ESSDEarth System Science DataESSDEarth Syst. Sci. Data1866-3516Copernicus PublicationsGöttingen, Germany10.5194/essd-10-1829-2018Mediterranean Sea climatic indices: monitoring long-term variability and climate changesMediterranean Sea climatic indicesIonaAthanasiasissy@hnodc.hcmr.grhttps://orcid.org/0000-0001-6878-4671TheodorouAthanasiosSofianosSarantisWateletSylvainTroupinCharleshttps://orcid.org/0000-0002-0265-1021BeckersJean-MarieHellenic Centre for Marine Research, Institute of Oceanography, Hellenic National Oceanographic Data Centre, 46,7 km Athens Sounio, Mavro Lithari P.O. Box 712 19013 Anavissos, Attica, GreeceUniversity of Thessaly, Department of Ichthyology & Aquatic Environment, Laboratory of Oceanography, Fytoko Street, 38445, Nea Ionia Magnesia, GreeceOcean Physics and Modelling Group, Division of Environmental Physics and Meteorology, University of Athens, University Campus, Phys–5, 15784 Athens, GreeceUniversity of Liège, GeoHydrodynamics and Environment Research, Quartier Agora, Allée du 6-Août, 17, Sart Tilman, 4000 Liège 1, BelgiumAthanasia Iona (sissy@hnodc.hcmr.gr)11October20181041829184211April201823April201823September201824September2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://essd.copernicus.org/articles/10/1829/2018/essd-10-1829-2018.htmlThe full text article is available as a PDF file from https://essd.copernicus.org/articles/10/1829/2018/essd-10-1829-2018.pdf
We present a new product composed of a set of
thermohaline climatic indices from 1950 to 2015 for the Mediterranean Sea
such as decadal temperature and salinity anomalies, their mean values over
selected depths, decadal ocean heat and salt content anomalies at selected
depth layers as well as their long time series. It is produced from a new
high-resolution climatology of temperature and salinity on a 1/8∘
regular grid based on historical high-quality in situ observations. Ocean
heat and salt content differences between 1980–2015 and 1950–1979 are
compared for evaluation of the climate shift in the Mediterranean Sea. The
two successive periods are chosen according to the standard WMO climate
normals. The spatial patterns of heat and salt content shifts demonstrate
that the climate changes differently in the several regions of the basin.
Long time series of heat and salt content for the period 1950 to 2015 are
also provided which indicate that in the Mediterranean Sea there is a net
mean volume warming and salinification since 1950 that has accelerated during
the last two decades. The time series also show that the ocean heat content
seems to fluctuate on a cycle of about 40 years and seems to follow the
Atlantic Multidecadal Oscillation climate cycle, indicating that the natural
large-scale atmospheric variability could be superimposed onto the warming
trend. This product is an observation-based estimation of the Mediterranean
climatic indices. It relies solely on spatially interpolated data produced
from in situ observations averaged over decades in order to smooth the
decadal variability and reveal the long-term trends. It can provide a
valuable contribution to the modellers' community, next to the
satellite-based products, and serve as a baseline for the evaluation of
climate-change model simulations, thus contributing to a better understanding
of the complex response of the Mediterranean Sea to the ongoing global
climate change. The product is available in netCDF at the following sources:
annual and seasonal T/S anomalies
(https://doi.org/10.5281/zenodo.1408832), annual and seasonal T/S
vertical averaged anomalies (https://doi.org/10.5281/zenodo.1408929),
annual and seasonal areal density of OHC/OSC anomalies
(https://doi.org/10.5281/zenodo.1408877), annual and seasonal linear
trends of T/S, OHC/OSC anomalies
(https://doi.org/10.5281/zenodo.1408917), annual and seasonal time
series of T/S, OHC/OSC anomalies
(https://doi.org/10.5281/zenodo.1411398), and differences of two
30-year averages of annual and seasonal T/S, OHC/OSC anomalies
(https://doi.org/10.5281/zenodo.1408903).
Introduction
During the twentieth century the Mediterranean Sea has undergone profound and
rapid changes. Temperature and salinity have increased with accelerating
trends in recent decades ,
reflecting apparently the global warming tendency . Because
of its geographical position, its small size (reduced volume to area size
ratio) and its being enclosed between continents, the Mediterranean Sea is
very sensitive and responds faster and more strongly to climate changes than
the open ocean, e.g. changes to atmospheric forcings and/or anthropogenic
influences . Moreover, the Mediterranean region
has been identified as one of the hotspots for future climate change in the
world where changes are expected to be largest. According to
the 5th assessment report, the observed global mean sea level
(GMSL) has changed since the mid-nineteenth century, with a larger rate than
the mean rate during the previous two millennia (high confidence). It is very
likely that the mean rate of global averaged sea level rise was 1.7 [1.5 to
1.9] mm yr-1 between 1901 and 2010, 2.0 [1.7 to 2.3] mm yr-1
between 1971 and 2010, and 3.2 [2.8 to 3.6] mm yr-1 between 1993 and
2010. The most important contributions to global and regional mean sea level
rise are a) increase in the ocean volume as a result of increase in the mass
of the water (due to melting of ice sheets and shrinking of glaciers), and b)
increase in the ocean volume as a result of decrease in ocean water density
(the ocean expands as it warms). However, ocean observations indicate that
the ocean is getting saltier and an increase in density should compensate for
the thermal expansion. Recent studies suggest that the water cycle has been
amplified because of the global warming, contributing to a saltier ocean
. The projected future changes show
that the GMSL will continue to rise during the twenty-first century with a
rate that will very likely exceed that observed during 1971 to 2010 due to
increased ocean warming and increased loss of mass from glaciers and ice
sheets. Sea level rise will not be uniform. In the Mediterranean region,
climate model projections show an acceleration of warming, salinification as
well as sea level rise during the twenty-first century
with a potential
strong impact on the marine environment, its effective management and thus
human welfare .
In turn, the Mediterranean Sea plays an essential role in influencing the
water formation processes and thermohaline circulation in the North Atlantic
. As a concentration basin (where
evaporation exceeds precipitation) it exports at intermediate depths salty
water through the Strait of Gibraltar to the Atlantic, a major site of dense
water formation for the global thermohaline circulation. In this context,
monitoring the changes of the ocean heat content (OHC) and ocean salt content
(OSC) of the Mediterranean Sea is of fundamental importance.
The ocean is the dominant component of the Earth's heat balance, and most of
the total warming caused by climate change is manifested in increased OHC.
Good estimates of past changes in OHC are essential for understanding the
role of the oceans in past climate change and for assessing future climate
change . However, accurate assessments of the OHC are still a
challenge, mainly because of insufficient and irregular data coverage.
The Mediterranean Sea (Fig. ) has a very high spatial and temporal
variability at all scales, from small turbulence to basin-scale processes
. Three main water masses are found, the surface, intermediate
and deep waters, which form a special flow regime characterized by an active
thermohaline (overturning) circulation: (a) one shallow cell that extends
over the two basins and communicates directly with the Atlantic Ocean and
consists of the inflowing Atlantic Water and the return flow of saltier
Mediterranean Water, and (b) two separate deep overturning cells, in the
western and eastern basins with several sites of deep water formation, e.g.
in the Gulf of Lions in the western and southern Adriatic, and the Aegean Sea
in the eastern basin and references therein. 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. The variability of the atmospheric circulation patterns induces
variations in the water masses either by changing temperature and salinity
properties through freshwater and heat fluxes or indirectly by changing the
main circulation pathways which in turn can produce changes in the
preconditioning phases previous to intermediate and deep water production or
redistributing salt and heat content in the water column and
references therein.
A major abrupt change has been recorded in the Mediterranean in the last
decades which induced important changes to the heat and salt contents.
Between the late 1980s and middle 1990s an interannual variation, the Eastern
Mediterranean Transient (EMT), strongly influenced the intermediate and deep
water masses' pathways and characteristics
. During that event,
the circulation of the eastern Mediterranean experienced a dramatic change
from the surface layers to the bottom. Dense water of Aegean origin replaced
the resident Eastern Mediterranean Deep Water (EMDW) of Adriatic origin.
Inducing the uplifting of the Ionian deep waters, the EMT significantly
modified the characteristics of the water masses flowing through the Sicily
Strait, while the remarkable presence of salty Cretan Intermediate Water
(CIW) in the Ionian Sea enhanced the salt export from the
eastern to western Mediterranean at the end of the 1990s. The EMT affected
not only the nearby Tyrrhenian Sea, but also the Western Mediterranean Deep
water production . After the late 1990s, the
dense waters of Aegean origin were no longer dense enough to reach the bottom
layer and the Adriatic Sea regained its role as the primary source of dense
water .
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. In small areas where the data coverage is sufficient, OHC/OSC
changes are calculated directly from the in situ measurements. 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.
, in using the World Ocean Database
(https://www.nodc.noaa.gov/OC5/WOD/pr_wod.html, last access: 9 October 2018), was the first who spoke about the warming of the global
oceans and quantified the interannual-to-decadal variability of the heat
content. 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 . The proposed corrections were included in
the World Ocean Database and in . Levitus showed that the
proposed corrections of bathythermographs reduce the interdecadal variability
but that the long-term trends remain similar . An analogous
study in the Mediterranean showed that including or not the bathythermographs
in the OHC estimates of the western Mediterranean does not significantly
change the results . reported that for
the period 1955–2010, the heat content of the world ocean for the 0–2000 m
layer increased by 24.0±1.9×1022 J, corresponding to a rate of
0.39 W m-2 (per unit area of the world ocean) and a volume mean
warming of 0.09 ∘C. This warming corresponds to a rate of
0.27 W m-2 per unit area of the Earth's surface. The heat content of
the world ocean for the 0–700 m layer increased by 16.7±1.6×1022 J, corresponding to a rate of 0.27 W m-2 (per unit area of
the world ocean) and a volume mean warming of 0.18 ∘C. They also
reported that the 0–700 m ocean layer accounted for approximately one-third
of the warming of the 0–2000 m layer of the world ocean .
It is worth mentioning that the ARGO array of profiling floats (their
deployment started in 2000) improved significantly the in situ observations'
spatial coverage and the subsequent assessments for 0–2000 m, but there are
still many regional seas uncovered (observations in these seas come mainly
from hydrographic cruises).
In the Mediterranean, many works since the late 1980s 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). Table 1 in and
, and Table 1 in , summarize the main
findings. An analysis of these results shows that there are differences
between them arising from (a) the input data (in situ or interpolated data or
model or satellite), (b) their spatial and temporal variability, (c) the
choice of the climatological reference, (d) the quality control procedures,
(e) the instruments' accuracy, and (f) the mapping techniques, e.g. the
gridding and infilling methodologies such as optimal interpolation or
variational inverse methods used to fill the data gaps and obtain a gridded
3-D continuous field and time series thereafter as well as which assumptions
are made in areas of missing data .
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.
, using the MEDATLAS climatology , found an
increase in OHC and OSC of about [1.3–1.5] 1021 J and [1.4–1.6]
1014 PSU m3, respectively, over the whole Mediterranean for the
period 1950–2000, corresponding to volume mean T and S anomalies of
about [0.09–0.10] ∘C and [0.035–0.04], respectively. 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 1950s, with the process accelerating
after the second half of the 1980s. The variation of the intermediate layers
is attributed to decadal variability. identified a strong
basin-scale multi-decadal salinification, particularly in the intermediate
and deep layers of order 0.015 practical salinity
scale(pss) decade-1, by analysing the
inter-annual objectively analysed gridded fields from EN4 from the Met Office
Hadley Centre (subversion En4.1.1.,
http://www.metoffice.gov.uk/hadobs/en4, last access: 9 October 2018) and MEDAR/MEDATLAS climatology , for two
reference periods, 1950–2002 and 1950–2015. , analysing
in situ data, found that over the period 1950–2010, the deep Western
Mediterranean Deep Water heat and salt contents increased almost steadily,
with an acceleration after the mid-1980s. Below 1000 m, the Mediterranean
underwent the strongest salinity gain in the world ocean .
The objective of this work is to provide estimates of T/S and OHC/OSC
variations using the latest SeaDataNet historical data sets combined with a
modern, numerically efficient interpolation technique that takes into account
constraints such as physical boundaries. The new product is expected to give
a more detailed insight into the spatial pattern of the changes for the whole
Mediterranean and the decadal variability of OHC/OSC. The originality of this
product compared to the existing ones is that we (a) use a higher spatial and
temporal resolution for gridded fields of T/S anomalies, (b) provide a
large, basin-scale spatial pattern for the trends of the decadal T/S and
OHC/OSC anomalies, (c) provide long-term time series of the decadal
anomalies, and (d) provide 30-year averages for evaluating the climate shift
in the Mediterranean. 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. Three
layers were considered in this work as representative of the main water
masses found in the Mediterranean: 0–150, 150–600, and 600–4000 m,
respectively, for the surface, intermediate and deep waters, as in
, and one additional 0–4000 m for the whole water column
and volume assessments.
The Mediterranean Sea and its main regions.
Data and methodsData sources
Gridded horizontal fields from a new high-resolution climatology of
temperature and salinity for the Mediterranean were used as
input data. These fields were produced using the SeaDataNet
temperature/salinity historical data collection V2 available at
http://sextant.ifremer.fr/record/8c3bd19b-9687-429c-a232-48b10478581c/, last access: 9 October 2018. The SeaDataNet collection
comprises 213 542 temperature and 138 691 salinity profiles from in situ
measurements for the 1911 to 2015 period. The gridded fields cover the
geographical region 6.25∘ W–36.5∘ E, 30–46∘ N on
31 standard depth levels from 0 to 4000 m: [0, 5, 10, 20, 30, 50, 75, 100,
125, 150, 200, 250, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200,
1300, 1400, 1500, 1750, 2000, 2500, 3000, 3500, 4000]. The spatial resolution
is 1/8∘×1/8∘. The seasonal scale is winter
(January–March), spring (April–June), summer (July–September), and autumn
(October–December). The gridding of the in situ observations was done with
the Data Interpolating Variational Analysis (DIVA) software tool that allows
the spatial interpolation of data in an optimal way, comparable to optimal
interpolation (OI) using a finite-element method .
The time filtering applied to the in situ observations (decadal averaging)
results in spatial correlations found in the data of the order 300–350 km,
much larger than the Rossby radius of the deformation scale (10–15 km)
associated with mesoscale motions. 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 0.08∘ (in the same units as the data locations) and the
characteristic time of weighting was set to be equal to 90 days (3 months).
Detrending was applied in the observations used for the reference
climatologies in order to remove the uneven spatial distributions in time.
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 1950 to 2015. This
climatology is used as a mean reference that is subtracted from the annual
decadal climatology to obtain the T/S anomalies. It is available here:
10.5281/zenodo.1146976.
Annual decadal climatology, obtained by analysing all data
regardless of month or season for each of the 57 running decades from
1950–1959 to 2006–2015. It is available here: 10.5281/zenodo.1146957.
Seasonal climatology (reference), obtained by analysing all data of
the whole period from 1950 to 2015 falling within each season. This
climatology is used as a mean reference that is subtracted from the seasonal
decadal climatologies to obtain the T/S anomalies. It is available here:
10.5281/zenodo.1146953.
Seasonal decadal climatology, obtained by analysing all data falling
within each season for each of the 57 running decades from 1950–1959 to
2006–2015. It is available here: 10.5281/zenodo.1146938.
The input data used for the current work have already been evaluated with
existing comparable products in the region such as SeaDataNet 2015 and WOA13
monthly climatologies, along with MEDAR/MEDATLAS 2002 monthly and decadal
climatologies, and are of higher spatial and temporal resolution
. 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.
Definitions
Anomalies
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.
Mean climatological references
Annual climatology used as a reference for the annual decadals.
Seasonal climatology used as a reference for the seasonal decadals.
Climates
The World Meteorological Organization (WMO) recommendation of using 30-year averages
(climate normals) to describe climate conditions was used in this study
. Climate shift is defined as the difference between two
successive 30-year averages.
Linear trends
They were computed by linear regression with a constant term.
It is noted that in the climate shifts presented in this work, the period
1950 to 1979 contains 3 decades and the period 1980 to 2015 contains 6 years
more because the period from 2000 to 2015 is treated as a decade. This was
done for two reasons: (a) not to exclude the recent data from the
representations of the regional patterns of the climate shifts (or the oldest
ones if the study period was shifted later than 1950), and (b) the averaging
of the additional recent years actually does not change the qualitative
results of the comparison of the two successive periods. Concerning the
quantitative differences, Tables 1 and 2 below show, for the two different
averaging periods, the mean values of the climate shifts for OHC and OSC
areal density over the whole Mediterranean. It can be seen that the inclusion
of the additional recent years (about 15 000 additional T/S on the about
150 000 T and 100 000 S profiles of the period 1980–2009) actually
reduces the T changes of the first 600 m. 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.
Mean values for the whole Mediterranean for areal density of ocean
heat content in 109 J m-2.
First, seasonal and annual decadal fields of temperature and salinity
anomalies (T/S) at each standard depth were generated. Next, T/S vertical
averages were calculated for the four layers, 0–150 m (and 5–150 m in the
case of the annual fields), 150–600, 600–4000, and 0–4000 m. The
thickness of the layers was used as weights for the vertical averaging
calculated as half of the distance between adjacent depths. The following
weights were used for the 31 standard depth levels: 2.5, 5, 7.5, 10, 15,
22.5, 25, 25, 25, 37.5, 50, 50, 75, 100, 100, 100, 100, 100, 100, 100, 100,
100, 100, 100, 175, 250, 375, 500, 500, 500, 500. 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. Each T/S anomaly at each standard depth is associated with a
volume which consists of the area of the 1/8∘×1/8∘
longitude–latitude grid multiplied by the thickness of each layer, e.g. the
vertical weights. 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:
OHC=ρCp∑i=1ndxdy∫z1z2ΔTdz.
The areal density of OHC (in J m-2) is obtained by integrating the
vertically averaged T anomaly over a depth layer according to the
equation
areal density OHC=ρCp∫z1z2ΔTdz,
where ρ=1028 kg m-3 is the density of reference seawater, Cp=3985 J kg-1∘C the specific heat of seawater, n the
number of grid cells and almost the same as the number of grid points as nx= length(lon), ny= length(lat) and n=(nx-1)×(ny-1), dx=10951.1 m, dy=13897.2 m, ΔT the temperature anomaly, and
z1 and z2 the upper and lower depths. 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. The dx,
dy are the longitude (1/8∘), latitude (1/8∘) steps of
the output grid transformed from degrees to metres. A mean basin volume is
estimated at 3.86×1015 m3 and corresponds to the mean wet
volume of the analysis grid of the interpolation. For the OSC, the same
methodology is used except that we do not multiply by (ρCp), the
term that converts temperature to thermal energy (heat). The OSC (in
ppt m3) is given from the equation
OSC=∑i=1ndxdy∫z1z2ΔSdz,
and the areal density of OSC (in ppt m) from the following equation:
areal density OSC=∫z1z2ΔSdz.
Climatic index content
The produced climatic indices for the whole Mediterranean Sea
(6.25∘ W–36.5∘ E, 30–46∘ N) consist of the
following.
Annual and seasonal T/S anomalies at 31 standard depths, for 57 running decades from 1950–1959 to 2006–2015.
Annual and seasonal T/S vertical averaged anomalies at four layers (surface, intermediate, deep and whole column), for 57 running decades from 1950–1959 to 2006–2015.
Annual and seasonal areal density of OHC/OSC anomalies in four layers (surface, intermediate, deep and whole column), for 57 running decades from 1950–1959 to 2006–2015.
Annual and seasonal linear trends of T/S, OHC/OSC anomalies at four layers (surface, intermediate, deep and whole column) for all 57 decades.
Annual and seasonal time series of T/S, OHC/OSC anomalies at four layers (surface, intermediate, deep and whole column) over the whole Mediterranean Sea.
Differences of two 30-year averages of annual and seasonal T/S anomalies at 31 standard depths for the period 1950 to 2015.
Differences of two 30-year averages of annual and seasonal T/S, OHC/OHC anomalies for the period 1950 to 2015, at four layers (surface, intermediate, deep and whole column).
All data are stored in netCDF files and are accessible using the following
DOIs.
Annual and seasonal T/S anomalies: 10.5281/zenodo.1408832.
Annual and seasonal T/S vertical averaged anomalies: 10.5281/zenodo.1408929.
Annual and seasonal areal density of OHC/OSC anomalies: 10.5281/zenodo.1408877.
Annual and seasonal linear trends of T/S, OHC/OSC anomalies: 10.5281/zenodo.1408917.
Annual and seasonal time series of T/S, OHC/OSC anomalies: 10.5281/zenodo.1411398.
Differences of two 30-year averages of annual and seasonal T/S, OHC/OSC anomalies: 10.5281/zenodo.1408903.
Climate shift of areal density of ocean heat content in
109 J m-2 between two 30-year periods 1980–2015 and 1950–1979 for
(a) 5–150 m, (b) 150–600 m, (c) 600–4000 m,
and (d) 5–4000 m.
Results
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.
The comparison of two successive 30-year averages of heat and salt content
anomalies for the period 1950 to 2015 can be used for the evaluation of the
Mediterranean Sea climate changes. The 30-year periods are averages of three
successive decades: the first one refers to the decades 1950–1959 to
1970–1979 and the second to 1980–1989 to 2000–2015. The two 30-year
successive periods were selected for consistency with the World
Meteorological Organization's recommendation of using as climate normals
30-year periods. Figure illustrates the geographical distribution
over the whole Mediterranean of the 30-year climate shift as the OHC
differences between the period 1980–2015 and 1950–1979 in the upper
5–150 m (Fig. a), 150–600 m (Fig. b), 600–4000 m
(Fig. c), and 5–4000 m (Fig. d).
From the surface layer down to 150 m the climate shift is not uniform. The
western Mediterranean surface layer (Fig. a) has experienced
warming almost everywhere expect the Gulf of Lions and the northern
Tyrrhenian–Ligurian eastern basins. The surface layer of the eastern
Mediterranean (Fig. a) is cooling with a noticeable warming spot
at the Ierapetra gyre. We observe the same with surface patterns at the
intermediate layers of 150–600 m but with about half the strength of the
surface (Fig. b). The deep waters are warming almost everywhere
except the southern Adriatic, the southern Levantine and the south-western
Ionian basin (Fig. c).
Regarding salinity, it is important to notice that in the whole western
Mediterranean there is a clear OSC increase throughout the whole water column
(Fig. a–d), while in the eastern basin we see that the spatial
pattern is not uniform and a notable salt content increase is observed in the
areas of deep water formation, e.g. the southern Adriatic and Aegean Sea.
According to the bibliography, this is the Eastern Mediterranean Transit
(EMT) signature on the intermediate and deep waters, not only in the eastern
Mediterranean, but also in the whole basin
. Notable salt increases are
found at the Shikmona gyre and the south-western Ionian Sea, following the
patterns of the heat content. To illustrate the temporal variability of the
thermohaline content, the annual OHC and OSC anomalies for six discrete
periods, for the three layers (5–150, 150–600, 600–4000 m) and the whole
water column (5–4000 m) are shown in Fig. a–b. We observe that
apart from the strong spatial variability shown in Figs. and
, there is a similar irregular pattern from one decade to another.
One remarkable feature in these distributions is the acceleration and the
substantial heat and salt gain of the deep layer (600–4000 m) starting from
1990. It is also found that the correlations (significant at the 95 %
confidence level) between the decadal Atlantic Multidecadal Oscillation (AMO)
index and the decadal OHC averages are 0.69 for 0–150 m, 0.64 for
150–600 m, 0.63 for 600–4000 m and 0.76 for the whole column: 5–4000 m.
The corresponding correlations with the North Atlantic Oscillation (NAO)
index are 0.23, -0.22, 0.50 and 0.38. These findings seem to be in
agreement with the bibliographical references, where observed acceleration
from satellite data of the Mediterranean waters' warming during the 1990s
could be attributed to the positive phase of AMO .
As in Fig. 2 for the climate shift of areal density of ocean salt
content in 102 ppt m between two 30-year periods 1980–2015 and
1950–1979 for (a) 5–150 m, (b) 150–600 m,
(c) 600–4000 m, and (d) 5–4000 m.
Decadal anomalies of OHC (a) and OSC (b) in
5–150, 150–600, 600–4000, and 5–4000 m. OHC anomalies are in
1020 J and OSC in 1013 ppt m3. (c) Volume
integrals of OHC (1020 J) and OSC (1013 ppt m3) anomalies
at 5–4000 m over the whole Mediterranean Sea. Trend values (per decade) are
given for OHC (in red) and OSC (in blue). AMO annual values (multiplied by 25
to resemble the OHC shape) are shown with green dots. The correlation between
annual AMO (normal and non-multiplied values) and decadal OHC significant at
the 95 % confidence level is shown in green.
57-year liner trend of temperature (a, c, e, g) and
salinity anomalies (b, d, f, h) averaged
over 5–150, 150–600, 600–4000, and 5–4000 m, in
∘C decade-1 and ppt decade-1, respectively. Regions where
the linear trend is not significant at the 95 % confidence level are not
plotted.
To get a more detailed insight into the long-term fluctuations we show in
Fig. c, time series of the decadal OHC and OSC anomalies were
integrated over the whole column depth and area of the Mediterranean Sea.
There are changes, slowdowns and accelerations throughout the study period
and we can distinguish three main periods: (a) from 1960 to the late 1970s
with an increasing trend in salt but a decreasing heat trend, (b) from 1980
to 1990 with no significant changes, and (c) from 1990 to 2015 with strong
OHC/OSC increasing trends .
For the study period there is an overall change in heat and salt content of
about +18.9 (1020 J) and +34.2 (1013 ppt m3) between the
last decade 2006–2015 and the first decade 1950–1959.
Finally we computed a 57-year trend for the period 1950–1959 to 2000–2015
based on the decadal T/S anomalies averaged over the four depth layers. As
reference, the annual climatology of all years was used. Figure
illustrates the statistically significant spatial pattern of the linear
trends for temperature (∘C decade-1, Fig. ,
left-hand side) and salinity (ppt decade-1, Fig. ),
right-hand side). The trend for the whole water column (Fig. g and
h) reveals that for the salinity (Fig. h) there is a positive
trend everywhere. The temperature pattern reveals two main areas of long-term
decreasing trends (Fig. d), the Aegean and southern Adriatic Sea.
The results show a noisy and patchy spatial patterns of the temperature
anomaly trend at the first 600 m (Fig. a and c) which are more
noisy than the corresponding ones for the salinity (Fig. b and d).
At the surface (Fig. b) the salinity trend is positive almost
everywhere in the Mediterranean Sea, while in the intermediate depths
(Fig. d) we distinguish the strong positive trends at the areas of
deep water formation at the eastern Mediterranean, southern Adriatic and
Aegean Sea. This strong signal can also be traced out at the Alboran Sea and
the southern Algerian basin (Fig. d), the outflow path of the LIW
towards the Atlantic Ocean, a result that is in agreement with the
bibliography .
Comparing with the spatial salinity linear trends at three layers 0–150,
150–600, and 600–400, presented in , we observe the
following. There are similarities to the patterns of the MEDATLAS 2002
climatology of 1/4∘ horizontal resolution expected from the
refreshing areas at the surface and intermediate layers of the northern
Aegean, northern Adriatic, and Gulf of Lions. Also, there are more spatial
maskings in the current indices because of the statistical significance of
the linear trend. SeaDataNet V2 data collection on which the current product
is based has almost double more salinity profiles for the period 1950–2015
than the MEDATLAS collection of the period 1950–2002.
Compared with the patterns of the EN4 Met Office climatology of 1∘
horizontal resolution, in the current product there are areas with decreasing
trends at the central Mediterranean only and more spatial variability at the
intermediate layers. EN4 climatology is based to a great extent on the World
Ocean Database and the latter for the common period 1950–2015 with the
current product has about 14 % more salinity profiles for the common period
1950–2015 with the current product.
The netCDF Operators (NCO) command-line programs and
the mathematical and statistical algorithms of the GSL (the GNU Scientific
Library) were used for the manipulation and analysis of the netCDF gridded
fields of temperature and salinity of the Mediterranean Atlas (functions
gsl_fit_linear, gsl_stats_covariance, gsl_stats_sd) NCO toolkit is
available here: http://nco.sourceforge.net/ (last access: 9 October 2018). The GSL is available here:
http://www.gnu.org/software/gsl (last access: 9 October 2018). The climatic indices are distributed through Zenodo at the following
links: 10.5281/zenodo.1408832, 10.5281/zenodo.1408929, 10.5281/zenodo.1408877, 10.5281/zenodo.1408917,
10.5281/zenodo.1411398, 10.5281/zenodo.1408903. The DIVA
interpolation software tool is distributed through Zenodo
(https://zenodo.org/record/836727, ) and GitHub
(https://github.com/gher-ulg/DIVA, last access: 9 October 2018).
Conclusions
We presented a new product of climatic indices for the Mediterranean Sea
oriented to the description and study of the long-term variability and
climate change of the area. The assessment of the T/S and OHC/OSC changes
is a key priority for monitoring the climate changes in a focal region such
as the Mediterranean. So far, the insufficient spatial and temporal coverage
of historical in situ data has induced large uncertainties and differences
among the used approaches, especially in large basin-scale estimations.
Thanks to data repositories such as SeaDataNet, which are improving
continuously in terms of abundance, quality and state-of-the-art mapping
techniques (implemented by the DIVA software tool), we were able to
interpolate in an optimal way and produce high-resolution products. These
products can fill data gaps and can be used in a more efficient way by many
applications, for the study of the past, present and future climate changes.
There is a total increase of 7 % in the number of profiles in the latest
SeaDataNet version V2 (2015) used in this study compared to the previous
version V1.1 (8 % increase in T and 15 % increase in S profiles).
SeaDataNet infrastructure includes data of more than 100 data providers which
are quality controlled, archived in data centres and distributed into the
infrastructure by the SeaDataNet participants. The data aggregation and
validation are performed by regional experts and in close collaboration with
the data originators for ensuring the highest quality of the delivered data
sets. To avoid duplicates, data not belonging to the SeaDataNet consortium
are not included in the repository. There is therefore a significant amount
of data such as bathythermographs (more than 100 000 T profiles, mainly
navy data) which were used in the World Ocean Atlas 2013 but not in this
study. In the next version of this product, these additional sources will be
combined with the SeaDataNet data. Future improvements include the use of
density climatological fields instead of a constant value at the OHC
estimations. Such a density gridded field is not currently available as input
since we interpolate T/S separately, but it would be possible to derive it
from the T/S gridded fields. Another improvement concerns the correction of
the historical bathythermograph data, although previous studies indicated
that it does not alter the final results.
AI created the climatic index product, wrote the first version of the manuscript and prepared the figures.
JMB, SW, CT, AT and SS reviewed the manuscript. AI, CT and SW formatted the document in LaTeX. CT prepared Fig. and the netCDF files for the time series.
The authors declare that they have no conflict of
interest.
It cannot be guaranteed that the product is free from errors or
omissions. Correct and appropriate product interpretation and usage are
solely the responsibility of data users.
Acknowledgements
Data were provided through the SeaDataNet Pan-European infrastructure for
ocean and marine data management
(http://www.seadatanet.org, last access: 9 October 2018).
The DIVA development received funding from the European Union Sixth Framework
Programme (FP6/2002–2006) under grant agreement no. 026212, SeaDataNet, the
Seventh Framework Programme (FP7/2007–2013) under grant agreement no.
283607, SeaDataNet II, SeaDataCloud and EMODnet (MARE/2008/03 – Lot 3
Chemistry – SI2.531432) from the Directorate-General for Maritime Affairs
and Fisheries.Edited by: Giuseppe M. R.
Manzella Reviewed by: Aristomenis Karageorgis and Roger
Proctor
ReferencesAdloff, F., Somot, S., Sevault, F., Jordà, G., Aznar, R., Déqué, M.,
Herrmann, M., Marcos, M., Dubois, C., Padorno, E., Alvarez-Fanjul, E., and
Damia, G.: Mediterranean Sea response to climate change in an ensemble of
twenty first century scenarios, Clim. Dynam., 45, 2775–2802,
10.1007/s00382-015-2507-3,
2015.Beckers, J.-M., Barth, A., Troupin, C., and Alvera-Azcárate, A.: Some
approximate and efficient methods to assess error fields in spatial gridding
with DIVA (Data Interpolating Variational Analysis), J. Atmos.
Ocean. Tech., 31, 515–530, 10.1175/JTECH-D-13-00130.1,
2014.Béthoux, J.-P., Gentili, B., and Tailliez, D.: Warming and freshwater
budget change in the Mediterranean since the 1940s, their possible relation
to the greenhouse effect, Geophys. Res. Lett., 25, 1023–1026,
10.1029/98gl00724,
1998.Béethoux, J. P., Gentili, B., Morin, P., Nicolas, E., Pierre, C., and Ruiz-Pino,
D.: The Mediterranean Sea: a miniature ocean for climatic and environmental
studies and a key for the climatic functioning of the North Atlantic,
Prog. Oceanogr., 44, 131–146, 10.1016/s0079-6611(99)00023-3,
1999.Durack, P. J. and Wijffels, S. E.: Fifty-Year Trends in Global Ocean
Salinities and Their Relationship to Broad-Scale Warming, J.
Climate, 23, 4342–4362, 10.1175/2010jcli3377.1,
2010.Durack, P. J., Wijffels, S. E., and Matear, R. J.: Ocean Salinities Reveal
Strong Global Water Cycle Intensification During 1950 to 2000, Science, 336,
455–458, 10.1126/science.1212222,
2012.Fusco, G., Manzella, G. M. R., Cruzado, A., Gacic, M., Gasparini, G. P.,
Kovacevic, V., Millot, C., Tziavos, C., Velasquez, Z. R., Walne, A.,
Zervakis, V., and Zodiatis, G.: Variability of mesoscale features in the
Mediterranean Sea from XBT data analysis, Ann. Geophys., 21, 21–32,
10.5194/angeo-21-21-2003, 2003.Füssel, H.-M., Jol, A., Marx, A., and Hildén, M.: Climate change, impacts and
vulnerability in Europe 2016, An indicator-based report, Tech. rep.,
European Environment Agency, 10.2800/534806,
2017.Giorgi, F.: Climate change hot-spots, Geophys. Res. Let., 33, L08707,
10.1029/2006gl025734,
2006.Giorgi, F. and Lionello, P.: Climate change projections for the Mediterranean
region, Global Planet. Change, 63, 90–104,
10.1016/j.gloplacha.2007.09.005,
2008.Gouretski, V. and Koltermann, K. P.: How much is the ocean really warming?,
Geophys. Res. Lett., 34, L01610, 10.1029/2006gl027834,
2007.Iona, A.: Mediterannean Sea-Temperature and Salinity Annual Climatology [Data set],
Zenodo,
10.5281/zenodo.1146976, 2018a.Iona, A.: Mediterannean Sea-Temperature and Salinity Annual Climatology for 57 running decades from 1950–1959 to 2006–2015 [Data set],
Zenodo,
10.5281/zenodo.1146957, 2018b.Iona, A.: Mediterannean Sea-Temperature and Salinity Seasonal Climatology [Data set], Zenodo, 10.5281/zenodo.1146953, 2018c.Iona, A.: Mediterannean Sea-Temperature and Salinity Seasonal Climatology for 57 running decades from 1950–1959 to 2006–2015 [Data set], Zenodo, 10.5281/zenodo.1146938,
2018d.Iona, A.: Mediterranean Sea Climatic Indices – T/S Anomalies [Data set],
Zenodo,
10.5281/zenodo.1408832, 2018e.Iona, A.: Mediterranean Sea Climatic Indices – T/S Vertical Averages [Data set],
Zenodo,
10.5281/zenodo.1408929, 2018f.Iona, A.: Mediterranean Sea Climatic Indices – Areal density of OHC/OSC [Data set],
Zenodo,
10.5281/zenodo.1408877, 2018g.Iona, A.: Mediterranean Sea Climatic Indices – Linear trends of T/S, OHC/OSC Anomalies [Data set], Zenodo,
10.5281/zenodo.1408917, 2018h.Iona, A.: Mediterranean Sea Climatic Indices – Time Series of T/S, OHC/OSC Anomalies [Data set],
Zenodo,
10.5281/zenodo.1411398, 2018i.Iona, A.: Mediterranean Sea Climatic Indices – Differences of two 30-years averages of T/S, OHC/OSC Anomalies [Data set],
Zenodo, 10.5281/zenodo.1408903, 2018j.Iona, A., Theodorou, A., Watelet, S., Troupin, C., Beckers, J.-M., and
Simoncelli, S.: Mediterranean Sea Hydrographic Atlas: towards optimal data
analysis by including time-dependent statistical parameters, Earth Syst. Sci.
Data, 10, 1281–1300, 10.5194/essd-10-1281-2018, 2018.IPCC: Climate Change 2013 – The Physical Science Basis: Working Group I
Contribution to the Fifth Assessment Report of the Intergovernmental Panel on
Climate Change, chap. Technical Summary, Cambridge University Press,
10.1017/CBO9781107415324, 2014.Jordà, G., Von Schuckmann, K., Josey, S., Caniaux, G., García-Lafuente,
J.,
Sammartino, S., Özsoy, E., Polcher, J., Notarstefano, G., Poulain, P.-M.,
and Adloff, F.: The Mediterranean Sea heat and mass budgets: Estimates,
uncertainties and perspectives, Prog. Oceanogr., 156, 174–208,
10.1016/j.pocean.2017.07.001,
2017.Klein, B., Roether, W., Manca, B. B., Bregant, D., Beitzel, V., Kovacevic, V.,
and Luchetta, A.: The large deep water transient in the Eastern
Mediterranean, Deep-Sea Res. Pt. I, 46,
371–414, 10.1016/s0967-0637(98)00075-2,
1999.Lascaratos, A., Roether, W., Nittis, K., and Klein, B.: Recent changes in deep
water formation and spreading in the eastern Mediterranean Sea: a review,
Prog. Oceanogr., 44, 5–36, 10.1016/s0079-6611(99)00019-1,
1999.Levitus, S.: Warming of the World Ocean, Science, 287, 2225–2229,
10.1126/science.287.5461.2225,
2000.Levitus, S., Antonov, J. I., Boyer, T. P., Locarnini, R. A., Garcia, H. E., and
Mishonov, A. V.: Global ocean heat content 1955–2008 in light of recently
revealed instrumentation problems, Geophys. Res. Lett., 36, L07608,
10.1029/2008gl037155,
2009.Levitus, S., Antonov, J. I., Boyer, T. P., Baranova, O. K., Garcia, H. E.,
Locarnini, R. A., Mishonov, A. V., Reagan, J. R., Seidov, D., Yarosh, E. S.,
and Zweng, M. M.: World ocean heat content and
thermosteric sea level change
(0–2000 m), 1955–2010, Geophys. Res. Lett., 39, L10603,
10.1029/2012gl051106,
2012.Lozier, M. S., Owens, W. B., and Curry, R. G.: The climatology of the North
Atlantic, Prog. Oceanogr., 36, 1–44,
10.1016/0079-6611(95)00013-5,
1995.Macias, D., Garcia-Gorriz, E., and Stips, A.: Understanding the Causes of
Recent Warming of Mediterranean Waters. How Much Could Be Attributed to
Climate Change?, PLoS ONE, 8, e81591, 10.1371/journal.pone.0081591,
2013.Malanotte-Rizzoli, P., Manca, B. B., d'Alcala, M. R., Theocharis, A.,
Brenner, S., Budillon, G., and Ozsoy, E.: The Eastern Mediterranean in the
80s and in the 90s: the big transition in the intermediate and deep
circulations, Dynam. Atmos. Oceans, 29, 365–395,
10.1016/s0377-0265(99)00011-1,
1999.Manca, B., Burca, M., Giorgetti, A., Coatanoan, C., Garcia, M.-J., and Iona,
A.: Physical and biochemical averaged vertical profiles in the Mediterranean
regions: an important tool to trace the climatology of water masses and to
validate incoming data from operational oceanography, J. Marine
Syst., 48, 83–116, 10.1016/j.jmarsys.2003.11.025,
2004.Manca, B. B.: Evolution of dynamics in the eastern Mediterranean affecting
water mass structures and properties in the Ionian and Adriatic Seas, J.
Geophys. Res., 108, 8102, 10.1029/2002jc001664,
2003.Mariotti, A., Zeng, N., Yoon, J.-H., Artale, V., Navarra, A., Alpert, P., and
Li, L. Z. X.: Mediterranean water cycle changes: transition to drier 21st
century conditions in observations and CMIP3 simulations, Environ.
Res. Lett., 3, 044001, 10.1088/1748-9326/3/4/044001,
2008.
MEDAR Group: MEDATLAS/2002 database, Mediterranean and Black Sea database
of
temperature salinity and bio-chemical parameters, Climatological Atlas,
Tech. rep., Ifremer, 4 Cdroms, 2002.Millot, C., Candela, J., Fuda, J.-L., and Tber, Y.: Large warming and
salinification of the Mediterranean outflow due to changes in its
composition, Deep-Sea Res. Pt. I, 53,
656–666, 10.1016/j.dsr.2005.12.017,
2006.Rahmstorf, S.: Influence of mediterranean outflow on climate, Eos, Transactions
American Geophysical Union, 79, 281–281, 10.1029/98eo00208,
1998.Rixen, M., Beckers, J.-M., Levitus, S., Antonov, J., Boyer, T., Maillard, C.,
Fichaut, M., Balopoulos, E., Iona, S., Dooley, H., Garcia, M.-J., Manca, B.,
Giorgetti, A., Manzella, G., Mikhailov, N., Pinardi, N., Zavatarelli, M., and
the Medar Consortium: The Western Mediterranean Deep Water: a proxy
for global climate change, Geophys. Res. Lett., 32, L12608,
10.1029/2005GL022702,
2005.Robinson, A., Leslie, W., Theocharis, A., and Lascaratos, A.: Mediterranean Sea
Circulation, Encyclopedia of Ocean Sciences, 1689–1705,
10.1006/rwos.2001.0376,
2001.Roether, W., Manca, B. B., Klein, B., Bregant, D., Georgopoulos, D., Beitzel,
V., Kovacevic, V., and Luchetta, A.: Recent Changes in Eastern Mediterranean
Deep Waters, Science, 271, 333–335, 10.1126/science.271.5247.333,
1996.Rohling, E. J. and Bryden, H. L.: Man-Induced Salinity and Temperature
Increases in Western Mediterranean Deep Water, J. Geophys.
Res., 97, 11191–11198, 10.1029/92jc00767,
1992.Schröder, K., Gasparini, G. P., Tangherlini, M., and Astraldi, M.: Deep
and intermediate water in the western Mediterranean under the influence of
the Eastern Mediterranean Transient, Geophys. Res. Lett., 33, L21607,
10.1029/2006gl027121,
2006.Schroeder, K., García-Lafuente, J., Josey, S. A., Artale, V., Nardelli,
B. B.,
Carrillo, A., Gačić, M., Gasparini, G. P., Herrmann, M., Lionello, P., and
Ludwig, W.: Circulation of the Mediterranean Sea and its Variability, The
Climate of the Mediterranean Region, 187–256,
10.1016/b978-0-12-416042-2.00003-3,
2012.Schroeder, K., Chiggiato, J., Bryden, H. L., Borghini, M., and Ben Ismail, S.:
Abrupt climate shift in the Western Mediterranean Sea, Sci. Rep.-UK,
6, 23009, 10.1038/srep23009,
2016.Schroeder, K., Chiggiato, J., Josey, S. A., Borghini, M., Aracri, S., and
Sparnocchia, S.: Rapid response to climate change in a marginal sea,
Sci. Rep.-UK, 7, 4065, 10.1038/s41598-017-04455-5,
2017.Simoncelli, S., Coatanoan, C., Myroshnychenko, V., Sagen, H., Bäck, O.,
Scory, S., Grandi, A., Barth, A., and Fichaut, M.: SeaDataNet, First Release
of Regional Climatologies. WP10 Third Year Report – DELIVERABLE D10.3,
Tech. rep., SeaDataNet, 10.13155/50381, 2015.Skliris, N., Marsh, R., Josey, S. A., Good, S. A., Liu, C., and Allan, R. P.:
Salinity changes in the World Ocean since 1950 in relation to changing
surface freshwater fluxes, Clim. Dynam., 43, 709–736,
10.1007/s00382-014-2131-7,
2014.Skliris, N., Zika, J. D., Nurser, G., Josey, S. A., and Marsh, R.: Global
water cycle amplifying at less than the Clausius-Clapeyron rate, Sci.
Rep.-UK, 6, 38752, 10.1038/srep38752,
2016.Skliris, N., Zika, J. D., Herold, L., Josey, S. A., and Marsh, R.:
Mediterranean sea water budget long-term trend inferred from salinity
observations, Clim. Dynam., 51, 2857–2876, 10.1007/s00382-017-4053-7,
2018.Somot, S., Sevault, F., Déqué, M., and Crépon, M.: 21st century climate
change scenario for the Mediterranean using a coupled atmosphere–ocean
regional climate model, Global Planet. Change, 63, 112–126,
10.1016/j.gloplacha.2007.10.003,
2008.Theocharis, A., Nittis, K., Kontoyiannis, H., Papageorgiou, E., and Balopoulos,
E.: Climatic changes in the Aegean Sea influence the eastern Mediterranean
thermohaline circulation (1986–1997), Geophys. Res. Lett., 26,
1617–1620, 10.1029/1999gl900320,
1999.Theocharis, A., Klein, B., Nittis, K., and Roether, W.: Evolution and status of
the Eastern Mediterranean Transient (1997–1999), J. Marine Syst.,
33-34, 91–116, 10.1016/s0924-7963(02)00054-4,
2002.Troupin, C., Sirjacobs, D., Rixen, M., Brasseur, P., Brankart, J.-M., Barth,
A., Alvera-Azcárate, A., Capet, A., Ouberdous, M., Lenartz, F.,
Toussaint, M.-E., and Beckers, J.-M.: Generation of analysis and consistent
error fields using the Data Interpolating Variational Analysis (Diva), Ocean
Model., 52–53, 90–101, 10.1016/j.ocemod.2012.05.002,
2012.Tsimplis, M. N., Zervakis, V., Josey, S. A., Peneva, E. L., Struglia, M. V.,
Stanev, E. V., Theocharis, A., Lionello, P., Malanotte-Rizzoli, P., Artale,
V., Tragou, E., and Oguz, T.: Chapter 4 Changes in the oceanography of the
Mediterranean Sea and their link to climate variability, in: Developments in
Earth and Environmental Sciences, edited by: Lionello, P., Malanotte-Rizzoli,
P., and Boscolo, R., Elsevier, 227–282,
10.1016/s1571-9197(06)80007-8,
2006.Vargas-Yáñez, M., Moya, F., Tel, E., García-Martínez, M. C., Guerber,
E.,
and Bourgeon, M.: Warming and salting in the western Mediterranean during
the second half of the 20th century: inconsistencies, unknowns and the effect
of data processing, Sci. Mar., 73, 7–28,
10.3989/scimar.2009.73n1007,
2008.Vargas-Yáñez, M., Moya, F., García-Martínez, M., Tel, E., Zunino, P.,
Plaza, F., Salat, J., Pascual, J., López-Jurado, J., and Serra, M.: Climate
change in the Western Mediterranean Sea 1900–2008, J. Marine
Syst., 82, 171–176, 10.1016/j.jmarsys.2010.04.013,
2010a.
Vargas-Yáñez, M., Zunino, P., Benali, A., Delpy, M., Pastre, F., Moya,
F.,
García-Martínez, M. d. C., and Tel, E.: How much is the western
Mediterranean really warming and salting?, J. Geophys. Res.,
115, C04001, 10.1029/2009jc005816,
2010b.Watelet, S., Troupin, C., Beckers, J.-M., Barth, A., and Ouberdous, M.: gher-ulg/DIVA: v4.7.1 (Version
v4.7.1),
Zenodo,
10.5281/zenodo.836727, 2017.WMO: Guide to Climatological Practices, Tech. rep., World Meteorological
Organization, Geneva, Switzerland,
available at: https://library.wmo.int/pmb_ged/wmo_100_en.pdf (last access: 9 October 2018), ISBN
978-92-63-10100-6, 2011.Zika, J. D., Skliris, N., Blaker, A. T., Marsh, R., Nurser, A. J. G., and
Josey, S. A.: Improved estimates of water cycle change from ocean salinity:
the key role of ocean warming, Environ. Res. Lett., 13, 074036,
10.1088/1748-9326/aace42,
2018.