Interannual variability of the summertime North Pacific subtropical high and its relation to atmospheric convection over the warm pool
Interannual variability of the summertime North Pacific subtropical high and its relation to atmospheric convection over the warm pool
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DOI：
10.2151/jmsj.79.771
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发表时间：
2001-06-01
影响因子：
3.1
通讯作者：
Lu, R
中科院分区：
文献类型：
--
作者：
Lu, R
利用美国国家环境预报中心/美国国家大气研究中心（NCEP/NCAR）再分析资料以及卫星观测的向外长波辐射（OLR）资料，我们研究了夏季北太平洋副热带高压（NPSH）的西伸和东缩。研究发现，NPSH在其西伸范围上在季节和年际时间尺度上都表现出很大的变异性。为了研究NPSH的年际变化，我们定义了一个NPSH指数，即6 - 7 - 8月（JJA）NPSH西边缘（110°E - 150°E，10°N - 30°N）平均的850 hPa位势高度距平。这个指数描述了NPSH逐年的纬向位移。基于这个NPSH指数的合成分析表明，NPSH的纬向位移与暖池上空的大气对流强度之间存在显著关系。一个与NPSH纬向移动密切相关的低层气旋性（反气旋性）异常出现在增强（减弱）的大气对流北侧，即涡度异常出现在散度异常北侧。从气候学角度来看，在第40候（7月15 - 19日）之后，NPSH迅速东缩。这种东缩与NPSH以及热带西太平洋暖池上空大气对流的向极移动密切相关。然而，NPSH和对流的这种季节变化在NPSH指数为正和为负的夏季之间表现出明显不同的特征。在NPSH指数为正的夏季，暖池上空的NPSH和对流没有表现出明显的季节演变。相比之下，在指数为负的夏季，它们在第40候之后表现出迅速的季节演变。最后，我们进行了涡度分析以解释年际时间尺度上散度和涡度异常之间的关系。分析表明，在对流层低层（925 hPa），相对涡度平流与拉伸相当，并且是环流异常相对于异常大气对流向北移动的原因。我们还讨论了与吉尔（1980）理论的差异。在对流层高层（200 hPa），在东亚和西北太平洋，平流比拉伸略小且符号相反，因此涡度异常的位置与拉伸异常的位置一致。
Using the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis data and satellite-observed outgoing long-wave radiation (OLR) data, we examined the westward extension and eastward contraction of the North Pacific subtropical high in summer (NPSH). It was found that the NPSH shows a great variability in its western extent, both on the seasonal and interannual time scales. In order to examine the interannual variations of NPSH, we defined a NPSH index as the June-July-August (JJA) mean geopotential height anomalies at 850 hPa averaged over the west edge (110 similar to 150 degreesE, 10 similar to 30 degreesN) of NPSH. This index describes the year-to-year zonal displacement of NPSH. Composite analysis based on this NPSH index showed that there is a significant relation between zonal displacement of NPSH and intensity of atmospheric convection over the warm pool. A low-level cyclonic (anticyclonic) anomaly that is closely associated with the zonal shift of NPSH appears north of enhanced (weakened) atmospheric convection, i.e., the vorticity anomaly is found north of the divergence one.Climatologically, the NPSH contracts eastward swiftly after pentad 40 (July 15 to 19). Such an eastward contraction is closely associated with the poleward shift of both NPSH and atmospheric convection over the tropical western Pacific warm pool. However, such seasonal variations of both NPSH and convection show distinct features between the summers with positive and negative NPSH indexes. During summers with positive NPSH index, NPSH and convection over the warm pool do not show an appreciable seasonal evolution. During summers with negative index, by contrast, they show a swift seasonal evolution after pentad 40.Finally, we performed a vorticity analysis to explain the relation between the divergence and vorticity anomalies on the interannual time scale. The analysis shows that in the lower troposphere (925 hPa), the advection of relative vorticity is comparable to the stretching and is responsible for the northward shift of the circulation anomaly relative to anomalous atmospheric convection. The difference from the theory of Gill (1980) is discussed. In the upper troposphere (200 hPa), the advection is slightly smaller than the stretching with opposite signs in East Asia and the western North Pacific, and thus the position of the vorticity anomaly is consistent with that of the stretching anomaly.