scholarly journals Assessment of Modes of Interannual Variability of Southern Hemisphere Atmospheric Circulation in CMIP5 Models

2014 ◽  
Vol 27 (21) ◽  
pp. 8107-8125 ◽  
Author(s):  
Simon Grainger ◽  
Carsten S. Frederiksen ◽  
Xiaogu Zheng
Keyword(s):  
Interannual Variability ◽  
Southern Hemisphere ◽  
Southern Oscillation ◽  
Slow Component ◽  
Coupled Model ◽  
Reanalysis Data ◽  
Cmip5 Models ◽  
Noise Component ◽  
Modes Of Variability ◽  
Multimodel Ensembles

Abstract An assessment is made of the modes of interannual variability in the seasonal mean summer and winter Southern Hemisphere (SH) 500-hPa geopotential height in the twentieth century in models from the Coupled Model Intercomparison Project (CMIP) phase 5 (CMIP5) dataset. Modes of variability of both the slow (signal) and intraseasonal (noise) components in the CMIP5 models are evaluated against those estimated from reanalysis data. There is general improvement in the leading modes of the slow (signal) component in CMIP5 models compared with the CMIP phase 3 (CMIP3) dataset. The largest improvement is in the spatial structures of the modes related to El Niño–Southern Oscillation variability in SH summer. An overall score metric is significantly higher for CMIP5 over CMIP3 in both seasons. The leading modes in the intraseasonal noise component are generally well reproduced in CMIP5 models, and there are few differences from CMIP3. A new total overall score metric is used to rank the CMIP5 models over both seasons. Weighting the seasons by the relative spread of overall scores is shown to be suitable for generating multimodel ensembles for further analysis of interannual variability. In multimodel ensembles, it is found that an ensemble of size 5 or 6 is sufficient in SH summer to reproduce well the dominant modes. In contrast, about 13 models are typically are required in SH winter. It is shown that it is necessary that the selected models individually reproduce well the leading modes of the slow component.

scholarly journals Origins of Biases in CMIP5 Models Simulating Northwest Pacific Summertime Atmospheric Circulation Anomalies during the Decaying Phase of ENSO

2018 ◽  
Vol 31 (14) ◽  
pp. 5707-5729 ◽  
Author(s):  
Weichen Tao ◽  
Gang Huang ◽  
Renguang Wu ◽  
Kaiming Hu ◽  
Pengfei Wang ◽  
...  
Keyword(s):  
Atmospheric Circulation ◽  
Southern Oscillation ◽  
Coupled Model ◽  
Tropical Indian Ocean ◽  
Shortwave Radiation ◽  
Cmip5 Models ◽  
Northwest Pacific ◽  
Circulation Anomalies ◽  
Atmospheric Circulation Anomalies ◽  
Sst Anomalies

Abstract The present study documents the biases of summertime northwest Pacific (NWP) atmospheric circulation anomalies during the decaying phase of ENSO and investigates their plausible reasons in 32 models from phase 5 of the Coupled Model Intercomparison Project. Based on an intermodel empirical orthogonal function (EOF) analysis of El Niño–Southern Oscillation (ENSO)-related 850-hPa wind anomalies, the dominant modes of biases are extracted. The first EOF mode, explaining 21.3% of total intermodel variance, is characterized by a cyclone over the NWP, indicating a weaker NWP anticyclone. The cyclone appears to be a Rossby wave response to unrealistic equatorial western Pacific (WP) sea surface temperature (SST) anomalies related to excessive equatorial Pacific cold tongue in the models. On one hand, the cold SST biases increase the mean zonal SST gradient, which further intensifies warm zonal advection, favoring the development and persistence of equatorial WP SST anomalies. On the other hand, they reduce the anomalous convection caused by ENSO-related warming, and the resultant increase in downward shortwave radiation contributes to the SST anomalies there. The second EOF mode, explaining 18.6% of total intermodel variance, features an anticyclone over the NWP with location shifted northward. The related SST anomalies in the Indo-Pacific sector show a tripole structure, with warming in the tropical Indian Ocean and equatorial central and eastern Pacific and cooling in the NWP. The Indo-Pacific SST anomalies are highly controlled by ENSO amplitude, which is determined by the intensity of subtropical cells via the adjustment of meridional and vertical advection in the models.

scholarly journals Interannual Variability of Stratospheric Final Warming in the Southern Hemisphere and its Tropospheric Origin

2021 ◽  
pp. 1-59
Author(s):  
Soichiro Hirano ◽  
Masashi Kohma ◽  
Kaoru Sato
Keyword(s):  
Interannual Variability ◽  
Southern Hemisphere ◽  
Reanalysis Data ◽  
Stationary Waves ◽  
High Latitudes ◽  
Wave Source ◽  
Barotropic Model ◽  
Stratospheric Final Warming ◽  
Zonal Wavenumber ◽  
Favorable Conditions

AbstractThe relation between interannual variability of stratospheric final warming (SFW) and tropospheric circulation in the Southern Hemisphere (SH) is explored using reanalysis data and a linear barotropic model. The analysis is focused on quasi-stationary waves with zonal wavenumber 1 (s = 1 QSWs; s is zonal wavenumber), which are the dominant component of the SH extratropical planetary waves.First, interannual variability of SFW is investigated in terms of amplitudes of stratospheric and tropospheric s = 1 QSWs, and wave transmission properties of the mean flow from the late austral winter to spring. Upward Eliassen–Palm flux due to s = 1 QSWs is larger from the stratosphere down to the middle troposphere in early-SFW years than late-SFW years. More favorable conditions for propagation of s = 1 stationary waves into the stratosphere are identified in early-SFW years. These results indicate that the amplification of tropospheric s = 1 QSWs and the favorable conditions for their propagation into the stratosphere lead to the amplification of stratospheric s = 1 QSWs, and hence earlier SFWs.Next, numerical calculations using a linear barotropic model are performed to explore how tropospheric s = 1 QSWs at high latitudes amplifies in early-SFW years. By using tropical Rossby wave source and horizontal winds in the reanalysis data as a source and background field, respectively, differences in s = 1 steady responses between early- and late-SFWs are examined at high latitudes. It is suggested that the larger amplitudes of tropospheric s = 1 QSWs in early-SFW years are attributed to differences in wave propagation characteristics associated with structure of the midlatitude jets in austral spring.

scholarly journals Simulating Southern Hemisphere extra-tropical climate variability with an idealised coupled atmosphere-ocean model

2012 ◽  
Vol 5 (5) ◽  
pp. 1161-1175 ◽  
Author(s):  
H. Kurzke ◽  
M. V. Kurgansky ◽  
K. Dethloff ◽  
D. Handorf ◽  
S. Erxleben ◽  
...  
Keyword(s):  
Climate Variability ◽  
Southern Hemisphere ◽  
Ocean Circulation ◽  
Coupled Model ◽  
Circulation Model ◽  
Horizontal Resolution ◽  
Global Ocean ◽  
Simplified Model ◽  
Cmip5 Models ◽  
Decadal Climate Variability

Abstract. A quasi-geostrophic model of Southern Hemisphere's wintertime atmospheric circulation with horizontal resolution T21 has been coupled to a global ocean circulation model with a resolution of 2° × 2° and simplified physics. This simplified coupled model reproduces qualitatively some features of the first and the second EOF of atmospheric 833 hPa geopotential height in accordance with NCEP data. The variability patterns of the simplified coupled model have been compared with variability patterns simulated by four complex state-of-the-art coupled CMIP5 models. The first EOF of the simplified model is too zonal and does not reproduce the right position of the centre of action over the Pacific Ocean and its extension to the tropics. The agreement in the second EOF between the simplified and the CMIP5 models is better. The total variance of the simplified model is weaker than the observational variance and those of the CMIP5 models. The transport properties of the Southern Ocean circulation are in qualitative accord with observations. The simplified model exhibits skill in reproducing essential features of decadal and multi-decadal climate variability in the extratropical Southern Hemisphere. Notably, 800 yr long coupled model simulations reveal sea surface temperature fluctuations on the timescale of several decades in the Antarctic Circumpolar Current region.

scholarly journals An Observational and Model Characterization of Vertical Structure of Wind Fields over Eastern United States: A Case Study of Sterling, Virginia

2016 ◽  
Vol 2016 ◽  
pp. 1-15
Author(s):  
Sium Gebremariam ◽  
Belay Demoz ◽  
Churchill Okonkwo ◽  
Ricardo K. Sakai
Keyword(s):  
Wind Speed ◽  
Vertical Distribution ◽  
Interannual Variability ◽  
Signal To Noise Ratio ◽  
Coupled Model ◽  
Cmip5 Models ◽  
Eastern United States ◽  
Wind Fields ◽  
Reanalysis Dataset ◽  
Radiosonde Observation

The performance of twenty GCMs that participated in the Coupled Model Intercomparison Phase 5 (CMIP5) is evaluated at Sterling, Virginia, by comparing model outputs with radiosonde observational dataset and reanalysis dataset. We evaluated CMIP5 models in their ability to simulate wind climatology, seasonal cycle, interannual variability, and trends at the pressure levels from 850 hPa to 30 hPa. We also addressed the question of the number of years required to detect statistically significant wind trends using radiosonde wind measurements. Our results show that CMIP5 models and reanalysis successfully reproduced the observed climatological annual mean zonal wind and wind speed vertical distribution. They also capture the observed seasonal zonal, meridional, and wind speed vertical distribution with stronger (weaker) wind during the winter (summer) season. However, there is some disagreement in the magnitude of vertical profiles among CMIP5 models, reanalysis, and radiosonde observation. Overall, the number of years to obtain statistically significant trend decreases with increasing pressure level except for upper troposphere. Although the vertical profile of interannual variability of CMIP5 models and reanalysis agree with the radiosonde observation, the wind trend is not statistically significant. This indicates that detection of trends on local scale is challenging because of small signal-to-noise ratio problems.

scholarly journals Does Global Warming Cause Intensified Interannual Hydroclimate Variability?

2012 ◽  
Vol 25 (9) ◽  
pp. 3355-3372 ◽  
Author(s):  
Richard Seager ◽  
Naomi Naik ◽  
Laura Vogel
Keyword(s):  
Global Warming ◽  
Interannual Variability ◽  
Southern Oscillation ◽  
Coupled Model ◽  
Scientific Basis ◽  
Specific Humidity ◽  
Forced Circulation ◽  
Almost Everywhere ◽  
Circulation Anomalies ◽  
The Impact

The idea that global warming leads to more droughts and floods has become commonplace without clear indication of what is meant by this statement. Here, the authors examine one aspect of this problem and assess whether interannual variability of precipitation P minus evaporation E becomes stronger in the twenty-first century compared to the twentieth century, as deduced from an ensemble of models participating in Coupled Model Intercomparison Project 3. It is shown that indeed interannual variability of P − E does increase almost everywhere across the planet, with a few notable exceptions such as southwestern North America and some subtropical regions. The variability increases most at the equator and the high latitudes and least in the subtropics. Although most interannual P − E variability arises from internal atmosphere variability, the primary potentially predictable component is related to the El Niño–Southern Oscillation (ENSO). ENSO-driven interannual P − E variability clearly increases in amplitude in the tropical Pacific, but elsewhere the changes are more complex. This is not surprising in that ENSO-driven P − E anomalies are primarily caused by circulation anomalies combining with the climatological humidity field. As climate warms and the specific humidity increases, this term leads to an intensification of ENSO-driven P − E variability. However, ENSO-driven circulation anomalies also change, in some regions amplifying but in others opposing and even overwhelming the impact of rising specific humidity. Consequently, there is sound scientific basis for anticipating a general increase in interannual P − E variability, but the predictable component will depend in a more complex way on both thermodynamic responses to global warming and on how tropically forced circulation anomalies alter.

Simulation and Projection of the Southern Hemisphere Annular Mode in CMIP5 Models

2013 ◽  
Vol 26 (24) ◽  
pp. 9860-9879 ◽  
Author(s):  
Fei Zheng ◽  
Jianping Li ◽  
Robin T. Clark ◽  
Hyacinth C. Nnamchi
Keyword(s):  
Southern Hemisphere ◽  
Coupled Model ◽  
Austral Summer ◽  
Cmip5 Models ◽  
Spatial And Temporal Variability ◽  
Positive Trend ◽  
Annular Mode ◽  
Future Evolution ◽  
Asymmetric Component ◽  
The Future

Abstract Climate variability in the Southern Hemisphere (SH) extratropical regions is dominated by the SH annular mode (SAM). Future changes in the SAM could have a large influence on the climate over broad regions. In this paper, the authors utilized model simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5) to examine projected future changes in the SAM during the austral summer [December–February (DJF)]. To start off, first, the ability of the models in reproducing the recently observed spatial and temporal variability was assessed. The 12 CMIP5 models examined were found to reproduce the SAM's spatial pattern reasonably well in terms of both the symmetrical and the asymmetric component. The CMIP5 models show an improvement over phase 3 of CMIP (CMIP3) in simulating the seesaw structure of the SAM and also give improvements in the recently observed positive SAM trend. However, only half the models appeared to be able to capture two major recent decadal SAM phases. Then, the future SAM trends and its sensitivity to greenhouse gas (GHG) concentrations using simulations based on the representative concentration pathways 4.5 (RCP4.5) and 8.5 (RCP8.5) were explored. With RCP4.5, a very weak negative trend for this century is found. Conversely, with RCP8.5, a significant positive trend was projected, with a magnitude similar to the recently observed trend. Finally, model uncertainty in the future SAM projections was quantified by comparing projections from the individual CMIP5 models. The results imply the response of SH polar region stratospheric temperature to GHGs could be a significant controlling factor on the future evolution of the SAM.

ENSO and SAM influence on the generation of long episodes of Rossby Wave Packets during Southern Hemisphere Summer

2021 ◽  
Author(s):  
Iago Perez ◽  
Marcelo Barreiro ◽  
Cristina Masoller
Keyword(s):  
Interannual Variability ◽  
Southern Hemisphere ◽  
Rossby Wave ◽  
Southern Oscillation ◽  
Wave Packets ◽  
Southern Annular Mode ◽  
Frequency Of Occurrence ◽  
Atmospheric Conditions ◽  
Mean Flow ◽  
Time Duration

<p>Rossby Wave Packets (RWPs) are key to the improvement of  long-range forecasting and for the prediction of sub-seasonal extremes. Several studies have focused on their properties, such as time duration, trajectory, areas of detection and dissipation as well as interannual variability in the northern hemisphere, but only a few of them have focused in the southern hemisphere. Here we study the influence of low-frequency climate modes on RWPs during southern hemisphere summer using NCEP DOE 2 Reanalysis data. Focusing on long-lived RWPs, which we define as RWPs with a lifespan above 8 days,  we determine how El Niño-Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) modify their frequency of occurrence and their main areas of detection and dissipation. We found that during El Niño and negative SAM years, the number of long-lived RWPs is maximum. In addition, years with the highest amount of long-lived RWPs show a zonally symmetric and narrow upper level jet that is shifted northward from its climatological position. On the other hand, when the jet is shifted southward, particularly in the southeastern Pacific, during positive SAM phases, only a small number of long-lived RWPs is detected. Therefore, negative SAM conditions provide a background mean flow that favours the occurrence of long-lived RWPs while positive SAM conditions have the opposite effect. The dependence on ENSO phase is not as symmetric: while El Niño sets atmospheric conditions that favour the formation of long-lived RWPs, La Niña years present high interannual variability in the frequency of occurrence. Furthermore, in El Niño events the main formation area is between 61-120ºE and the main dissipation area between 300-359ºE. During La Niña events, the main formation area is located by 241-300ºE and no main dissipation area is identified. In the case of positive SAM two main formation areas appear at 61-120ºE and 241-300ºE and two main dissipation areas within 121-180 and 301-359ºE. Lastly in the case of negative SAM one main formation area at 241-300ºE is detected and no main dissipation area is detected. The robustness of the results was tested repeating the analysis using data from the ERA5 Reanalysis and supports the finding that the maximum number of long-lived RWPs occur during negative SAM and El Niño years</p>

Diverse influences of spring Arctic Oscillation on the following winter El Niño-Southern Oscillation in CMIP5 models

2020 ◽  
Author(s):  
Yuqiong Zheng
Keyword(s):  
North Pacific ◽  
Arctic Oscillation ◽  
Climate Models ◽  
Southern Oscillation ◽  
Storm Track ◽  
Coupled Model ◽  
Cmip5 Models ◽  
Northwestern Pacific ◽  
Westerly Wind ◽  
Sst Cooling

<p>This study evaluates the ability of 35 climate models, which participate in the Coupled Model Intercomparison Project Phase 5 (CMIP5) historical climate simulations, in reproducing the connection between boreal spring Arctic Oscillation (AO) and its following winter El Niño-Southern Oscillation (ENSO). The spring AO-winter ENSO correlations range from -0.41 to 0.44 among the 35 models for the period of 1958-2005. Ensemble means of the models with significant positive and negative AO-ENSO correlations both show strong spring sea surface temperature (SST) cooling in the subtropical North Pacific during a positive phase of spring AO, which is conducive to occurrence of a La Niña event in the following winter. However, the models with positive AO-ENSO relations produce a pronounced spring cyclonic anomaly over the subtropical northwestern Pacific and westerly anomalies over the tropical western Pacific (TWP). These westerly wind anomalies would lead to SST warming and positive precipitation anomalies in the tropical central-eastern Pacific (TCEP) during the following summer, which could maintain and develop into an El Niño-like pattern in the following winter via a positive air-sea feedback. By contrast, the models with negative AO-ENSO connections fail to reproduce the spring AO-related cyclonic anomaly over the subtropical northwestern Pacific and westerly wind anomalies in the TWP. Thus, these models would produce a La Niña-like pattern in the subsequent winter. Difference in the spring AO-associated atmospheric anomalies over the subtropical North Pacific among the CMIP5 models may be attributed to biases of the models in simulating the spring climatological storm track.</p>

scholarly journals Skill in Simulating Australian Precipitation at the Tropical Edge*

2016 ◽  
Vol 29 (4) ◽  
pp. 1477-1496 ◽  
Author(s):  
Penelope Maher ◽  
Steven C. Sherwood
Keyword(s):  
Large Scale ◽  
Climate Models ◽  
Linear Independence ◽  
Southern Oscillation ◽  
Coupled Model ◽  
Southern Annular Mode ◽  
Reanalysis Data ◽  
Hadley Cell ◽  
Model Errors ◽  
Skill Scores

Abstract Expansion of the tropics will likely affect subtropical precipitation, but observed and modeled precipitation trends disagree with each other. Moreover, the dynamic processes at the tropical edge and their interactions with precipitation are not well understood. This study assesses the skill of climate models to reproduce observed Australian precipitation variability at the tropical edge. A multivariate linear independence approach distinguishes between direct (causal) and indirect (circumstantial) precipitation drivers that facilitate clearer attribution of model errors and skill. This approach is applied to observed precipitation and ERA-Interim reanalysis data and a representative subset of four models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) and their CMIP3 counterparts. The drivers considered are El Niño–Southern Oscillation, southern annular mode, Indian Ocean dipole, blocking, and four tropical edge metrics (position and intensity of the subtropical ridge and subtropical jet). These models are skillful in representing the covariability of drivers and their influence on precipitation. However, skill scores have not improved in the CMIP5 subset relative to CMIP3 in either respect. The Australian precipitation response to a poleward-located Hadley cell edge remains uncertain, as opposing drying and moistening mechanisms complicate the net response. Higher skill in simulating driver covariability is not consistently mirrored by higher precipitation skill. This provides further evidence that modeled precipitation does not respond correctly to large-scale flow patterns; further improvements in parameterized moist physics are needed before the subtropical precipitation responses can be fully trusted. The multivariate linear independence approach could be applied more widely for practical model evaluation.

scholarly journals Improvement of ENSO Simulation Based on Intermodel Diversity

2015 ◽  
Vol 28 (3) ◽  
pp. 998-1015 ◽  
Author(s):  
Yoo-Geun Ham ◽  
Jong-Seong Kug
Keyword(s):  
Interannual Variability ◽  
El Niño ◽  
Climate Models ◽  
El Nino ◽  
Southern Oscillation ◽  
Global Climate ◽  
Coupled Model ◽  
Global Climate Models ◽  
Climate Simulation ◽  
Central Pacific

Abstract In this study, a new methodology is developed to improve the climate simulation of state-of-the-art coupled global climate models (GCMs), by a postprocessing based on the intermodel diversity. Based on the close connection between the interannual variability and climatological states, the distinctive relation between the intermodel diversity of the interannual variability and that of the basic state is found. Based on this relation, the simulated interannual variabilities can be improved, by correcting their climatological bias. To test this methodology, the dominant intermodel difference in precipitation responses during El Niño–Southern Oscillation (ENSO) is investigated, and its relationship with climatological state. It is found that the dominant intermodel diversity of the ENSO precipitation in phase 5 of the Coupled Model Intercomparison Project (CMIP5) is associated with the zonal shift of the positive precipitation center during El Niño. This dominant intermodel difference is significantly correlated with the basic states. The models with wetter (dryer) climatology than the climatology of the multimodel ensemble (MME) over the central Pacific tend to shift positive ENSO precipitation anomalies to the east (west). Based on the model’s systematic errors in atmospheric ENSO response and bias, the models with better climatological state tend to simulate more realistic atmospheric ENSO responses. Therefore, the statistical method to correct the ENSO response mostly improves the ENSO response. After the statistical correction, simulating quality of the MME ENSO precipitation is distinctively improved. These results provide a possibility that the present methodology can be also applied to improving climate projection and seasonal climate prediction.

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