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Ocean bottom pressure (OBP) variability serves as a proxy of ocean mass variability, the knowledge of which is needed in geophysical applications. The question of how well it can be modeled by the present general ocean circulation models on time scales in excess of one day is addressed here by comparing the simulated OBP variability with the observed one. To this end, a new multi-year data set is used, obtained with an array of bottom pressure gauges deployed deeply along a transect across the Southern Ocean. We present a brief description of OBP data and show large scale correlations over several thousand kilometres at all time scales red using daily and monthly averaged data. Annual and semi-annual cycles are weak. Close to the Agulhas Retroflection, signals of up to 30 cm equivalent water height (EWH) are detected. Further south, signals are mostly intermittent and noisy. It is shown that the models simulate consistent patterns of bottom pressure variability on monthly and longer scales except for areas with high mesoscale eddy activity, where high resolution is needed to capture the variability due to eddies. Furthermore, despite good agreement in the amplitude of variability, the in situ and simulated OBP show almost no correlation.

Liquid freshwater transports of the shelfbreak East Greenland Current (EGC) and the separated EGC are determined from mooring records from the Kogur section north of Denmark Strait between August 2011 and July 2012. The 11-month mean freshwater transport (FWT), relative to a salinity of 34.8, was 65 ± 11 mSv to the south. Approximately 70% of this was associated with the shelfbreak EGC and the remaining 30% with the separated EGC. Very large southward FWT ranging from 160 mSv to 120 mSv was observed from September to mid-October 2011 and was foremost due to anomalously low upper-layer salinities. The FWT may, however, be underestimated by approximately 5 mSv due to sampling biases in the upper ocean. The FWT on the Greenland shelf was estimated using additional inshore moorings deployed from 2012-14. While the annual mean ranged from nearly zero during the first year to 18 mSv to the south during the second year, synoptically the FWT on the shelf can be significant. Furthermore, an anomalous event in autumn 2011 caused the shelfbreak EGC to reverse, leading to a large reduction in FWT. This reversed circulation was due to the passage of a large, 100 km wide anticyclone originating upstream from the shelfbreak. The late summer FWT of -131 mSv is 150% larger than earlier estimates based on sections in the late-1990s and early-2000s. This increase is likely the result of enhanced freshwater flux from the Arctic Ocean to the Nordic Seas during the early 2010s. This article is protected by copyright. All rights reserved.

[1] The objective of this paper is to evaluate the baroclinic and barotropic components of sea surface height (SSH) anomalies in the Southern Ocean and investigates the causes for the weak correlation between hydrographically derived and satellite measured SSH anomalies. To this end, data obtained by six Pressure Inverted Echo Sounders (PIES) deployed south of Africa were used to derive baroclinic and barotropic SSH anomalies. Our results show that the barotropic component accounts for 30%–60% of the variability between the jets of the Antarctic Circumpolar Current (ACC). In contrast, the jets associated with the major ACC fronts are predominantly baroclinic. The deep baroclinic component is important in the ACC and accounts for the major part of the baroclinic variability. The comparison with along-track satellite altimetry (Jason 1/2) shows correlations coefficients of 0.24–0.92, with low values in regions of high-barotropic variability. The comparison with gridded Aviso satellite altimetry generally shows higher correlation coefficients (0.33–0.92) and lower root mean square (RMS) errors compared to the along-track product. In conclusion the barotropic SSH anomaly plays a major role in this region and has to be accounted for when assimilating SSH into ocean models. Due to their high-baroclinic component, the correct representation of the time and space varying ACC fronts seems to be crucial for the right SSH anomaly partitioning used for assimilation purposes. Gridded products, namely Aviso, seems to be more suitable compared to along-track products (Jason 1/2) in representing the variability of SSH anomalies.

GRACE observes the temporally changing gravity field of the Earth with unprecedented accuracy. Compared to the gravity signals of the continental hydrological cycle, local ocean mass variability reflecting ocean current, density and sea level changes are a challenge for the GRACE mission. Hence, validation of GRACE with in-situ observations of ocean bottom pressure is critical to evaluate the capability of GRACE to observe oceanic mass redistribution. Here, GRACE data is compared with in-situ ocean bottom pressure at a hundred sites located in all of the world’s oceans. The advances made by recent GRACE product releases are shown, and gravity fields provided from different data centres are compared. In some regions, particularly at high-latitude sites with comparatively strong ocean bottom pressure variability and dense satellite coverage, GRACE captures oceanic variability quite well. This is a robust feature for all gravity field solutions. In contrast, some other regions show remarkably large differences between different GRACE solutions, suggesting that discrepancies in de-aliasing models play a major role in defining the skill of GRACE to realistically observe local oceanic mass variability.

In spite of the fundamental role the Atlantic Meridional Overturning Circulation (AMOC) plays for global climate stability, no direct current measurement of the Denmark Strait Overflow, which is the densest part of the AMOC, has been available until recently that resolve the cross-stream structure at the sill for long periods. Since 1999, an array of bottom-mounted acoustic instruments measuring current velocity and bottom-to-surface acoustic travel times was deployed at the sill. Here, the optimization of the array configuration based on a numerical overflow model is discussed. The simulation proves that more than 80% of the dense water transport variability is captured by two to three acoustic current profilers (ADCPs). The results are compared with time series from ADCPs and Inverted Echo Sounders deployed from 1999 to 2003, confirming that the dense overflow plume can be reliably measured by bottom-mounted instruments and that the overflow is largely geostrophically balanced at the sill.

The global thermohaline circulation is an important part of Earth's climate system. Cold, dense water formed in the Nordic Seas enters the Atlantic Ocean as overflows across the sills of the Greenland-Scotland Ridge. The Denmark Strait Overflow (DSO) is one of the main sources of North Atlantic Deep Water. Until now the DSO has been believed to be stable on interannual timescales. Here, for the first time, evidence is presented from a 4-year program of observations showing that overflow transports in 1999/2000 were approximately 30% higher than previous estimates. Later, transports decreased remarkably during the observation period, coincident with a temporary temperature increase of about 0.5°C.

Data from repeat hydrographic surveys over the 25-year period 1993 to 2017, together with satellite altimetry data, are used to quantify the temporal and spatial variability of the North Icelandic Irminger Current (NIIC), East Icelandic Current (EIC), and the water masses they advect around northern Iceland. We focus on the warm, salty Atlantic Water (AW) flowing northward through Denmark Strait and the cooler, fresher, denser Atlantic-origin Overflow Water (AtOW) which has circulated cyclonically around the rim of the Nordic Seas before being advected to the Iceland slope via the EIC. The absolute geostrophic velocities reveal that approximately half of the NIIC recirculates just north of Denmark Strait, while the remaining half merges with the EIC to form a single current that extends to the northeast of Iceland, with no further loss in transport of either component. The AW percentage decreases by 75% over this distance, while the AtOW percentage is higher than that of the AW in the merged current. The NIIC and merged NIIC-EIC are found to be baroclinically unstable, which causes the flow to become increasingly barotropic as it progresses around Iceland. A seasonal accounting of the water masses within the currents indicates that only in springtime is the NIIC dominated by AW inflow north of Denmark Strait. Overall, there is considerably more seasonal and along-stream variability in the properties of the flow prior to the merging of the NIIC and EIC. Over the 25-year time period, the NIIC became warmer, saltier, and increased in volume transport. publishedVersion

A coordinated atmosphere-ocean research project, centered on a rare wintertime field campaign to the Iceland and Greenland Seas, seeks to determine the location and causes of dense water formation by cold-air outbreaks. The Iceland Greenland Seas Project (IGP) is a coordinated atmosphere-ocean research program investigating climate processes in the source region of the densest waters of the Atlantic Meridional Overturning Circulation. During February and March 2018, a field campaign was executed over the Iceland and southern Greenland Seas that utilized a range of observing platforms to investigate critical processes in the region – including a research vessel, a research aircraft, moorings, sea gliders, floats and a meteorological buoy. A remarkable feature of the field campaign was the highly-coordinated deployment of the observing platforms, whereby the research vessel and aircraft tracks were planned in concert to allow simultaneous sampling of the atmosphere, the ocean and their interactions. This joint planning was supported by tailor-made convection-permitting weather forecasts and novel diagnostics from an ensemble prediction system. The scientific aims of the IGP are to characterize the atmospheric forcing and the ocean response of coupled processes; in particular, cold-air outbreaks in the vicinity of the marginal-ice zone and their triggering of oceanic heat loss, and the role of freshwater in the generation of dense water masses. The campaign observed the lifecycle of a long-lasting cold-air outbreak over the Iceland Sea and the development of a cold-air outbreak over the Greenland Sea. Repeated profiling revealed the immediate impact on the ocean, while a comprehensive hydrographic survey provided a rare picture of these subpolar seas in winter. A joint atmosphere-ocean approach is also being used in the analysis phase, with coupled observational analysis and coordinated numerical modelling activities underway.