Amos Winter

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.

The 13C Suess effect in scleractinian corals mirror changes in the anthropogenic CO2 inventory of the surface oceans

[1] New δ 13 C data are presented from 10 coral skeletons collected from Florida and elsewhere in the Caribbean (Dominica, Dominican Republic, Puerto Rico, and Belize). These corals range from 96 to 200 years in age and were collected between 1976 and 2002. The change in the δ 13 C of the skeletons from these corals between 1900 and 1990 has been compared with 27 other published coral records from the Atlantic, Pacific, and Indian Oceans. The new data presented here make possible, for the first time, a global comparison of rates of change in the δ 13 C value of coral skeletons. Of these records, 64% show a statistically significant (p < 0.05) decrease in δ 13 C towards the modem day (23 out of 37). This decrease is attributable to the addition of anthropogenically derived CO 2 ( 13 C Suess effect) to the atmosphere. Between 1900 and 1990, the average rate of change of the δ 13 C in all the coral skeletons living under open oceanic conditions is approximately - 0.01%o yr ―1 . In the Atlantic Ocean the magnitude of the decrease since 1960,―0.019 yr ―1 ±0.015‰, is essentially the same as the decrease in the δ 13 C of atmospheric CO 2 and the δ 13 C of the oceanic dissolved inorganic carbon (-0.023 to -0.029‰ yr ―1 ), while in the Pacific and Indian Oceans the rate is more variable and significantly reduced (-0.007‰ yr ―1 ±0.013). These data strongly support the notion that (i) the δ 13 C of the atmosphere controls ambient δ 13 C of the dissolved inorganic carbon which in turn is reflected in the coral skeletons, (ii) the rate of decline in the coral skeletons is higher in oceans with a greater anthropogenic CO 2 inventory in the surface oceans, (iii) the rate of δ 13 C decline is accelerating. Superimposed on these secular variations are controls on the δ 13 C in the skeleton governed by growth rate, insolation, and local water masses.

Sea surface temperatures and coral reef bleaching off La Parguera, Puerto Rico (northeastern Caribbean Sea)

Abstract Much recent attention has been given to coral reef bleaching because of its widespread occurrence, damage to reefs, and possible connection to global change. There is still debate about the relationship between temperature and widespread bleaching. We compared coral reef bleaching at La Parguera, Puerto Rico to a 30-y (1966–1995) record of sea surface temperature (SST) at the same location. The last eight years of the La Parguera SST record have all had greater than average maximum temperatures; over the past 30 y maximum summer temperature has increased 0.7 °C. Coral reef bleaching has been particularly frequent since the middle 1980s. The years 1969, 1987, 1990, and 1995 were especially noteworthy for the severity of bleaching in Puerto Rico. Seven different annual temperature indices were devised to determine the extent to which they could predict severe coral bleaching episodes. Three of these, maximum daily SST, days >29.5 °C, and days >30 °C predict correctly the four years with severe bleaching. A log-log linear relationship was found between SST and the number of days in a given year above that SST at which severe coral beaching was observed. However, the intra-annual relationship between temperature and the incidence of bleaching suggests that no one simple predictor of the onset of coral bleaching within a year may be applicable.

Caribbean sea surface temperatures: Two‐to‐three degrees cooler than present during the Little Ice Age

We determined the seasonal environmental conditions during the Little Ice Age (LIA) by interpreting isotope proxies in the coral skeleton of Montastrea faveolata from the northeast Caribbean. The oxygen isotope composition was determined for three time intervals during the LIA (1700–1705, 1780–1785, 1810–1815), thought to correspond to the coldest intervals. The period 1984–1989 was used to represent modern calibration conditions. We determined that SSTs from the LIA intervals are nearly 2–3°C cooler than present. LIA cooling in the Caribbean may have resulted from regional oceanic and atmospheric circulation differences, especially in winter. We propose that a trough of cold air from the north Atlantic may have extended farther south than present, into the northern Caribbean. Although there are indications from Pacific corals that seasonality was greater during the LIA, this does not seem to be the case for the Caribbean.

El Niño–Southern Oscillation and North Atlantic Oscillation Control of Climate in Puerto Rico

Abstract Many studies have shown that the El Nino–Southern Oscillation (ENSO) has a significant influence on climate in many parts of the globe, mostly in the Pacific Basin. The objective of this study is to examine the possible impact of ENSO on climatic patterns on the island of Puerto Rico in the Caribbean. The authors find that annual mean air temperatures are controlled by ENSO since 1914. El Nino years are associated with warm air temperatures, whereas El Viejo (La Nina) years, which are the opposite of El Nino, are cooler. On the other hand, since 1911 fluctuations in annual rainfall amounts are synchronous with variations in the North Atlantic Oscillation (NAO) during the winter and are not controlled by ENSO. During years of a high winter NAO index, when the axis of moisture transport in the North Atlantic changes to a more southwesterly–northeasterly orientation, annual precipitation in Puerto Rico is lower than average.

A centennial-scale variability of tropical North Atlantic surface hydrography during the late Holocene

Abstract Sea-surface temperature (SST) and sea-surface salinity (SSS) fluctuations in the northeastern Caribbean have been reconstructed through the last 2000 yr using an artificial neural network and δ 18 O analyses of planktonic foraminifera. A warmer period prevailed in the NE Caribbean from AD ∼700–950, which may reflect the occurrence of stronger and/or more frequent El Nino events. A ∼2°C cooling of winter SSTs, from AD ∼1400 to 1550, coincides with the occurrence of reduced solar output, the Sporer event. Episodes of lower SSSs with marked minima at the onsets of the Dark Ages in Europe (AD ∼500–600) and Little Ice Age (AD ∼1400) are cyclically recurrent at intervals of 200–400 yr, and coincide with drier periods in Mexico. This may indicate that the tropical Atlantic evaporation–precipitation budget and SSSs are affected by a centennial-scale modulation involving the freshwater export (import) from (into) the Atlantic Ocean. Coeval changes recorded in the deep North Atlantic circulation indicate that low-latitude SSS anomalies may be advected polewards by the North Atlantic current system, thus affecting deep-ocean convection and strength of the thermohaline circulation.

Climate Zonation in Puerto Rico Based on Principal Components Analysis and an Artificial Neural Network

Abstract The authors analyzed climate data, seasonal averages of precipitation, and maximum, mean, and minimum temperatures over the years 1960–90, from 18 stations spread around the island of Puerto Rico in the Caribbean, to determine whether these distinguish the existence of climate zones in Puerto Rico. An R-mode principal components analysis (PCA), with varimax rotation to the seasonal data in order to reduce their dimensionality, was applied. The first five principal components, found by cross validation to be statistically significant, account for 99% of the variability in the 16 variables included in the analysis. These five components are related to annual variation in mean and minimum temperature (first PC), annual maximum temperature (second PC), and spring, summer, and fall precipitation (third through fifth PCs). A self-organizing map, an artificial neural network algorithm, was then employed to classify the first five PC scores in an optimal fashion. The scores were classified by the neural ...

Evaluation of the fidelity of isotope records as an environmental proxy in the coral Montastraea

Abstract. Many studies of climate variability in the Tropical Ocean have used high-resolution chemical tracer records contained in coral skeletons. The complex architecture of coral skeletons may lead to the possibility of biases in coral records and it is therefore important to access the fidelity of coral geochemical records as environmental proxies. Coral skeletal records from the same coral colony, and even the same corallite, may show large variation due to differing extension rates, formational timing of the skeletal elements, colony topography, and sampling resolution. To assess the robustness of the skeletal record, we present δ13C and δ18O data based on different sampling resolutions, skeletal elements, and coral colonies of Montastraea faveolata species complex, the primary coral used for climate reconstruction in the Atlantic. We show that various skeletal elements produce different isotopic records. The best sampling rate to resolve the full annual range of sea surface temperature (SST) is 40 samples per year. This sampling frequency also consistently recovered SST variability measured at weekly intervals. A sampling rate of 12 times per year recovered 84% of the annual range recording average monthly SST changes through the year. Six samples per year significantly decreased the ability to resolve the annual SST range. The δ18O recorded from two adjacent colonies were very similar, suggesting that this isotope can be trusted to record environmental changes. The δ13C, on the other hand, remained highly variable, perhaps as a result of the activity of symbiotic algae (zooxanthellae).

CARBON AND OXYGEN ISOTOPE TIME SERIES FROM AN 18-YEAR CARIBBEAN REEF CORAL

Colonies of Montastrea annularis live near La Parguera, Puerto Rico, and may be 700 years old. Time series from 1964 to 1982 of δ13C and δ18O from a continuous core of these corals are compared to an adjacent environmental record. At the intraannual level, δ18O correlates well with water temperature. Changes in the amplitude of the δ18O signal between 1967 and 1976 are attributed to sampling frequency but may be also due to environmental changes such as salinity. Average annual δ18O, δ13C and sea surface temperature show similar trends for the period from 1964 to 1982 but especially from 1969 onwards. Changes in average annual values during this time interval are most likely due to water mass changes brought about by interannual variability of the North Atlantic circulation. Since water temperatures at La Parguera are representative of changes occurring in the wider Caribbean, the isotope record from La Parguera corals could be used as a proxy for large-scale environmental changes beyond the historical record through the Little Ice Age.

Subtropical coccolithophores in the Weddell Sea

Abstract Eleven subtropical coccolithophore species were identified in three samples taken in the austral autumn from the Weddell Sea, Antarctic, between 69°S and 70°S, just south of the Antarctic Slope Front. This is the first report of coccolithophores present at such southern latitudes. We provide three hypotheses for their occurrence in the Weddell Sea: (1) Coccolithophore species have wider temperature tolerances than previously believed. (2) Coccolithophores found in the Weddell Sea were part of a remnant community from the Agulhas Current. (3) Coccolithophores were transported by a N–S eddy crossing the Brazil–Malvinas confluence region and then subsequently transported to the east by warm water eddies of the Antarctic Circumpolar Current to the study location. Further temperature tolerance experiments with coccolithophores are recommended.