On the relationship between cloud feedbacks and circulation changes in the tropics

Thesis

Clouds and their radiative effects are strongly coupled to the large-scale circulation, and this coupling plays a fundamental role in the climate response to CO2 forcing. This thesis examines this interaction from two complementary perspectives: how circulation changes influence tropical cloud feedbacks inferred from observations, and how cloud-radiative changes drive circulation and hydrological responses in a warming climate.

Using top-of-atmosphere radiative flux observations and an atmospheric reanalysis, we estimate tropical cloud feedbacks based on trends from 1985 to 2020. Decomposing the feedbacks into dynamic and non-dynamic components reveals that regional dynamic feedbacks from narrowing and strengthening of ascent are large but, as in high- and low-resolution climate models, these dynamic feedbacks largely cancel in the tropical mean. This highlights the value of isolating non-dynamic feedback components when estimating feedbacks for individual cloud types. Feedbacks inferred from multi-decadal trends differ from those inferred from inter-annual variability, reflecting distinct physical drivers. As previous estimates of tropical cloud feedbacks relied on inter-annual variability, we provide a new trend-based estimate of the combined anvil area and albedo feedback.

Next, we use radiation-locking experiments with a global climate model to isolate how radiative changes in CO2, water vapour, and clouds shape the tropical precipitation–minus-evaporation, P−E, response to increased CO2. Cloud-radiative changes drive a robust wet-gets-drier, dry-gets-wetter pattern that offsets an opposing water vapour-driven response, leaving the overall P −E response closely resembling the comparatively weak CO2-driven pattern of drying in climatologically wet and dry regions. Comparing fixed-SST and slab-ocean simulations allows the separation of cloud-driven P−E changes into those occurring via the atmospheric/land pathway versus via the SST-mediated pathway. The wet-gets-drier, dry-gets-wetter response occurs via the atmospheric/land pathway: upper-tropospheric heating from rising high clouds increases static stability, weakens the overturning circulation, and reduces precipitation in wet regions and evaporation in dry regions. In contrast, the SST-mediated pathway drives poleward precipitation shifts – most notably over the Indian and West Pacific Oceans – reflecting spatial gradients in cloud-driven SST changes.

Authors: Van de Koot, E

• DOI: 10.5287/ora-ao8bnknv2
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English