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Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC San Diego Electronic Theses and Dissertations (2023)

This thesis advances the understanding of the global carbon cycle and atmospheric tracer transport by optimizing the use of CO2 and O2 measurements from global airborne campaigns and surface stations. The first three chapters provide toolboxes that leverage the use of airborne data in many aspects. Chapter 1 introduces a novel transformed isentropic coordinate system, Mθe, initially designed to organize airborne data, correct for meteorological variability, and calculate atmospheric inventories of trace gases. This system is versatile and has enabled the development of several toolboxes and models to assess large-scale atmospheric tracer transport and surface tracer flux rates. In Chapter 2, I present the first comprehensive application of Mθe coordinate, focusing on estimating atmospheric potential oxygen (APO ~ O2 + 1.1 CO2) and its seasonal air-sea fluxes, contributing valuable insights into oceanic carbon cycles, and benchmarking existing observation-based and modeled APO flux estimations. In chapter 3, I tackle the challenge of uncertainties in atmospheric transport models (ATMs), by developing two evaluative constraints based on atmospheric CO2 gradients across Mθe and parameterized diabatic mixing rates (tracer transport timescales across Mθe). These constraints lead to a more intuitive evaluation of ATMs, and more accurate descriptions of large-scale tracer transport, particularly over the extratropical Southern Ocean (SO). I also applied the diabatic mixing rates derived from reanalyses to improve seasonal air-sea CO2 flux estimations across different latitudes over the SO. In Chapter 4, I explore the role of large-scale CO2 transport in interpreting long-term CO2 variability at low-latitude stations like Mauna Loa (MLO). It identifies circulation changes as key contributors to variations in CO2 seasonal cycle amplitude at MLO and establishes correlations with climate modes like the Pacific Decadal Oscillation. The implications are significant for ecosystem studies, suggesting that using observed CO2 at low-latitude to constrain large-scale ecosystem changes requires a correct representation of large-scale CO2 transport.Collectively, this thesis represents a multifaceted approach to promote our understanding of the global carbon cycle and atmospheric tracer transport, by leveraging new data types and developing novel analytical tools. The findings support technical advancement in atmospheric science and have broad implications for carbon-climate feedback, climate variability, and global biogeochemical cycles.

    • Article
    • Peer Reviewed
    UC San Diego Previously Published Works (2022)
    Long-term measurements at the Mauna Loa Observatory (MLO) show that the CO2 seasonal cycle amplitude (SCA) increased from 1959 to 2019 at an overall rate of 0.22  ±  0.034 ppm decade-1 while also varying on interannual to decadal time scales. These SCA changes are a signature of changes in land ecological CO2 fluxes as well as shifting winds. Simulations with the TM3 tracer transport model and CO2 fluxes from the Jena CarboScope CO2 Inversion suggest that shifting winds alone have contributed to a decrease in SCA of -0.10  ±  0.022 ppm decade-1 from 1959 to 2019, partly offsetting the observed long-term SCA increase associated with enhanced ecosystem net primary production. According to these simulations and MIROC-ACTM simulations, the shorter-term variability of MLO SCA is nearly equally driven by varying ecological CO2 fluxes (49%) and varying winds (51%). We also show that the MLO SCA is strongly correlated with the Pacific Decadal Oscillation (PDO) due to varying winds, as well as with a closely related wind index (U-PDO). Since 1980, 44% of the wind-driven SCA decrease has been tied to a secular trend in the U-PDO, which is associated with a progressive weakening of westerly winds at 700 mbar over the central Pacific from 20°N to 40°N. Similar impacts of varying winds on the SCA are seen in simulations at other low-latitude Pacific stations, illustrating the difficulty of constraining trend and variability of land CO2 fluxes using observations from low latitudes due to the complexity of circulation changes.
      Cover page: Impact of Changing Winds on the Mauna Loa CO2 Seasonal Cycle in Relation to the Pacific Decadal Oscillation
      • Article
      • Peer Reviewed
      UC San Diego Previously Published Works (2024)
      We present improved estimates of air-sea CO2 exchange over three latitude bands of the Southern Ocean using atmospheric CO2 measurements from global airborne campaigns and an atmospheric 4-box inverse model based on a mass-indexed isentropic coordinate (Mθe). These flux estimates show two features not clearly resolved in previous estimates based on inverting surface CO2 measurements: a weak winter-time outgassing in the polar region and a sharp phase transition of the seasonal flux cycles between polar/subpolar and subtropical regions. The estimates suggest much stronger summer-time uptake in the polar/subpolar regions than estimates derived through neural-network interpolation of pCO2 data obtained with profiling floats but somewhat weaker uptake than a recent study by Long et al. [Science 374, 1275-1280 (2021)], who used the same airborne data and multiple atmospheric transport models (ATMs) to constrain surface fluxes. Our study also uses moist static energy (MSE) budgets from reanalyses to show that most ATMs tend to have excessive diabatic mixing (transport across moist isentrope, θe, or Mθe surfaces) at high southern latitudes in the austral summer, which leads to biases in estimates of air-sea CO2 exchange. Furthermore, we show that the MSE-based constraint is consistent with an independent constraint on atmospheric mixing based on combining airborne and surface CO2 observations.
        Cover page: Improved atmospheric constraints on Southern Ocean CO2 exchange.
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