(PhD Thesis) Influence of the sea surface temperature pattern effect on the global top of the atmosphere energy budget and on global warming
Published in PhD Thesis - Université de Toulouse III - Paul Sabatier, 2023
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Abstract
Anthropogenic emissions of greenhouse gases are disrupting the Earth energy balance. As a consequence, more energy is received from the sun than what the Earth emits to space. The Earth thus stores energy and warms up. Global warming affects the various flows of water and energy in the climate, which in turn influence the top of atmosphere energy imbalance, through the radiative response. The Earth’s radiative response determines global climate change over time, as well as its amplitude at equilibrium. It depends on the global mean surface temperature, and on the magnitude of climate feedbacks. The latter are notably impacted by the pattern of sea surface temperature, specifically in the tropical Pacific which is associated with the tropical atmospheric circulation. Changes in the radiative response affect the Earth’s energy imbalance, and therefore the amount of energy stored in the climate system which eventually modifies global warming. In my thesis, I aim to understand and quantify the influence of the pattern of surface temperature on global warming through the changes in the radiative response. To this end, I formulate a new multilinear approximation of the radiative response, which isolates the contribution of surface temperature pattern. This approach is validated for moderate warming (below 5°C) using climate model simulations. The multilinear approximation of the radiative response is then used to determine the influence of the changing pattern of surface temperature on global warming. I show that, through its influence on the radiative response, the changing pattern of surface temperature affects global warming in the same way as a radiative forcing. In this sense, the climate system responds similarly to changing warming patterns as it does to a radiative forcing. Therefore, global warming can be written as the sum of the direct response to the radiative forcing and the additional response induced by changes in the pattern of surface temperature. The global surface temperature changes induced by the surface temperature patterns are quantified in simulations in which the atmospheric concentration of CO2 is increased by 1% per year for 150 years (1pctCO2), carried out with CMIP5 and CMIP6 climate models. The pattern of warming slightly damps global warming over the first century of simulations. In particular, the Transient Climate Response (TCR), reached at year 70, is reduced by 15 ± 9%. After 100 years, the influence of the pattern of warming evolves, and some models show an amplified global warming in the following decades. In 1pctCO2 simulations, the forced response dominates the signal. However, modes of internal variability of the coupled ocean-atmosphere dynamics, associated with change of surface temperature pattern, also influence the radiative response, and hence global warming. I show that high-frequency variability, for example associated with ENSO, is largely attenuated, and has little impact on global warming. In contrast, lower-frequency variability, notably associated with the PDO, is propagated by its radiative effect in global warming. Overall, this thesis provides a new analytical framework for quantifying the impact of the surface temperature pattern, whether anthropogenically forced or associated with internal climate variability, on the evolution of the Earth’s energy imbalance and on global surface warming.
Recommended citation: Robin Guillaume-Castel, "Influence of the sea surface temperature pattern effect on the global top of the atmosphere energy budget and on global warming." PhD Thesis, 2023.