Hemispheric-Scale Seasonality of the Southern Annular Mode and Impacts on the Climate of New Zealand

2009 ◽  
Vol 22 (18) ◽  
pp. 4759-4770 ◽  
Author(s):  
J. Kidston ◽  
J. A. Renwick ◽  
J. McGregor
Keyword(s):  
New Zealand ◽  
Wind Speed ◽  
Climate Variability ◽  
Zonal Wind ◽  
Large Scale ◽  
Southern Annular Mode ◽  
Near Surface ◽  
Annular Mode ◽  
Zonal Wavenumber ◽  
Australian Continent

Abstract The seasonality of the southern annular mode (SAM) and the resulting impacts on the climate variability of New Zealand (NZ) are investigated. As with previous studies, during summer the SAM is found to be largely zonally symmetric, whereas during winter it exhibits increased zonal wavenumber 2–3 variability. This is consistent with seasonal variations in the mean state, and the authors argue that the seasonal cycle of near-surface temperature over the Australian continent plays an important role, making the eddy-driven jet, and hence the SAM, more zonally symmetric during summer than winter. During winter, the SAM exhibits little variability over the South Pacific and southeast of Australia. Dynamical reasons for this behavior are discussed. For the NZ region this seasonality implies that fluctuations in the SAM are associated with a zonal wind speed anomaly during summer but a more meridional wind speed anomaly during winter. This behavior is well captured by temperature and rainfall station data, which serves to corroborate the seasonal changes seen in the large-scale analysis. Moreover, the mode of climate variability that corresponds to a fluctuation of the zonal wind speed is well correlated with the SAM during the summer only and exhibits less variance during the winter. This is consistent with the notion that the seasonality of the SAM significantly impacts modes of climate variability in the region.

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

2013 ◽  
Vol 26 (14) ◽  
pp. 5220-5241 ◽  
Author(s):  
Isla R. Simpson ◽  
Theodore G. Shepherd ◽  
Peter Hitchcock ◽  
John F. Scinocca
Keyword(s):  
Negative Feedback ◽  
Planetary Wave ◽  
Southern Annular Mode ◽  
Summer Season ◽  
Substantial Contribution ◽  
Mean Flow ◽  
Annular Mode ◽  
Planetary Scale ◽  
Zonal Wavenumber ◽  
Climatological Circulation

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.

scholarly journals The Temporal Autocorrelation Structure of Sea Surface Winds

2012 ◽  
Vol 25 (19) ◽  
pp. 6684-6700 ◽  
Author(s):  
Adam H. Monahan
Keyword(s):  
Wind Speed ◽  
Zonal Wind ◽  
Large Scale ◽  
Strong Dependence ◽  
Meridional Wind ◽  
Sea Surface ◽  
Small Scale ◽  
Wind Component ◽  
Temporal Autocorrelation ◽  
Vector Wind

Abstract The temporal autocorrelation structures of sea surface vector winds and wind speeds are considered. Analyses of scatterometer and reanalysis wind data demonstrate that the autocorrelation functions (acf) of surface zonal wind, meridional wind, and wind speed generally drop off more rapidly in the midlatitudes than in the low latitudes. Furthermore, the meridional wind component and wind speed generally decorrelate more rapidly than the zonal wind component. The anisotropy in vector wind decorrelation scales is demonstrated to be most pronounced in the storm tracks and near the equator, and to be a feature of winds throughout the depth of the troposphere. The extratropical anisotropy is interpreted in terms of an idealized kinematic eddy model as resulting from differences in the structure of wind anomalies in the directions along and across eddy paths. The tropical anisotropy is interpreted in terms of the kinematics of large-scale equatorial waves and small-scale convection. Modeling the vector wind fluctuations as Gaussian, an explicit expression for the wind speed acf is obtained. This model predicts that the wind speed acf should decay more rapidly than that of at least one component of the vector winds. Furthermore, the model predicts a strong dependence of the wind speed acf on the ratios of the means of vector wind components to their standard deviations. These model results are shown to be broadly consistent with the relationship between the acf of vector wind components and wind speed, despite the presence of non-Gaussian structure in the observed surface vector winds.

scholarly journals Southern Hemisphere Annular Mode Variability and the Role of Optimal Nonmodal Growth

2005 ◽  
Vol 62 (6) ◽  
pp. 1947-1961 ◽  
Author(s):  
Harun A. Rashid ◽  
Ian Simmonds
Keyword(s):  
Southern Hemisphere ◽  
Zonal Wind ◽  
Positive Feedback ◽  
General Model ◽  
Initial Perturbation ◽  
Low Frequency ◽  
Southern Annular Mode ◽  
Feedback Mechanism ◽  
Annular Mode ◽  
Positive Feedback Mechanism

Abstract The southern annular mode is the leading mode of Southern Hemisphere circulation variability, the temporal evolution of which is characterized by large amplitudes and significant persistence. Previous investigators have suggested a positive feedback mechanism that explains some of this low-frequency variance. Here, a mechanism is proposed, involving transient nonmodal growths of the anomalies, that is at least as effective as the positive feedback mechanism in increasing the low-frequency variance of the southern annular mode. Using the vector autoregressive modeling technique, a number of linear inverse models of southern annular mode variability from National Centers for Environmental Prediction–Department of Energy (NCEP–DOE) Reanalysis 2 is derived. These models are then analyzed applying the ideas of the generalized stability theory. It is found that, as a consequence of the nonnormality of the system matrices, a significant increase in the low-frequency variance of the southern annular mode occurs through optimal nonmodal growth of the zonal wind anomalies. The nonnormality arises mainly from the relative dominance of the eddy forcing, while the nonmodal growth is caused by the interference of the nonorthogonal eigenvectors of the nonnormal system matrix. These results are demonstrated first in a simple model that retains only the two leading modes of the zonally averaged zonal wind and eddy-forcing variability, and then in a more general model that includes all the important modes. Using the more general model the authors have determined, among other things, the optimal initial perturbation and the time scale over which it experiences the maximum nonmodal growth to evolve into the pattern associated with the southern annular mode.

scholarly journals Variability of Daily Maximum Wind Speed across China, 1975–2016: An Examination of Likely Causes

2020 ◽  
Vol 33 (7) ◽  
pp. 2793-2816 ◽  
Author(s):  
Gangfeng Zhang ◽  
Cesar Azorin-Molina ◽  
Deliang Chen ◽  
Jose A. Guijarro ◽  
Feng Kong ◽  
...  
Keyword(s):  
Wind Speed ◽  
Large Scale ◽  
Southern Oscillation ◽  
Vertical Wind Shear ◽  
Maximum Wind Speed ◽  
The Arctic ◽  
Risk Governance ◽  
Daily Maximum ◽  
Near Surface ◽  
Maximum Wind

AbstractAssessing change in daily maximum wind speed and its likely causes is crucial for many applications such as wind power generation and wind disaster risk governance. Multidecadal variability of observed near-surface daily maximum wind speed (DMWS) from 778 stations over China is analyzed for 1975–2016. A robust homogenization protocol using the R package Climatol was applied to the DMWS observations. The homogenized dataset displayed a significant (p < 0.05) declining trend of −0.038 m s−1 decade−1 for all China annually, with decreases in winter (−0.355 m s−1 decade−1, p < 0.05) and autumn (−0.108 m s−1 decade−1; p < 0.05) and increases in summer (+0.272 m s−1 decade−1, p < 0.05) along with a weak recovery in spring (+0.032 m s−1 decade−1; p > 0.10); that is, DMWS declined during the cold semester (October–March) and increased during the warm semester (April–September). Correlation analysis of the Arctic Oscillation, the Southern Oscillation, and the west Pacific modes exhibited significant correlation with DMWS variability, unveiling their complementarity in modulating DMWS. Further, we explored potential physical processes relating to the atmospheric circulation changes and their impacts on DMWS and found that 1) overall weakened horizontal airflow [large-scale mean horizontal pressure gradient (from −0.24 to +0.02 hPa decade−1) and geostrophic wind speed (from −0.6 to +0.6 m s−1 decade−1)], 2) widely decreased atmospheric vertical momentum transport [atmospheric stratification thermal instability (from −3 to +1.5 decade−1) and vertical wind shear (from −0.4 to +0.2 m s−1 decade−1)], and 3) decreased extratropical cyclones frequency (from −0.3 to 0 month decade−1) are likely causes of DMWS change.

Barotropic and Baroclinic Annular Variability in the Southern Hemisphere

2014 ◽  
Vol 71 (4) ◽  
pp. 1480-1493 ◽  
Author(s):  
David W. J. Thompson ◽  
Jonathan D. Woodworth
Keyword(s):  
Kinetic Energy ◽  
Climate Variability ◽  
Southern Hemisphere ◽  
Large Scale ◽  
Principal Component ◽  
Southern Annular Mode ◽  
Eddy Kinetic Energy ◽  
Propagating Waves ◽  
Vertically Propagating ◽  
High Degree

Abstract The leading patterns of large-scale climate variability in the Southern Hemisphere are examined in the context of extratropical kinetic energy. It is argued that variability in the Southern Hemisphere extratropical flow can be viewed in the context of two distinct and largely independent structures, both of which exhibit a high degree of annularity: 1) a barotropic structure that dominates the variance in the zonal-mean kinetic energy and 2) a baroclinic structure that dominates the variance in the eddy kinetic energy. The former structure corresponds to the southern annular mode (SAM) and has been extensively examined in the literature. The latter structure emerges as the leading principal component time series of eddy kinetic energy and has received seemingly little attention in previous work. The two structures play very different roles in cycling energy through the extratropical troposphere. The SAM is associated primarily with variability in the meridional propagation of wave activity, has a surprisingly weak signature in the eddy fluxes of heat, and can be modeled as Gaussian red noise with an e-folding time scale of approximately 10 days. The baroclinic annular structure is associated primarily with variations in the amplitude of vertically propagating waves, has a very weak signature in the wave fluxes of momentum, and exhibits marked quasi periodicity on time scales of approximately 25–30 days. Implications for large-scale climate variability are discussed.

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 Wind speed stilling and its recovery due to internal climate variability

2021 ◽  
Vol 12 (4) ◽  
pp. 1239-1251
Author(s):  
Jan Wohland ◽  
Doris Folini ◽  
Bryn Pickering
Keyword(s):  
Wind Speed ◽  
Climate Variability ◽  
Internal Variability ◽  
Systematic Investigation ◽  
Max Planck ◽  
Near Surface ◽  
Climate Conditions ◽  
Wind Speeds ◽  
Internal Climate Variability ◽  
Long Run

Abstract. Near-surface winds affect many processes on planet Earth, ranging from fundamental biological mechanisms such as pollination to man-made infrastructure that is designed to resist or harness wind. The observed systematic wind speed decline up to around 2010 (stilling) and its subsequent recovery have therefore attracted much attention. While this sequence of downward and upwards trends and good connections to well-established modes of climate variability suggest that stilling could be a manifestation of multidecadal climate variability, a systematic investigation is currently lacking. Here, we use the Max Planck Institute Grand Ensemble (MPI-GE) to decompose internal variability from forced changes in wind speeds. We report that wind speed changes resembling observed stilling and its recovery are well in line with internal climate variability, both under current and future climate conditions. Moreover, internal climate variability outweighs forced changes in wind speeds on 20-year timescales by 1 order of magnitude, despite the fact that smaller, forced changes become relevant in the long run as they represent alterations of mean states. In this regard, we reveal that land use change plays a pivotal role in explaining MPI-GE ensemble-mean wind changes in the representative concentration pathways 2.6, 4.5, and 8.5. Our results demonstrate that multidecadal wind speed variability is of greater relevance than forced changes over the 21st century, in particular for wind-related infrastructure like wind energy.

scholarly journals Interactions between Antarctic sea ice and large-scale atmospheric modes in CMIP5 models

2016 ◽  
Author(s):  
Serena Schroeter ◽  
Will Hobbs ◽  
Nathaniel L. Bindoff
Keyword(s):  
Sea Ice ◽  
Large Scale ◽  
Coupled Model ◽  
Southern Annular Mode ◽  
Cmip5 Models ◽  
Atmospheric Variability ◽  
Annular Mode ◽  
Antarctic Sea Ice ◽  
Ice Retreat ◽  
Sea Ice Retreat

Abstract. The response of Antarctic sea ice to large-scale patterns of atmospheric variability varies according to sea ice sector and season. In this study, interannual atmosphere-sea ice interactions were explored using observation-based data and compared with simulated interactions by models in the Coupled Model Intercomparison Project Phase 5. Simulated relationships between atmospheric variability and sea ice variability generally reproduced the observed relationships, though more closely during the season of sea ice advance than the season of sea ice retreat. Atmospheric influence on sea ice is known to be strongest during its advance, with the ocean emerging as a dominant driver of sea ice retreat; therefore, while it appears that models are able to capture the dominance of the atmosphere during advance, simulations of ocean-atmosphere-sea ice interactions during retreat require further investigation. A large proportion of model ensemble members overestimated the relative importance of the Southern Annular Mode compared with other modes on high southern latitude climate, while the influence of tropical forcing was underestimated. This result emerged particularly strongly during the season of sea ice retreat. The amplified zonal patterns of the Southern Annular Mode in many models and its exaggerated influence on sea ice overwhelm the comparatively underestimated meridional influence, suggesting that simulated sea ice variability would become more zonally symmetric as a result. Across the seasons of sea ice advance and retreat, 3 of the 5 sectors did not reveal a strong relationship with a pattern of large-scale atmospheric variability in one or both seasons, indicating that sea ice in these sectors may be influenced more strongly by atmospheric variability unexplained by the major atmospheric modes, or by heat exchange in the ocean.

scholarly journals Decadal Changes of Wind Stress over the Southern Ocean Associated with Antarctic Ozone Depletion

2007 ◽  
Vol 20 (14) ◽  
pp. 3395-3410 ◽  
Author(s):  
Xiao-Yi Yang ◽  
Rui Xin Huang ◽  
Dong Xiao Wang
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
Zonal Wind ◽  
Ozone Depletion ◽  
Surface Wind ◽  
Southern Annular Mode ◽  
Mean Flow ◽  
Annular Mode ◽  
Surface Wind Stress ◽  
Antarctic Ozone

Abstract Using 40-yr ECMWF Re-Analysis (ERA-40) data and in situ observations, the positive trend of Southern Ocean surface wind stress during two recent decades is detected, and its close linkage with spring Antarctic ozone depletion is established. The spring Antarctic ozone depletion affects the Southern Hemisphere lower-stratospheric circulation in late spring/early summer. The positive feedback involves the strengthening and cooling of the polar vortex, the enhancement of meridional temperature gradients and the meridional and vertical potential vorticity gradients, the acceleration of the circumpolar westerlies, and the reduction of the upward wave flux. This feedback loop, together with the ozone-related photochemical interaction, leads to the upward tendency of lower-stratospheric zonal wind in austral summer. In addition, the stratosphere–troposphere coupling, facilitated by ozone-related dynamics and the Southern Annular Mode, cooperates to relay the zonal wind anomalies to the upper troposphere. The wave–mean flow interaction and the meridional circulation work together in the form of the Southern Annular Mode, which transfers anomalous wind signals downward to the surface, triggering a striking strengthening of surface wind stress over the Southern Ocean.

scholarly journals Multiple Regimes of Wind, Stratification, and Turbulence in the Stable Boundary Layer

2015 ◽  
Vol 72 (8) ◽  
pp. 3178-3198 ◽  
Author(s):  
Adam H. Monahan ◽  
Tim Rees ◽  
Yanping He ◽  
Norman McFarlane
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Cloud Cover ◽  
Large Scale ◽  
Potential Temperature ◽  
Surface Wind ◽  
Geostrophic Wind ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Near Surface

Abstract A long time series of temporally high-resolution wind and potential temperature data from the 213-m tower at Cabauw in the Netherlands demonstrates the existence of two distinct regimes of the stably stratified nocturnal boundary layer at this location. Hidden Markov model (HMM) analysis is used to objectively characterize these regimes and classify individual observed states. The first regime is characterized by strongly stable stratification, large wind speed differences between 10 and 200 m, and relatively weak turbulence. The second is associated with near-neutral stratification, weaker wind speed differences between 10 and 200 m, and relatively strong turbulence. In this second regime, the state of the boundary layer is similar to that during the day. The occupation statistics of these regimes are shown to covary with the large-scale pressure gradient force and cloud cover such that the first regime predominates under clear skies with weak geostrophic wind speed and the second regime predominates under conditions of extensive cloud cover or large geostrophic wind speed. These regimes are not distinguished by standard measures of stability, such as the Obukhov length or the bulk Richardson number. Evidence is presented that the mechanism generating these distinct regimes is associated with a previously documented feedback resulting from the existence of an upper limit on the maximum downward heat flux that can be sustained for a given near-surface wind speed.

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