Ocean Lagrangian Trajectories (OLTraj): Lagrangian analysis for non-expert users

Lagrangian analysis is becoming increasingly important to better understand the ocean's biological and biogeochemical cycles. Yet, biologists and chemists often lack the technical skills required to set up such analyses. Here, we present a new product of pre-computed ocean Lagrangian trajectories (OLTraj) targeting non-expert users, and demonstrate how to use it by means of worked examples. OLTraj is based on satellite-derived geostrophic currents, which allows one to directly compare it with other in-situ or satellite products. We anticipate that OLTraj will foster a new interest in Lagrangian applications in ocean biology and biogeochemistry.

Lasting impact of winds on Arctic sea ice through the ocean's memory

In this paper we studied the impact of winds on Arctic sea ice through the ocean's memory by using numerical simulations. We found that the changes in halosteric height induced by wind perturbations can significantly affect the Arctic sea ice drift, thickness, concentration and deformation rates regionally even years after the wind perturbations. Changes in the Arctic liquid freshwater content and thus in halosteric height can cause changes in the sea surface height and surface geostrophic currents, which further enforce a lasting and strong impact on sea ice. The changes in both sea surface height gradient force (due to changes in sea surface height) and ice–ocean stress (due to changes in surface geostrophic currents) are found to be important in determining the overall ocean effects. The revealed ocean effects are mainly associated with changes in sea ice dynamics, not thermodynamics. Depending on the preceding atmospheric mode driving the ocean, the ocean's memory of the wind forcing can lead to changes in Arctic sea ice characteristics with very different spatial patterns. We obtained these spatial patterns associated with Arctic Oscillation, Arctic Dipole Anomaly and Beaufort High modes through dedicated numerical simulations. The dynamical impact of the ocean has strong seasonal variations, stronger in summer and weaker in winter and spring. This implies that declining trends of Arctic sea ice will very possibly allow a stronger ocean impact on the sea ice in a warming climate.

Ocean Lagrangian Trajectories (OLTraj): Lagrangian analysis for non-expert users

Lagrangian analysis is becoming increasingly important to better understand the ocean's biological and biogeochemical cycles. Yet, biologists and chemists often lack the technical skills required to set up such analyses. Here, we present a new product of pre-computed ocean Lagrangian trajectories (OLTraj) targeting non-expert users, and demonstrate how to use it by means of worked examples. OLTraj is based on satellite-derived geostrophic currents, which allows one to directly compare it with other in-situ or satellite products. We anticipate that OLTraj will foster a new interest in Lagrangian applications in ocean biology and biogeochemistry.

On the Structure and Kinematics of an Algerian Eddy in the Southwestern Mediterranean Sea

An Algerian Eddy, anticyclonic vortex generated by the instability of the Algerian Current in the southwestern Mediterranean Sea, is studied using data provided by drifters (surface currents), Argo floats (temperature and salinity profiles), environmental satellites (absolute dynamic topography maps and ocean color images) and operational oceanography products. The eddy was generated in May 2018 and lasted as an isolated vortex until November 2018. Its morphology and kinematics are described in June–July 2018 when drifters were trapped in its core. During that period, the eddy was slowly moving to the NE (~2 km/day), with an overall diameter of about 200 km (slowly growing with time) and maximal surface swirl velocity of ~50 cm/s at a radius of ~50 km. Geostrophic currents derived from satellite altimetry data compare well with low-pass filtered drifter velocities, with only a slight overestimation, which is expected as its maximum vorticity corresponds to a small Rossby number of ~0.6. Satellite ocean color images and some drifters show that the eddy has an elliptical spiral structure. The looping tracks of the drifters trapped in the eddy were analyzed using two statistical methods: least-squares ellipse fitting and wavelet ridge analysis, revealing a typical eccentricity of about 0.5, a wide range of inclination and a rotation period between 3 and 10 days. Clusters of drifters on the northeastern limb of the eddy were also considered to estimate divergence and vorticity. The results indicate convergence (divergence) and downwelling (upwelling) at scales of 20–50 km near the northeastern (northwestern) edge of the eddy, in agreement with the quasi-geostrophic theory. Vertically, the eddy extends mostly down to 250 m depth, with a warm, low-salinity and low-density signature and with geostrophic currents near 50 cm/s in the top layer (down to ~80 m) reducing to less than 10 cm/s near 250 m. Near the surface, colder water is advected into it.

Influence of the Arctic Oscillation on the Formation of Water Circulation Regimes in the Sector of the North, Norwegian and Barents Seas

The article studies the influence of wind forcing associated with the Arctic Oscillation on the water circulation regimes in the sector of the World Ocean (65–81.5 N, 0–70 E), which consolidates the North, Norwegian and Barents Seas. The study aims at establishing quantitative patterns of variability of the ocean level and surface geostrophic current velocities depending on the value of the Arctic Oscillation index. In general, the response of the sea level averaged over the ocean sector under consideration is in an antiphase with this index. However, there are periods of mismatch between antiphase fluctuations of the sea level and the Arctic Oscillation index. After 2009, an increase in the amplitude and a decrease in the duration of the phases of the Arctic Oscillation index are noted. The difference between the areas of positive and negative values of sea level anomalies creates a pressure gradient that causes surface geostrophic currents carrying Atlantic waters along the shelf edge eastward in a cyclonic regime (the Arctic Oscillation index is greater than 0) and westward in an anticyclonic regime (the index is less than 0). The article provides estimates of the linear regression coefficients: for the sea level they are ~ 2 cm in the shelf zone and about minus 1 cm in the deep-water part of the sector. Thus, the level difference between the shelf and the deeper part of the considered water area is ~ 3 cm per 1 unit of the Arctic Oscillation index. Estimates of the linear regression coefficients for anomalies of the geostrophic currents velocity were ~ 0.5 cm/s per 1 unit of the index. Analysis of the longterm variability of the steric component of the ocean level showed a better relationship with the interannual variability of the Arctic Oscillation index as compared to the ocean level.

Lasting impact of winds on Arctic sea ice through the ocean's memory

In this paper we studied the impact of winds on Arctic sea ice through the ocean’s memory by using numerical simulations. We found that the changes in ocean freshwater content induced by wind perturbations can significantly affect the Arctic sea ice drift, thickness, concentration and deformation rates regionally even years after the wind perturbations. Changes in the Arctic liquid freshwater content cause changes in the sea surface height and surface geostrophic currents, which further enforce a lasting and strong impact on sea ice. Both the changes in sea surface height gradient force (due to changes in sea surface height) and ice-ocean stress (due to changes in surface geostrophic currents) are found to be important in determining the overall ocean effects. The revealed ocean effects are mainly associated with changes in sea ice dynamics, not thermodynamics. Depending on the preceding atmospheric mode driving the ocean, the ocean’s memory of the wind forcing can lead to changes in Arctic sea ice characteristics with very different spatial patterns. We identified these spatial patterns associated with Arctic Oscillation, Arctic Dipole Anomaly and Beaufort High modes through dedicated numerical simulations. The dynamical impact of the ocean has strong seasonal variations, stronger in summer and weaker in winter and spring. It implies that declining trends of Arctic sea ice will very possibly allow a stronger ocean impact on the sea ice in a warming climate.

Surface circulation in the Ligurian Sea: an assessment of satellite radar altimeter-derived geostrophic currents.

<p>Recent improvements of satellite altimeter observations allow to approach investigations on the surface ocean circulation even in those regions where the slope associated to dynamic structures is reduced. The capability to detect the main dynamic features and their variability from satellite radar altimetry in the Ligurian Sea (Western Mediterranean) is here assessed.</p><p>Altimeter data from X-TRACK products recently released are used for this study: the time series of satellite-based- currents along the track n.044, which crosses the Ligurian Sea from the Corsica Channel to the Ligurian coast, is analysed. The temporal sampling is about 10 days and the along-track resolution is 7 km. Geostrophic currents computed from satellite radar altimetry are checked for consistency against the dynamic topography obtained from concurrent CTD casts collected during recent oceanographic campaigns carried out by the Italian Hydrographic Institute along the track. A more detailed assessment of the computed current velocities is based on the analysis of long-term ADCP measurements from a fixed mooring deployed from 2004 to 2006 in the Central Ligurian Sea (43°47.77’ N; 9°02.85’ E) 40 nm from the coast, quite close to the altimeter track. An RD&I 300 kHz upward-looking ADCP sampled the upper layer at 8 m vertical resolution. Currents in the upper layer (0-100 m) are almost barotropic with the variability due to the wind confined to the upper few meters. In order to define an appropriate metrics to compare currents from different measuring systems, EOF analysis of ADCP profiles have proved to be a good tool to filter out the high frequency and wind driven currents, thus enhancing the contribution of the geostrophic component.</p>

Seasonal cycle of Arctic Ocean circulation inferred from satellite altimetry

<p>In recent decades, the retreat of the Arctic sea ice has modified vertical momentum fluxes from the atmosphere to the ice and the ocean, in turn affecting the surface circulation. Satellite altimetry has contributed in the past ten years to understand these changes. Most oceanographic datasets are however to date limited either to open ocean and ice-covered regions, given that different techniques are required to track sea surface height over these two surfaces. Hence, efforts to generate unified Arctic-wide datasets are still required to further basin-wide studies of the Arctic Ocean surface circulation.</p><p>We present here the assessment of a new Arctic-wide gridded dataset of the Sea Level Anomaly (SLA) and SLA-derived geostrophic velocities. This dataset is based on Cryosat-2 observations over ice-covered and open ocean areas in the Arctic during 2011 to 2018.</p><p>We compare the SLA and geostrophic currents derived hereof to in situ observations of ocean bottom pressure, steric height and near-surface ocean velocity, in three regions: the Fram Strait, the shelf break north of the Arctic Cape and the Laptev Sea continental slope. Good agreement in SLA is shown at seasonal time scales, with the dominant component of SLA variability being steric height both in Fram Strait and at the Arctic Cape. On the other hand, ocean bottom pressure dominates SLA changes at the Laptev Sea site. The comparison of velocity at two mooring transects, one in Fram Strait and the other at the Laptev Sea continental slope, reveals that the correlation is highest at the moorings closest to the shelf break, where currents are faster and the seasonal cycle is enhanced.</p><p>The seasonal cycle of SLA and geostrophic currents as derived from the altimetric product is in favourable agreement with previous results. A quasi-simultaneous occurrence of the SLA maximum happens between October and January; similar phase has been found in steric height seasonal cycle by studies using hydrographic profiles in several regions of the Arctic Ocean. We thereby find the highest SLA amplitude over the shelves, which other studies point to be possibly related to winter-enhanced shoreward water mass transport. Seasonal variability in the geostrophic currents is most pronounced along the shelf edges, representing a basin wide, coherent seasonal acceleration of the Arctic slope currents in winter and a deceleration in summer. This is consistent with the shelf-amplified SLA seasonal cycle described above. Density driven coastal currents near Alaska and Siberia have variable cycle, consistent with the cycle of river runoff and local wind forcing. Enhanced south-western limb of the Beaufort Gyre in early winter is in agreement with a combination between the Beaufort High buildup and relatively thin sea ice.</p><p>In summary, we provide evidence that the altimetric data set has skills to reproduce the seasonal cycle of SLA and geostrophic currents consistently with in situ data and findings from other studies. We suggest that this dataset could be used not only for large scale studies but also to study Arctic boundary currents.  </p>