM anomaly in the stratosphere and troposphere. Previous work further suggests that tropical Indian Ocean (TIO) warming is associated with a positive NAM anomaly, which is of opposite sign to the TPO case. The TIO case is, however, difficult to interpret because the TPO and TIO warmings are not independent. To better understand the dynamics of tropical influences on the NAM, the current study investigates the NAM response to imposed TPO and TIO warmings in a general circulation model. The NAM responses to the two warmings have opposite sign and can be of surprisingly similar amplitude even though the TIO forcing is relatively weak. It is shown that the sign and strength of the NAM response is often simply related to the phasing, and hence the linear interference, between the Rossby wave response and the climatological stationary wave. The TPO (TIO) wave response reinforces (attenuates) the climatological wave and therefore weakens (strengthens) the stratospheric jet and leads to a negative (positive) NAM response. In additional simulations, it is shown that decreasing the strength of the climatological stationary wave reduces the importance of linear interference and increases the importance of nonlinearity. This work demonstrates that the simulated extratropical annular mode response to climate forcings can depend sensitively on the amplitude and phase of the climatological stationary wave and the wave response.

scholarly journals The Emergence of Shallow Easterly Jets within QBO Westerlies

2018 ◽  
Vol 75 (1) ◽  
pp. 21-40 ◽  
Author(s):  
Peter Hitchcock ◽  
Peter H. Haynes ◽  
William J. Randel ◽  
Thomas Birner
Keyword(s):  
Potential Vorticity ◽  
General Circulation ◽  
Circulation Model ◽  
Shear Zones ◽  
Wave Activity ◽  
Mean Flow ◽  
Quasi Biennial Oscillation ◽  
Westerly Jet ◽  
Eddy Fluxes ◽  
Biennial Oscillation

A configuration of an idealized general circulation model has been obtained in which a deep, stratospheric, equatorial, westerly jet is established that is spontaneously and quasi-periodically disrupted by shallow easterly jets. Similar to the disruption of the quasi-biennial oscillation (QBO) observed in early 2016, meridional fluxes of wave activity are found to play a central role. The possible relevance of two feedback mechanisms to these disruptions is considered. The first involves the secondary circulation produced in the shear zones on the upper and lower flanks of the easterly jet. This is found to play a role in maintaining the aspect ratio of the emerging easterly jet. The second involves the organization of the eddy fluxes by the mean flow: the presence of a weak easterly anomaly within a tall, tropical, westerly jet is demonstrated to produce enhanced and highly focused wave activity fluxes that reinforce and strengthen the easterly anomalies. The eddies appear to be organized by the formation of strong potential vorticity gradients on the subtropical flanks of the easterly anomaly. Similar wave activity and potential vorticity structures are found in the ERA-Interim for the observed QBO disruption, indicating this second feedback was active then.

scholarly journals Forced Annular Mode Patterns in a Simple Atmospheric General Circulation Model

2007 ◽  
Vol 64 (10) ◽  
pp. 3611-3626 ◽  
Author(s):  
Michael J. Ring ◽  
R. Alan Plumb
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
Zonal Flow ◽  
General Circulation Models ◽  
Dominant Mode ◽  
Annular Mode ◽  
Annular Modes ◽  
Symmetric Version ◽  
Direct Forcing ◽  
Eddy Fluxes

Abstract Previous studies using simplified general circulation models have shown that “annular modes” arise as the dominant mode of variability. A simple GCM is used here to explore to what extent these modes are also the preferred response of the system to generic forcing. A number of trials are conducted under which the model is subjected to an artificial, zonally symmetric angular momentum forcing, and the climatologies of these trials are compared to that of the control. The forcing location is varied among the several trials. It is found that the changes in the model’s climatology are generally annular mode–like, as long as the imposed forcing projects strongly upon the annular modes of the unforced model. The role of changes to the eddy–zonal flow feedback versus the action of direct forcing is also considered through the use of a zonally symmetric version of the model. It is found that the direct responses to forcing are insufficient to capture either the strength or the structure of the annular mode responses. Instead, the changes in eddy fluxes are needed to produce the correct responses.

scholarly journals Pacific Influences on Tropical Atlantic Teleconnections to the Southern Hemisphere High Latitudes

2016 ◽  
Vol 29 (18) ◽  
pp. 6425-6444 ◽  
Author(s):  
Graham R. Simpkins ◽  
Yannick Peings ◽  
Gudrun Magnusdottir
Keyword(s):  
Climate Variability ◽  
Southern Hemisphere ◽  
Rossby Wave ◽  
General Circulation ◽  
Wave Train ◽  
Circulation Model ◽  
Hadley Circulation ◽  
Walker Circulation ◽  
Tropical Atlantic ◽  
Dynamical Mechanism

Abstract Several recent studies have connected Antarctic climate variability to tropical Atlantic sea surface temperatures (SST), proposing a Rossby wave response from the Atlantic as the primary dynamical mechanism. In this investigation, reanalysis data and atmospheric general circulation model experiments are used to further diagnose these dynamical links. Focus is placed on the possible mediating role of Pacific processes, motivated by the similar spatial characteristics of Southern Hemisphere (SH) teleconnections associated with tropical Atlantic and Pacific SST variability. During austral winter (JJA), both reanalyses and model simulations reveal that Atlantic teleconnections represent a two-mechanism process, whereby increased tropical Atlantic SST promotes two conditions: 1) an intensification of the local Atlantic Hadley circulation (HC), driven by enhanced interaction between SST anomalies and the ITCZ, that increases convergence at the descending branch, establishing anomalous vorticity forcing from which a Rossby wave emanates, expressed as a pattern of alternating positive and negative geopotential height anomalies across the SH extratropics (the so-called HC-driven components); and 2) perturbations to the zonal Walker circulation (WC), driven primarily by an SST-induced amplification, that creates a pattern of anomalous upper-level convergence across the central/western Pacific, from which an ENSO-like Rossby wave train can be triggered (the so-called WC-driven components). While the former are found to dominate, the WC-driven components play a subsidiary yet important role. Indeed, it is the superposition of these two separate but interrelated mechanisms that gives the overall observed response. By demonstrating an additional Pacific-related component to Atlantic teleconnections, this study highlights the need to consider Atlantic–Pacific interactions when diagnosing tropical-related climate variability in the SH extratropics.

scholarly journals The Response of a Simplified GCM to Axisymmetric Forcings: Applicability of the Fluctuation–Dissipation Theorem

2008 ◽  
Vol 65 (12) ◽  
pp. 3880-3898 ◽  
Author(s):  
Michael J. Ring ◽  
R. Alan Plumb
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
Quantitative Relationship ◽  
Model Atmosphere ◽  
Annular Mode ◽  
Annular Modes ◽  
Fluctuation Dissipation ◽  
Fluctuation Dissipation Theorem ◽  
Theoretical Predictions ◽  
The Tropics

Abstract Following on their previous work, in which they found the annular modes to be a preferred response of a simplified general circulation model atmosphere to a number of mechanical forcings, the authors now explore the quantitative relationship between forcing and response. In particular, the applicability of the fluctuation–dissipation theorem to this problem is investigated. First, the set of model trials is expanded by including runs in which the applied forcings are thermal rather than mechanical. For thermal forcings confined to the extratropics, “annular mode–like” responses, reminiscent of those found in earlier work, are found, but, as found in previous studies, the response is less like an annular mode when the forcing has significant amplitude in the tropics. Assuming small departures from the control climatology, and making a few further assumptions, the authors derive a theoretical relationship between forcing and response. This relationship is a statement of the fluctuation–dissipation theorem for this problem. The response of the model is found to be qualitatively consistent with the theoretical predictions. However, several aspects of the response diverge quantitatively from the theoretical expectation.

scholarly journals Impacts of a Midlatitude Oceanic Frontal Zone for the Baroclinic Annular Mode in the Southern Hemisphere

2021 ◽  
pp. 1-63
Author(s):  
MORIO NAKAYAMA ◽  
HISASHI NAKAMURA ◽  
FUMIAKI OGAWA
Keyword(s):  
Southern Hemisphere ◽  
General Circulation ◽  
Frontal Zone ◽  
Circulation Model ◽  
Southern Annular Mode ◽  
Internal Dynamics ◽  
Near Surface ◽  
Annular Mode ◽  
Major Mode ◽  
Sst Gradient

AbstractAs a major mode of annular variability in the Southern Hemisphere, the baroclinic annular mode (BAM) represents the pulsing of extratropical eddy activity. Focusing mainly on sub-weekly disturbances, this study assesses the impacts of a midlatitude oceanic frontal zone on the BAM and its dynamics through a set of “aqua-planet” atmospheric general circulation model experiments with zonally uniform sea-surface temperature (SST) profiles prescribed. Though idealized, one experiment with realistic frontal SST gradient reasonably well reproduces observed BAM-associated anomalies as a manifestation of a typical lifecycle of migratory baroclinic disturbances. Qualitatively, these BAM features are also simulated in the other experiment where the frontal SST gradient is removed. However, the BAM-associated variability weakens markedly and shifts equatorward, in association with the corresponding modifications in the climatological-mean stormtrack activity. The midlatitude oceanic frontal zone amplifies and anchors the BAM variability by restoring near-surface baroclinicity through anomalous sensible heat supply from the ocean and moisture supply to cyclones, although the BAM is essentially a manifestation of atmospheric internal dynamics. Those experiments and observations further indicate that the BAM modulates momentum flux associated with transient disturbances to induce a modest but robust meridional shift of the polar-front jet, suggesting that the BAM can help maintain the southern annular mode. They also indicate that the quasi-periodic behavior of the BAM is likely to reflect internal dynamics in which atmospheric disturbances on both sub-weekly and longer time scales are involved.

scholarly journals Atmospheric Blocking: The Impact of Topography in an Idealized General Circulation Model

2020 ◽  
Author(s):  
Veeshan Narinesingh ◽  
James F. Booth ◽  
Spencer K. Clark ◽  
Yi Ming
Keyword(s):  
High Pressure ◽  
General Circulation ◽  
Storm Track ◽  
Circulation Model ◽  
Wave Activity ◽  
Stationary Waves ◽  
Atmospheric Blocking ◽  
The Pacific ◽  
The Impact ◽  
Wave Activity Flux

Abstract. Atmospheric blocking can have important impacts on weather hazards, but the fundamental dynamics of blocking are not yet fully understood. As such, this work investigates the influence of topography on atmospheric blocking in terms of dynamics, spatial frequency, duration and displacement. Using an idealized GCM, an aquaplanet integration, and integrations with topography are analyzed. Block-centered composites show midlatitude aquaplanet blocks exhibit similar wave activity flux behavior to those observed in reality, whereas high-latitude blocks do not. The addition of topography significantly increases blocking and determines distinct regions where blocks are most likely to occur. These regions are found near high-pressure anomalies in the stationary waves and near storm track exit regions. Focusing on block duration, blocks originating near topography are found to last longer than those that are formed without or far from topography but have qualitatively similar evolutions in terms of nearby geopotential height anomalies and wave activity fluxes in composites. Integrations with two mountains have greater amounts of blocking compared to the single mountain case, however, the longitudinal spacing between the mountains is important for how much blocking occurs. Comparison between integrations with longitudinally long and short ocean basins show that more blocking occurs when storm track exits spatially overlap with high-pressure maxima in stationary waves. These results have real-world implications, as they help explain the differences in blocking between the Northern and Southern Hemisphere, and the differences between the Pacific and Atlantic regions in the Northern Hemisphere.

Sign in / Sign up

Export Citation Format

Share Document