Abstract The alkenone‐based U37K′ ${\mathrm{U}}{37}^{\mathrm{K}\prime }$ index is widely used to reconstruct sea surface temperature (SST), including past SST variability associated with the East Asian Winter Monsoon (EAWM). However, its ability to record and preserve seasonal signals remains uncertain. Here, we present alkenone flux and U37K′ ${\mathrm{U}}{37}^{\mathrm{K}\prime }$ temperature data from two sediment traps in the oligotrophic northern South China Sea (SCS). The U37K′ ${\mathrm{U}}{37}^{\mathrm{K}\prime }$ records show dampened seasonal variability and temporal lags compared to the weekly SST record, resembling time‐averaged SST over 20–40 weeks, which indicates long residence times of alkenones. High U37K′ ${\mathrm{U}}{37}^{\mathrm{K}\prime }$ temperatures during winter suggest that the winter flux contains material produced outside of winter and/or transported from adjacent continental shelf. Consequently, flux‐weighted U37K′ ${\mathrm{U}}{37}^{\mathrm{K}\prime }$ temperatures align closely with annual mean SSTs, despite elevated alkenone flux during the EAWM months. These findings suggest that U37K′ ${\mathrm{U}}{37}^{\mathrm{K}\prime }$‐based reconstructions in the northern SCS likely reflect annual mean condition rather than EAWM variability.

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