302
J.P. Bethoux
2 Climatic and Environmental Forcing
2.1 Temperature and Salinity Trends in Deep Waters, and
Corresponding Surface Trends
In the Algero-Proven9al basin, when the temperature and salinity characteristics
were considered as constant from the early century, the accurate measurements made
since year 1959 show continuous increases in temperature and salinity. At depth
greater than 2000m, over the 1959-2000 period, the annual trends are: 3.4xlO- 3 cC
for potential temperature and 1.05x10 3 for salinity, i.e., increases of 0.14 'c and
0.043 salinity. More than the last deep layer, these increasing trends concern the
whole deep water column, since about 800m depth. As the deep water originates
from the winter dense water formation occurring at the sea surface, the trends
originate in a multi-decennial evolution in surface temperature and in air-sea
exchanges, the origin of trends stands before year 1944 due to a residence time of
about 15 years. In the intermediate water (originating from the eastern basin), the
trends appears to be about 2 to 3 times those in the deep layer (Sparnoccia et aI.,
1994; Bethoux and Gentili, 1996), but space and time changes of characteristics
complicate the determination of mean trends. In the surface layer, the inter-annual
variability of temperature and salinity characteristics are rather high, but in a few
places, more or less long time series allow a first determination of surface trends.
Over the last decades, temperature trends were found comprised between 9-15xlO- 3
'C yr-l at Villefranche (French Riviera) in the 0-75m layer, and at Medes Islands
(Spanish coast, north of Barcelona) the trend was equal to 26xlO- 3 C y-l in the 080m layer. In a 20 box-model, the deep water trends were used to calculate the
surface trends in different parts of the Mediterranean Sea (Bethoux and Gentili,
1999). These calculated annual trends for the surface layer stand between 0.9 and
13.8xlO- 3 C, and 0.2 and 5.4xlO- 3 for the salinity. The lowest values correspond to
the Alboran Sea area where the inflowing Atlantic waters smooth the local climatic
effect, the highest values concerns the surface layer of the Aegean Sea, i.e., the
oldest surface waters due to the Mediterranean circulation. Changes in the dense
water formation and circulation in the Aegean Sea in early 1990 (Rother et aI.,
1996, Della Vedova et aI., 1997) result from major temperature and salinity changes
in this area and confirm the previous calculations.
From deep water trends in the western basin, it was calculated the corresponding
surface changes in heat and water budgets capable to explain the deep trends. Over
the whole Mediterranean, it was calculated a change in surface heat budget of 1.5
Wm- 2 and a change in freshwater budget of 0.1 m. The change in heat budget was
linked, via the air and sea surface temperature changes, to an increase in greenhouse
effect of 1. 7 W m- 2 over the 1940-1995 period (Bethoux et aI., 1998). This estimate
from marine data corresponds to the calculation from the changes in the atmospheric
content of radiative gas (IPCC, 1995). The change in freshwater budget (about
O.lm) was linked to the increase of anthropic use offreshwater from rivers (e.g., 4
J.P. Bethoux
2 Climatic and Environmental Forcing
2.1 Temperature and Salinity Trends in Deep Waters, and
Corresponding Surface Trends
In the Algero-Proven9al basin, when the temperature and salinity characteristics
were considered as constant from the early century, the accurate measurements made
since year 1959 show continuous increases in temperature and salinity. At depth
greater than 2000m, over the 1959-2000 period, the annual trends are: 3.4xlO- 3 cC
for potential temperature and 1.05x10 3 for salinity, i.e., increases of 0.14 'c and
0.043 salinity. More than the last deep layer, these increasing trends concern the
whole deep water column, since about 800m depth. As the deep water originates
from the winter dense water formation occurring at the sea surface, the trends
originate in a multi-decennial evolution in surface temperature and in air-sea
exchanges, the origin of trends stands before year 1944 due to a residence time of
about 15 years. In the intermediate water (originating from the eastern basin), the
trends appears to be about 2 to 3 times those in the deep layer (Sparnoccia et aI.,
1994; Bethoux and Gentili, 1996), but space and time changes of characteristics
complicate the determination of mean trends. In the surface layer, the inter-annual
variability of temperature and salinity characteristics are rather high, but in a few
places, more or less long time series allow a first determination of surface trends.
Over the last decades, temperature trends were found comprised between 9-15xlO- 3
'C yr-l at Villefranche (French Riviera) in the 0-75m layer, and at Medes Islands
(Spanish coast, north of Barcelona) the trend was equal to 26xlO- 3 C y-l in the 080m layer. In a 20 box-model, the deep water trends were used to calculate the
surface trends in different parts of the Mediterranean Sea (Bethoux and Gentili,
1999). These calculated annual trends for the surface layer stand between 0.9 and
13.8xlO- 3 C, and 0.2 and 5.4xlO- 3 for the salinity. The lowest values correspond to
the Alboran Sea area where the inflowing Atlantic waters smooth the local climatic
effect, the highest values concerns the surface layer of the Aegean Sea, i.e., the
oldest surface waters due to the Mediterranean circulation. Changes in the dense
water formation and circulation in the Aegean Sea in early 1990 (Rother et aI.,
1996, Della Vedova et aI., 1997) result from major temperature and salinity changes
in this area and confirm the previous calculations.
From deep water trends in the western basin, it was calculated the corresponding
surface changes in heat and water budgets capable to explain the deep trends. Over
the whole Mediterranean, it was calculated a change in surface heat budget of 1.5
Wm- 2 and a change in freshwater budget of 0.1 m. The change in heat budget was
linked, via the air and sea surface temperature changes, to an increase in greenhouse
effect of 1. 7 W m- 2 over the 1940-1995 period (Bethoux et aI., 1998). This estimate
from marine data corresponds to the calculation from the changes in the atmospheric
content of radiative gas (IPCC, 1995). The change in freshwater budget (about
O.lm) was linked to the increase of anthropic use offreshwater from rivers (e.g., 4