The Physical Mechanism of the Wintertime Turbulent Heat Flux Variability over the Northwestern Pacific

The physical mechanism of the wintertime turbulent (latent and sensible) heat flux variability over the northwestern Pacific is investigated. For this purpose, CSEOF analysis is conducted on various physical variables derived from the daily NCEP reanalysis dataset over the period of 1948-2011. Turbulent heat flux anomaly over the northwestern Pacific is inversely proportional to anomalous thermal advection. Thus, increased thermal advection decreases turbulent heat flux released from the surface of the ocean and vice versa; correlation between the two is higher than 0.9 over the Northwestern Pacific. Anomalous thermal advection, in turn, is determined primarily by the strength of the Aleutian Low, which essentially dictates the direction of meridional wind anomaly over the northwestern Pacific. On top of the lower-frequency variability of the turbulent heat flux over the northwestern Pacific, higher-frequency variability associated with Rossby waves is clearly observed, which gives the characteristic eastward propagation of the train of alternating turbulent heat flux anomalies. CSEOF analysis also identifies an interannual mode, of which the PC time series exhibits interannual variability on top of a conspicuous linear trend. The corresponding loading vector shows that the release of turbulent heat flux over the marginal seas (East/Japan Sea) in the northwestern Pacific has been decreasing due to the increasing thermal advection over the marginal seas. This is one of plausible reasons why the rate of warming in the East/Japan Sea is higher than most other places of the world oceans. The decreased turbulent heat flux also reduces the variance of storm tracks and shifts them slightly northward.