The salinity of the inflow in Gibraltar decreases
(-9.0·10-3 psuyr-1) due to the
freshening of the inflowing Atlantic waters. This fresher Atlantic water
is the main contributor to the surface freshening in the GoL. Otherwise,
the outflow waters become saltier (+2.0 ·10-3psuyr-1) because of the salinization of LIW and deeper
waters as shown in Figure 3, thus the salt transport through the Strait
to Atlantic Ocean increases (Table 1).
4 Discussion and Conclusions
The response of the NWMed deep water formation to climate change has
been explored in Soto-Navarro et al. (2020). The authors found in an
ensemble of six Regional Climate Models a robust dramatic decrease of
DWF by the end of the 21st century. However, the mechanisms involved
were not revealed. As the time evolution of the DWF is strongly model
dependent, single model runs can be used to identify in a physically
consistent way the causes of this collapse. Therefore, we study these
mechanisms in one of the simulations used in Soto-Navarro et al. (2020).
Specifically, we analyze the simulation with the regionally coupled
system ROM under the RCP8.5 scenario.
Our simulation is able to reproduce the several DWF episodes during the
present period: deep convection occurs every year from 2009 to 2014
(Figure 2a and Table S1), what is statistically in good agreement with
observations (Houpert et al., 2016; Somot et al., 2018; Margirier et
al., 2020).
Starting from the early 2040s, we find a dramatic reduction of
MLDmax in the GoL (Figure 2a). The maximum MLD for the
present climate (1976-2005) is 2385 ± 630 m while for 2070-2099 under
the RCP8.5 scenario is 297± 47 m. That is, the MLDmaxsimulated by ROM experiences a reduction of almost 90% in the GoL.
These values are in close agreement with values reported in Soto-Navarro
et al. (2020).
Exploring the possible mechanisms (t. e. air-sea fluxes and ocean
preconditioning) that lead to this dramatic MLDmaxdecrease we find that the air-sea fluxes do not seem to be the factor
responsible for the DWF collapse, as the buoyancy loss does not change
significantly (Figure 2b). Changes in the buoyancy loss also cannot
explain the changes in DWF strength, as strong or weak DWF events can
happen with similar BL values. In agreement with Somot et al. (2018),
our results show that in any case no deep water is formed
(MLDmax < 1000 m) when BL < 0.6
m2s-2 during the 2006-2013 period.
Our results indicate that the changes in properties of the upper and
intermediate water masses affecting the ocean preconditioning are key in
the DWF collapse. In our simulation, the temperatures of MAW and LIW are
projected to increase 2.6ºC and 2.3ºC, respectively (Figure 3a and 3b).
In turn, the salinity decreases slightly (-0.01 psu) on the surface,
while deeper waters become saltier (Figure 3c and 3d). As shown
previously in Parras-Berrocal et al. (2020), the warming and freshening
signal of MAW comes to the GoL from the Eastern Atlantic, while the
increase of LIW temperature and salinity is propagated from its
formation area in the Eastern Mediterranean. These changes increase the
vertical density gradient between the MAW and LIW, strongly reducing the
vertical mixing between these water masses. This is reflected in the
stratification index for the 0-1000 m water column (Figure 3e), which
from 2040s onward experiences a positive trend (0.02
m2s-2y-1). Our
results do not show a significant change in atmospheric fluxes, changes
in MAW and LIW characteristics play the main role in the DWF collapse in
the NWMed. The recent study of Margirier et al. (2020) lends some
support to our hypothesis: they found in glider and other platforms
profiles collected over 2007-2017 that an abrupt jump of LIW temperature
and salinity provoked a strong reduction of vertical mixing in the NWMed
in 2014. The authors concluded that under those conditions, stronger
atmospheric forcing is needed to trigger deep convection. Amitai et al.
(2021) have also recently demonstrated that LIW characteristics play a
key role in enabling or disabling the deep convection in the GoL. In
agreement with Margirier et al. (2020) and Amitai et al. (2021), our
results indicate that the projected deep water formation collapse in the
21st century is controlled by the change in LIW characteristics, but we
found also that changes in MAW properties play a role. In order to
assess the relative contribution of LIW and MAW to the deep water
formation collapse we compare the SI calculated from spatially and
temporally averaged vertical profiles in the GoL in four cases (Figure
S2): (i) using the values corresponding to the pre-collapse period
(2006-2041); (ii) using the values corresponding to the post-collapse
period (2070-2099); and creating additionally two synthetic profiles,
(iii) one containing the pre-collapse characteristics of MAW (0-200 m
depth) with the post-collapse properties of deeper layers (200-1000 m
depth; Figure S2g), and (iv) a second one with the post-collapse MAW and
pre-collapse deeper layers. As seen in Figure S2, there is a clear
increase in the post-collapse SI (see also Fig. 3e), but interestingly
the SI for the (iv) situation (1.20
m2s-2, Figure S2h) is greater than
for the (iii) (0.97 m2s-2, Figure
S2g). This results suggest that in the future the change in MAW
properties will play a role at least as relevant as LIW in the deep
water formation collapse. This collapse of NWMed deep water formation
seems to have an impact on the ventilation and on the thermohaline
circulation of the Mediterranean Sea.
Both inflow and outflow water transport at the Strait of Gibraltar show
a decreasing trend which is larger in the outflow, increasing the net
flow (Table 1). Moreover, the incoming surface Atlantic jet will
transport more heat and less salt into the Mediterranean basin, causing
the hydrographic changes of surface waters (MAW) in the GoL represented
in Figure 3. At the same time, the salinization of intermediate (LIW) in
the Mediterranean Sea contributes to the increase of MO salinity. As a
result, the MO will be warmer and saltier by the end of the 21st
century. During the period with DWF episodes (2006-2041) the warming of
MO is gradual and does not present noticeable changes, however after the
collapse of DWF it accelerates reaching an increase rate of
0.034ºCyr-1 (Figure S1). Then, the collapse of DWF
could be one of the driving factors of the changes in characteristics of
fluxes at Gibraltar Strait which reflects changes in the Mediterranean
overturning circulation at large scale.
Acknowledgments and Data
I. M. Parras-Berrocal, O. Álvarez and M. Bruno were supported by the
Spanish National Research Plan through project TRUCO
(RTI2018-100865-BC22). R. Vázquez was supported through a doctoral grant
at the University Ferrara and University of Cádiz. W. Cabos have been
funded by the Spanish Ministry of Science, Innovation and Universities,
the Spanish State Research Agency and the European Regional Development
Fund, through grant CGL2017-89583-R. D. Sein was supported in the
framework of the state assignment of the Ministry of Science and Higher
Education of Russia (0128-2021-0014). The model data are available
online (at https://doi.org/10.5281/zenodo.5151396).
The authors declare that they have no conflict of interest.
References
Adloff, F., Somot, S., Sevault, F., Jordà, G., Aznar, R., Déqué, M.,
Herrmann, M., Marcos, M., Dubois, C., Padorno, E., & Alvarez-Fanjul, E.
(2015). Mediterranean Sea response to climate change in an ensemble of
twenty first century scenarios. Climate Dynamics, 45, 2775–2802.
https://doi.org/10.1007/s00382-015-
2507-3
Amitai. Y., Ashkenazy, Y., & Gildor, H. (2021). The Effect of the
Source of Deep Water in the Eastern Mediterranean on Western
Mediterranean Intermediate and Deep Water. Frontiers in Marine Science,
7:615975. https://doi.org/10.3389/fmars.2020.615975
Bethoux, J.P., & Gentili, B. (1999). Functioning of the Mediterranean
Sea: past and present changes related to freshwater input and climate
changes. Journal of Marine Systems, 20, 33–47.
https://doi.org/10.1016/S09247963(98)00069-4
Darmaraki, S., Somot, S., Sevault, F., Nabat, P., Cabos Nar- vaez, W.
D., Cavicchia, L., Djurdjevic, V., Li, L., Sannino, G., & Sein, D. V.
(2019). Future evolution of Marine Heatwaves in the Mediterranean Sea,
Climate Dynamics, 53, 1371–1392.
https://doi.org/10.1007/s00382-019-04661-z
D’Ortenzio, F., Iudicone, D., de Boyer Montegut, C., Testor, P.,
Antoine, D., Marullo, S., Santoreli, R., & Madec, G. (2005). Seasonal
variability of the mixed layer depth in the mediterranean sea as derived
from in situ profiles. Geophysical Research Letter, 32, L12605.
https://doi.org/10.1029/2005GL022463
Durrieu de Madron, X., Houpert, L., Puig, P., Sanchez-Vidal, A., Testor,
P., Bosse, A., Estournel, C., Somot, S., Bourrin, F., Bouin, M. N.,
Beauverger, M., Beguery, L., Calafat, A., Canals, M., Cassou, C.,
Coppola, L., Dausse, D., D’Ortenzio, F., Font, J., Heussner, S.,
Kunesch, S., Lefevre, D., Goff, H. L., Martin, J., Mortier, L.,
Palanques, A., & Raimbault, P. (2013). Interaction of dense shelf water
cascading and open-sea convection in the Northwestern Mediterranean
during winter 2012. Geophysical Research Letter, 40, 1379–1385.
https://doi.org/10.1002/grl.50331
Giorgetta, M. A., Jungclaus, J., Reick, C. H., Legutke, S., Bader, J.,
Böttinger, M., Brovkin, V., Crueger, T., Esch, M., Fieg, K., Glushak,
K., Gayler, V., Haak, H., Hollweg, H.-D., Ily- ina, T., Kinne, S.,
Kornblueh, L., Matei, D., Mauritsen, T., Mikolajewicz, U., Mueller, W.,
Notz, D., Pithan, F., Raddatz, T., Rast, S., Redler, R., Roeckner, E.,
Schmidt, H., Schnur, R., Segschneider, J., Six, K. D., Stockhause, M.,
Timmreck, C., Wegner, J., Widmann, H., Wieners, K.-H., Claussen, M.,
Marotzke, J., & Stevens, B. (2013). Climate and carbon cycle changes
from 1850 to 2100 in MPI-ESM simulations for the Coupled Model
Intercomparison Project phase 5. Journal of Advances in Modelling Earth
Systems, 5, 572–597, https://doi.org/10.1002/jame.20038
Giorgi, F. (2006). Climate change hot-spots. Geophysical Research
Letter, 33, L08707. https://doi.org/10.1029/2006GL025734
Hagemann, S., & Dümenil-Gates, L. (1998). A parameterization of the
lateral waterflow for the global scale Climate Dynamics, 14, 17–31.
https://doi.org/10.1007/s003820050205
Hagemann, S. & Dümenil-Gates, L. (2001). Validation of the hydrological
cycle of ECMWF and NCEP reanalysis using the MPI hydrological discharge
model. Journal Geophysical Research, 106, 1503–1510.
https://doi.org/10.1029/2000JD900568
Herrmann, M., Sevault, F., Beuvier, J., & Somot, S. (2010). What
induced the exceptional 2005 convection event in the northwestern
Mediterranean basin? Answers from a modeling study. Journal Geophysical
Research: Oceans, 115(C12)051. https://doi.org/10.1029/2010JC006162
Hibler, W. D. (1979). A dynamic thermodynamic sea ice model. Journal of
Physical Oceanography, 9, 815–846. https://doi.org/10.1175/1520-
0485(1979)009<0815:ADTSIM>2.0.CO;2
Houpert, L., Durrieu de Madron, X., Testor, P., Bosse, A., D’Ortenzio,
F., Bouin, M. N., Dausse, D., Le Goff, H., Kunesch, S., Labaste, M., &
Coppola, L. (2016). Observations of open‐ocean deep convection in the
northwestern Mediterranean Sea: Seasonal and interannual variability of
mixing and deep water masses for the 2007‐2013 period. Journal of
Geophysical Research: Oceans, 121(11), 8139-8171.
https://doi.org/10.1002/2016JC011857
IPCC. (2013). Climate Change 2013: The Physical Science Basis.
Contribution of Working Group I to the Fifth Assessment Report of the
Intergovernmental Panel on Climate Change (Stocker TF, Qin D, Plattner
G-K, Tignor M, Allen SK, Boschung J, Nauels A, Xia Y, Bex V and Midgley
PM (eds)). Cambridge University Press, Cambridge and New York, pp 1535
Jacob, D. (2001). A note to the simulation of the annual and in-
terannual variability of the water budget over the Baltic Sea drainage
basin. Meteorology and Atmospheric Physics, 77, 61–73.
https://doi.org/10.1007/s007030170017
Jungclaus, J. H., Fischer, N., Haak, H., Lohmann, K., Marotzke, J.,
Matei, D., Mikolajewicz, U., Notz, D., & von Storch, J. S. (2013).
Characteristics of the ocean simulations in MPIOM, the ocean component
of the MPI-Earth system model. Journal of Advances in Modelling Earth
Systems, 5, 422–446. https://doi.org/10.1002/jame.20023
Lascaratos, A., Williams, R., & Tragou, E. (1993). A mixed-layer study
of the formation of Levantine Intermediate Water. Journal Geophysical
Research, 98(C8), 14739–14749. https://doi.org/10.1029/93JC00912
Leaman, K., & Schott, F. (1991). Hydrographic structure of the
convection regime in the Gulf of Lions: Winter 1987. Journal of Physical
Oceanography, 21(4), 575–598.
Léger, F., Lebeaupin Brossier, C., Giordani, H., Arsouze, T., Beuvier,
J., Bouin, M.-N., Bresson, E., Ducrocq, V., Fourrié, N., & Nuret, M.
(2016). Dense water formation in the north- western Mediterranean area
during HyMeX-SOP2 in 1/36° ocean simulations: Sensitivity to initial
conditions. Journal Geophysical Research: Oceans, 121(8), 5549–5569.
https://doi.org/10.1002/2015JC011542
L’Hévéder, B. Li, L. Sevault, F. & Somot, S. (2013). Interannual
variability of deep convection in the Northwestern Mediterranean
simulated with a coupled AORCM. Climate Dynamics, 41(3–4), 937–960.
https://doi.org/10.1007/s00382-012-1527-5
Maier-Reimer, E., Kriest, I., Segschneider, J., & Wetzel, P. (2005).
The HAMburg Ocean Carbon Cycle Model HAMOCC5.1 Technical Description
Release 1.1, Ber. Erdsystemforschung, 14, available at:
http://hdl.handle.net/11858/00-001M-0000-0011-FF5C-D
Margirier, F., Testor, P., Heslop, E., Mallil, K., Bosse, A., Houpert,
L., Mortier, L., Bouin, M.-B., Coppola, L., D’Ortenzio, F., Durrie de
Madron, X., Mourre, B., Prieur, L., Raimbault, P. & Taillandier, V.
(2020). Abrupt warming and salinification of intermediate waters
interplays with decline of deep convection in the Northwestern
Mediterranean Sea. Scientific Reports 10, 20923.
https://doi.org/10.1038/s41598-020-77859-5
Marshall, J., & Schott, F. (1999). Open-ocean convection: observations,
theory, and models. Reviews of Geophysics, 37(1), 1–64.
https://doi.org/10.1029/98RG02739
Marsland, S. J., Haak, H., Jungclaus, J. H., Latif, M., & Roeske, F.
(2003). The Max-Planck- Institute global ocean/sea ice model with
orthogonal curvilinear coordinates. Ocean Modelling, 5 (2), 91–127,
https://doi.org/10.1016/S1463-5003(02)00015-X
MEDOC Group. (1970). Observations of formation of deep-water in the
Mediterranean Sea. Nature, 227, 1037–1040.
https://doi.org/10.1038/2271037a0
Menna, M. & Poulain, P. M. (2010). Mediterranean intermediate
circulation estimated from Argo data in 2003–2010. Ocean Science, 6,
331–343. https://doi.org/10.5194/os-6-331-2010
Mertens, C., & Schott, F. (1998). Interannual variability of deep-water
formation in the northwestern Mediterranean. Journal of Physical
Oceanography, 28, 1410–1423,
https://doi.org/10.1175/1520-0485(1998)028<1410:IVODWF>2.0.CO;2
Mikolajewicz, U., Sein, D. V., Jacob, D., Königk, T., Podzun, R. &
Semmler, T. (2005). Simulating Arctic sea ice variability with a coupled
regional atmosphere-ocean-sea ice model. Meteorologische Zeitschrift, 14
(6), 793–800. https://doi.org/10.1127/0941-2948/2005/0083
Millot, C. (1999). Circulation in the Western Mediterranean Sea, Journal
of Marine Systems, 20 (1-4), 423-440.
https://doi.org/10.1016/S0924-7963(98)00078-5
Millot, C. (2005). Circulation in the mediterranean sea: Evidences,
debates and unanswered questions, Scientia Marirna, 69, 5–21.
https://doi.org/10.3989/scimar.2005.69s15
Millot, C. (2014). Levantine Intermediate Water characteristics: an
astounding general misunderstanding! (addendum), Scientia Marina, 78(2),
165–171. https://doi.org/10.3989/scimar.04045.30H
Parras-Berrocal, I. M., Vazquez, R., Cabos, W., Sein, D., Mañanes, R.,
Perez-Sanz, J., & Izquierdo, A. (2020). The climate change signal in
the Mediterranean Sea in a regionally coupled atmosphere–ocean model,
Ocean Science, 16, 743–765. https://doi.org/10.5194/os-16-743-2020
Rechid, D., & Jacob, D.: Influence of monthly varying vegetation on the
simulated climate in Europe. Meteorologische Zeitschrift, 15(1),
99–116. https://doi.org/10.1127/0941-2948/2006/0091
Rhein, M. (1995). Deep water formation in western Mediterranean. Journal
of Geophysical Research, 100-C4, 6943-6959.
https://doi.org/10.1029/94JC03198
Robinson, A., Leslie, W., Theocharis, A., & Lascaratos, A. (2001).
Encyclopedia of ocean sciences. Academic Press Ltd., London chap
Mediterranean Sea Circulation
Sánchez-Gómez, E., Somot, S., Josey, S. A., Dubois, C., Elguindi, N., &
Déqué, M. (2011). Evaluation of Mediterranean Sea water and heat budgets
simulated by an ensemble of high resolution regional climate models.
Climate Dynamics, 37, 2067–2086.
https://doi.org/10.1007/s00382-011-1012-6
Schott, F., & Leaman, K. D. (1991). Observations with Moored Acoustic
Doppler Current Profilers in the Convection Regime in the Golfe du Lion.
Journal of Physical Oceanography, 21, 558–574,
https://doi.org/10.1175/1520-
0485(1991)021<0558:OWMADC>2.0.CO;2
Sein, D. V., Mikolajewicz, U., Gröger, M., Fast, I., Cabos, W., Pinto,
J. G., Hagemann, S., Semmler, T., Izquierdo, A., & Jacob, D. (2015).
Regionally coupled atmosphere-ocean- sea ice-marine biogeochemistry
model ROM: 1. Description and validation, Journal of Advance in
Modelling Earth Systems, 7(1), 268–304.
https://doi.org/10.1002/2014MS000357
Sein, D. V., Gröger, M., Cabos, W., Alvarez-Garcia, F. J., Hagemann, S.,
Pinto, J. G., Izquierdo, A., de la Vara, A., Koldunov, N. V., Dvornikov,
A. Y., Limareva, N., Alekseeva, E., Martinez-Lopez, B. & Jacob, D.
(2020). Regionally coupled atmosphere-ocean- sea ice-marine
biogeochemistry model ROM: 2. Studying the Climate Change Signal in the
North Atlantic and Europe, Journal of Advance in Modelling Earth
Systems, 12, e2019MS001646. https://doi.org/10.1029/2019Ms001646
Seyfried, L., Marsaleix, P., Richard, E., & Estournel, C. (2017).
Modelling deep-water formation in the north-west Mediterranean Sea with
a new air–sea coupled model: sensitivity to turbulent flux
parameterizations. Ocean Science, 13, 1093–1112.
https://doi.org/10.5194/os-13-1093-2017
Shaltout, M., & Omstedt, A. (2014). Recent sea surface temperature
trends and future scenarios for the Mediterranean Sea. Oceanologia, 56,
411–443. https://doi.org/10.5697/oc.56-3.411
Somot, S., Sevault, F., & Déqué, M. (2006). Transient climate change
scenario simulation of the Mediterranean Sea for the 21st century using
a high-resolution ocean circulation model. Climate Dynamics, 27,
851–879. https://doi.org/10.1007/s00382-006-0167-z
Somot, S., Sevault, F., Déqué, M., & Crépon, M. (2008). 21st century
climate change scenario for the Mediterranean using a coupled
atmosphere–ocean regional climate model. Global Planetary Change, 63,
112–126. https://doi.org/10.1016/j.gloplacha.2007.10.003
Somot, S., Houpert, L., Sevault, F., Testor, P., Bosse, A.,
Taupier-Letage, I., Bouin, M. N., Waldman, R., Cassou, C.,
Sanchez-Gomez, E., Durrieu de Madron, X., Adloff, F., Nabat, P. &
Herrman, M. (2018). Characterizing, modelling and understanding the
climate variability of the deep water formation in the North-Western
Mediterranean Sea. Climate Dynamics, 51, 1179–1210.
https://doi.org/10.1007/s00382-016-3295-0
Soto-Navarro, J., Jordá, G., Amores, A., Cabos, W., Somot, S., Sevault,
F., Macias, D., Djurdjevic, V., Sannino, G., Li, L. & Sein, D. (2020).
Evolution of Mediterranean Sea water properties under climate change
scenarios in the Med-CORDEX ensemble. Climate Dynamics, 54, 2135–2165.
https://doi.org/10.1007/s00382-019-05105-4
Taylor, K., Stouffer, R., & Meehl, G. (2012). An overview of CMIP5 and
the experiments design. Bulletin of the American Meteorological Society,
93, 485–498. https://doi.org/10.1175/BAMS-D-11-00094.1
Valcke, S. (2013). The OASIS3 coupler: a European climate modelling
community software. Geoscientific Model Development, 6, 373–388.
https://doi.org/10.5194/gmd-6-373-2013
Turner, J. (1973). Buoyancy effects in fluids: Cambridge monographs on
mechanics and applied mathematics. Cambridge University Press, Cambridge
Vargas-Yáñez, M., Zunino, P., Schroeder, K., López-Jurado, J. L., Plaza,
F., Serra, M., Castro, C., García-Martínez, M. C., Moya, F. & Salat, J.
(2012). Extreme Western Intermediate Water formation in winter 2010.
Journal of Marines Systems, 105-10, 52-59.
https://doi.org/10.1016/j.jmarsys.2012.05.010
Waldman, R., Somot, S., Herrmann, M., Bosse, A., Caniaux, G., Estournel,
C., Houpert, L., Prieur, L., Sevault, F., & Testor, P. (2017).
Modelling the intense 2012–2013 dense water formation event in the
northwestern Mediterranean Sea: Evaluation with an ensemble a simulation
approach. Journal of Geophysical Research: Oceans, 122, 1297–1324.
https://doi.org/10.1002/2016JC012437
Waldman, R., Somot, S., Herrmann, M., Sevault, F., & Isachsen, P. E.
(2018). On the Chaotic Variability of Deep Convection in the
Mediterranean Sea. Geophysical Research Letters, 45,2433-2443.
https://doi.org/10.1002/2017GL076319
Wetzel, P., Haak, H., Jungclaus, J., & Maier-Reimer, E. (2004). The
Max-Planck-institute global ocean/sea ice model. Model MPI-OM Technical
report. http://www.mpimet.mpg.de/fileadmin/models/
MPIOM/DRAFT_MPIOM_TECHNICAL_REPORT.pdf