Frontiers in Decadal WATER S CIENCE AND TECHNOLOGY BOARD Climate - PowerPoint PPT Presentation

Frontiers in Decadal WATER S CIENCE AND TECHNOLOGY BOARD Climate Variability: Proceedings of a Workshop Monday, July 25 th , 2pm EDT Gerald A. Meehl National Center for Atmospheric Research (NCAR) Organizing Committee Chair

Today’s webinar discusses the recently released Frontiers in Decadal Climate Variability: Proceedings of a Workshop . Not a report: Proceedings: • evidence-based consensus of an • chronicle the presentations and authoring committee of experts discussions at a workshop, • typically include findings, symposium, or other convening conclusions, and recommendations event based on information gathered by • statements and opinions contained the committee and committee are those of the participants and are deliberations not necessarily endorsed by other • peer reviewed and approved by participants, the planning the National Academies of committee, or the National Sciences, Engineering, and Academies of Sciences, Engineering, Medicine and Medicine • peer reviewed For information about other products and activities of the Academies, please visit nationalacademies.org/whatwedo.

The Workshop: • Topic: Decadal climate variability and the role of the ocean in variability of the GMST trend • Organized jointly by the Academies’ Board on Atmospheric Sciences and Climate (BASC) & the Ocean Studies Board (OSB) • Planning Committee Membership: Gerald A. (Jerry) Meehl, Chair Lisa Goddard (BASC), Columbia (BASC), NCAR University Kevin Arrigo (OSB), Stanford Robert Hallberg (OSB), NOAA Shuyi S. Chen (BASC), University of David Halpern (OSB), NASA Jet Miami Propulsion Laboratory • Workshop held September 3-4, 2015 at NAS Jonsson Center in Woods Hole, MA

Workshop Goals 1. Examine our understanding of the processes governing decadal-scale variability in key climate parameters, observational evidence of decadal variability and potential forcings, and model-based experiments to explore possible factors affecting decadal variations; 2. Identify key science, observing, and modeling gaps ; 3. Consider the utility and accuracy of various observations for tracking long-term climate variability , anticipating the onset and end of hiatus regimes, and closing the long-term heat budget; 4. Consider the utility of hiatus regimes as a metric for evaluating performance of long-term climate models ; and 5. Consider how best to communicate current understanding of climate variability , including potential causes and consequences, to non-expert audiences.

Workshop Participants • Kevin Arrigo , Stanford University • Brian Kahn , Climate Central Antonietta Capotondi , Cooperative Institute for Tom Knutson , GFDL • • Research in Environmental Sciences • Yochanan Kushnir , Lamont Doherty Earth (CIRES)/National Oceanic and Atmospheric Observatory (LDEO) Administration (NOAA) • James Overland, NOAA Pacific Marine • Shuyi S. Chen , University of Miami Environmental Laboratory (PMEL) • Kim Cobb , Georgia Institute of Technology • Michael Mann , Pennsylvania State University • Gokhan Danabasoglu , National Center for • John Marshall , Massachusetts Institute of Atmospheric Research (NCAR) Technology (MIT) • Tom Delworth , Geophysical Fluid Dynamics Gerald A. Meehl , NCAR • Laboratory (GFDL) • Matthew Menne , NOAA Baylor Fox-Kemper , Brown University • • Veronica Nieves , NASA JPL • John Fyfe , Canadian Centre for Climate Modelling • Susan Solomon , MIT and Analysis • Diane Thompson , Boston University • Lisa Goddard , International Research Institute for Mingfang Ting , LDEO • Climate and Society (IRI) • Jim Todd , NOAA • Robert Hallberg , NOAA • Caroline Ummenhofer , Woods Hole • David Halpern , National Aeronautics and Space Oceanographic Institution Administration Jet Propulsion Laboratory (NASA • Shang-Ping Xie , Scripps Institution of JPL) Oceanography Susan Hassol , Climate Communication • • Huai-min Zhang , NOAA • Patrick Heimbach , University of Texas at Austin

Acknowledgements • Thank you to: – Planning committee (especially Jerry!) and staff – NASA, NOAA, NSF, and DOE for their support – Reviewers: • Lisa Goddard, Columbia University • Philip Jones, University of East Anglia • Veronica Nieves, NASA Jet Propulsion Laboratory • Gavin Schmidt, NASA Goddard Institute for Space Studies

Frontiers in Decadal Climate Variability Gerald A. Meehl National Center for Atmospheric Research Biological and Energy Research Regional and Global Climate Modeling Program

Decadal climate variability science problems: 1. What are the relative contributions of internally generated decadal timescale variability and externally forced response to the observed time evolution of global climate on decadal timescales? 2. What are the processes and mechanisms in the climate system that produce internally generated climate variability? 3. Can these processes and mechanisms, if properly initialized, provide increased prediction skill of the time evolution of regional climate in the near-term, over and above that from the externally forced response? The workshop focused on 1 and 2 Workshop report prepared by NRC staff (thanks to Amanda Purcell and Nancy Huddleston)

Attention on decadal climate variability was brought into focus by the reduced rate of global surface warming in the early 21 st century. This has been variously referred to as a “hiatus”, “pause”, or “slowdown”.

Slowdown periods have occurred before in observations and models and are a naturally-occurring part of climate variability in combination with contributions from external forcings (Easterling and Wehner, 2009, GRL). Mid-1970s shift And the flip side of hiatus periods are accelerated warming periods.

Interpretation of trends related to decadal climate variability must use a process-based approach. There is evidence that the phase of the Interdecadal Pacific Oscillation (IPO) influences global surface temperature trends. If the IPO is the process-based decadal climate variability framework, global temperature trends can be compared for different IPO phases to see if they are different.

Following Zhang, Wallace and Battisti (1997, J. Climate) the Interdecadal Pacific Oscillation (IPO, Power et al., 1999) defined for entire Pacific; the Pacific Decadal Oscillation PDO (Mantua et al 1997, BAMS) is defined for the North Pacific but patterns are comparable (sometimes both referred to as “PDV” – Pacific Decadal Variability ) Climate model simulations indicate IPO is internally generated Observations Unforced model control run (CCSM4) Early-2000s slowdown Big hiatus The observed IPO pattern resembles internally-generated decadal pattern from an unforced model control run (pattern correlation= +0.63) Mid-70s (Meehl et al., 2009, J. Climate; Meehl and Shift Arblaster, 2011, J. Climate)

NOAA press release on Karl et al Science paper published in Science Express on June 3, 2015:

The early-2000s slowdown (2001-2014, negative phase of the Interdecadal Pacific Oscillation, IPO) is characterized by a trend that is significantly less than the previous positive IPO period from 1972-2001 (Fyfe et al., 2016, Nature Clim. Chg). Recent slow down in global surface temperature increase

We understand what produces slowdown decades in the model (opposite for accelerated warming decades): • relatively greater trends of ocean heat content below 300m • surface temperature trends indicate negative phase of the IPO • 3 ocean mixing processes: subtropical cells in Pacific, Southern Ocean Antarctic Bottom Water formation; Atlantic Meridional Overturning Circulation (Meehl et al., 2011, Nature Climate Change: Meehl et al., 2013, J. Climate)

Global warming does not stop during slowdown decades—heat content of the climate system continues to increase but we don’t see as much warming if the heat goes into the subsurface ocean during negative IPO. (Meehl et al., 2011, Nature Climate Change: Meehl et al., 2013, J. Climate)

Forcing from volcanic eruptions and stratospheric water vapor also could be playing a role in the early-2000s slowdown. Solomon et al., 2010, Science: maybe 25% of the early-2000s slowdown was due to decreased stratospheric water vapor since 2000; and ~30% of the accelerated warming from 1980-2000 due to increased stratospheric water vapor Santer et al., 2014 Nat. Geo.; 2015 GRL: perhaps at least 15% of the slowdown was due to stratospheric aerosols from several moderate sized volcanoes Maher et al., 2015, GRL: models show a lagged La Niña-like response the third year after a composite large tropical volcanic eruption associated with global cooling

Some CMIP5 uninitialized Slowdown as observed from 2000-2013: 10 members out of 262 possible realizations models actually simulated the slowdown Tend to be characterized by a negative phase of the IPO. Internally generated variability in those model simulations happened to sync with observed internally generated variability. Total: 262 possible simulations 2000-2012 slowdown: 21 2000-2014 slowdown: 9 2000-2015 slowdown: 6 2000-2016 slowdown: 6 2000-2017 slowdown: 1 2000-2018: 1 (Meehl et al., 2014, Nature Climate Change)