北太平洋风暴轴“深冬抑制”现象的能量分析

An energy analysis of midwinter suppression of the North Pacific storm track

采用欧洲中期天气预报中心逐日再分析资料(ERA-40),从局地能量变化方程出发,通过分析北太平洋风暴轴区域对流层不同层次局地能量的季节演变过程,对风暴轴区域各能量项在"深冬抑制"现象中的作用进行了深入探讨。结果表明,天气尺度扰动动能的季节变化可以很好地反映北太平洋风暴轴的"深冬抑制"现象,并且该现象在对流层上层最为显著,其发生概率约为80%,其中20世纪70年代中后期到80年代前期抑制最强。从同期各能量项的变化来看,扰动动能的变化主要受斜压能量转换项、涡动非地转位势通量的散度项和正压能量转换项的影响。在深冬季节,由于消耗扰动动能的正压能量转换项虽有些微弱减少从而使得扰动动能有所增加,但为风暴轴提供扰动动能的斜压能量转换项和涡动非地转位势通量的散度项减少的幅度却更大,因而总的效果是扰动动能大为减小,这可能是造成北太平洋风暴轴"深冬抑制"现象的直接原因。

Based on the daily reanalysis data of European Center for Medium Weather Forecast （ERA- 40）, by using local energy change equations to calculate the seasonal variation of the local energy in different tropospheric layers over the North Pacific storm track, the role of internal dynamic mechanism in the maintainance of the track is discussed in detail. Results show that the seasonal variation of eddy kinetic energy can well reflect the phenomenon of midwinter suppression of the North Pacific storm track, which, with an occurrence probability of nearly 80%, is most prominent on the upper levels of the troposphere and was the strongest through the mid and late 1970s to the early 1980s. As viewed from the simultaneous variation of each energy terms averaged over the track, by adding up the stronger loss resulted from the larger decrease of the baroclinic energy conversion and eddy ageostrophic geopotential flux and the weaker gain resulted from the smaller decrease of the barotropic energy conversion, the re- markable net loss of the eddy kinetic energy during midwinter is possibly the direct reason for midwin- ter suppression of the North Pacific storm track.