The role of shallow convection in tropical circulation: a simple analytic approach

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题名:	The role of shallow convection in tropical circulation: a simple analytic approach
作者:	鄧東坡;Pha, Dang Dong
贡献者:	大氣科學學系
关键词:	shallow convection;convective-quasi equilibrium
日期:	2022-01-10
上传时间:	2022-07-13 16:40:41 (UTC+8)
出版者:	國立中央大學
摘要:	摘要在此研究中，我們基於第一和第二斜壓模的對流準平衡(CQE)假設，提出了垂直濕靜能(MSE)平流的近似理論。熱帶地區的兩種主要垂直運動模式分別為深、淺對流，在此理論中皆能精確地被制定出來。且此新的近似理論能夠診斷大尺度垂直濕靜能平流，其在熱帶輻合帶(ITCZ)的相關係數為0.6。此理論表明，第一斜壓模為西太平洋輻合帶的主導項，而在東太平洋輻合帶地區中第二斜壓模反而較為重要。然而，CQE近似理論無法在ITCZ以外地區再現垂直濕靜能平流的特徵。一般來說，與垂直乾靜能平流項相比，第二斜壓的貢獻主要是來自垂直水氣平流項。總體而言，該模型理論提供了有用的工具以了解深、淺對流在氣候時間尺度上分別扮演的角色。奇異值分解(SVD)在本研究中被用以測試邊界條件（海面溫度-SST）和大氣穩定度（粗濕穩定度-M 和粗濕分層-Mq）之間的耦合關係。透過此研究發現，後者與東太平洋海溫異常(正距平)的聖嬰-南方震盪現象有強烈相關性。在聖嬰事件期間，西太平洋上空的對流以Bottom-heavy、負的淺對流結構為主。而在東太平洋上空則以Top-heavy、正的淺對流結構為主，且在此部分研究中也明顯展示出M與Mq的增加分別與第一、第二斜壓模有關。此外，SVD分解出的第二個模態顯示了全球暖化對垂直運動變化的影響。此結果表明，海表面溫度越暖的區域其Top-heavy、正的淺對流結構將會增加。 ;Abstract In this study, we present a theoretical approximation for the vertical moist static energy (MSE) advection based on convective quasi-equilibrium (CQE) assumption corresponding to the 1st and 2nd baroclinic mode. The two major vertical motion modes in the tropics are explicitly formulated, representing the role of deep and shallow convection. The novel approximation is capable of diagnosing the large-scale pattern of vertical MSE advection with a correlation of 0.6 for the intertropical convergence zones (ITCZs). The first baroclinic mode is dominant over the Western Pacific Convergence Zone, while the second baroclinic mode is indispensable on the Eastern Pacific Convergence Zone. Outside the ITCZ, the CQE approximation fails to reproduce the vertical MSE advection. In general, the contribution of the second baroclinic is mostly accounted by the vertical moisture advection compared to the vertical dry static energy (DSE) advection. Overall, the model provides a useful tool to understand the role of deep and shallow convection on climatological time scales. A singular value decomposition (SVD) is conducted to examine the coupled pattern between boundary conditions (sea surface temperature - SST) and atmospheric stability (gross moist stability - M and gross moist stratification - M_q). It is found that the latter is strongly correlated with El Niño-Southern Oscillation characterized by positive SST anomalies over the Eastern Pacific. During El Niño events, convection over the Western Pacific is dominated by bottom-heavy and negative shallow structures. On the eastern side, the increase in M associated with 1st mode and M_q associated with 2nd mode indicates a top-heavy and positive shallow convection. Furthermore, the second SVD modes show the impact of global warming on the vertical motion variation. The results imply that warmer SST regions tend to enhance top-heavy deep and positive shallow convection.
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