Circulation Patterns Linked to the Positive Sub-Tropical Indian Ocean Dipole

Abstract

The positive phase of the subtropical Indian Ocean dipole (SIOD) is one of the climatic modes in the subtropical southern Indian Ocean that influences the austral summer inter-annual rainfall variability in parts of southern Africa. This paper examines austral summer rain-bearing circulation types (CTs) in Africa south of the equator that are related to the positive SIOD and the dynamics through which specific rainfall regions in southern Africa can be influenced by this relationship. Four austral summer rain-bearing CTs were obtained. Among the four CTs, the CT that featured (i) enhanced cyclonic activity in the southwest Indian Ocean; (ii) positive widespread rainfall anomaly in the southwest Indian Ocean; and (iii) low-level convergence of moisture fluxes from the tropical South Atlantic Ocean, tropical Indian Ocean, and the southwest Indian Ocean, over the south-central landmass of Africa, was found to be related to the positive SIOD climatic mode. The relationship also implies that positive SIOD can be expected to increase the amplitude and frequency of occurrence of the aforementioned CT. The linkage between the CT related to the positive SIOD and austral summer homogeneous regions of rainfall anomalies in Africa south of the equator showed that it is the principal CT that is related to the inter-annual rainfall variability of the south-central regions of Africa, where the SIOD is already known to significantly influence its rainfall variability. Hence, through the large-scale patterns of atmospheric circulation associated with the CT, the SIOD can influence the spatial distribution and intensity of rainfall over the preferred landmass through enhanced moisture convergence.

摘 要

副热带印度洋偶极子(SIOD)的正位相是影响南半球夏季南非部分地区降水年际变率的副热带南印度洋气候模态之一. 本文研究了与SIOD正位相相关的南半球夏季赤道以南非洲地区的降水环流型 (CT) 及其影响机制. 共得出四种南半球夏季降水 CT, 其中三种与 SIOD 正位相相关, 其特征分别表现为 (i) 西南印度洋气旋性活动增强, (ii) 西南印度洋大范围的正降水异常, 以及 (iii) 来自热带南大西洋, 热带印度洋和西南印度洋的水汽在非洲中南部大陆上空低层辐合. 这种联系还意味着 SIOD 正位相被认为可以提高上述 CT 的强度和发生频率. 与 SIOD 正位相相关的 CT 和南半球夏季赤道以南非洲地区的降水异常均质区之间的联系表明, 这是与非洲中南部地区降水年际变率相关的最主要的 CT, 而在非洲中南部地区 SIOD 被广泛认识到它显著影响降水变率. 因此, SIOD 可以通过与 CT 联系的大尺度大气环流模态以及增强的水汽辐合从而影响大陆上空降水的空间分布和强度.