ALBERT

Description: Projected future shoaling of the wintertime mixed layer in the Northeast (NE) Atlantic has been shown to induce a regime shift in the main nutrient supply pathway from the Atlantic to the Northwest European Shelf (NWES) near the end of the 21st century. While reduced winter convection leads to a substantial decrease in the vertical nutrient supply and biological productivity in the open ocean, vertical mixing processes at the shelf break maintain a connection to the subpycnocline nutrient pool and thus productivity on the shelf. Here we investigate how meltwater discharge from the Greenland ice sheet (GIS) not yet taken into account impacts the mixed layer shoaling and the regime shift in terms of spatial distribution and temporal variability. To this end we have downscaled sensitivity experiments by a global earth system model for various GIS melting rates with a regionally coupled ocean-atmosphere climate system model. The model results indicate that increasing GIS meltwater discharge leads to a general intensification of the regime shift. Atlantic subpycnocline water masses mixed up at the shelf break become richer in nutrients and thus limit the projected nutrient decline on the shelf. Moreover, the stronger vertical nutrient gradient through the pycnocline results in an enhanced interannual variability of on-shelf nutrient fluxes which, however, do not significantly increase variations in nutrient concentrations and primary production on the shelf. Moreover, due to the impact of the GIS meltwater discharge on the NE Atlantic mixed layer depth, the regime shift becomes initiated earlier in the century by about 1–2 decades, depending on the discharge rate. The effect on the onset timing, though, is found to be strongly damped by the weakening of the Atlantic meridional overturning circulation. A GIS melting rate that is even 10 times higher than expected for emission scenario RCP8.5 would lead to an onset of the regime shift not until the 2070s.

Description: The Humboldt Large Marine Ecosystem (HLME) and Patagonian Large Marine Ecosystem (PLME) are the two largest marine ecosystems of the Southern Hemisphere, respectively located along the Pacific and Atlantic coasts of southern America. This work investigates the exchange between these two LMEs and its variability, employing numerical model results and offline particle tracking algorithms. 27 years of a 1/12° ROMS configuration (CMM) show a general poleward transport on the Southern region of HLME, and equatorward on the Patagonian Shelf (PS). A mean transport across Cape Horn's shelf (68.1° W) is 0.95 Sv. Lagrangian simulations show that the majority of the southern PS waters originate from the upper layer in the southeast South Pacific (〉 200 m), mainly from the southern Chile and Cape Horn shelves. The exchange takes place through Le Maire Strait, Magellan Strait, and the shelf-break. These inflows account to a net northeastward transport of 0.88 Sv at 51° S in the southern PLME. The transport across Magellan strait is small (0.1 Sv) but due to its relatively low salinity it impacts greatly the density and surface circulation of the coastal waters of the southern PLME. The water masses flowing into the Malvinas Embayment eventually reach the PLME through the Malvinas Shelf and occupy the outer part of the shelf. The seasonal and interannual variability of the transport are also addressed. On the southern PLME, the interannual variability of the shelf exchange is partly explained by the large-scale wind variability, which in turn is partly associated with the SAM index (r = 0.52).

Description: The Kuroshio Current System in the North Pacific displays path transitions on a decadal time scale. It is known that both internal variability involving barotropic and baroclinic instabilities and remote Rossby waves induced by North Pacific wind-stress anomalies are involved in these path transitions. However, the precise coupling of both processes and its consequences for the dominant decadal transition time scale are still under discussion. Here, we analyse the output of a multi-centennial long high-resolution global climate model simulation and study phase synchronisation between Pacific zonal wind-stress anomalies and Kuroshio Current System path variability. We apply the Hilbert transform technique to determine the phase and find epochs where such phase synchronisation appears. The physics of this synchronisation is shown to occur through the effect of the vertical motion of isopycnals, as induced by the propagating Rossby waves, on the instabilities of the Kuroshio Current System.

Description: A dynamically passive inert tracer was released in the interior South Pacific Ocean at latitudes of the Antarctic Circumpolar Current. Observational cross sections of the tracer were taken over four consecutive years as it drifted through Drake Passage and into the Atlantic Ocean. The tracer was released within a region of high salinity relative to surrounding waters at the same density. In the absence of irreversible mixing a tracer remains at constant salinity and temperature on an isopycnal surface. To investigate the process of irreversible mixing we analysed the tracer in potential density versus salinity-anomaly coordinates. Observations of high tracer concentration tended to be collocated with isopycnal salinity anomalies. With time an initially narrow peak in tracer concentration as a function of salinity at constant density, broadened with the tracer being found at ever fresher salinities, consistent with diffusion-like behaviour in that coordinate system. The second moment of the tracer as a function of salinity suggested an initial period of slow spreading for approximately 2 years in the Pacific, followed by more rapid spreading as the tracer entered Drake Passage and the Scotia Sea. Analysis of isopycnal salinity gradients based on the Argo programme suggests that part of this apparent change can be explained by changes in background salinity gradients while part of the change may be explained by geographical changes in background mixing.

Description: Surface currents are poorly known over most of the oceans. Satellite-borne Doppler Waves and Current Scatterometers (DWCS) can be used to fill this observation gap. The Sea surface KInematics Multiscale (SKIM) proposal, is the first satellite concept built on a DWCS design at near-nadir angles, and now one of the two candidates to become the 9th mission of the European Space Agency Earth Explorer program. As part of the detailed design and feasibility studies (phase A) funded by ESA, airborne measurements were carried out with both a Ku-Band and a Ka-Band Doppler radars looking at the sea surface at near nadir-incidence in a real-aperture mode, i.e. in a geometry and mode similar to that of SKIM. The airborne radar KuROS was deployed to provide simultaneous measurements of the radar backscatter and Doppler velocity, in a side-looking configuration, with an horizontal resolution of about 5 to 10 m along the line of sight and integrated in the perpendicular direction over the real-aperture 1-way 3-dB footprint diameter (about 580 m). The KaRADOC system has a much narrower beam and footprint that only about 45 m in diameter. The experiment took place in November 2018 off the French Atlantic coast, with sea states representative of the open ocean and a well known tide-dominated current regime. The data set is analyzed to explore the contribution of non-geophysical velocities to the measurement and how the geophysical part of the measured velocity combines wave-resolved and wave-averaged scales. We find that the measured Doppler velocity contains a characteristic wave phase speed, called here C0 that is analogous to the Bragg phase speed of coastal High Frequency radars that use a grazing measurement geometry, with little variations ΔC associated to changes in sea state. The Ka-band measurements at an incidence of 12° are 10 % lower than the theoretical estimate C0 ~ 2.4 m/s for typical oceanic conditions defined by a wind speed of 7 m/s and a significant wave height of 2 m. For Ku-band the measured data is 30 % lower than the theoretical estimate 2.8 m/s. ΔC is of the order of 0.2 m/s for a 1 m change in wave height, and cannot be confused with a 1 m/s change in tidal current. The actual measurement of the current velocity from an aircraft at 4 to 18° incidence angle is, however, made difficult by uncertainties on the measurement geometry, which are much reduced in satellite measurements.

Description: The Coastal-Ocean Carbon Exchange in the Canary Region Project (COCA) arises in order to analyse and get to understand the impact of lateral export of nutrients and organic matter from the highly productive Coastal Upwelling System off NW Africa in the biogeochemical cycles during two different seasons. The circulation patterns off NW African Upwelling System are examined by applying an inverse model to two hydrographic datasets gathered in fall 2002 and spring 2003. The mass transports estimated by model are consistent with the thermal wind equation and the conservation of mass in a closed volume. Besides, the Ekman transport and the freshwater flux are also considered. These estimates show a seasonal variability in the circulation patterns at central levels, particularly in the southern boundary of the domain, where the Cape Verde Frontal Zone is located. In the beginning of fall, this circulation is deeper and northward with a net transport of 6 ± 3 Sv and, in the late spring, it is shallower and southward with a similar intensity. At intermediate levels important differences are also observed between the two seasons. In fall, the Antarctic Intermediate Waters reaches higher latitudes with 2 ± 2 Sv flowing northward. During spring, there is no significant northward flow of AAIW. However, there is a moderate westward mass transport which impacts both the lateral transports of inorganic nutrients and organic matter at intermediate layers and also the shallowest lateral transports of organic matter. Seasonal variability in circulation patterns are also reflected in lateral transports of inorganic nutrients and dissolved organic carbon. Therefore, the changes in the circulation patterns between the two seasons have allowed us to assess the variability in the contributions of SiO2, NO3, PO4 and DOC from the first to the second season. In fall, the transports are mainly northward from the south with −0.80 ± 0.34, −1.11 ± 0.47 and −0.07 ± 0.03 kmol s-1 of SiO2, NO3 and PO4, respectively. In spring, however, lateral transports off-shore are favoured with 0.75 ± 0.37, 1.34 ± 0.66 and 0.08 ± 0.04 kmol s-1 of SiO2, NO3 and PO4, respectively. This westward transport stimulates in turn an intensified westward DOC transport at shallow layers, specifically 0.50 ± 0.25 x 108 mol C day-1.

Description: Satellite data from the equatorial Pacific is compared with results from a high resolution ocean model during a period including the strong El Niño of 1997–98. The results show that the ocean model realistically captures the changes in sea surface temperature and the propagation of the annual Rossby wave although it may overestimate the reduced energy of tropical instability waves during development of an El Niño. The results provide additional confidence in the oceanic mechanisms which model analysis implicated as being responsible for the development of both the 1982–83 and the 1997–98 El Niños.

Description: In May 2012, we conducted a hydrographic survey over the Carlsberg Ridge in the northwest Indian Ocean. In this paper, we use these station data, in combination with some free-floating Argo profiles, to obtain the sectional temperature and salinity fields, and subsequently, the hydrographic characteristics are comprehensively analyzed. Through the basic T-S diagram, three salty water masses, Arabian Sea High-Salinity Water, Persian Gulf Water, and Red Sea Water, are identified. The sectional data show a clear ventilation structure associated with Arabian Sea High-Salinity Water. The 35.8 psu salty water sinks at 6.9° N and extends southward to 4.4° N at depths around the thermocline, where the thermocline depth is in the range of 100 to 150 m. This salty thermocline extends much further south than the climatology indicates. Furthermore, the temperature and salinity data are used to compute the absolute geostrophic current over the specific section, and the results show meso-scale eddy vertical structure different from some widely used oceanic reanalysis data. We also find a west-propagating disturbance at 6° N, and the related features are described in terms of phase speed, horizontal and vertical structures.

Description: Assessments of ocean data assimilation (DA) systems and observing system design experiments typically rely on identical or fraternal twin experiments. The identical twin approach has been recognized as yielding biased impact assessments in atmospheric predictions but these shortcomings are not sufficiently appreciated for oceanic DA applications. Here we present the first direct comparison of the fraternal and identical twin approach in an ocean DA application. We assess the assimilation impact for both approaches in a DA system for the Gulf of Mexico that uses the Ensemble Kalman Filter. Our comparisons show that, despite a reasonable error growth rate in both approaches, the identical twin produces a biased skill assessment overestimating the improvement from assimilating sea surface height and sea surface temperature observations while underestimating the value of assimilating temperature and salinity profiles. Such biases can lead to an undervaluation of some observing assets (in this case profilers) and thus misguided distribution of observing system investments.

Description: The variability of the sea surface temperature (SST) in the Northwest Pacific has been studied on seasonal, annual and interannual scales based on the monthly datasets of ERSST 3b (1854–2017, 164 years) and OISST V2 (1988–2017, 30 years). The overall trends, spatial-temporal distribution characteristics, regional differences in seasonal trends, and seasonal differences of SST in the Northwest Pacific have been calculated over the past 164 years based on these datasets. In the past 164 years, the SST in the Northwest Pacific has been increasing linearly year by year with a trend of 0.033 °C/10 yr. The period from 1880 to 1910 is a slow decreasing trend period in the past 164 years and the SST during the 1910–1930 period was a trough of the past 164 years. After 1930, SST has continued to increase until now. The increasing trend in the past 30 years has reached 0.132 °C/10 yr and the increasing trend in the past 10 years is 0.306 °C/10 yr, which is around ten times in the past 164 years. The SST in most regions of the Northwest Pacific showed a linear increasing trend year by year, and the increasing trend in the offshore region was stronger than that in the ocean and deep-sea region. The change trend of the SST in the Northwest Pacific shows a large seasonal difference, and the increasing trend in autumn and winter is larger than that in spring and summer. There are some correlations between the SST and some climate indexes and atmospheric parameters, the correlation between the SST and some atmospheric parameters have been discussed, such as NAO, PDO, SOI anomaly, TCW, Nino 3.4, SLP, Precipitation, T2 and wind speed. The lowest SST in the Near China Sea basically occurred in February and the highest in August. The SST fluctuation in the Bohai Sea and Yellow Sea (BYS) is the largest with a range from 5 °C to 22 °C, the SST in the East China Sea (ECS) is from 18 °C to 27 °C, the smallest fluctuations occurs in the South China Sea (SCS) maintained at range of 26 °C to 29 °C. There are large differences between the mean and standard deviation in different sea regions.