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The life history and oceanic impact of three very large icebergs that escaped together from the Weddell Sea sea ice, near the tip of the Antarctic Peninsula, are traced from March 2014. Despite the initial proximity of these three icebergs, they followed very different trajectories across the South Atlantic until their eventual break-up and melting 1 year later. The largest, giant iceberg, B17a, spent extensive periods grounded near two different islands. The triplet's gradual melting is examined through the impact on the icebergs' dimensions, but also the meltwater's oceanic influence on the local salinity and primary productivity. It is found that there was generally a significant local surface and mixed-layer freshening of a few tenths of a practical salinity unit, up to several hundred kilometres away from the 10–20 km-sized icebergs. In contrast, the chlorophyll impact was highly temporally variable, although it tended to be larger in the summer. Break-up of these large icebergs did not occur until near the end of their life. We also show that modelling the trajectories of individual very large icebergs can be reasonable for up to 2 weeks if the characteristics of the iceberg and the local ocean and atmospheric forcing are well known.

Funding information: This work was funded by the Natural Environment Research Council consortium grant BRITICE-CHRONO NE/J009768/1 and by the NERC Radio- carbon Facility and the NERC Cosmogenic Isotope Analysis Facility. The project benefited from the PalGlac team of researchers with funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme to CDC (Grant Agreement No. 787263) and supporting SB and AI. JCE acknowledges support from a NERC independent fellowship award (NE/R014574/1). We thank the PISM team of ice-sheet modellers and Evan Gowan for ICESHEET, and note that the development of PISM was supported by NSF grants PLR- 1603799 and PLR-1644277 and NASA grant NNX17AG65G. DP acknowledges funding from the Italian PNRA project IPY GLAMAR (grant number 2009/A2.15), and from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 656821 (project SEAGAS). The BRITICE‐CHRONO consortium of researchers undertook a dating programme to constrain the timing of advance, maximum extent and retreat of the British–Irish Ice Sheet between 31 000 and 15 000 years before present. The dating campaign across Ireland and Britain and their continental shelves, and across the North Sea included 1500 days of field investigation yielding 18 000 km of marine geophysical data, 377 cores of sea floor sediments, and geomorphological and stratigraphical information at 121 sites on land; generating 690 new geochronometric ages. These findings are reported in 28 publications including synthesis into eight transect reconstructions. Here we build ice sheet‐wide reconstructions consistent with these findings and using retreat patterns and dates for the inter‐transect areas. Two reconstructions are presented, a wholly empirical version and a version that combines modelling with the new empirical evidence. Palaeoglaciological maps of ice extent, thickness, velocity, and flow geometry at thousand‐year timesteps are presented. The maximum ice volume of 1.8 m sea level equivalent occurred at 23 ka. A larger extent than previously defined is found and widespread advance of ice to the continental shelf break is confirmed during the last glacial. Asynchrony occurred in the timing of maximum extent and onset of retreat, ranging from 30 to 22 ka. The tipping point of deglaciation at 22 ka was triggered by ice stream retreat and saddle collapses. Analysis of retreat rates leads us to accept our hypothesis that the marine‐influenced sectors collapsed rapidly. First order controls on ice‐sheet demise were glacio‐isostatic loading triggering retreat of marine sectors, aided by glaciological instabilities and then climate warming finished off the smaller, terrestrial ice sheet. Overprinted on this signal were second order controls arising from variations in trough topographies and with sector‐scale ice geometric readjustments arising from dispositions in the geography of the landscape. These second order controls produced a stepped deglaciation. The retreat of the British–Irish Ice Sheet is now the world’s most well‐constrained and a valuable data‐rich environment for improving ice‐sheet modelling. Publisher PDF

Icebergs are known to have a significant fertilizing impact on primary productivity in the Southern Ocean, but this link is yet to be investigated in the Northern Hemisphere. This study combines sightings of icebergs with satellite-derived ocean colour data from 1998 to 2015, to seek such a relationship in the NW Atlantic. Despite the obscuring coincidence of the seasonal iceberg flux with the spring bloom of chlorophyll, it is shown that there is a large-scale, one-month-lagged regional correlation between iceberg flux and chlorophyll levels. In addition, a spatial time-lag analysis is consistent with the main cause for the iceberg-chlorophyll relationship being through advection of the nutrients entrained in iceberg meltwater. This leads to a delayed fertilization response of 2–4 weeks. There are a range of possible sources for the nutrients likely leading to this delayed response. The Northern Hemisphere impact of iceberg meltwater on primary production is much less pronounced than in the Southern Ocean, but it is discernible.

The British and Irish Ice Sheet (BIIS) was highly dynamic during the Late Quaternary, with considerable regional differences in the timing and extent of its change. This was reflected in equally variable offshore ice‐rafted debris (IRD) records. Here we reconcile these two records using the FRUGAL intermediate complexity iceberg–climate model, with varying BIIS catchment‐level iceberg fluxes, to simulate change in IRD origin and magnitude along the western European margin at 1000‐year time steps during the height of the last BIIS glaciation (31–6 ka bp). This modelled IRD variability is compared with existing IRD records from the deep ocean at five cores along this margin. There is general agreement of the temporal and spatial IRD variability between observations and model through this period. The Porcupine Bank off northwestern Ireland was confirmed by the modelling as a major dividing line between sites possessing exclusively northern or southern source regions for offshore IRD. During Heinrich events 1 and 2, the cores show evidence of a proportion of North American IRD, more particularly to the south of the British Isles. Modelling supports this southern bias for likely Heinrich impact, but also suggests North American IRD will only reach the British margin in unusual circumstances.

Highlights: • NW Pacific ice-rafted debris found further south than previously thought. • Largest iceberg events were comparable in magnitude to N Atlantic Heinrich Events. • Iceberg flux to ODP Site 1207 was episodic throughout the late Quaternary. • Kamchatka-Koryak bergs frequently reached south of the N Pacific Subarctic Front. • Ice-rafting and climate contrast sharply between the last two glacial periods. Abstract: The ice-rafted-debris (IRD) record of the open Northwest Pacific points towards the existence of substantial glacial ice on the Northeast Siberian coast during the late Quaternary. However, the scale and timing of glaciation and de-glaciation remains controversial due to the dearth of both onshore and offshore records. Existing IRD data suggests at least one event of dynamic and abrupt change during mid-late Marine Isotope Stage (MIS 3) which mimics the massive collapse of the Laurentide ice sheet during Heinrich Events. It is uncertain whether other events of this magnitude occurred during the late Quaternary. Here we present a ∼160,000 yr IRD series, planktic foraminiferal counts and an age model, derived from a benthic O curve, radiocarbon dates and tephrochronology, from core ODP 1207A (37.79°N, 162.75°E), revealing the presence of low but episodic flux of IRD. We conclude that glacial Northwest Pacific icebergs spread further south than previously thought, with icebergs emanating from Northeast Siberia being transported to the transition region between the subpolar and subtropical waters, south of the subarctic front during at least the Quaternary's last two glacial periods. The episodic nature of the 1207A IRD record during the last glacial, combined with coupled climate-iceberg modelling, suggests occasional times of much enhanced ice flux from the Kamchatka-Koryak coast, with other potential sources on the Sea of Okhotsk coast. These findings support the hypothesis of a variable but extensive ice mass during the last glacial over Northeast Siberia, particularly early in the last glacial period, behaving independently of North American and Eurasian ice masses. In strong contrast, IRD was absent during much of the penultimate glacial Marine Isotope Stage (MIS) 6 suggesting the possibility of very different Northeast Siberian ice coverage between the last two glacial periods.

Palaeoceanographic evidence suggests that glacial periods of the Mid to Late Pleistocene were characterized by markedly different global ocean circulation patterns to modern; in the Atlantic basin, deep waters of Southern Ocean origin increased in volume while above them the core of the North Atlantic Deep Water (NADW) shoaled. Whilst proxy records and modelling efforts continue to clarify this picture, an evidence alluding to the origin of this phenomenon remains elusive. Because of this, our understanding of the sequence of events leading to global glacial conditions remains incomplete. Here we present multi-proxy evidence showing that northward shifts in Antarctic iceberg melt in the Indian–Atlantic Southern Ocean (0–50°E) systematically preceded deep-water mass reorganizations by 1-2 thousand years during Pleistocene-era glaciations. With the aid of iceberg-trajectory model experiments, we demonstrate that such a shift in iceberg trajectories during glacial periods can result in a considerable redistribution of freshwater in the Southern Ocean. This, in concert with increased sea-ice cover, may have enabled positive buoyancy anomalies to effectively escape into the ‘upper’ Atlantic overturning circulation limb, providing a teleconnection between surface Southern Ocean conditions and the formation of NADW. Furthermore, we observe a distinct obliquity pacing of Antarctic iceberg melt both preceding and following the Mid-Pleistocene Transition, become obscured during this interval. With new and existing data we investigate the evolution of orbital forcing at the Agulhas Plateau, considering the implications for ‘Southern Escape’ of freshwater as a key feedback in the transition to the ‘100-kyr world’.

A systematic comparison of El Nino Southern Oscillation reconstructions during the early to mid-nineteenth century is presented using a range of proxy and documentary sources. Reconstructions of the boreal winter Southern Oscillation Index (SOI) using data from ships’ logbooks presented in a companion paper are evaluated and compared to previous ENSO reconstructions. Comparisons between ENSO reconstructions and the instrumental SOI during a period of overlap (1876–1977) are made. These same proxy and documentary reconstructions are then compared to the logbook-based reconstructions, over 1815–1854. The logbook-based reconstructions compare best with a recent multi-proxy reconstruction that used signals taken from different teleconnection regions, and they have an improved agreement with multi-proxy records compared to a previous attempt to reconstruct the SOI from ships’ logbook data. The logbook-based and the multi-proxy reconstructions are found to capture El Nino events better than La Nina events, and East Pacific El Nino events better than Central Pacific El Nino events, thus suggesting a degree of bias in the historical reconstructions. These findings have important implications for future ENSO reconstructions, with a need for an increased understanding of the effects of different ENSO flavours for future reconstructions.

Iceberg discharge is a major component of the mass balance of the Greenland Ice Sheet (GrIS). While bulk estimates of discharge variation over time exist, inferred remotely from measurements of grounding line ice velocities or surface mass balance calculations, few detailed measurements of discharge itself from individual marine-terminating glaciers existed until recent years. Recently, it has been shown, through a combination of ocean-iceberg modelling and non-linear system identification, that the century-long record of iceberg numbers crossing 48oN in the West Atlantic is a good first-order proxy for discharge from at least south and west Greenland. Here, we explore the varying relative importance of ice sheet, oceanic and climatic forcing of iceberg discharge from these areas over the twentieth century, by carrying out sensitivity studies of a non-linear auto-regressive mathematical model of the 48oN time series. We find that the relationships are mainly non-linear, with the contribution of the GrIS surface mass balance to iceberg discharge likely to be dominant in the first half of the century. This period is followed by several decades where oceanic temperature effects are most important in determining the model variation in iceberg discharge. In recent decades, all physical processes play a non-negligible part in explaining the iceberg discharge and the model suggests that the glacial response time to environmental changes may have decreased. © 2015.

With the polar regions opening up to more marine activities but iceberg numbers\ud more likely to increase than decline as a result of global warming, the risk from icebergs to\ud shipping and offshore facilities is increasing. The NW Atlantic iceberg hazard has been\ud well monitored by the International Ice Patrol for a century, but many other polar regions\ud have little detailed climatological knowledge of the iceberg risk. Here, we develop a\ud modelling approach to assessing iceberg hazard. This uses the region of the Falklands\ud Plateau and its shipping routes for a case study, but the approach has general geographical\ud applicability and can be used for assessing iceberg hazard for routes or fixed locations. The\ud iceberg risk for a number of locations selected from the main shipping routes in the SW\ud Atlantic is assessed by using an iceberg model, forced by the output from a high-resolution\ud ocean model. The iceberg model was seeded with icebergs around the edge of the modelled\ud region using a number of scenarios for the seeding distribution, based on a combination of\ud idealised, modelled and observed iceberg fluxes from the Southern Ocean. This enabled us\ud to determine measures of iceberg risk linked to a mix of starting location and the likelihood\ud of icebergs being encountered in such a position. For our study area, the main area of\ud iceberg risk is linked to the East Falklands Current, but small, yet nonzero, risk covers\ud much of the east and north of the region.

We have used a coupled ocean-iceberg model to study the variation in global ocean circulation and North Atlantic iceberg flux from 1900 to 2008. The latter component of the study focused particularly on Greenland icebergs feeding into the Labrador Current and past Newfoundland. The model was forced with daily heat, freshwater and wind fluxes from the Twentieth Century Reanalysis. The reanalysis heat fluxes were shown to be offset from the, shorter, NCEP reanalysis and a grid-point correction was applied to this component of the forcing. The model produces a generally realistic ocean circulation, although with an enhanced Atlantic Meridional Overturning largely due to the forcing. The modelled iceberg flux at 48°N is well correlated with the long-term observed flux when using a modelled iceberg discharge that varies in a similar fashion to the highly variable observed flux at 48°N. From this model we infer changes in the spatial and temporal variability of iceberg calving from western Greenland. During the first third of the twentieth century the majority of modelled icebergs reaching 48°N derive from southern Greenland, while only after 1930 is the traditional perspective of a majority of such icebergs originating from Baffin Bay consistent with model results. Decadal-scale changes in the dominant regional sources are found, with oscillations between western Greenland and northern Baffin Bay. The latter origin was modelled to be most important in the last third of the twentieth century, although west Greenland sources have increased in importance in recent years. The model correctly reproduces the pronounced late spring peak in flux at 48°N for southern Greenland icebergs, but has an approximately six month offset for icebergs from Baffin Bay, most likely due to resolution issues leading to model icebergs not being delayed in shallow coastal waters, whereas in reality they may be grounded for some time or trapped in coastal sea-ice. © 2015 The Authors.

In this paper we investigate the effect on sea-ice movement of changes in the synoptic atmospheric conditions in late boreal summer 2010. Our study area is the western Fram Strait, a crucial passage for the transport of ice out of the Arctic basin. Ice dynamics here affect the movement of ice in the East Greenland Current, the transpolar drift and ice extent in the Arctic Ocean. In contrast to other times of the year, when the Fram Strait wind field is characterized by strong, persistent northerlies, we show that the weaker, more variable winds typical during late summer for the Fram Strait can slow movement of ice floes out of the area, thus slowing the export of ice from the Arctic Ocean at the end of summer, a time crucial for ice export. The Arctic Ocean could lose even more of the ice that survives the summer if this was not the case. This would leave the Arctic Ocean in an even more vulnerable position with regard to the amount of multi-year ice remaining the following summer.

Nutrient input from icebergs can fertilize productivity in the ocean. Ten years of satellite measurements reveal that giant icebergs could be responsible for up to 20% of carbon export to depth in the Southern Ocean.

At 2340h local time (0310 GMT) on the cold, moonless, night of 14 April 1912, near 41°47 N, 49°55 W (Marine Accident Investigation Branch, 1992), the crow’s nest lookouts on board RMS Titanic sighted a large iceberg only 500m ahead. Despite quick action on the bridge to slow the ship, and turn to port, as well as the closing of the water-tight doors, the slow response of a large vessel meant that the iceberg still struck the ship aft of the bows. Some 100m of her hull below the waterline buckled, allowing water to flood into the ship across several compartments (Howells, 1992). In little more than two and a half hours she had sunk, with the loss of 1514 lives (Havern, 2012). A distress call requesting assistance was transmitted only 20 minutes after the collision, and RMS Carpathia turned and raced towards the Titanic at a speed of 17.5kn, 20% above her normal maximum speed. However, she did not arrive at the scene until around 0330h (15 April; 0700 GMT), by which time only 710 people remained to be rescued from the 20 lifeboats that had been able to be launched (Howells, 1992). The weather, ice conditions and time of year combined to increase the iceberg hazard on that fateful day. High pressure had dominated the mid-latitude, central Atlantic for several days (Howells, 1992) and by the time of the collision a ridge linking two high-pressure centres over Nova Scotia and the south of Ireland extended across the entire Atlantic (Figure 1). This resulted in northnorthwest winds transporting nearfreezing air from northeast Canada over the western Atlantic south of Newfoundland (Figure 1). These winds and temperatures, assisted by the prevailing southward flow of the ocean’s Labrador Current on the Grand Banks, led to transport of icebergs and sea ice further south than is currently normal for the time of year, but not beyond the known limits to icebergs during the twentieth century (Figure 2). However, note that Figure 2. The average sea-ice limit for April 1979–2013 (dotted), a typical Newfoundland maximum sea-ice limit for the early twentieth century (dashed and denoted as 1912; from Hill and Jones, 1990) and the maximum iceberg limit for 1900–2000 are shown, in addition to the 48°N line. The location of the Titanic is shown by an ‘X’. The blue shading shows depth, with the lightest blue denoting the continental shelf (

In this study, the principal controls on the coastal environment around the Pacific island of Okinawa, typical of the many mountainous islands of the sub-tropical and tropical northwest Pacific in the arc from Melanesia to the Ryukyu Islands, are investigated. Two principal satellite remote sensing data products were used to observe properties at regional-scale and in the Okinawan coastal zone: (1) AVHRR sea surface temperature (SST) for 1985–2006 and (2) a SeaWiFS ocean colour variable to estimate combined sediment and chlorophyll-a loadings for 1998–2006, along with direct observations (e.g. climatological data, river discharge, suspended sediment and typhoon tracks). Analysis suggested that coastal SST is strongly influenced by the outer ocean SST, indicating a regional control. However, the local winds, with associated summer rainfall and copious sediment-laden run-off, had a significant secondary impact. Typhoons are also an important factor leading to ocean colour variation. Along the western coastal zone of Okinawa, the winter East Asian Monsoon causes significant upwelling and seafloor sediment removal, while terrestrial impact is a significant secondary control in summer on ocean colour variability via sediment-laden river run-off. Previously noted changes in the magnitude of extreme rainfall events in the area suggest that sediment delivery to the coastal zones of islands in the region has been increasing in recent decades, providing a combined climate and terrestrial impact on the coastal environment. Although typhoons and heavy rainfall are episodic events, appropriate land management on the many hilly and mountainous islands of the sub-tropical northwestern Pacific is needed if such events are not to lead to deterioration, and decrease of resilience, of coastal ecosystems.

The marine-based Atlantic periphery of the last NW European Ice Sheet experienced significant advances and retreats of its marine sector during its existence. It therefore had considerable potentia ...

Observations from satellites play an important role in tracking sea ice movement. Synthetic aperture radar (SAR) images are preferred because they are neither weather nor daylight dependent. Tracking the movement of individual objects (icebergs or sea ice floes) that may threaten ships or installations is of particular interest. The Ice Tracking from SAR Images (ITSARI) algorithm was originally developed to track large icebergs in the Antarctic. An adaptation of this algorithm to identify and track the movement of sea ice floes in Arctic conditions is presented, using ENVISAT wide swath SAR images of the Fram Strait acquired at 1–3-day intervals during the month of February 2008. It is shown that the algorithm can be successfully adapted to gain a general sense of the direction of movement of the ice due to both winds and ocean currents and, significantly, to identify and track specific objects of interest over a series of images.

Past changes in the density and momentum structure of oceanic circulation are an important aspect of changes in the Atlantic Meridional Overturning Circulation and consequently climate. However, very little is known about past changes in the vertical density structure of the ocean, even very extensively studied systems such as the North Atlantic. Here we exploit the physical controls on the settling depth of the dense Mediterranean water plume derived from the Strait of Gibraltar to obtain the first robust, observations-based, probabilistic reconstruction of the vertical density gradient in the eastern North Atlantic during the last 30,000 years. We find that this gradient was weakened by more than 50%, relative to the present, during the last Glacial Maximum, and that changes in general are associated with reductions in AMOC intensity. However, we find only a small change during Heinrich Event 1 relative to the Last Glacial Maximum, despite strong evidence that overturning was substantially altered. This implies that millennial-scale changes may not be reflected in vertical density structure of the ocean, which may be limited to responses on an ocean-overturning timescale or longer. Regardless, our novel reconstruction of Atlantic density structure can be used as the basis for a dynamical measure for validation of model-based AMOC reconstructions. In addition, our general approach is transferrable to other marginal sea outflow plumes, to provide estimates of oceanic vertical density gradients in other locations.

Ice-rafted debris (IRD) seeded into the ocean from Northern Hemisphere ice sheets is found in ocean cores along the southwestern European margin through the last glacial period. It is known that the origin of this IRD, especially off Iberia, can vary between North America and western Europe during short-lived episodes of greatly enhanced iceberg flux, known as Heinrich events, although in most Heinrich events the IRD has a North American source. During the longer times of much lower IRD fluxes between Heinrich events, use of an intermediate complexity climate model, coupled to an iceberg dynamic and thermodynamic model, shows that background levels of IRD most likely originate from western Europe, particularly the British Irish Ice Sheet. Combining modelling with palaeoceanographic evidence supports reconstructions of a short-lived, but substantial, Celtic and Irish Sea Ice Stream around 23 ka.

The presence of thick (∼ 200 m) carbonaceous shales of Cretaceous age in the deep waters of the eastern Equatorial Atlantic was first revealed by drilling at DSDP sites 367 and 368 near the Cape Verde Islands. More recent seismic investigations show that black shales are widely distributed in this region. In the Gambia Basin they are associated with a group of seismic reflectors lying at depths of 400–600 m below the sea floor. These can be traced southwards from the drill sites into the Sierra Leone Basin and eastwards beneath the African continental rise to depths at which ambient temperatures fall within the oil generation window. In the Gambia Basin the reflectors continue westwards from the Cape Verde Archipelago before terminating on Turonian oceanic basement (∼ 92 Ma). South of the Guinea Fracture Zone near 10° N extension of the same seismic sequence on to younger oceanic crust indicates that deep-sea carbonaceous deposits formed close to the present equator as late as the early Campanian (∼ 80 Ma), long after a series of oceanic anoxic events in the Aptian–Albian (OAE1) and over the Cenomanian–Turonian boundary (OAE2) led to widespread accumulation of black shales. This late period of deposition may correspond to the more locally developed OAE3. High surface productivity in equatorial upwelling zones during the late Cretaceous probably played an important role in delivering large amounts of organic carbon to the sea floor when meridional movement of deep water between the South and North Atlantic was restricted by closely-spaced, east–west fracture zones. In some areas seismic data show that the black shales level out rather than drape pre-existing topography, implying significant downslope transport of organic-rich material from upwelling cells near the African margin.

An established iceberg module, ICB, is used interactively with the Nucleus for European Modelling of the Ocean (NEMO) ocean model in a new implementation, NEMO–ICB (v1.0). A 30-year hindcast (1976–2005) simulation with an eddy-permitting (0.25°) global configuration of NEMO–ICB is undertaken to evaluate the influence of icebergs on sea ice, hydrography, mixed layer depths (MLDs), and ocean currents, through comparison with a control simulation in which the equivalent iceberg mass flux is applied as coastal runoff, a common forcing in ocean models. In the Southern Hemisphere (SH), drift and melting of icebergs are in balance after around 5 years, whereas the equilibration timescale for the Northern Hemisphere (NH) is 15–20 years. Iceberg drift patterns, and Southern Ocean iceberg mass, compare favourably with available observations. Freshwater forcing due to iceberg melting is most pronounced very locally, in the coastal zone around much of Antarctica, where it often exceeds in magnitude and opposes the negative freshwater fluxes associated with sea ice freezing. However, at most locations in the polar Southern Ocean, the annual-mean freshwater flux due to icebergs, if present, is typically an order of magnitude smaller than the contribution of sea ice melting and precipitation. A notable exception is the southwest Atlantic sector of the Southern Ocean, where iceberg melting reaches around 50% of net precipitation over a large area. Including icebergs in place of coastal runoff, sea ice concentration and thickness are notably decreased at most locations around Antarctica, by up to ~ 20% in the eastern Weddell Sea, with more limited increases, of up to ~ 10% in the Bellingshausen Sea. Antarctic sea ice mass decreases by 2.9%, overall. As a consequence of changes in net freshwater forcing and sea ice, salinity and temperature distributions are also substantially altered. Surface salinity increases by ~ 0.1 psu around much of Antarctica, due to suppressed coastal runoff, with extensive freshening at depth, extending to the greatest depths in the polar Southern Ocean where discernible effects on both salinity and temperature reach 2500 m in the Weddell Sea by the last pentad of the simulation. Substantial physical and dynamical responses to icebergs, throughout the global ocean, are explained by rapid propagation of density anomalies from high-to-low latitudes. Complementary to the baseline model used here, three prototype modifications to NEMO–ICB are also introduced and discussed.

We model iceberg flow paths from the Eurasian Ice Sheet, and the associated meltwater production and sedimentation rates within the Norwegian–Greenland Sea during the last glaciation. Results from a numerical ice sheet model, an atmospheric general circulation model and an ocean general circulation model are collated and used to provide iceberg production rate, wind field and surface current forcings to an iceberg trajectory model. The iceberg model then determines how icebergs issuing from the Eurasian Ice Sheet travel across the ocean and eventually melt. In addition the release of iceberg sediments is also predicted. The results show that iceberg trajectories are complex and that common features of iceberg movement are clustering in zones of convergence and exit into the North Atlantic through the Iceland–Faeroes Channel. Eurasian icebergs do not penetrate into the interior of the Arctic Ocean. The gathering of icebergs produces a complex meltwater pattern that does not follow the conceptual idea of decreasing meltwater production with distance from the ice sheet margin. Sedimentation results are compared with the meltwater configuration and are found to be a poor indicator of past zones of iceberg melt, with zones of sedimentation extending significantly less far.

“Great Salinity Anomalies” (GSAs) have been observed to propagate around the North Atlantic subpolar gyre. Similar anomalies occur in the Third Hadley Centre Coupled Atmosphere–Ocean GCM (HadCM3) of preindustrial climate. It has been hypothesized that these salinity anomalies result from the advection of anomalously low salinity waters around the subpolar gyre. Here, the consequences of using passive tracers in the HadCM3 climate model to tag the anomalously low salinity water associated with a GSA in the Greenland and Labrador Seas are reported. Rather than predominantly advecting around the modeled subpolar gyre in accordance with the upper-ocean salinity anomaly, the tracers mix to intermediate depths, before becoming incorporated into the model's North Atlantic Deep Water. Horizontal advection of the tracer in the upper ocean is limited to around 1000 km, compared with the gyre-scale propagation of the salinity anomalies. It is concluded that GSAs are unlikely to be caused by the advection of salinity anomalies; rather anomalous oceanic currents or surface fluxes are responsible.

A global fine resolution curvilinear ocean model, forced by NCEP Re-Analysis fluxes, is used to study changes in the circulation of the Nordic Seas and surrounding ocean basins during 1994–2001. The model fields exhibit regionally distinct temporal variability, mostly determined by atmospheric forcing but in regions of significant sea-ice longer timescale variability is found. Some abrupt circulation changes accompany the relaxation of the westerlies following the peak North Atlantic Oscillation Index phase of the mid 1990s. The Greenland gyre spins up over the following years, with the increased circulation partially exiting through the Denmark Strait into the northern Atlantic as well as re-circulating within the Nordic Seas. This resulted in a distinct freshening around northern Iceland and an increase in the East Icelandic Current. However, these latter increases steadied after 1998, as the increased Greenland Sea gyre circulation led to a greater proportion of water leaving through the Denmark Strait...

Icebergs play an important role in climate through the transfer of fresh water and heat between ice sheets and the oceans. Synthetic aperture radar (SAR) images offer a robust way of observing icebergs in the often dark and cloud-covered polar areas. A computer-based technique is proposed to identify icebergs at higher resolution than in the past, allowing the shape of small to medium icebergs to be retained and used for tracking between images acquired at different times. The identification consists of (i) automatic segmentation, followed by (ii) classification of these objects as icebergs or nonicebergs. The classification stage requires user intervention as the icebergs are not always a separable class from other objects, most notably sea ice. The icebergs are then matched between images using their size and shape similarities, and their position tracked. The system performance was assessed by applying the technique to three wintertime ERS-1 PRI images, around Kapp Norvegia (10°W), on the Eastern Weddell Sea coast. The results show both the identification and the tracking to be effective.

A 16-box model of the global ocean circulation is constructed to investigate the sensitivity of the thermohaline circulation to upper-ocean density perturbations. The model attempts to represent the main geographical components of the global ocean and is tuned to give a present-day circulation broadly compatible with observed volume fluxes. Extensive tests are made of the model's sensitivity to upper-ocean density perturbations, equivalent to a range of ±3 psu in salinity about the current climate. It is found that there are seven separate modes of thermohaline circulation that can occur within the model's constraints, as well as the possibility of significant variations in the strength of the current circulation mode. These modes are related to possible changes in surface forcing, such as the abrupt increase in freshwater from ice-sheet melting, where such a possibility exists. A mode with North Atlantic upwelling rather than deep- or bottom-water formation was found to occur with large addition...

Understanding the evolution of the surface circulation and the patterns of upwelling in the Atlantic is important not only because of the ocean’s crucial role in the present-day thermohaline circulation, in supplying heat to high latitudes, but also because of the significant petroleum reservoirs found around the basin. In this paper we examine the upper ocean circulation during four time-slices – the Kimmeridgian (c. 155 Ma), the Cenomanian (c. 100 Ma), the Maastrichtian (c. 70 Ma) and the Early Eocene (c. 50 Ma) – using a 1.5-layer, reduced-gravity, ocean model. For each time-slice the upwelling fields predicted from simulations of wind-driven upper layer circulation are compared with the deep-sea and continental margin sedimentary records. The boundary configurations at each time-slice are the subject of sensitivity tests, where matching of both deep-sea and coastal sedimentary records with the model-predicted upwelling fields is used to suggest best fits between model and geological data. Our model runs are consistent with there being a deep-water connection to the Pacific at each time period after the Kimmeridgian, with such a link being most important to the geological–ocean-model match in the Maastrichtian. No period after the Kimmeridgian produced a stable ocean simulation with a deep-water connection to Tethys. Our simulations c. 100 Ma suggest that a deep-water connection between the North and South Atlanctic was not established until after the Cenomanian. All time-slices show a clear western boundary current in the North Atlantic, although generally only a third to half the strength of today’s. A similar difference is found between tropical palaeo-productivity at each run, and today’s more productive Atlantic Ocean. We conclude that upwelling and anoxia are not necessarily independent means of producing organic-rich marine sediments. Most such sedimentary deposits laid down during the four time-slices can be linked to the presence of upwelling zones. Thus, where anoxia is well-established we suggest that upwelling may help supply organic material to the de-oxygenated bottom waters.

An ocean general-circulation model with an orthogonal curvilinear grid is presented, with the grid north pole over Greenland to achieve improved resolution in the Greenland and Labrador Seas. The model is integrated for 4500 years in five different experiments, with open and closed connections to the Arctic via the Bering Strait (BS) and Canadian Archipelago (CA). When the BS and CA connections are open the Atlantic overturning circulation has a realistic strength (20 Sv), whereas there is stronger overturning with either (25–31 Sv) or both (39 Sv) the connections closed due to additional deep-water formation in the Labrador Sea. The volume fluxes across the Greenland–Iceland–Scotland ridge are reasonably close to those observed with the two connections either open or closed. With only the BS closed, low-salinity waters leave the Arctic via the CA, giving a weak East Greenland Current, whereas with only the CA closed deep-water formation in the Nordic Seas is weak due to the passage of excess low-salinity water originating from the Pacific via the BS. The simulations suggest that both the CA and BS connections to the Arctic Influence the North Atlantic circulation. Closing the BS reduces the effective freshwater input to the Arctic, leading to a strong Atlantic overturning circulation. Closing the CA leads to increased salinity in the Labrador Sea, and additional deep-water formation. It is therefore recommended that both these seaways should be represented in ocean general-circulation models. Copyright © 2002 Royal Meteorological Society

Small to medium sized icebergs (200 m to 10 km across) in two areas of the Weddell Sea were tracked using satellite-borne Synthetic Aperture Radar (SAR) from 22 January–21 February 1994 and 29 January–25 February 1992 respectively. The westward mass flux of icebergs in the Antarctic Coastal Current was estimated at the eastern entrance to the Weddell Sea as being 50–70 Gta−1. A large contrast was found between observations over the narrow shelf here and the off-shelf area. The latter region had very much reduced iceberg density, and the bergs moved with slower and less coherent velocities compared with the fast, but narrow (10–20 km wide) flow in the Coastal Current. This region is a promising site for the monitoring of decadal trends in iceberg fluxes. The second study area in the south-western Weddell Sea showed iceberg motion consistent with non-contemporaneous observations of tagged icebergs, with steady flow parallel to the shelf-ice edge, at significantly lower densities and speeds than in the Coastal Current but almost double that observed off-shelf in the eastern Weddell Sea. We also suggest that shipboard observations of icebergs need careful analysis to avoid their over-estimating true iceberg concentrations by a substantial amount.

Present-day (PD) and Last Glacial Maximum (LGM) simulations of the global ocean are presented, with the oxygen-18 isotope included as a passive tracer. The gradient of the PD North Atlantic surface y 18 O:salinity relationship is found to result from different processes at low and high latitudes. At low latitudes, the balance between surface 18 O flux and oceanic advection and mixing sets the surface y 18 O:salinity gradient, whereas at high latitudes, mixing between 18 O-depleted runoff and precipitation to the Arctic, Bering Strait inflow, and waters from lower latitudes, controls the y 18 O:salinity gradient. The importance of the Bering Strait contribution has not previously been recognised. These gradients change significantly at the LGM, and are found to be sensitive to both Arctic runoff y 18 O concentrations and changes in oceanic advection, particularly the rate of exchange of North Atlantic deep water with the global ocean. It is concluded that reconstructions of past climates from records of sea surface y 18 O based on analogues of the PD y 18 O:salinity relationship are likely to be in error. D 2002 Elsevier

A general climatology of the main mechanisms involved in Mediterranean cyclogenesis is presented. A diagnostic study of both composite means and case studies is performed to analyze processes occurring in different seasons, and in different cyclogenetic regions within the same season. It is shown that cyclones that developed over the three most active areas in winter—the Gulf of Genoa, the Aegean Sea, and the Black Sea—are essentially subsynoptic lows, triggered by the major North Atlantic synoptic systems being affected by local orography and/or low-level baroclinicity over the northern Mediterranean coast. It is also suggested that cyclones in two, or all three, of these regions often occur consecutively, linked to the same synoptic system. In spring and summer, thermally induced lows become progressively more important, despite the existence of other factors, such as the Atlas Mountains contributing to lee cyclogenesis in northern Africa, or the extension of the Asian monsoon into the eastern ...

We examine variations in the ice-rafted sources for sediments in the Iceland/East Greenland offshore marine archives by utilizing a sediment unmixing model and link the results to a coupled iceberg-ocean model. Surface samples from around Iceland and along the E/NE Greenland shelf are used to define potential sediment sources, and these are examined within the context of the down-core variations in mineralogy in the

This is the first large-scale modeling study of iceberg trajectories and melt rates in the Southern Ocean. An iceberg model was seeded with climatological iceberg calving rates based on a calculation of the net surface accumulation from each snow catchment area on the Antarctic continent. In most areas, modeled trajectories show good agreement with observed patterns of iceberg motion, though discrepencies in the Weddell Sea have highlighted problems in the ocean general circulation model output used to force the iceberg model. The Coriolis force is found to be important in keeping bergs entrained in the coastal current around Antarctica, and topographic features are important in causing bergs to depart from the coastal regions. The modeled geographic distribution of iceberg meltwater joining the ocean has been calculated and is found in many near-coastal regions to be comparable in magnitude to the excess of precipitation over evaporation (P-E).

The precipitation fields of a palaeoatmospheric general circulation model are used to derive estimates of the geographical distribution, and flux, of icebergs from the Laurentide, Fennoscandinavian and eastern Siberian ice-sheets at the Last Glacial Maximum (LGM). The atmospheric model fields from LGM simulations using CLIMAP or Peltier (ICE-4G) ice orography were studied, to test the sensitivity of the predicted flux. The estimated Northern Hemispheric LGM iceberg flux is 3500–4000 km3 yr−1, of which about 60% issued directly into the North Atlantic. The iceberg flux from the St Lawrence area is of similar significance to that issuing from Hudson Strait in all estimates. Both the North Pacific and the Arctic received substantial iceberg fluxes (ca. 700 km3 yr−1), with relatively minor differences occurring between the two ice-sheet reconstructions. Apparent discrepancies between Arctic deep-sea core samples of ice-rafted debris and our estimates of mean glacial iceberg flux may be ascribed to coastal trapping of bergs, the existence of floating ice tongues or a rapid exit of icebergs from the Arctic basin into the Greenland Sea through the Fram Strait. Copyright © 2001 John Wiley & Sons, Ltd.

Climate and ocean-only models have shown that the ocean will respond abruptly to significant perturbations in surface forcing. Centennial-scale oscillation is a characteristic of circulation in large semi-enclosed ocean basins such as the Arctic, whereas millennial-scale adjustment to changes in surface forcing has been found in the global ocean component of climate models. We show that the millennial time-scale in climate models is likely to be intrinsic to the ocean through its presence in an ocean-only model. The strength of the thermohaline circulation is shown to be very sensitive to the magnitude of ice albedo and, to a lesser extent, perturbation in the surface freshwater flux. Modelled glacial ocean circulation, in contrast to present-day simulations, requires an enhanced freshwater flux over the northern Atlantic, even in its non-Heinrich state, to obtain realistic overturning in the North Atlantic. Copyright © 2001 John Wiley & Sons, Ltd.

An anomalous climatic event in the tropical Atlantic, starting in autumn 1995 and continuing into the autumn of 1997, is investigated. Using remotely sensed datasets and a reduced-gravity model, it is shown that this event contained both a warm and a cold phase, each of roughly 12 months duration. The propagating signal within each phase is identified in the sea surface height and temperature signatures as both equatorial and off-equatorial waves in the ocean, with coupling to atmospheric convection. The strength, geographical pattern and timing of the anomalous period is consistent with it being the first observed example of a coupled mode of interaction between the ocean and atmosphere found in two previous coupled ocean-atmosphere models of the tropics. This interpretation means that the two-phased climate event, which we will call the Equatorial Atlantic Oscillation (EAO), was associated with purely internal atmospheric and oceanographic variability within the Atlantic basin and hence was independent of the Pacific climate. It can also be inferred, from long-term monthly sea surface temperature and sea-level pressure datasets, that there were potentially several previous EAO events during the past century.

Causal mechanisms of mesocyclone and synoptic cyclone characteristics over the Northeast Atlantic and Nordic Seas are investigated using a 2-year database compiled from infrared satellite imagery. A strong seasonality is found, with peak activity in winter and a minimum in spring. A distinct interannual variability is also found. Smaller mesocyclones (

Polar mesocyclones are subsynoptic-scale low pressure systems that form over high-latitude oceans in a cold air mass. It is known that these systems can be very vigorous but only relatively short time periods have been studied in the past to estimate their characteristics. Here satellite imagery is used to study mesocyclones observed over the North-East Atlantic and Nordic Seas during the 2-year period October 1993–September 1995. An unexpectedly large number (4054) of mesocyclones were found, occurring throughout the year, although winter was the most active season. Most mesocyclones were observed in the northern regions of the study area, near the ice edge, and were small, with 86% of cyclones having diameters of

An objective cyclone detection and tracking analysis is performed over an 18-yr period, for the Mediterranean basin. The high-resolution (1.125° × 1.125° grid) European Centre for Medium-Range Weather Forecasts data used in this study proved to be particularly suitable for the detection and tracking techniques and to identify subsynoptic-scale Mediterranean lows, which have often been underestimated in previous studies. The major characteristics of Mediterranean cyclones are examined and compared with other Northern Hemisphere depressions. Both cyclogenesis and cyclolysis regions are identified in the domain of study. In addition, characteristics of Mediterranean depressions, such as cyclone duration and intensity, as well as their persistence throughout the year, are shown to be quite variable for different formation areas. Overall, the regions where cyclogenesis is mainly controlled by topography, like the Gulf of Genoa and south of the Atlas Mountains, seem to generally account for the most in...

Using several relationships between calving velocity and physical parameters at the glacier's entry to the ocean, estimates are made of the iceberg flux from the Greenland ice sheet. The flux from each major glacier is estimated, leading to total iceberg fluxes of 170-270 Tkg yr-~. This figure is rather less than recent estimates of this flux derived from an assumption of equilibrium in the ice sheet mass balance. The iceberg flux is consistent with airborne altimeter measurements.

A reduced-gravity ocean modelis used to deter-mine the pattern of upwelling in the Cenomanian Atlantic.The deposition of organic-rich sediments on the margins ofthe equatorial region is found to be consistent with the pat-terns of upwelling in simulations which incorporate only ashallow ( 300m) equatorial connection between the Northand South Atlantic. Fracture zone ridges were probably re-sponsible for the e ective shallowness of the gateway. If adeep equatorial channel is assumed the modeling results areconsistent only with the morerecent, Turonian, sedimentaryrecord. We suggest, therefore, that reduced-gravity modelsprovide a mean of predicting areas of the continental mar-gins which may be underlain by hydrocarbon-rich sedimen-tary rocks. Introduction The equatorial region of the Atlantic is a major route forwater transfer between the two hemispheres. Studies of itssedimentary cover have provided abundant evidence for ma-jor environmental changeswithin this critical oceanographicgateway from late Jurassic time. Among the important sed-imentary consequences ofcirculation and sea level variationswas the development of petroleum-rich sequences along thecontinental margins and in the adjacent deep-water basins.On the present continental shelves these occur over a widestratigraphic range, from the early Cretaceous to Oligocene[Reyre , 1966; Nairn and Stehli, 1973; Akpati , 1978; Kesse,1984; Dumestre and Carvalho, 1985; Herbin et al., 1986;Koutsoukos et al. , 1991], while in the deep-sea organic-richshales ranging in age from Albian to late Cretaceous havebeen found at several Deep Sea Drilling Project (DSDP)sites (Figure 1). The deep sea deposits have been attributedto oceanwide anoxic events (OAEs)[ Pedersen and Calvert,1990; Arthur and Sageman, 1994]. The stratigraphic rangeof the continental margin black shales is much broader thanthe deep sequences however. We have to look for othermechanisms apartfrom general oceananoxia to explain this.These sequences could be ascribed to enhanced primary pro-ductivity arising from upwelling of nutrients into the eu-photic zone[Barron, 1985;Parrish, 1995]. Parrish[1995]be

Hydrodynamic modelling of water movement in Hunts Bay, a protected part of Kingston Harbour, Jamaica, shows that depth averaged tidal flows are very low. In the northeast corner of Hunts Bay, water is essentially stagnant. Even under high flow conditions, much of the Bay bottom water is `bypassed' by buoyant, lower salinity surface flows. The muddy sediments of Hunts Bay reflect these sluggish to stagnant conditions; sediment cores from the northeast corner of the Bay contain progressively higher amounts of organic matter in their upper parts (∼last 15–20 years sedimentation). Combined C/N ratios and stable carbon isotope compositions of this organic matter imply a sewage origin. Both lead and chromium metal concentrations and enrichment factors relative to average crustal shales show geographically related patterns that reflect hydrodynamic circulation predicted by modelling. In particular, metal concentrations and enrichment factors are highest at the northern end of the bay, especially in the northeast corner. Modelling confirms that stagnant conditions would occur in the northeast part of the bay even without the presence of a major causeway. The causeway may contribute to low flow conditions, but is not the principal cause of organic contamination, which is simply an excessive input of sewage.

Two stable thermohaline circulation states were produced in simulations of the last glacial maximum (LGM) ocean, both forced by the same prescribed atmosphere. Direct application of this atmosphere led to an equilibrium state with strong North Atlantic Deep Water formation (northern sinking state (NSS)). Upon applying a weak freshwater flux anomaly to the North Atlantic for a short time a state was entered with little North Atlantic, but significant Southern Ocean, Deep Water formation (southern sinking state (SSS)). A fully dynamic and thermodynamic iceberg trajectory model was developed, and icebergs were seeded into both states. Distinctly different pathways were found, particularly in the eastern Atlantic where the SSS icebergs tend to move south along the European coast while they move north in the NSS. While neither ocean state is claimed to be a definitive model for the LGM ocean, paleoclimate data are more consistent with the characteristics of the NSS.

In paleoceanography, a traditional application of oxygen isotope ratios in fossil carbonate from deep-sea cores concerns the reconstruction of paleotemperatures. Recently, isotopic data have been increasingly employed in the reconstruction of paleosalinities as well. This application involves a number of basic assumptions, a critical review of which is presented here. As the calculation of paleosalinity from δ18O residuals assumes constant and linear surface water salinity (S): δ18O relationships, we investigate this basic assumption for areas with important and variable freshwater budget and/or sea-ice influences. We show that it is particularly unwarranted to assume such temporally invariant linearity in temperate and subpolar seas because of the effects of freezing and melting. More critically, we argue that the determinant of many regions' S:δ18O is advection, not the local water balance, so that the basic δ18O of seawater in many regions of the world ocean would likely have changed in a manner somewhat different from that expected from the ice volume record. Although the differences may not be significant for the gross reconstruction of such features as past sea level, we believe they do potentially bias paleosalinity reconstructions, mostly toward high salinities. To fully exploit the potential of oxygen isotope records as quantitative paleosalinity indicators, both the spatial and temporal variabilities need to be constrained. Hence we recommend that (1) for any time slice of specific interest to paleoceanographic and paleoclimatological studies, a coupled ocean-atmosphere-isotope model, or at least an ocean-isotope model driven by a paleoclimate-isotope model, should be used to define the distribution of the base, mixing end-member, δ18O of sea water; (2) much further regional-scale research is needed on flux rates and isotopic compositions of the various terms in the freshwater budget, as well as on the variability in the various advective terms. The latter variability may be assessed from reconstructions of past seaice distributions and of volume fluxes/isotopic compositions of outflow from marginal seas and through sea straits/passages.

Icebergs are a significant hazard for polar shipping, and, geophysically, are significant components of the mass balance of continental ice sheets while providing major freshwater inputs to the polar oceans. Some modelling of iceberg trajectories has been undertaken in the past, principally in the Labrador Sea, but here we present a hemispheric-wide attempt to model iceberg motion in the Arctic and North Atlantic Oceans. We show that the basic force balance in iceberg motion is between water drag and water advection, but with the pure geostrophic balance being only a minor component of the latter. Iceberg density maps essentially demonstrate the effect of the major boundary currents but we show that the time and size of calving from individual tidewater glaciers are important variables in determining the ultimate fate of bergs. The biggest bergs never leave the Arctic Ocean. All modelled icebergs have melted after about 5 years from their release date, although most melt over the first year. During their lifetime most, but not all bergs, overturn several times. Our model shows good agreement with the limited observational data. We therefore suggest that icebergs, both modelled and observed, may be exploited as previously little-used geophysical tracers.