MIDLATITUDE EXTREME WEATHER EVENTS SUCH AS COLD AIR OUTBREAKS HEAT WAVES FLOODS AND DROUGHTS ARE RESPONSIBLE FOR DISPROPORTIONATELY LARGE CLIMATE-RELATED DAMAGE. IN RECENT DECADES NORTHERN HEMISPHERE MIDLATITUDE CONTINENTS EXPERIENCED SEVERAL UNUSUALLY COLD WINTERS CONCURRENT WITH RAPID ARCTIC WARMING AND SEA ICE LOSS. THE RAPID TRANSITION FROM PROLONGED DROUGHT IN CALIFORNIA IN 2012-2016 TO SEVERE FLOODING IN 2017 WAS ASSOCIATED WITH A SWITCH FROM ATMOSPHERIC RIDGE TO TROUGH IN THE NORTHEASTERN PACIFIC. SUMMER HEAT WAVES AND RELATED EXTREMES IN RECENT YEARS WERE ASSOCIATED WITH QUASI-RESONANCE AMPLIFICATION OF PLANETARY ROSSBY WAVES IN THE ATMOSPHERE. HOWEVER IT REMAINS UNCLEAR HOW AND TO WHAT EXTENT THE OBSERVED CHANGES IN MIDLATITUDE CIRCULATION AND EXTREMES WERE CAUSED BY ANTHROPOGENIC CLIMATE CHANGE OR INTERNAL VARIABILITY IN THE CLIMATE SYSTEM. WE PROPOSE TO DEVELOP A COMPREHENSIVE UNDERSTANDING OF ATMOSPHERIC PROCESSES RESPONSIBLE FOR CHANGES IN MIDLATITUDE CIRCULATION AND WEATHER EXTREMES (E.G. BLOCKING AND ATMOSPHERIC RIVERS) IN A CHANGING CLIMATE INCLUDING THE INTERNAL ATMOSPHERIC VARIABILITY THE RESPONSE TO TROPICAL AND ARCTIC FORCINGS AND THE ROLE OF THE STRATOSPHERE IN THE UNFORCED AND FORCED VARIABILITY. WE PLAN TO FIRST EXTEND OUR PREVIOUS WORK ON LOCAL ROSSBY WAVE ACTIVITY TO A UNIFIED SET OF DYNAMICS-BASED METRICS FOR MIDLATITUDE WEATHER SYSTEMS AND MOISTURE TRANSPORT BASED ON REANALYSIS PRODUCTS RETRIEVALS FROM THE ATMOSPHERIC INFRARED SOUNDER (AIRS) ON NASA S AQUA SATELLITE AND GLOBAL PRECIPITATION MEASUREMENT (GPM). THEN WE WILL USE THESE METRICS TO EVALUATE MIDLATITUDE SUBSEASONAL AND INTERANNUAL ATMOSPHERIC VARIABILITY IN SEVERAL ENSEMBLES OF CLIMATE MODEL SIMULATIONS. IN PARTICULAR WE WILL ANALYZE GISS MODEL E2.1/E2.2 SIMULATIONS ON THE INFLUENCE OF THE STRATOSPHERE ON SURFACE EXTREME EVENTS. FINALLY WE WILL EXAMINE THE COUPLED MODEL INTERCOMPARISON PROJECT PHASE 6 (CMIP6) SIMULATIONS OF MIDLATITUDE EXTREMES IN THE HISTORICAL PERIOD IN THE NEAR FUTURE AND AT THE END OF THE 21ST CENTURY. THE PROPOSED RESEARCH DIRECTLY ADDRESSES THE THEME OF EXTREMES IN THE EARTH SYSTEM IN THE MODELING ANALYSIS AND PREDICTION (NNH20ZDA001N-MAP). IN PARTICULAR WE WILL DEVELOP ROBUST OBSERVATION-BASED MEASURES TO EVALUATE THE DEGREE TO WHICH MIDLATITUDE WEATHER SYSTEMS AND MOISTURE TRANSPORT AND THEIR IMPACTS ON TEMPERATURE AND PRECIPITATION EXTREMES ARE PROPERLY REPRESENTED IN EARTH SYSTEM MODELS WE STRIVE TO UNDERSTAND THE ROLE OF INTERNAL VARIABILITY TROPIC-MIDLATITUDE-ARCTIC INTERACTIONS AND STRATOSPHERE-TROPOSPHERE COUPLING IN THE OBSERVED CHANGES IN EXTREME EVENTS. THE OUTCOME OF THIS STUDY WILL HELP IMPROVE THE MODEL REPRESENTATION OF MIDLATITUDE CIRCULATION AND EXTREMES AND THUS PRODUCE MORE CREDIBLE PROJECTIONS FOR THE FUTURE. BY COLLABORATING WITH GISS AND JPL SCIENTISTS THIS WILL CONTRIBUTE TO NASA S MODELING EFFORTS THROUGH THE USE OF SATELLITE RETRIEVALS OF WATER VAPOR AND PRECIPITATION AS OBSERVATIONAL CONSTRAINTS OF MIDLATITUDE CIRCULATION AND MOISTURE TRANSPORT IN CLIMATE MODELS.