Examining the composites of wind velocity and air-sea gradients individually permits us to diagnose contributions from each to the observed flux trends. In particular, the meridional wind velocity difference map displays the pattern of enhanced northerly wind components near the northern coasts and in the Ionian and Levantine basins.
While the meridional wind velocity composite difference shows smaller increases in the northern coastal areas, orography may locally modify and intensify winds generated by the large scale atmospheric pressure centers, as described by [ 27 ]. Even with little or no change in air-sea humidity and temperature, local wind changes could produce larger trends in LHF and SHF immediately adjacent to the northern coasts, as displayed in Figure 2.
The air-sea temperature gradient difference map Figure 6 c , when scaled by the wind velocity differences, produces a pattern that resembles the SHF trends in Figure 2 b in the central Mediterranean but does not agree with that in the northwestern Mediterranean and in the eastern Levantine basin. Examining Figures 2 b , 6 c , and 6 f reveals that even in regions with the largest air-sea temperature differences and the most northerly winds e. Wind velocity enhancements are smaller as well and do not extend to the Gulf of Lions and the northern Adriatic.
The NAO difference composite air-sea temperature gradient pattern does not show the large increase around the coast of Greece seen in the Azores High difference map.
The spatial patterns of the difference composites shown in Figures 6 a , 6 c , 6 e , and 6 f do not agree perfectly with the LHF and SHF trends in Figure 2. This may result from the influence of other teleconnection patterns on Mediterranean turbulent fluxes e. As the Azores High strengthens and moves eastward, the anticyclonic wind field associated with it also strengthens and shifts eastward.
Because the Mediterranean Sea is located just eastward of the Azores High, the longitudinal position of the Azores High has a large impact on the strength and direction of wind in different parts of the basin. This produces the wind vectors shown in Figures 7 a and 7 b , which demonstrate that intensification and eastward displacement of the Azores High are associated with stronger northerly winds over the entire basin, with the largest differences found over the eastern portion. Recent studies have found an upward trend of SST in the entire Mediterranean [ 17 , 24 , 48 ]. The paper [ 24 ] attributed this change to enhanced inflow of warm water from the Atlantic related to variability in the EA and the Atlantic Multidecadal Oscillation AMO , a pattern of variability of the North Atlantic.
The anomalously warm waters are advected eastward by the surface currents of the Mediterranean, eventually affecting the entire basin. Trends are everywhere positive, in agreement with [ 48 ], but there is substantial spatial variability, particularly in LHF. Trend maxima are found near the northern coastlines, in the central Mediterranean and the southern Levantine basin. Minima occur in the central basins of the Alboran Sea in the western Mediterranean, the Tyrrhenian and Ionian Seas in the central Mediterranean, and especially the northern Levantine basin. These patterns resemble patterns in evaporation trends for a shorter time interval presented by [ 17 ], suggesting that the LHF trend spatial pattern has remained consistent over the longer time interval.
Time series of Azores High pressure and longitudinal position indices are presented, along with time series of the NAO and EAWR, two teleconnections known to influence Mediterranean climate. The two Azores High indices that have positive trends over the study period are found to covary such that strong Azores High pressure centers occur further to the east, while weak Azores High pressure centers are located to the west.
This suggests that the additional information on spatial variability in the location of the Azores High improves understanding of the long term trends in Mediterranean Sea LHF and SHF, compared to using only the pressure difference, as in the NAO index. Examining Mediterranean climate variability with respect to Azores High variability rather than one of the other ways of describing large scale atmospheric circulation variability that have fixed modes e.
Similar results might be obtained using another representation of the meridional pressure dipole, as it is the dominant mode of variability in the region. However, the center of action method used here is not computationally burdensome, is directly related to observed physical quantities, and is appropriate for investigating the effect of fluctuations in the location of the centers of action on regional climate. Indeed, we find that adding information on the position of the Azores High improves the outcome of our analysis.
Previous studies agree that the strength of the pressure dipole is intrinsically related to shifts in the location of the centers of action, in the past [ 51 ] and current climates [ 9 , 11 ], and in model simulations of past and present climates [ 50 , 51 ]. It is unclear whether this relationship will continue in the future. The relationship of the Mediterranean heat fluxes with the Azores High is interesting for the additional reason that the Azores High represents the downward branch of the Hadley circulation in the North Atlantic sector.
The increasing strength of the Azores High is an example of the strengthening and expansion of Hadley circulation that has been attributed to anthropogenic warming [ 53 , 54 ]. Because of large natural variability, it may not be possible to conclusively attribute late 20th century trends in North Atlantic large scale atmospheric circulation to external forcing [ 14 , 50 ].
If the current trend in the DJF Azores High pressure continues, we can expect the trend of increasing surface heat fluxes in the Mediterranean in the future, which could have large impacts on many aspects of Mediterranean climate, including circulation, biogeochemistry, biological productivity, and hydrography [ 30 — 32 , 55 , 56 ] and on the global thermohaline circulation [ 38 , 39 ].
The authors declare that there is no conflict of interests regarding the publication of this paper. Advances in Meteorology. Indexed in Science Citation Index Expanded. Journal Menu. Special Issues Menu. Subscribe to Table of Contents Alerts. Table of Contents Alerts. Introduction Low-frequency variability of the large scale atmospheric circulation over the North Atlantic and Europe has long been recognized as a critical driver of climate variability in the region.
Only trends which are significant at are shown. Figure 4: a , c , and e Correlations between detrended monthly mean latent heat flux and detrended Azores High sea level pressure, Azores High longitude, and NAO index; b , d , and f correlations between detrended monthly mean sensible heat flux and detrended Azores High sea level pressure, Azores High longitude, and NAO index, DJF — Only correlations which are significant at are shown.
Only differences which are significant at are shown. References C. Wallace and D. Barnston and R. Kutiel and Y. Krichak and P. Hurrell and C. Hurrell and H. Cassou, L. Terray, J. Hurrell, and C.
Moore, I. Renfrew, and R. Kostopoulou and P. Jung, M. Hilmer, E. Ruprecht, S. Kleppek, S. Gulev, and O. Johnson, S. Feldstein, and B. Josey, S. Somot, and M. Romanski, A. Romanou, M. Bauer, and G. Zveryaev and A. Krichak, P. Kishcha, and P.
Skliris, S. Sofianos, A. Gkanasos et al. Kazmin, A. Zatsepin, and H. Tsimplis, F. Calafat, M. Complexity arises from multiple driving forces, strong topographic and coastal influences and internal dynamical processes that interact on several temporal and spatial scales basin, sub-basin and mesoscale to form an extremely complex and variable circulation. The seasonal, interannual and decadal variabilities are associated with the internal variability of the climatic system.
A major abrupt change has been recorded in the Mediterranean in the last decades which induced important changes to the heat and salt contents. During that event, the circulation of the eastern Mediterranean experienced a dramatic change from the surface layers to the bottom. Heat and salt contents are calculated from temperature and salinity differences in relation to mean climatological reference values integrated over a particular reference depth and study area see the next section for more details.
To detect their long-term tendency, long time series extending to more than a few decades are needed in order to identify the natural climate long-term oscillations and quantify any remaining trends related to global warming. But at the large basin scale, where the coverage is not good enough, we need to interpolate the data to fill the gaps.
In such cases, the noise from the interpolation schemes is an additional source of uncertainty. Since then, periodical updates are released based on additional data, updated estimations of corrections for the time-varying systematic bias in expendable bathythermograph data and corrections of some ARGO float data. The first publication on the time-dependent warm bias of the bathythermograph data was by Gouretski and Koltermann This warming corresponds to a rate of 0.
In the Mediterranean, many works since the late s have been carried out trying to quantify the trends of temperature and salinity and determine which causes underlie these such as global warming or anthropogenic climate change due to main rivers damming. Some of the above findings are outlined below. The increasing trend is more evident in the salinity than the temperature. The temperature and salinity of the deep waters of the western Mediterranean are increasing.
In the eastern Mediterranean, for the intermediate layer there is no general consensus. During the last decades, the western Mediterranean OHC and OSC have been increasing with an accelerating tendency of the western deep waters towards higher temperatures and salinities since the s, with the process accelerating after the second half of the s. The variation of the intermediate layers is attributed to decadal variability. The finer spatial resolution of the input data climatology filters out the noise induced by the mesoscale features, but at the same time is such that it smoothes less the large-scale features.
The temporal resolution is such that it smoothes the strong seasonal, interannual and decadal variability so that the final product is able to resolve in more detail the climatic variability and identify possible warming trends. The gridded fields cover the geographical region 6. The mesoscale features and other smaller patches are therefore filtered out and the climatologies used for the indices' calculations focus on the variability of the large-scale features. The influence of the uneven distribution in space where a large number of data points are concentrated in a very small area and within a very short period is controlled by applying different weights and lower than 1 to each of these data points because such points cannot be considered independent in a climatological analysis.
The characteristic length of weighting was set to be equal to 0. Detrending was applied in the observations used for the reference climatologies in order to remove the uneven spatial distributions in time.
In book: The Climate of the Mediterranean Region: From the Past to the Future, Chapter: 3, Publisher: Elsevier, Editors: Lionello, P, pp -. Circulation of the Mediterranean Sea and its variability. In: The . lar focus on the topics that have been described in this chapter, we schematically list . w; i1sE;i ;i41 JO uo!lBtn:iJf:i ;i:iEpns. 'O. IOZ. '"3 ·zi;iJ!;iH ··v 'JllZll'l. ··A.
The input gridded data are listed below. They are stored in netCDF files and are accessible from the Zenodo platform, a research platform where papers, data, software codes or any other object contributing to the reproducibility of scientific results can be uploaded and then cited using a digital object identifier DOI.
Annual climatology reference , obtained by analysing all data regardless of month or season for the whole period from to Annual decadal climatology , obtained by analysing all data regardless of month or season for each of the 57 running decades from — to — Seasonal climatology reference , obtained by analysing all data of the whole period from to falling within each season. Seasonal decadal climatology , obtained by analysing all data falling within each season for each of the 57 running decades from — to — It is important to note that in the used input climatology, each gridded field is accompanied by an error field that allows one to assess the reliability of the input data.
This helps to objectively identify areas with poor data coverage, mask them and exclude them from further processing. In all products, temperature and salinity anomalies have been used. Anomaly is defined as the difference between the value of a grid point and a mean climatological reference. Climate shift is defined as the difference between two successive year averages.
It is noted that in the climate shifts presented in this work, the period to contains 3 decades and the period to contains 6 years more because the period from to is treated as a decade. The user of course can choose between any period and average the decades according to their needs of each study since the available product includes all 57 running decades from where the climates are computed.
The thickness of the layers was used as weights for the vertical averaging calculated as half of the distance between adjacent depths.
Lamarque, Y. Morcos, These conceptual experiments help to evaluate the impact of specific observations on the EOV core product estimates. Among various applications, Argo float data have been used to study convection and dense water formation in the Adriatic Sea see for instance Bensi et al. Macrocrustacea provide examples of early colonization of the East Mediterranean by Indo-Pacific species.
The following weights were used for the 31 standard depth levels: 2. In the case of the annual fields, 30 weights are used starting from the second value 5. The used weights are available with the netCDF files used for the computation of the indices. For the estimation of the OHC anomalies the following methodology was used. By multiplying the volume by the T anomalies, by the density of seawater, and by the specific heat, we obtain the OHC anomaly of a specific grid point at each standard depth.
By integrating over a depth layer and over all of the analysis area, we obtain the OHC anomaly in Joules for the whole Mediterranean Sea according to the following equation:. In the current climatologies density and specific heat of seawater are not calculated separately, but it would be possible to derive them from T and S gridded fields.
Such calculations will be available in future releases of the indices. A mean basin volume is estimated at 3. The produced climatic indices for the whole Mediterranean Sea 6. We outline below some of the capabilities of the new product. The explanation of the long-term variability patterns that are revealed and attribution of possible causes is out of the scope of this work. A short overview of these was given in the introduction to facilitate the viewing of the products for those readers who are not familiar with the Mediterranean complex dynamics. Free download. Large-scale atmospheric forcing influencing the long-term variability of Mediterranean heat and freshwater budgets: climatic indices.
Energy of marine currents in the Strait of Gibraltar and its potential as a renewable energy resource, Renewable and Sustainable Energy Reviews , 34, Climatic indices influencing the long-term variability of Mediterranean heat and freshwater fluxes: The North-Atlantic and the Mediterranean oscillations. Atmosphere-Ocean, , Evaluation of regional ocean circulation models for the Mediterranean Sea and the Strait of Gibraltar: volume transport and thermohaline properties of the outflow, Climate Dynamics, DOI From local processes in the Strait of Gibraltar to basin-scale effects.
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