Research
Uplifted reef with corals of Holocene ages near Tanavusvus, Vanuatu in the southwest Pacific
Hover over images for captions
TROPICAL CLIMATE VARIABILITY
El Niño-Southern Oscillation (ENSO) Variability
What is the Response of ENSO to external forcing?
CESM1 simulated changes in Holocene ENSO variability. (A) The 30-year running SD of Niño 3.4 monthly SSTA for 9, 6, 3, and 0 ka. Lower and upper bounds of the boxes respectively correspond to the 25th and 75th percentiles, and the center line indicates the median (50th percentile). Whiskers represent the 1.5 × interquartile range, and outliers are indicated with a black dot. The range of the box and whiskers (Fig. 2A) captures intervals with higher and lower ENSO variability that arise purely from internal variability within the simulated climate system of each time interval. (B) Probability density functions (PDFs) of Niño 3.4 monthly SSTA for the Holocene time slices (see legend for labels). (C and D) Scatter plot of SD versus the number of extreme El Niño events in (C) 100-year and (D) 30-year windows. Extreme El Niño events are defined when the November-December-January (NDJ) average SSTA exceeds the 95th percentile (p95) of monthly SSTA for that time interval. Solid symbols in (C) indicate the average number of events/century for the full-length time slice simulations. Open symbols in (C) and (D) represent individual nonoverlapping windows. In this and all subsequent figures, the reported monthly anomalies are 9-year high-pass filtered, climatology-removed, and 5-month running mean anomalies to isolate interannual variability and facilitate comparison with coral proxy data. (From Fig. 2 in Lawman et al. (2022)).
The El Niño-Southern Oscillation (ENSO) is the largest mode of interannual (year-to-year) climate variability. Uncertainty surrounding the future response of El Niño-Southern Oscillation (ENSO) variability to anthropogenic warming necessitates the study of past ENSO sensitivity to substantial climate forcings over geological history. The Holocene Epoch (11.65 thousand years ago to present) provides an opportunity to test how ENSO responds to changes in orbital forcing. Although several climate model simulations for the mid-Holocene show a reduction in ENSO variance, proxy observations paint a more nuanced picture. To better understand the response of ENSO to external forcing, this study combines new climate model simulations, advances in coral proxy system modeling, and coral proxy data from the central tropical Pacific to show that ENSO amplitude and frequency intensified over the Holocene, driven by an increase in extreme El Niño events.
Publication: Lawman et al., (2022), Unraveling forced responses of extreme El Niño variability over the Holocene, Science Advances.
Drilling an in situ fossil coral head at Tasmaloum, Vanuatu (15.9°S, 166.9°E). The core extracted from this coral head is used to reconstruct climate variability during the Medieval Climate Anomaly (950-1250 CE). Photo courtesy of Jud Partin & Fred Taylor.
Top. Southwest Pacfiic coral Sr/Ca-SST reconstructions for two Medieval Climate Anomaly fossil corals (11-TM-S5: green; 11-TM-I1: gray). The corals have ~24 years of overlap. Triangles indicate the 230Th ages ± 2σ analytical error. Bottom. Climatology-removed Sr-Ca-SST anomalies for 11-TM-S5. The anomalies are 9-year high-pass filtered to isolate ENSO variability at internal (>1 - 9 yr) timescales.
What is the range of natural ENSO variability?
The instrumental record of ENSO is too short (<150 years) to quantify the full range of natural variability. Corals are a paleoclimate archive poised to address the limitations of instrumental data sets as they can provide decades to centuries of monthly-resolved climate data from the tropics.
This study uses replicated corals from the southwest Pacific to reconstruct ENSO-related sea surface temperature (SST) variability. We compare ENSO variability during the 20th century to variability ~900 years ago during the Medieval Climate Anomaly (MCA), a time interval when orbital, solar, and volcanic climate forcings were similar to pre-industrial values. Our results provide new insight to the range of observed ENSO variability by documenting a century of reduced ENSO variability during the MCA that is similar to the early 20th century.
Publication: Lawman, et al. (2020), A century of reduced ENSO variability during the Medieval Climate Anomaly. Paleoceanography and Paleoclimatology.
Link to the archived coral data: NOAA
Learn more about this project from my AGU talk: Link to AGU On-Demand
How do uncertainties inherent to the coral archive impact our ability to reconstruct ENSO variability?
Coral records of surface ocean conditions extend our knowledge of ENSO variability to the pre-instrumental period. That said, the ability to detect forced changes in ENSO using coral Sr/Ca and oxygen isotope records is challenging due to multiple sources of uncertainties inherent to the coral archive. This work uses surface temperature and sea surface salinity output from the Community Earth System Model to simulate coral geochemical time series. We incorporate different sources of uncertainty into a coral proxy system model that allows us to quantify how these factors impact estimates of interannual variance in a coral climate reconstruction. Although different processes and assumptions impact estimates of interannual variance recorded by corals, our PSM work highlights the strength of corals in their ability to capture changes in ENSO variability on decadal and longer timescales (decadal+).
Correlation between Niño 3.4 SST anomalies (SSTA) and values at each grid point. Monthly Niño 3.4 SSTA correlated with monthly (A) SSTA and (B) δ18O generated using the sensor model of Thompson et al. (2011). The 20‐year running SD of Niño 3.4 SSTA (i.e. decadal+ changes in ENSO variability) correlated with (C) SST and (D) pseudocoral δ18O anomalies. The 20‐year running SD of Niño 3.4 SSTA correlated with (E) SSTSr/Ca and (F) pseudocoral δ18O anomalies perturbed by the three coral PSM sub-models. Statistically significant correlations (p<0.01) are stippled. The gold diamond (C-F) indicates the average correlation coefficient for the Niño 3.4 region (white box). Figure from Lawman et al. (2020).
Publication: Lawman, et al. (2020), Developing a coral proxy system model to compare coral and climate model estimates of changes in paleo-ENSO variability, Paleoceanography and Paleoclimatology.
Code and example scripts are provided Here on GitHub!
Abrupt Climate Change
![Wet (blue) and dry (red) rainfall changes (units: mm/day) for the HadCM3 [Gordon et al., 2000; Pope et al. 2000] and IPSL [Marti et al., 2010] hosing experiments. Rainfall changes are relative to the Last Glacial Maximum background state (~21 ka). M…](https://images.squarespace-cdn.com/content/v1/5bd8800494d71a5b19ee2741/1541381113977-8GC8W3VL7UP63MDVI4BH/websiteFigsEdited.003.jpeg)
Wet (blue) and dry (red) rainfall changes (units: mm/day) for the HadCM3 [Gordon et al., 2000; Pope et al. 2000] and IPSL [Marti et al., 2010] hosing experiments. Rainfall changes are relative to the Last Glacial Maximum background state (~21 ka). Maps modified from Kageyama et al. [2013], Clim. Past.
What is the response of tropical precipitation to abrupt climate change?
Paleoclimate data reveals coherent patterns of hydroclimate change during intervals when the high-latitude North Atlantic cools. Inspired by proxy evidence showing the reduced strength of the Atlantic Meridional Overturning Circulation (AMOC) during Heinrich Stadials “hosing” experiments in which freshwater is added to the high-latitude North Atlantic are widely performed to investigate the mechanisms that cause abrupt climate change
In this collaborative study involving both paleoclimate data and modelers, we investigate the global response of tropical precipitation during Heinrich Stadial 1 (~17-15 thousand years ago) by synthesizing published paleoclimate data. To explore the dynamical mechanisms responsible for communicating high-latitude signals to the global tropics, we incorporate climate model output from a suite of fifteen freshwater hosing experiments designed to simulate a reduction in AMOC.
What is the spatial and temporal pattern of abrupt CLIMATE changes during the Holocene?
Abrupt climate transitions that operate on the timescales of decades to a century impact both humans and ecosystems. That said, our understanding of abrupt changes during the Holocene is limited. To develop a more comprehensive picture of abrupt climate change, this work investigates key factors for simulating abrupt climate change under Holocene background conditions. Given that orbital forcing varies gradually through the Holocene, smaller and higher-frequency external forcings (e.g., solar irradiance changes or volcanic eruptions) or additional feedbacks and processes are likely needed to generate abrupt transitions. To test this hypothesis, we use Bayesian methods to identify abrupt transitions (changepoints) in transient climate simulations forced with and without solar and volcanic forcing. This work sets-up a framework for a model-data comparison with available Holocene temperature and hydroclimate-sensitive paleoclimate records, and will help resolve long-standing model-data disagreements.
Coral Reef Ecosystem Vulnerability WITH FUTURE Climate Change
Coral reefs are one of the most sensitive ecosystems on Earth to climate change. Rising temperatures and ocean acidification pose ominous threats to precious coral reef resources. The once structurally complex coral reefs of the Gulf of Mexico and the Caribbean have declined since the 1970s, and recent bleaching events occurred in 2014-2017. Urgent mitigation efforts are needed to protect vulnerable coral reef ecosystems in these areas. My research uses climate model simulations to quantify the rates and magnitudes of future climate change impacts on coral reefs in the Gulf of Mexico and the Caribbean Sea.
Publication: Lawman et al. (2022), Rates of Future Climate Change in the Gulf of Mexico and the Caribbean Sea: Implications for coral reef ecosystems, Journal of Geophysical Research - Biogeosciences.
Coral colony from the Flower Garden Banks National Marine Sanctuary. (Image Credit: NOAA)