Inversions of observations near the Azores Front

CTD data from Discovery Cruise 138 of late June, 1983 in the Madeira Abyssal Plain show a strong, but isolated, Mediterranean Water (MW) signal around 35N, 25W. This water mass gives rise to an anticyclonic eddy at its southward tip and, through double diffusive processes, to modification of the density field a few hundred meters below. Current estimates have been calculated from the cruise data using the Bernoulli inverse technique. Extensions of this inverse method, which take into account the depth dependence of dynamically informative density and potential vorticity functions, are developed to investigate the flow field and the consistency of the inversions. The Bernoulli inversions all show a strong eastward current in the upper 500 m around 34–35N. In the south of our study area the upper level flow is predominantly southward, consistent with current meter records from the NEADS 1 site at 33N, 22W. It is speculated that during the summer of 1983 there was a southward excursion of the Azores Front into this area, as has been recorded in the past

Modelling twentieth century global ocean circulation and iceberg flux at 48°N: Implications for west Greenland iceberg discharge

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

Modelling abrupt glacial North Atlantic freshening: Rates of change and their implications for Heinrich events

The abrupt delivery of large amounts of freshwater to the North Atlantic in the form of water or icebergs has been thought to lead to significant climate change, including abrupt slowing of the Atlantic Ocean meridional overturning circulation. In this paper we examine intermediate complexity coupled modelling evidence to estimate the rates of change, and recovery, in oceanic climate that would be expected for such events occurring during glacial times from likely sources around the North Atlantic and Arctic periphery. We show that rates of climate change are slower for events with a European or Arctic origin. Palaeoceanographic data are presented to consider, through the model results, the origin and likely strength of major ice-rafting, or Heinrich, events during the last glacial period. We suggest that Heinrich events H1-H3 are likely to have had a significant contribution from an Arctic source as well as Hudson Strait, leading to the observed climate change. In the case of H1 and H2, we hypothesise that this secondary input is from a Laurentide Arctic source, but the dominant iceberg release for H3 is hypothesised to derive from the northern Fennoscandian Ice Sheet, rather than Hudson Strait. Earlier Heinrich events are suggested to be predominantly Hudson Strait in origin, with H6 having the lowest climate impact, and hence iceberg flux, but H4 having a climate signal of geographically variable length. We hypothesise that this is linked to a combination of climate-affecting events occurring around the globe at this time, and not just of Laurentide origin. (C) 2010 Elsevier B.V. All rights reserved

Polar mesoscale cyclones in the northeast Atlantic: Comparing climatologies from ERA-40 and satellite imagery

Polar mesoscale cyclones over the subarctic are thought to be an important component of the coupled atmosphere–ocean climate system. However, the relatively small scale of these features presents some concern as to their representation in the meteorological reanalysis datasets that are commonly used to drive ocean models. Here polar mesocyclones are detected in the 40-Year European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis dataset (ERA-40) in mean sea level pressure and 500-hPa geopotential height, using an automated cyclone detection algorithm. The results are compared to polar mesocyclones detected in satellite imagery over the northeast Atlantic, for the period October 1993–September 1995. Similar trends in monthly cyclone numbers and a similar spatial distribution are found. However, there is a bias in the size of cyclones detected in the reanalysis. Up to 80% of cyclones larger than 500 km are detected in MSL pressure, but this hit rate decreases, approximately linearly, to ∼40% for 250-km-scale cyclones and to ∼20% for 100-km-scale cyclones. Consequently a substantial component of the associated air–sea fluxes may be missing from the reanalysis, presenting a serious shortcoming when using such reanalysis data for ocean modeling simulations. Eight maxima in cyclone density are apparent in the mean sea level pressure, clustered around synoptic observing stations in the northeast Atlantic. They are likely spurious, and a result of unidentified shortcomings in the ERA-40 data assimilation procedure

High resolution (1 km) positive degree-day modelling of Greenland ice sheet surface mass balance, 1870–2012 using reanalysis data

We show results from a positive degree-day (PDD) model of Greenland ice sheet (GrIS) surface mass balance (SMB), 1870–2012, forced with reanalysis data. The model includes an improved daily temperature parameterization as compared with a previous version and is run at 1 km rather than 5 km resolution. The improvements lead overall to higher SMB with the same forcing data. We also compare our model with results from two regional climate models (RCMs). While there is good qualitative agreement between our PDD model and the RCMs, it usually results in lower precipitation and lower runoff but approximately equivalent SMB: mean 1979–2012 SMB (± standard deviation), in Gt a−1, is 382 ± 78 in the PDD model, compared with 379 ± 101 and 425 ± 90 for the RCMs. Comparison with in situ SMB observations suggests that the RCMs may be more accurate than PDD at local level, in some areas, although the latter generally compares well. Dividing the GrIS into seven drainage basins we show that SMB has decreased sharply in all regions since 2000. Finally we show correlation between runoff close to two calving glaciers and either calving front retreat or calving flux, this being most noticeable from the mid-1990s

Prediction of iceberg trajectories for the North Atlantic and Arctic Oceans

Icebergs are a well-known hazard for shipping. Their study also provides information about diverse geophysical processes, as varied as ocean circulation, air-sea fluxes, calving rates of glaciers or the mass balance of ice sheets. As a first step to obtaining this information from iceberg data we have developed a model of iceberg drift driven by ocean and atmospheric forcing derived from general circulation models. We have applied the drift model to a distribution of typical icebergs released from the main tidewater glaciers of the North Atlantic and Arctic Oceans. We demonstrate that the main driving force of iceberg motion is rooted in the unsteady component of oceanic advection. From simulated trajectories we are able to reproduce the observed southwards limit of iceberg penetration and demonstrate sometimes surprising geographical links between iceberg origin and ultimate melting zones. Copyright 1996 by the American Geophysical Union

Prospects for seasonal forecasting of iceberg distributions in the North Atlantic

An efficient approach to ocean–iceberg modelling provides a means for assessing prospects for seasonal forecasting of iceberg distributions in the northwest Atlantic, where icebergs present a hazard to mariners each spring. The stand-alone surface (SAS) module that is part of the Nucleus for European Modelling of the Ocean (NEMO) is coupled with the NEMO iceberg module (ICB) in a “SAS-ICB” configuration with horizontal resolution of 0.25°. Iceberg conditions are investigated for three recent years, 2013–2015, characterized by widely varying iceberg distributions. The relative simplicity of SAS-ICB facilitates efficient investigation of sensitivity to iceberg fluxes and prevailing environmental conditions. SAS-ICB is provided with daily surface ocean analysis fields from the global Forecasting Ocean Assimilation Model (FOAM) of the Met Office. Surface currents, temperatures and height together determine iceberg advection and melting rates. Iceberg drift is further governed by surface winds, which are updated every 3 h. The flux of icebergs from the Greenland ice sheet is determined from engineering control theory and specified as an upstream flux in the vicinity of Davis Strait for January or February. Simulated iceberg distributions are evaluated alongside observations reported and archived by the International Ice Patrol. The best agreement with observations is obtained when variability in both upstream iceberg flux and oceanographic/atmospheric conditions is taken into account. Including interactive icebergs in an ocean–atmosphere model with sufficient seasonal forecast skill, and provided with accurate winter iceberg fluxes, it is concluded that seasonal forecasts of spring/summer iceberg conditions for the northwest Atlantic are now a realistic prospect

Inferring the variation of climatic and glaciological contributions to West Greenland iceberg discharge in the twentieth century

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