ALBERT

Beschreibung: The Aegean water masses and circulation structure are studied via two large-scale surveys performed during the late winters of 1988 and 1990 by the R/V Yakov Gakkel of the former Soviet Union. The analysis of these data sheds light on the mechanisms of water mass formation in the Aegean Sea that triggered the outflow of Cretan Deep Water (CDW) from the Cretan Sea into the abyssal basins of the eastern Mediterranean Sea (the so-called Eastern Mediterranean Transient). It is found that the central Aegean Basin is the site of the formation of Aegean Intermediate Water, which slides southward and, depending on their density, renews either the intermediate or the deep water of the Cretan Sea. During the winter of 1988, the Cretan Sea waters were renewed mainly at intermediate levels, while during the winter of 1990 it was mainly the volume of CDW that increased. This Aegean water mass redistribution and formation process in 1990 differed from that in 1988 in two major aspects: (i) during the winter of 1990 the position of the front between the Black Sea Water and the Levantine Surface Water was displaced farther north than during the winter of 1988 and (ii) heavier waters were formed in 1990 as a result of enhanced lateral advection of salty Levantine Surface Water that enriched the intermediate waters with salt. In 1990 the 29.2 isopycnal rose to the surface of the central basin and a large volume of CDW filled the Cretan Basin. It is found that, already in 1988, the 29.2 isopycnal surface, which we assume is the lowest density of the CDW, was shallower than the Kassos Strait sill and thus CDW egressed into the Eastern Mediterranean.

Beschreibung: Published

Beschreibung: 1841-1859

Beschreibung: JCR Journal

Beschreibung: reserved

Beschreibung: In this paper results from the application of an ocean data assimilation (ODA) system, combining a multivariate reduced-order optimal interpolator (OI) scheme with a global ocean general circulation model (OGCM), are described. The present ODA system, designed to assimilate in situ temperature and salinity observations, has been used to produce ocean reanalyses for the 1962–2001 period. The impact of assimilating observed hydrographic data on the ocean mean state and temporal variability is evaluated. A special focus of this work is on the ODA system skill in reproducing a realistic ocean salinity state. Results from a hierarchy of different salinity reanalyses, using varying combinations of assimilated data and background error covariance structures, are described. The impact of the space and time resolution of the background error covariance parameterization on salinity is addressed.

Beschreibung: This work has been funded by the ENACT Project (Contract EVK2-CT2001-00117) for A. Bellucci and P. Di Pietro, and partially by the ENSEMBLES Project (Contract GOCE-CT-2003-505539) for A. Bellucci.

Beschreibung: Published

Beschreibung: 3785-3807

Beschreibung: 3.7. Dinamica del clima e dell'oceano

Beschreibung: JCR Journal

Beschreibung: reserved

Beschreibung: Ensemble experiments are performed with five coupled atmosphere–ocean models to investigate the potential for initial-value climate forecasts on interannual to decadal time scales. Experiments are started from similar model-generated initial states, and common diagnostics of predictability are used. We find that variations in the ocean meridional overturning circulation (MOC) are potentially predictable on interannual to decadal time scales, a more consistent picture of the surface temperature impact of decadal variations in the MOC is now apparent, and variations of surface air temperatures in the North Atlantic Ocean are also potentially predictable on interannual to decadal time scales, albeit with potential skill levels that are less than those seen for MOC variations. This intercomparison represents a step forward in assessing the robustness of model estimates of potential skill and is a prerequisite for the development of any operational forecasting system.

Beschreibung: Published

Beschreibung: 1195-1203

Beschreibung: JCR Journal

Beschreibung: reserved

Beschreibung: In the first part of this work, the dominant time scales that explain the tropical variability of the first SINTEX simulation (ECHAM4(T30)-ORCA) are identified through a spectral analysis. Higher order spectral analysis is used to examine the interactions among these time scales. The time series analyzed are an average of sea surface temperature over the Niño3 region. The time scales obtained are compared with those identified in another coupled GCM simulation (ECHAM4(T42)-OPYC3). The higher importance of the biannual time scale in this last is explained partly by the strength of the coupling between the annual and the biannual time scales. There is no such strong coupling in the SINTEX simulation. Important differences among the generation of the simulated warm (or cold) event suggest the need of a systematic classification to isolate their relevant features. Therefore in the second part of this work, we address this problem. A space-time cluster analysis is performed on a data set built by collecting the values of the heat content anomalies in the tropical Pacific region, in the fifteen months previous to a peak in the Niño3 Index that has been identified as a warm (or cold) event. In the case of the warm events, three types of generation schemes are found. In two of them, there are anomalies of heat content in the west, north and south of the equator, more than nine months before the events start. In the third case, the anomalies appear and grow in the central equatorial Pacific. Only two types are needed to classify the generation of cold events. Negative sea level height anomalies appear six months before the Niño3 Index reaches the (local) minimum. They are located north of the equator in one of the groups, and south of it in the other. Some of these characteristic traits also appear in observations of warm and cold events.

Beschreibung: Published

Beschreibung: JCR Journal

Beschreibung: open

Beschreibung: Warm and cold events in the Gulf of Guinea are an important feature in the interannual variability of the tropical Atlantic Ocean, and partly a manifestation of the equatorial Atlantic system's intrinsic variability. Due to the relatively reduced zonal extension of this ocean, the latter variability is comparatively weak and thus strongly modified by other factors at play, either local or remote, like the seasonal cycle or ENSO. We present here an analysis of the tropical Atlantic variability in a 100-year-long chunk of the output of a coupled GCM. Through it, we obtain a better understanding of this variability and of its interactions with the seasonal cycle and with the ENSO signal. Following hints in the observations, we separate warm or cold events of the simulation in a few types, according to their similarities and differences. This classification is carried out as a spatio-temporal cluster analysis of the values, from nine months before up to the peak of the event, of the heat content anomalies. This is an optimal variable to monitor the generation of the events. One of the warm event classes can be explained by ENSO interactions. One of the cold event types can be explained by this influence as well, while the seasonal interactions might explain the characteristics of another of them.

Beschreibung: Published

Beschreibung: JCR Journal

Beschreibung: open

Beschreibung: Examples of current research into systematic errors in climate models are used to demonstrate the importance of scale interactions on diurnal,intraseasonal and seasonal timescales for the mean and variability of the tropical climate system. It has enabled some conclusions to be drawn about possible processes that may need to be represented, and some recommendations to be made regarding model improvements. It has been shown that the Maritime Continent heat source is a major driver of the global circulation but yet is poorly represented in GCMs. A new climatology of the diurnal cycle has been used to provide compelling evidence of important land-sea breeze and gravity wave effects, which may play a crucial role in the heat and moisture budget of this key region for the tropical and global circulation. The role of the diurnal cycle has also been emphasized for intraseasonal variability associated with the Madden Julian Oscillation (MJO). It is suggested that the diurnal cycle in Sea Surface Temperature (SST) during the suppressed phase of the MJO leads to a triggering of cumulus congestus clouds, which serve to moisten the free troposphere and hence precondition the atmosphere for the next active phase. It has been further shown that coupling between the ocean and atmosphere on intraseasonal timescales leads to a more realistic simulation of the MJO. These results stress the need for models to be able to simulate firstly, the observed tri-modal distribution of convection, and secondly, the coupling between the ocean and atmosphere on diurnal to intraseasonal timescales. It is argued, however, that the current representation of the ocean mixed layer in coupled models is not adequate to represent the complex structure of the observed mixed layer, in particular the formation of salinity barrier layers which can potentially provide much stronger local coupling between the atmosphere and ocean on diurnal to intraseasonal timescales.

Beschreibung: Published

Beschreibung: JCR Journal

Beschreibung: open

Beschreibung: This report summarizes the work, under the auspices of the WOCE Hydrographic Programme (WHP), which took place on the R/V Akademik Vernadsky in June-July 1991. The goal of the exercise was an international comparison/training cruise to obtain comparisons of water sampling methods and analytical techniques employed by several groups for the oceanographic measurement of salinity and oxygen in seawater. The training aspect was formalized in a pre-cruise practicum held on board the vessel by Dr. Fred Culkin and Mr. Paul Ridout of Ocean Scientific International, the manufacturer of IAPSO Standard Seawater, At-sea comparisons of Russian Reference Water, manufactured in Moscow, were made with IAPSO water. Further work involved comparisons of a Guildline and a pair of SOKOL salinometers using reference water and natural seawater collected on the cruise. While the agreement among the salnometers was quite good and at the level of acceptability for the WHP, it was discovered that lack of thermal equilbration of the salinity samples run on the SOKOLs led to substantial "errors" at large depths (or for cold water samples). This could not have been anticipated before the cruise and would have been missed in a shore-based or laboratory exercise. All five groups making oxygen comparisons benefitted from the technical exchange afforded by the cruise and, as a result of reconciling inter-group differences, have identified procedural changes they wil make in the future in order to achieve the high standards required by the WHP, The technical interaction, which amounted to "training" for all groups, was greatly facilitated by the cooperation of Captain Malnovsky, Chief Scientist Panteleyev, the scientists from MHI, Sevastopol, and the crew of the Vernadsky. On the Vernadsky comparison cruise, a shore-based practicum was held prior to sailing in order to review the theory and measurement of salinity. The cruise itself, which took place between 27 June and 8 July 1991, was in the NE Atlantic to the west of Madeira. An international group of experts in salnity and oxygen measurements was drawn from the United Kingdom (UK), Spain, Russia, Ukraine, and the United States (US). Except for one of the five groups, measured oxygen values in the range of 3-5 mIll (concentration units are used throughout) agreed with one another within :11%, which exceeds the WOCE requirements by a factor of 2. Subsequent analyses of the differences by all five groups could account for much of the observed inter-group variations, Intra-group precision was generally in the range of 0,1-0.4%, As a result, of a low oxygen sparging experiment in which sample oxygens 0: 1.2 mIll were generated, substantial inter-group differences of :1.08 mIll were found indicating that oxygen specifications for WOCE need to be expressed as both a percent "error" and a low oxygen bias. The salinity comparison component of the cruise enabled comparison of various batches of Russian Reference Water (RRW) of various salnities (10, 20, 30, 35 and 40) against IAPSO Standard Seawater (SSW) of various salinities (10, 30, 35 and 38); inferences could then also be drawn on the relative performance of the two types of salinometer (Guildline Autosal 8400 and SOKOL 4602) used during the experiment. Under quasilaboratory conditions, the machines produced mean results different by no more than 0.001 in salinity for salinity 35 RRW, standardizing against salinity 35 SSW. The other salnities produced less conclusive results, but the three salinometers deviated by no more than 0.003 from the expected value over a wide range of salinities. Under operational conditions (analysis of duplicate samples collected from CTD casts) a mean bias emerged between machines of approximately 0.005 in salinity (SOKOL fresher than Guildline), Subsequent investigations tentatively ascribed this bias to non-equilbration of deep (cold) samples in the Russian system.

Beschreibung: Prepared for the National Science Foundation under Grants OCE89-07815, and OCE92-06184, T. Joyce, Principal Investigator; Vetlesen Funds, Woods Hole Oceanographic Institution; and with support from the Intergovernmental Oceanographic Commission.