THE GOALS OF THIS PROPOSED RESEARCH ARE (1) TO CHARACTERIZE AND UNDERSTAND THE NATURE AND CAUSES OF THE LARGE-SCALE CIRCULATION VARIABILITY IN THE PACIFIC OCEAN ON TIME SCALES FROM INTERANNUAL TO DECADAL AND (2) TO CLARIFY THE INTERACTION OF OCEANIC MOTIONS OF DIFFERENT TIME SCALES AND SPATIAL DOMAINS. AN UNDERSTANDING OF THESE TOPICS WILL LEAD TO INCREASED PREDICTABILITY OF THE OCEAN CIRCULATION AND DENSITY STRUCTURES BOTH OF WHICH ARE IMPORTANT TO THE CLIMATE VARIABILITY ON LONGER TIME SCALES. THE CONCURRENT SATELLITE ALTIMETER MISSIONS FROM THE PAST 2.5 DECADES HAVE HELPED RESOLVE THE ENERGY-CONTAINING MESOSCALE EDDY SIGNALS THAT ARE CRUCIAL FOR THE EQUILIBRATION OF THE LARGE-SCALE OCEANIC GYRE CIRCULATIONS. ALTHOUGH DESCRIPTIONS ABOUT THE MESOSCALE EDDY SIGNALS HAVE IMPROVED SIGNIFICANTLY THANKS TO THE ACCUMULATING ALTIMETER DATA EDDY'S ROLES IN FEEDBACKING TO THE SLOWLY MODULATING LARGER-SCALE CIRCULATION ARE YET TO BE ADEQUATELY QUANTIFIED. WITH THE GROWING TIME SERIES OF THE SATELLITE ALTIMETER DATA THAT RESOLVES BOTH THE MESO- AND LARGE-SCALE SSH SIGNALS THE TIMING BECOMES NOW RIPE FOR ADDRESSING THIS EDDY-MEAN FLOW INTERACTION ISSUE IN AN OBSERVATIONALLY-CONSTRAINED FUNDAMENTAL WAY. THIS STUDY PROPOSES TO CONDUCT CAREFUL ANALYSES OF THE MULTI-DECADE ALTIMETRY DATA FROM ALL AVAILABLE EXISTING/FUTURE MISSIONS. WE PLAN TO FIRST DOCUMENT THE LARGE-SCALE SSH FLUCTUATIONS ON THE DECADAL TIME SCALES. THE OBSERVED SIGNALS WILL BE COMPARED AGAINST OTHER OCEANIC DATA (SUCH AS SEA SURFACE TEMPERATURE AND ARGO PROFILING FLOAT DATA) AND THE ATMOSPHERIC VARIABLES (E.G. THE SURFACE WIND AND BUOYANCY FORCING) WITH THE GUIDANCE OF OUR UNDERSTANDING OF THE OCEAN DYNAMICS. OUR NEXT STEP IS TO CLARIFY THE CAUSES FOR THE OBSERVED DECADAL CHANGES. TO DO SO WE PROPOSE TO USE BOTH SIMPLIFIED DYNAMIC MODELS AND OCEAN GENERAL CIRCULATION MODEL OUTPUTS WITH REALISTIC TOPOGRAPHY AND SURFACE BOUNDARY CONDITIONS. THROUGH COMBINING THE ALTIMETRIC DATA AND THE OCEAN MODELS WITH VARIOUS COMPLEXITY WE SEEK TO IDENTIFY THE ROLES PLAYED BY DIFFERENT PHYSICAL PROCESSES SUCH AS THE INTERIOR OCEAN DYNAMICS THE SURFACE WIND AND BUOYANCY FORCINGS AND REGIONAL NONLINEAR AND INSTABILITY PROCESSES. PARTICULAR ATTENTION WILL BE DIRECTED TO CLARIFY HOW THE OBSERVED TIME-VARYING CIRCULATION SIGNALS OF VARIOUS TIME AND SPACE SCALES MUTUALLY INTERACT. BY COMBINING THE DATA ANALYSIS AND NUMERICAL MODELING THIS PROPOSED RESEARCH WILL DIRECTLY CONTRIBUTE TO THE TWO RESEARCH THEMES SET OUT BY THE JOINT NASA/NOAA/CNES OCEAN SURFACE TOPOGRAPHY SCIENCE TEAM (OSTST): 1) TO SUPPORT STUDIES IN PHYSICAL OCEANOGRAPHY UTILIZING JASON-SERIES MISSION DATA AS WELL AS THE COMBINED 20-YEAR TP/JASON/JASON-2/ JASON-3 DATA PREFERABLY JOINTLY WITH OTHER SATELLITE AND IN-SITU DATA AND/OR MODELS IN SUPPORT OF BOTH BASIC RESEARCH AND OPERATIONAL APPLICATIONS AND 2) TO SUPPORT STUDIES OF HIGH-RESOLUTION MERGED ALTIMETRIC DATA SETS TO EXAMINE THE ROLE OF MESOSCALE EDDIES AND WESTERN BOUNDARY CURRENTS IN THE GENERAL CIRCULATION OF THE OCEAN.