Cooling trend over the past 4 centuries in northeastern Hong Kong waters as revealed by alkenone-derived SST records

• Three ∼400 years-long SST records reconstructed in HK coastal waters.
• A general cooling trend evident, opposite to the global temperature rise.
• The cooling results from stronger upwelling induced by summer monsoon.

The climate history over the past few centuries is important to be used to assess how regional climate responds to global forcing. Here we report three high-resolution alkenone-based sea surface temperature (SST) records over the past 4 centuries from three sediment cores collected in the Mirs Bay, northeastern Hong Kong. All three SST records consistently show a general cooling trend toward the present, with most of cooling occurring over the last century. Alkenone-derived SST values stayed around 26.5–27 °C at the three sites prior to 1900s and decreased into the range of 25–26 °C. The magnitude of cooling approximately from the Little Ice Age (LIA) to present tends to be dampened from ∼2 °C nearshore to ∼1 °C offshore. The cooling trend, as identified in all three SST records, is thus opposite to the global temperature rise over the last century. Assisted with modern observations, we interpret that the alkenone-derived SST reflects increasing upwelling in the Mirs Bay, which likely results from the strengthened East Asian summer monsoon, in the context of global warming over the last century.