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Found 7 results

  1. Atmospheric Dynamics Feedback: Concept, Simulations, and Climate Implications Authors: Michael P. Byrne and Tapio Schneider First Published: January 12th, 2018 Published on line: March 26th, 2018 Abstract: The regional climate response to radiative forcing is largely controlled by changes in the atmospheric circulation. It has been suggested that global climate sensitivity also depends on the circulation response, an effect called the “atmospheric dynamics feedback.” Using a technique to isolate the influence of changes in atmospheric circulation on top-of-the-atmosphere radiation, the authors calculate the atmospheric dynamics feedback in coupled climate models. Large-scale circulation changes contribute substantially to all-sky and cloud feedbacks in the tropics but are relatively less important at higher latitudes. Globally averaged, the atmospheric dynamics feedback is positive and amplifies the near-surface temperature response to climate change by an average of 8% in simulations with coupled models. A constraint related to the atmospheric mass budget results in the dynamics feedback being small on large scales relative to feedbacks associated with thermodynamic processes. Idealized-forcing simulations suggest that circulation changes at high latitudes are potentially more effective at influencing global temperature than circulation changes at low latitudes, and the implications for past and future climate change are discussed. Link to Paper:
  2. El Nino, El Nino Modoki, La Nina, Indian Ocean Dipole - YouTube Presentation Presentation Team from: PMF IAS Presentation Date: 29th October, 2015 Link to YouTube presentation (24 minutes): or click on the chart below: Link to Written Notes and Slides:
  3. Large-Scale Atmospheric Forcing of Vertical Wind Shear in the Tropics - Poster 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: Jhordanne J. Jones, M. M. Bell and P. J. Klotzbach Presentation Date: 17th April, 2018 Poster Presentation Summary: It has been well-established that vertical wind shear (VWS) plays a key role in modulating tropical cyclone activity, but there are multiple large-scale processes that can produce VWS in the tropics and their interactions are not fully understood. One of the strongest inter-annual drivers of tropical VWS is the El-Nino Southern Oscillation (ENSO) which modulates shear through variations in the Walker circulation. VWS may be further modulated through midlatitude intrusions of high potential vorticity (PV) streamers and Rossby wave breaking into lower latitudes. In this study, we investigate how both ENSO and PV streamer activity influence the seasonal and spatial variations in tropical VWS. The relative contributions from sources of ‘tropical’ ENSO and ‘extra-tropical’ high PV shear will be discussed, along with potential relationships between the two sources of VWS. Furthermore, we look at how tropical cyclone activity in the Atlantic basin is influenced when VWS is driven by ENSO, PV streamers, or combinations of both sources. Link to Handout: To download the handout, go the conference session and click on "Handout" - Link below: Link to full conference agenda:
  4. The AMO as a Driver of Multi-decadal ENSO Variability This was a presentation at the AMS 97th Annual Conference held at Washington State Convention Centre, Seattle, USA between 21st and 26th January, 2017. Presenters: Aaron F. Z. Levine, NOAA, Seattle, WA; and M. J. McPhaden and D. M. Frierson Presentation Date: 26th January, 2017 Presentation Summary: Multi-decadal shifts in ENSO variability have been observed, but it is unclear if this variability is just a random variation in the ENSO cycle or whether it is forced from outside the tropical Pacific basin. El Niño events represent a collapse of the typical tropical Pacific annual cycle. Using the ENSO spring persistence barrier strength to represent the tropical Pacific annual cycle, we find an inverse relationship between ENSO variance and the amplitude of the annual cycle can be found on decadal timescales. Using a combination of observations, reconstructions, reanalysis, proxy data, and coupled ocean-atmosphere model experiments, we find that the cause of this inverse relationship is the extent of the northward excursion of ITCZ during the boreal summer and its effect on ENSO stability seasonally and annually. Furthermore, we find that a major control of the northward extent of the boreal summer ITCZ on these timescales is strength of the Pacific trade winds, which in turn are significantly influenced by tropical Atlantic SSTs. On these multidecadal timescales, the long term pattern of tropical Atlantic SSTs are controlled by the Atlantic Multidecadal Oscillation. Link to conference video presentation (15 minutes): Link to full conference agenda:
  5. The impact of the AMO on multidecadal ENSO variability Authors: Aaron F. Z. Levine, Michael J. McPhaden and Dargan M. W. Frierson Published: 17th April, 2017 Abstract: Multidecadal shifts in El Niño–Southern Oscillation (ENSO) variability have been observed, but it is unclear if this variability is just a random variation in the ENSO cycle or whether it is forced by other modes of climate variability. Here we show a strong influence of the Atlantic on the multidecadal variability of ENSO. The Atlantic Multidecadal Oscillation (AMO) is the dominant mode of multidecadal sea surface temperature (SST) variability in the Atlantic Ocean. Changes in AMO‐related tropical Atlantic SSTs are known to force changes in the Walker circulation in the tropical Pacific Ocean. Using conceptual and coupled model experiments, we show that these changes to the Walker circulation modify ENSO stability on both annual and multidecadal time scales leading to a distinctive pattern of multidecadal ENSO variability that we find in observations and ocean reanalyses. Plain Language Summary: El Niño events have significant global impacts. Over the observed record, periods of both enhanced and reduced El Niño activity exist. The question is whether these changes come purely from random chance or if they are forced from elsewhere in the climate system. Here we will explore the hypothesis that Atlantic multidecadal variability is important for these periods of enhanced and reduce El Niño activity. We will show that the periods of enhanced El Niño activity correspond with periods of increased El Niño predictability and that this signal is consistent with the observed changes in El Niño over the last century and with Atlantic forcing of these changes. The role of the Atlantic sea surface temperature variability will be further confirmed by targeted coupled model experiments. Link to full paper:
  6. Relationship of Multidecadal Global Temperatures to Multidecadal Oceanic Oscillations Authors: J.S. D'Aleo and D.J. Easterbrook Published: First edition 2011, revised second edition 2016. Abstract: Evidence shows that observed climate changes can be explained by natural factors, including secular changes in multidecadal cycles in the oceans and solar activity. The correlation of two decades of climate changes in the 1980 and 1990s to increasing CO2 is coincidental, as five of the last seven decades since the beginning of the boom following the end of World War II have actually seen temperature cool or remain steady even as CO2 increased. Multidecadal cycles in the ocean correlate with the frequency and strength of the shorter-term El Niño Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) phases and through them, U.S. temperatures. Total solar irradiance is shown to vary with both these multidecadal ocean cycles and thus the temperatures, suggesting that the sun is the ultimate driver. Link to full paper:
  7. The Dynamics of the ENSO–Atlantic Hurricane Teleconnection: ENSO-Related Changes to the North African–Asian Jet Affect Atlantic Basin Tropical Cyclogenesis Authors: Jeffrey Shaman Steven, K. Esbensen and Eric D. Maloney Published: 9th June, 2008 Abstract: The nature of the teleconnection linking ENSO variability with Atlantic basin tropical storm formation is investigated. Solutions of the linearized barotropic vorticity equation forced with August–October El Niño event divergence produce upper-tropospheric vorticity anomalies over the Sahel and at the mouth of the North African–Asian (NAA) jet over the tropical Atlantic. These responses are similar in magnitude and orientation to observed ENSO vorticity variability for this region. Further investigation reveals that the vorticity anomalies over the subtropical Atlantic develop primarily in response to very low wavenumber, westward-propagating stationary Rossby waves excited by El Niño–related convective activity over the equatorial Pacific Ocean. However, the dynamics of this teleconnection change as the Atlantic basin hurricane season progresses. In August and September the response is dominated by the westward-propagating stationary Rossby waves that alter vorticity within the NAA jet and to its south. The upper-tropospheric nondivergent zonal wind anomalies produced by these vorticity anomalies are similar in pattern to observed zonal wind and vertical zonal wind shear anomalies, which suppress Atlantic basin tropical cyclogenesis. By October, eastward-propagating signals also develop over the tropical Atlantic Ocean in response to El Niño conditions. Over the main development region of Atlantic basin tropical cyclogenesis, these eastward-propagating Rossby waves appear to destructively interfere with the vorticity changes produced by the westward-propagating Rossby waves within the NAA jet. In addition, the NAA jet has shifted south by October. Consequently, the resultant upper-tropospheric nondivergent zonal wind perturbations for October are weak and suggest that ENSO should have little effect on rates of Atlantic basin tropical cyclogenesis during October. Statistical analyses of monthly ENSO-related changes in Atlantic basin tropical storm formation support this hypothesis. Link to full paper:
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