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Browsing by Author "Hurrell, Jim, committee member"

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    The effect of projected sea surface temperature change on MJO activity in a warmer climate
    (Colorado State University. Libraries, 2023) Bowden, Amanda Francine Marie, author; Maloney, Eric D., advisor; Hurrell, Jim, committee member; Ross, Matthew, committee member
    The Madden Julian Oscillation (MJO) consists of a convective region that propagates eastward in the tropics on repeat every 30-90 days with peak amplitude during the Boreal Winter (November - March). Since the MJO modulates extreme weather such as tropical cyclones, atmospheric rivers, and monsoon variability, future MJO changes in a warmer climate have implications for prediction of extreme events. Understanding precipitation pattern changes in a changing climate is critical for fresh-water resources and societal planning for oceanic regions. Decadal variability in the climate system causes patterns of sea surface temperature (SST) change in the tropical Pacific and associated precipitation, humidity, and wind pattern changes to vary from one decade to the next. MJO changes are strongly dependent on the pattern of SST change, and so understanding uncertainty in MJO change in future decades in the context of this decadal variability is the primary motivation for this investigation. Since climate models contain climate variability on decadal timescales, different initial conditions across ensemble members can result in diverse projection outcomes in any given decade. This investigation examines the impact of projected SST and moisture pattern changes over the 21st Century on MJO precipitation and zonal wind (850 mb) amplitude changes using 80 members with the SSP370 radiative forcing scenario from the Community Earth System Model 2 (CESM2) Large Ensemble. The projected SST and moisture pattern changes can be weighted more toward the central or eastern equatorial Pacific in earlier parts of the 21st Century across ensemble members, although becomes strongly El Niño-like later in the century. Ensemble members with stronger MJO precipitation amplitude in a given period are characterized by stronger El Niño-like east Pacific warming, associated with a strengthened meridional moisture gradient. As interpreted through moisture mode theory, greater east Pacific warming supports a stronger MJO by enhancing propagation through a stronger meridional moisture gradient, and enhancing MJO amplitude through a stronger vertical moisture gradient. The investigation supports the hypothesis that projected SST and moisture pattern changes influence MJO activity, and also highlights the importance of understanding decadal climate variability for interpreting changes in water resources of oceanic regions.
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    The hydroclimate and environmental response to warming in the southwestern US: a study across the Mid-Miocene Climate Optimum
    (Colorado State University. Libraries, 2022) Spaur, Siânin, author; Rugenstein, Jeremy Caves, advisor; Sertich, Joe, committee member; Hurrell, Jim, committee member
    Predictions for the effects of modern climate change on the southwestern US tend to suggest increased aridity, which is incompatible with paleoclimate data from other warm, high pCO2 periods. The Mid-Miocene Climate Optimum (MMCO; ~17-14 Ma) represents a period of warm global temperatures and high pCO2 similar to the projected pCO2 for future decades. We present new stable isotope records of mid-Miocene terrestrial carbonates from the Española basin in northern NM, along with new 40Ar/39Ar ages that establish an updated, high resolution age model for the Miocene-aged basin sediments. Our δ18O and δ13C records span 17-12 Ma, recording the extent of the MMCO and the beginning of late Miocene cooling. We use δ18O as a measure of the balance between summertime and wintertime precipitation and δ13C as a reflection of soil productivity. We find evidence for an increasingly winter-wet climate in the southwest US during the MMCO; when compared to modern precipitation δ18O, the carbonate δ18O record suggests that the region received more westerly-derived, wintertime precipitation than it does today. This indicates that El Niño Southern Oscillation (ENSO) was operating during the MMCO, and may have even been stronger than today; it seems to have been particularly strong during cooler periods during the MMCO, suggesting that cooler temperatures and high pCO2 may be favorable to ENSO. We also find that increases in wintertime precipitation are highly correlated with increases in soil productivity, suggesting that the amount of cool-season precipitation is a main control on vegetation for the region. Changes in the seasonal hydroclimate and soil productivity agree well with the paleontological record at the site, which show a diverse and dynamic faunal assemblage that evolved with the hydroclimate. Collectively our data do not support increased aridity in the southwest US during warm, high pCO2 periods, instead suggesting a shift towards increased cool-season precipitation that drives higher soil productivity, causing dynamic changes in the faunal and floral assemblage of the region.