The Structure and Interannual Variability of the Global Subtropical Underwater from 1950 to 2014 in the EN4 Data Product

This study examines the subtropical salinity maximum water, or Subtropical Underwater (STUW), in the subtropical regions of the North and South Atlantic, North and South Pacific and Indian Oceans. These salty water masses are important components of the upper-ocean overturning and global water cycle. In this work, we use the global, observation-based, version 4 of the Met Office Hadley Centre ‘‘EN’’ series of data (EN4) from 1950 to 2014 to investigate 1) the basic structure and property of STUW in each ocean basin in the climatological fields 2) the seasonal and interannual variability of the global STUWs. We discuss how these five STUWs differ. For example, in a climatological view, the North Pacific STUW has the largest outcrop, the South Pacific STUW has the most volume, the Indian Ocean STUW reaches the deepest depth, the South Atlantic STUW has the least surface outcrop, shallowest depth and therefore the smallest volume. During the 64 years examined, the interannual variability of the global STUWs’ properties shows distinct year to year variations and the overall trends of all the five STUWs’ salinity are increasing, which is in agreement of the surface salinity trend under the strengthened global water cycle.