Tropospheric O3 columns retrieved from OMI and MLS measurements, NO2 columns from OMI, and upper tropospheric O3 concentrations from TES over North America and the western North Atlantic from April to August 2005 are analyzed using the Regional chEmical and trAnsport Model (REAM). Large enhancements of column and upper tropospheric O3 over the western North Atlantic comparable to those over the eastern United States are found in the satellite measurements and REAM simulations. The O3 enhancement region migrates northward from spring to summer. Model analysis indicates that the northward migration is driven by seasonal shifts of O3 transported from the stratosphere and that produced through photochemistry from surface emissions and lightning NOx. As their uncertainties improve, satellite measurements of O3 and its precursors will be able to provide more quantitative constraints on pollutant outflow from the continents.