In situ data vs. current satellite products
The L’Estartit sea surface temperature (SST) time series agrees quite well with the SST time series derived from the various satellite-based data sets (Fig. 11), with an RMS difference of consistently just over 0.5 °C (Table 3 and Table 4). This is comparable with the typical minimum uncertainty in satellite SST measurement of 0.3–0.5 °C. The satellite SST is cooler than the L’Estartit SST by 0.1 to 0.2 °C on average. A potential cause for this bias is the diurnal warming of sea temperature: while the satellite data sets are aiming to report the pre-dawn temperature, the L’Estartit measurements are performed mostly in the late morning to noon. We note that the feature resolution (smoothness of the SST field) of the satellite data sets used here varies vastly from around 1 up to 100 km, with the OSTIA and NCEI data sets having a similar feature resolution of around 100 km despite having fairly different grid spacing (Reynolds and Chelton 2010; Chin et al. 2017). The comparison statistics (RMS and bias) are remarkably consistent across feature resolutions up to 25 km, and degrades only by 0.1 to 0.2 °C for the OSTIA and NCEI data sets. This indicates that the temporal variability of the L’Estartit time series is representative of the areal average SST from the surrounding sea.
The good statistical comparison of sea surface temperature values (Table 3 and Table 4) and trends (Table 5) obtained from L’Estartit data set and from available satellite products confirms that the reported sea surface temperature trends are robust. Additionally, their good agreement for such a long-term (more than 5 consecutive years) is a first step towards the cross-validation of these products. That opportunity to validate the multi-decadal trends that can now be observed in satellite data gives more confidence to use these data to model and/or parameterize time changes in oceanic states (which have often assumed to be recurring in order to develop theories, e.g. for spatial patterns).
Comparison with previous and other estimates of climate trends
Most of the articles published within the two last decades show the Western Mediterranean has increased in temperature and salinity during the twentieth century. Most of these articles, however, focused on the intermediate and deep layers; works dealing with changes in the upper layers and shallow waters remain scarce. Vargas-Yáñez et al. (2005) analysed coastal data obtained at several fixed stations and detected strong warming trends in the near surface waters, usually ranging from 0.02 to 0.05 °C/year, up to almost 0.1 °C/year (in the Ligurian sea) during the 1990s. These results (of almost one order of magnitude higher than those found at intermediate and deep waters) contrasted with those from Krahmann and Schott (1998) and Sparnocchia et al. (1994) that found no significant changes in the temperature evolution of the upper layer in the Western Mediterranean since the mid-1970s. More recent works, that included data from satellite infrared data, evidenced the warming of the surface layer, both in the Western and Eastern Mediterranean since the mid-1980s to the first decade of the twenty-first century (Skliris et al. 2012; Nykjaer 2009). The trend estimates in these two papers are close to those obtained from our station. Similarly, Rixen et al. (2005), using data from the MEDATLAS database, found positive trends of about 0.02 °C/year in the upper layer temperatures since mid-1980s, after an almost steady (zero trend) situation in the previous 30 years. Near the coast in the western side of the NW Mediterranean (where L’Estartit station is located), SST showed an average increase of nearly 0.7 °C from 1960 to 2000.
Vargas Yáñez et al. (2009, 2010) attempted to estimate the evolution of air and sea temperatures during the whole twentieth century, up to 2008, in the Spanish waters of the Western Mediterranean. The authors merged the information from MEDATLAS with some of the more recent time series at fixed stations (RADMED) in the region. The estimates for the whole period and the whole area showed a non-significant trend for the upper layer of sea temperature of 0.0038 ± 0.0032 °C/year (partially due to the many gaps in the available series) and a significant trend of 0.0074 ± 0.0013 °C for the air temperature. The evolution, however, showed an acceleration of the warming trends since around 1975 (from when data started to be more abundant although many locations still presented gaps). Trends similar to ours were only detected in the Balearic Sea, while much lower trends were found in the rest of NW Mediterranean areas. These analyses were updated in Vargas-Yáñez et al. (2017), and described the results for the period 1943–2015 in four separate areas, from the Alboran Sea to near the Gulf of Lions. Again, the upper layer temperatures did not show any significant trend, but while in the Alboran and Balearic areas, these trends were slightly negative, in the SE of Spain and in the north trends were slightly positive.
The Copernicus Marine Environment Monitoring Service (CMEMS) provided annual reports of the state of the global ocean and European regional seas based on all the available observations from 1993. In the 2016 report (von Schuckmann et al. 2016), the observed 1993–2015 SST trends for the whole ocean indicate a global increase of 0.016 ± 0.002 °C/year (99% confidence interval), that was quite sensitive to the particular strong increase in SST during the 2015 El Niño event. The Arctic region also showed a high SST increase during 2015 while this signal was much weaker in the North Atlantic. Within the European regional seas, the Mediterranean (including the Black sea) is where trends are the highest (0.039 ± 0.009 °C/year (95% confidence interval). However, the Western Mediterranean basin presented lower trends, in good agreement with other observations (e.g. Vargas-Yáñez et al. 2009 and 2010). Again, the relative maxima of the NW basin appeared in open sea while at the coastal zone, including L’Estartit, remained within 0.02–0.03 °C/year. The CMEMS paper also mentioned that trends for subsurface layers down to 100 m are dependent on SST. For that reason, there is no information about the evolution at 80 m. The update of the CMEMS report for 2018 (von Schuckmann et al. 2018) enlarged the length of the time series to 2016 and obtained a slightly higher trend for SST with reduced uncertainty 0.040 ± 0.004 °C/year. This may be attributed to the increase of the time series and the similarly high temperatures in the last years of the series.
In terms of sea level rise, despite the global increasing trend, sea level in the Mediterranean sea has been decreasing from the 1960s to the 1990s, at an average rate of − 0.5 to − 1 mm/year, reaching rates as low as − 1.3 mm/year in some locations (Marcos and Tsimplis 2008). Such negative trend was attributed to an increase of the atmospheric pressure in the Mediterranean sea region, associated to the positive phase of the North Atlantic Oscillation (Gomis et al. 2008). Although Tsimplis and Baker (2000) suggested the increase would be associated to the increasing salinities of the sea during that period, Jordà and Gomis (2013) demonstrated that the influence of the reported changes in salinity was only residual. Since the mid-1990s, the atmospheric pressure recovered to its normal values for the Mediterranean region and the sea level started to rise at trends higher than 2 mm/year, reaching up to 10 mm/year (Criado-Aldeanueva et al. 2008; Gomis et al. 2008). Marcos et al. (2016), using satellite altimetry data, estimated an average increase in absolute (geocentric) sea level of 2.6 ± 0.2 mm/year for the period 1993–2015.
The CMEMS report (von Schuckmann et al. 2016) estimated the sea level rising trend for 1993–2015 in the whole Mediterranean was 2.9 ± 0.9 mm/year (95% confidence interval), of which about one half corresponded to the thermosteric component. This figure is very similar to ours (Table 2). Interestingly, the whole-Mediterranean time series of sea level anomalies reproduced most of the peaks observed in our time series (Fig. 8)—namely in 1996, 1997, 2010 and 2013, but not that of 2000 which apparently appears 1 year later. The update of the CMEMS report for 2018 (von Schuckmann et al., 2018) that included one more year (2016) slightly reduced the previous estimate for the whole Mediterranean to 2.7 ± 0.9 mm/year, as for other regional seas. The estimate for L’Estartit zone lies within 2.5 and 3.5 mm/year, in good agreement with CMEMS.
In terms of specific aspects of the annual cycle, such as the seasonal distribution of trends, the evolution of the difference between sea surface and air temperatures or stratification periods dealt in our paper, we hardly find any paper that specifically addressed these aspects in the Mediterranean, with the exception of those that used this set of data (e.g. Coma et al. 2009). Only scattered references to some of these subjects in the above-mentioned papers. In particular, it is worth to mention that in Skliris et al. (2012) seasonal trends were estimated for the Western, Eastern and the whole Mediterranean sea. In the Western Mediterranean, these trends were found to be much higher and significant in spring and summer than it was found in our series. The same paper also highlights the role of the latent heat exchanges as the dominant factor among the drivers of seasonality and long-term trends. In particular, they found that the temperature difference between air and sea surface is very well correlated with the latent heat exchanges, whose annual maximum falls in winter, as widely assumed. The relation of these temperature differences with latent heat exchanges, hence with evaporation, is a good support for our hypothesis. In particular, the decrease in spring evaporation may be related to the decrease in local precipitation during this season. Although we did not find any paper that highlighted this relation, many papers on Mediterranean climate (e.g. De Luis et al. 2009, 2010) show a decreasing trend in annual precipitation in the Western Mediterranean region that is more larger in spring.
Specific events such as the heat waves of summer 2003 and 2006 have been clearly reflected in the series and, although it is a shallow coastal station, the mild to very mild 1990, 2014, 2016 and 2017 winters as well as the exceptional heat loss in the 2004–2005 winter. While this extreme heat loss conducted to an unprecedent volume of dense water formation in the Western Mediterranean (Canals et al. 2006; Font et al. 2007), the previously mentioned mild winters only produced small amounts of water not dense enough to get the deep layers (Mertens and Schott, 1998; Schoreder et al. 2017).
Discrepancies among results reported above, as well as with ours, can be attributed to different factors, including uncertainties resulting from merging data from wide areas, the use of different methodologies to calculate the trends and the collection of data from different seasons. Last but not least, the high variability in the upper ocean layer introduces large standard deviations in data. The good statistical comparison of sea surface temperature values and trends obtained from L’Estartit data set and from available satellite products is encouraging.