Characterizing the structure of the surface layer in the Pacific Ocean

Abstract

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 more than 18,000 temperature casts to 800 m, with station spacing of 10 to 40 km along transects between Auckland and Seattle (beginning in 1986), San Francisco and Taiwan (1991), Auckland and Valparaiso (1993), and Honolulu and Valdez (1993). The surface layer can assume many different shapes. It can include strongly stratified layers, actively mixed layers, salinity barrier layers, fossil mixed layers, and inversions. The spatial and temporal distribution of these features within the XBT and XCTD profiles is examined. Fossil 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 are less seasonally dependent and most commonly related to interleaving of different 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 subpolar regions of the Pacific Ocean that is strongly influenced by the salinity stratification. This highlights the need for complementary salinity and density information in these areas to accurately categorize the true nature of the active mixed layer.