Decadal scale variability of sea surface temperature in the Mediterranean Sea in relation to atmospheric variability

Abstract

Twenty-four years of AVHRR-derived sea surface temperature (SST) data and 35 years of NOCS (V.2) in situ-based SST data were used to investigate the decadal scale variability of this parameter in the Mediterranean Sea in relation to local air-sea interaction and large-scale atmospheric variability. Satellite and in situ-derived data indicate a strong eastward increasing sea surface warming trend from the early 1990s onwards. The satellite-derived mean annual warming rate is about 0.037°C year -1 for the whole basin, about 0.026°C year -1 for the western sub-basin and about 0.042°C year -1 for the eastern sub-basin over 1985-2008. NOCS-derived data indicate similar variability but with lower warming trends for both sub-basins over the same period. The long-term Mediterranean SST spatiotemporal variability is mainly associated with horizontal heat advection variations and an increasing warming of the Atlantic inflow. Analysis of SST and net heat flux interannual variations indicates a negative correlation, with the long-term SST increase, driving a net air-sea heat flux decrease in the Mediterranean Sea through a large increase in the latent heat loss. Empirical orthogonal function (EOF) analysis of the monthly average anomaly satellitederived time series showed that the first EOF mode is associated with a long-term warming trend throughout the whole Mediterranean surface and it is highly correlated with both the Eastern Atlantic (EA) pattern and the Atlantic Multidecadal Oscillation (AMO) index. On the other hand, SST basin-average yearly anomaly and NAO variations show low and not statistically significant correlations of opposite sign for the eastern (negative correlation) and western (positive correlation) sub-basins. However, there seems to be a link between NAO and SST decadal-scale variations that is particularly evidenced in the second EOF mode of SST anomalies. NOCS SST time series show a significant SST rise in the western basin from 1973 to the late 1980s following a large warming of the inflowing surface Atlantic waters and a long-term increase of the NAO index, whereas SST slowly increased in the eastern basin. In the early 1990s, there is an abrupt change from a very high positive to a low NAO phase which coincides with a large change in the SST spatiotemporal variability pattern. This pronounced variability shift is followed by an acceleration of the warming rate in the Mediterranean Sea and a change in the direction (from westward to eastward) of its spatial increasing tendency.

Recent warming in the Balearic Sea and Spanish Mediterranean coast. Towards an earlier and longer summer

“…This rate is consistent with the trend obtained for the SST, where a lower rate (0.026 ºC/year) has been registered for the period 1985-2009, which translates into an overall increase of 0.7 ºC in 25 years. The annual warming rate in SST is in agreement with the findings of Nykjaer (2009), López García and Camarasa (2011) and Skliris (2012) for the western Mediterranean, who gave a rate of about 0.03 ºC/year for about the same period; and with the trends given by Macías et al (2013) for the whole Mediterranean (0.037 ºC/year). Although the warming of the climate system since the middle of the 19th century is accepted by almost all scientists, the observed trends in oceanic SST differ depending on the ocean, the period of reference and also the database employed in the analysis.…”

Sea Surface Temperature in the Mediterranean: Trends and Spatial Patterns (1982–2016)

“…4 shows the SST trend clustering, from this map it can be seen that the warming is not uniform across the basin; two main regions appear with different warming intensity. The lower, but still positive, warming trend is found in the central Mediterranean around Italy (most of Tyrrhenian and Ionian basins) and parts of Algerian and Morocco coast, being higher in the centre and north of the Western Mediterranean while the highest warming is located in the Levantine basin, in agreement with the studies of Nykjaer (2009), Skliris et al (2012) and Shaltout and Omstedt (2014) that found higher values for SST trend in the eastern Mediterranean basin.…”

“…To check the consistency of these results with SST trends previously found by other authors, we have run the trend analysis for different periods inside our database temporal span. The total SST increase derived from our analysis for the 1985-2008 period, trend of (1.06 ± 0.17) 9 10 -4°C /day, for the whole Mediterranean basin gives a value of 0.93°C, which is in good agreement with the 0.89°C value found in Skliris et al (2012), although this latter value was calculated for monthly SST versus our daily data. There is also good agreement with the trend calculated by Shaltout and Omstedt (2014) for the 1982-2012 period yielding a value of 0.35°C/decade while in our case it would be of 0.36°C/decade.…”

“…In the case of the Western basin he found a 0.03°C/year and a higher 0.05°C/year for the Eastern basin. Skliris et al (2012) used two different data bases to investigate decadal variability of SST in the Mediterranean. They found different total increases in SST for the 1985-2008 period for the whole Mediterranean (0.89°C), the Western (0.62°C) and Eastern (1.01°C) basins.…”

Dense intermediate water outflow from the Cretan Sea: A salinity driven, recurrent phenomenon, connected to thermohaline circulation changes

“…Recent studies [Criado-Aldanueva et al, 2008;Nykjaer, 2009, Skliris et al, 2012 based on satellite-derived SST data have shown that during roughly the last two decades there has been a sea surface warming trend of about 0.4-0.6 C/decade over the EMed and Aegean Seas. This warming effect could enhance stratification thus reducing DWF rates.…”