Seasonal Turnabout of the Air-Sea Correlation in Mid-latitudes and Its Effects on the Baiu Frontal Activity

In mid-latitudes, a shallow mixed layer is formed on the seasonal thermocline from spring to summer. Since the mixed layer depth (MLD) represents thermal inertia of the upper ocean, the development of shallow mixed layer alternates the polarity of air-sea correlation seasonally. When the MLD is deep (winter), the upper ocean has a potential to force atmospheric variations, while, when the MLD is shallow (summer), atmospheric variations easily modify the thermal condition in the upper ocean. Seasonal anomalies of Baiu precipitation tend to reverse the sign from May to June in the western North Pacific. The turnabout is led by the development of seasonal shallow mixed layer along 20N and to the north. The positive sea surface temperature anomalies (SSTAs) in earlier months and surrounding regions where MLD is deep form the low-level convergence on regions with the shallow mixed layer, resulting in the stronger-(weaker-)than- normal frontal activity in May, which leaves the negative (positive) SSTAs under it. The negative (positive) SSTAs then lead to weak (strong) stability and decrease (increase) Baiu precipitation in June. In the following months, the weaker (stronger) Baiu activity yields positive (negative) SSTAs in the shallow mixed layer. Thus, the seasonal anomalies of Baiu precipitation reverse the sign from May to June.