Long-term changes in the seasonal cycle of sea surface temperature (SST) can have far-reaching ecological and societal implications. Previous studies suggest that the amplitude of the SST seasonal cycle has intensified globally. Yet, how these changes vary regionally and whether the timing and lengths of seasons have shifted remain poorly understood. Using high-resolution satellite observations over the past four decades, we find that the amplitude of the SST seasonal cycle in the western North Pacific has intensified in the subpolar western Pacific and the East China Sea, while weakening along the Kuroshio region. Preliminary analyses show that this intensification is not just confined to the surface and mixed layer, but also appears to extend to the subsurface in the upper ocean. To examine changes in the phase of the SST seasonal cycle, we removed the linear warming trend from the time series at each location, and the results reveal a prolonged winter phase in many parts of the western North Pacific, accompanied by a delayed spring transition. Reanalysis products further suggest that changes in mixed-layer depth, in addition to surface heat flux forcing, play an important role in driving these regional contrasts, with potential consequences for ecosystems and fisheries across the basin.
 

Seasonal cycle,western north pacific