Further investigation of the impact of idealised continents and SST distributions on the Northern Hemisphere storm tracks

Building on studies by Brayshaw et al. (2009, 2011) of the basic ingredients of the North Atlantic storm track (land-sea contrast, orography and SST), this article investigates the impact of Eurasian topography and Pacific SST anomalies on the North Pacific and Atlantic storm tracks through a hierarchy of atmospheric GCM simulations using idealised boundary conditions in the HadGAM1 model. The Himalaya-Tibet mountain complex is found to play a crucial role in shaping the North Pacific storm track. The northward deflection of the westerly flow around northern Tibet generates an extensive pool of very cold air in the north-eastern tip of the Asian continent, which strengthens the meridional temperature gradient and favours baroclinic growth in the western Pacific. The Kuroshio SST front is also instrumental in strengthening the Pacific storm track through its impact on near-surface baroclinicity, while the warm waters around Indonesia tend to weaken it through the impact on baroclinicity of stationary Rossby waves propagating poleward from the convective heating regions. Three mechanisms by which the Atlantic storm track may be affected by changes in the boundary conditions upstream of the Rockies are discussed. In the model configuration used here, stationary Rossby waves emanating from Tibet appear to weaken the North Atlantic storm track substantially, whereas those generated over the cold waters off Peru appear to strengthen it. Changes in eddy-driven surface winds over the Pacific generally appear to modify the flow over the Rocky Mountains, leading to consistent modifications in the Atlantic storm track. The evidence for each of these mechanisms is, however, ultimately equivocal in these simulations.

Wave-breaking characteristics of Northern Hemisphere winter blocking: a two-dimensional approach

This paper generalises and applies recently developed blocking diagnostics in a two- dimensional latitude-longitude context, which takes into consideration both mid- and high-latitude blocking. These diagnostics identify characteristics of the associated wave-breaking as seen in the potential temperature (θ) on the dynamical tropopause, in particular the cyclonic or anticyclonic Direction of wave-Breaking (DB index), and the Relative Intensity (RI index) of the air masses that contribute to blocking formation. The methodology is extended to a 2-D domain and a cluster technique is deployed to classify mid- and high-latitude blocking according to the wave-breaking characteristics. Mid-latitude blocking is observed over Europe and Asia, where the meridional gradient of θ is generally weak, whereas high-latitude blocking is mainly present over the oceans, to the north of the jet-stream, where the meridional gradient of θ is much stronger. They occur respectively on the equatorward and poleward flank of the jet- stream, where the horizontal shear ∂u/∂y is positive in the first case and negative in the second case. A regional analysis is also conducted. It is found that cold-anticyclonic and cyclonic blocking divert the storm-track respectively to the south and to the north over the East Atlantic and western Europe. Furthermore, warm-cyclonic blocking over the Pacific and cold-anticyclonic blocking over Europe are identified as the most persistent types and are associated with large amplitude anomalies in temperature and precipitation. Finally, the high-latitude, cyclonic events seem to correlate well with low- frequency modes of variability over the Pacific and Atlantic Ocean.

Winter and Summer Northern Hemisphere blocking in CMIP5 models

The frequencies of atmospheric blocking in both winter and summer and the changes in them from the 20th to the 21st centuries as simulated in twelve CMIP5 models is analysed. The RCP 8.5 high emission scenario runs are used to represent the 21st century. The analysis is based on the wave-breaking methodology of Pelly and Hoskins (2003a). It differs from the Tibaldi and Molteni (1990) index in viewing equatorward cut-off lows and poleward blocking highs in equal manner as indicating a disruption to the westerlies. 1-dimensional and 2-dimensional diagnostics are applied to identify blocking of the mid-latitude storm-track and also at higher latitudes. Winter blocking frequency is found to be generally underestimated. The models give a decrease in the European blocking maximum in the 21st century, consistent with the results in other studies. There is a mean 21st century winter poleward shift of high- latitude blocking, but little agreement between the models on the details. In summer, Eurasian blocking is also underestimated in the models, whereas it is now too large over the high-latitude ocean basins. A decrease in European blocking frequency in the 21st century model runs is again found. However in summer there is a clear eastward shift of blocking over Eastern Europe and Western Russia, in a region close to the blocking that dominated the Russian summer of 2010. While summer blocking decreases in general, the poleward shift of the storm track into the region of frequent high latitude blocking may mean that the incidence of storms being obstructed by blocks may actually increase.

Cryptic hammerhead shark lineage occurrence in the western South Atlantic revealed by DNA analysis

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Past and present distribution, densities and movements of blue whales Balaenoptera musculus in the Southern Hemisphere and northern Indian Ocean

1. Blue whale locations in the Southern Hemisphere and northern Indian Ocean were obtained from catches (303 239), sightings (4383 records of ≥ 8058 whales), strandings (103), Discovery marks (2191) and recoveries (95), and acoustic recordings. 2. Sighting surveys included 7 480 450 km of effort plus 14 676 days with unmeasured effort. Groups usually consisted of solitary whales (65.2%) or pairs (24.6%); larger feeding aggregations of unassociated individuals were only rarely observed. Sighting rates (groups per 1000 km from many platform types) varied by four orders of magnitude and were lowest in the waters of Brazil, South Africa, the eastern tropical Pacific, Antarctica and South Georgia; higher in the Subantarctic and Peru; and highest around Indonesia, Sri Lanka, Chile, southern Australia and south of Madagascar. 3. Blue whales avoid the oligotrophic central gyres of the Indian, Pacific and Atlantic Oceans, but are more common where phytoplankton densities are high, and where there are dynamic oceanographic processes like upwelling and frontal meandering. 4. Compared with historical catches, the Antarctic (‘true’) subspecies is exceedingly rare and usually concentrated closer to the summer pack ice. In summer they are found throughout the Antarctic; in winter they migrate to southern Africa (although recent sightings there are rare) and to other northerly locations (based on acoustics), although some overwinter in the Antarctic. 5. Pygmy blue whales are found around the Indian Ocean and from southern Australia to New Zealand. At least four groupings are evident: northern Indian Ocean, from Madagascar to the Subantarctic, Indonesia to western and southern Australia, and from New Zealand northwards to the equator. Sighting rates are typically much higher than for Antarctic blue whales.

Is Drosera meristocaulis a pygmy sundew? Evidence of a long-distance dispersal between Western Australia and northern South America

South America and Oceania possess numerous floristic similarities, often confirmed by morphological and molecular data. The carnivorous Drosera meristocaulis (Droseraceae), endemic to the Neblina highlands of northern South America, was known to share morphological characters with the pygmy sundews of Drosera sect. Bryastrum, which are endemic to Australia and New Zealand. The inclusion of D. meristocaulis in a molecular phylogenetic analysis may clarify its systematic position and offer an opportunity to investigate character evolution in Droseraceae and phylogeographic patterns between South America and Oceania. was included in a molecular phylogenetic analysis of Droseraceae, using nuclear internal transcribed spacer (ITS) and plastid rbcL and rps16 sequence data. Pollen of D. meristocaulis was studied using light microscopy and scanning electron microscopy techniques, and the karyotype was inferred from root tip meristem. The phylogenetic inferences (maximum parsimony, maximum likelihood and Bayesian approaches) substantiate with high statistical support the inclusion of sect. Meristocaulis and its single species, D. meristocaulis, within the Australian Drosera clade, sister to a group comprising species of sect. Bryastrum. A chromosome number of 2n approx. 3236 supports the phylogenetic position within the Australian clade. The undivided styles, conspicuous large setuous stipules, a cryptocotylar (hypogaeous) germination pattern and pollen tetrads with aperture of intermediate type 78 are key morphological traits shared between D. meristocaulis and pygmy sundews of sect. Bryastrum from Australia and New Zealand. The multidisciplinary approach adopted in this study (using morphological, palynological, cytotaxonomic and molecular phylogenetic data) enabled us to elucidate the relationships of the thus far unplaced taxon D. meristocaulis. Long-distance dispersal between southwestern Oceania and northern South America is the most likely scenario to explain the phylogeographic pattern revealed.

Changing correlation structures of the Northern Hemisphere atmospheric circulation from 1000 to 2100 AD

Atmospheric circulation modes are important concepts in understanding the variability of atmospheric dynamics. Assuming their spatial patterns to be fixed, such modes are often described by simple indices from rather short observational data sets. The increasing length of reanalysis products allows these concepts and assumptions to be scrutinised. Here we investigate the stability of spatial patterns of Northern Hemisphere teleconnections by using the Twentieth Century Reanalysis as well as several control and transient millennium-scale simulations with coupled models. The observed and simulated centre of action of the two major teleconnection patterns, the North Atlantic Oscillation (NAO) and to some extent the Pacific North American (PNA), are not stable in time. The currently observed dipole pattern of the NAO, its centre of action over Iceland and the Azores, split into a north–south dipole pattern in the western Atlantic with a wave train pattern in the eastern part, connecting the British Isles with West Greenland and the eastern Mediterranean during the period 1940–1969 AD. The PNA centres of action over Canada are shifted southwards and over Florida into the Gulf of Mexico during the period 1915–1944 AD. The analysis further shows that shifts in the centres of action of either teleconnection pattern are not related to changes in the external forcing applied in transient simulations of the last millennium. Such shifts in their centres of action are accompanied by changes in the relation of local precipitation and temperature with the overlying atmospheric mode. These findings further undermine the assumption of stationarity between local climate/proxy variability and large-scale dynamics inherent when using proxy-based reconstructions of atmospheric modes, and call for a more robust understanding of atmospheric variability on decadal timescales.

Accumulation rates for western Dronning Maud Land from firn cores and snow pits

As a result of intensive field activities carried out by several nations over the past 15 years, a set of accumulation measurements for western Dronning Maud Land, Antarctica, was collected, based on firn-core drilling and snow-pit sampling. This new information was supplemented by earlier data taken from the literature, resulting in 111 accumulation values. Using Geographical Information Systems software, a first region-wide mean annual snow-accumulation field was derived. In order to define suitable interpolation criteria, the accumulation records were analyzed with respect to their spatial autocorrelation and statistical properties. The resulting accumulation pattern resembles well known characteristics such as a relatively wet coastal area with a sharp transition to the dry interior, but also reveals complex topographic effects. Furthermore, this work identifies new high-return shallow drilling sites by uncovering areas of insufficient sampling density.

Chronology and stable carbon isotopic ratios of sediments from the Western Pacific Warm Pool region

Instrumental data suggest that major shifts in tropical Pacific atmospheric dynamics and hydrology have occurred within the past century, potentially in response to anthropogenic warming. To better understand these trends, we use the hydrogen isotopic ratios of terrestrial higher plant leaf waxes (DDwax) in marine sediments from southwest Sulawesi, Indonesia, to compile a detailed reconstruction of central Indo-Pacific Warm Pool (IPWP) hydrologic variability spanning most of the last two millennia. Our paleodata are highly correlated with a monsoon reconstruction from Southeast Asia, indicating that intervals of strong East Asian summer monsoon (EASM) activity are associated with a weaker Indonesian monsoon (IM). Furthermore, the centennial-scale oscillations in our data follow known changes in Northern Hemisphere climate (e.g., the Little Ice Age and Medieval Warm Period) implying a dynamic link between Northern Hemisphere temperatures and IPWP hydrology. The inverse relationship between the EASM and IM suggests that migrations of the Intertropical Convergence Zone and associated changes in monsoon strength caused synoptic hydrologic shifts in the IPWP throughout most of the past two millennia.

Sedimentology and Cd/Ca ratios of benthic foraminifera from ODP Site 172-1060 in the western Nort Atlantic

The Western Boundary Undercurrent (WBUC), off eastern America, is an important component of the Atlantic Meridional Overturning circulation and is the principal route for southward transport of North Atlantic waters and southward return of Southern Source Water (SSW). Here a direct flow speed proxy (mean grain size of the sortable silt) is used to infer the vigour of flow of the palaeo-WBUC at Blake Outer Ridge, (ODP Site 1060, depth 3481 m) during Marine Isotope Stage (MIS) 3. The overall shape of the flow speed proxy record shows a complex pattern of variability, with generally more vigorous flow and larger-scale flow variations between 35 and 60 ka than in the younger part of MIS 3 and MIS 2 (b35 ka). Six events of reduced bottom flow vigour (Slow Events, SEs) occur. These appear uncorrelated with Heinrich events, but are instead synchronous with the warming phases of Antarctic Warm Events A-1 to A-4 (with one new one, A-1a and one poorly defined, 'A-0'). This indicates that Antarctic climate exerts a stronger control on deep flow vigour in the North Atlantic during MIS 3 than Northern Hemisphere climate. The correspondence of SEs with Antarctic warming suggests a weaker WBUC flow due to reduced volume flux at SSW source or reduced SSW density. Because the variability of the lower limb of the WBUC was not connected to sharp North Atlantic changes in temperature, it is unlikely that the Dansgaard/Oeschger cycles were associated with a mode of MOC variation involving wholeocean overturn, but more likely with perturbations of only the shallow Glacial Gulf Stream-Glacial Northern Source Intermediate Water cell. Nutrient proxies (benthic carbon isotopes and Cd/Ca of Uvigerina peregrina) at this site show similar trends to the GRIP delta18O record. This correlation has previously been attributed mainly to hydrographic and flow changes but is here shown to be better explained by variations in surface ocean productivity and subsequent decomposition of 12C rich organic material on the sea floor.

Determination of millennial-scale climate variability based on alkenones in the western Atlantic Ocean

High-resolution records of alkenone-derived sea surface temperatures and elemental Ti/Ca ratios from a sediment core retrieved off northeastern Brazil (4° S) reveal short-term climate variability throughout the past 63,000 a. Large pulses of terrigenous sediment discharge, caused by increased precipitation in the Brazilian hinterland, coincide with Heinrich events and the Younger Dryas period. Terrigenous input maxima related to Heinrich events H6-H2 are characterized by rapid cooling of surface water ranging between 0.5 and 2° C. This signature is consistent with a climate model experiment where a reduction of the Atlantic meridional overturning circulation (AMOC) and related North Atlantic cooling causes intensification of NE trade winds and a southward movement of the Intertropical Convergence Zone, resulting in enhanced precipitation off northeastern Brazil. During deglaciation the surface temperature evolution at the core site predominantly followed the Antarctic warming trend, including a cooling, prior to the Younger Dryas period. An abrupt temperature rise preceding the onset of the Bølling/Allerød transition agrees with model experiments suggesting a Southern Hemisphere origin for the abrupt resumption of the AMOC during deglaciation caused by Southern Ocean warming and associated with northward flow anomalies of the South Atlantic western boundary current.

Stable isotopes, core logging data, relative paleointensity, dry bulk density, biogenic opal, CN-data (TC, TOC, CaCO3, TN), element concentrations, and coarse fraction of three sediment cores from the western Bering Sea

We used piston cores recovered in the western Bering Sea to reconstruct millennial-scale changes in marine productivity and terrigenous matter supply over the past ~180 kyr. Based on a geochemical multi-proxy approach, our results indicate closely interacting processes controlling marine productivity and terrigenous matter supply comparable to the situation in the Okhotsk Sea. Overall, terrigenous inputs were high, whereas export production was low. Minor increases in marine productivity occurred during intervals of Marine Isotope Stage 5 and interstadials, but pronounced maxima were recorded during interglacials and Termination I. The terrigenous material is suggested to be derived from continental sources on the eastern Bering Sea shelf and to be subsequently transported via sea ice, which is likely to drive changes in surface productivity, terrigenous inputs, and upper-ocean stratification. From our results we propose glacial, deglacial, and interglacial scenarios for environmental change in the Bering Sea. These changes seem to be primarily controlled by insolation and sea-level forcing which affect the strength of atmospheric pressure systems and sea-ice growth. The opening history of the Bering Strait is considered to have had an additional impact. High-resolution core logging data (color b*, XRF scans) strongly correspond to the Dansgaard-Oeschger climate variability registered in the NGRIP ice core and support an atmospheric coupling mechanism of Northern Hemisphere climates.

Alkenone analyses of sediment cores from the western Arabian Sea

Records of total organic carbon (TOC) and C37 alkenones were used as indicators for past primary productivity in the western and eastern Arabian Sea. Data from GeoB 3005, an open ocean site in the western Arabian Sea upwelling area, are compared with similar records of GeoB 3007 from the Owen Ridge, Ocean Drilling Program (ODP) Site 723 from the continental margin off Oman and MD 900963 from the eastern Arabian Sea. TOC/C37 alkenone records together with other proxies used to reconstruct upwelling intensity, indicate periods of high productivity in tune with precessional forcing all over the Arabian Sea. Based on their phase-relationship to variations in boreal summer insolation they can be divided into three groups. In the western Arabian Sea the precession-related phasing is different between productivity proxies and those for summer monsoon wind strength and upwelling intensity. TOC and C37 alkenone records from the western Arabian Sea lag the other monsoonal indicators by about 5 kyr, but lead productivity indicators from the eastern Arabian Sea by 3 kyr. Based on the differences in phase relationships associated with the precessional cycling between productivity and monsoonal proxies in the western Arabian Sea it is proposed that the TOC/C37 alkenone signal in the western Arabian Sea document a combined signal of moderate SW monsoon winds and of strengthened and prolonged NE monsoon winds. In the eastern Arabian Sea the phasing hints to coincidence between maximum productivity and stronger NE monsoon winds associated with precession-related maxima in ice volume. In contrast, variations in paleoproductivity at site GeoB 3007 from the Owen Ridge indicate productivity maxima during glacial substages 8.2, 6.2 and 2.2, whereas precessionrelated changes are of only minor importance at this location. The results of frequency analyses confirm that productivity at site GeoB 3007 responds predominantly to glacialinterglacial climate changes, while site GeoB 3005 from the open ocean upwelling region near the Gulf of Aden is dominated by precessional insolation. A possible explanation for the pattern revealed at the Owen Ridge is the periodic NW-SE displacement of the Findlater Jet axis, which separates the region of open ocean upwelling to the northwest from downwelling to the southeast ofthe jet. The carbon isotopes of planktic foraminifera reflect nutrient related d13C variations of dissolved inorganic carbon. The difference between the planktic foraminifera Globigerinoides ruber (w), living in the upper 50 m of the water column, and the deeper Iiving Neogloboquadrina dutertrei (Delta d13Cr-d) of core GeoB 3005 displays nutrient variations in the upwelling area near the Gulf of Aden. The results of cross-spectral analyses between Deltad13Cr-d of GeoB 3005 and proxies for SW monsoon intensity indicate, too, a dissociation of productivity from monsoonal upwelling intensity. Instead, productivity depends mainly on the availability of nutrients, while upwelling intensity of sub-surface water masses seems to be of only secondary importance. Additionally, sea surface temperatures (SSTs) were reconstructed using the unsaturation ratio of C37 alkenones. Alkenone SSTs reflect annual mean temperatures rather than explicitly the season of upwelling. This is evident from alkenone SSTs in a transect of surface sediments extending from the inner Gulf of Aden into the western Arabian Sea. The alkenone-derived SST records of GeoB 3005 from the open ocean upwelling region near the Gulf of Aden and GeoB 3007 from the Owen Ridge reveal similar variations with high SSTs during interglacial and low SSTs during glacial periods. The glacial marine oxygen isotope stage (MIS) 6 remains relatively warm and was not as cold as MIS 3 to 4 and 8 according to the alkenone SST. Similar variation-patterns were reconstructed in the coastal upwelling area off Oman for ODP Site 723 as weIl as in the eastern Arabian Sea for MD 900963, where upwelling is not as pronounced as in the western Arabian Sea. Spectral-analyses indicate that SST changes are in good agreement with the modulation of low-latitude precessional insolation changes by eccentricity. However, they do not show the pronounced cydicity in the precessional frequency band, which is characteristic for variations in paleoproductivity. Although the overall variation pattern is very similar, a dose comparison between the western (GeoB 3005) and the eastern Arabian Sea (MD 900963) shows larger differences between both sites during cold intervals than during periods of warm SSTs. This is attributed to a more effective cooling of surface waters in the western Arabian Sea by prolonged NE monsoon winds during times of expanded Northern Hemisphere ice-sheets, thereby lowering the annual mean SSTs stronger than in the eastern Arabian Sea.

A taxonomic tool for identifying needle remains of south-western European Pinus species of the Late Quaternary

This work provides a tool whereby the needle remains of native, south-western European Pinus spp. can be easily identified from species-specific epidermal features. To construct this tool, the needles of P. uncinata, P. sylvestris, P. nigra, P. pinaster, P. pinea and P. halepensis were gathered across the Northern Hemisphere range of each taxon and compared with non-indigenous trees growing in two South Australian Botanic Gardens. Three needles from each of these species were taken from three adult trees growing at three different localities. Light microscopy was used to observe the key epidermal and stomatal features of the needles. To improve interpretation, additional scanning electron microscopy samples were prepared. Epidermal features, including variation in the diameter of the epistomatal chamber aperture (pore), are described. A taxonomic key based on the size, shape and arrangement of the subsidiary cells of the stomatal complexes was constructed. This key enables the identification of pine needle fragments at the species level (except those belonging to the group P. gr. nigra-uncinata). Despite their overlapping range, pore size was helpful in distinguishing between P. nigra and P. uncinata and between three groups of species. Isolated stomata were also observed. Cluster and discriminant analyses of stomatal variables described in earlier studies were performed. Overlap in guard cell variables hampers species-level identification of isolated stomata. Species discrimination is improved if groups of ecological affinity are considered.

Planktonic foraminiferal preservation in Plio/Pleistocene sediments from the western equatorial Atlantic and Caribbean

Records of mean sortable silt and planktonic foraminiferal preservation from the Ceará Rise (western equatorial Atlantic) and from the Caribbean are presented to analyze the Pliocene (3.5-2.2 Ma) to Pleistocene (1.6-0.3 Ma) evolution of near-bottom current strength and the carbonate corrosiveness of deep water. During the mid-Pleistocene climate transition (~1 Ma) a drastic decrease in glacial bottom current strength and an increase in carbonate corrosiveness is registered, demonstrating a substantial decrease in the glacial contribution of the Lower North Atlantic Deep Water (LNADW) to the Atlantic Ocean. Also, an increased sensitivity to eccentricity orbital forcing is registered after the MPT. By contrast, carbonate preservation increases considerably in the deep Caribbean in response to a strong and persistent stable contribution of Upper North Atlantic Deep Water (UNADW). We found evidence for the strongest and most stable circulation within the LNADW cell during the Northern Hemisphere cooling period between ~3.2 and 2.75 Ma. This is in agreement with the 'superconveyor model' which postulates that the highest NADW production took place prior to ~2.7 Ma. A considerable decrease in bottom current strength and planktonic foraminiferal preservation is observed synchronous with the first occurrence of large-scale continental ice sheets in the Northern Hemisphere. This documents the final termination of the 'superconveyor' at ca. 2.75 Ma. However, our data do not support a 'superconveyor' in the interval between 3.5 and 3.2 Ma when high-amplitude fluctuations in bottom current flow and preservation in planktonic foraminifera are observed. Because of the great sensitivity of NADW production to changes in surface water salinity, we assume that the high-amplitude fluctuations of LNADW circulation prior to ~3.2 Ma are linked to changes in the Atlantic salinity budget. After 2.75 Ma they are primarily controlled by ice-sheet forcing. In contrast to the stepwise deterioration of planktonic foraminiferal preservation in the western deep Atlantic, a trend toward better preservation from the Pliocene to Pleistocene is observed in the deep Caribbean. This indicates a long-term increase in the contribution of UNADW to the Atlantic Ocean.