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为什么索马里如此干旱?

大多数沙漠是由雨影效应、冷流、亚热带山脊或这些因素综合而成的。知道了这些，索马里沙漠是如何形成的呢?

值得注意的是，它是靠近赤道的沙漠气候，以潮湿而闻名。此外，它位于非洲东海岸，东纬0度至45度以内的东海岸几乎总是潮湿的。

我搜索并阅读了埃塞俄比亚高原造成的雨影效应。然而，信风吹向东方，这将使索马里更加潮湿。我读过关于季风效应的文章，这可以解释雨影效应……夏天的时候。然而，在冬天，风会把潮湿的空气带回索马里。因此，如果有什么不同的话，索马里应该属于热带地中海气候。不过，有报道称，全年都可能不下雨。

总之，我的问题是索马里沙漠如此干燥的原因，以及为什么它不是地中海气候。

回答

尽管索马里靠近赤道(这为全年降雨提供了充足的机会)，但它的干旱让大多数气象学家感到困惑。在下面的回答中，我们将尝试探索这一悖论背后的一些原因。有三种主要气流:刚果气流(刚果空气边界的一部分)，为该地区带来西风和西南风，以及东北季风和东南季风。这两种季风的问题是，水流与海岸平行。6月至9月季风(JJAS)的气候学图显示850风平行于海岸。风遇到了所谓的摩擦差，即陆地上的摩擦大于海洋表面的摩擦。假设低气压在陆地上(白天由于太阳辐射)，由此产生的摩擦差导致风在沿海地区发散。关于这篇参考文献(4)的详细信息解释了它背后的数学。[!下面的图显示了东非上空850 hPa的流函数，这是一个气候学图。 Positive values of the streamfunction in the northern hemisphere correspond to low level divergence(enhances subsidence) and negative values to low level wind convergence(enhances ascent). This low level divergence induces subsidence of the air and prevents precipitation. [![enter image description here][2]][2] The climatology of the lower level stream function is shown for the whole year but it does not change much if you take it for any of the two seasons (JJAS) or (NDJF) individually. There is only lower level wind divergence. [![enter image description here][3]][3] The contour lines are the isobars. I have made my version of Fig 2.29 in (3)(due to copyright) that shows off shore winds parallel to the shore and winds diverging onshore due to the frictional differential. This phenomenon of winds diverging at the coast is not unique to Sonalia but is present everwhere you have coastal deserts and winds flowing parallel to the shore. If the winds did approach the coastline either perpendicular or at an angle then there would be greater low level wind convergence (850 hPa level)as the air would "pile up" at the shore promoting ascent of the air and precipitation. That would be reflected in the climatological 850 hPa wind streamfunction values. The same off shore south easterly winds cause upwelling along the coast and bring in cooler SSTs. The phenomenon is reversed during the North East Monsoon (Oct - March) the NE winds flow parallel to the shore. Please note that the lower level wind divergence shown here is for large scale winds and Somalia may indeed get some rains due to a mesoscale phenomenon known as the [Sea Breeze front][4] but that depends on whether the coastline is convex or concave over a short distance. Somalia does get some rains due to the westerly flow due to some combination of mid latitude disturbances and tropical influences and these are known as the [Gu rains][5]. The Ethopian Highlands and the [East African Rift Valley][6] act as a barrier for the westerly moist laden flow (and the Somalian region is the leeward region) but some amount of moisture does slip by through the transition period in between the two seasons. Finally reference (2) also talks of the Pacific Warm Pool extending into t he Indian Ocean and the Walker Cell due to the Indian Monsoon westerlies rising over the East Indian Ocean and causing subsidence over the Arabian Sea. References : 1) Nicholson, S. E. (2011), Dryland Climatology, 516 pp., Cambridge Univ. Press, New York.[Dryland Climatology][7] 2) Nicholson, S. E. (2017), Climate and climatic variability of rainfall over eastern Africa, Rev. Geophys., 55,590–635,[East African climate and rainfall ][8] 3) Warner, T. T., 2004: Desert Meteorology. Cambridge University Press, Cambridge, 595 pp [Desert Meteorology][9] 4) Bryson, R. A., and P. M. Kuhn, 1961: Stress-differential induced divergence with application to littoral precipitation. Erdkunde, 15, 287–294.[Friction Induced Differential][10] [1]: https://i.stack.imgur.com/kO7Pw.gif [2]: https://i.stack.imgur.com/ihjQV.gif [3]: https://i.stack.imgur.com/lFwrV.jpg [4]: https://en.wikipedia.org/wiki/Sea_breeze [5]: https://en.wikipedia.org/wiki/Somali_Flash_Floods [6]: https://en.wikipedia.org/wiki/Great_Rift_Valley [7]: https://www.cambridge.org/core/books/dryland-climatology/AD650508A319ABA023FF3132D958F9F5 [8]: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016RG000544 [9]: https://www.cambridge.org/core/books/desert-meteorology/830636E147078337D24957845F6A6855 [10]: https://www.jstor.org/stable/25636783

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\ begingroup美元我错过了一个很好的答案。在对其他答案的推理不满意之后，我开始尝试在那个时候一起工作，但并没有想出一个一致的画面，但你的答案很好。如果尼克尔森博士是研究生院的教授，他应该更明白! \ endgroup美元
- - - - - -JeopardyTempest
2021年10月12日4:21
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\ begingroup美元谢谢你，我的朋友。回顾过去，我认为南加州也有与海岸平行的风。最终结果——下加利福尼亚沙漠? \ endgroup美元
- - - - - -gansub
2021年10月12日5:59

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