A role of the yellow and east China Seas in the development of extratropical cyclones in winter

To investigate whether the relatively warm Yellow and East China Seas play an active role in the deepening of extratropical cyclones over East Asia during winter, surface wind vectors downloaded from the Quick Scatterometer (QuikSCAT) website are used to compute the standard deviation of surface vorticity at 1/4°resolution. In addition, a regional numerical atmospheric model is adopted to find atmospheric and/or oceanic conditions favorable for development of extratropical cyclones over thestudy area. These satellite-derived and modeled vorticity fields demonstrate that, on average, extratropical cyclone activity is moderate over the warm Yellow and East China Seas. This is because enhanced lower-level baroclinicity over these ocean areas is transferred as far as the shelf break of the East China Sea by strong northwesterly surface winds. Based on the numericalmodel results, the weak northwesterly surface wind condition is required for enhancing lowerlevel baroclinicity over the Yellow and East China Seas. This baroclinicity may contribute to enhancing cyclone development near Japan, with a simultaneous increase of lower-level baroclinicity over the Sea of Japan.