Recent Changes in the Mediterranean Water Cycle: A Pathwaytoward Long-Term Regional Hydroclimatic Change?

ANNARITA MARIOTTI

Earth System Science Interdisciplinary Center, University of Maryland, College Park,

College Park, Maryland, and ENEA, Rome, Italy

(Manuscript received 19 May 2009, in final form 9 October 2009)

ABSTRACT

An observational analysis of Mediterranean Sea water cycle variability based on recently available datasets

provides new insights on the long-term changes that affected the region since the 1960s. Results indicate an

overall increase in evaporation during 1958–2006, with a decrease up until the mid-1970s and an increase

thereafter. Precipitation variability is characterized by substantial interdecadal variations and a negative long-

term trend. Evaporation increase, primarily driven by SST variability, together with precipitation decrease

resulted in a substantial increase in the loss of freshwater from the Mediterranean Sea toward the overlying

atmosphere. An increase in the freshwater deficit is consistent with observed Mediterranean Sea salinity

tendencies and has broad implications for the Mediterranean water cycle and connected systems.

These observational results are in qualitative agreement with simulated Mediterranean Sea water cycle be-

havior from a large ensemble of models from the Coupled Model Intercomparison Project Phase 3 (CMIP3).

However, simulated anomalies are about one order of magnitude smaller than those observed. This in-

consistency and the large uncertainties associated with the observational rates of change highlight the need for

more research to better characterize and understand Mediterranean water cycle variations in recent decades,

and to better simulate the crucial underlying processes in global models.

1. Introduction

The Fourth Assessment Report (AR4) of the Intergov-

ernmental Panel on Climate Change (IPCC; Solomon

et al. 2007) and related investigations project major

changes in the Mediterranean region, in particular, as

a ‘‘hot spot’’ in hydrological change with significant

impacts on both mean precipitation and variability

(Gibelin and Deque 2003; Giorgi 2006; Giorgi and

Lionello 2008; Sheffield and Wood 2008; Ulbrich et al.

2006). In a recent study, Mariotti et al. (2008) show how

in the Coupled Model Intercomparison Project Phase 3

(CMIP3) multimodel simulations, the combination of

projected twenty-first-century precipitation reduction

and warming-enhanced evaporation concurs to deter-

mine even greater alterations of Mediterranean water

cycle characteristics with significant increases in land sur-

face dryness and loss of freshwater over the Mediterra-

nean Sea. In these simulations, the ‘‘transition’’ toward

such conditions is seen to be ongoing, with an acceler-

ation around the turn of the century toward the larger

rates projected for the twenty-first century. This study

also finds consistency between these tendencies and ob-

servational evidence of twentieth-century-long negative

trends in regionally averaged precipitation, the Palmer

Drought Severity Index, and discharge from numerous

Mediterranean rivers. However, recent long-term changes

in sea evaporation (the biggest single component of the

Mediterranean water cycle) and sea surface freshwater

fluxes (potentially a combination of precipitation and

evaporation changes), as suggested by the CMIP3 sim-

ulations, have yet to be explored. In view of the semi-

enclosed nature of the Mediterranean–Black Sea system,

connected to the Atlantic Ocean via the Strait of

Gibraltar, and the semiarid and/or arid conditions in

land regions downstream of Mediterranean moisture

fluxes, the impacts of such changes could be substantial.

Increases in sea evaporation (and associated latent heat)

and freshwater loss effect the salt, water, and energy

budgets with potentially important implications for Med-

iterranean Sea salinity (note that Mediterranean Sea

Corresponding author address: Dr. Annarita Mariotti, 5825

University Research Court, Earth System Science Interdisciplinary

Center, University of Maryland, College Park, College Park, MD

20740-3823.

E-mail: amariott@essic.umd.edu

15 MARCH 2010 M A R I O T T I 1513

DOI: 10.1175/2009JCLI3251.1

� 2010 American Meteorological Society

salinity is among the highest globally), circulation, and sea

level (e.g., Skliris et al. 2007; Tsimplis et al. 2008), and

Atlantic circulation via changes in Strait of Gibraltar

water fluxes (Lozier and Stewart 2008; Millot et al.

2006; Potter and Lozier 2004; Reid 1979). Additionally,

an increase in evaporation (i.e., the amount of moisture

the Mediterranean Sea injects into the overlying atmo-

sphere) would enhance moisture fluxes to downstream

regions, potentially affecting precipitation there [e.g., in

the Sahel (Jung et al. 2006)]. In spite of the high CMIP3

intermodel consistency regarding the Mediterranean re-

gion, substantiating the tendencies suggested by the

models with observational evidence is crucial in view

of the significant limitations in state-of the-art global

models’ representation of the Mediterranean Sea (e.g.,

Marcos and Tsimplis 2008).

A number of previous studies have indicated a long-

term increase in Western Mediterranean Deep Water

salinity and temperatures during the latter half of the

twentieth century (e.g., Bethoux et al. 1998; Krahmann

1998; Rixen et al. 2005). Several of these studies have

evidenced the linkage between this salinity increase and

the long-term decrease in Mediterranean precipitation

during the period from the mid-1970s to the early 1990s,

primarily in connection with the decadal variations of

the North Atlantic Oscillation (Hurrell 1995; Mariotti

et al. 2002). Long-term salinity increase has also been

connected to a reduction in river discharge (e.g., dam-

ming of the Nile River in the 1960s) and Black Sea

freshwater inputs (Rohling and Bryden 1992; Skliris

et al. 2007). Interannual evaporation anomalies and as-

sociated cooling have been identified as a key factor in

the eastern Mediterranean transient event of 1991–93

(Josey 2003; Roether et al. 2007). However, decadal evap-

oration changes over the Mediterranean Sea are still vir-

tually unknown, because previous attempts were limited

by data availability (e.g., Krahmann 1998; Mariotti et al.

2002). In fact, both oceanic precipitation and evaporation

estimates and their interdecadal variability have long re-

mained elusive in the absence of suitable climatic datasets.

Substantial data developments in the last decade, with

some datasets now going back 25 yr or more, have brought

major new opportunities to investigate long-term water

cycle variability in oceanic regions. For instance, Adler

et al. (2008) use Global Precipitation Climatology Pro-

ject data (GPCP; Adler et al. 2003) to investigate the

SST–precipitation relationship on interannual-to-decadal

time scales. Yu (2007) and Yu and Weller (2007) explore

global long-term oceanic evaporation changes in the ob-

jectively analyzed air–sea fluxes (OAFlux) dataset and

find widespread increases in many regions. Wentz et al.

(2007) investigate overall changes in the global water

cycle in relation to global warming.

Research presented here explores the regional mani-

festations of these long-term global water cycle changes

in the sensitive Mediterranean system, exploiting recent

progress in data availability. The focus is on the com-

bined effects of precipitation and evaporation changes

on the Mediterranean water cycle. A major question

that this work addresses is whether the behavior ob-

served during the last few decades is consistent with the

‘‘transition’’ phase suggested by the CMIP3 simulations

for the Mediterranean as a pathway toward future pro-

jected changes. The attribution of observed changes and

the analysis of related impacts is beyond the scope of this

paper. The paper is organized as follows: Section 2 pres-

ents the data used in the analyses and the basic method-

ologies. Section 3 depicts mean characteristics during

recent decades, defining a baseline for change. Follow-

ing sections describe long-term changes since 1960 and

their consistency with the CMIP3 results for evaporation

(section 4), precipitation (section 5), and freshwater flux

(section 6). Finally, summary and concluding remarks

are presented in section 7.

2. Data and methodology

A variety of different data sources are used in this study

taking advantage of recently available datasets combin-

ing in situ, satellite, and reanalyses data for global water

cycle climatic studies. We use monthly GPCP oceanic

precipitation estimates based on blended gauge–satellite

products. The data, available since 1979 on a 2.58 3 2.58

grid, are combined using microwave-based estimates

from the Special Sensor Microwave Imager (SSM/I), in-

frared rainfall estimates, and surface rain gauges. Among

the satellite-based datasets, we use monthly precipitation

estimates and other air–sea fluxes derived by Remote

Sensing Systems (REMSS) using a variety of passive

microwave radiometers, including SSM/I retrievals

[available 1987–2006, at 2.58 3 2.58 resolution (Wentz

et al. 2007)]. To extend the analysis of Mediterranean

Sea precipitation variability back in time, we analyze a

recent reconstruction of oceanic precipitation (Smith

et al. 2008), which aims at capturing the large-scale fea-

tures of global precipitation since 1900 (REOFS here-

after). The data are based on a covariance analysis of

satellite microwave estimates (SSM/I) merged with at-

mospheric reanalyses for the period of 1992–2001. Re-

sults from this analysis are used jointly with historical

land gauge precipitation to reconstruct oceanic precipi-

tation back to the presatellite era. REOFS data are

available on a 58 3 58 grid for the period of 1900–2006.

Because the Mediterranean Sea is semienclosed by land,

land gauge precipitation estimates from surrounding re-

gions are also analyzed for comparison. We analyze the

1514 J O U R N A L O F C L I M A T E VOLUME 23

monthly precipitation dataset from the National Climatic

Data Center (NCDC) Global Historical Climatology

Network (GHCN) version 2 data (Vose et al. 1992; 1900–

2007, 58 3 58 resolution), the Climatic Research Unit

(CRU) TS2.1 data (Mitchell and Jones 2005; 1901–2002,

0.58 3 0.58 resolution), and the Climate Prediction

Center (CPC) Precipitation Reconstruction over Land

(PRECL) data (Chen et al. 2002; since 1948, 2.58 3 2.58

resolution).

Evaporation is derived from the novel air–sea fluxes

dataset OAFlux, which objectively synthesizes surface

meteorology obtained from satellite products [including

retrievals from SSM/I, Quick Scatterometer (QuikSCAT),

Advanced Very High Resolution Radiometer (AVHRR),

and Tropical Rainfall Measuring Mission (TRMM)]

and model reanalyses (Yu et al. 2008). SST in OAFlux

is originally from the Reynolds et al. (2007) product.

OAFlux data are available monthly since 1958 on a 18 3 18

grid. The OAFlux dataset does not use in situ data as

a direct input. Hence, in situ evaporation estimates de-

rived from the National Oceanography Centre South-

ampton (NOCS) new air–sea flux dataset, version 2

[NOCS Flux Dataset v2.0; hereafter NOCS (Berry and

Kent 2008)] is analyzed here as an independent data

source. These data use the International Comprehensive

Ocean–Atmosphere Data Set (ICOADS) to derive

monthly oceanic air–sea fluxes (Worley et al. 2005; 1973–

2006, 18 3 18 grid). For both OAFlux and NOCS, we

compute sea surface humidity based on the Coupled

Ocean–Atmosphere Response Experiment (COARE

3.0) bulk formula (Fairall et al. 2003) using SST and

the Hadley Centre’s mean sea level pressure (HadSLP;

Allan and Ansell 2006) data. Among the satellite-based

air–sea fluxes datasets, we analyze the Goddard Satellite-

Based Surface Turbulent Fluxes version 2.0 (GSSTF 2.0)

data, which use SSM/I and National Centers for Envi-

ronmental Prediction–National Center for Atmospheric

Research (NCEP–NCAR) reanalyses (hereafter NCEP;

Chou et al. 2003; 1987–2000, 18 3 18 resolution), and the

Hamburg Ocean Atmosphere Parameters and Fluxes

from Satellite Data, version 3 (HOAPS-3; Andersson

et al. 2007), which uses SSM/I data and National Oceanic

and Atmospheric Administration’s (NOAA’s) Pathfinder

SSTs (1987–2005, 0.58 3 0.58 resolution). Meteorological

reanalyses are used only to estimate climatologies, be-

cause these data are less well suited to study long-term

variability and trends of regional water cycle charac-

teristics. NCEP (Kalnay et al. 1996) monthly data are

available on a spectral grid at about 1.98 resolution; 40-yr

European Centre for Medium-Range Weather Fore-

casts (ECMWF) Re-Analysis (ERA-40; Uppala et al.

2005) covers the period of 1957–2002, with data avail-

able at a 2.58 3 2.58 resolution. The twentieth-century

CMIP3 model simulations analyzed in this study are

coupled runs with various observed forcings (climate of

the twentieth-century experiment; details available on-

line at http://www-pcmdi.llnl.gov/ipcc/standard_output.

html#Experiments). For the twenty-first century, we ana-

lyze CMIP3 projections for the Special Report on Emis-

sions Scenarios (SRES) A1B. Ensemble means include

data from 14 different models and multiple runs from

each model [refer to Mariotti et al. (2008) for more in-

formation regarding these simulations and their valida-

tion over the Mediterranean region].

In view of data availability, we compute climatologies

over the common period of 1988–2000, although the re-

sults presented in sections 4–6 show that this was a period

of substantial regional water cycle changes. Our approach

is to use data intercomparison to evaluate the uncer-

tainties associated with the various Mediterranean wa-

ter cycle estimates (for individual dataset evaluation, see

the above-mentioned references). In addition to data

quality, resolution disparities also contribute to differ-

ences among individual estimates. Climatological esti-

mates are computed by averaging all available data

sources. Uncertainties are evaluated by computing the

standard error representing the deviation of various

estimates from the datasets’ mean. Area averages are

computed by using individual datasets’ land–sea masks

for the Mediterranean Sea and the surrounding land in

the domain 288–478N, 108W–408E. The disparities in

land–sea mask specifications also add to differences

between the estimates (observational evaporation is

provided as sea only; a fractional cover land–sea mask is

used for CMIP3 model output and for observational

precipitation, if available). Eastern Mediterranean means

are area averages over the sea in the 308–408N, 138–408E

domain; western Mediterranean sea-only means are in

the 328–448N, 58W–128E domain. Anomalies are relative

to the monthly 1988–2000 climatology. Six-year running

means are used to depict low-frequency variations (on

decadal or longer time scales). Although the focus is on

these variations, annual mean values are displayed for

selected datasets as an indicator of interannual variabil-

ity. Linear trends of annual mean values are computed in

order to quantify long-term changes; this does not imply

that the original signal is best represented by a linear

increase in time.

3. Mean characteristics

Annual mean precipitation over the Mediterranean

Sea for the period 1988–2000 is 1.12 mm day21, based

on the mean of various available datasets (Table 1).

Mean evaporation for the same period is 2.94 mm day21,

15 MARCH 2010 M A R I O T T I 1515

that is, well over twice the precipitation rate. These lead

to an estimated annual mean loss of freshwater by the

Mediterranean Sea (E 2 P) of 1.71 mm day21. Uncer-

tainties on these mean values based on the standard error

are 18% and 13% of climatology for precipitation and

evaporation, respectively. Evaporation has a sharp min-

imum in May (;1.5 mm day21); after that there is a

steady increase up until the November–December max-

imum (;4 mm day21), with evaporation remaining high

through February (over 3 mm day21) and then rapidly

decreasing into spring (Fig. 1). Among the various data-

sets, NOCS gives the lowest evaporation rates; GSSTF2.0

and NCEP are consistently above the mean. In contrast

to evaporation, precipitation is low in June–August (with

a minimum in July, ;0.3 mm day21), and increases rap-

idly during the autumn season. Precipitation is about

2 mm day21 in November–December and then starts

to gradually decline. GPCP gives the highest precipita-

tion, while ERA-40 and REMSS are among the lowest.

Overall precipitation has a smaller seasonal cycle com-

pared to evaporation (1.7 mm day21 vis-a-vis 2.5 mm

day21), and E 2 P is positive year-round, minimum in

late spring (;0.7 mm day21), and peaks in late summer

(;2.5 mm day21) when precipitation is still low, while

evaporation has already significantly increased from its

May minimum. All datasets show a roughly similar be-

havior, however monthly estimates vary significantly

and uncertainties are high. Biases tend to follow those of

evaporation, with E 2 P estimates using NCEP and

GSSTF2.0 generally higher than the mean and those

using NOCS being the lowest. Annual mean, E 2 P

fluxes are highest in the southeastern Mediterranean (to

2–3 mm day21, according to GPCP and OAFlux; not

shown) and lowest in the Adriatic and Ligurian Seas (less

than 0.5 mm day21). The E 2 P rates over the eastern

Mediterranean are roughly double those of the western

Mediterranean (see Table 1). This is due to differences

in E rates, because P rates are similar over the two

regions.

4. Long-term mean evaporation changes

We now explore long-term Mediterranean Sea evap-

oration changes relative to the 1988–2000 mean values

for the period since 1958 (Fig. 2). For the period of 1996–

2005, a variety of evaporation data sources are available.

All datasets unanimously show an increase in evapora-

tion leading up to the current period. However, the

increase over this period varies significantly among

datasets, with OAFlux giving the highest increase and

REMSS the lowest. Only the OAFlux and NOCS

datasets are available going back to the 1970s. Accord-

ing to OAFlux, the evaporation increase discussed above

is part of a long-term progressive evaporation increase,

which started in the mid-1970s, when E was about

0.1 mm day21 below the 1988–2000 mean. Similarly to

OAFlux, NOCS also shows mid-1970s evaporation rates

TABLE 1. Mean precipitation (P), evaporation (E), and E 2 P

over the Mediterranean Sea (Med) and major subbasins (eastern

Mediterranean: EMed; western Mediterranean: WMed). Reported

in each cell, annual mean averages for the period of 1988–2000

computed as the mean of available data sources (for precipitation:

GPCP, REMSS, NCEP, ERA-40; for evaporation: GSSTF,

HOAPS-3, OAFlux, NOCS, REMSS, NCEP, ERA-40) and the std

error of various estimates from the datasets’ mean. Units: mm day21.

P E E 2 P

Med 1.12 6 0.21 2.94 6 0.37 1.71 6 0.38

EMed 1.04 6 0.22 3.25 6 0.42 2.10 6 0.42

WMed 1.08 6 0.21 2.51 6 0.37 1.30 6 0.38

FIG. 1. Seasonal cycle of Mediterranean Sea mean (top) evapo-

ration (E), (middle) precipitation (P), and (bottom) E 2 P for the

period of 1988–2000. Individual data sources (various colors; see

legend) and their average (thick solid black line) are reported to-

gether with overall annual averages (dashed black horizontal

lines). Units: mm day21.

1516 J O U R N A L O F C L I M A T E VOLUME 23

lower than the 1988–2000 mean that progressively in-

crease. However, during the 1980s, before the common

evaporation increase of the 1990s, the two datasets dis-

agree: while OAFlux evaporation remains roughly level,

NOCS 6-yr means show a local maximum in 1985. The

two datasets give an overall trend for the period of 1979–

2006 of 0.1–0.2 mm day21 decade21 (4%–8% of cli-

matology per decade; these trends are significant and

statistically consistent, see Table 2). Only OAFlux is

available since the 1960s. According to this dataset,

evaporation rates in the 1960s were comparable to those

of the late 1990s, and subsequently decreased up until

the mid-1970s before the increase leading up to the

current period. Considering the entire 1958–2006 pe-

riod, these interdecadal variations have amounted to

an equivalent linear mean evaporation increase of 2%

decade21 (or 0.06 mm day21 decade21). Evaporation

that is separately area averaged over the eastern and

western Mediterranean gives similar results to those

described above for the entire Mediterranean (not shown).

CMIP3 models’ behavior since the 1960s is broadly similar

to that of OAFlux: an initial steep decrease (1965–1975)

and then a progressive increase since the mid-1970s up

until present times (CMIP3 leads OAFlux by about

5 yr). However, CMIP3 rates are about one order of

magnitude smaller than those of OAFlux.

The humidity gradient at the air–sea interface (Qs 2

Qa) and surface winds are primary factors controlling

sea evaporation. SST influences evaporation by directly

affecting humidity at the sea surface (Qs) and may also

impact the large-scale structure of air humidity (Qa).

The analysis of long-term Mediterranean SST anomalies

from OAFlux and NOCS consistently shows that the

mid-1970s–present evaporation increase described above

was associated with a progressive SST increase during this

period (see Fig. 3 and Table 3). Rates of increase for the

1979–2006 period are in the range of 0.2–0.3 K decade21

(various estimates are statistically consistent). The SST in-

crease over the 1958–2006 period is about 0.1 K decade21.

Both OAFlux and NOCS datasets show an increase in

Qs and Qa since the mid-1970s (not shown). An increase

in Qa is consistent with the increased moisture holding

capacity of the atmosphere at higher temperatures (fol-

lowing the Clausius–Clapeyron equation). During the

period of 1996–2005, both NOCS and OAFlux show an

increase in humidity gradient and evaporation. During

the 1980s, Qs 2 Qa differences between datasets reflect

those in evaporation and are in part associated with SST

differences. From 1960 to mid-1970, OAFlux shows a

steep decrease in Qs 2 Qa, consistently with decreasing

SST, which resulted in decreased evaporation during

this period. An increase in wind speed since the 1990s,

seen in both OAFlux and NOCS, also contributed to

increased evaporation during this period. Before that,

the two datasets show inconsistent behavior, so results

regarding the long-term wind change contribution are

inconclusive.

Figure 4 illustrates how the increase in evaporation

between the decades of 1979–88 and 1996–2005 (by about

TABLE 2. Linear trends of (top to bottom) annual mean evapo-

ration (E), precipitation (P), and E 2 P for the periods of 1958–

2006 and 1979–2006 (mm day21 decade21) using various data

sources. Statistically significant results are in bold.

E

1958–2006 1979–2006

OAFlux 0.063 6 20.039 0.235 6 0.073

NOCS 0.107 6 0.058

CMIP3 0.003 6 20.004 0.011 6 0.007

P

1958–2006 1970–2006

GPCP 20.046 6 0.084

REOFS 20.041 6 0.032 0.007 6 0.078

CRU 20.031 6 0.023 20.021 6 0.071

PRECL 20.036 6 0.018 20.033 6 0.044

GHCN 20.018 6 0.025 0.006 6 0.052

CMIP3 20.011 6 0.006 20.009 6 0.016

E 2 P

1958–2006 1979–2006

OAFlux/GPCP 0.276 6 0.077

OAFlux/REOFS 0.104 6 0.046 0.228 6 0.097NOCS/GPCP 0.148 6 0.090

NOCS/REOFS 0.100 6 0.095

CMIP3 0.014 6 0.006 0.019 6 0.014

FIG. 2. Decadal variations in Mediterranean Sea mean evapo-

ration over the period of 1958–2007. Shown are 6-yr running means

of Mediterranean Sea area-averaged evaporation anomalies rela-

tive to the period of 1988–2000 (lines, mm day21). Various ob-

servational sources are used (see legend; left-hand scale). For

OAFlux, annual mean values are also represented (symbols).

CMIP3 models ensemble running mean averages are also displayed

(note different scale at right).

15 MARCH 2010 M A R I O T T I 1517

0.4 mm day21 according to OAFlux) affected the mean

seasonal cycle. Evaporation increased year-round, with

most of the increase occurring in September–December.

NOCS evaporation (not shown) gives similar results,

although the anomalies are about half those of OAFlux.

Considering the pattern of evaporation change during

1979–2006 (Fig. 5), OAFlux displays increases every-

where over the Mediterranean Sea, and also extending to

the Black Sea and the neighboring Atlantic Ocean waters

[see Yu and Weller (2007) for global evaporation trends].

Annual mean increases are greatest in the Ligurian Sea,

the Adriatic, and parts of the southeastern Mediter-

ranean (up to 0.4–0.5 mm day21 decade21). Other parts

of the Mediterranean show evaporation increases of at

least 0.1–0.2 mm day21 decade21. The October–March

pattern of increase is similar to that of the annual means,

except that regions with increases of 0.4–0.5 mm day21

decade21 are widespread and most parts of the Medi-

terranean see an increase of at least 0.2–0.3 mm day21

decade21. NOCS evaporation trends (not shown) give

similar patterns of change as those described above,

except the values are lower (maximum annual trends are

up to 0.3–0.4 mm day21 decade21, with trends gen-

erally in the range of 0.1–0.2 mm day21 decade21).

The Mediterranean Sea evaporation trends described

FIG. 3. Factors controlling decadal Mediterranean Sea evapo-

ration changes. (top) Mediterranean-averaged SST (K), (middle)

specific humidity gradient (g g21), and (bottom) 10-m winds (m s21).

Anomalies are for the period of 1958–2007 relative to 1988–2000.

Data are 6-yr running means of area-averaged values from OAFlux

(solid) and NOCS (dashed) datasets. (top) SST from CMIP3 simu-

lations are also displayed (dotted).

TABLE 3. Same as in Table 2, but for SST. OAFlux SST is from

Reynolds et al. (2007), NOCS SST is from ICOADS (Worley et al.

2005). Data are in K decade21.

SST

1958–2006 1979–2006

OAFlux 0.086 6 0.057 0.304 6 0.098NOCS 0.245 6 0.098

CMIP3 0.113 6 0.022 0.211 6 0.035

FIG. 4. Changes in the seasonal cycle of Mediterranean Sea av-

eraged evaporation (E), precipitation (P), and E 2 P. Repre-

sented, 1979–88 means (solid) and 1996–2005 means (dashed).

Annual mean values for the two periods are also reported (hori-

zontal lines). Evaporation is from OAFlux and precipitation is

from GPCP (mm day21).

1518 J O U R N A L O F C L I M A T E VOLUME 23

above were primarily due to local increases in SST

and associated changes in Qs 2 Qa (see Fig. 6). The

greatest annual SST increases during 1979–2006 are

seen in the Ligurian Sea and southern parts of the

Mediterranean (up to 0.4 K decade21 according to

OAFlux). The Qs 2 Qa increases broadly follow the

SST pattern of increases, although differences exist

(e.g., in the Alboran Sea).

5. Long-term mean precipitation changes

Long-term mean precipitation changes in the Medi-

terranean region for the period since 1958 are presented

in Fig. 7. As discussed in section 2, only indirect esti-

mates of Mediterranean Sea precipitation are available

before 1979, based on REOFS reconstruction and land

gauge precipitation from regions surrounding the sea.

The analysis presented here considers together Medi-

terranean Sea estimates (GPCP, REMSS, and REOFS)

and those from land gauges (PRECL, CRU TS2.1, GHCN).

During 1979–2006, decadal variations characterize the

low-frequency variability of Mediterranean precipita-

tion. These are quite similarly represented in the various

datasets: a substantial decrease of 0.2–0.3 mm day21

(about 15%–25% of climatology) during the 1980s,

followed by a rapid increase in precipitation (about

0.2 mm day21) until the mid-1990s; after that, precipi-

tation is seen to decrease slightly up until the turn of the

twentieth century, with a recent tendency to increase.

Despite the general similarity, differences exist among

the various estimates, owing in part to the different areas

considered in the spatial averages (land-only versus sea

averages) and differing spatial resolutions. Overall,

GPCP precipitation suggests larger anomalies even com-

pared to the other sea-only averages, with an especially

large decrease during the 1980s. Between 1958 and 1979,

land gauge estimates show roughly constant precipitation

amounts, at about 0.1 mm day21 above the 1988–2000

FIG. 5. Linear trends for evaporation over the Mediterranean Sea for the period of 1979–

2006. Trends are for (top) annual means and (bottom) October–March means based on

OAFlux data. Units: mm day21 decade21.

15 MARCH 2010 M A R I O T T I 1519

means; REOFS gives higher precipitation compared to

the land gauge estimates (about 0.15 mm day21 above

the mean). Overall, for the period of 1958–2006 both

REOFS and the land gauge estimates suggest that these

decadal variations were superimposed on a long-term

negative trend (0.03–0.04 mm day21 decade21 or ;4%

of climatology per decade; various estimates are sta-

tistically consistent). In contrast, on the shorter 1979–

2006 period there is no significant precipitation trend.

Overall, CMIP3 model behavior is quite similar to

that depicted by the observational datasets; however,

the anomalies are about 5 times smaller than those

observed.

Changes in the mean seasonal cycle of precipitation

between the decades of 1979–88 and 1996–2005 are

mixed (see Fig. 4). Based on GPCP, precipitation de-

creased in January–March (;0.4 mm day21) and in-

creased during early fall. The spatial pattern of the

linear precipitation trend over the period of 1979–2006

is shown in Fig. 8. Annual mean decreases are found

only in the Adriatic Sea and parts of the southeastern

Mediterranean (0.1–0.2 mm day21 decade21). Nega-

tive trends become more substantial and widespread

October–March, with most of the Mediterranean ex-

periencing a precipitation decrease (0.1–0.2 mm day21

decade21), especially in the eastern Mediterranean (up

to 0.3 mm day21 decade21).

6. Long-term mean surface freshwater flux changes

The combination of the evaporation and precipitation

changes described in previous sections resulted in sig-

nificant long-term changes in Mediterranean Sea surface

freshwater fluxes during the period of 1958–2006 (Fig. 9).

FIG. 6. Same as Fig. 5, but for annual mean (top) Qs 2 Qa (g g21 decade21) and (bottom) SST

(K decade21).

1520 J O U R N A L O F C L I M A T E VOLUME 23

Estimates based on OAFlux/REOFS suggest a substantial

increase in E 2 P over this period (;0.5 mm day21 in

total). Considering the 1979–2006 subperiod, the E 2 P

rate of increase is estimated as 0.1–0.3 mm day21 decade21

(see Table 2; estimates are mostly statistically consis-

tent). The E 2 P increase during the 1980s is primarily

driven by the decrease in precipitation during this pe-

riod. Similarly, the ‘‘dip’’ in E 2 P during the mid-1990s

is also precipitation driven, and it is depicted quite

consistently across data sources. In contrast, the most

recent E 2 P increase is dominated by an evaporation

increase. The observational E 2 P results discussed here

are broadly consistent with those from the CMIP3 sim-

ulations, with an overall tendency for Mediterranean

E 2 P to increase during 1958–2006. However, CMIP3

E 2 P anomalies are about one order of magnitude

smaller than observed.

The comparison of the E 2 P seasonal cycle during the

decades of 1979–88 and 1996–2005, based on OAFlux/

GPCP, indicates that most of the E 2 P increase

occurred during September–March (see Fig. 4). Evapo-

ration increase was the main cause of the September–

December E 2 P increase, with the precipitation decrease

significantly contributing to the January–March increase.

Results using NOCS/GPCP are similar, although their

rates of change are smaller (not shown).

During 1979–2006, annual mean E 2 P increased

everywhere in the Mediterranean Sea, and most sub-

stantially in the Ligurian Sea, Adriatic Sea, and parts

of the southeastern Mediterranean (up to 0.4–0.5

mm day21 decade21 based on OAFlux and GCPC esti-

mates; Fig. 10). Increases of 0.2–0.3 mm day21 decade21

were widespread. October–March means shows a similar

pattern of increase but the rates of increase are much

higher (over 0.5 mm day21 decade21) in vast parts of

the Mediterranean. A similar analysis based on NOCS/

GPCP (not shown), gives E 2 P trend patterns that are

consistent with those described above, except that the

rates of change are generally more modest (maximum

annual rates are 0.3–0.4 mm day21 decade21).

7. Summary and concluding remarks

The analysis of the Mediterranean water cycle vari-

ability presented here provides new insights on the long-

term changes that have affected the region since the

1960s. Sea evaporation significantly increased since the

mid-1970s (0.1–0.2 mm day21 decade21) with a ten-

dency toward higher rates of increase during the 1990s.

According to OAFlux, this long-term increase followed

a period of evaporation decrease during 1965–75, but

overall evaporation has increased since the 1960s by

about 10% in total (0.06 mm day21 decade21). Much of

the evaporation increase since the mid-1970s has been in

early winter, especially in the Ligurian Sea, Adriatic

Sea, and southeastern Mediterranean. Increases in SSTs

have primarily driven these evaporation changes via

changes in the surface humidity gradient. Based on

OAFlux data, the estimated Mediterranean mean rate

of evaporation change in relation to the warming is

;0.7 mm day21 K21 (or 25% K21) over the period of

1958–2006 (similarly during 1979–2006). Significant in-

terdecadal variations characterized precipitation dur-

ing the period of 1979–2006. Most noticeable is the

well-know precipitation decrease during the 1980s, which

has been attributed to the regional influence of the

North Atlantic Oscillation (e.g., Hurrell 1995). Average

Mediterranean Sea precipitation decreased during the

period of 1958–2006 (0.03–0.04 mm day21 decade21)

while there is no annual precipitation trend during

1979–2006. The combination of the evaporation and

precipitation changes described above resulted in sub-

stantial long-term changes in Mediterranean Sea surface

freshwater fluxes with an overall increase estimated in

the range of 0.1–0.3 mm day21 decade21 during the

period of 1979–2006. This increase affected the Ligurian

Sea in particular, the Adriatic and parts of the south-

eastern Mediterranean, and especially the October–

March means. Concerning the longer 1958–2006 period,

OAFlux/REOFS datasets give an E 2 P decrease during

the period from 1958 to the mid-1970s, but indicate an

overall mean increase in E 2 P since 1958 of 0.1 mm

day21 decade21 (;0.5 mm day21 in total).

FIG. 7. Decadal variations in Mediterranean mean precipitation

over the period of 1958–2007. Shown are 6-yr running means of

area-averaged precipitation anomalies relative to the period 1988–

2000 (lines, mm day21) from various observational sources (see

legend). PRECL, CRU, and GHCN are land-only averages for the

region surrounding the Mediterranean Sea; REOFS, GPCP, and

REMSS are Mediterranean Sea–only averages. For GPCP, annual

mean values are also shown (symbols). Running mean averages

based on CMIP3 simulations are also displayed (note different

scale at right).

15 MARCH 2010 M A R I O T T I 1521

The dataset intercomparison highlights qualitative

agreement among the datasets on the long-term be-

havior described above, albeit large uncertainties on the

quantitative estimates of the rate of change. These un-

certainties are less well defined for the presatellite era

because fewer data sources are available for compari-

son. Among the caveats is that many of the analyzed

datasets, and especially the satellite-based ones, share

common data sources, although data are often diversely

processed and combined (see section 2). Other datasets

(e.g., OAFlux) instead use reanalyses for their deriva-

tion, and hence are not purely observational. At a global

level, the OAFlux-derived evaporation increase since

1958 (;30% K21) is quite high (Stephens and Ellis 2008;

Takahashi 2009; Vecchi and Soden 2007; Wentz et al.

2007). However, over the 1979–2006 period, OAFlux

global evaporation rates are similar to those of NOCS

evaporation and global oceanic precipitation rates by

GPCP (L. Yu 2009, personal communication). At a

regional level, the changes in Mediterranean Sea evap-

oration described here reflect the Mediterranean Sea

warming observed in recent decades. Similarly, an in-

crease in Mediterranean Sea surface freshwater loss

is broadly consistent with previous studies suggesting

an increase in the salinity of various components of

the Mediterranean Sea system (e.g., Millot et al. 2006;

Potter and Lozier 2004; Rixen et al. 2005). For instance,

Rixen et al. (2005) estimate a mean Mediterranean

salinity increase of 0.03–0.04 psu over the period of

1950–2000. A simplified form of the salt conservation

equation, neglecting all dynamical terms, indicates that

an increase in freshwater loss of 0.06–0.15 mm day21

decade21 (0–0.09 mm day21 decade21) during 1958–

2006 (1958–2000) would result in a 0.07–0.15 psu (0.-0.08

psu) increase in mean Mediterranean salinity. While

other studies have emphasized the effects of decreased

precipitation and river discharge on observed salin-

ity changes (Krahmann 1998; Rohling and Bryden

1992; Skliris et al. 2007), our results suggest an im-

portant role for evaporation increase, which needs to

be further investigated. Similarly, underlying causes

and broader implications of the water cycle changes

FIG. 8. Same as Fig. 5, but trends are for GPCP precipitation data.

1522 J O U R N A L O F C L I M A T E VOLUME 23

described here also need to be explored in future

work.

Results from a large ensemble of CMIP3 model

simulations are in qualitative agreement with the ob-

servational results. However, simulated water cycle

anomalies are about one order of magnitude smaller

than that observed despite the fact that CMIP3 SST

anomalies are comparable to the observations. A muted

response of surface evaporation and precipitation to

warming in global climate simulations compared to

observations has previously been noted (Richter and

Xie 2008; Wentz et al. 2007). Internal decadal climate

variability (e.g., related to NAO variability), not cap-

tured by the CMIP3 ensemble means, also contributes

to discrepancies between CMIP3 and observations and

needs to be further investigated. CMIP3 projections for

the twenty-first century indicate that the Mediterranean

Sea loss of freshwater will accelerate in future decades

(Mariotti et al. 2008). Confidence in these projections

will depend on our ability to reconcile differences be-

tween observations and simulations, better defining past

observed changes and improving our understanding and

simulation capability. Long-term monitoring is needed

to ensure that future long-term changes in the Medi-

terranean water cycle do not go undetected.

FIG. 9. Same as Fig. 2, but for E 2 P fluxes area-averaged over

the Mediterranean Sea. Annual E 2 P means based on OAFlux

and GPCP data are also shown (symbols).

FIG. 10. Same as Fig. 5, but trends are for E 2 P fluxes based on OAFlux and GPCP datasets.

15 MARCH 2010 M A R I O T T I 1523

Acknowledgments. The author would like to thank

Rong-Hua Zhang, Jin-Ho Yoon, Ning Zeng, Lisan Yu,

Lucrezia Ricciardulli, and Volgango Rupolo for their

valuable input and the two anonymous reviewers for

their comments; the modeling groups, the Program

for Climate Model Diagnosis and Intercomparison

(PCMDI) and the WCRP’s Working Group on Coupled

Modeling (WGCM) for their roles in making available

the WCRP CMIP3 multimodel dataset. Support of this

dataset is provided by the Office of Science, U.S. De-

partment of Energy. The author thankfully acknowl-

edges all those who provided data for this study and the

EC for financial support under CIRCE Integrated Pro-

ject (Contract 036961).

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