The SST multidecadal variability in the Atlantic-Mediterranean region and its relation to AMO

Abstract Two sea surface temperature (SST) time series, the Extended Reconstructed SST version 3 (ERSST.v3) and the Hadley Centre Sea Ice and Sea Surface Temperature dataset (HadISST), are used to investigate SST multidecadal variability in the Mediterranean Sea and to explore possible connections with other regions of the global ocean. The consistency between these two time series and the original International Comprehensive Ocean–Atmosphere Dataset version 2.5 (ICOADS 2.5) over the Mediterranean Sea is investigated, evaluating differences from monthly to multidecadal scales. From annual to longer time scales, the two time series consistently describe the same trends and multidecadal oscillations and agree with Mediterranean ICOADS SSTs. At monthly time scales the two time series are less consistent with each other because of the evident annual cycle that characterizes their difference. The subsequent analysis of the Mediterranean annual SST time series, based on lagged-correlation analysis, multitaper method (MTM), and singular spectral analysis (SSA), revealed the presence of a significant oscillation with a period of about 70 yr, very close to that of the Atlantic multidecadal oscillation (AMO). An extension of the analysis to other World Ocean regions confirmed that the predominance of this multidecadal signal with respect to longer period trends is a unique feature of the Mediterranean and North Atlantic Ocean, where it reaches its maximum at subpolar latitudes. Signatures of multidecadal oscillations are also found in the global SST time series after removing centennial and longer-term components. The analysis also reveals that Mediterranean SST and North Atlantic indices are significantly correlated and coherent for periods longer than about 40 yr. For time scales in the range 40–55 yr the coherence between the Mediterranean and subpolar gyre temperatures is higher than the coherence between the Mediterranean SST and North Atlantic Oscillation (NAO) or AMO. Finally, the results of the analysis are discussed in the light of possible climate mechanisms that can couple the Mediterranean Sea with the North Atlantic and the Global Ocean

Komponenta projekta ADRICOSM – sustav promatranja na velikoj skali – satelitski sustav

In the framework of the ADRICOSM project, the Satellite Oceanography Group (GOS) of Rome developed a Fast Delivery System (FDS) for providing the partner modeling centres with remotelysensed ocean colour and sea surface temperature (SST) data. Data are processed, mapped and binned on the Adriatic Sea area in order to be assimilated into both ecosystem models and circulation models for ocean forecasting. Further technological improvements permitted the building and optimization of a system suitable for meeting the increasing demand for near-real-time ocean colour and SST products for applications in operational oceanography. Real-Time Images of SeaWiFS chlorophyll concentration, clouds/case I/case II water flags and true colour images are obtained by processing the satellite passes using climatological ancillary data. These images are provided daily through an ad hoc automatic system that processes the raw satellite data and makes it available on the web within an hour of satellite overpass acquisition. All of the images are stored in a gallery web archive organized in a calendar chart. Accurate chlorophyll maps for assimilation are produced in near real time (typically after 4 days) as soon as daily meteorological ancillary data are made available on the NASA website. Each chlorophyll map is flagged for clouds or other contamination factors using the corresponding 24 quality flag maps. This implies that case-2 waters and spurious atmospheric effects have been removed from the pigment data set. This final product is binned on the Adriatic model grid and made available for the ADRICOSM project on the GOS web site. NOAA/AVHRR data are also acquired by the GOS ground station in Rome and managed by the FDS from their reception up to their distribution. Daily SST maps of the Adriatic Sea binned over the AREG model grid at 1/16° resolution are distributed weekly in Near-Real-Time along with the daily SST maps of the eastern Mediterranean Sea delivered at 1/8° resolution to the MFSTEP project. Real-Time SST maps of the Adriatic Sea at 1km resolution are posted daily in GIF format on the GOS website.U okviru projekta ADRISOSM, GOS (Grupa za satelitsku oceanografiju) iz Rima razvila je Sustav za brzu isporuku FDS, snabdijevanje partnerskih centara za modeliranje satelitskim snimcima boje mora i tem-peraturnim podacima površine mora (SST). Podaci za Jadran su obrađeni, pretvoreni u grafičke produkte i digitalizirani kako bi se mogli asimilirati u model strujanja i model ekosistema, te koristiti oceanografskoj prognozi. Daljnja tehnološka poboljšanja su omogućila izgradnju i optimalizaciju sustava, zbog rastućih potreba za produktima boje mora i površinske temperature za različite primjene u operativnoj oceanografiji. Slike koncentracije klorofila od senzora SeaWiFS, slike oblaka te Case1 i Case2 oznake, kao i slike prave boje dobivaju se procesiranjem satelitskih scena uz popratne klimatološke podatke. Slike se procesiraju dnevno kroz ad-hoc automatski sustav koji obrađuje sirove satelitske podatke i omogućuje njihovu isporuku na mrežu, sat vremena nakon prikupljanja satelitskih podataka tj. nakon prolaska satelita. Sve se slike spremaju u arihvu na mreži koja je organizirana prema datumima. Korigirane slike koncentracije klorofila za asimilaciju u model proizvode se u skoro realnom vremenu (tipično 4 dana kasnije) čim se dobiju popratni meteorološki podaci s mreže NASA-e. Na svakoj slici klorofila su označeni oblaci ili drugi kontaminirajući faktori, prema 24 kategorije kvalitete slika. To znači da su Case 2 slučajevi piksela uklonjeni iz snimaka kao i atmosferske sme-tnje. Konačni produkt se usklađuje s koordinatnom mrežom Jadrana i stavlja na raspolaganje na stranicama ADRICOSM-a preko GOS-ove Internet stranice. GOS zemaljska stanica u Rimu prikuplja i podatke NOAA/ AVHRR koji se procesiraju kroz FDS sustav, od prijema do konačne distribucije podataka. Dnevne se slike površinske temperature mora (SST), usklađene preko koordinatne mreže AREG-a pri prostornom razlučivanju od 1/16 stupnja, distribuiraju tjedno u skoro realnom vremenu, zajedno sa slikama istočnog Sredozemlja koje imaju razlučivanje od 1/8 stupnja prema MFSTEP projektu. Dnevno, u skoro realnom vremenu, isporučuju se slike SST za Jadran uz prostorno razlučivanje od 1 km u GIF formatu na Internet stranici GOS-a

An Artificial Neural Network to Infer the Mediterranean 3D Chlorophyll-a and Temperature Fields from Remote Sensing Observations

Remote sensing data provide a huge number of sea surface observations, but cannot give direct information on deeper ocean layers, which can only be provided by sparse in situ data. The combination of measurements collected by satellite and in situ sensors represents one of the most effective strategies to improve our knowledge of the interior structure of the ocean ecosystems. In this work, we describe a Multi-Layer-Perceptron (MLP) network designed to reconstruct the 3D fields of ocean temperature and chlorophyll-a concentration, two variables of primary importance for many upper-ocean bio-physical processes. Artificial neural networks can efficiently model eventual non-linear relationships among input variables, and the choice of the predictors is thus crucial to build an accurate model. Here, concurrent temperature and chlorophyll-a in situ profiles and several different combinations of satellite-derived surface predictors are used to identify the optimal model configuration, focusing on the Mediterranean Sea. The lowest errors are obtained when taking in input surface chlorophyll-a, temperature, and altimeter-derived absolute dynamic topography and surface geostrophic velocity components. Network training and test validations give comparable results, significantly improving with respect to Mediterranean climatological data (MEDATLAS). 3D fields are then also reconstructed from full basin 2D satellite monthly climatologies (1998–2015) and resulting 3D seasonal patterns are analyzed. The method accurately infers the vertical shape of temperature and chlorophyll-a profiles and their spatial and temporal variability. It thus represents an effective tool to overcome the in-situ data sparseness and the limits of satellite observations, also potentially suitable for the initialization and validation of bio-geophysical models

Seasonal distributions of ocean particulate optical properties from spaceborne lidar measurements in Mediterranean and Black sea

Assessing the oceanic surface layer's optical properties through CALIOP has been one of the reasons of the extension of the CALIOP mission for 3 more years (2018-2020). This is the first work evaluating the potential use of CALIOP for ocean applications at regional scale in mid-latitude regions (i.e. Mediterranean, MED, and Black Sea, BS) and investigating the added information on ocean particles given by the column integrated depolarization ratio (delta(T)) parameter. We implemented and refined a retrieval procedure to estimate this parameter at 1/4 degree of spatial resolution, comparing 7 years of CALIOP observations (2011-2017) to the corresponding Copernicus multi-sensor L3 ocean colour products of the surface particle backscattering coefficient (b(bp)) and chlorophyll-a concentration (Chl-a). This study pointed out that the current CALIOP sampling is inadequate to detect subtle day-night difference due to plankton diel variability for these basins. At a basin scale, delta(T) covaries with b(bp) for b(bp) >= 0.0015 m(-1). This is more evident for BS (R = 0.84) than for MED (R = 0.61). The analysis of seasonal distributions confirm this result for BS, where dT has a semi-annual cycle in very good agreement with bbp. In the MED, characterized by different trophic regimes, delta(T) shows also some similarities with Chl-a annual cycle. The combined characterization in the MED bioregions of the annual patterns of b(bp):Chl-a, delta(T):Chl-a and delta(T):b(bp) ratios suggested that delta(T) parameter can provide valuable information about the non-sphericity and the size of ocean particles

The role of Internal Solitary Waves on deep-water sedimentary processes. The case of up-slope migrating sediment waves off the Messina Strait

Subaqueous, asymmetric sand waves are typically observed in marine channel/canyon systems, tidal environments, and continental slopes exposed to strong currents, where they are formed by current shear resulting from a dominant unidirectional flow. However, sand-wave fields may be readily observed in marine environments where no such current exists; the physical processes driving their formation are enigmatic or not well understood. We propose that internal solitary waves (ISWs) induced by tides can produce an effective, unidirectional boundary “current” that forms asymmetric sand waves. We test this idea by examining a sand-wave field off the Messina Strait, where we hypothesize that ISWs formed at the interface between intermediate and surface waters are refracted by topography. Hence, we argue that the deflected pattern (i.e., the depth-dependent orientation) of the sand-wave field is due to refraction of such ISWs. Combining field observations and numerical modelling, we show that ISWs can account for three key features: ISWs produce fluid velocities capable of mobilizing bottom sediments; the predicted refraction pattern resulting from the interaction of ISWs with bottom topography matches the observed deflection of the sand waves; and predicted migration rates of sand waves match empirical estimates. This work shows how ISWs may contribute to sculpting the structure of continental margins and it represents a promising link between the geological and oceanographic communities

Improving the Altimeter-Derived Surface Currents Using Sea Surface Temperature (SST) Data: A Sensitivity Study to SST Products

Measurements of ocean surface topography collected by satellite altimeters provide geostrophic estimates of the sea surface currents at relatively low resolution. The effective spatial and temporal resolution of these velocity estimates can be improved by optimally combining altimeter data with sequences of high resolution interpolated (Level 4) Sea Surface Temperature (SST) data, improving upon present-day values of approximately 100 km and 15 days at mid-latitudes. However, the combined altimeter/SST currents accuracy depends on the area and input SST data considered. Here, we present a comparative study based on three satellite-derived daily SST products: the Remote Sensing Systems (REMSS, 1/10 ∘ resolution), the UK Met Office OSTIA (1/20 ∘ resolution), and the Multiscale Ultra-High resolution SST (1/100 ∘ resolution). The accuracy of the marine currents computed with our synergistic approach is assessed by comparisons with in-situ estimated currents derived from a global network of drifting buoys. Using REMSS SST, the meridional currents improve up to more than 20% compared to simple altimeter estimates. The maximum global improvements for the zonal currents are obtained using OSTIA SST, and reach 6%. Using the OSTIA SST also results in slight improvements (≃1.3%) in the zonal flow estimated in the Southern Ocean (45 ∘ S to 70 ∘ S). The homogeneity of the input SST effective spatial resolution is identified as a crucial requirement for an accurate surface current reconstruction. In our analyses, this condition was best satisfied by the lower resolution SST products considered

Using overlapping VIIRS scenes to observe short term variations in particulate matter in the coastal environment

Abstract In coastal areas, the concentrations and the optical properties of the water components have a large spatial and temporal variability, due to river discharges and meteo-marine conditions, such as wind, wave and current, and their interaction with shallow water bathymetry. This large temporal variability cannot be captured using the standard Ocean Colour Radiometry (OCR) polar orbiting satellites, the latter providing almost one image per day. On the contrary, the use of OCR geostationary sensors, like the Geostationary Ocean Colour Imager (GOCI), centred above the Korean Peninsula, enable to capture the short-term variability of the optical properties. To compensate the lack of a geostationary sensor similar to GOCI over other coastal environments, like the North Adriatic Sea (NAS), the multiple observations provided during the same day by the Visible Infrared Imaging Radiometer Suite (VIIRS) mounted on the SUOMI NPP satellite, can be exploited. Indeed, due to its large swath of 3060 km, the VIIRS orbits can overlap over the NAS during the same day within 1 h and 42 min, an important feature that can be useful in capturing the short term variability of the optical properties. A large number of VIIRS overlaps in the NAS are characterized by high sensor zenith angle (SZA) of the observation, resulting in a large portion of images masked by the high satellite zenith flag. In order to make available those observations and, in general, to reduce the dependence of the VIIRS observations from the SZA, an adjustment based on a multi linear regression scheme, which exploits radiometric in situ observations, was here applied. This study aims to prove the suitability of the adjusted overlapping VIIRS in capturing the short time scale dynamics of particulate backscattering, and this was demonstrated by the analysis of a case study for the 21st and 22nd of March 2013. In order to evaluate the advantages in using multiple observations during the same day, also the ~24 h dynamics was analysed, comparing the overlapping VIIRS results with the ones obtained from the daily product

Air–Sea Interaction in the Central Mediterranean Sea: Assessment of Reanalysis and Satellite Observations

Air–sea heat fluxes are essential climate variables, required for understanding air–sea interactions, local, regional and global climate, the hydrological cycle and atmospheric and oceanic circulation. In situ measurements of fluxes over the ocean are sparse and model reanalysis and satellite data can provide estimates at different scales. The accuracy of such estimates is therefore essential to obtain a reliable description of the occurring phenomena and changes. In this work, air–sea radiative fluxes derived from the SEVIRI sensor onboard the MSG satellite and from ERA5 reanalysis have been compared to direct high quality measurements performed over a complete annual cycle at the ENEA oceanographic observatory, near the island of Lampedusa in the Central Mediterranean Sea. Our analysis reveals that satellite derived products overestimate in situ direct observations of the downwelling short-wave (bias of 6.1 W/m2) and longwave (bias of 6.6 W/m2) irradiances. ERA5 reanalysis data show a negligible positive bias (+1.0 W/m2) for the shortwave irradiance and a large negative bias (−17 W/m2) for the longwave irradiance with respect to in situ observations. ERA5 meteorological variables, which are needed to calculate the air–sea heat flux using bulk formulae, have been compared with in situ measurements made at the oceanographic observatory. The two meteorological datasets show a very good agreement, with some underestimate of the wind speed by ERA5 for high wind conditions. We investigated the impact of different determinations of heat fluxes on the near surface sea temperature (1 m depth), as determined by calculations with a one-dimensional numerical model, the General Ocean Turbulence Model (GOTM). The sensitivity of the model to the different forcing was measured in terms of differences with respect to in situ temperature measurements made during the period under investigation. All simulations reproduced the true seasonal cycle and all high frequency variabilities. The best results on the overall seasonal cycle were obtained when using meteorological variables in the bulk formulae formulations used by the model itself. The derived overall annual net heat flux values were between +1.6 and 40.4 W/m2, depending on the used dataset. The large variability obtained with different datasets suggests that current determinations of the heat flux components and, in particular, of the longwave irradiance, need to be improved. The ENEA oceanographic observatory provides a complete, long-term, high resolution time series of high quality in situ observations. In the future, more similar sites worldwide will be needed for model and satellite validations and to improve the determination of the air–sea exchange and the understanding of related processes