From Temperature Extremes to Ocean Circulation: Insights from the Transport of Tracers

Abstract

Tracers are ubiquitous in both the atmosphere and the ocean. This dissertation develops a tracer-transport-based perspective to provide a new quantitative toolbox and novel insights into two key problems in atmospheric and oceanic sciences. The first problem concerns atmospheric temperature extremes (Chapter 1). By treating temperature as a tracer governed by its transport equation, I propose a new framework to quantify how different physical processes control the probability distribution functions (PDFs) of temperature at any given location. Using this framework, we find that horizontal temperature advection is the dominant control on local temperature extremes, governing extreme events over the mid-to-high-latitude oceans and approximately 60 % of the global land area. We further use this framework to decompose the contributions of different eddy and mean components of advection to the global distribution of temperature standard deviation and skewness. In particular, we show that the eddy-mean interaction term, v′ · ∇ T̄, makes a significant contribution to skewness—a contribution that has been largely overlooked in previous studies. This term highlights the potential amplification of temperature skewness due to changes in the waveness of atmospheric circulation and polar amplification. Finally, we briefly discuss the broader implications of this framework: it can be extended to understand the spatial distribution of other key tracers in the atmosphere and ocean, such as pollutants. The second problem addresses the relationship between ocean circulation and the spatial distribution of ocean water age (Chapters 2 and 3). Starting from the age-tracer transport equation, I derive a conservation law for age: for any control volume, the total age flux across its boundary equals the volume of the control domain (Chapter 2). This conservation law allows us to infer diapycnal mixing and overturning circulation directly from the spatial distribution of age. Using a set of idealized single-basin MITgcm simulations, we demonstrate that the global overturning rate across different density levels can be quantitatively estimated from the age difference between upwelling and downwelling regions. We further show that this framework can be applied to regional ocean circulation. For example, by analyzing the zonal-mean vertical profile of age at the equator, we estimate the deep overturning rate in the North Pacific, yielding results consistent with previous studies. This age framework is also applied to investigate the physical controls on the ventilation of mid-depth North Pacific waters—the oldest waters in the modern ocean (Chapter 3). We find that diapycnal mixing is the dominant control on ventilation in this region, while isopycnal mixing and overturning circulation play secondary roles. Based on this finding, we propose a method to estimate basin-scale effective diapycnal diffusivity from the vertical profile of horizontally averaged age. Applying this method to observational radiocarbon data, we obtain a bottom-enhanced diffusivity profile with magnitudes on the order of 10⁻⁴ m² s⁻¹, consistent with previous estimates. However, our inferred diffusivities are somewhat larger than those from state-of-the-art bottom-up estimates, highlighting the uncertainty in basin-scale diffusivity and the need for further investigation. The three applications of the age framework—global overturning, deep North Pacific circulation, and mid-depth North Pacific ventilation—demonstrate the potential of the age framework to enhance our understanding and quantification of past and present ocean circulation.