Characterizing the structure of the surface layer in the Pacific Ocean

The structure and statistics of the oceanic surface layer are characterized using quarterly surveyed, eddy-resolving expendable bathythermograph (XBT) and expendable conductivity-temperature-depth (XCTD) data collected along a number of routes spanning the Pacific Ocean. This data set consists of mo re than 18,000 temperature casts to 800 mi with station spacing of 10 to 40 km along transects between Auckland and Seattle (beginning in 1986), San F rancisco and Taiwan (1991), Auckland and Valparaiso (1993), and Honolulu an d Valdez (1993). The surface layer can assume many different shapes. It can include strongly stratified layers, actively mixed layers. salinity barrie r layers, fossil mixed layers, and inversions. The spatial and temporal dis tribution of these features within the XBT and XCTD profiles is examined. F ossil layers are predominantly a springtime feature and are associated with regions of Subtropical Mode Water formation in the southwest Pacific (near New Zealand) and the northeast Pacific (near San Francisco). Inversions ar e less seasonally dependent and most commonly related to interleaving of di fferent water masses in the high shear regions of the California Transition Zone and its counterpart eastern boundary systems in the far northeast and southeast Pacific and in the tropical zonal current system. The XCTD casts show a rich and varied surface layer structure in the equatorial and subpo lar regions of the pacific Ocean chat is strongly influenced by the salinit y stratification. This highlights the need for complementary salinity and d ensity information in these areas to accurately categorize the true nature of the active mixed layer.