Search

International audience; The circulation and stratification in the shallow semi-enclosed bay of Paracas located downstream of the main upwelling cell off the Peruvian coast were studied during the summer season using a regional circulation model and in situ observations. A downscaling strategy based on a series of three embedded grids, from 10 km to 500 m resolution in the bay allows to take into account the influence of remote perturbations on the bay dynamics. Debiased surface winds from a high-resolution regional atmospheric model were used to force the model. The shortwave absorption depth was parameterized using satellite measurements of surface chlorophyll. Sensitivity experiments to the model forcing and parameterizations were performed to investigate the impact of the wind diurnal variability, tidal forcing, freshwater discharge from a nearby river and shortwave absorption depth on the bay stratification. Results show that: debiasing the wind intensity reduced the model cold bias in the bay and increase the stratification; a shallow shortwave absorption depth induced a cooling of the subsurface water, increasing the stratification; freshwater discharge from the Pisco river north of the bay increased slightly the stratification in the bay during days of weak wind. The high sensitivity of the bay stratification to the atmospheric forcing calls for the need to use more realistic wind forcing products. The circulation in the bay under strong (>5.5 m s-1) and weak (-1) winds was also examined. The summer circulation during strong upwelling-favorable wind conditions was characterized by northward surface currents transporting the bay surface waters outward and subsurface currents transporting cold deeper waters into the bay along its western shore. During weak wind conditions, the current is outward in the bottom layer and a surface southward current related to the poleward undercurrent flowing over the continental slope and shelf transported warm waters into the bay, generating a cyclonic circulation in the bay.

A long simulation over the period 1901–2010 with an eddy-permitting ocean circulation model is used to study the variability of the upper layer circulation in the North Ionian Gyre (NIG) in the Eastern Mediterranean Sea (EMed). The model is driven by the atmospheric forcing from the twentieth century reanalysis data set ERA-20C, ensuring a consistent performance of the model over the entire simulation period. The main modes of variability known in the EMed, in particular the decadal reversals of the NIG upper layer circulation observed since the late 1980s are well reproduced. We find that the simulated NIG upper layer circulation prior to the observational period is characterized by long-lasting cyclonic phases with weak variability during years 1910–1940 and 1960–1985, while in the in-between period (1940–1960) quasi-decadal NIG circulation reversals occur with similar characteristics to those observed in the recent decades. Our simulation indicates that the NIG upper layer circulation is rather prone to the cyclonic mode with occasional kicks to the anticyclonic mode. The coherent variability of the NIG upper layer circulation mode and of the Adriatic Deep Water (AdDW) outflow implies that atmospheric forcing triggering strong AdDW formation is required to kick the NIG into an anticyclonic circulation 1–2 years later. A sensitivity experiment mimicking a cold winter event over the Adriatic Sea supports this hypothesis. Our simulation shows that it is the multi-decadal variability of the salinity in the Adriatic Sea that leads to periods where low salinity prevents strong AdDW formation events. This explains the absence of quasi-decadal NIG reversals during 1910–1940 and 1960–1985.

In August 2018, a remarkable polynya was observed off the north coast of Greenland, a perennial ice zone where thick sea ice cover persists. In order to investigate the formation process of this polynya, satellite observations, a coupled ice-ocean model, ocean profiling data, and atmosphere reanalysis data were applied. We found that the thinnest sea ice cover in August since 1978 (mean value of 1.1 m, compared to the average value of 2.8 m during 1978−2017) and the modest southerly wind caused by a positive North Atlantic Oscillation (mean value of 0.82, compared to the climatological value of −0.02) were responsible for the formation and maintenance of this polynya. The opening mechanism of this polynya differs from the one formed in February 2018 in the same area caused by persistent anomalously high wind. Sea ice drift patterns have become more responsive to the atmospheric forcing due to thinning of sea ice cover in this region.

Measurements of collocated fields of atmospheric forcing, surface waves, and mean and turbulent velocities associated with passage of Tropical Storm (TS) Barry over the U.S. Navy Tower R2 on the Georgia continental shelf are presented. A vertical-beam ADCP enables computation of directional surface wave spectra and hence of directional Stokes functions of depth and time, as well as mean (including tidal) and turbulent velocities throughout the water column. Full-depth turbulent velocity and backscatter structures observed during TS Barry are determined to be Langmuir supercells (LS). The LS appear in the present observations and in similar observations from a shallower site only when a surface growth rate g* exceeds a critical value, providing a means of predicting how deep an unstratified water column must be before LS will not be expected. When observed, LS structures at Tower R2 are less organized than archetypical LS structures: we suggest that this result is due primarily to smaller near-bottom growth rate in the deeper water column. Despite g* values above the critical value, and appropriate values of Langmuir and Rayleigh numbers, full-depth velocity/backscatter structures disappear completely for a time between the two wind maxima associated with the TS, as wind veers rapidly clockwise with eye passage to the west of Tower R2. From the observations, the most likely explanation for this hiatus is decreased wave breaking during the period of wind veering, reducing surface supply of “effective” vertical vorticity that dominates generation of Langmuir circulation (LC). This result has significant implications for LES modeling of LC.

Radiales del Mediterráneo, RADMED-TRES, SI

The response of snowpack to a +2°C global warming relative to the present climate was estimated in Hokkaido, Japan, using a physical snowpack model driven by dynamically downscaled (DDS) data, after model evaluation. The evaluation revealed that the snowpack model successfully reproduced the height of snow cover (HS), snow water equivalent (SWE) and snow-covered days (SCDs), but had a moderate bias in the thickness ratios of melt form (MF) and hoar category (HC). The DDS-forced simulation predicted that the seasonal-maximum HS and SWE would decrease by 30–40% in the southwestern and eastern parts of Hokkaido due to a large decrease in snowfall during the accumulation period, and that the HS and SWE in the north would decrease, albeit not significantly due to uncertain atmospheric forcing. The number of SCDs in Hokkaido was predicted to decline by ~30 d. Additionally, ~50% of snowpack thickness during a season would be MF in most areas, whereas HC would be

Translation of atmospheric forcing variability into the ocean interior via ocean ventilation is an important aspect of transient climate change. On a seasonal timescale in the subtropics, this translation is mediated by a so‐called “Demon” that prevents access to all except late‐winter mixed‐layer water. Here, we use an eddy‐permitting numerical circulation model to investigate a similar process operating on longer (interannual) timescales in the subpolar North Atlantic. We find that variations in atmospheric forcing are mediated in their translation to the ocean interior, with year‐to‐year changes in the late‐winter mixed layer depth being the critical factor. The signature of persistent strong atmospheric forcing driving deep mixed layers is preferentially ventilated to the interior when the forcing is ceased. Susceptibility to this effect depends on the location and density of subduction — with the rate at which newly ventilated water escapes its region of subduction being the crucial factor. Plain Language Summary Water that leaves the ocean's surface boundary layer — where water is in direct contact with the overlying atmosphere — to be transported into the subsurface, is said to be “ventilated” (the name arising from the abundance of oxygen in newly ventilated water). The ventilation process, which carries implications for the ocean storage of climate‐relevant substances such as carbon dioxide, occurs only at certain times and under certain conditions. In describing a mechanism for the selective nature of ventilation over the seasonal cycle, Henry Stommel imagined a Demon sitting at the base of the surface boundary layer, granting access only to parcels of water that meet certain characteristics (namely their speed of “escape”). Thus, “Stommel's Demon” was born. Here, we investigate this same process as it operates in more northerly regions and on longer timescales. In so doing we give birth to a new “interannual Demon”, and describe its characteristics.

The density and salinity of High Salinity Shelf Water, a key component of Antarctic Bottom Water emanating from the Ross Sea, are intensified by brine rejection induced by ice formation within the Terra Nova Bay (TNB) polynya. Ocean mooring data from 2007, meteorological observations from automatic weather stations and a satellite-derived history of the opening of TNB polynya delineate variability in water column salinity linked to atmospheric forcing, with a period on the order of 10 days. Lagged correlation analysis indicates that on average salinity response lags the polynya opening by 2 days and the wind forcing by 5 days. We find stronger correlations of salinity with the wind during March through May and with the polynya open-water fraction during June through October, with decreasing lags in the latter period. A one-dimensional mixed-layer model incorporating thermodynamic ice formation captures the oscillations in salinity. A process study shows that the variability in the polynya open-water fraction governs the final salinity attained by the model as well as the salinity cycling. Variability in surface heat fluxes modulates that effect. Our work suggests that there is a more complex relationship between salinity, the polynya open-water fraction, and atmospheric forcing than previously suggested.

To improve Antarctic sea-ice simulations and estimations, an ensemble-based Data Assimilation System for the Southern Ocean (DASSO) was developed based on a regional sea ice–ocean coupled model, which assimilates sea-ice thickness (SIT) together with sea-ice concentration (SIC) derived from satellites. To validate the performance of DASSO, experiments were conducted from 15 April to 14 October 2016. Generally, assimilating SIC and SIT can suppress the overestimation of sea ice in the model-free run. Besides considering uncertainties in the operational atmospheric forcing data, a covariance inflation procedure in data assimilation further improves the simulation of Antarctic sea ice, especially SIT. The results demonstrate the effectiveness of assimilating sea-ice observations in reconstructing the state of Antarctic sea ice, but also highlight the necessity of more reasonable error estimation for the background as well as the observation.

Recently, we highlighted the presence of a strong west‐east asymmetry in sea ice thickness across Hudson Bay that is driven by cyclonic circulation. Building on this work, we use satellite altimetry and a unique set of in situ observations of ice thickness from three moored upward looking sonars to examine the role of atmospherically driven ice dynamics in producing contrasting regional ice thickness patterns. Ultimately, north‐northwesterly winds coupled with numerous reversals during winter 2016/2017 led to thicker ice in southern Hudson Bay, while enhanced west‐northwesterly winds during winter 2017/2018 led to thicker ice in eastern Hudson Bay that delayed breakup and onset of the summer shipping season to coastal communities. Extending the analysis over the 40‐year satellite observation period, we find that these two different patterns of atmospheric forcing alter the timing of breakup by 30 days in eastern Hudson Bay and offer some skill in seasonal predictions of breakup.

Mechanisms driving the North Atlantic meridional overturning circulation (AMOC) variability at low frequency are of central interest for accurate climate predictions. Although the subpolar gyre region has been identified as a preferred place for generating climate time-scale signals, their southward propagation remains under consideration, complicating the interpretation of the observed time series provided by the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array–Western Boundary Time Series (RAPID–MOCHA–WBTS) program. In this study, we aim at disentangling the respective contribution of the local atmospheric forcing from signals of remote origin for the subtropical low-frequency AMOC variability. We analyze for this a set of four ensembles of a regional (20°S–55°N), eddy-resolving (1/12°) North Atlantic oceanic configuration, where surface forcing and open boundary conditions are alternatively permuted from fully varying (realistic) to yearly repeating signals. Their analysis reveals the predominance of local, atmospherically forced signal at interannual time scales (2–10 years), whereas signals imposed by the boundaries are responsible for the decadal (10–30 years) part of the spectrum. Due to this marked time-scale separation, we show that, although the intergyre region exhibits peculiarities, most of the subtropical AMOC variability can be understood as a linear superposition of these two signals. Finally, we find that the decadal-scale, boundary-forced AMOC variability has both northern and southern origins, although the former dominates over the latter, including at the site of the RAPID array (26.5°N).

This work presents an 18-year-long (2002&ndash, 2019) tide-gauge dataset collected on the Island of Ischia (Gulf of Naples, Southern Tyrrhenian Sea) that can contribute to the analysis of the basic features of sea-level variability in this region. Analysis of tidal constituents shows that the Gulf of Naples is characterized by the absence of any amphidromic system. In this area, sea-level changes due to the astronomical component of the tide are generally limited to &plusmn, 20 cm with respect to the mean sea level, but the impact of this variability is enhanced by global sea-level increase and the effect of regional atmospheric perturbations that might also triple sea-level variations. The effects of these events, whose frequency has increased in recent decades, has been dramatic in coastal areas where intense social and economic activity occurs, e.g., in Ischia. On interannual time scales, the results indicate that the relative sea-level rise in Ischia has a magnitude of 3.9 mm/year. Special attention is dedicated to the &ldquo, acqua alta&rdquo, episodes and to their linkage with long-term sea-level trends and atmospheric forcing.

This article reports the results of three field campaigns conducted in Lake Issyk-Kul in 2015, 2016, and 2017. During the campaigns, CTD profiling and water sampling were performed at 34 locations all over the lake. A total of 75 CTD profiles were obtained. Some biogeochemical and thermohaline parameters at the lake surface were also mapped at high horizontal resolution along the ship's track. In addition, thermistor chains were deployed at three mooring stations in the eastern littoral region of the lake, yielding 147-day-long records of temperature data. The measurements revealed that – while the thermal state of the active layer, as well as some biogeochemical characteristics, were subject to significant interannual variability mediated by atmospheric forcing – the haline structure of the entire lake was remarkably stable at the interannual scale. Our data do not confirm the reports of progressive warming of the deep Issyk-Kul waters as suggested in some previous publications. However, they do indicate a positive trend of salinity in the lake's interior over the last 3 decades. A noteworthy newly found feature is a weak but persistent salinity maximum below the thermocline at a depth of 70–120 m, from where salinity slightly decreased downwards. The data from the moored thermistor chains support the previously published hypothesis about the significant role of the submerged ancient riverbeds on the eastern shelf in advecting littoral waters into the deep portion of the lake during differential cooling period. We hypothesize that the less saline littoral water penetrating into the deep layers due to this mechanism is responsible for the abovementioned features of salinity profile, and we substantiate this hypothesis by estimates based on simple model assumptions.

The Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM), which uses unstructured grids, is set up for the North and Baltic Seas. With a resolution of ∼100 m in the narrow straits connecting the two basins, this model accurately resolves the inter-basin exchange. Validation against observations in the straits shows the model has good skill in simulating the transport and vertical profiles of temperature, salinity and currents. The timing and magnitude of the major inflow event in 2014–2015 is also realistically simulated. The analysis is focused on the two-layer exchange, its dependence on the atmospheric forcing, and dominant physical balances. The two-layer flows in the three connecting straits show different dependencies upon the net transport. The spatial variability of this dependence is also quite pronounced. The three-strait system developed specific dynamics, with time lags and differences between currents in the individual straits during inflow and outflow conditions. Analysis on the impact of resolution indicates that the performance of the model changes depending on whether the narrow parts of the straits are resolved with a resolution of 500 m or 100 m. With this ultra-fine resolution, gravity flows and variability of salinity in deep layers is generally more adequately simulated. This paper identifies the needs for more profound analysis of the coupled dynamics of Baltic and North Seas with a focus on the Danish straits.

Two numerical experiments on reconstructing velocity fields, sea level, temperature, and salinity were conducted with account for real atmospheric forcing in autumn 2007 using the Marine Hydrophysical Institute (MHI) hydrodynamic model with an open boundary (northwestern shelf) adjusted to the coastal region of the Black Sea. A high spatial resolution of 500 m and 1.6 km was used, while the bottom topography had a resolution of ~1.6 km. The higher spatial resolution made it possible to reconstruct detailed mesoscale and submesoscale structures of the hydrophysical fields in the upper and deep layers over the northwestern shelf and to obtain quantitative and qualitative characteristics of eddies and jets that are more accurate compared to previous calculations. It was shown that improvement of the spatial resolution up to a few hundred meters makes it possible to take into account the detailed bottom topography and shape of the coastline in the numerical model, which in turn yields a more accurate quantitative and qualitative reconstruction of the mesoscale and submesoscale properties of coastal circulation.

In July–August 2021, intense marine heatwaves (MHWs) occurred at the sea surface over extensive areas of the northwestern Pacific Ocean, including the entire Sea of Japan and part of the Sea of Okhotsk. In extent and intensity, these MHWs were the largest since 1982, when satellite measurements of global sea surface temperatures started. The MHWs in summer 2021 were observed at the sea surface and occurred concomitantly with a stable shallow oceanic surface boundary layer. The distribution of the MHWs was strongly related to heat fluxes at the sea surface, indicating that the MHWs were generated mainly by atmospheric forcing. The MHWs started to develop after around 10 July, concurrent with an extreme northward shift of the atmospheric westerly jet. The MHWs developed rapidly under an atmospheric high-pressure system near the sea surface, associated with a northwestward expansion of the North Pacific Subtropical High. The MHWs exhibited peaks around 30 July to 1 August. Subsequently, following the southward displacement of the westerly jet, the MHWs weakened and then shrank abruptly, synchronously with rapid deepening of the oceanic surface boundary layer. By 18 August, the MHWs had disappeared.

The sensitivity of different atmospheric forcing on the simulation of Sea Surface Temperature (SST) in the Indian Ocean is examined using Regional Ocean Modeling System (ROMS). Model simulations using three different atmospheric forcings from the National Centers for Environmental Prediction (NCEP; 2.5 deg), National Centre for Medium Range Weather Forecasting (NCMRWF; 0.25 deg) and TropFlux (0.5 deg) are analyzed here. Model sensitivity to the atmospheric forcing is studied by analyzing the response of SST and mixed layer depth (MLD) using statistical methods. Results show that the response of NCMRWF and TropFlux forcing was almost similar in capturing the variability of SST in comparison with the corresponding observations. But NCEP was unable to capture SST variability, especially over the central part of the Arabian Sea (AS). It is shown that deeper MLD simulations by NCEP forcing due to the high magnitude of wind resulted in an unrealistic simulation of SST.

The Turkish Strait System, which is the only connection between the Black Sea and the Mediterranean Sea, is a challenging region for ocean circulation models due to topographic constraints and water mass structure. We present a newly developed high resolution unstructured finite element grid model to simulate the Turkish Strait System using realistic atmospheric forcing and lateral open boundary conditions. We find that the jet flowing from the Bosphorus Strait into the Marmara creates an anticyclonic circulation. The eddy kinetic energy field is high around the jets exiting from the Bosphorus Strait, Dardanelles Strait, and also the leeward side of the islands in the Marmara Sea. The model successfully captures the two-layer structure of the Sea of Marmara. The volume transport at the Bosphorus is around 120 km3/year which is consistent with the recent observations. The largest bias in the model is at the interface depth due to the shallower mixed layer.

The influence of high-frequency atmospheric forcing on the formation of a well-mixed summer warm water column in the central Bohai Sea is investigated comparing model simulations driven by daily surface forcing and those using monthly forcing data. In the absence of high-frequency atmospheric forcing, numerical simulations have repeatedly failed to reproduce this vertically uniform column of warm water measured over the past 35 years. However, high-frequency surface forcing is found to strongly influence the structure and distribution of the well-mixed warm water column, and simulations are in good agreement with observations. Results show that high frequency forcing enhances vertical mixing over the central bank, intensifies downward heat transport, and homogenizes the water column to form the Bohai central warm column. Evidence presented shows that high frequency forcing plays a dominant role in the formation of the well-mixed warm water column in summer, even without the effects of tidal and surface wave mixing. The present study thus provides a practical and rational way of further improving the performance of oceanic simulations in the Bohai Sea and can be used to adjust parameterization schemes of ocean models.

Measurements of near-surface oxygen (O2) concentrations and mixed layer depth from the K1 mooring in the central Labrador Sea are used to calculate the change in column-integrated (0–1700 m) O2 content over the deep convection winter 2014/2015. During the mixed layer deepening period, November 2014 to April 2015, the oxygen content increased by 24.3 ± 3.4 mol m−2, 40% higher than previous results from winters with weaker convection. By estimating the contribution of respiration and lateral transport on the oxygen budget, the cumulative air-sea gas exchange is derived. The O2 uptake of 29.1 ± 3.8 mol m−2, driven by persistent undersaturation (≥5%) and strong atmospheric forcing, is substantially higher than predicted by standard (nonbubble) gas exchange parameterizations, whereas most bubble-resolving parameterizations predict higher uptake, comparable to our results. Generally large but varying mixed layer depths and strong heat and momentum fluxes make the Labrador Sea an ideal test bed for process studies aimed at improving gas exchange parameterizations.

Global satellite sea surface temperature (SST) measurements and NCEP/NCAR reanalysis wind data for the period of 1982–2009 have been used to study the seasonal and interannual variability of the main climatic oceanic frontal zones (OFZ; subpolar, subtropical and equatorial) associated with the large-scale atmospheric forcing. The seasonal variability of the OFZ is manifested in variations of their intensity (magnitude of the meridional gradient of SST) and latitudinal position of the cores (defined as areas of maximum SST gradient). The maximum intensity of the subpolar OFZ is observed in summer of the corresponding hemisphere, while subtropical OFZ are intensified synchronously in both hemispheres during boreal winter. Subtropical OFZ cores in both hemispheres shift synchronously to the south/north during the winter/summer of the Northern hemisphere, which is caused by the seasonal meridional migration of the areas of the maximum convergence of Ekman transport. All subpolar and subtropical OFZ reveal a pronounced quasi-decadal (7–10 years) variability, manifested in the variations of their intensity and latitudinal position of the zones’ cores. Strengthening of the SST gradient is accompanied by a displacement of the zones’ cores to the north in both hemispheres for subpolar OFZ, while subtropical OFZ cores migrate to the poles in this situation. Positive correlations between the maximum magnitude of the meridional gradient of zonally averaged SST and meridional shear of zonal wind (which is an estimate of the Ekman convergence intensity) were found for all subpolar and subtropical OFZ. Variability of the latitudinal position of subpolar OFZ cores is associated with the Ekman advection variability due to zonal wind variations (strengthening of zonal wind results in a shift of subpolar OFZ cores to the south/north in the Northern/Southern hemispheres). A period of the variability of the North Pacific equatorial OFZ is 4–5 years and is determined by the variability of the ENSO system. During the El Nino events, the sharp decrease of the intensity of the equatorial OFZ take place.

The 2015-2016 El Nino contributed to large anomalies across the California Current System (CalCS), but these anomalies ceased unexpectedly in late 2015. Here, we use a suite of three hindcast simulations with the Regional Oceanic Modeling System (ROMS) to assess the responsible mechanisms for this development. We find that the early build-up was primarily driven by the early onset of this event in the tropical Pacific, driving anomalies in the CalCS through the propagation of coastally trapped waves. In contrast, the abrupt end in the central CalCS was caused by the unusual onset of upwelling favorable winds in the fall of 2015, which offset the continuing remote forcing through the coastal waveguide. Nevertheless, low nutrient anomalies persisted, causing anomalously low phytoplankton abundance in the upwelling season of 2016. This is a recurring pattern for all El Nino events over the last 37 years, suggesting predictive skill on seasonal timescales.

This study analyzes the atmospheric forcing of upwelling occurrence along differently oriented coastlines of the southern Baltic Sea basin. The mean daily sea surface temperature (SST) data from the summer seasons (June&ndash, August) of the years 1982&ndash, 2017 made the basis for the detection of upwelling cases. For the atmospheric part of the analysis, monthly indices of four macroscale circulation patterns were used: North Atlantic Oscillation (NAO), Scandinavian (SCAND), East Atlantic (EA) and East Atlantic/Western Russia (EATL/WRUS). In order to identify the local airflows and wind conditions, zonal and meridional regional circulation indices were constructed and introduced to the analysis. Within the southern Baltic Sea basin, upwelling occurs most frequently along the zonally oriented southern coasts of Sweden, and least frequently along the southern (Polish) and eastern (Lithuanian-Latvian) coasts. Among the macroscale circulation patterns, the SCAND has the strongest impact on the horizontal flow of surface sea waters in the southern Baltic, which triggers upwelling. The summer NAO and EA appeared to have a weak effect on upwelling occurrence, and EATL/WRUS have the smallest impact. Local circulation indices allowed us to recognize the atmospheric control of upwelling frequency better than the indices of the macroscale patterns. Anomalies in upwelling frequency are their highest at the positive/negative phase of the zonal circulation, particularly along the southern and eastern coast of the southern Baltic Sea basin.

Purpose. The aim of the paper was to compare the results of numerical experiments on reconstructing seasonal thermal evolution of the sea ice thickness with the data of in situ observations of the ice state in the northeastern part of the Taganrog Bay. Methods and Results. Characteristics of the ice state in the northeastern part of the Taganrog Bay were studied using the previously developed thermodynamical model of sea ice. The data of the European Center for Medium-Range Weather Forecasts ERA-Interim, regional prognostic model SKIRON and the array of daily eight (with 3-hour intervals) observations of the basic meteorological parameters (All-Russian Research Institute of Hydrometeorological Information – World Data Center (RIHMI – WDC)) obtained at the meteorological station Taganrog, were used in the numerical experiments as the atmospheric forcing. The modeling results were compared with the in situ data for the winter seasons in 2007/2008–2010/2011. It is shown that the characteristics of the snow-ice cover resulted from application of various meteorological data as the external forcing, can be significantly different. Conclusions. The highest similarity between the modeled ice thickness seasonal variation and the one reconstructed using the observations data was obtained at applying the RIHMI – WDC data array. In this case, both thickness and the basic stages of the snow-ice cover evolution in the Taganrog Bay were adequately reproduced in the model. As compared to the in situ data, the results of the models based on the SKIRON and ERA-Interim data were mainly overestimating and underestimating, respectively. It is related, to a great extent, to determination of the precipitation amount, the prognostic values of which in ERA-Interim are higher than those in SKIRON. However, even in the calculations taking no account of atmospheric precipitation or in those for the ice seasons when the atmospheric precipitation is very insignificant, the SKIRON based model provides the higher values of ice thickness than the values resulted from the ERA-Interim based model. Analysis of the modeling results shows that adequate reconstruction of the ice state characteristics in the Azov Sea requires preliminary setting of the thermodynamic model depending on the chosen data array used as the atmospheric forcing.

Marine-terminating glaciers in the Canadian Arctic Archipelago have retreated in response to atmospheric, not oceanic, warming., The Canadian Arctic Archipelago contains >300 glaciers that terminate in the ocean, but little is known about changes in their frontal positions in response to recent changes in the ocean-climate system. Here, we examine changes in glacier frontal positions since the 1950s and investigate the relative influence of oceanic temperature versus atmospheric temperature. Over 94% of glaciers retreated between 1958 and 2015, with a region-wide trend of gradual retreat before ~2000, followed by a fivefold increase in retreat rates up to 2015. Retreat patterns show no correlation with changes in subsurface ocean temperatures, in clear contrast to the dominance of ocean forcing in western Greenland and elsewhere. Rather, significant correlations with surface melt indicate that increased atmospheric temperature has been the primary driver of the acceleration in marine-terminating glacier frontal retreat in this region.

In recent decades, the global warming drives the huge ice sheet melting in northern hemisphere. Previous studies on long-term melting rates of Greenland have basically reached agreement, but there is still a lack of the knowledge of the pattern the short-term ice mass change, makes it difficult to fulfill the understanding of the mechanism of the ice sheet. In this study, we used Gravity Recovery and Climate Experiment (GRACE) RL06 data to derive the time series of mass variation of Greenland Ice Sheet (GrIS), from January 2003 to December 2015. We derived the short-term mass change rate during 2003-2015 and found that the GrIS has experienced four melting stages, i.e., a steady melting (-204.4 Gt/yr) phase form 2003 to 2009, an abrupt accelerating (-384.6 Gt/yr) phase form 2010 to 2012, an abnormal pause (+24.8 Gt/yr) year in 2013 and a recovering (-207.3 Gt/yr) phase from 2014 to 2015. In addition, using ensemble empirical mode decomposition (EEMD), the high frequency signal has been removed and the annual mass change has been studied, which shows inter-annual variability. Meanwhile, we correlated the annual mass change with the Greenland Blocking Index (GBI) and North Atlantic Oscillation (NAO) during summertime and the correlation for 2003-2015 was found at r=-0.63 between the summer GBI and the annual mass change, while r=0.57 between summer NAO and the annual mass change. Our results indicate that the short-term mass change of GrIS mainly forced by atmospheric variability.

Deep convection is presumed to be vital for the North Atlantic Meridional Overturning Circulation,even though observational evidence for the link remains inconclusive. Modeling studies have suggested thatconvection will weaken as a result of enhanced freshwater input. The emergence of anomalously low seasurface temperature in the subpolar North Atlantic has led to speculation that this process is already at work.Hereweshow that strong atmospheric forcing in the winter of 2014–2015, associated with a high North AtlanticOscillation (NAO) index, produced record mixed layer depths in the Irminger Sea. Local mixing removed thestratification of the upper 1400mand ventilated the basin to middepths resembling a state similar to themid-1990s when a positive NAO also prevailed. We show that the strong local atmospheric forcing ispredominantly responsible for the negative sea surface temperature anomalies observed in the subpolar NorthAtlantic in 2015 and that there is no evidence of permanently weakened deep convection.

High-resolution satellite sea surface temperature measurements (PATHFINDER dataset) indicate that the fronts at the boundary of the East China Sea (Taiwan front, Kuroshio frontal zone, and South Korean coastal front) appear as a unified dominating frontal structure when climatological averaging is applied. This structure is about 1200 km in length, spreads over the continental shelf from Taiwan to the Tsushima Islands, and separates productive seawaters from the oligotrophic oceanic waters. The Kuroshio frontal zone, incorporated into this structure, reveals interannual variability with periods consistent with El Nino–Southern Oscillation (4–5 years).

A numerical simulation using the Regional Ocean Modeling System (ROMS) indicates that there was significant interannual-to-interpentadal variability of along-shelf transport and water properties over the Middle Atlantic Bight (MAB) from 2004 to 2013. To examine the relative contribution from local atmospheric forcing versus remote oceanic open boundary forcing to such low-frequency variability, we implement a suite of process oriented numerical experiments. Results show that the interannual variability is dominated by remote forcing from the open boundaries of the region rather than by local atmospheric forcing. The penetration of the Labrador Current into the region contributes to a significant increase of along-shelf transport in the winters of 2009 and 2010. By contrast, the anti-cyclonic mesoscale eddies associated with the Gulf Stream decrease the background along-shelf jet and, in certain cases, even reverse the along-shelf transport. In addition, the along-shelf transport appears to possess an interpentadal variation, i.e., weaker during 2004–2008 but stronger during 2009–2013, which is found caused by the migration of the Gulf Stream.

Global satellite sea surface temperature (SST) measurements and NCEP/NCAR reanalysis wind data for the period of 1982–2009 have been used to study the relationship between long-term variability of oceanic frontal zones (OFZ) and large-scale atmospheric forcing. Statistically significant positive correlations between the maximum magnitude of the meridional gradient of zonally averaged SST and meridional shear of zonal wind (which is an estimate of the Ekman convergence intensity) were found for all subpolar and subtropical OFZ of the World Ocean. Variability of the latitudinal position of OFZ cores may be associated with Ekman advection variability due to zonal wind variations. Strengthening of zonal wind results in a shift of subpolar OFZ cores to the south/north in the Northern/Southern hemispheres.

Large-scale patterns of positive temperature anomalies persisted throughout the surface waters of the North Pacific Ocean during 2014–2015. In the Southern California Current System, measurements by our sustained network of underwater gliders reveal the coastal effects of the recent warming. Regional upper ocean temperature anomalies were greatest since the initiation of the glider network in 2006. Additional observed physical anomalies included a depressed thermocline, high stratification, and freshening; induced biological consequences included changes in the vertical distribution of chlorophyll fluorescence. Contemporaneous surface heat flux and wind strength perturbations suggest that local anomalous atmospheric forcing caused the unusual oceanic conditions.

A series of meteotsunamis hit a few locations in the Mediterranean and Black Seas during 22– 27 June 2014. Meteot- sunamis were particularly numerous on 25 and 26 June in the Adriatic Sea, where at least six harbours and bays were stricken by ˇ powerful waves: strongest events occurred in Vela Luka (Korcula Island), a known meteotsunami hot-spot, where waves reached ˇ height of *3 m, and in Rijeka dubrovacka Bay, where strong *5 m/s currents accompanied *2.5 m high waves. Inten- sification of high-frequency sea level activity was observed at both the eastern and western Adriatic tide gauge stations, with maximum recorded wave heights reaching *68 cm (Ortona, Italy). A series of individual air pressure disturbances characterized by pronounced rates of air pressure change (up to 2.4 hPa/5 min), limited spatial extent (*50 km) and high temporal variability, propagated over the Adriatic on 2 days in question. Numerical hydrodynamic model SCHISM forced by measured and idealised air pressure distur- bances was utilised to reproduce the observed Adriatic sea level response. Several important conclusions were reached: (1) meteotsunamis occurring at various parts of the coast were gen- erated by different atmospheric air pressure disturbances ; (2) topographic influence can be removed from sea level spectra by computing spectral signal- to-background ratios ; the result, being related to the external forcing, resembles atmospheric pressure spectra ; (3) sea response is strongly dependant on details of atmospheric forcing ; and (4) over complex bathymetries, like the middle and south Adriatic ones, numerous effects, including Proudman resonance, edge waves, strong topographical enhance- ment and refractions on the islands placed on the pathway of atmospheric disturbances should be taken into account to fully understand meteotsunami generation and dynamics. An in- depth numerical study is planned to supplement the latter conclusion and to quantify contribution of each process.

Observational analyses reveal that wintertime variations in sea surface temperatures (SST) in the Kuroshio–Oyashio Extension (KOE) region of the North Pacific are associated with two distinct and robust patterns of atmospheric variability: 1) a pattern that peaks in amplitude approximately 2–3 weeks prior to large KOE SST anomalies and is consistent with atmospheric forcing of the SST field and 2) a very different pattern that lags SST anomalies in the KOE region by approximately a month. The latter pattern is dominated by low sea level pressure anomalies and turbulent heat fluxes directed into the atmosphere over warm SST anomalies and is interpreted as the transient atmospheric response to SST anomalies over the KOE region. The results contribute to a growing body of evidence that suggests variations in SSTs in the midlatitude oceans are capable of significantly influencing the large-scale atmospheric circulation, especially near western boundary currents.

In Eastern Boundary Upwelling Systems, cold coastal waters are separated from offshore by a strong cross-shore Sea Surface Temperature (SST) gradient zone. This upwelling front plays a major role for the coastal ecosystem. This paper proposes a method to automatically identify the front and define its main characteristics (position, width, and intensity) from high resolution data. The spatio-temporal variability of the front characteristics is then analyzed in a region off Central Chile (37°S), from 2003 to 2016. The front is defined on daily 1 km-resolution SST maps by isotherm T0 with T0 computed from mean SST with respect to the distance from the coast. The probability of detecting a front, as well as the front width and intensity are driven by coastal wind conditions and increased over the 2007–2016 period compared to the 2003–2006 period. The front position, highly variable, is related to the coastal jet configuration and does not depend on the atmospheric forcing. This study shows an increase by 14% in the probability of detecting a front and also an intensification by 17% of the cross-front SST difference over the last 14 years. No trend was found in the front position.

The Yucatan Channel connects the Caribbean Sea with the Gulf of Mexico and is the main outflow region of the Caribbean Sea. Moorings in the Yucatan Channel show high-frequent variability in kinetic energy (50–100 days) and transport (20–40 days), but the physical mechanisms controlling this variability are poorly understood. In this study, we show that the short-term variability in the Yucatan Channel transport has an upstream origin and arises from processes in the North Brazil Current. To establish this connection, we use data from altimetry and model output from several high resolution global models. A significant 40–70 day variability is found in the sea surface height in the North Brazil Current retroflection region with a propagation toward the Lesser Antilles. The frequency of variability is generated by intrinsic processes associated with the shedding of eddies, rather than by atmospheric forcing. This sea surface height variability is able to pass the Lesser Antilles, it propagates westward with the background ocean flow in the Caribbean Sea and finally affects the variability in the Yucatan Channel volume transport.

Interannual-to-decadal variability and trends of sea level in the South China Sea (SCS) are studied using altimetric data during 1993–2012 and reconstructed sea level data from 1950–2009. The interannual variability shows a strong seasonality. Surface wind anomalies associated with El Nino-Southern Oscillation explain the sea-level anomaly pattern in the interior SCS, while Rossby waves radiated from the eastern boundary dominate the sea-level variability in the eastern SCS. Decadal variability of sea level in the SCS follows that in the western tropical Pacific, with large variance found west of Luzon Island. Local atmospheric forcing makes a negative contribution to decadal variability in the central SCS, and Rossby waves radiated from the eastern boundary appear to be important. During 1993–2012, decadal sea level averaged in the SCS is significantly correlated with the Pacific Decadal Oscillation (PDO) (r = −0.96). The decadal variability associated with the PDO accounts for most part of sea-level trends in the SCS in the last two decades.

The transport of Upper Labrador Sea Water (ULSW) at Flemish Cap (47°N/45°W) is investigated in the period 1960–2009 using the output from an 8 km resolution numerical ocean model. The average model transport of ULSW decreases southward from 6.7 Sv at 53°N to 4.5 Sv at 45°N due to interior pathways. The largest fraction of the total ULSW volume transport goes around Flemish Cap within the Deep Western Boundary Current (DWBC, 72%) but a significant part goes through Flemish Pass (20%). At interannual timescales, the variability at Flemish Pass shows a distinct behavior when compared to the variability in the DWBC and to the upstream fluctuations. A running correlation method is applied to obtain the connection of the transport variability at Flemish Pass with several quantities, representative for different physical mechanisms: (1) the North Atlantic Oscillation index, (2) the Ekman transport, (3) the rate of ULSW formation in the Labrador Sea, (4) the position of the North Atlantic Current (NAC) relative to the slope and (5) the averaged transport in the subpolar gyre. Weakened or strengthened transport of ULSW through Flemish Pass coincides with changes of the atmospheric forcing or with changes of the NAC‘s position. Strong meandering of the NAC close to the DWBC reduces the transport off Flemish Cap, and the ULSW flow is “redirected” into the Flemish Pass, enhancing the transport there. In contrast, the transport variability in the DWBC is mainly caused by upstream fluctuations and changes according to the rate of ULSW formation.

The development of coastal polynyas, areas of enhanced heat flux and sea ice production strongly depend on atmospheric conditions. In Antarctica, measurements are scarce and models are essential for the investigation of polynyas. A robust quantification of polynya exchange processes in simulations relies on a realistic representation of atmospheric conditions in the forcing dataset. The sensitivity of simulated coastal polynyas in the south-western Weddell Sea to the atmospheric forcing is investigated with the Finite-Element Sea ice-Ocean Model (FESOM) using daily NCEP/NCAR reanalysis data (NCEP), 6 hourly Global Model Europe (GME) data and two different hourly datasets from the high-resolution Consortium for Small-Scale Modelling (COSMO) model. Results are compared for April to August in 2007–09. The two coarse-scale datasets often produce the extremes of the data range, while the finer-scale forcings yield results closer to the median. The GME experiment features the strongest winds and, therefore, the greatest polynya activity, especially over the eastern continental shelf. This results in higher volume and export of High Salinity Shelf Water than in the NCEP and COSMO runs. The largest discrepancies between simulations occur for 2008, probably due to differing representations of the ENSO pattern at high southern latitudes. The results suggest that the large-scale wind field is of primary importance for polynya development.

The main objective of this study is to investigate the variability of the thermohaline characteristics of the deep-water masses in the Aegean Sea and the possible impact of the regional atmospheric forcing variability by analyzing the available oceanographic and atmospheric datasets for the period of 1960–2012. During this period the variability of the deep water characteristics of the Aegean sub-basins is found to be very large as well as the diversity of the deep water characteristics among the sub-basins. The Central Aegean seems to play the key role in the Aegean deep water formation processes. Due to its small size, the Aegean Sea surface responds rapidly to the meteorological changes and/or the variability of the lateral fluxes and this variability propagates in the thermohaline characteristics of the deep water masses of the basin through deep water formation processes. There are many episodes characterized by a tight coupling of the atmosphere and the ocean during the examined period, with the Eastern Mediterranean Transient (EMT) being the most prominent case. We suggest that deep water formation is triggered mostly by the combination of preconditioning during early winter and/or previous winters together with the number of subsequent extreme events during present winter and not only by the total amount of the extreme heat loss winter days.

Summary In the context of the environmental monitoring of the Concordia wreck removal project, measurements of currents, winds and sea level height were made along the eastern coast of the Giglio Island, Tyrrhenian Sea (Italy), during 2012–2013. The aim of the study was to investigate the effect of atmospheric forcing and periodic sea-level changes on the coastal currents. Normalised Cross-Correlation Function analysis allowed us to correlate these observations. A marked inter-seasonal variability was found in both current and local wind velocity observations but a significant level of correlation between the data was only found during strong wind events. Current and wind directions appeared to be uncorrelated and current measurements showed a predominant NW–SE direction, presumably linked to the shape and orientation of Giglio Island itself. During strong winds from the SSE, current flow was towards the NNW but it suddenly switched from the NNW to the SE at the end of wind events. The results show that, at Giglio Island, currents are principally dominated by the general cyclonic Tyrrhenian circulation, and, secondly, by strong wind events. The sea level had no effects on the current regime.

The paper investigates wintertime dynamics of the coastal northeastern Adriatic Sea, and is based on numerical modelling and in situ data collected through field campaigns executed during the winter and spring of 2015. The data have been collected by a variety of instruments and platforms (ADCPs, CTDs, glider, profiling float), and have been accompanied with a one-way coupled ALADIN/ROMS modelling effort. Research focus has been put on dense water formation (DWF), thermal changes and circulation, and water exchange between the coastal and open Adriatic. According to both observations and modelling results, dense waters are formed in the northeastern coastal Adriatic during cold bora outbreaks, even during milder-than-average winters (as was the winter of 2015). However, dense water formed in this coastal region has, due to lower salinities, lower densities than dense water formed at the open Adriatic. Since the sea is deeper in the coastal area than at the open Adriatic, dense waters from the open Adriatic occasionally enter the coastal area near the bottom of the connecting passages, while the surface flow is mostly outward from the coastal area. Median residence time of the coastal area is estimated to about 1–2 months, indicating that the coastal area may be relatively quickly renewed by the open Adriatic waters. The model significantly underestimates currents and transports in connecting channels, which may be a result of a too coarse resolution of atmospheric forcing, misrepresentation of bathymetry or absence of the air-sea feedback in the model. Obtained data represents a comprehensive marine dataset, pointing to a number of interesting phenomena to be investigated in the future.

We propose a new framework for parameterization of ocean convection processes. The new framework is termed “patchy convection” since our aim is to represent the heterogeneity of mixing processes that take place within the horizontal scope of a grid cell. We focus on applying this new scheme to represent the effect of pre-conditioning for deep convection by subgrid scale eddy variability. The new parameterization separates the grid-cell into two regions of different stratification, applies convective mixing separately to each region, and then recombines the density profile to produce the grid-cell mean density profile. The scheme depends on two parameters: the areal fraction of the vertically-mixed region within the horizontal grid cell, and the density difference between the mean and the unstratified profiles at the surface. We parameterize this density difference in terms of an unresolved eddy kinetic energy. We illustrate the patchy parameterization using a 1D idealized convection case before evaluating the scheme in two different global ocean-ice simulations with prescribed atmospheric forcing; (i) diagnosed eddy velocity field applied only in the Labrador Sea (ii) diagnosed global eddy velocity field. The global simulation results indicate that the patchy convection scheme improves the warm biases in the deep Atlantic Ocean and Southern Ocean. This proof-of-concept study is a first step in developing the patchy parameterization scheme, which will be extended in future to use a prognostic eddy field as well as to parameterize convection due to under-ice brine rejection.

The effect of the high frequency (synoptic) variability of wind and heat fluxes upon the surface ocean off south-central Chile (west coast of South America) is investigated using a regional ocean model. We focus our analysis in austral summer, when the regional wind experiences significant day-to-day variability superimposed on a mean, upwelling favorable flow. To evaluate the nature and magnitude of these effects, we performed three identical simulations except for the surface forcing: the climatological run, with long-term monthly mean wind-stresses and heat fluxes; the wind-synoptic run, with daily wind stresses and climatological heat fluxes; and the full-synoptic run, with daily wind-stresses and daily fluxes. The mean currents and surface geostrophic EKE fields show no major differences between simulations, and agree well with those observed in this ocean area. Nevertheless, substantially more ageostrophic EKE is found in the simulations which include synoptic variability of wind-stresses, impacting the total surface EKE and diffusivities, particularly south of Punta Lavapie (37° S), where the lack of major currents implies low levels of geostrophic EKE. Summer mean SSTs are similar in all simulations and agree with observations, but SST variability along the coast is larger in the runs including wind-stress synoptic variability, suggesting a rather linear response of the ocean to cycles of southerly wind strengthening and relaxation. We found that coastal SST variability does not change significantly in the first tenths of kilometers from the shore when including daily heat fluxes, highlighting the prominent role of wind-driven upwelling cycles. In contrast, in the offshore region situated beyond the 50 km coastal strip, it is necessary to include synoptic variability in the heat fluxes to account for a realistic SST variability.

The response of the oceanic circulation to mesoscale atmospheric forcing is studied by comparing integrations of a global sea ice-ocean model with high-resolution European Centre for Medium-Range Weather Forecasts analysis data (0.4◦) to those with the same forcing coarse grained to a resolution typically employed in climate models and atmospheric reanalyses (1.8◦). It is shown that the representation of mesoscale features in atmospheric forcing fields leads to an increase in the strength of the wind-driven gyres in the North Atlantic and North Pacific regions of about 5–10% of its mean value. An increase of similar magnitude is found for the Atlantic meridional overturning circulation. From the results of this study it is argued that small-scale atmospheric phenomena such as fronts, mesoscale cyclones, and topographic jets play an important role in driving the mean oceanic circulation.