Sea Height Anomalies Research Articles

The impact of climate change on circulation patterns in the Eastern Mediterranean Sea upper layer using Med-ROMS model

This paper deals with three hindcasting simulations of circulation and sea level elevation, using Med-ROMS (Mediterranean- Regional Ocean Model System, with high horizontal -resolution (8.6 km). Each simulation has started at the end of previous 100-years spin-up and control run. The model surface boundary conditions, from the corrected ERA-40 re-analyses (Pettenuzzo et al., 2010) and three momentum flux ensemble members are based on a surface wind Bayesian Hierarchical Models (BHM) - Mediterranean Forecasting System (MFS-Wind-BHM) described in Berliner and Kim (2007) and Milliff et al. (2011), cover the period January 1958 to December 2001. The results have been validated by an Empirical Orthogonal Function (EOF) analysis of surface sea height anomalies for satellite and Med-ROMS Hindcasts. The correlation coefficients between Med-ROMS (Principal Component) PC1 (Sea Surface Height anomaly) SSHa hindcasts 1, 2, 3 and AVISO satellite are 0.985, 0.97 and 0.98 respectively, all values are significant at 95% confidence limit. We have defined a Bimodal Oscillating System mechanism responsible for decadal reversals of the Ionian basin-wide circulation (BiOS) (Gacic et al., 2011, 2014) as a pattern of Eastern Mediterranean climate change. The ROMS model successfully simulated the observed general circulation pattern in the Eastern Mediterranean. Over the period from July 1997 till December 2001, the Med-ROMS has successfully simulated the north Ionian inversion; SSHa same as Satellite SSH with an averaged RMSE between them of 0.0792 m. The spatial correlation maps between wind stress curl and Med-ROMS-SSHa/EOF1-SSHa are made and found to be significantly correlated at a 95% confidence limit. The highest significant correlation coefficient for Med-ROMS-SSHa was approximately 0.8 and located off the western Turkish coast inside the Aegean Sea. This strong significant positive correlation reflects the cyclonic motion of Rhodes Gyre. A high significant correlation coefficient for Med-ROMS-EOF1-SSHa, of around 0.51, was found along the western coast of Greece in the northeast Ionian Sea. For the first time, we could correlate the North Ionian Sea inversion to wind stress curl, which was significantly correlated with the EOF1 Med-ROMS SSHa averaged hindcasts during the inversion period. In this transient period, we noticed a significant correlation coefficient, at a 95% confidence limit, between NAO indices moving on average over 12 months and Med-ROMS-SSHa -PC1 over the Eastern Mediterranean.

Assimilation of satellite surface-height anomalies data into a Hybrid Coordinate Ocean Model (HYCOM) over the Atlantic Ocean

The data of sea height anomalies calculated along the tracks of the Jason-1 and Jason-2 satellites are assimilated into the HYCOM hydrodynamic ocean model developed at the University of Miami, USA. We used a known method of data assimilation, the so-called ensemble method of the optimal interpolation scheme (EnOI). In this work, we study the influence of the assimilation of sea height anomalies on other variables of the model. The behavior of the time series of the analyzed and predicted values of the model is compared with a reference calculation (free run), i.e., with the behavior of model variables without assimilation but under the same initial and boundary conditions. The results of the simulation are also compared with the independent data of observations on moorings of the Pilot Research Array in the Tropical Atlantic (PIRATA) and the data of the ARGO floats using objective metrics. The investigations demonstrate that data assimilation under specific conditions results in a significant improvement of the 24-h prediction of the ocean state. The experiments also show that the assimilated fields of the ocean level contain a clearly pronounced mesoscale variability; thus they quantitatively differ from the dynamics obtained in the reference experiment.

Spatial and environmental determinants of the distribution of Striped Marlin (Kajikia audax) in the western and central North Pacific Ocean

Striped Marlin is a highly migratory species distributed throughout tropical and temperate waters in the North Pacific Ocean. The habitat characteristics of Striped Marlin in the western and central North Pacific Ocean were examined using generalized additive models by modelling fishery catch-rates as a function of remotely-sensed environmental covariates, including sea surface temperature (SST), chlorophyll-a concentration (CHL), mixed layer depth (MLD) and sea height anomalies (SHA). SST explained the largest proportion of the deviance, and is therefore considered the best predictor for the habitat of Striped Marlin. Spatial distributions of the relative density of Striped Marlin indicated that there is a seasonal north–south migration, and that the highest densities occur in the central North Pacific Ocean. The preferred habitat characteristics of Striped Marlin in high density areas were identified as SST between 23 and 26 °C, MLD within 30 m depth, and CHL around 0.08 mg m−3, while no preferred range was found for SHA. The results of this study could improve our understanding of Striped Marlin spatial distributions and habitat characteristics.

Investigation of the Brazil Current front variability from altimeter data

The southwestern Atlantic Ocean is characterized by the confluence of the Brazil and Malvinas Currents, which form very strong surface and subsurface fronts that can be detected from hydrographic and remote sensing observations. Three data sets, consisting of TOPEX/Poseidon‐derived sea height anomalies, the climatologically derived depth of the 10°C isotherm, and reduced gravity, are used in conjunction with a two‐layer dynamical ocean scheme to monitor the Brazil Current front and to investigate its variability during a 6 year period (1993–1998). Results reveal that the fronts exhibit motions that are larger zonally than meridionally, showing strong interannual variability with annual mean amplitudes that range from 1° to 6°. The annual and semiannual components account for more than 75% of the variability of the frontal oscillations. In the annual cycle the frontal motions appear to be related closely to fluctuations in the baroclinic transport of the Brazil Current and are only influenced by the Malvinas Current when the Brazil Current transport is very small.

Transition regions and their role in the relationship between sea surface height and subsurface temperature structure in the Atlantic Ocean

Expendable bathythermograph (XBT) profiles and TOPEX/Poseidon altimeter data (T/P) are compared for the years 1993 through 1997 to determine how much can be understood about water column variability from XBT's given only sea height anomalies (SHA) from T/P. Our focus is on the annual cycle along two well sampled XBT sections in the Atlantic Ocean from 10°S to 40°N. Regions of transition are identified that separate the mid‐latitudes where surface buoyancy fluxes dominate the forcing of sea level, from those in the equatorial region where thermocline effects dominate. Zones of transition occur in the vicinity of troughs where small fluctuations in SHA belie the true nature of water column variability. Here, surface and thermocline variability tend to cancel each other. Thus, the character of SHA in transition regions emphasizes how important direct observations can be in interpreting satellite altimetric observations correctly when both surface and thermocline variability are important but are compensating in nature.

Multi-sensor remote sensing of eddy-induced upwelling in the southern coastal region of Sicily

We identify upwelling signatures caused by eddy interactions with bottom topography on the downstream of the Malta Plateau, using space-borne remote sensing Advanced Very High Resolution Radiometer (AVHRR) imagery, ERS-1 Synthetic Aperture Radar (SAR) imagery, Topex/Poseidon/ERS-2 (TPE) altimeter maps, and Orbview2/SeaWiFS imagery. The cyclonically rotating eddy contributes to upwelling, reducing the sea surface temperature (SST) by about 1-2°C inside the eddy with respect to the ambient ocean. Ocean colour imagery shows increases in pigment concentrations (0.5 mg m-3) associated with the eddy. In TPE altimeter maps, the eddy is manifested as a 40-50 km wide 'bowl-shaped' structure, with negative sea height anomalies ranging between 6 and 10 cm. The eddy-type motion is further evidenced in SAR imagery by the appearance of striations forming along the boundaries of the eddy as well as reduced backscatter at its centre. The study demonstrates the utility of sensor fusion and identifies a set of generic indicators for upwelling identification and tracking using multiple sensors.

Dynamics of the Brazil‐Malvinas Confluence based on inverted echo sounders and altimetry

We use data from Geosat altimeter and from 10 inverted echo sounder (IES) moorings deployed in the SW Atlantic Ocean off the Argentine continental shelf to investigate several aspects of the dynamics of the upper layer in the Brazil‐Malvinas Confluence region. We use the altimeter data to estimate the sea height anomalies at each IES location and use the IES data to compute the upper layer thickness, taken in this work to go to the depth of the 8°C isotherm. We first discuss the sea height and upper layer thickness variations caused by the passage of the Brazil Current, Malvinas Current, and warm anticyclonic and cold cyclonic eddies. We introduce a two‐layer model in which we decompose the sea height into its baroclinic and barotropic contributions. We then propose a method to monitor the thickness of the upper layer and the barotropic and baroclinic transports as a function of the sea height anomalies and the statistics of the upper layer thickness and reduced gravity for the region. We compute the reduced gravity values from the slope of a linear fit between the sea height anomalies and the upper layer thicknesses. We estimate the reduced gravity values for this region to range from 0.005 to 0.011 m s−2. We also estimate the mean barotropic sea height difference using two methods: conservation of mass and conservation of potential vorticity. Finally, we compute the time series for the baroclinic and barotropic transports during the Geosat Exact Repeal Mission time period. Our results suggest that the mean baroclinic transport in the upper layer decreases from 12 Sv at around 35°S to 7 Sv at 37°S. Our results also indicate that there is a significant barotropic contribution to the upper layer transport in the confluence region.

Ocean surface topography measured by the Geosat radar altimeter during the Frontal Air‐Sea Interaction Experiment

An objective analysis (OA) technique was used to analyze Geosat Geodetic Mission residual sea height anomalies (RSHA) for the Frontal Air‐Sea Interaction Experiment (FASINEX), conducted in the western North Atlantic. Preprocessing of the Geosat data included removal of a mean sea surface and orbit error and corrections for tides, wave height, inverse barometer, and ionospheric and dry tropospheric effects. Geosat data were not corrected for water vapor. Central to the analysis was the computation of a spatially anisotropic, time‐dependent autocorrelation function directly from the Geosat data. Results show a single autocorrelation function peak, having a diameter of ∼270 km at 0‐day time lag. A 15‐month time series of RSHA topography was constructed at 10‐day intervals, forming a set of 47 OA maps. These maps show a field of topographic features propagating to the west at ∼4 km d−1. The features exhibited spatial and temporal scales of ∼200 km and ∼50 days, respectively. The root‐mean‐square (rms) of the OA‐derived feature amplitudes fluctuated slowly between 0.05 and 0.09 m with a period of ∼200 days over the 15‐month interval. These fluctuations appeared to be related to the passage of large‐scale, northeast‐southwest trending sea surface temperature (SST) anomaly features reported for FASINEX, which were surface manifestations of large‐scale, first‐mode baroclinic Rossby waves. The rms amplitude of the OA‐derived features was maximal (minimal) during periods of negative (positive) SST anomaly, when the depths of the mixed layer and main thermocline in the upper ocean shoaled (deepened). The fluctuations also appeared to be correlated with the structure of the field. Periods of decreased feature amplitudes corresponded to a “closely packed” structure, while during times of increased amplitudes, the field structure became “zonal” with regions of alternating sign. During FASINEX (January–June 1986) the field was characterized by the closely packed structure and decreasing feature amplitudes. This corresponded to the period of positive SST anomaly and high upper ocean heat content reported for FASINEX.