Eddy Mean Flow Feedbacks Research Articles

Links between Climate Sensitivity and the Large-Scale Atmospheric Circulation in a Simple General Circulation Model

Abstract Thermodynamical and dynamical aspects of the climate system response to anthropogenic forcing are often considered in two distinct frameworks: the former in the context of the forcing–feedback framework, and the latter in the context of eddy–mean flow feedbacks and large-scale thermodynamic constraints. Here we use experiments with the dynamical core of a general circulation model (GCM) to provide insights into the relationships between these two frameworks. We first demonstrate that the climate feedbacks and climate sensitivity in a dynamical core model are determined by its prescribed thermal relaxation time scales. We then perform two experiments: one that explores the relationships between the thermal relaxation time scale and the climatological circulation, and a second that explores the relationships between the thermal relaxation time scale and the circulation response to a global warming–like forcing perturbation. The results indicate that shorter relaxation time scales (i.e., lower climate sensitivities in the context of a dynamical core model) are associated with 1) a more vigorous large-scale circulation, including increased thermal diffusivity and stronger and more poleward storm tracks and eddy-driven jets, and 2) a weaker poleward displacement of the storm tracks and eddy-driven jets in response to the global warming–like forcing perturbation. Interestingly, the circulation response to the forcing perturbation effectively disappears when the thermal relaxation time scales are spatially uniform, suggesting that the circulation response to homogeneous forcing requires spatial inhomogeneities in the local feedback parameter. Implications for anticipating the circulation response to global warming and thermodynamic constraints on the circulation are discussed.

Understanding the Basin Asymmetry in Surface Response to Sudden Stratospheric Warmings from an Ocean–Atmosphere Coupled Perspective

AbstractThe canonical tropospheric response to a weakening of the stratospheric vortex—an equatorward shift of the eddy-driven jet—is mostly limited to the North Atlantic following sudden stratospheric warmings (SSWs). A coherent change in the Pacific eddy-driven jet is notably absent. Why is this so? Using daily reanalysis data, we show that air–sea interactions over the North Pacific are responsible for the basin-asymmetric response to SSWs. Prior to the onset of some SSWs, their tropospheric precursors produce a dipolar SST pattern in the North Pacific, which then persists as the stratospheric polar vortex breaks down following the onset of the SSW. By reinforcing the lower-tropospheric baroclinicity, the dipolar SST pattern helps sustain the generation of baroclinic eddies, strengthening the near-surface Pacific eddy-driven jet and maintaining its near-climatological-mean state. This prevents the jet from being perturbed by the downward influence of the stratospheric anomalies. As a result, these SSWs exhibit a highly basin-asymmetric surface response with only the Atlantic eddy-driven jet shifted equatorward. For SSWs occurring without the atmospheric precursors in the North Pacific troposphere, the dipolar SST pattern is absent due to the lack of the atmospheric forcing. In the absence of the dipolar SST pattern and the resultant eddy–mean flow feedbacks, these SSWs exhibit a basin-symmetric surface response with both the Atlantic and the Pacific eddy-driven jets shifted equatorward. Our results provide an ocean–atmosphere coupled perspective on stratosphere–troposphere interaction following SSW events and have potential for improving subseasonal to seasonal forecasts for surface weather and climate.

Tropospheric jet variability in different flow regimes

AbstractA transition in the variability properties of the jet stream is studied using a two‐layer quasi‐geostrophic model. The eddy energy is increased by controlling two parameters that affect baroclinic instability, causing the jet to shift poleward and leading to a stronger and less persistent annular mode. The transition in the jet variability properties as the eddy energy increases is associated with a transition from a merged (subtropical and eddy‐driven) jet regime to an eddy‐driven jet regime, and is accompanied by a transition in the eddy spectral properties. In the merged jet regime the spectrum is dominated by a single synoptic‐scale wave mode, whereas in the eddy‐driven jet regime the spectrum includes a wider range of planetary‐ and synoptic‐scale waves. An eddy–mean flow feedback mechanism explains the relation between the transition in the eddy spectrum and the jet variability properties. In the merged jet regime synoptic‐scale waves maintain the jet close to its climatological position at all times and act as a positive feedback on the weak annular mode, increasing its persistence. In the eddy‐driven jet regime the jet fluctuates between an equatorward‐shifted state, which is similar to the merged jet, and a poleward‐shifted state, where the eddy‐driven jet is separated from the subtropical jet. The low persistence of the annular mode in the eddy‐driven jet regime is due to a negative feedback by planetary‐scale waves, which grow barotropically on the poleward flank of the jet during the equatorward‐shifted jet state. The results of this study highlight the need to capture the dynamics of planetary‐scale waves correctly in general circulation models in order to prevent the emergence of an overly persistent annular mode.

The Role of Extratropical Air–Sea Interaction in the Autumn Subseasonal Variability of the North Atlantic Oscillation

Abstract Considerable progress has been made in understanding the internal eddy–mean flow feedback in the subseasonal variability of the North Atlantic Oscillation (NAO) during winter. Using daily atmospheric and oceanic reanalysis data, this study highlights the role of extratropical air–sea interaction in the NAO variability during autumn when the daily sea surface temperature (SST) variability is more active and eddy–mean flow interactions are still relevant. Our analysis shows that a horseshoe-like SST tripolar pattern in the North Atlantic Ocean, marked by a cold anomaly in the Gulf Stream and two warm anomalies to the south of the Gulf Stream and off the western coast of northern Europe, can induce a quasi-barotropic NAO-like atmospheric response through eddy-mediated processes. An initial southwest–northeast tripolar geopotential anomaly in the North Atlantic forces this horseshoe-like SST anomaly tripole. Then the SST anomalies, through surface heat flux exchange, alter the spatial patterns of the lower-tropospheric temperature and thus baroclinicity anomalies, which are manifested as the midlatitude baroclinicity shifted poleward and reduced baroclinicity poleward of 70°N. In response to such changes of the lower-level baroclinicity, anomalous synoptic eddy generation, eddy kinetic energy, and eddy momentum forcing in the midlatitudes all shift poleward. Meanwhile, the 10–30-day low-frequency anticyclonic wave activities in the high latitudes decrease significantly. We illustrate that both the latitudinal displacement of midlatitude synoptic eddy activities and intensity variation of high-latitude low-frequency wave activities contribute to inducing the NAO-like anomalies.

A Role for Barotropic Eddy–Mean Flow Feedbacks in the Zonal Wind Response to Sea Ice Loss and Arctic Amplification

Abstract Previous studies have suggested that, in the zonal mean, the climatological Northern Hemisphere wintertime eddy-driven jet streams will weaken and shift equatorward in response to Arctic amplification and sea ice loss. However, multiple studies have also pointed out that this response has strong regional differences across the two ocean basins, with the North Atlantic jet stream generally weakening across models and the North Pacific jet stream showing signs of strengthening. Based on the zonal wind response with a fully coupled model, this work sets up two case studies using a barotropic model to test a dynamical mechanism that can explain the differences in zonal wind response in the North Pacific versus the North Atlantic. Results indicate that the differences between the two basins are due, at least in part, to differences in the proximity of the jet streams to the sea ice loss, and that in both cases the eddies act to increase the jet speed via changes in wave breaking location and frequency. Thus, while baroclinic arguments may account for an initial reduction in the midlatitude winds through thermal wind balance, eddy–mean flow feedbacks are likely instrumental in determining the final total response and actually act to strengthen the eddy-driven jet stream.

Roles of Intraseasonal Disturbances and Diabatic Heating in the East Asian Jet Stream Variabilities Associated with the East Asian Winter Monsoon

This study evaluates the relative importance of diabatic heating and intraseasonal disturbances with regard to the variabilities of the East Asian jet stream (EAJS) associated with the East Asian winter monsoon (EAWM). First, strong and weak monsoon years are selected based on the EAWM index of Jhun and Lee, which is highly correlated with the monsoon northerlies between the Eurasian continent and the Pacific. The EAJS is stronger and narrower in strong monsoon years and weaker and wider in weak monsoon years. Model experiments were performed to investigate the atmospheric response to the diabatic heating and the eddy–mean flow feedback from the intraseasonal disturbances. The diabatic heating is closely related to the convective activities. The intraseasonal disturbances include high-frequency components with periods of 3–10 days and low-frequency components with periods of 10–90 days. The model results indicate that the diabatic heating plays a major role maintaining the stronger and weaker EAJS in the strong and weak monsoon years, respectively, whereas the impacts of the eddy feedback are relatively small.

The Linear Response Function of an Idealized Atmosphere. Part II: Implications for the Practical Use of the Fluctuation–Dissipation Theorem and the Role of Operator’s Nonnormality

Abstract A linear response function (LRF) relates the mean response of a nonlinear system to weak external forcings and vice versa. Even for simple models of the general circulation, such as the dry dynamical core, the LRF cannot be calculated from first principles owing to the lack of a complete theory for eddy–mean flow feedbacks. According to the fluctuation–dissipation theorem (FDT), the LRF can be calculated using only the covariance and lag-covariance matrices of the unforced system. However, efforts in calculating the LRFs for GCMs using FDT have produced mixed results, and the reason(s) behind the poor performance of the FDT remain(s) unclear. In Part I of this study, the LRF of an idealized GCM, the dry dynamical core with Held–Suarez physics, is accurately calculated using Green’s functions. In this paper (Part II), the LRF of the same model is computed using FDT, which is found to perform poorly for some of the test cases. The accurate LRF of Part I is used with a linear stochastic equation to show that dimension reduction by projecting the data onto the leading EOFs, which is commonly used for FDT, can alone be a significant source of error. Simplified equations and examples of 2 × 2 matrices are then used to demonstrate that this error arises because of the nonnormality of the operator. These results suggest that errors caused by dimension reduction are a major, if not the main, contributor to the poor performance of the LRF calculated using FDT and that further investigations of dimension-reduction strategies with a focus on nonnormality are needed.

Influence of the Quasi-Biennial Oscillation and Sea Surface Temperature Variability on Downward Wave Coupling in the Northern Hemisphere

Abstract Downward wave coupling occurs when an upward-propagating planetary wave from the troposphere decelerates the flow in the upper stratosphere and forms a downward reflecting surface that redirects waves back to the troposphere. To test this mechanism and potential factors influencing the downward wave coupling, three 145-yr sensitivity simulations with NCAR’s Community Earth System Model [CESM1(WACCM)], a state-of-the-art high-top chemistry–climate model, are analyzed. The results show that the quasi-biennial oscillation (QBO) and SST variability significantly impact downward wave coupling. Without the QBO, the occurrence of downward wave coupling is significantly suppressed. In contrast, stronger and more persistent downward wave coupling occurs when SST variability is excluded. The above influence on the occurrence of downward wave coupling is mostly due to a direct influence of the QBO and SST variability on stratospheric planetary wave source and propagation. The strengths of the tropospheric circulation and surface responses to a given downward wave coupling event, however, behave differently. The surface anomaly is significantly weaker (stronger) in the experiment with fixed SSTs (without QBO), even though the statistical signal of downward wave coupling is strongest (weakest) in this experiment. This apparent mismatch is explained by the differences in the strength of the synoptic-scale eddy–mean flow feedback and the possible contribution of SST anomalies in the North Atlantic during the downward wave coupling event. The weaker synoptic-scale eddy–mean flow feedback and the absence of the positive NAO-related SST-tripole pattern in the fixed SST experiment are consistent with a weaker tropospheric response to downward wave coupling. The results highlight the importance of synoptic-scale eddies in setting the tropospheric response to downward wave coupling.

Southern Annular Mode Dynamics in Observations and Models. Part II: Eddy Feedbacks

Abstract Many global climate models (GCMs) have trouble simulating southern annular mode (SAM) variability correctly, particularly in the Southern Hemisphere summer season where it tends to be too persistent. In this two-part study, a suite of experiments with the Canadian Middle Atmosphere Model (CMAM) is analyzed to improve the understanding of the dynamics of SAM variability and its deficiencies in GCMs. Here, an examination of the eddy–mean flow feedbacks is presented by quantification of the feedback strength as a function of zonal scale and season using a new methodology that accounts for intraseasonal forcing of the SAM. In the observed atmosphere, in the summer season, a strong negative feedback by planetary-scale waves, in particular zonal wavenumber 3, is found in a localized region in the southwest Pacific. It cancels a large proportion of the positive feedback by synoptic- and smaller-scale eddies in the zonal mean, resulting in a very weak overall eddy feedback on the SAM. CMAM is deficient in this negative feedback by planetary-scale waves, making a substantial contribution to its bias in summertime SAM persistence. Furthermore, this bias is not alleviated by artificially improving the climatological circulation, suggesting that climatological circulation biases are not the cause of the planetary wave feedback deficiency in the model. Analysis of the summertime eddy feedbacks in the models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) confirms that this is indeed a common problem among GCMs, suggesting that understanding this planetary wave feedback and the reason for its deficiency in GCMs is key to improving the fidelity of simulated SAM variability in the summer season.

A Mechanism for the Effect of Tropospheric Jet Structure on the Annular Mode–Like Response to Stratospheric Forcing

Abstract For many climate forcings the dominant response of the extratropical circulation is a latitudinal shift of the tropospheric midlatitude jets. The magnitude of this response appears to depend on climatological jet latitude in general circulation models (GCMs): lower-latitude jets exhibit a larger shift. The reason for this latitude dependence is investigated for a particular forcing, heating of the equatorial stratosphere, which shifts the jet poleward. Spinup ensembles with a simplified GCM are used to examine the evolution of the response for five different jet structures. These differ in the latitude of the eddy-driven jet but have similar subtropical zonal winds. It is found that lower-latitude jets exhibit a larger response due to stronger tropospheric eddy–mean flow feedbacks. A dominant feedback responsible for enhancing the poleward shift is an enhanced equatorward refraction of the eddies, resulting in an increased momentum flux, poleward of the low-latitude critical line. The sensitivity of feedback strength to jet structure is associated with differences in the coherence of this behavior across the spectrum of eddy phase speeds. In the configurations used, the higher-latitude jets have a wider range of critical latitude locations. This reduces the coherence of the momentum flux anomalies associated with different phase speeds, with low phase speeds opposing the effect of high phase speeds. This suggests that, for a given subtropical zonal wind strength, the latitude of the eddy-driven jet affects the feedback through its influence on the width of the region of westerly winds and the range of critical latitudes on the equatorward flank of the jet.

Regime Transitions of Steady and Time-Dependent Hadley Circulations: Comparison of Axisymmetric and Eddy-Permitting Simulations

Abstract Steady-state and time-dependent Hadley circulations are investigated with an idealized dry GCM, in which thermal forcing is represented as relaxation of temperatures toward a radiative-equilibrium state. The latitude ϕ0 of maximum radiative-equilibrium temperature is progressively displaced off the equator or varied in time to study how the Hadley circulation responds to seasonally varying forcing; axisymmetric simulations are compared with eddy-permitting simulations. In axisymmetric steady-state simulations, the Hadley circulations for all ϕ0 approach the nearly inviscid, angular-momentum-conserving limit, despite the presence of finite vertical diffusion of momentum and dry static energy. In contrast, in corresponding eddy-permitting simulations, the Hadley circulations undergo a regime transition as ϕ0 is increased, from an equinox regime (small ϕ0) in which eddy momentum fluxes strongly influence both Hadley cells to a solstice regime (large ϕ0) in which the cross-equatorial winter Hadley cell more closely approaches the angular-momentum-conserving limit. In axisymmetric time-dependent simulations, the Hadley cells undergo transitions between a linear equinox regime and a nonlinear, nearly angular-momentum-conserving solstice regime. Unlike in the eddy-permitting simulations, time tendencies of the zonal wind play a role in the dynamics of the transitions in the axisymmetric simulation. Nonetheless, the axisymmetric transitions are similar to those in the eddy-permitting simulations in that the role of the nonlinear mean momentum flux divergence in the zonal momentum budget shifts from marginal in the equinox regime to dominant in the solstice regime. As in the eddy-permitting simulations, a mean-flow feedback—involving the upper-level zonal winds, the lower-level temperature gradient, and the poleward boundary of the cross-equatorial Hadley cell—makes it possible for the circulation fields to change at the transition more rapidly than can be explained by the steady-state response to the thermal forcing. However, the regime transitions in the axisymmetric simulations are less sharp than those in the eddy-permitting simulations because eddy–mean flow feedbacks in the eddy-permitting simulations additionally sharpen the transitions.

Southern “Annular Modes” Simulated by a Climate Model—Patterns, Mechanisms, and Uses

Abstract Both high-latitude (HLM) and low-latitude modes (LLM) of variability of zonal wind in the Southern Hemisphere have been identified. Through an analysis of a simulation for 1871–2200 by the CSIRO Mark 3 climate model, the extent to which these might both be described as “annular modes,” based on their statistical patterns, physical mechanisms, and usefulness in climate study, is assessed. The modes are determined as EOF1 and EOF2 of vertically integrated zonal and monthly mean zonal wind, for 1871–1970. These match well those from ECMWF Re-Analysis (ERA) data and also from the earlier Mark 2 model. The mode index time series relate to largely annular patterns of local wind and surface pressure anomalies [with HLM giving the familiar southern annular mode (SAM)], and other simulated quantities. While modes calculated from 90° sectors are only moderately correlated (mostly in the polar region) for HLM, the link increases with time scale. There is little such relationship for LLM. A momentum equation analysis using daily data confirms that both zonal modes are driven by eddies, but only HLM features a positive eddy–mean flow feedback. Variation in feedback and surface damping through the seasonal cycle relate well to that in index autocorrelation, with the HLM being more persistent in summer. Stratospheric winds feature a long-lived component that tends to lead the HLM. The HLM drives sea surface temperature anomalies that persist for months, and coupling with the ocean increases variability on longer time scales. The annular variability in the warmer climate of the twenty-second century is barely changed, but the mean climate change in the far south projects strongly on the HLM. The LLM features some statistical annularity and may have some uses. However, only the HLM can be considered to be a physically based mode—the zonal-wind equivalent to the one southern annular mode.

Eddy–Zonal Flow Feedback in the Southern Hemisphere Winter and Summer

Abstract The eddy–zonal flow feedback in the Southern Hemisphere (SH) winter and summer is investigated in this study. The persistence time scale of the leading principal components (PCs) of the zonal-mean zonal flow shows substantial seasonal variation. In the SH summer, the persistence time scale of PC1 is significantly longer than that of PC2, while the persistence time scales of the two PCs are quite similar in the SH winter. A storm-track modeling approach is applied to demonstrate that seasonal variations of eddy–zonal flow feedback for PC1 and PC2 account for the seasonal variations of the persistence time scale. The eddy feedback time scale estimated from a storm-track model simulation and a wave-response model diagnostic shows that PC1 in June–August (JJA) and December–February (DJF), and PC2 in JJA, have significant positive eddy–mean flow feedback, while PC2 in DJF has no positive feedback. The consistency between the persistence and eddy feedback time scales for each PC suggests that the positive feedback increases the persistence of the corresponding PC, with stronger (weaker) positive feedback giving rise to a longer (shorter) persistence time scale. Eliassen–Palm flux diagnostics have been performed to demonstrate the dynamics governing the positive feedback between eddies and anomalous zonal flow. The mechanism of the positive feedback, for PC1 in JJA and DJF and PC2 in JJA, is as follows: an enhanced baroclinic wave source (heat fluxes) at a low level in the region of positive wind anomalies propagates upward and then equatorward from the wave source, thus giving momentum fluxes that reinforce the wind anomalies. The difference of PC2 between DJF and JJA is because of the zonal asymmetry of the climatological flow in JJA. For PC2 in DJF, wind anomalies reinforce the climatological jet, thus increasing the barotropic shear of the jet flow. The “barotropic governor” plays an important role in suppressing eddy generations for PC2 in DJF and thus inhibiting the positive eddy–zonal flow feedback.

Influences of ENSO-Induced Indo–Western Pacific SST Anomalies on Extratropical Atmospheric Variability during the Boreal Summer

Abstract The causes for the observed occurrence of anomalous zonally symmetric upper-level pressure ridges in the midlatitude belts of both hemispheres during the year after warm El Niño–Southern Oscillation (ENSO) events have been investigated. Sea surface temperature (SST) anomalies in the Indo–western Pacific (IWP) sector were simulated by allowing an oceanic mixed layer model for that region to interact with local atmospheric changes forced remotely by observed ENSO episodes in the eastern/central tropical Pacific. The spatiotemporal evolution of these SST conditions through a composite ENSO cycle was then inserted as lower boundary conditions within the IWP domain in an ensemble of atmospheric general circulation model (GCM) integrations. This experimental setup is seen to reproduce zonally symmetric geopotential height anomalies with maximum amplitudes being attained over the extratropics in the boreal summer after the peak phase of ENSO. The model evidence hence supports the notion that these global-scale atmospheric changes are primarily responses to SST perturbations in IWP, which are in turn linked to ENSO variability in the equatorial Pacific by the “atmospheric bridge” mechanism. Experimentation with a stationary wave model indicates that the Eastern Hemisphere portion of the aforementioned atmospheric signals are attributable to forcing by tropical heat sources and sinks associated with precipitation anomalies in the IWP region, which are closely related to the underlying SST changes. Diagnosis of the output from the GCM integrations reveals that these circulation changes due to diabatic heating are accompanied by alterations of the propagation path and intensity of the high-frequency eddies at locations farther downstream. The geopotential tendencies associated with the latter disturbances bear some resemblance to the anomalous height pattern in the Western Hemisphere. Such local eddy–mean flow feedbacks hence contribute to the zonal symmetry of the atmospheric response pattern to forcing in the IWP region. Analysis of zonally averaged circulation statistics indicates that the mean meridional circulation induced by divergence of anomalous transient eddy momentum fluxes in ENSO events could also generate zonally symmetric perturbations in midlatitudes. The model-simulated precipitation and surface temperature anomalies in the North American sector in response to SST changes in IWP suggest an increased frequency of droughts and heat waves in that region during the summer season after warm ENSO events.