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  1. Fast and Slow Components of the Extratropical Atmospheric Circulation Response to CO2 Forcing Authors: Paulo Ceppi, Giuseppe Zappa, Theodore G. Shepherd and Jonathan M. Gregory First Published: September 15th, 2017 Published on line: January 19th, 2018 Abstract: Poleward shifts of the extratropical atmospheric circulation are a common response to CO2 forcing in global climate models (GCMs), but little is known about the time dependence of this response. Here it is shown that in coupled climate models, the long-term evolution of sea surface temperatures (SSTs) induces two distinct time scales of circulation response to steplike CO2 forcing. In most GCMs from phase 5 of the Coupled Model Intercomparison Project as well as in the multimodel mean, all of the poleward shift of the midlatitude jets and Hadley cell edge occurs in a fast response within 5–10 years of the forcing, during which less than half of the expected equilibrium warming is realized. Compared with this fast response, the slow response over subsequent decades to centuries features stronger polar amplification (especially in the Antarctic), enhanced warming in the Southern Ocean, an El Niño–like pattern of tropical Pacific warming, and weaker land–sea contrast. Atmosphere-only GCM experiments demonstrate that the SST evolution drives the difference between the fast and slow circulation responses, although the direct radiative effect of CO2 also contributes to the fast response. It is further shown that the fast and slow responses determine the long-term evolution of the circulation response to warming in the representative concentration pathway 4.5 (RCP4.5) scenario. The results imply that shifts in midlatitude circulation generally scale with the radiative forcing, rather than with global-mean temperature change. A corollary is that time slices taken from a transient simulation at a given level of warming will considerably overestimate the extratropical circulation response in a stabilized climate. Link to Paper: https://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-17-0323.1
  2. Seasonal and Regional Variations of Long-Term Changes in Upper-Tropospheric Jets from Reanalyses Authors: Gloria L. Manney and Michaela I. Hegglin First Published: September 15th, 2017 Published on line: December 19th, 2017 Abstract: Long-term changes in upper-tropospheric jet latitude, altitude, and strength are assessed for 1980–2014 using five modern reanalyses: MERRA, MERRA-2, ERA-Interim, JRA-55, and NCEP CFSR. Changes are computed from jet locations evaluated daily at each longitude to analyze regional and seasonal variations. The changes in subtropical and polar (eddy driven) jets are evaluated separately. Good agreement among the reanalyses in many regions and seasons provides confidence in the robustness of the diagnosed trends. Jet shifts show strong regional and seasonal variations, resulting in changes that are not robust in zonal or annual means. Robust changes in the subtropical jet indicate tropical widening over Africa except during Northern Hemisphere (NH) spring, and tropical narrowing over the eastern Pacific in NH winter. The Southern Hemisphere (SH) polar jet shows a robust poleward shift, while the NH polar jet shifts equatorward in most regions/seasons. Both subtropical and polar jet altitudes typically increase; these changes are more robust in the NH than in the SH. Subtropical jet wind speeds have generally increased in winter and decreased in summer, whereas polar jet wind speeds have weakened (strengthened) over Africa and eastern Asia (elsewhere) during winter in both hemispheres. The Asian monsoon has increased in area and appears to have shifted slightly westward toward Africa. The results herein highlight the importance of understanding regional and seasonal variations when quantifying long-term changes in jet locations, the mechanisms for those changes, and their potential human impacts. Comparison of multiple reanalyses is a valuable tool for assessing the robustness of jet changes. Link to Paper: https://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-17-0303.1
  3. How Global Warming and Arctic Ice Melt Intensify Hurricanes - YouTube Presentation Presented By: Dr Jennifer Francis (Research Professor at Rutgers University's Institute of Marine and Coastal Sciences since 1994) Interviewed By: Greg Wilpert Broadcast Team: The Real News Network Presentation Date: 21st September, 2018 Link to YouTube presentation (13 minutes): You can click on the chart above or use this link: https://www.youtube.com/watch?time_continue=45&v=cZTiFA39T8I
  4. The Influence of the Upper-Level Environmental Flow on Tropical Cyclone Structure and Intensity Predictability - Presentation A presentation at the AMS 33rd Conference on “Hurricanes and Tropical Meteorology” held at Ponte Vedra, Florida, USA between 16th and 20th April, 2018 Presenters: Yi Dai, S. J. Majumdar and D. S. Nolan Presentation Date: 19th April, 2018  Presentation Summary: Features of the upper-level environmental flow, such as mid-latitude jets, troughs, and tropical upper-tropospheric troughs (TUTT), have long been thought to be important in regulating tropical cyclone (TC) intensity, although the exact physical processes are still to be clarified. This study uses idealized WRF simulations of TCs that interact with an upper-level trough (jet, and TUTT), to elucidate the mechanisms of their interactions and impact on TC structure and intensity. The TC outflow, which is the upper-part of the TC secondary circulation, mediates this interaction. We present a range of simulations where the simulated trough interacts with TCs at different stages (weak or strong), different characteristics (such as compact or big), and different distances. More specifically, the spatial distribution of the horizontal wind and convection is modulated by the enhanced asymmetric outflow, which is due to the trough-TC-interaction. The spatially asymmetric features will be highlighted in this study. Lastly, the potential role of TCs on regulating the state of upper-level environmental flow, such as the trough (jet) strength and phase, is explored.  Link to conference video presentation (14 minutes): https://ams.confex.com/ams/33HURRICANE/videogateway.cgi/id/46896?recordingid=46896&uniqueid=Paper339800&entry_password=845295 Link to full conference agenda: https://ams.confex.com/ams/33HURRICANE/webprogram/33HURRICANE.html
  5. Dependence of MJO Phase Speed on Background Flow - Presentation A presentation at the AMS 33rd Conference on “Hurricanes and Tropical Meteorology” held at Ponte Vedra, Florida, USA between 16th and 20th April, 2018 Presenters: Paul E. Roundy and R. M. Setzenfand Presentation Date: 18th April, 2018 Presentation Summary: Three lines of evidence are presented in support of the view that a large fraction of the variance in the phase speeds of Madden Julian oscillation (MJO) events is governed by advection by the background circulation in which anomalous MJO convection and circulation are embedded. First, regression of fields of data against outgoing longwave radiation (OLR) signals at zonal wavenumber 2 filtered for a variety of phase speeds via the space-time wavelet transform shows the preferred spatial/temporal structures of the MJO at those phase speeds. Reconstruction of the momentum budget terms regressed against indexes wavelet filtered for the different phase speeds suggests that advection of and by the background wind in regions of stronger upper tropospheric background flow is associated with faster eastward propagation. Second, a new algorithm of lag regression is applied, relating OLR anomalies in the equatorial plane to a rotated RMM index, to analyze the relationship between MJO phase speed and the background circulation. In standard linear regression, the slope coefficient is the ratio between the covariance between the predictor and the predictand to the variance of the predictor. In the new algorithm, these variance and covariance quantities are modeled as they respond to background wind signals in different parts of the world, yielding regression slope coefficients between the RMM index and field variables as the relationship varies with the background flow. The result demonstrates how MJO phase speed varies with these background quantities. Finally, third, simple observations of the background wind associated with individual MJO events that are fast or slow are consistent with the general conclusions: Anomalous background westerly winds in the upper troposphere near the longitudes of the convection but over a broad range of latitudes in the tropics are associated with more rapid eastward propagation while anomalous easterly wind in the same region is associated with slow propagation. Results suggest that the slowest MJO convective events over the Pacific basin occur to the east of exit regions in the background jet stream when the regional base states continue to favor convection. This analysis is part of broader study of the association of the global atmospheric circulation with the phase speed of MJO convection. Link to conference video presentation (14 minutes): https://ams.confex.com/ams/33HURRICANE/videogateway.cgi/id/46754?recordingid=46754&uniqueid=Paper338997&entry_password=435416 Link to full conference agenda: https://ams.confex.com/ams/33HURRICANE/webprogram/33HURRICANE.html
  6. Modulation of the Northern Hemisphere Midlatitude Flow and Extreme Events By the Madden-Julian Oscillation - Presentation A presentation at the AMS 33rd Conference on “Hurricanes and Tropical Meteorology” held at Ponte Vedra, Florida, USA between 16th and 20th April, 2018 Presenters: Eric D. Maloney, E. A. Barnes, C. F. Baggett, K. C. Tseng, B. D. Mundhenk, S. Henderson and B. Wolding Presentation Date: 16th April, 2018 Presentation Summary: Recent work on teleconnections between the Madden-Julian oscillation (MJO) and northern Hemisphere midlatitude geopotential height anomalies, blocking, and atmospheric rivers (ARs) are discussed. It is first demonstrated using reanalysis fields and a linear baroclinic model that MJO teleconnections to higher latitudes are more robust during certain MJO phases due to the spatial configuration of MJO heating anomalies relative to the North Pacific jet. This robustness is also reflected in excellent ensemble agreement in prediction of North Pacific geopotential height anomalies in a leading numerical weather prediction model at 3-week lead times for certain MJO phases. It is demonstrated that climate and weather forecasting models can have difficulty simulating the spatial pattern and strength of such teleconnections not only due to poor MJO performance, but also due to biases in the spatial extent of the North Pacific jet that affect the pathway of Rossby wave propagation into high latitudes. The modulation of atmospheric blocking and atmospheric river (AR) activity associated with these MJO teleconnections are discussed. It is also shown that the nature of the MJO teleconnection to the Northern Hemisphere depends on the phase of the tropical quasi-biennial oscillation (QBO). The QBO phase-dependent modulation of AR activity by the MJO along the west coast of North America area is presented. A statistical prediction scheme for anomalous AR activity using the initial state of the MJO and QBO as the sole predictors is developed. When evaluated over 36 boreal winters, it is found that certain combinations of MJO and QBO phases produce predictive skill for anomalous AR activity up to 5 weeks in advance that exceeds that produced by a state-of-the-art numerical weather prediction model. Finally, recent modeling results suggest that the MJO teleconnection to higher latitudes may weaken in a warmer climate. The implications of these weaker teleconnections for subseasonal prediction of blocking and ARs will be discussed. Link to conference video presentation (15 minutes): https://ams.confex.com/ams/33HURRICANE/videogateway.cgi/id/46438?recordingid=46438&uniqueid=Paper339784&entry_password=812548 Link to full conference agenda: https://ams.confex.com/ams/33HURRICANE/webprogram/33HURRICANE.html
  7. Variability of the African Easterly Jet and Its Influence on Easterly Waves - Poster Presentation This was a short (poster) presentation at the AMS 97th Annual Conference incorporating the 16th Annual Student Conference held at Washington State Convention Centre, Seattle, USA between 21st and 26th January, 2017. Presenters: Amana Hosten, Tallahassee, FL and S. E. Nicholson Poster Presentation Date: 22nd January, 2017 Presentation Summary: The African Easterly Jet (AEJ) is believed to be one of the phenomena driving regional as well as global atmospheric circulation. Specifically, within the Sahel region, the AEJ and African Eastern Waves (AEWs) are important features associated with the West African Monsoon. Both features determine weather patterns and storm generation in this region. The goal of this study is to identify distinct cases of the AEJ structure and investigate how it affects the AEW formation. Current studies have shown that the AEJ possesses a double-core structure during the month of August when the jet is the strongest, which impacts local climatic conditions in the Sahel as well as influence the formation of AEWs. For this study, we obtained zonal and meridional wind data measured at 850 hPa and 600 hPa during the month of August from 1948 to 2016. Plots of the monthly mean of zonal wind were created and the years where the double core was apparent were chosen for further analysis. The years showing the double core structure were used to construct Hövmoller diagrams of the meridional wind in order to determine AEW track and lifespan. Preliminary results identified distinct cases where the structure of the AEJ deviated from its mean structure, including the western (eastern) core being located north relative to the eastern (western) core. Comparisons of the different cases can provide valuable insight about the relationship between the AEJ, easterly waves and rainfall variability over the Sahel as well as tropical cyclogenesis at the West African coast. Link to conference poster presentation (charts only): not available Link to full conference agenda: https://ams.confex.com/ams/97Annual/webprogram/meeting.html
  8. Characterization of convective systems and their association with African easterly waves Authors: Hilawe Semunegus, Ademe Mekonnen and Carl J. Schreck III Published: 3rd May, 2017 Abstract: This study investigates the relationship between African easterly waves (AEWs) and different types of deep convection. It is known that AEWs impact the development of deep convection over tropical North Africa and tropical cyclone formation over the eastern Atlantic. However, the process of how AEWs interact with deep convection is not well understood. Composite analysis based on a 24‐year data set of cloud systems (CS) from the International Satellite Cloud Climate Project shows that the relationship changes with various types of convection over this region. This phase change relationship analysis may shed light into the dynamics of AEWs and improve the ability of forecasters to anticipate associated rainfall over the Sahel. Weak and disorganized convective systems (WDCSs; 50 km < radius < 100 km) are most common within the southerly phase of the AEWs over East Africa. Mesoscale convective systems (MCSs) with cloud radii >100 km increase in frequency within and to the west of the AEW‐trough zone. MCSs are common features of summer in northwestern Africa. Our results indicate that the association between AEWs and deep convection is different and changes across North Africa. Weak AEWs over East Africa have a stronger relationship with WDCSs, while mature AEWs over West Africa have more MCS activity. This evolution suggests that the organization of convection from WDCS to MCS may play a critical role in AEW development. This hypothesis contrasts the traditional view that treats convection uniformly. Link to full paper: https://rmets.onlinelibrary.wiley.com/doi/epdf/10.1002/joc.5085
  9. The Intraseasonal Variability of African Easterly Wave Energetics Authors: Ghassan J. Alaka Jr. and Eric D. Maloney Published: 28th August, 2014 Abstract: African easterly waves (AEWs) and associated perturbation kinetic energy (PKE) exhibit significant intraseasonal variability in tropical North Africa during boreal summer. Consistent with East Africa (e.g., east of Lake Chad) being an initiation region for AEWs, previous studies have shown that increased East African PKE precedes and leads to increased West African AEW activity on intraseasonal time scales. In this study, reanalysis budgets of PKE and perturbation available potential energy (PAPE) are used to understand this behavior. The variability of PKE and PAPE sources is analyzed as a function of Madden–Julian oscillation (MJO) phase and a local 30–90-day West African PKE index to diagnose when and where eddy energy conversions terms are important to periods of increased or decreased intraseasonal AEW activity. In East Africa, an increased meridional temperature gradient locally enhances baroclinic energy conversion anomalies to initiate periods of increased intraseasonal AEW activity. Downstream barotropic and baroclinic energy conversions associated with strong AEWs are important for the maintenance of intraseasonal AEW activity in West Africa. Barotropic energy conversions dominate south of the African easterly jet (AEJ), while baroclinic energy conversions are most important north of the AEJ. In both East and West Africa, diabatic heating does not appear to aid intraseasonal PKE creation. Instead, negative PAPE tendency anomalies due to the diabatic heating–temperature covariance act as a negative feedback to increased baroclinic energy conversion downstream in the AEJ. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-14-00146.1
  10. The Influence of the MJO on Upstream Precursors to African Easterly Waves Authors: Ghassan J. Alaka Jr. and Eric D. Maloney Published: 8th May, 2012 Abstract: The Madden–Julian oscillation (MJO) produces alternating periods of increased and reduced precipitation and African easterly wave (AEW) activity in West Africa. This study documents the influence of the MJO on the West African monsoon system during boreal summer using reanalysis and brightness temperature fields. MJO-related West African convective anomalies are likely induced by equatorial Kelvin and Rossby waves generated in the Indian Ocean and West Pacific by the MJO, which is consistent with previous studies. The initial modulation of tropical African convection occurs upstream of West Africa, near the entrance of the African easterly jet (AEJ). Previous studies have hypothesized that an area to the east of Lake Chad is an initiation region for AEWs. Called the “trigger region” in this study, this area exhibits significant intraseasonal convection and wave activity anomalies prior to the wet and dry MJO phases in the West African monsoon region. In the trigger region, cold tropospheric temperature anomalies and high precipitable water, as well as an eastward extension of the African easterly jet, appear to precede and contribute to the wet MJO phase in West Africa. An anomalous stratiform heating profile is observed in advance of the wet MJO phase with anomalous PV generation maximized at the jet level. The opposite behavior occurs in advance of the dry MJO phase. The moisture budget is examined to provide further insight as to how the MJO modulates and initiates precipitation and AEW variability in this region. In particular, meridional moisture advection anomalies foster moistening in the trigger region in advance of the wet MJO phase across West Africa. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-11-00232.1
  11. Progress in research of stratosphere-troposphere interactions: Application of isentropic potential vorticity dynamics and the effects of the Tibetan Plateau Authors: Rongcai Ren, Guoxiong Wu, Ming Cai, Shuyue Sun, Xin Liu and Weiping Li Published: 19th October, 2014 Abstract: This paper reviews recent progress in understanding isentropic potential vorticity (PV) dynamics during interactions between the stratosphere and troposphere, including the spatial and temporal propagation of circulation anomalies associated with the winter polar vortex oscillation and the mechanisms of stratosphere-troposphere coupling in the global mass circulation framework. The origins and mechanisms of interannual variability in the stratospheric circulation are also reviewed. Particular attention is paid to the role of the Tibetan Plateau as a PV source (via its thermal forcing) in the global and East Asian atmospheric circulation. Diagnosis of meridional isentropic PV advection over the Tibetan Plateau and East Asia indicates that the distributions of potential temperature and PV over the east flank of the Tibetan Plateau and East Asia favor a downward and southward isentropic transport of high PV from the stratosphere to the troposphere. This transport manifests the possible influence of the Tibetan Plateau on the dynamic coupling between the stratosphere and troposphere during summer, and may provide a new framework for understanding the climatic effects of the Tibetan Plateau. Link to full paper: http://www.lasg.ac.cn/staff/gxwu/docs/2014/7. Ren Wu etc JMR_QXXB.pdf
  12. The Karakoram/Western Tibetan vortex: seasonal and year-to-year variability Authors: Xiao-Feng Li, Hayley J. Fowler, Nathan Forsythe, Stephen Blenkinsop and David Pritchard Published: 21st February, 2018 Abstract: The “Karakoram Vortex” (KV), hereafter also referred to as the “Western Tibetan Vortex” (WTV), has recently been recognized as a large-scale atmospheric circulation system related to warmer (cooler) near-surface and mid-lower troposphere temperatures above the Karakoram in the western Tibetan Plateau (TP). It is characterized by a deep, anti-cyclonic (cyclonic) wind anomaly associated with higher (lower) geopotential height in the troposphere, during winter and summer seasons. In this study, we further investigate the seasonality and basic features of the WTV in all four seasons, and explore its year-to-year variability and influence on regional climate. We find the WTV accounts for the majority of year-to-year circulation variability over the WTP as it can explain over 50% (R2⩾0.5R2⩾0.5) variance of the WTP circulation on multiple levels throughout the troposphere, which declines towards the eastern side of the TP in most seasons. The WTV is not only more (less) active but also has a bigger (smaller) domain area, with a deeper (shallower) structure, in winter and spring (summer and autumn). We find that the WTV is sensitive to both the location and intensity of the Subtropical Westerly Jet (SWJ), but the relationship is highly dependent on the climatological mean location of SWJ axes relative to the TP in different seasons. We also show that the WTV significantly modulates surface and stratospheric air temperatures, north–south precipitation patterns and total column ozone surrounding the western TP. As such, the WTV has important implications for the understanding of atmospheric, hydrological and glaciological variability over the TP. Link to full paper: https://link.springer.com/content/pdf/10.1007%2Fs00382-018-4118-2.pdf
  13. North Atlantic winter eddy-driven jet and atmospheric blocking variability in the Community Earth System Model version 1 Large Ensemble simulations Authors: Young-Oh Kwon, Alicia Camacho, Carlos Martinez and Hyodae Seo Published: 18th January, 2018 Abstract: The atmospheric jet and blocking distributions, especially in the North Atlantic sector, have been challenging features for a climate model to realistically reproduce. This study examines climatological distributions of winter (December–February) daily jet latitude and blocking in the North Atlantic from the 40-member Community Earth System Model version 1 Large Ensemble (CESM1LE) simulations. This analysis aims at examining whether a broad range of internal climate variability encompassed by a large ensemble of simulations results in an improved representation of the jet latitude distributions and blocking days in CESM1LE. In the historical runs (1951–2005), the daily zonal wind at 850 hPa exhibits three distinct preferred latitudes for the eddy-driven jet position as seen in the reanalysis datasets, which represents a significant improvement from the previous version of the same model. However, the meridional separations between the three jet latitudes are much smaller than those in the reanalyses. In particular, the jet rarely migrates to the observed southernmost position around 37°N. This leads to the bias in blocking frequency that is too low over Greenland and too high over the Azores. These features are shown to be remarkably stable across the 40 ensemble members with negligible member-to-member spread. This result implies the range of internal variability of winter jet latitude and blocking frequency within the 55-year segment from each ensemble member is comparable to that represented by the full large ensemble. Comparison with 2046–2100 from the RCP8.5 future projection runs suggests that the daily jet position is projected to maintain the same three preferred latitudes, with a slightly higher frequency of occurrence over the central latitude around 50°N, instead of shifting poleward in the future as documented in some previous studies. In addition, the daily jet speed is projected not to change significantly between 1951–2005 and 2046–2100. On the other hand, the climatological mean jet is projected to become slightly more elongated and stronger on its southern flank, and the blocking frequency over the Azores is projected to decrease. Link to full paper: https://link.springer.com/content/pdf/10.1007%2Fs00382-018-4078-6.pdf
  14. Analysis of the variability of the North Atlantic eddy-driven jet stream in CMIP5 Authors: Waheed Iqbal, Wai-Nang Leung and Abdel Hannachi Published: 22nd September, 2017 Abstract: The North Atlantic eddy-driven jet is a dominant feature of extratropical climate and its variability is associated with the large-scale changes in the surface climate of midlatitudes. Variability of this jet is analysed in a set of General Circulation Models (GCMs) from the Coupled Model Inter-comparison Project phase-5 (CMIP5) over the North Atlantic region. The CMIP5 simulations for the 20th century climate (Historical) are compared with the ERA40 reanalysis data. The jet latitude index, wind speed and jet persistence are analysed in order to evaluate 11 CMIP5 GCMs and to compare them with those from CMIP3 integrations. The phase of mean seasonal cycle of jet latitude and wind speed from historical runs of CMIP5 GCMs are comparable to ERA40. The wind speed mean seasonal cycle by CMIP5 GCMs is overestimated in winter months. A positive (negative) jet latitude anomaly in historical simulations relative to ERA40 is observed in summer (winter). The ensemble mean of jet latitude biases in historical simulations of CMIP3 and CMIP5 with respect to ERA40 are −2.43∘−2.43∘ and −1.79∘−1.79∘ respectively. Thus indicating improvements in CMIP5 in comparison to the CMIP3 GCMs. The comparison of historical and future simulations of CMIP5 under RCP4.5 and RCP8.5 for the period 2076–2099, shows positive anomalies in the jet latitude implying a poleward shifted jet. The results from the analysed models offer no specific improvements in simulating the trimodality of the eddy-driven jet. Link to full paper: https://link.springer.com/content/pdf/10.1007%2Fs00382-017-3917-1.pdf
  15. Response of the mid-latitude jets and storm tracks to projected future warming Workshop Presentation: Aspen Global Change Institute, Aspen, Colorado Workshop Programme: “Understanding the Causes and Consequences of Polar Amplification” - June 12th -16th, 2017 Presenter: Tim Woollings Presentation Date: 15th June, 2017 Link to full presentation (25 minute video): https://www.agci.org/lib/17s1/response-mid-latitude-jets-and-storm-tracks-projected-future-warming Link to presentation (slides and charts only): https://www.agci.org/sites/default/files/pdfs/lib/main/Aspen_Arctic_mtg.pdf Link to full agenda and presentations: https://www.agci.org/event/17s1
  16. Improving weather forecasts via impacts of turbulent orographic form & small-scale orographic gravity wave drag on boundary layers Authors: Maarten Minkman (Master thesis) Published: November, 2017 Abstract: Numerical Weather Prediction (NWP) models have difficulties with representing stable boundary layer conditions. During these conditions NWP models need more drag than physically observed to increase the representation of cyclonics. Therefore, NWP models use a non-physical enhanced momentum drag (long-tail formulation), which negatively affects the skill for the near-surface wind speeds and boundary layer depth. To describe the extra needed drag for NWP models in a more physical way, this study investigates whether the already developed parameterizations of turbulent orographic form drag (TOFD) and small-scale orographic wave drag (GWDSBL) could describe the needed drag instead of the longtail formulation. On top of the short-tail formulation TOFD and GWDSBL were added individually and combined. The short-tail formulation is supported by observations and LES simulations. Polar WRF (version 3.7.1.) was used in a 10-day case study during the northern hemispheric winter over North America. Individual GWDSBL and TOFD both reduced the wind speeds with a bias reduction of 2% and 13% respectively compared to the short-tail formulation. As a result the 2-meter temperatures reduced locally and increased the general cold bias within WRF by 1-4◦C where the wind speeds reduced due to GWDSBL and TOFD. Besides the boundary layer the synoptic scale was affected to by GWDSBL and TOFD. The cyclonic core pressure was improved and TOFD and the combination of GWDSBL and TOFD even outperformed the long-tail formulation by reducing the biases with approx. 5%. The 500 hPa geopotential heigth was raised with more drag added to short-tail and decreased the bias. However, the Arctic and number of model levels showed a significant impact in the representation of the 500 hpa geopotential height. TOFD also reduced on average the jet stream by 1-4 m s−1 and resulted in a northward shift. In the end the combination of GWDSBL and TOFD is seen as the best option to describe the extra needed drag instead of the long-tail formulation. Link to full paper: http://edepot.wur.nl/427452
  17. The Role of Zonal Asymmetry in the Enhancement and Suppression of Sudden Stratospheric Warming Variability by the MJO Authors: Wanying Kang and Eli Tziperman Published: 20th February, 2018 Abstract: Sudden stratospheric warming (SSW) events influence the Arctic Oscillation and midlatitude extreme weather. Previous work showed the Arctic stratosphere to be influenced by the Madden–Julian oscillation (MJO) and that the SSW frequency increases with an increase of the MJO amplitude, expected in a warmer climate. It is shown here that the zonal asymmetry in both the background state and forcing plays a dominant role, leading to either enhancement or suppression of SSW events by MJO-like forcing. When applying a circumglobal MJO-like forcing in a dry dynamic core model, the MJO-forced waves can change the general circulation in three ways that affect the total vertical Eliassen–Palm flux in the Arctic stratosphere. First, weakening the zonal asymmetry of the tropospheric midlatitude jet, and therefore preventing the MJO-forced waves from propagating past the jet. Second, weakening the jet amplitude, reducing the waves generated in the midlatitudes, especially stationary waves, and therefore the upward-propagating planetary waves. Third, reducing the Arctic lower-stratospheric refractory index, which prevents waves from upward propagation. These effects stabilize the Arctic vortex and lower the SSW frequency. The longitudinal range to which the MJO-like forcing is limited plays an important role as well, and the strongest SSW frequency increase is seen when the MJO is located where it is observed in current climate. The SSW suppression effects are active when the MJO-like forcing is placed at different longitudinal locations. This study suggests that future trends in both the MJO amplitude and its longitudinal extent are important for predicting the Arctic stratosphere response. Link to full paper: https://www.seas.harvard.edu/climate/eli/reprints/Kang-Tziperman-2018.pdf
  18. Effects of the Tibetan Plateau - A Chapter from the book "The Asian Monsoon" Authors: Michio Yanai and Guo-Xiong Wu First Published: January, 2006; updated on: 26th August, 2017 Abstract: The Tibetan Plateau (Qinghai–Xizang Plateau) extends over the latitude–longitude domain of 25–458N, 70–1058E, with a size of about one-quarter of the Chinese territory and a mean elevation of more than 4,000 m above sea level (Figure 13.1, color section). Surface elevation changes rapidly across the boundaries of the Plateau, especially the southern boundary, and strong contrasts exist between the western and eastern parts in land surface features, vegetation, and meteorological characteristics (e.g., Ye and Gao, 1979b; Smith and Shi, 1995). At these altitudes the mass of the atmosphere over the surface is only 60% that at sea level. Because of the lower densities, various radiative processes over the Plateau, particularly in the boundary layer, are quite distinct from those over lower elevated regions (e.g., Liou and Zhou, 1987; Smith and Shi, 1992; Shi and Smith, 1992). Therefore, the Tibetan Plateau exerts profound thermal and dynamical influences on atmospheric circulation. Link to full paper: file:///C:/Users/David/Downloads/4.YanaiWu_effectsofTPChapter13AsianMonsoon.pdf
  19. Impact of Mongolian Plateau versus Tibetan Plateau on the westerly jet over North Pacific Ocean Authors: Zhengguo Shi, Xiaodong Liu, Yimin Liu, Yingying Sha and Tingting Xu Published: 28th June, 2014 Abstract: Mountains have long been considered to play an important role in the formation of modern climate. Particularly in the Asia-Pacific sector, the existence of Tibetan Plateau (TP) is believed to maintain the stationary planetary wave patterns and to intensify the westerly jet over North Pacific. However, the potential role of Mongolian Plateau (MP) has often been neglected in these studies. With an atmospheric general circulation model, we show that the MP, despite its smaller size, exerts a great influence on the planetary-scale circulation and the subtropical westerly jet. The MP amplifies the meridional thermal gradient at the mid-upper troposphere and thus strengthens the jet, which is primarily facilitated by its high-latitude location. By blocking westerly winds, the MP forces their northern branch further northward, which allows the southward penetration of cold air at the lee side, and intensifies the East Asian trough. In contrast, the impact of the TP itself is not as large as expected. Hence, one should not simply ascribe all the mountain-induced climate change to the TP uplift, because other smaller topography might be also very important. Link to full paper: Please note that the full paper is currently behind a paywall. The pdf is only available in Chinese. Here's a link to Springer website: https://link.springer.com/article/10.1007/s00382-014-2217-2
  20. Mongolian Mountains Matter Most: Impacts of Latitude & Height of Asian Orography on Pacific Wintertime Atmospheric Circulation Authors: R. H. White Published: 12th January, 2017 Abstract: The impacts of Asian orography on the wintertime atmospheric circulation over the Pacific are explored using altered-orography, semi-idealized, general circulation model experiments. The latitude of orography is found to be far more important than height. The Mongolian Plateau and nearby mountain ranges, centered at ~48°N, have an impact on the upper-level wintertime jet stream that is approximately 4 times greater than that of the larger and taller Tibetan Plateau and Himalayas to the south. Key contributing factors to the importance of the Mongolian mountains are latitudinal variations in the meridional potential vorticity gradient and the strength of the impinging wind—both of which determine the amplitude of the atmospheric response—and the structure of the atmosphere, which influences the spatial pattern of the downstream response. Interestingly, while the Mongolian mountains produce a larger response than the Tibetan Plateau in Northern Hemisphere winter, in April–June the response from the Tibetan Plateau predominates. This result holds in two different general circulation models. In experiments with idealized orography, varying the plateau latitude by 20°, from 43° to 63°N, changes the response amplitude by a factor of 2, with a maximum response for orography between 48° and 53°N, comparable to the Mongolian mountains. In these idealized experiments, the latitude of the maximum wintertime jet increase changes by only ~6°. It is proposed that this nearly invariant spatial response pattern is due to variations in the stationary wavenumber with latitude leading to differences in the zonal versus meridional wave propagation. Link to full paper: https://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-16-0401.1
  21. Distinct impacts of the Mongolian and Tibetan Plateaus on the evolution of the East Asian monsoon Authors: Yingying Sha, Zhengguo Shi, Xiaodong Liu and Zhisheng An Published: 4th May, 2015 Abstract: The Mongolian Plateau (MP), which is relatively lower in altitude and smaller in extent than the Tibetan Plateau (TP), has received little attention about its climate effect. Building upon previous work in which we highlighted the role of the MP on the high‐level westerly jet stream, the response of surface‐level features of the Asian climate is examined in this study. The results show that the Indian and East Asian summer monsoonal and inland precipitation are mainly enhanced by the uplift of the TP. The precipitation during the onset of the summer monsoon is also intensified over India and eastern China. In addition, the East Asian monsoon domain is significantly expanded with the uplift of the TP, while the Indian summer monsoon domain does not change obviously. The MP plays a significant role in the strengthening of the East Asian winter monsoon, which is larger than the TP. With the uplift of the MP, the cold northerly wind in winter intensifies significantly in East Asia from higher latitudes to the South China Sea. The Siberian high is also enhanced and moves remarkably northward to its modern location. The strengthening of the Asian winter monsoon is related to the MP‐induced diversion of westerly wind. The bypassing flows around the plateau modify the temperature advections over middle latitudes and the atmosphere thermal structure in winter, which leads to the strengthening of the East Asian winter monsoon. Link to full paper: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014JD022880
  22. The Circulation of the High Troposphere over China in the Winter of 1945–46 Authors: Tu‐Cheng Yeh Published: 25th April, 1950 Abstract: The paper studies the structure of the high troposphere over China in the autumn of 1945 and winter of 1945–46. The principal feature is the existence of two belts of maximum westerlies, one flowing around the southern and the other around the northern edge of the Tibetan Plateau. The southern jet stream (the main one) is extremely stationary in position. Its speed increases downstream beyond the edge of the Asiatic continent. The onset of this southern jet stream is abrupt in the middle of October over central and southwestern China and advances downstream at a speed of about 3° longitude per day. South of this main jet stream is a belt of extremely uniform absolute vorticity which is zero in December and January. In spite of the existence of zero absolute vorticity the circulation above 15,000 feet is strikingly stable. Below 10,000 feet is the regular procession of warm troughs and cold ridges. Comparison with the conditions along 76° E and the east coast of United States has also been made. The difference in the structure of the basic current along the east coasts of the two continents suggests an explanation of the observed formation of winter typhoons off the Chinese coasts versus the non‐existence of such storms in the Caribbean Sea in the cold season. Link to full paper: https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.2153-3490.1950.tb00329.x
  23. On the Influence of the Earth's Orography on the General Character of the Westerlies Authors: B. Bolin Published: 15th May, 1950 Abstract: The upper westerlies in middle latitudes possess, in the mean, a wave‐like character which has been explained as a result of thermal contrasts between land and sea. On the other hand, recent theoretical investigations by Queney, Charney and Eliassen have shown that an obstacle of the dimensions of the Rocky Mountains generates a wave pattern downstream whose scale is comparable to the observed mean waves. In the present paper these theoretical studies are extended, and an attempt is made to discuss, in a general way, the influence of the northern hemisphere mountains on the character of the westerlies. Finally the paper points out some of the climatic consequences of the proposed dynamic‐topographic control of the prevailing flow patterns. REFERENCES Link to full paper: https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.2153-3490.1950.tb00330.x
  24. Baroclinic-to-Barotropic Pathway in ENSO Teleconnections from the Viewpoint of a Barotropic Rossby Wave Source Authors: Xuan Ji, J. David Neelin, and C. Roberto Mechoso Published: 18th July, 2016 Abstract: The baroclinic-to-barotropic pathway in ENSO teleconnections is examined from the viewpoint of a barotropic Rossby wave source that results from decomposition into barotropic and baroclinic components. Diagnoses using the NCEP–NCAR reanalysis are supplemented by analysis of the response of a tropical atmospheric model of intermediate complexity to the NCEP–NCAR barotropic Rossby wave source. Among the three barotropic Rossby wave source contributions (shear advection, vertical advection, and surface drag), the leading contribution is from shear advection and, more specifically, the mean baroclinic zonal wind advecting the anomalous baroclinic zonal wind. Vertical advection is the smallest term, while surface drag tends to cancel and reinforce the shear advection in different regions through damping on the baroclinic mode, which spins up a barotropic response. There are also nontrivial impacts of transients in the barotropic wind response to ENSO. Both tropical and subtropical baroclinic vorticity advection contribute to the barotropic component of the Pacific subtropical jet near the coast of North America, where the resulting barotropic wind contribution approximately doubles the zonal jet anomaly at upper levels, relative to the baroclinic anomalies alone. In this view, the barotropic Rossby wave source in the subtropics simply arises from the basic-state baroclinic flow acting on the well-known baroclinic ENSO flow pattern that spreads from the deep tropics into the subtropics over a scale of equatorial radius of deformation. This is inseparably connected to the leading deep tropical Rossby wave source that arises from eastern Pacific climatological baroclinic winds advecting the tropical portion of the same ENSO flow pattern. Link to full paper: https://dept.atmos.ucla.edu/sites/default/files/csi/files/ji_etal_2016_rws_revsd_jas.pdf?m=1524862597
  25. The MJO‐SSW Teleconnection: Interaction Between MJO‐Forced Waves and the Midlatitude Jet Authors: Wanying Kang and Eli Tziperman Published: 19th April, 2018 Abstract: The Madden‐Julian Oscillation (MJO) was shown to affect both present‐day sudden stratospheric warming (SSW) events in the Arctic and their future frequency under global warming scenarios, with implications to the Arctic Oscillation and midlatitude extreme weather. This work uses a dry dynamic core model to understand the dependence of SSW frequency on the amplitude and longitudinal range of the MJO, motivated by the prediction that the MJO will strengthen and broaden its longitudinal range in a warmer climate. We focus on the response of the midlatitude jets and the corresponding generated stationary waves, which are shown to dominate the response of SSW events to MJO forcing. Momentum budget analysis of a large ensemble of spinup simulations suggests that the climatological jet response is driven by the MJO‐forced meridional eddy momentum transport. The results suggest that the trends in both MJO amplitude and longitudinal range are important for the prediction of the midlatitude jet response and for the prediction of SSWs in a future climate. Plain Language Summary: Sudden stratospheric warming (SSW) events occur in the Arctic stratosphere during winter approximately every other year, featuring an abrupt warming and a breakdown of the polar vortex. These events affect midlatitude extreme weather events and are therefore of a societal relevance, making it important to be able to predict a change in their frequency in a future climate change scenario. In the present climate, these events seem to be only weakly influenced by the Madden‐Julian Oscillation (MJO), the dominant intraseasonal variability in the tropics. The authors have previously shown that a strengthening of the MJO, which is expected in a warmer future climate, may lead to more frequent SSW events. The mechanism behind such enhanced future SSW frequency is shown here to involve a nonlinear interaction of MJO‐forced atmospheric waves with the midlatitude tropospheric jet. The waves make the midlatitude jet more asymmetric in longitude, therefore causing it to emit stronger stationary waves that reach the polar Arctic stratosphere, therefore leading to the more frequent occurrence of SSW events. This motivates studying this teleconnection between the tropical MJO and the Arctic SSW events using more detailed models, to increase our confidence in the prediction of future climate and weather regimes. Link to full paper: https://www.seas.harvard.edu/climate/eli/reprints/Kang-Tziperman-2018b-preprint.pdf
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