Causes of the tropical mean climate pattern biases: water vapor shortwave absorptivity and Southern Ocean albedo

• Main Author: Kim, Hanjun
• Other Authors: Kang, Sarah M.
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Ulsan National Institute of Science and Technology 2022
• Subjects: Climate model uncertainty; tropical precipitation; Energetics Framework; Southern Ocean cloud bias; atmospheric shortwave absorption; double ITCZ problem; Pacific; Southern Ocean
• Online Access: https://scholarworks.unist.ac.kr/handle/201301/59374; http://unist.dcollection.net/common/orgView/200000642632

Department of Urban and Environmental Engineering (Environmental Science and Engineering) The majority of the climate models erroneously simulate insufficient precipitation near the equator and excessive precipitation over the southern tropics. These tropical mean state biases have been persistent for three decades, limiting the skill of climate predictions and projections. In this thesis, I examine how the tropical mean climate pattern is influenced by the shortwave radiation biases. I start by characterizing the inter-model spread in tropical zonal-mean precipitation pattern via Empirical Orthogonal Function (EOF) analysis, which divides the tropical precipitation pattern into hemispherically symmetric and anti-symmetric components. The symmetric component, which dominates the total inter-model spread, is tightly linked to the seasonal amplitude of the maximum precipitation position. The Energetics Framework relates the symmetric component to the inter-model spread in equatorial cold tongue strength and atmospheric shortwave absorption (SWA), where the latter is newly highlighted in this thesis. By contrast, the anti-symmetric component is partially attributed to the Southern Ocean warm bias in the literature but its importance is being debated. Therefore, I examine the role of SWA and Southern Ocean albedo on the tropical mean climate pattern biases through targeted modeling experiments. I first find that the inter-model spread in the climatological SWA is primarily caused by the inter-model differences in water vapor shortwave (SW) absorptivity. To elucidate the role of SWA on the tropical mean climate pattern, the climate model experiments are performed in which the extent to water vapor absorbs SW is altered. Enhancing the water vapor SW absorptivity leads to global-mean precipitation reduction and a La Ni??a-like cooling over the tropical Pacific. A La Ni??a-like cooling pattern is ascribed to the zonal asymmetry in climatological evaporative damping efficiency and the low cloud enhancement over the ...