Enhanced warming of the subtropical mode water in the North Pacific and North Atlantic

Yearly time series of NPSTMW temperature (red).
TitleEnhanced warming of the subtropical mode water in the North Pacific and North Atlantic
Publication TypeJournal Article
Year of Publication2017
AuthorsSugimoto S., Hanawa K., Watanabe T., Suga T., Xie SP
JournalNature Climate Change
Date Published2017/09
Type of ArticleArticle
ISBN Number1758-678X
Accession NumberWOS:000408769500017
Keywordsdataset; impacts; ocean; oscillation; part i; sea; variability

Over the past six decades, the subtropical surface ocean has warmed at rates close to those of global mean surface ocean temperature(1) except in western boundary current regions where the surface warming is locally enhanced by a factor of two(2). Changes in the subsurface ocean, however, remain unclear because of lack of data. Compiling historical temperature measurements-some available for the first time-here we show that the subtropical mode water has warmed over the past six decades in both the North Pacific and North Atlantic. The rate of the warming is twice as large in the mode waters than at the surface. Subtropical mode waters are important water masses of vertically uniform temperature that are a few hundred metres thick and distributed widely in the main thermocline of the subtropical oceans(3). The enhanced warming of subtropical mode waters can be traced back to the surface warming in the formation regions along the western boundary current extensions. Furthermore, we detect increased temperature stratification and decreased dissolved oxygen in the subtropical mode waters. The latter change has clear implications for predicting biogeochemical responses to climate warming.

Short TitleNat. Clim. Chang.

We conclude that the North Pacific subtropical mode water (NPSTWM) and North Atlantic subtropical mode water (NASTMW) have warmed over the past six decades, at twice the rate of the surface warming averaged over the broad subtropical and global ocean. The enhanced warming of NPSTMW and NASTMW was due to the marked wintertime surface warming in the western boundary current extensions where the subtropical mode waters form. The enhanced warming of the subtropical mode waters in both the North Pacific and North Atlantic suggests that it is a robust feature of global warming, a notion climate model simulations support. Our study showed that the enhanced warming of these important water masses is already detectable from observations.

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