Preconditioning of the wintertime mixed layer at the Kuroshio Extension Observatory

Summertime surface heat flux and upper ocean state in 2004, 2005, and 2006 obtained from the Kuroshio Extension Observatory (KEO) buoy were investigated, focusing on the summertime preconditioning of the following winter's mixed layer. Summertime net shortwave radiation at the surface shows large year‐to‐year variations that resulted in anomalous heating in 2005 and anomalous cooling in 2006. Covariation of the surface heat flux and upper ocean stratification was found and suggests that year‐to‐year variations of summertime heat flux induce corresponding changes in the near surface stratification. Cold core rings, observed in 2006, tend to intensify both the near surface (<100 m depth) density stratification and the density stratification below the seasonal thermocline (>100 m depth). Lateral and vertical heat fluxes evaluated from the imbalance between the observed heat storage rate and the net heat flux and entrainment also have a significant role in determination of upper ocean stratification and can intensify year‐to‐year variation of the mixed layer. The physical mechanism that determines the precondition of the next winter mixed layer can change each year. In 2005, near surface stratification induced by anomalous summertime heating has a dominant role compared to deeper stratification. On the other hand, in 2006, the much deeper stratification below the seasonal thermocline (>100 m depth) associated with cold core rings contributes to make the maximum vertical density stratification.