Björn A. Malmgren

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.

Subregional precipitation climate of the Caribbean and relationships with ENSO and NAO

[1]xa0Thirty-five meteorological stations encompassing the Caribbean region (Cuba, Bahamas, Jamaica, Dominican Republic, Puerto Rico, US Virgin Islands, St. Maarten, and Barbados) were analyzed over the time interval 1951–1981 to assess regional precipitation patterns and their relationships with the North Atlantic Oscillation (NAO) and El Nino-Southern Oscillation (ENSO). Application of factor analysis to these series revealed the existence of four geographically distinct precipitation regions, (C1) western Cuba and northwestern Bahamas, (C2) Jamaica, eastern Cuba, and southeastern Bahamas, (C3) Dominican Republic and northwestern Puerto Rico, and (C4) eastern Puerto Rico, US Virgin Islands, St. Maarten, and Barbados. This regionalization is related to different annual cycles and interannual fluctuations of rainfall. The annual cycle is more unimodal and largest in the northwest Caribbean (C1) and becomes increasingly bimodal toward lower latitudes (C4) as expected. Year-to-year variations of precipitation are compared with two well-known climatic indices. The ENSO relationship, represented by Nino 3.4 sea surface temperatures (SST), is positive and stable at all lags, but tends to reverse over the SE Caribbean (C4) in late summer. The NAO influence is weak and seasonally dependent. Early summer rainfall in the northwest Caribbean (C1) increases under El Nino conditions. Clusters 2 and 3 are less influenced by the global predictors and more regional in character.

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.

Variability of North Atlantic heat transfer during MIS 2

[1]xa0Short-term changes in sea surface conditions controlling the thermohaline circulation in the northern North Atlantic are expected to be especially efficient in perturbing global climate stability. Here we assess past variability of sea surface temperature (SST) in the northeast Atlantic and Norwegian Sea during Marine Isotope Stage (MIS) 2 and, in particular, during the Last Glacial Maximum (LGM). Five high-resolution SST records were established on a meridional transect (53°N–72°N) to trace centennial-scale oscillations in SST and sea-ice cover. We used three independent computational techniques (SIMMAX modern analogue technique, Artificial Neural Networks (ANN), and Revised Analog Method (RAM)) to reconstruct SST from planktonic foraminifer census counts. SIMMAX and ANN reproduced short-term SST oscillations of similar magnitude and absolute levels, while RAM, owing to a restrictive analog selection, appears less suitable for reconstructing “cold end” SST. The SIMMAX and ANN SST reconstructions support the existence of a weak paleo-Norwegian Current during Dansgaard-Oeschger (DO) interstadials number 4, 3, 2, and 1. During the LGM, two warm incursions of 7°C water to occurred in the northern North Atlantic but ended north of the Iceland Faroe Ridge. A rough numerical estimate shows that the near-surface poleward heat transfer from 53° across the Iceland-Faroe Ridge up to to 72° N dropped to less than 60% of the modern value during DO interstadials and to almost zero during DO stadials. Summer sea ice was generally confined to the area north of 70°N and only rarely expanded southward along the margins of continental ice sheets. Internal LGM variability of North Atlantic (>40°N) SST in the GLAMAP 2000 compilation [Sarnthein et al., 2003b; Pflaumann et al., 2003] indicates maximum instability in the glacial subpolar gyre and at the Iberian Margin, while in the Nordic Seas, SST was continuously low.

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 ...

Seasonal sea surface temperature contrast between the Holocene and last glacial period in the western Arabian Sea (Ocean Drilling Project Site 723A): Modulated by monsoon upwelling

[1]xa0Annual, summer, and winter sea surface temperatures (SSTs) in the western Arabian Sea were reconstructed through the last 22 kyr using artificial neural networks (ANNs) based on quantitative analyses of planktic foraminifera. Down-core SST estimates reveal that annual, summer, and winter SSTs were 2, 1.2, and 2.6°C cooler, respectively, during the last glacial period than in the Holocene. A 2.5°C SST increase during Termination 1A (hereinafter referred as glacial to Holocene transition) in the western Arabian Sea. The study reveals a strong seasonal SST contrast between winter and summer from 18 to 14 calendar kyr owing to the combined effect of weak upwelling and strong cold northeasterly winds. Minor or no seasonal SST changes were noticed within the Holocene period, which is attributed to the intense upwelling during the summer monsoon. This causes a lowering of SST to values similar to those of the winter season in analogy with the present day. A 3°C rise in winter SSTs during the glacial to Holocene transition coincides with a strengthening of the monsoon, suggesting a link between winter SST and monsoon initiation from the beginning of the Holocene. Strikingly, annual, summer, and winter SSTs show a cooling trend from ∼8 ka to the present day, implying tropical cooling in the late Holocene.

Differences between evolution of mean form and evolution of new morphotypes; an example from Late Cretaceous planktonic Foraminifera

Morphological evolution in the Late Cretaceous (Maastrichtian) Contusotruncana lineage of planktonic foraminifera was studied at DSDP Sites 525 (South Atlantic) and 384 (North Atlantic). A multivariable approach was used to separate aspects of form controlled by geographical variation (size, spiral roundness of the test, percentage of kummerform specimens) from those due to changes that occurred simultaneously in geographically distant populations of the lineage (shell conicity, number of chambers in the last whorl). A gradual increase in mean shell conicity was observed over the last 3 million years of the Cre- taceous. It arose from the combination of a rapid development of highly conical shells after 68.5 Ma and a long-term trend of progressive disappearance of the ancestral morphotype. Therefore, despite the gradual change in mean form, the morphological evolution in the Contusotruncana lineage differs from the classical image of phyletic gradualism. The gradual increase in mean shell conicity in the lineage was accompanied by a remarkable decrease in its absolute abundance (shell accumulation rate), suggesting that the changes in shell morphology might not have been neutral with respect to natural selection. Apparently, gradual change in mean form of fossil lineages does not require an equally gradual development of morphological novelties. It may be caused by natural selection operating on a constant range of variation in populations living in environments without geographical barriers.

Recent benthic foraminifera as tracers of water masses along a transect in the Skagerrak, north-eastern North Sea

Abstract Recent benthic foraminifera have been investigated in surface samples (0–8 cm) from thirteen sites along a profile in the Skagerrak, between Norway and Denmark. The investigated transect is sampled in an oceanographically well investigated area. Principal component analysis based on the 32 most abundant taxa is used to recognize similarities within the large data set of benthic foraminifera (a total of 93 samples), which is shown to cluster into four assemblage groups (A–D). The spatial distribution of these foraminiferal groups indicates that they inhabit areas that correspond with the delimitations of different water masses. We have identified areas where it is possible to investigate effects on the sea floor of different tracers, natural or anthropogenic contaminants, related to specific water masses.

Biostratigraphy of planktic Foraminifera from the Maastrichtian white chalk of Sweden

Abstract Three bore-hole cores (30–200 m long) from the latest Cretaceous chalk of southern Sweden (Limhamn area, Skane) have been analyzed for planktic Foraminifera. The sequences are referable to the Upper Maastrichtian (Nephrolithus frequens coccolith Zone). Planktic foraminiferal assemblages, mostly dominated by Heterohelix striata and Globigerinelloides multispina, are of a typical high-latitude (Boreal) character. Rugoglobigerina rugosa and the Pseudotextularia elegans-Racemiguembelina fructicosa complex are occasionally abundant. The P. elegans-R. fructicosa complex is abundant in the uppermost parts of the sequences; this increase has been suggested by other workers to represent a climatically related migration from the Tethys. Other taxa are all rare (Hedbergella monmouthensis, Praeglobotruncana havanensis, Guembelitria cretacea, Heterohelix glabrans, Globigerinelloides sp., and Globotruncana sp.). The proportions between the two dominating species, G. multispina and H. striata, change systematic...