Recent decadal change in the North Atlantic subtropical underwater associated with the poleward expansion of the surface salinity maximum

Abstract

Yu et al. (2017, https://doi.org/10.1002/2017GL075772) reported that the annual mean sea surface salinity maximum (SSS‐max) in the North Atlantic expanded northward by 0.35 ± 0.11° per decade over the 34‐year data record (1979–2012). The expansion shifted and expanded the ventilation zone northward and increased the production of the Subtropical Underwater (STUW). As a result, the STUW became deeper, thicker, and saltier. In this study, the seasonal characteristics of the poleward expansion of the North Atlantic SSS‐max and their effects on the STUW are examined. The results show that the SSS‐max expansion occurred primarily during boreal spring (April, May, and June) and expanded northward by 0.43 ± 0.21° per decade over the 34‐year period. The annual volume of the STUW increased by 0.21 ± 0.09 1014 m3 per decade over the same period, and the spring (April, May, and June) volume increased by 0.31 ± 0.02 1014 m3 per decade (a relative increase of 48 ± 1%). The characteristics of the decadal changes in STUW were attributable to the increased subduction rate associated with the northward expansion of the SSS‐max. The annual subduction rate increased by 0.29 ± 0.07 Sv per decade over the 34 years, and the greatest increase of 1.73 ± 0.61 Sv per decade occurred in April. The change in subduction associated with the expansion of the SSS‐max appeared to be consistent with the Atlantic Multidecadal Oscillation.