Terrence M. Quinn

Low Atlantic hurricane activity in the 1970s and 1980s compared to the past 270 years

Hurricane activity in the North Atlantic Ocean has increased significantly since 1995 (refs 1, 2). This trend has been attributed to both anthropogenically induced climate change and natural variability, but the primary cause remains uncertain. Changes in the frequency and intensity of hurricanes in the past can provide insights into the factors that influence hurricane activity, but reliable observations of hurricane activity in the North Atlantic only cover the past few decades. Here we construct a record of the frequency of major Atlantic hurricanes over the past 270 years using proxy records of vertical wind shear and sea surface temperature (the main controls on the formation of major hurricanes in this region) from corals and a marine sediment core. The record indicates that the average frequency of major hurricanes decreased gradually from the 1760s until the early 1990s, reaching anomalously low values during the 1970s and 1980s. Furthermore, the phase of enhanced hurricane activity since 1995 is not unusual compared to other periods of high hurricane activity in the record and thus appears to represent a recovery to normal hurricane activity, rather than a direct response to increasing sea surface temperature. Comparison of the record with a reconstruction of vertical wind shear indicates that variability in this parameter primarily controlled the frequency of major hurricanes in the Atlantic over the past 270 years, suggesting that changes in the magnitude of vertical wind shear will have a significant influence on future hurricane activity.

Geology and Hydrogeology of Carbonate Islands

Geology and hydrogeology of Bermuda (H.L. Vacher, M.P. Rowe). Geology of the Bahamas (J.L. Carew, J.E. Mylroie). Geology and hydrogeology of the Florida Keys (R.B. Halley et al. ). Geology of coastal islands, northeastern Yucatan Peninsula (W.C. Ward). Geology and hydrogeology of St. Croix, Virgin Islands (I.P. Gill et al. ). Geomorphology and hydrogeology of selected islands of French Polynesia: Tikehau (Atoll) and Tahiti (Barrier Reef) (F. Rougerie et al. ). Hydrogeology of northern Guam (J.F. Mink, H.L. Vacher). Geology of reef islands of the Great Barrier Reef, Australia (D. Hopley). Hydrogeology of Diego Garcia (C.D. Hunt).

A reconstruction of sea surface temperature variability in the southeastern Gulf of Mexico from 1734 to 2008 C.E. using cross-dated Sr/Ca records from the coral Siderastrea siderea

This study uses skeletal variations in coral Sr/Ca from three Siderastrea siderea coral colonies within the Dry Tortugas National Park in the southeastern Gulf of Mexico (24°42′N, 82°48′W) to reconstruct monthly sea surface temperature (SST) variations from 1734 to 2008 Common Era (C.E.). Calibration and verification of the replicated coral Sr/Ca-SST reconstruction with local, regional, and historical temperature records reveals that this proxy-temperature relationship is stable back to 1879 C.E. The coral SST reconstruction contains robust interannual (~2.0°C) and multidecadal variability (~1.5°C) for the past 274 years, the latter of which does not covary with the Atlantic Multidecadal Oscillation. Winter SST extremes are more variable than summer SST extremes (±2.2°C versus ±1.6°C, 2σ) suggesting that Loop Current transport in the winter dominates variability on interannual and longer time scales. Summer SST maxima are increasing (+1.0°C for 274 years, σMC = ±0.5°C, 2σ), whereas winter SST minima contain no significant trend. Colder decades (~1.5°C) during the Little Ice Age (LIA) do not coincide with decades of sunspot minima. The coral SST reconstruction contains similar variability to temperature reconstructions from the northern Gulf of Mexico (planktic foraminifer Mg/Ca) and the Caribbean Sea (coral Sr/Ca) suggesting areal reductions in the Western Hemisphere Warm Pool during the LIA. Mean summer coral SST extremes post-1985 C.E. (29.9°C) exceeds the long-term summer average (29.2°C for 1734–2008 C.E.), yet the warming trend after 1985 C.E. (0.04°C for 24 years, σMC = ±0.5, 2σ) is not significant, whereas Caribbean coral Sr/Ca studies contain a warming trend for this interval.

A coral‐based reconstruction of sea surface salinity at Sabine Bank, Vanuatu from 1842 to 2007 CE

Climate variability associated with the El Nino Southern Oscillation (ENSO) results in large sea-surface temperature (SST) and sea-surface salinity (SSS) anomalies in many regions of the tropical Pacific Ocean. We investigate interannual changes in SSS driven by ENSO in the southwestern Pacific at Sabine Bank, Vanuatu (SBV, 166.04°E, 15.94°S) using monthly variations in coralδ18O from 1842 to 2007 CE. We develop and apply a coral δ18O-SSS transfer function, which is assessed using a calibration-verification exercise (1970-2007 CE). The 165-year reconstructed SSS record contains a prominent trend toward freshening from 1842 to 2007 CE; mean SSS for 1842-1872 CE is 35.46 ± 0.28 psu, which contrasts with a mean value of 34.85 ± 0.31 psu for 1977-2007 CE, with a freshening trend during the latter part of the 20th century that is not unprecedented with respect to the overall record. Variance in the record is concentrated in the interannual (42%) and interdecadal (29%) bands. The SBV-SSS record matches well with a similarly reconstructed SSS time series at Malo Channel, Vanuatu, which is located ∼120 km to the east of SBV. This regional signal is likely driven by ENSO-related changes in the SPCZ and interdecadal changes in surface water advection. The pattern of interdecadal variability at SBV agrees reasonably well with coral records of interdecadal variability from Fiji and Tonga, especially in the pre-1940 portions of the records, further evidence for the regional extent of the salinity signal at Sabine Bank, Vanuatu.

Globigerinoides ruber morphotypes in the Gulf of Mexico: A test of null hypothesis

Planktic foraminifer Globigerinoides ruber (G. ruber), due to its abundance and ubiquity in the tropical/subtropical mixed layer, has been the workhorse of paleoceanographic studies investigating past sea-surface conditions on a range of timescales. Recent geochemical work on the two principal white G. ruber (W) morphotypes, sensu stricto (ss) and sensu lato (sl), has hypothesized differences in seasonal preferences or calcification depths, implying that reconstructions using a non-selective mixture of morphotypes could potentially be biased. Here, we test these hypotheses by performing stable isotope and abundance measurements on the two morphotypes in sediment trap, core-top, and downcore samples from the northern Gulf of Mexico. As a test of null hypothesis, we perform the same analyses on couplets of G. ruber (W) specimens with attributes intermediate to the holotypic ss and sl morphologies. We find no systematic or significant offsets in coeval ss-sl δ18O, and δ13C. These offsets are no larger than those in the intermediate pairs. Coupling our results with foraminiferal statistical model INFAUNAL, we find that contrary to previous work elsewhere, there is no evidence for discrepancies in ss-sl calcifying depth habitat or seasonality in the Gulf of Mexico.

Coral record of reduced El Niño activity in the early 15th to middle 17th centuries

El Nino–Southern Oscillation (ENSO) powers global interannual climate variability through changes in trade wind strength, temperature and salinity anomalies, sea level, and atmospheric circulation patterns. ENSO variability is well characterized in modern times, but instrumental records cannot fully describe natural ENSO variability due to the imprint of anthropogenic climate forcing. ENSO activity may also be affected by solar variability, but the response of ENSO to such changes is difficult to predict. We constructed a continuous, monthly resolved, spliced fossil Porites coral δ18O and Sr/Ca record from Misima Island, Papua New Guinea, in the Western Pacific Warm Pool, spanning 233 yr (1411–1644 CE [Common Era]). The Misima coral record indicates that the surface ocean in this region experienced a small change in hydrologic balance with no change in temperature, extended periods of quiescence in El Nino activity, and no change in average amplitudes of El Nino events relative to signals captured in regional modern records. The reduced El Nino variability occurs during a known change in solar forcing, the initiation of the Little Ice Age. However, there is no clear relationship between the timing of changes in solar forcing and ENSO activity, implying that ENSO variability changes arise from internal dynamics. The century-scale switch between active and inactive El Nino states has not previously been recorded, and provides a new baseline for climate models and reconstructions.

Coral windows onto seasonal climate variability in the northern Caribbean since 1479

Mean surface ocean conditions in the Caribbean were up to ∼2°C cooler than today at times during the Little Ice Age. The seasonal context for such mean state changes is important for determining the mechanisms involved. We reconstructed surface ocean conditions in southwest Puerto Rico at approximately monthly resolution over eight 4–12 year periods during the last ∼520 years to test if the seasonal cycles of temperature or salinity varied with mean state. We carried out paired analyses of Sr/Ca and δ18O for two coral cores. The δ18O data contained clear annual cycles and were significantly correlated to temperature during the 20th century calibration periods (1993–2004 and 1902–1912, r = 0.73). The Sr/Ca data contained high-frequency noise that obscured the seasonal cycles, although the centennial variability matched that of the coral δ18O, indicating a common forcing that is likely temperature. The seasonal coral δ18O amplitude averaged 0.60 ± 0.17‰, with none of the periods significantly different from the most recent. The simplest explanation is that the amplitudes of seasonal seawater δ18O and temperature variations were not different from today. Previous work in the southern Caribbean indicates that the Intertropical Convergence Zone was shifted southward or weaker during the Little Ice Age, and we speculate about how this could occur with no apparent affect on seasonality in the northern Caribbean.

Constraining past seawater δ18O and temperature records developed from foraminiferal geochemistry

Paired measurements of magnesium-to-calcium ratios (Mg/Ca) and the stable oxygen isotopic composition (δ18O) in foraminifera have significantly advanced our knowledge of the climate system by providing information on past temperature and seawater δ18O (δ18Osw, a proxy for salinity and ice volume). However, multiple sources of uncertainty exist in transferring these downcore geochemical data into quantitative paleoclimate reconstructions. Here, we develop a computational toolkit entitled Paleo-Seawater Uncertainty Solver (PSU Solver) that performs bootstrap Monte Carlo simulations to constrain these various sources of uncertainty. PSU Solver calculates temperature and δ18Osw, and their respective confidence intervals using an iterative approach with user-defined errors, calibrations, and sea-level curves. Our probabilistic approach yields reduced uncertainty constraints compared to theoretical considerations and commonly used propagation exercises. We demonstrate the applicability of PSU Solver for published records covering three timescales: the late Holocene, the last deglaciation, and the last glacial period. We show that the influence of salinity on Mg/Ca can considerably alter the structure and amplitude of change in the resulting reconstruction and can impact the interpretation of paleoceanographic time series. We also highlight the sensitivity of the records to various inputs of sea-level curves, transfer functions, and uncertainty constraints. PSU Solver offers an expeditious yet rigorous approach to test the robustness of past climate variability inferred from paired Mg/Ca-δ18O measurements.

Nyberg et al. reply

Replying to: U. Neu 451, 10.1038/nature06576 (2008).Neu suggests that the reconstruction of Atlantic major hurricane activity (MHA) (that is, frequency) in Nyberg et al. overestimates past MHA because it differs significantly from the known observational records of tropical storms and MHA before 1945 and overestimates the influence of vertical windshear |Vz|.

Chapter 21 – Geology of Anewetak Atoll, Republic of the Marshall Islands

This chapter describes the geology of Anewetak atoll, republic of the Marshall Islands. Anewetak Atoll (formally Enewetak and Eniwetok), the northwesternmost member of the Marshall Islands, is located in the western equatorial Pacific Ocean at 162°E, 11°N. Anewetak was first sighted by Spanish explorers in the mid-1500s and later resighted by English explorers in the late 1700s. It provides independent constraints on the records of sea-level change inferred from continental-margin stratigraphies and from deep-sea foraminiferal oxygen isotope stratigraphies. Extensive scientific and geological studies of Anewetak were conducted as part of Operation Crossroads that coincided with the nuclear program at Anewetak. It is a great natural laboratory for studying marine diagenesis because marine waters are currently circulating through the atoll and apparently have done so for many millions of years. Anewetak provides independent constraints on the records of sea-level change inferred from continental-margin stratigraphies and from deep-sea foraminiferal oxygen isotope stratigraphies.