Grant L. Harley

Shipwreck rates reveal Caribbean tropical cyclone response to past radiative forcing

Significance Twenty-first-century North Atlantic tropical cyclone (TC) projections are crucial for the development of adaptation and mitigation strategies, but they are subject to large uncertainties, particularly with respect to TC response to radiative forcing. We used a combination of tree-ring data and historical shipwreck data to show that TC activity in the Caribbean was distinctly suppressed during the Maunder Minimum (1645–1715 CE), a period when solar irradiance was severely reduced. This solar fingerprint on decadal-scale Caribbean TC variability implies modulation by a combination of basin-wide climatic phenomena. Our findings highlight the need to enhance our understanding of the response of atmospheric circulation patterns to radiative forcing and climate change to improve the skill of future TC projections. Assessing the impact of future climate change on North Atlantic tropical cyclone (TC) activity is of crucial societal importance, but the limited quantity and quality of observational records interferes with the skill of future TC projections. In particular, North Atlantic TC response to radiative forcing is poorly understood and creates the dominant source of uncertainty for twenty-first-century projections. Here, we study TC variability in the Caribbean during the Maunder Minimum (MM; 1645–1715 CE), a period defined by the most severe reduction in solar irradiance in documented history (1610–present). For this purpose, we combine a documentary time series of Spanish shipwrecks in the Caribbean (1495–1825 CE) with a tree-growth suppression chronology from the Florida Keys (1707–2009 CE). We find a 75% reduction in decadal-scale Caribbean TC activity during the MM, which suggests modulation of the influence of reduced solar irradiance by the cumulative effect of cool North Atlantic sea surface temperatures, El Niño–like conditions, and a negative phase of the North Atlantic Oscillation. Our results emphasize the need to enhance our understanding of the response of these oceanic and atmospheric circulation patterns to radiative forcing and climate change to improve the skill of future TC projections.

On the declining relationship between tree growth and climate in the Midwest United States: the fading drought signal

Tree rings are widely considered to be a reliable proxy record of variations in climate and soil moisture. Here, using data from the Midwest United States (US), we provide documentation of a deteriorating relationship between radial tree growth and drought that is consistent across multiple species and locations. We find that traditional methods for drought reconstructions produce models that have rapidly declining validation statistics in recent decades. Split-sample calibration-verification that uses the first and second halves of the record can be problematic, as those two samples may not represent a sufficiently wide range of soil moisture conditions. To investigate this problem, we develop a randomized validation procedure that generates an empirical distribution of calibration and validation statistics. We place validation statistics derived from traditional methods in the generated distribution and compare them to a stratified approach that ensures each calibration model is composed of a sample that includes both dry and wet years. We find that the deteriorating relationship between tree growth and soil moisture is an artifact of the absence of drought over an extended period of time. A model that forces each calibration period to contain extreme drought years is statistically validated.. Nonetheless, if the current pluvial continues in the Midwest US, the linear relationship between tree rings and soil moisture will likely continue to deteriorate to the point where tree rings in the region will have a reduced ability to estimate past drought conditions.

A GIS-based modeling approach to predicting cave disturbance in karst landscapes: a case study from west-central Florida

We present a GIS-based cave disturbance model (CDM) that can be used as a tool to predict the disturbance of cave systems. To validate our model, we compare results to an in situ cave disturbance index (CDI) that was applied to 36 terrestrial caves during the year 2007 in west-central Florida. The CDM demonstrated high-performance skill and produced similar disturbance scores compared to the CDI. The CDM disturbance scores ranged from 0.22 (minimum) to 0.78 (maximum), and the mean disturbance score for all 36 caves was 0.45. The mean of the CDI scores was 0.40, but the difference between means was not significantly different (p = 0.147, t-test). Disturbance scores from the CDM and CDI were significantly correlated (R = 0.45, R2 = 0.20; p < 0.001), but a linear model of the data only explains ca. 20% of the variance. We demonstrate that land managers, who are unable to conduct an in situ inventory of caves to determine inherent disturbance, can use the GIS-based CDM tool as a proxy. In theory, the design of the CDM makes it applicable to other areas, but more research is needed to increase model accuracy in west-central Florida, as well as in other karst regions around the world.

Fire Synchrony and the Influence of Pacific Climate Variability on Wildfires in the Florida Keys, United States

We investigated relationships between climate variability and wildfires in endangered pine rockland communities in the Florida Keys, United States, using fire-scarred samples from the canopy dominant Pinus elliottii var. densa. To test broad-scale, spatiotemporal relationships between wildfires and climate, we compared cross-dated fire-scar chronologies from two islands in the lower Florida Keys, Big Pine Key (BPK) and No Name Key (NNK), to measured values of the Atlantic Multidecadal Oscillation (AMO), North Atlantic Oscillation (NAO), El Niño-Southern Oscillation (ENSO; NIÑO3.4), Pacific Decadal Oscillation (PDO), Interdecadal Pacific Oscillation (IPO), and divisional temperature and precipitation over the period from 1856 to 1956. Large-scale climate anomalies captured by ENSO (NIÑO3.4) and IPO indexes had combined effects on widespread fires. Superposed epoch analysis revealed that widespread fires on BPK occurred during years that were drier than average and when constructive phases (years of combined warm [positive] or cool [negative] phases) of ENSO and the IPO occurred three years and one year prior to fires. Positive phases of the PDO were also significantly associated with widespread fires three years prior to events, but the PDO was not influential one year prior to fires. Although fire years were temporally synchronous between the two islands during the period between 1818 and 1924 (n = 10), we did not find significant relationships between climate and fire on the smaller NNK, which suggests that island size influences the ability to detect broad-scale climate forcing of wildfires in the Florida Keys.

Recording Tropical Cyclone Activity from 1909 to 2014 along the Northern Gulf of Mexico using Maritime Slash Pine Trees (Pinus elliottii var. elliottii Engelm.)

ABSTRACT Tucker, C.S.; Trepanier, J.C.; Harley, G.L., and DeLong, K.L., 2018. Recording Tropical Cyclone Activity from 1909 to 2014 along the Northern Gulf of Mexico using Maritime Slash Pine Trees (Pinus elliottii var. elliottii Engelm.). The temporally incomplete tropical cyclone (TC) observational record limits the understanding of recurrence intervals and the impacts of repeated TCs on coastal ecosystems. Growth of maritime trees near the Gulf of Mexico may be affected by high winds, precipitation, and storm surges from TCs. Proxy records, such as tree growth recorded in annual ring widths, can be used to extend TC records temporally but must be verified with the observational record first. This study develops chronologies of total ring width (TRW), earlywood ring width (ERW), and latewood ring width (LRW) from slash pine (Pinus elliottii var. elliottii Engelm.) trees located 2 km from an open saltwater bay in the Grand Bay National Estuarine Research Reserve in coastal Mississippi, U.S.A. These chronologies are compared with TC storm surge and wind speed records using superposed epoch analysis. The year after a TC occurrence, tree growth is significantly (p < 0.05) suppressed (narrower than average annual growth rings) for TRW, ERW, and LRW with respect to storm surge and shows no statistical significance with high wind speeds. Individual trees display suppression in growth for 1–6 years after TC occurrences. This study demonstrates that slash pine trees in close proximity to salt water can be used to produce chronologies of TC occurrences for the northern Gulf of Mexico region and thus can eventually be used to extended and supplement temporally the existing TC observational record.

Using Proxy Records to Document Gulf of Mexico Tropical Cyclones from 1820-1915

Observations of pre-1950 tropical cyclones are sparse due to observational limitations; therefore, the hurricane database HURDAT2 (1851–present) maintained by the National Oceanic and Atmospheric Administration may be incomplete. Here we provide additional documentation for HURDAT2 from historical United States Army fort records (1820–1915) and other archived documents for 28 landfalling tropical cyclones, 20 of which are included in HURDAT2, along the northern Gulf of Mexico coast. One event that occurred in May 1863 is not currently documented in the HURDAT2 database but has been noted in other studies. We identify seven tropical cyclones that occurred before 1851, three of which are potential tropical cyclones. We corroborate the pre-HURDAT2 storms with a tree-ring reconstruction of hurricane impacts from the Florida Keys (1707–2009). Using this information, we suggest landfall locations for the July 1822 hurricane just west of Mobile, Alabama and 1831 hurricane near Last Island, Louisiana on 18 August. Furthermore, we model the probable track of the August 1831 hurricane using the weighted average distance grid method that incorporates historical tropical cyclone tracks to supplement report locations.