Understanding the role of ocean dynamics in climate variability

Abstract

The ocean plays a key role in regulating Earth's mean climate, both because of its massive heat capacity, but also its heat transport by slow-moving circulations and other dynamics. In principle, fluctuations in such ocean heat transport can influence the variability in the climate, by impacting the sea-surface temperature (SST) variability and in turn the atmospheric variability through surface heat exchange, but this is incompletely understood, particularly in the extratropics. The goal of this dissertation is to clarify the role of ocean dynamics in climate variability, first focusing on the role of ocean dynamics in SST variability across the global oceans (Chapters 1 and 2), and then on the impact of midlatitude ocean-driven SST anomalies on the atmospheric circulation (Chapter 3). In Chapter 1, the contributions of ocean dynamics to ocean-mixed layer temperature variance are quantified on monthly to multiannual timescales across the globe. To do so, two methods are used: 1) a method in which monthly ocean heat transport anomalies are estimated directly from a state-of-the-art ocean state estimate spanning 1992-2015; and 2) a method in which they are estimated indirectly using the energy budget of the mixed layer with monthly observations of SSTs and air-sea heat fluxes between 1980-2017. Consistent with previous studies, both methods indicate that ocean dynamics contribute notably to mixed layer temperature variance in western boundary current regions and tropical regions on monthly to interannual timescales. However, in contrast to previous studies, the results also suggest that ocean dynamics reduce the variance of Northern Hemisphere mixed layer temperatures on timescales longer than a few years. In Chapter 2, the role of ocean dynamics in midlatitude SST variability is further understood using Hasselmann's model of climate variability, wherein midlatitude SST anomalies are driven entirely by atmospheric processes. Motivated by the results of Chapter 1, here Hasselmann's climate model is extended to include the forcing and damping of SST variability by ocean processes, which are estimated indirectly from monthly observations. It is found that the classical Hasselmann model driven only by observed surface heat fluxes generally produces midlatitude SST power spectra that are too red compared to observations. Including ocean processes in the model reduces this discrepancy by decreasing the low-frequency SST variance and increasing the high-frequency SST variance, leading to a whitening of the midlatitude SST spectra. This happens because ocean forcing increases the midlatitude SST variance across many timescales but is outweighed by ocean damping at timescales > 2 years, particularly away from the western boundary currents. It is also shown that the whitening of midlatitude SST variability by ocean dynamical processes operates in NCAR's Community Earth System Model (CESM). In the final chapter, the atmospheric circulation response to midlatitude ocean-forced SST anomalies is explored. In particular, the extended Hasselmann model is used to isolate the oceanic and atmospheric-forced components of the observed SST variability in the Kuroshio-Oyashio Extension (KOE) region. The associated atmospheric circulation anomalies are diagnosed by lagged-regression of monthly sea-level pressure (SLP) anomalies onto the KOE-averaged SST anomalies, and their oceanic and atmospheric-forced components. Consistent with previous studies, a large-scale SLP pattern is found to lag the KOE SST anomalies by one month. Here it is shown that this pattern is linked to the oceanic-forced component of the SST variability, but not the atmospheric-forced component. The results hence suggest that the midlatitude ocean dynamical processes in the North Pacific influence the variability of the large-scale atmospheric circulation.