The transient-simulated interannual climate variability in the Mediterranean Sea during 1901-2010

Past climate and biogeochemical changes can be derived from sediment records through transfer functions. Traditionally, these transfer functions are constructed by evaluating the spatial relationships between the present-day sediment proxy data and environmental parameters. This approach is based on the assumption that temporal variations leave the same signature in the proxy as spatial variations. Usually this cannot be shown to be the case due to the coarse temporal resolution of the proxy records and the shortness of the instrumental observations.

Here we propose to infer a novel spatio-temporal transfer function based on the model simulation in the Mediterranean Sea during the present-day time slice (1901-2010). As a first step, we analyze the model results in order to obtain a clear understanding of the interannual variability of the physical states and the related biogeochemical cycles and to describe how this variability is recorded in marine sediments. The analysis of the sediment variations is focused on the Gulf of Taranto. Because there are several sediment cores with a sub-decadal resolution that are available for model validation[1]

We use a high-resolution regional ocean circulation and biogeochemistry model (MPIOM-HAMOCC) with an advanced sediment component in the Mediterranean Sea and the Black Sea (Fig.1). The horizontal resolution is approx. 9 km and vertically there are 30 layers. The model is run for the period of 1901-2010, driven by the newly generated long-term atmospheric reanalysis forcing ERA20C[2], transient river input (discharge and nutrient load)[3] and atmospheric nitrogen deposition[4]. A sponge zone with restoring to monthly climatology is applied at the western margin of the Atlantic box.

                                    

Fig.1 Model domain and the bathymetry. Adr.: Adriatic Sea, Ion.: Ionian Sea, Lev.: Levantine Basin. Black box represents the Gulf of Taranto. Black line represents the Strait of Otranto

Adriatic Dense Water (AdDW) outflow 

The Adriatic Dense Water (AdDW) is the major source of deep water formation in the eastern Mediterranean Sea. The meridional overturning stream function across the Adriatic and the northern Ionian Sea is used to assess the spreading of the AdDW. As compared with the 110-year average condition (Fig.2a), a larger amount of AdDW is formed and the transport is strongly enhanced during a cold winter year (1987). The location of the maximum transport deepens from ~300m to ~1000m, allowing the outflow of AdDW to reach the bottom layers of the northern Ionian Sea (Fig.2b).

Fig.2(c) shows that the AdDW outflow has a significant interannual variability. Transport occurring below 1000m is referred as a strong AdDW outflow event, which is only found in selective years with cold winters.
                                      

Fig.2 Meridional overturning stream function integrated across the Adriatic and the northern Ionian Sea calculated on a 110-year averaged velocity field (a) and  during one cold winter year (1987) (b). The grey color represents the topography. (c) Hovmöller diagram of the annual mean water transport rate across the Strait of Otranto (black line in Fig. 1) for the 110 years of simulation. Note the nonlinear spacing of the color bar.

Surface circulation in the northern Ionian Sea

The North Ionian Gyre (NIG) changes its upper layer circulation from cyclonic to anti-cyclonic and vice versa on a decadal time scale. The inversion of the circulation leads to the advection of Modified Atlantic Water or of the Levantine/Eastern Mediterranean waters into the Gulf of Taranto as indicated by the current fields and salinity patterns in Fig.3.

The simulated change of the NIG circulation from anti-cyclonic in 1990s to cyclonic in early 2000s is consistent with observations obtained in the past three decades[5].During the time before observations, we found the NIG is dominated by cyclonic circulation over the period of the simulation. Anti-cyclonic circulation occurs only in the years having strong AdDW outflow events.

                                    

Fig.3 Annual mean currents (vectors in m s-1) and salinity (colored field) averaged over the upper 250m depth for the year 1992 and 2001.

Biogeochemical dynamics in the Gulf of Taranto

The primary production is increasing over the simulation period (Fig.4a). This tendency can be attributed to the nutrient increase caused by the anthropogenic input (Fig.4b-c).

Significant decadal and inter-annual variability overlays the general tendency. High primary production takes place when the nutriclines are uplifted (Fig.4b-c). The uplifting of the nutriclines is due to the strong AdDW intrusion into the bottom layers of the Gulf of Taranto (Fig.2c).

                                 

Fig.4 (a) Time series of the yearly maximum vertically integrated primary production averaged over the Gulf of Taranto. (b) and (c) Hovmöller diagrams of the winter nitrate and phosphate concentration in the upper 700m depth of the Gulf of Taranto. Black lines represent the nutriclines (NO3=3mmol m-3, PO4=0.15mmol m-3).

Sediment process dynamics in the Gulf of Taranto

The organic carbon concentration in the surface sediment shows a general increasing trend together with a decadal variability (Fig.5a). It can be partly related to the local productivity (Fig.4a).

Large flux of detritus to the sediments is found in selective years (Fig.5b). Such variation shows the response of sediments to the climate variability and can be used in the construction of the transfer function.
                                

Fig.5 Time series of the annual mean organic carbon concentration in the surface sediment (a) and the detritus sedimentation rate (b) in the Gulf of Taranto. Calculation is based on areas shallower than 500m.

References:

[1] Menke, V.,  Ehrmann, W., Milker, Y., Brzelinski, S., Möbius, J., Mikolajewicz, U., Zolitschka, B., Zonneveld, K., Emeis, K.C., and Schmiedl, G., 2017. Combined North Atlantic and anthropogenic forcing of changes in the marine environments in the Gulf of Taranto (Italy) during the last millennium. Clim. Past, Manuscript submitted for publication.

[2] Poli, P., Hersbach, H., Tan, D., Dee, D., Thepaut, J.N., Simmons, A., Peubey, C., Laloyaux, P., Komori, T., Berrisford, P. and Dragani, R., 2013. The data assimilation system and initial perfor-mance evaluation of the ECMWF pilot reanalysis of the 20th-century assimilating surface observa-tions only (ERA-20C).

[3] Ludwig, W., Dumont, E., Meybeck, M. and Heussner, S., 2009. River discharges of water and nutrients to the Mediterranean and Black Sea: major drivers for ecosystem changes during past and future decades? Progress in Oceanography, 80(3).

[4] Forcing Databases in Support of CMIP6, https://pcmdi.llnl.gov/search/input4mips/

[5] Civitarese, G., Gačić, M., Lipizer, M., and Eusebi Borzelli, G. L., 2010. On the impact of the Bimodal Oscillating System (BiOS) on the biogeochemistry and biology of the Adriatic and Ionian Seas (Eastern Mediterranean). Biogeosciences, 7(12), 3987-3997.