Theses Doctoral

Asymmetric and Non-monotonic Response of the Climate System to Idealized CO₂ Forcing

Mitevski, Ivan

In this thesis, I explore the climate system's response to symmetric abrupt and transient CO₂ forcing across a range of concentrations, from ⅛ ⨉ to 8⨉CO₂, relative to pre-industrial levels. I use two CMIP6 class models: the CESM Large Ensemble (CESM-LE) model configuration and the NASA Goddard Institute for Space Studies Model E2.1-G (GISS-E2.1-G). I use a hierarchy of (1) fully coupled atmosphere-ocean-sea-ice-land, (2) slab ocean, and (3) prescribed sea surface temperature simulations to analyze and support the findings.

First, I find an asymmetric response in global mean surface air temperature (𝚫𝜯_s) and effective climate sensitivity (EffCS) between colder and warmer experiments. The 𝚫𝜯_s response at 8⨉CO₂ is more than a third larger than the corresponding cooling at ⅛⨉CO₂. I attribute this assymetry primarily due to the non-logarithmic CO₂ forcing, not to changes in the radiative feedbacks.

Second, I identify a non-monotonic response of EffCS in the warmer scenarios, with a minimum occurring at 4⨉CO₂ (3⨉CO₂) in CESM-LE (GISS-E2.1-G). This minimum in the warming simulations is associated with a non-monotonicity in the radiative feedback. Similar non-monotonic responses in Northern Hemisphere sea-ice, precipitation, the latitude of zero precipitation-minus-evaporation, and the strength of the Hadley cell are also identified. Comparing the climate response over the same CO₂ range between fully coupled and slab-ocean versions of the same models, I demonstrate that the climate system’s non-monotonic response is linked to changes in ocean dynamics, associated with a collapse of the Atlantic Meridional Overturning Circulation (AMOC).

Third, to establish the significance of North Atlantic cooling in driving the non-monotonic changes in the radiative feedback, I conducted additional atmosphere-only (AMIP) simulations using the same models but with prescribed sea surface temperatures (SSTs) restricted to different regions. Through these simulations, I uncovered that the minimum EffCS value, characterized by notably negative radiative feedbacks, primarily originates from relative cooling of the sea surface temperature (SST) in the tropical and subtropical North Atlantic. This cooling of SSTs contributes to an increase in low-level cloud content in the eastern region of the North Atlantic, subsequently leading to a pronounced negative (stabilizing) feedback response.

Furthermore, I investigated the state dependence of the effective radiative forcing (ERF) from 1/16 ⨉ to 16⨉CO₂. I found that ERF increases with CO₂ concentration due to the increase in Instantaneous Radiative Forcing (IRF). Specifically, the IRF increases at higher CO₂ values primarily due to stronger stratospheric cooling induced by CO₂ forcing. On the other hand, the radiative adjustments counteract the IRF increase, causing the ERF to rise at a slower pace compared to the corresponding increase in IRF induced by higher CO₂ concentrations.

Lastly, I studied the winter storm tracks in the Southern Hemisphere, focusing on experiments up to 8⨉CO₂. Through this analysis, I identified a non-linear response in the low latitude storm tracks. It is projected that the storm tracks will experience an intensification by the end of the century. However, my findings reveal that this intensification does not scale linearly with CO₂ forcing. In fact, the storm tracks shift poleward, including a reduction of the storm tracks at low-mid latitudes and intensification at mid-high latitudes.

Geographic Areas


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More About This Work

Academic Units
Applied Physics and Applied Mathematics
Thesis Advisors
Polvani, Lorenzo M.
Orbe, Clara
Ph.D., Columbia University
Published Here
August 23, 2023