Barotropic aspects of ITCZ breakdown
Barotropic aspects of ITCZ breakdown
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DOI：
10.1175/1520-0469(1997)054
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发表时间：
1997-01-15
影响因子：
3.1
通讯作者：
Schubert, WH
中科院分区：
文献类型：
--
作者：
Ferreira, RN;Schubert, WH
在卫星图像中，热带辐合带（ITCZ）有时会出现起伏，并分解成一系列热带扰动。热带气旋可能会在这些扰动中发展并向高纬度移动，使ITCZ得以改革。有人提出ITCZ破裂是由平均气流的正压和斜压不稳定性的对流修正形式引起的。一个不稳定的意思是如何可以产生的ITCZ对流在短短几天内。在这个意义上，ITCZ为它自身的不稳定和破裂创造了有利的条件。在浅水模式中，ITCZ是由一个指定的纬向拉长质量汇赤道附近模拟。质量汇产生的气旋性位涡（PV）异常，有一个反向的纬向PV梯度在其极侧。根据Ripa定理，在其PV的对角梯度中具有反转的流动可能在存在小扰动时变得不稳定。在模式模拟中，不稳定的PV带要么起伏并分裂成几个气旋，要么轴对称地分裂成一个大气旋，模式结果表明，ITCZ的分裂可能在产生热带风暴在时间上聚集并在ITCZ中心纬度的极向和现有热带风暴以东形成的观测趋势中起作用。其他实验表明，在中美洲海岸以西观测到的热带气旋生成频率较高，可能是由于该地区ITCZ的水平形态。在东太平洋，ITCZ是一条纬向拉长的对流线，其东侧的气候范围较宽。有人提出，轴对称的PV带产生这样的ITCZ是热带气旋生成的频率增加的原因就在西部的中美洲。最后，在这项研究中得到的结果点的纬向不对称固有的ITCZ在确定流的演变的重要性，这表明需要进一步研究这种效果。本文还讨论了强迫模拟在流动稳定性研究中的重要性。
In satellite images the ITCZ (intertropical convergence zone) is sometimes observed to undulate and break down into a series of tropical disturbances. Tropical cyclones may later develop within these disturbances and move into higher latitudes allowing the ITCZ to reform. It has been proposed that ITCZ breakdown results from a convectively modified form of combined barotropic and baroclinic instability of the mean flow. An unstable mean how can be produced by ITCZ convection in just a couple of days. In this sense, the ITCZ produces favorable conditions for its own instability and breakdown.In this study, a nonlinear shallow water model on the sphere is used to simulate barotropic aspects of ITCZ breakdown. In the shallow-water model, the ITCZ is simulated by a prescribed zonally elongated mass sink near the equator. The mass sink produces a cyclonic potential vorticity (PV) anomaly that has a reversed meridional PV gradient on its poleward side. According to the Ripa theorem, a flow that has a reversal in its meridional gradient of PV may become unstable in the presence of small disturbances. In the model simulations the unstable PV strip either undulates and breaks down into several cyclones or axisymmetrizes into one large cyclone.The model results suggest that ITCZ breakdown may play a role in producing the observed tendencies for tropical storms to cluster in time and form poleward of the central latitude of the ITCZ and to the east of existing tropical storms. Additional experiments indicate that the observed higher frequency of tropical cyclogenesis just west of the Central American coast may be due to the horizontal morphology of the ITCZ in that region. In the eastern Pacific, the ITCZ is a zonally elongated line of convection that is climatologically wider on its eastern side. It is proposed that axisymmetrization of the PV strip produced by such an ITCZ is the cause of the increased frequency of tropical cyclogenesis just west of Central America.Finally, the results obtained in this study point to the importance of zonal asymmetries inherent to the ITCZ in determining the flow evolution, suggesting the need for further studies of this effect. The importance of using forced simulations in the study of flow stability is also discussed.