Southern North Atlantic Research Articles

Mg/Ca thermometry in planktic foraminifera: Improving paleotemperature estimations for G. bulloides and N. pachyderma left

AbstractPlanktic foraminiferal Mg/Ca ratios have become a fundamental seawater temperature proxy in past climate reconstructions, due to the temperature dependence of Mg uptake into foraminiferal calcite. However, empirical calibrations for single species from methodologically consistent data are still lacking. Here we present species‐specific calibrations of Mg/Ca versus calcification temperature for two commonly used species of planktic foraminifera: Globigerina bulloides and Neogloboquadrina pachyderma left, based on a series of Southern Ocean and North Atlantic core tops. Combining these new data with previously published data, we derive an integrated G. bulloides Mg/Ca‐temperature calibration for mid and high latitudes of both hemispheres between 2 and 18°C, where Mg/Ca = 1.006 ± 0.032 * e0.065 ± 0.003*Tiso (R2 = 0.82). G. bulloides is found to calcify deeper in the Southern Ocean (∼ 200 m) than in the North Atlantic (top 50 m). We also propose a Mg/Ca temperature calibration to describe the temperature response in N. pachyderma left that calcified away from the influence of sea ice in the Southern Ocean, valid between ∼ −1 and 9°C, of the form Mg/Ca = 0.580 ± 0.016 * e0.084 ± 0.006*Tiso (R2 = 0.70). These calibrations account for uncertainties on Mg/Ca measurements and calcification temperature that were carefully estimated and propagated using Monte Carlo iterations. The 1σ propagated error in Mg/Ca‐derived temperatures is 1.1°C for G. bulloides and 0.9°C for N. pachyderma left for the presented data sets. Geographical extension of genotypes must be assessed when choosing to develop regional or global calibrations.

Ice melt, sea level rise and superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2 °C global warming could be dangerous

Abstract. We use numerical climate simulations, paleoclimate data, and modern observations to study the effect of growing ice melt from Antarctica and Greenland. Meltwater tends to stabilize the ocean column, inducing amplifying feedbacks that increase subsurface ocean warming and ice shelf melting. Cold meltwater and induced dynamical effects cause ocean surface cooling in the Southern Ocean and North Atlantic, thus increasing Earth's energy imbalance and heat flux into most of the global ocean's surface. Southern Ocean surface cooling, while lower latitudes are warming, increases precipitation on the Southern Ocean, increasing ocean stratification, slowing deepwater formation, and increasing ice sheet mass loss. These feedbacks make ice sheets in contact with the ocean vulnerable to accelerating disintegration. We hypothesize that ice mass loss from the most vulnerable ice, sufficient to raise sea level several meters, is better approximated as exponential than by a more linear response. Doubling times of 10, 20 or 40 years yield multi-meter sea level rise in about 50, 100 or 200 years. Recent ice melt doubling times are near the lower end of the 10–40-year range, but the record is too short to confirm the nature of the response. The feedbacks, including subsurface ocean warming, help explain paleoclimate data and point to a dominant Southern Ocean role in controlling atmospheric CO2, which in turn exercised tight control on global temperature and sea level. The millennial (500–2000-year) timescale of deep-ocean ventilation affects the timescale for natural CO2 change and thus the timescale for paleo-global climate, ice sheet, and sea level changes, but this paleo-millennial timescale should not be misinterpreted as the timescale for ice sheet response to a rapid, large, human-made climate forcing. These climate feedbacks aid interpretation of events late in the prior interglacial, when sea level rose to +6–9 m with evidence of extreme storms while Earth was less than 1 °C warmer than today. Ice melt cooling of the North Atlantic and Southern oceans increases atmospheric temperature gradients, eddy kinetic energy and baroclinicity, thus driving more powerful storms. The modeling, paleoclimate evidence, and ongoing observations together imply that 2 °C global warming above the preindustrial level could be dangerous. Continued high fossil fuel emissions this century are predicted to yield (1) cooling of the Southern Ocean, especially in the Western Hemisphere; (2) slowing of the Southern Ocean overturning circulation, warming of the ice shelves, and growing ice sheet mass loss; (3) slowdown and eventual shutdown of the Atlantic overturning circulation with cooling of the North Atlantic region; (4) increasingly powerful storms; and (5) nonlinearly growing sea level rise, reaching several meters over a timescale of 50–150 years. These predictions, especially the cooling in the Southern Ocean and North Atlantic with markedly reduced warming or even cooling in Europe, differ fundamentally from existing climate change assessments. We discuss observations and modeling studies needed to refute or clarify these assertions.

A Track Pattern–Based Seasonal Prediction of Tropical Cyclone Activity over the North Atlantic

Abstract A seasonal prediction model of tropical cyclone (TC) activities for the period August–October over the North Atlantic (NA) has been developed on the basis of TC track patterns. Using the fuzzy c-means method, a total of 432 TCs in the period 1965–2012 are categorized into the following four groups: 1) TCs off the U.S. East Coast, 2) TCs over the Gulf of Mexico, 3) TCs that recurve into the open ocean of the central NA, and 4) TCs that move westward in the southern NA. The model is applied to predict the four TC groups separately in conjunction with global climate forecasts from the National Centers for Environmental Prediction (NCEP) Climate Forecast System, version 2 (CFSv2). By adding the distributions of the four TC tracks with precalculated weighting factors, this seasonal TC forecast model provides the spatial distribution of TC activities over the entire NA basin. Multiple forecasts initialized in six consecutive months from February to July are generated at monthly intervals to examine the applicability of this model in operational TC forecasting. Cross validations of individual forecasts show that the model can reasonably predict the observed TC frequencies over NA at the 99% confidence level. The model shows a stable spatial prediction skill, proving its advantage for forecasting regional TC activities several months in advance. In particular, the model can generate reliable information on regional TC counts in the near-coastal regions as well as in the entire NA basin.

Assessing the impact of orogenic inheritance on the architecture, timing and magmatic budget of the North Atlantic rift system: a mapping approach

To investigate the impact of orogenic inheritance on the characteristics of the North Atlantic rift system we develop new mapping methods that highlight the first-order architecture and timing of rifts, as well as the distribution of heterogeneities inherited from the Palaeozoic Caledonian and Variscan orogenies. These maps demonstrate major differences in the behaviour of the North Atlantic rift system relative to the two orogens, with the Variscan front appearing to be an important boundary: the rift cuts through the Caledonian orogen and is parallel to its structural grain to the north, whereas it circumvents the core of the Variscides to the south. In addition, rifting is protracted and polyphase with break-up being magma-rich north of the Variscan front, as opposed to the south, where a single, apparently continuous extensional event led to magma-poor break-up in less than 50 myr. Also, the North Atlantic rift system reactivates sutures corresponding to large (>2000 km) former oceans, whereas sutures of small (<500–1000 km) oceanic basins are little affected in both the northern and southern North Atlantic. These observations point to a major influence of orogenic inheritance on the characteristics of rift systems.

Changes in the large‐scale thermodynamic instability and connection with rain shower frequency over Romania: verification of the Clausius–Clapeyron scaling

The goal of this article is twofold: first, changes in the spring and summer thermodynamic instability over the North Atlantic European region and the connection with frequency of rain showers at 81 locations across Romania for the period 1961–2010 are analysed; second, the relationship between hourly precipitation extremes and air temperature with respect to the validity of Clausius–Clapeyron (CC) scaling is investigated. The Best Lifted Index (lftx4) has been selected to quantify the thermodynamic instability. The mechanisms responsible for the rain showers' variability in Romania were analysed using the canonical correlation analysis (CCA). Results show that the regional distribution of the lftx4 changes is quite similar in the two seasons, with a transition towards thermodynamic stability over most of Mediterranean regions – central Europe and instability in northern Europe, Black Sea, North Atlantic (summer), southern North Atlantic (spring). The frequency of rain showers in Romania exhibits a significant increasing trend in both seasons, which is in agreement with the increase in frequency of Cumulonimbus clouds. In contrast, the frequency of rainfall days does not exhibit any significant trend that shows a shift in the nature of precipitation towards more showers. The CCA shows that the optimum large-scale mechanisms responsible for this behaviour are given by the first CCA pair with the lftx4 pattern represented by a dipole structure with a nucleus of thermodynamic instability centred over Romania. The dynamic factor is an additional ingredient for spring, as precipitable water is for summer. The extreme hourly rainfall intensities reveal a similar behaviour over the entire country: the 90th percentile shows dependence close to the CC relation for all temperatures; the 99th and 99.9th percentiles exhibit rates close to double the CC rate for temperatures between ∼10 and ∼22 °C and negative scaling rates for higher temperatures. The daily intensities have a less clearly defined scaling behaviour.

ANÁLISES CLIMÁTICAS: O FILTRO HODRICK-PRESCOTT APLICADO AOS ÍNDICES ATMOSFÉRICOS DA OSCILAÇÃO SUL E DA OSCILAÇÃO DO ATLÂNTICO NORTE

A existência de ciclos ou periodicidades, aparentes nas séries temporais longas de variáveis meteorológicas, auxilia no entendimento dos processos físicos que controlam o clima e os ciclos identificados, eventualmente, servem como prognosticadores do clima regional. Com essa motivação, foi aplicado o filtro de Hodrick-Prescott (HP) às séries dos índices de Oscilação Sul (IOS) e Oscilação do Atlântico Norte (OAN) no período 1900-2010. O filtro HP, aplicado a uma série temporal longa, gera duas novas séries, uma contendo os sinais da tendência e a outra, os da parte cíclica/aleatória da variável. Posteriormente, cada uma das séries foi submetida à análise das transformadas de ondeletas de Morlet para identificação dos ciclos aparentes. A série da tendência dos IOS apresentou ciclos de 6, 9, 17 e 28 anos, o primeiro associado ao Ciclo El Niño/La Niña, os dois seguintes possivelmente ao Ciclo Nodal Lunar (Ciclo Saros) de 18 anos e o último, à Oscilação Decadal do Pacífico (ODP). A série da tendência de OAN apresentou ciclos de 6, 9 e 33 anos, os dois últimos possivelmente relacionados ao Ciclo Saros. Os coeficientes de correlação linear entre a série cíclica/aleatória do IOS, adiantado de 6 meses, e a precipitação no Brasil apresentaram valores altos, de até 0,72 e -0,67 para áreas do Norte e Sul do País, respectivamente, no período 1948-1999 (52 anos), com nível de significância superior a 99%. Esse período cobre as duas fases da ODP. Considerando que os modelos de clima ainda não apresentam destreza adequada para previsão regional no Brasil, sugere-se que a previsão de clima, utilizando a série cíclica/aleatória do IOS adiantado de até 6 meses, seja explorada e, eventualmente, validada.

Biogeography of jellyfish in the North Atlantic, by traditional and genomic methods

Abstract. Scientific debate on whether or not the recent increase in reports of jellyfish outbreaks represents a true rise in their abundance has outlined a lack of reliable records of Cnidaria and Ctenophora. Here we describe different jellyfish data sets produced within the EU programme EURO-BASIN. These data were assembled with the aim of creating an improved baseline and providing new data that can be used to evaluate the current diversity and standing stocks of jellyfish in the North Atlantic region. Using a net adapted to sample gelatinous zooplankton quantitatively, cnidarians and ctenophores were collected from the epipelagic layer during spring–summer 2010–2013, in inshore and offshore waters between lat 59 and 68° N and long 62° W and 5° E. Jellyfish were also identified and counted in samples opportunistically collected by other sampling equipment in the same region and at two coastal stations in the Bay of Biscay and in the Gulf of Cádiz. Continuous Plankton Recorder (CPR) samples collected in 2009–2012 were re-analysed with the aim of identifying the time and location of cnidarian blooms across the North Atlantic Basin. Overall the data show high variability in jellyfish abundance and diversity, mainly in relation to different water masses and bathymetry. Higher densities were generally recorded on the shelves, where the communities tend to be more diverse due to the presence of meropelagic medusae. Comparison of net records from the G.O. Sars transatlantic cruise shows that information on jellyfish diversity differs significantly depending on the sampling gear utilised. Indeed, the big trawls mostly collect relatively large scyphozoan and hydrozoan species, while small hydrozoans and early stages of Ctenophora are only caught by smaller nets. Based on CPR data from 2009 to 2012, blooms of cnidarians occurred in all seasons across the whole North Atlantic Basin. Molecular analysis revealed that, contrary to previous hypotheses, the CPR is able to detect blooms of meroplanktonic and holoplanktonic hydrozoans and scyphozoans. Through combination of different types of data, key jellyfish taxa for the spring–summer period were identified in the northern North Atlantic regions. Key species for the central and southern North Atlantic could be inferred based on the blooms identified by the CPR survey, although this should be confirmed further by comparison with quantitative data. The identification by DNA barcoding of 23 jellyfish specimens collected during the EURO-BASIN cruises contributes to increasing the still very limited number of jellyfish sequences available on GenBank. All observations presented here can be downloaded from PANGAEA (http://doi.pangaea.de/10.1594/PANGAEA.835732).

POPs in free-ranging pilot whales, sperm whales and fin whales from the Mediterranean Sea: Influence of biological and ecological factors

The pilot whale Globicephala melas, the sperm whale Physeter macrocephalus, and the fin whale Balaenoptera physalus are large cetaceans permanently inhabiting the Mediterranean Sea. These species are subjected to numerous anthropogenic threats such as exposure to high levels of contaminants. Therefore, selected persistent organic pollutants POPs (29 PCBs, 15 organochlorine compounds, 9 PBDEs and 17 PCDD/Fs) were analysed in blubber biopsies of 49 long-finned pilot whales, 61 sperm whales and 70 fin whales sampled in the North Western Mediterranean Sea (NWMS) from 2006 to 2013. Contamination profile and species feeding ecology were then combined through the use of stable isotopes. δ13C, δ15N values and POPs levels were assessed through IR-MS and GC–MS respectively. To assess the toxic potency of the dioxin-like compounds, the TEQ approach was applied. δ15N values were 12.2±1.3‰ for sperm whales, 10.5±0.7‰ for pilot whales and 7.7±0.8‰ in fin whales, positioning sperm whales at higher trophic levels. δ13C of the two odontocetes was similar and amounted to −17.3±0.4‰ for sperm whales and −17.8±0.3‰ for pilot whales; whilst fin whales were more depleted (−18.7±0.4‰). This indicates a partial overlap in toothed-whales feeding habitats, while confirms the differences in feeding behaviour of the mysticete. Pilot whales presented higher concentrations than sperm whales for ΣPCBs (38,666±25,731ngg−1lw and 22,849±15,566ngg−1lw respectively), ΣPBDEs (712±412ngg−1lw and 347±173ngg−1lw respectively) and ΣDDTs (46,081±37,506ngg−1 lw and 37,647±38,518ngg−1lw respectively). Fin whales presented the lowest values, in accordance with its trophic position (ΣPCBs: 5721±5180ngg−1 lw, ΣPBDEs: 177±208ngg−1 lw and ΣDDTs: 6643±5549ngg−1 lw). Each species was characterized by large inter-individual variations that are more related to sex than trophic level, with males presenting higher contaminant burden than females. The discriminant analysis (DA) confirmed how DDTs and highly chlorinated PCBs were influential in differentiating the three species. Pollutant concentrations of our species were significantly higher than both their Southern Hemisphere and North Atlantic counterparts, possibly due to the particular Mediterranean geomorphology, which influences pollutants distribution and recycle. Dioxin-like PCBs accounted for over 80% of the total TEQ. This study demonstrated (1) an important exposure to pollutants of Mediterranean cetaceans, often surpassing the estimated threshold toxicity value of 17,000ngg−1 lw for blubber in marine mammals; and (2) how the final pollutant burden in these animals is strongly influenced not only by the trophic position but also by numerous other factors such as sex, age, body size and geographical distribution.

North Atlantic Eddy-Driven Jet in Interglacial and Glacial Winter Climates

AbstractThe atmospheric westerly flow in the North Atlantic (NA) sector is dominated by atmospheric waves or eddies generating via momentum flux convergence, the so-called eddy-driven jet. The position of this jet is variable and shows for the present-day winter climate three preferred latitudinal states: a northern, central, and southern position in the NA. Here, the authors analyze the behavior of the eddy-driven jet under different glacial and interglacial boundary conditions using atmosphere–land-only simulations with the CCSM4 climate model. As state-of-the-art climate models tend to underestimate the trimodality of the jet latitude, the authors apply a bias correction and successfully extract the trimodal behavior of the jet within CCSM4. The analysis shows that during interglacial times (i.e., the early Holocene and the Eemian) the preferred jet positions are rather stable and the observed multimodality is the typical interglacial character of the jet. During glacial times, the jet is strongly enhanced, its position is shifted southward, and the trimodal behavior vanishes. This is mainly due to the presence of the Laurentide ice sheet (LIS). The LIS enhances stationary waves downstream, thereby accelerating and displacing the NA eddy-driven jet by anomalous stationary momentum flux convergence. Additionally, changes in the transient eddy activity caused by topography changes as well as other glacial boundary conditions lead to an acceleration of the westerly winds over the southern NA at the expense of more northern areas. Consequently, both stationary and transient eddies foster the southward shift of the NA eddy-driven jet during glacial winter times.

Controls on the silicon isotope distribution in the ocean: New diagnostics from a data‐constrained model

AbstractThe global distributions of the silicon isotopes within silicic acid are estimated by adding isotope fractionation to an optimized, data‐constrained model of the oceanic silicon cycle that is embedded in a data‐assimilated steady circulation. Including fractionation during opal dissolution improves the model's ability to capture the approximately linear relation between isotope ratio, δ30Si, and inverse silicic acid concentration observed in the deep Atlantic. To quantify the importance of hydrographic control on the isotope distribution, δ30Si is partitioned into contributions from preformed and regenerated silicic acid, further partitioned according to euphotic zone origin. We find that the large‐scale features of the isotope distribution in the Atlantic basin are dominated by preformed silicic acid, with regenerated silicic acid being important for setting vertical gradients. In the Pacific and Indian Oceans, preformed and regenerated silicic acid make roughly equally important contributions to the pattern of the isotope ratio, with gradients of the preformed and regenerated contributions tending to cancel each other in the deep Pacific. The Southern Ocean euphotic zone is the primary origin of both the preformed and regenerated contributions to δ30Si. Nearly the entire preformed part of δ30Si is of Southern Ocean and North Atlantic origin. The regenerated part of δ30Si in the Atlantic basin also has a contribution of Central Atlantic (∼40°S–40°N) origin that is comparable in magnitude to the North Atlantic contribution. In other basins, the Central Pacific and Indian Ocean are the second largest contributors to the regenerated part of δ30Si.

A-Train Observations of Maritime Midlatitude Storm-Track Cloud Systems: Comparing the Southern Ocean against the North Atlantic

Abstract Cloud and precipitation properties of the midlatitude storm-track regions over the Southern Ocean (SO) and North Atlantic (NA) are explored using reanalysis datasets and A-Train observations from 2007 to 2011. In addition to the high-level retrieval products, lower-level observed variables—CloudSat radar reflectivity and Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar attenuated backscatter—are directly examined using both contoured frequency by altitude diagrams (CFADs) and contoured frequency by temperature diagrams (CFTDs) to provide direct insight into thermodynamic phase properties. While the wintertime temperature profiles are similar over the two regions, the summertime environment is warmer over the NA. The NA atmosphere is generally moister than the SO, while the SO boundary layer is moister during winter. The results herein suggest that although the two regions exhibit many similarities in the prevalence of boundary layer clouds (BLCs) and frontal systems, notable differences exist. The NA environment exhibits stronger seasonality in thermodynamic structure, cloud, and precipitation properties than the SO. The regional differences of cloud properties are dominated by microphysics in winter and thermodynamics in summer. Glaciated clouds with higher reflectivities are found at warmer temperatures over the NA. BLCs (primarily below 1.5 km) are a predominant component over the SO. The wintertime boundary layer is shallower over the SO. Midlevel clouds consisting of smaller hydrometeors in higher concentration (potentially supercooled liquid water) are more frequently observed over the SO. Cirrus clouds are more prevalent over the NA. Notable differences exist in both the frequencies of thermodynamic phases of precipitation and intensity of warm rain over the two regions.

Spatial and temporal evolution of hyperextended rift systems: Implication for the nature, kinematics, and timing of the Iberian-European plate boundary

We focus on the Iberian-European plate boundary (IEPB), whose nature, age, and evolution are strongly debated. In contrast to previous interpretations of the IEPB as a major lithospheric-scale left-lateral strike-slip fault, we propose a more complex deformation history. The mapping of rift domains at the transition between Iberia and Europe emphasizes the existence of spatially disconnected rift systems. Based on their restoration, we suggest that the deformation was partitioned between a set of distinct left-lateral transtensional rift systems from the Late Jurassic to Early Cretaceous. A plate kinematic reorganization at Aptian-Albian time resulted in the onset of sea-floor spreading in the western Bay of Biscay and extreme crustal and lithosphere thinning in intra-continental rift basins to the east. The formation and reactivation of the IEPB is interpreted as the result of the polyphase evolution of a diffuse transient plate boundary that failed to localize. The results of this work may provide new insights on (1) processes preceding breakup and the initiation of segmented and strongly oblique shear margins, (2) the deformation history of nascent divergent plate boundaries, and (3) the kinematics of the southern North Atlantic and Alpine domain in western Europe.

Distinct roles of the Southern Ocean and North Atlantic in the deglacial atmospheric radiocarbon decline

In the context of the atmospheric CO214C/C (ΔCatm14) changes since the last ice age, two episodes of sharp ΔCatm14 decline have been related to either the venting of deeply sequestered low-14C CO2 through the Southern Ocean surface or the abrupt onset of North Atlantic Deep Water (NADW) formation. In model simulations using an improved reconstruction of 14C production, Atlantic circulation change and Southern Ocean CO2 release both contribute to the overall deglacial ΔCatm14 decline, but only the onset of NADW can reproduce the sharp ΔCatm14 declines. To fully simulate ΔCatm14 data requires an additional process that immediately precedes the onsets of NADW. We hypothesize that these “early” ΔCatm14 declines record the thickening of the ocean's thermocline in response to reconstructed transient shutdown of NADW and/or changes in the southern hemisphere westerly winds. Such thermocline thickening may have played a role in triggering the NADW onsets.

Bipolar Atlantic deepwater circulation in the middle-late Eocene: Effects of Southern Ocean gateway openings

We present evidence for Antarctic Circumpolar Current (ACC)-like effects on Atlantic deepwater circulation beginning in the late-middle Eocene. Modern ocean circulation is characterized by a thermal differentiation between Southern Ocean and North Atlantic deepwater formation regions. In order to better constrain the timing and nature of the initial thermal differentiation between Northern Component Water (NCW) and Southern Component Water (SCW), we analyze benthic foraminiferal stable isotope (δ18Obf and δ13Cbf) records from Ocean Drilling Program Site 1053 (upper deep water, western North Atlantic). Our data, compared with published records and interpreted in the context of ocean circulation models, indicate that progressive opening of Southern Ocean gateways and initiation of a circum-Antarctic current caused a transition to a modern-like deep ocean circulation characterized by thermal differentiation between SCW and NCW beginning ~38.5 Ma, in the initial stages of Drake Passage opening. In addition, the relatively low δ18Obf values recorded at Site 1053 show that the cooling trend of the middle-late Eocene was not global, because it was not recorded in the North Atlantic. The timing of thermal differentiation shows that NCW contributed to ocean circulation by the late-middle Eocene, ~1–4 Myr earlier than previously thought. We propose that early NCW originated in the Labrador Sea, based on tectonic reconstructions and changes in foraminiferal assemblages in this basin. Finally, we link further development of meridional isotopic gradients in the Atlantic and Pacific in the late Eocene with the Tasman Gateway deepening (~34 Ma) and the consequent development of a circumpolar proto-ACC.

Modeling the relationship between climate oscillations and drought by a multivariate GARCH model

Typical multivariate time series models may exhibit comovement in mean but not in variance of hydrologic and climatic variables. This paper introduces multivariate generalized autoregressive conditional heteroscedasticity (GARCH) models to capture the comovement of the variance or the conditional covariance between two hydroclimatic time series. The diagonal vectorized and Baba-Engle-Kroft-Kroner models are developed to evaluate the covariance between drought and two atmospheric circulations, Southern Oscillation Index (SOI) and North Atlantic Oscillation (NAO) time series during 1954-2000. The univariate generalized autoregressive conditional heteroscedasticity model indicates a strong persistency level in conditional variance for NAO and a moderate persistency level for SOI. The conditional variance of short-term drought index indicates low level of persistency, while the long-term index drought indicates high level of persistency in conditional variance. The estimated conditional covariance between drought and atmospheric indices is shown to be weak and negative. It is also observed that the covariance between drought and atmospheric indices is largely dependent on short-run variance of atmospheric indices rather than their long-run variance. The nonlinearity and stationarity tests show that the conditional covariances are nonlinear but stationary. However, the degree of nonlinearity is higher for the covariance between long-term drought and atmospheric indices. It is also observed that the nonlinearity of NAO is higher than that for SOI, in contrast to the stationarity which is stronger for SOI time series. Key Points Multivariate heteroscedastic models are developed for drought analysis Conditional covariance between drought, SOI, and NAO is not strong Time-varying correlations between drought and atmospheric indices are estimated.

Long-term trends in ocean plankton production and particle export between 1960–2006

Abstract. We analyse long-term trends in marine primary and particle export production and their link to marine phytoplankton community composition for the period 1960–2006 using a hindcast simulation of the Biogeochemical Elemental Cycling Model coupled to the ocean component of the Community Climate System Model. In our simulation, global primary and export production decrease significantly over the last 50 yr, by 6.5% and 8% respectively. These changes are associated with an 8.5% decrease in small phytoplankton biomass and 5% decrease in zooplankton biomass. Diatom biomass decreases globally by 3%, but with strong temporal and spatial variability. The strongest decreases in primary and export production occur in the western Pacific, where enhanced stratification leads to stronger nutrient limitation and a decrease in total phytoplankton. The concurrent decrease in diatom fraction and in zooplankton biomass causes a lower export efficiency in this region. Substantial phytoplankton composition changes also occur in the Southern Ocean and North Atlantic, although these are masked in part by a high degree of interannual variability. In these regions, stronger wind stress enhances mixing, reducing the biomass of small phytoplankton, while diatoms profit from higher nutrient inputs and lower grazing pressure. The relative fraction of diatoms correlates positively with the export efficiency (r = 0.8, p < 0.05) in most areas except for the North Pacific and Antarctic Circumpolar Current, where the correlation is negative (r = –0.5, p < 0.05). However, the long-term trends in global export efficiency are ultimately driven by the reduction in small phytoplankton and particularly decreases in coccolithophore biomass. The diagnosed trends point toward a substantial sensitivity of marine primary production and export to climatic variations and trends.

Response of Tropical Cyclones to Idealized Climate Change Experiments in a Global High-Resolution Coupled General Circulation Model

Abstract The authors present an assessment of how tropical cyclone activity might change owing to the influence of increased atmospheric carbon dioxide concentrations, using the U.K. High-Resolution Global Environment Model (HiGEM) with N144 resolution (~90 km in the atmosphere and ~40 km in the ocean). Tropical cyclones are identified using a feature-tracking algorithm applied to model output. Tropical cyclones from idealized 30-yr 2×CO2 (2CO2) and 4×CO2 (4CO2) simulations are compared to those identified in a 150-yr present-day simulation that is separated into a five-member ensemble of 30-yr integrations. Tropical cyclones are shown to decrease in frequency globally by 9% in the 2CO2 and 26% in the 4CO2. Tropical cyclones only become more intense in the 4CO2; however, uncoupled time slice experiments reveal an increase in intensity in the 2CO2. An investigation into the large-scale environmental conditions, known to influence tropical cyclone activity in the main development regions, is used to determine the response of tropical cyclone activity to increased atmospheric CO2. A weaker Walker circulation and a reduction in zonally averaged regions of updrafts lead to a shift in the location of tropical cyclones in the Northern Hemisphere. A decrease in mean ascent at 500 hPa contributes to the reduction of tropical cyclones in the 2CO2 in most basins. The larger reduction of tropical cyclones in the 4CO2 arises from further reduction of the mean ascent at 500 hPa and a large enhancement of vertical wind shear, especially in the Southern Hemisphere, North Atlantic, and northeast Pacific.

New discovery of seafloor hydrothermal activity on the Indian Ocean Carlsberg Ridge and Southern North Atlantic Ridge—progress during the 26th Chinese COMRA cruise

The 26th Chinese COMRA (China Ocean Mineral Resources Research & Development Association) cruise was an important cruise. The Carlsberg Ridge (CR) of the Northwest Indian Ocean and the North Atlantic Ridge (NAR), in which less investigation has been carried out for hydrothermal activities, were investigated and studied during the first two legs of the 26th COMRA cruise. During the first leg, we found one hydrothermal activity field located in the CR at 3.5°–3.8°N on the Northwest Indian Ocean Ridge (NWIR), and sampled seafloor polymetallic sulfide deposits where only abnormalities were found before. During the second leg, we found a new hydrothermal anomaly field located in the NAR at 4°–7°N. The discovery of two hydrothermal and anomaly fields filled in the gap of hydrothermal investigation and study in the corresponding regions for China.

Integrated remote sensing and wavelet analyses for screening short-term teleconnection patterns in northeast America

Summary Global sea surface temperature (SST) anomalies have an inherent effect on vegetation dynamics and precipitation processes throughout the continental United States (U.S.). SST variations have been correlated with precipitation patterns via ocean–atmospheric interactions known as climate teleconnections. Prior research has demonstrated that understanding excitation mechanisms of the teleconnection patterns can be instrumental for climate prediction across a wide region at sub-continental scales, yet these studies tend to have large uncertainties in estimates by assuming linearity when examining teleconnection signals. The co-existence of non-stationary and nonlinear signals embedded in SST anomalies makes the identification of the teleconnection patterns difficult at the local scale. This study explores the short-term (10-year) frequencies (i.e., interannual and seasonal ) embedded in the non-stationary teleconnection signals between SST at the North Atlantic and North Pacific oceans and the responses of terrestrial greenness and precipitation along multiple pristine sites in northeast U.S., including (1) White Mountain National Forest – Pemigewasset Wilderness, (2) Green Mountain National Forest – Lye Brook Wilderness, and (3) Adirondack State Park – Siamese Ponds Wilderness. Each site was selected to avoid anthropogenic influences that may otherwise mask climate teleconnection signals. Lagged pixel-wise linear teleconnection analysis based on remote sensing satellite images across anomalous global SST datasets found significant correlation regions between SST and these terrestrial sites. With the aid of wavelet analyses including continuous wavelet transform, cross-wavelet analysis, and wavelet coherency analysis, nonlinear and non-stationary signals exhibit salient covariations at biennial and triennial frequencies between terrestrial responses and SST anomalies across oceanic regions in agreement with the El Nino Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) signals. Multiple regression analysis of the combined ocean indices explained up to 50% of the greenness and 42% of the precipitation in the study sites. These identified short-term signals in association with some hydrometeorological forcing processes of circumglobal teleconnection can improve the understanding and projection of the climate change impacts at local scales and harness the interannual periodicity information for future precipitation and greenness projections.

Influences of ENSO on Stratospheric Variability, and the Descent of Stratospheric Perturbations into the Lower Troposphere

Abstract The linkage between El Niño–Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) through the stratospheric pathway is examined using a global coupled climate model [GFDL Climate Model version 3 (CM3)], with increased vertical resolution and extent in the stratosphere as compared to an earlier model [GFDL Climate Model version 2 (CM2)]. It is demonstrated that the relationship between ENSO and NAO is stronger in CM3 than in CM2. It is found that ENSO plays an important role in modulating the frequency of occurrence of the stratospheric polar vortex anomalies through enhancement/attenuation of the amplitudes of zonal wavenumbers 1 and 2, especially in late winter. A higher frequency of weak (strong) stratospheric vortex events is simulated in CM3 during El Niño (La Niña) episodes. The weak vortex events during El Niño winters are preceded by enhancement of the zonal wave-1 pattern and weakening of zonal wave-2 pattern. These modified tropospheric planetary waves propagate upward and then weaken the stratospheric polar vortex through eddy–mean flow interaction. The zonal-mean geopotential response in the stratosphere propagates downward and weakens the polar vortex throughout the troposphere. The effects of planetary wave refraction in the upper troposphere on the zonally averaged circulation cells in the tropospheric meridional plane, and the linkage between the lower branches of these cells and the near-surface wind patterns, play an important role in the flow pattern over the region corresponding to the southern lobe of the NAO. Specifically, a negative annular mode and NAO response is discernible in weak stratospheric vortex events during El Niño. Conversely, the positive annular mode and NAO is evident in strong vortex events during La Niña.