Browsing by Author "Stevenson, S"

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    Equatorial Pacific coral geochemical records show recent weakening of the Walker Circulation
    (American Geophysical Union, 2014-11-10) Carilli, JE; McGregor, HV; Gaudry, JJ; Donner, SD; Gagan, MK; Stevenson, S; Wong, HKY; Fink, D
    Equatorial Pacific ocean-atmosphere interactions affect climate globally, and a key component of the coupled system is the Walker Circulation, which is driven by sea surface temperature (SST) gradients across the equatorial Pacific. There is conflicting evidence as to whether the SST gradient and Walker Circulation have strengthened or weakened over the late twentieth century. We present new records of SST and sea surface salinity (SSS) spanning 1959–2010 based on paired measurements of Sr/Ca and δ18O in a massive Porites coral from Butaritari atoll in the Gilbert Islands, Republic of Kiribati, in the central western equatorial Pacific. The records show 2–7 year variability correlated with the El Niño–Southern Oscillation (ENSO) and corresponding shifts in the extent of the Indo-Pacific Warm Pool, and decadal-scale signals related to the Pacific Decadal Oscillation and the Pacific Warm Pool Index. In addition, the Butaritari coral records reveal a small but significant increase in SST (0.39°C) from 1959 to 2010 with no accompanying change in SSS, a trend that persists even when ENSO variability is removed. In contrast, larger increases in SST and SSS are evident in coral records from the equatorial Pacific Line Islands, located east of Butaritari. Taken together, the equatorial Pacific coral records suggest an overall reduction in the east-west SST and SSS gradient over the last several decades, and a recent weakening of the Walker Circulation. © 2014, American Geophysical Union. All Rights Reserved.
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    Paleoclimate data-model comparison and the role of climate forcings over the past 1500 Years
    (American Meterological Society, 2013-09-01) Phipps, SJ; McGregor, HV; Gergis, J; Gallant, AJE; Neukom, R; Stevenson, S; Ackerley, D; Brown, JR; Fischer, MJ; van Ommen, TD
    The past 1500 years provide a valuable opportunity to study the response of the climate system to external forcings. However, the integration of paleoclimate proxies with climate modeling is critical to improving the understanding of climate dynamics. In this paper, a climate system model and proxy records are therefore used to study the role of natural and anthropogenic forcings in driving the global climate. The inverse and forward approaches to paleoclimate data-model comparison are applied, and sources of uncertainty are identified and discussed. In the first of two case studies, the climate model simulations are compared with multiproxy temperature reconstructions. Robust solar and volcanic signals are detected in Southern Hemisphere temperatures, with a possible volcanic signal detected in the Northern Hemisphere. The anthropogenic signal dominates during the industrial period. It is also found that seasonal and geographical biases may cause multiproxy reconstructions to overestimate the magnitude of the long-term preindustrial cooling trend. In the second case study, the model simulations are compared with a coral O-18 record from the central Pacific Ocean. It is found that greenhouse gases, solar irradiance, and volcanic eruptions all influence the mean state of the central Pacific, but there is no evidence that natural or anthropogenic forcings have any systematic impact on El Nino-Southern Oscillation. The proxy climate relationship is found to change over time, challenging the assumption of stationarity that underlies the interpretation of paleoclimate proxies. These case studies demonstrate the value of paleoclimate data-model comparison but also highlight the limitations of current techniques and demonstrate the need to develop alternative approaches. © 2013, American Meteorological Society.
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    Reply to comment by Karnauskas et al. on “Equatorial Pacific coral geochemical records show recent weakening of the Walker circulation”
    (American Geophysical Union, 2015-05-18) Carilli, JE; McGregor, HV; Gaudry, JJ; Donner, SD; Gagan, MK; Stevenson, S; Wong, HKY; Fink, D
    In our paper describing a new coral record from Butaritari, we hypothesized that comparing the temporal trends in our records to coral records from farther east in the equatorial Pacific may support the evidence for a weakening of a Walker circulation, documented elsewhere in the literature [Power and Smith, 2007; Tokinaga et al., 2012]. Weakening of the Walker circulation is expected under global warming due to an imbalance in the rate of change in different aspects of the hydrological cycle [Vecchi and Soden, 2007]. We thank Karnauskas et al. [2015] for recognizing the value of our Butaritari coral climate reconstruction, and we appreciate their critique of our study. The Karnauskas et al. [2015] analyses strengthen our argument regarding the utility of interisland coral-proxy derived sea surface temperature (SST) gradients as a Walker circulation metric, but we disagree with their interpretation of decadal variability in our records. Here we provide additional analyses, which confirm that our reconstruction [Carilli et al., 2014] shows a long-term weakening of the Walker circulation over 1972–1998. We also document that significant decadal variations in Walker circulation strength, and for particular choices of start and end years over which trends are calculated, are able to show slight Walker strengthening. Overall, we conclude that Walker circulation variations are more nuanced than either our original publication [Carilli et al., 2014] or the subsequent Karnauskas et al. [2015] comment would suggest. Karnauskas et al. [2015] also provide a detailed analysis of Equatorial Undercurrent (EUC) activity near the Gilbert Islands and argue that the EUC does not strongly affect Butaritari. Our original publication did not claim to find significant EUC/Butaritari linkages, and we appreciate the diligence of Karnauskas et al. [2015] for ruling this out as a possibility. © 2015, American Geophysical Union.
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    Salinity information from hydro-sensitive, biogenic marine carbonate δ18O records
    (American Geophysical Union (AGU), 2021-12-14) Thompson, DM; Conroy, JL; Williams, B; Konecky, BL; Stevenson, S; DeLong, KL; McKay, N; Dassie, EP; Fischer, MJ; Jonkers, L; Martrat, B; Pages ISO2K Project Members
    Stable oxygen isotope ratios in marine biogenic carbonates (δ18Ocarb, e.g., from corals, coralline algae, bivalves, sclerosponges) have greatly extended the instrumental record, providing invaluable information about climate variability and change from the tropics to high latitudes. These δ18Ocarb records reflect seawater temperature and δ18O (δ18Osw) at the time of calcification, which occurs sufficiently rapidly to permit reconstruction of sub-annual to annual variability over the lifetime of the organism. δ18Osw is strongly related to salinity, as both are similarly impacted by hydroclimate processes such as precipitation, evaporation, and advection. Many studies have leveraged δ18Ocarb and the tendency for temperature and salinity to covary constructively in the tropics (e.g., warm with wet or low salinity, and vice versa) to reconstruct changes in major modes of climate variability and responses to forcings. However, with limited networks of in situ seawater temperature, salinity, and δ18O measurements, quantifying the relative contribution of temperature and δ18Osw to δ18Ocarb variability remains a large source of uncertainty. Here we use ‘pseudo-carbonate’ δ18O records modeled from temperature and salinity to identify sites where salinity contributes substantially to δ18Ocarb variance. These ‘hydro-sensitive’ δ18Ocarb records are located predominantly in the Indo-Pacific Warm Pool, Caribbean Sea, Gulf of Mexico, and Aleutian Archipelago. Notably, we find that temperature and salinity vary destructively in δ18Ocarb (i.e., warm with drier or higher salinity) in many regions for which high-resolution paleoclimate reconstructions have provided key constraints on ocean circulation—the eastern equatorial Pacific, the North Atlantic, and at depth. However, the inferred salinity variability at these sites is sensitive to uncertainties in the δ18Osw-salinity relationship, emphasizing the need for a coordinated network of in situ salinity and δ18Osw measurements. Plain-language Summary Ocean salinity integrates valuable information about changes in the global water cycle, including changes in precipitation, evaporation, and ocean currents. However, our understanding of recent salinity changes are hampered by the limited coverage of direct salinity measurements. Many marine organisms incorporate information about their local environment in their skeleton as they grow, providing indirect evidence of past ocean salinity. The interpretation of these records is dependent on the relative magnitude of temperature and salinity signal recorded, and in turn, how these variables relate to one another. We show that samples from the western Pacific Ocean, eastern Indian Ocean, Caribbean Sea, Gulf of Mexico, and Aleutian Archipelago are most likely to provide reliable salinity information across seasons, years, and decades, as the organisms in these regions primarily record salinity at all timescales. Finally, we show that direct seawater observations are critical to improve the salinity information obtained from the skeletons of marine organisms.
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    Solar and volcanic forcing of the Southern Hemisphere climate over the past 1500 years
    (Past Global Changes, 2013-02-13) Phipps, SJ; Ackerley, D; Brown, JR; Curran, MAJ; Fischer, MJ; Gallant, A; Gergis, J; McGregor, HV; Neukom, R; Plummer, C; Stevenson, S; van Ommen, TD
    The past 1500 years provides a valuable opportunity to study the role of external forcings in driving the global climate. Significant changes have taken place within the climate system over this period, and proxy data that records these changes covers a wide geographical area and has high temporal resolution. Natural and anthropogenic forcings are also reasonably well constrained. While previous detection and attribution studies have found a significant role of volcanic eruptions in driving the pre-industrial Northern Hemisphere climate, the drivers of the Southern Hemisphere climate are much less well understood. Here, the CSIRO Mk3L climate system model is used to simulate the global climate of the past 1500 years. Different combinations of natural and anthropogenic forcings are applied, including changes in the Earth’s orbital parameters, solar irradiance, volcanic emissions and anthropogenic greenhouse gases. The simulations are then compared with a multi-proxy reconstruction of Southern Hemisphere temperature. We find strong solar and volcanic influences on the Southern Hemisphere climate during the pre-industrial period, with the anthropogenic signal becoming increasingly dominant after 1850 CE. However, the results are sensitive to the specific reconstructions of solar and volcanic activity that are used to drive the model. The choice of volcanic reconstruction is particularly critical, and we find that the dating of major eruptions can impact significantly upon the agreement between the model and the proxy record. If we are to learn all that we can from the climate of recent millennia, a critical challenge is therefore to develop better reconstructions of past climatic forcings − particularly volcanic eruptions.