THE GOAL OF THIS PROJECT IS TO INVESTIGATE OZONE-CLIMATE INTERACTIONS IN THE NORTHERN HEMISPHERE (NH) USING THE GODDARD EARTH OBSERVING SYSTEM (GEOS) CHEMISTRY-CLIMATE MODEL (CCM). THIS PROJECT IS MOTIVATED ON THE ONE HAND BY THE 2011 SEVERE ARTIC OZONE HOLE WHICH POTENTIALLY CONTRIBUTED TO THE OBSERVED EXTREME NH CLIMATE ANOMALIES IN SPRING AND ON THE OTHER HAND BY THE FACT THAT THEROLE OF OZONE CLIMATE INTERACTIONS IN FUTURE NH CLIMATE CHANGE IS NOT UNDERSTOOD. THE GEOS CCM CONFIGURATION INCLUDES THE GEOS-5 ATMOSPHERE-OCEAN GCM COUPLED TO THE GMI COMBO CHEMISTRY MECHANISM. IN THIS PROJECT THE GEOS CCM WILL BE USED MAINLY IN COUPLED ATMOSPHERE-OCEAN MODE (GEOS AO-CCM). BASED ON A SET OF NUMERICAL EXPERIMENTS ACCOMPANIED BY ANALYSIS OF MERRA DATA AND SATELLITE OBSERVATIONS WE WILL ADDRESS THE FOLLOWING RESEARCH QUESTIONS: 1) HOW SENSITIVE IS OZONE CHANGE TO A RANGE OF FACTORS IN THE MODEL? 2) HOW HAVE OZONE CHANGES IMPACTED SURFACE CLIMATE IN THE EARLY 21ST CENTURY? 3) HOW WILL OZONE AND CLIMATE INTERACT IN THE LATE 21ST CENTURY?THE PROJECT WILL BEGIN WITH A CHARACTERIZATION OF TROPOSPHERIC AND STRATOSPHERIC CLIMATE AND TROPOSPHERE-STRATOSPHERE INTERACTIONS IN THE NH BASED ON EXISTING SIMULATIONS OF THE ATMOSPHERIC CCM AND THE AO-GCM. THE MERRA REANALYSIS WILL BE USED TO EVALUATE THESE MODELS. THIS ANALYSIS WILL CHARACTERIZE THE REALISM OF GEOS-5 AS WELL AS THE DIFFERENT IMPACTS OF OZONE-CIRCULATION FEEDBACKS AND AIR-SEA INTERACTIONS. THE EVALUATION ALSO INCLUDES PARAMETER-SPACE EXPERIMENTS WITH THE CCM FOR THE FIRST DECADE OF THE 21ST CENTURY TO TEST THE SENSITIVITY OF SIMULATED CHEMISTRY-CIRCULATION TO CHANGES IN MODEL PARAMETER WITH EOS-AURA AND OTHER SATELLITE OBSERVATIONSUSED FOR COMPARISON. THE BULK OF THE NEW WORK FOR THIS PROJECT WILL BE TO CONDUCT TWO UNIQUE SETS OF MULTI-DECADAL SIMULATIONS CARRIED OUT IN A LARGE ENSEMBLE MODE OF THE GEOS AO-CCM AND AO-GCM. THESE TWO SETS OF EXPERIMENTS WILL BE USED TO STUDY THE IMPACTS OF OZONE (AND OTHER CHEMICAL CHANGES) ON RECENT AND FUTURE CHANGES IN NH CLIMATE. THE ENSEMBLES WILL COVER UNCERTAINTY IN INITIAL CONDITIONS AND ALSO SPAN A RANGE OF VALUES OF MODEL PARAMETERS ALLOWING FOR (I) A ROBUST ASSESSMENT OF THE INTERACTIONS AND (II)EXAMINATION OF THEIR DEPENDENCE ON MODEL FORMULATION AS WELL AS (III) THE SENSITIVITY TO REPRESENTATIVE CONCENTRATION PATHWAY (RCP) FOR FUTURE CHANGES.AN IMPORTANT CONTRIBUTION OF THIS PROJECT IS TO ASSESS THE ROLE OF OZONE CHANGES IN THE OCCURRENCE OF EXTREME TROPOSPHERIC CLIMATE EVENTS BY CONDUCTING LARGE ENSEMBLES THAT ALLOW STUDYING THE PROPERTIES OF PROBABILITY DISTRIBUTION FUNCTIONS (PDFS) OF TROPOSPHERIC CLIMATEPARAMETERS INCLUDING THE BEHAVIOR OF THESE PDFS IN THEIR TAILS WHICH REPRESENT THE OCCURRENCE OF EXTREME EVENTS.BY UTILIZING THE GEOS CCM THE PROJECT WILL LINK DIRECTLY TO GMAO S ACTIVITIES AND USE THE CHEMISTRY MODULES DEVELOPED BY GMI INCLUDING STUDIES THAT RELATE UNCERTAINTY IN MODELING ATMOSPHERIC COMPOSITION TO THE PARAMETER SENSITIVITY OF THE UNDERLYING GCM. THE PROPOSED WORK DIRECTLY ADDRESSES SEVERAL RESEARCH THEMES IDENTIFIED IN THE MAP SOLICITATION AND ITS OVERALL EARTH SYSTEM GOAL: THISPROJECT WILL ENHANCE OUR UNDERSTANDING ABOUT THE ROLE OF OZONE-CLIMATE FEEDBACKS IN HEMISPHERIC AND REGIONAL CLIMATE CHANGE IN THE CONTEXT OF GLOBAL CLIMATE CHANGE. FURTHERMORE IT WILL ASSESS THE IMPORTANCE OF REPRESENTING FEEDBACKS RELATED TO AIR-SEA INTERACTION AND CHEMISTRY CLIMATE INTERACTIONS FOR SIMULATING FUTURE EVOLUTION OF THE OZONE LAYER.