N. Ayoub

Major changes in Mediterranean Sea level variability from 7 years of TOPEX/Poseidon and ERS-1/2 data

Abstract Seven years of combined maps of TOPEX/Poseidon (T/P) and ERS-1/2 altimeter data are used to describe the surface circulation variability in the Mediterranean Sea. As described in the past, the Mediterranean Sea level variability is a complex combination of a wide range of spatial and temporal scales. However, this paper gives an improved estimation of sea level statistics (rms, annual, semiannual cycle). Moreover, the longer period (1993–1999) and the merging of T/P and ERS-1/2 altimeter data allows us to observe with a good accuracy the major changes that occurred in the Mediterranean Sea and, in particular, at basin and sub-basin scales. First, important interannual signals were found in the Ionian basin where the cyclonic circulation has clearly intensified since 1997. In the Levantine basin, although the Ierapetra eddy exhibits a clear seasonal cycle, it is not always present during our period of observation. These interannual changes seem to be correlated with the variability of the Mid-Mediterranean Jet, which produces strong meanders. A new view of the circulation in the south of the basin is indeed suggested by considering that the eddies observed in the Mersa-Matruh and Shikmona areas represent meanders of the Mid-Mediterranean Jet rather than permanent structures as commonly described. Secondly, the seasonal cycle of the Alboran gyres is confirmed. Moreover, these gyres and the Ierapetra eddy constitute the most intense signals of the Mediterranean Sea variability. Finally, this descriptive study illustrates the need to continue monitoring the surface circulation in order to better understand the dynamics of the Mediterranean Sea.

Mean sea level and surface circulation variability of the Mediterranean Sea from 2 years of TOPEX/POSEIDON altimetry

We describe the circulation and mean sea level variations of the Mediterranean Sea from 2 years of TOPEX/POSEIDON altimetric data. It is first shown that the response of the Mediterranean Sea to atmospheric pressure forcing is close to an inverse barometer (except at high frequencies) which means that the adjustment is accompanied by a flow through the Straits of Sicily and Gibraltar. We then use TOPEX/POSEIDON to study the mean sea level variations, representing steric effects and integrated large-scale changes of the mass of the Mediterranean Sea. We observe an annual cycle with a fast drop during winter. Steric effects account for about half of the observed variations. The remaining signal is believed to be driven by evaporation minus precipitation (E - P) forcing and internal hydraulic control in the Straits of Gibraltar. Using suboptimal space-time objective analysis, the classic components of the Mediterranean surface circulation are recovered, despite low signal-to-noise ratio (the rms of sea level variability is less than 10 cm). The variable Mediterranean circulation is seen as a complex combination of mesoscale and large-scale variations. The surface circulation is more complex in the eastern basin than in the western basin. In the east it is composed of subbasin-scale gyres, such as the so-called Mersa-Matruh and Shikmona gyres, which do not have an obvious recurrence period. We also observe an intensification of the large-scale cyclonic winter circulation in the western and in the Ionian basins. Several mesoscale structures, such as the Alboran gyres and the Ierepetra gyre, show a clear seasonal cycle, with a maximum in summer. The good qualitative and quantitative agreement of the results with previous data from the Mediterranean illustrates the improved accurary of TOPEX/POSEIDON over its predecessors.

Interannual long equatorial waves in the tropical Atlantic from a high‐resolution ocean general circulation model experiment in 1981–2000

We investigate the tropical Atlantic vertical structure variability (1981-2000) based on the CLIPPER ocean general circulation model (OGCM). We aim at determining to what extent the observed interannual variability can be explained by the low-frequency wave dynamics. The linear vertical modes of the OGCM climatological stratification are estimated along the equator. The baroclinic mode contributions to surface zonal current and sea level anomalies are calculated and analyzed at interannual timescales. The second baroclinic mode is the most energetic. The first (third) mode exhibits a variability peak in the west (east). The summed-up contribution of the high-order baroclinic modes (4-6) is as energetic as the gravest modes and is largest in the east. Wave components are then derived by projection onto the associated meridional structures. The effect of longitudinal boundaries near the equator is taken into consideration. Equatorial Kelvin and Rossby waves propagations, with phases speed close to the theory, are identified for the first three baroclinic modes. The comparison with a multimode linear simulation corroborates the propagating properties of the OGCM waves coefficients. An estimation of the meridional boundary reflection efficiency indicates that wave reflections take place at both boundaries. A 65% reflection efficiency is found at the eastern boundary. Our study suggests that low-frequency wave dynamics is to a large extent at work in the tropical Atlantic. On the basis of what is known on the Pacific El Nino-Southern Oscillation mode this may provide a guidance for investigating ocean-atmosphere mechanisms that can lead to the Atlantic zonal equatorial mode.

The 1996 equatorial Atlantic warm event: Origin and mechanisms

We investigate the interannual warm event that occurred in the equatorial Atlantic in boreal spring-summer 1996. The role of local coupled air-sea interactions versus Tropical Pacific remote forcing is analysed using observations and ensemble experiments of an intermediate coupled model of the Tropical Atlantic. Results show that the persistent anomalous cold conditions in the Tropical Pacific over 1995-96 were favorable to the growth of the local air-sea interactions that led to the 1996 warming in the equatorial Atlantic. Based on the estimation of the changes in the Walker circulation over the Pacific and Atlantic for the meteorological reanalyses and the coupled model, a mechanism of Pacific-Atlantic equatorial connection is proposed to explain this particular warm episode.

Coastal Ocean Forecasting: science foundation and user benefits

The advancement of Coastal Ocean Forecasting Systems (COFS) requires the support of continuous scientific progress addressing: (a) the primary mechanisms driving coastal circulation; (b) methods to achieve fully integrated coastal systems (observations and models), that are dynamically embedded in larger scale systems; and (c) methods to adequately represent air-sea and biophysical interactions. Issues of downscaling, data assimilation, atmosphere-wave-ocean couplings and ecosystem dynamics in the coastal ocean are discussed. These science topics are fundamental for successful COFS, which are connected to evolving downstream applications, dictated by the socioeconomic needs of rapidly increasing coastal populations.

Characterisation of errors of a regional model of the Bay of Biscay in response to wind uncertainties: a first step toward a data assimilation system suitable for coastal sea domains

This study is aimed at exploring the errors of a regional model of the Bay of Biscay, a regional zoom of the IBI configuration of the ocean model NEMO, with the ultimate objective of guiding the choice and implementation of a data assimilation system in that region. An ensemble experiment was carried out by randomly perturbing winds along a base of EOFs with the aim to mimic a potential source of error in the model forecasts. A characterisation was attempted with proxy forecast errors by using statistical moments of order 1 to 4. The temporal variability of model correction patterns in a hypothetical data assimilation system was also illustrated. Significant departures from linear/Gaussian response were found, as well as well-marked non-stationarities in the error patterns. Within the limits of the experimental protocol, this could be technically applicable to other coastal areas as the study illustrates the likely limits of stationary/Gaussian data assimilation approaches in the Bay of Biscay.