Cole Frederick Persch

Nitrogen and oxygen (δ ¹⁵ N and δ ¹⁸ O) stable isotopes of nitrate are critical for identifying and quantifying nitrogen sources and their associated chemistry in the environment. Current techniques for nitrate isotope analysis, such as bacterial denitrification or Cd-azide reduction, are labor-intensive. A recent study demonstrated that nitrate can be converted to N ₂ O for isotopic analysis using a simple one-step chemical reduction with titanium(III) chloride. However, challenges remain in accurately quantifying δ ¹⁸ O due to strong matrix effects. Moreover, both δ ¹⁵ N and δ ¹⁸ O values vary with the concentration of titanium used in the reduction solution. Here, we present a revised methodology for the Ti(III) reduction that employs the addition of potassium iodide to reduce the influence of solution matrix components on δ ¹⁸ O and identifies an optimized composition for the reduction solution that minimizes variation in both δ ¹⁵ N and δ ¹⁸ O. Results from our new methodology yield uncertainties of ± 0.2‰ for δ ¹⁵ N and ± 1.0‰ for δ ¹⁸ O, even in samples containing chloride, demonstrating that this approach can be applied to complex environmental samples. Compared to traditional methods, the one-step chemical conversion of nitrate to N ₂ O is faster, lower-cost, and easier to scale for routine measurements. Together, these improvements enhance the accessibility of high-precision nitrate isotope analysis and support broader applications in oceanography, hydrology, and atmospheric science.

Orbital precession modifies the intensity of the annual cycle at millennial time scales and is a major external driver of El Niño–Southern Oscillation (ENSO) variability in both proxy records and climate model simulations. We examine precession’s influence on ENSO through a subtropical pathway, the Pacific meridional mode (PMM), using a suite of NCAR Community Earth System Model, version 1.2 (CESM1.2), experiments that simulate five precessional extremes: perihelion at autumnal equinox (AE), winter solstice (WS), vernal equinox (VE), summer solstice (SS), and zero eccentricity (E0). We investigate mechanisms that may moderate the PMM’s influence on ENSO such as the strength of midlatitude stochastic forcing via the North Pacific Oscillation, changes in the climatological mean state, and the wind–evaporation–sea surface temperature (SST) (WES) feedback. We find that orbital precession strongly influences PMM variability, the PMM’s ability to trigger El Niño events, and ENSO diversity. Precessional extremes characterized by a more southerly intertropical convergence zone (ITCZ) and stronger trade winds (WS and AE) have more variable PMM behavior and a PMM that is more effective at triggering El Niño events, particularly central Pacific events. Precessional extremes characterized by a more northerly ITCZ and weaker trade winds (SS and VE) have reduced PMM variability and a PMM that is a less-reliable precursor to El Niño events. The PMM response to precession is driven by variations in surface wind fields that moderate the strength of WES feedback, the mechanism by which PMM anomalies grow and propagate. Understanding the sensitivity of ENSO to subtle shifts in the mean state contextualizes past variability and aids in anticipating future change. Significance Statement: Orbital precession alters the seasonal distribution of solar insolation around Earth and has vast impacts on the climate system including El Niño–Southern Oscillation (ENSO). Here, we examine how precession influences ENSO through a subtropical ENSO precursor: the Pacific meridional mode (PMM). We use climate models to test how the influence of warm PMM events on El Niño events may vary under extreme states of orbital precession. We find precession moderates the variability of the PMM, the ability of the PMM to trigger El Niño events, and the spatial diversity of El Niño events. These results help us to interpret past climatic changes and understand the sensitivity of the tropical Pacific to small variations in external forcing.

Understanding external drivers of the El-Nino Southern Oscillation (ENSO) is essential for predicting its future evolution. Orbital precession has been identified as a driver of ENSO variability through both proxy records and climate model simulations, yet the exact mechanics remain unclear. This orbital cycle moderates the seasonal timing of insolation relative to Earth's revolution around the Sun, thereby adjusting the magnitude of the seasonal cycle experienced by each hemisphere. Here, we analyze output from a suite of simulations in NCAR CESM 2.1.1 designed to analyze ENSO under different precessional extremes that significantly modify the meridional temperature gradients and the cold tongue seasonal cycle in the Pacific ocean. Variations in orbital precession have a strong impact on the magnitude, periodicity, and spatial expression of tropical Pacific variability. We find a critical role for both the North and South Pacific Meridional Modes (NPMM and SPMM) in explaining changes in ENSO and decadal variability by propagating subtropical anomalies to the equatorial Pacific along with a shift in the meridional structure of equatorial winds. As an example, when the perihelion of orbit occurs during boreal winter creating a dampened (strengthened) seasonal cycle in the Northern (Southern) Hemisphere, the SPMM becomes significantly more active while the NPMM weakens. This precessional state experiences a shift toward amplified decadal variability and a greater prevalence of Eastern El Nino events in comparison with the other orbital configurations tested. Understanding the precessional control of tropical variability via subtropical pathways may help explain developments that have occurred in the past, as well as future changes which may be observed due to shifts in meridional temperature gradients.

Orbital precession has been linked to glacial cycles and the atmospheric carbon dioxide (CO₂) concentration, yet the direct impact of precession on the carbon cycle is not well understood. We analyze output from an Earth system model configured under different orbital parameters to isolate the impact of precession on air‐sea CO₂ flux in the Southern Ocean—a component of the global carbon cycle that is thought to play a key role on past atmospheric CO₂ variations. Here, we demonstrate that periods of high precession are coincident with anomalous CO₂ outgassing from the Southern Ocean. Under high precession, we find a poleward shift in the southern westerly winds, enhanced Southern Ocean meridional overturning, and an increase in the surface ocean partial pressure of CO₂ along the core of the Antarctic Circumpolar Current. These results suggest that orbital precession may have played an important role in driving changes in atmospheric CO₂.

Orbital precession has been linked to glacial cycles and the atmospheric carbon dioxide (CO2) concentration, yet the direct impact of precession on the carbon cycle is not well understood. We analyze output from an Earth system model configured under different orbital parameters to isolate the impact of precession on air-sea CO2 flux in the Southern Ocean – a component of the global carbon cycle that is thought to play a key role on past atmospheric CO2 variations. Here, we demonstrate that periods of high precession are coincident with anomalous CO2 outgassing from the Southern Ocean. Under high precession, we find a poleward shift in the southern westerly winds, enhanced Southern Ocean meridional overturning, and an increase in the surface ocean partial pressure of CO2 along the core of the Antarctic Circumpolar Current. These results suggest that orbital precession may have played an important role in driving changes in atmospheric CO2