A Comparison of Gulf Stream Sea Surface Height Fields Derived from Geosat Altimeter Data and Those Derived from Sea Surface Temperature Data

Two types of satellite data, Geosat altimeter data and sea surface temperature data (SST), are compared and evaluated for their usefulness in assimilation into a numerical model of the Gulf Stream region. Synoptic sea surface height (SSH) fields are derived from the SST data in the following way: first three-dimensional temperature and salinity analysis fields are obtained through the Optimum Thermal Interpolation System (OTIS), and then SSH fields are calculated using a primitive equation, free-surface, numerical model running in a diagnostic mode. The aforementioned SSH fields are compared with SSH fields obtained from the Geosat altimeter data. Use of Geosat data requires an estimate of the cream SSH field relative to the earth geoid. Three different methods to obtain the mean SSH field are demonstrated. The first method uses altimetry and SST data, the second uses a diagnostic calculation with climatological data; and the third uses prognostic numerical calculations. The three estimates compared favorably with each other and with estimates obtained elsewhere. The comparison of the synoptic SSH fields derived from both data types reveals similarity in the Gulf Stream meanders and some mesoscale features, but shows differences in strength of eddies and in variability far from the Gulf Stream. Due to the smoothed nature of the OTIS analysis fields, the SSH derived from altimetry data has larger variability amplitudes compared to that derived from SST data. The statistical interpolation method, which is used to interpolate altimetry data from satellite tracks onto the model grid, is also evaluated for its filtering effect and its sensitivity to different parameters. The SSH variability of the Gulf Stream was calculated from two years of the exact repeat mission of the Geosat satellite, where altimeter data were interpolated daily onto the model grid. It is suggested here that some of the underestimation of mesoscale variations by statistical interpolation methods, as indicated by previous studies, may be explained by the filtering effect of the scheme

The Equatorial Thermocline Outcroppin - A Seasonal Control on the Tropical Pacific Ocean-Atmosphere Instability Strength

One of the major factors determining the strength and extent of ENSO events is the instability state of the equatorial Pacific coupled ocean-atmosphere system and its seasonal variations. This study analyzes the coupled instability in a hybrid coupled model of the Indo-Pacific region, using the adjoint method for sensitivity studies. It is found that the seasonal changes in the ocean-atmosphere instability strength in the model used here are related to the outcropping of the thermocline in the east equatorial Pacific. From July to December, when the thermocline outcrops over a wide area in the east Pacific, there is a strong surface-thermocline connection and anomalies that arrive as Kelvin waves from the west along the thermocline can reach the surface and affect the SST and thus the coupled system. Conversely, from February to June, when the thermocline outcropping is minimal, the surface decouples from the thermocline and temperature anomalies in the thermocline depth range do not affect the surface and dissipate within the thermocline. The role of vertical mixing rather than upwelling in linking vertical thermocline movements to SST changes is emphasized. It is therefore suggested that the seasonal ocean-atmosphere instability strength in the equatorial Pacific is strongly influenced by the thermocline outcropping and its seasonal modulation, a physical mechanism that is often neglected in intermediate coupled models and that can be represented properly only in models that employ the full dynamics of the mixed layer

Optimal surface salinity perturbations of the meridional overturning and heat transport in a global ocean general circulation model

Recent observations and modeling studies have stressed the influence of surface salinity perturbations on the North Atlantic circulation over the past few decades. As a step toward the estimation of the sensitivity of the thermohaline circulation to salinity anomalies, optimal initial surface salinity perturbations are computed and described for a realistic mean state of a global ocean general circulation model [Océan Parallélisé (OPA)]; optimality is defined successively with respect to the meridional overturning circulation intensity and the meridional heat transport maximum. Although the system is asymptotically stable, the nonnormality of the dynamics is able to produce a transient growth through an initial stimulation. Optimal perturbations are calculated subject to three constraints: the perturbation applies to surface salinity; the perturbation conserves the global salt content; and the perturbation is normalized, to remove the degeneracy in the linear maximization problem. Maximization using Lagrangian multipliers leads to explicit solutions (rather than eigenvalue problems), involving the integration of the model adjoint for each value to maximize.The most efficient transient growth for the intensity of the meridional overturning circulation appears for a delay of 10.5 yr after the perturbation by the optimal surface salinity anomaly. This optimal growth is induced by an initial anomaly located north of 50°N. In the same way, the most efficient transient growth for the intensity of the meridional heat transport appears for a shorter delay of 2.2 yr after the perturbation by the optimal surface salinity anomaly. This initial optimal perturbation corresponds to a zonal salinity gradient around 24°N. The optimal surface salinity perturbations studied herein yield upper bounds on the intensity of the response in meridional overturning circulation and meridional heat transport. Using typical amplitudes of the Great Salinity Anomalies, the upper bounds for the associated variability are 0.8 Sv (1 Sv ? 106 m3 s?1) (11% of the mean circulation) and 0.03 PW (5% of the mean circulation), respectively

Altimeter processing tools for analyzing mesoscale ocean features

Satellite altimeters provide many opportunities for oceanographers to supplement their research with a valuable new data set. The recent GEOSAT exact repeat mission is the first of several altimeter missions proposed during the next decade. To utilize this new data, a software package was developed at the Woods Hole Oceanographic Institution and the University of Hawaii to facilitate the extraction of useful information from the NODC distributed GEOSAT data tapes. This software package was written with portability and modularity in mind. It should be possible to use this package with little or no modifications on data from future altimeters. The code was written in C and tested on Sun workstations and is oriented toward UNIX operating systems. However, since standard code was used, the programs should port easily to other computer systems. The modularity of the code should enable users to create additional programs. Additional programs designed to handle collocated water vapor corrections are also included for comparison.Funding was provided by the Office of Naval Research through Grant No. N00014-86-K-0751

The Global ocean circulation during 1992-1997, estimated from ocean observations and a general circulation model

We discuss the three-dimensional oceanic state estimated for the period 1992- 1997 as it results from bringing together large-scale ocean data sets with a general circulation model. To bring the model into close agreement with ocean data, its initial temperature and salinity conditions where changed as well as the time-dependent surface fluxes of momentum, heat and freshwater. Resulting changes of those control fields are largely consistent with accepted uncertainties in the hydrographic climatology and meteorological analyses. Our results show that the assimilation procedure is able to correct for the traditional shortcomings of the flow field by changing the surface boundary conditions. Changes of the resulting flow field are predominantly on the gyre scale and affect many features which are often poorly simulated in traditional numerical simulations, such as the strengths of the Gulf Stream and its extension, the Azores Current and the anticyclonic circulation associated with the Labrador Sea. A detailed test of the results and their consistency with prior error assumptions shows that the constrained model has moved considerably closer to those observations which have been imposed as constraints, but also to independent data from the World Ocean Circulation Experiment not used in the assimilation procedure. In some regions where the comparisons remain indeterminate, not enough ocean observations are available. And in such situations, it is difficult to ascribe the residuals to either the model or the observations. We conclude from this experiment that we can find an acceptable solution to the global time-dependent ocean state estimation problem. As the estimates improve through the evolution of numerical models, computer power increases, and better assimilation schemes, improved and routine estimates will become possible

Interpreting Soft Sediment Deformation and Mass Transport Deposits as Seismites in the Dead Sea Depocenter

Peer reviewedPublisher PD

Variational assimilation of SSH variability from TOPEX/POSEIDON and ERSI into an eddy-permitting model of the North Atlantic

A first step for improving the climatological state of high‐resolution general circulation models by means of data assimilation is presented. A method developed for the assimilation of statistical characteristics into chaotic ocean models is applied to assimilate SSH variability from TOPEX/POSEIDON and ERS1 in association with temperature and salinity from the World Ocean Atlas 1997 in order to estimate the underlying mean circulation. The method requires a parameterization of SSH variability which derives from the approach of Green and Stone. By estimating initial conditions for temperature and salinity, a mean state is achieved which, although not fully consistent with the altimetric and climatological data, is markedly improved on time scales of one year in comparison to the control run. The assimilation of SSH variability data introduces complementary information about the main frontal structures consistent with climatological observations. The state is however not an equilibrium state and returns back to the first guess quasi‐equilibrium state for longer integration periods

Using the Adjoint Method With the Ocean Component of Coupled Ocean-Atmosphere Models

A Primitive Equation Ocean General Circulation Model (PE OGCM) in a global configuration similar to that used in coupled ocean-atmosphere models is fitted to climatological data using the adjoint method. The ultimate objective is the use of data assimilation for the improvement of the ocean component of coupled models, and for the calculation of initial conditions for initializing coupled model integrations. We argue that ocean models that are used for coupled climate studies are an especially appropriate target for data assimilation using the adjoint method