Publication Date

2018-11-15

Availability

Open access

Embargo Period

2018-11-15

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Meteorology and Physical Oceanography (Marine)

Date of Defense

2018-08-20

First Committee Member

Amy C. Clement

Second Committee Member

Benjamin Kirtman

Third Committee Member

Brian J. Soden

Fourth Committee Member

Brian Medeiros

Abstract

Contributions from dynamical ocean heat transport and cloud radiative feedbacks to internal variations in sea surface temperature (SST) are explored using various climate modeling configurations in conjunction with intermodel comparison. The experimental modeling configurations include those with ocean heat transport and/or cloud radiative feedbacks disabled, resulting in a different, freely-evolving climate. By comparing the variability between experiment and control simulations, it is determined that the influence from either ocean dynamics or cloud feedback depends on the region and timescale. To begin, global-scale decadal changes in temperature do not require dynamical ocean heat transport, but instead, low-frequency variations in global mean temperature (GMT) may arise from atmospheric noise interacting with the mixed layer of the ocean. This contradicts numerous studies suggesting that anomalous deep ocean heat uptake is responsible for decadal cooling in GMT that led to the global warming hiatus. On the other hand, it is well- known that El Nino Southern Oscillation (ENSO) is driven by ocean dynamics and may lead to decadal variations in global climate. We find that around half of model disagreement in the magnitude of decadal variability can be explained by model spread in ENSO magnitude, which suggests other processes are contributing to decadal variability in climate models, like cloud radiative feedbacks, for example. Cloud radiative feedbacks as a source of decadal variability is largely overlooked in the literature and could explain some of the low frequency SST variability produced in a climate model configuration without ocean dynamics. To understand cloud radiative feedbacks’ impact on SST variability, we implement cloud-locking in the National Center for Atmospheric Research (NCAR) Community Atmosphere Model, version 5 (CAM5). This method disables cloud radiative feedbacks by prescribing clouds in the radiation module, preventing them from interacting with their environment, but allows the climate to evolve freely and, therefore, enables analysis of the variability without cloud radiative feedbacks. There are many practical uses for cloud- locking, but this is one of the few studies that utilizes the tool for understanding the relationship between cloud radiative feedbacks and internal variability. SST variability changes in the tropics and subtropics due to cloud-locking are the focus of half of this dissertation. Disabling cloud radiative feedbacks produces the largest changes in global SST variability via ENSO. Without cloud radiative feedbacks, the periodicity of ENSO events shifts from 2-7 years to approximately 10 years. Damping by negative shortwave feedback, which aids in the termination of ENSO, is not active in the cloud-locking configuration, so each ENSO event continues to grow unabated until the ocean “discharges”. Furthermore, in the northeastern subtropical ocean basins, a region important for decadal modes of variability, we find that positive cloud radiative feedback enhances SST variability by up to 40%, but that the magnitude depends on the role of the ocean. In the Atlantic Ocean, dynamical ocean heat transport enhances positive SST anomalies, leaving a smaller role for positive cloud radiative feedback. In the Pacific Ocean, ocean heat transport damps positive SST anomalies, so positive cloud radiative feedback competes with heat flux from the ocean. Altogether, the results of this dissertation suggest that more attention should be paid to the role of cloud radiative feedbacks when examining mechanisms of variability. The climate is very sensitive to cloud radiative feedbacks in the model used in this study, and different models and configurations exhibit diverse sensitivities. The spatially varying contribution of ocean dynamics further complicates the importance of cloud feedbacks. This study motivates an intermodel comparison of cloud-locking experiments to better constrain the role of cloud radiative feedbacks on the climate system.

Keywords

Climate variability; cloud radiative feedbacks; ocean heat transport; sea surface temperature; clouds

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