Warming of newly-subducted subtropical mode water in the North Pacific was suppressed from 1999 to 2017

Abstract

Subtropical mode waters are important water masses influencing the regional and global climate. By tracing water masses from the mixed layer into the deeper ocean in late winter, we identified the newly-subducted North Pacific Subtropical Mode Water (NPSTMW) and therefore investigated its temperature changes. The temperature of newly-subducted NPSTMW showed significant warming during 1981-1999, but the warming was suppressed during 1999-2017. The newly-subducted NPSTMW temperature generally agreed with the March-April mixed layer temperature in the subduction region, but was half a degree lower. A mixed layer heat budget analysis indicates that the suppressed warming was caused by the combination of reduced warming effects of shortwave radiation due to the deepened mixed layer, strengthening of sea surface latent heat loss due to the warmer sea surface and enhanced cooling effects of vertical entrainment due to the strengthened subsurface stratification in the subduction region. Our results imply that the oceanic response to global warming can exert a cooling effect on the newly-subducted NPSTMW through these three mechanisms. The shift in trends of the newly-subducted NPSTMW temperature has important implications for the climate variabilities and the marine ecosystem in the North Pacific.