Oscillations of the intertropical convergence zone and the genesis of easterly waves. Part I: diagnostics and theory
Oscillations of the intertropical convergence zone and the genesis of easterly waves. Part I: diagnostics and theory
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DOI：
10.1007/s00382-009-0584-x
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发表时间：
2008-07
期刊：
影响因子：
4.6
通讯作者：
V. Toma;P. Webster
中科院分区：
文献类型：
--
作者：
V. Toma;P. Webster
通过分析资料和简单的理论，我们研究了热带辐合带（ITCZ）的平均和瞬态。我们集中在热带东太平洋注意到，在这个地区存在一个发达的平均ITCZ。此外，这是一个区域，在文献中有相当多的讨论，东风波是否在原地发展或从大西洋向西传播。该地区是典型的热带地区，有一个强大的跨赤道气压梯度（CEPG）：平均对流远离赤道，但位于赤道的最高海表温度（SST）和最低海平面气压（MSLP）。再往西，在日界线附近，对流较弱，且与SST和MSLP极值共存。有人认为，在显着的CEPG，近赤道热带系统是惯性不稳定的地区，一个给定的CEPG的不稳定性的纠正决定的气候ITCZ的位置和强度。使用简单的理论参数，我们开发了一个表达式的ITCZ的平均纬度作为一个功能的CEPG。我们注意到，在一天一天的基础上，ITCZ是高度瞬态的变化发生在3-8天的时间尺度。振幅约为平均ITCZ一半的瞬变，从近赤道南半球向北传播为异常纬向振荡，最终放大平均ITCZ附近的对流。有人认为，在这些强烈的CEPG的平均ITCZ是持续惯性不稳定的反气旋涡度平流横跨赤道，导致创建一个振荡的发散-收敛偶极子。低层辐合产生对流，涡管伸展产生气旋性涡度，抵消反气旋性涡度的北向平流。在一个周期中，10ºN附近对流层中层加热以平均对流纬度的惯性频率在6 - 12 K/天之间振荡。因此，存在着一个异常的和浅，振荡的纬向环流的大小约50%的平均ITCZ与稳定状态的反气旋涡度的产生后。此外，有人认为，ITCZ的不稳定性直接与在原地发展的东风波发生的惯性周期的平均ITCZ的纬度。在更远的东部地区，动力序列和东风波的成因是不存在的，那里的CEPG是非常弱或完全没有。在同伴研究（第二部分）中，数值实验进行了测试在本研究中提出的假设。
We examine the mean and transient state of the intertropical convergence zone (ITCZ) by analyzing data and using simple theory. We concentrate on the tropical eastern Pacific Ocean noting that there exists in this region a well-developed mean ITCZ. Furthermore, it is a region where there has been considerable discussion in the literature of whether easterly waves develop in situ or propagate westwards from the Atlantic Ocean. The region is typical of tropical regions where there is a strong cross-equatorial pressure gradient (CEPG): mean convection well removed from the equator but located equatorward of the maximum sea-surface temperature (SST) and minimum sea level pressure (MSLP). Further to the west, near the dateline where the CEPG is much smaller, convection is weaker and collocated with SST and MSLP extrema. It is argued that in regions of significant CEPG that the near-equatorial tropical system is inertially unstable and that the rectification of the instability for a given CEPG determines the location and intensity of the climatological ITCZ. Using simple theoretical arguments, we develop an expression for the mean latitude of the ITCZ as a function of the CEPG. We note on a day-by-day basis that the ITCZ is highly transient state with variability occurring on 3–8 day time scales. Transients with amplitudes about half of the mean ITCZ, propagate northwards from the near-equatorial southern hemisphere as anomalous meridional oscillations, eventually amplifying convection in the vicinity of the mean ITCZ. It is argued that in these longitudes of strong CEPG the mean ITCZ is continually inertially unstable with advections of anticylonic vorticity across the equator resulting in the creation of an oscillating divergence–convergence doublet. The low-level convergence produces convection and the resultant vortex tube stretching generates cyclonic vorticity which counteracts the northward advection of anticylonic vorticity. During a cycle, the mid-troposphere heating near 10ºN oscillates between 6 and 12 K/day at the inertial frequency of the latitude of the mean convection. As a result, there exists an anomalous and shallower, oscillating meridional circulation with a magnitude about 50% of the mean ITCZ associated with the stable state following the generation of anticylonic vorticity. Further, it is argued that the instabilities of the ITCZ are directly associated with in situ development of easterly waves which occur with the inertial period of the latitude of the mean ITCZ. The dynamical sequences and the genesis of easterly waves are absent in the regions further to the east where the CEPG is much weaker or absent altogether. In a companion study (Part II), numerical experiments are conducted to test the hypothesis raised in the present study.