1. Introduction
Sea surface temperature (SST) is one of the essential climate variables (ECVs), as defined by the Global Climate Observing System (GCOS): it plays a key role in regulating climate and its variability. SST modulates the exchange of heat between the ocean and the atmosphere, and reflects, together with sea surface salinity, the modifications of the thermohaline motions in the ocean.
From seasonal to longer timescales (i.e., interannual, decadal, and multidecadal), SST responds to both natural climate variability [
1] and human-driven climate change [
2]. SST annual cycle is the dominant oscillation induced by the solar forcing. However, the annual cycle is not the sole regular response to the solar radiation as is modulated by other external environmental factors, such as changes in wind forcing [
3], cloudiness [
4], and ocean–atmosphere interactions [
5]. Also, on seasonal timescales, a change in the seasonal amplitude/phase can substantially contribute to the interannual variability in mean temperatures [
6]. On interannual and (multi-)decadal timescales, the nonseasonal or slow variation in SST is modulated by atmospheric and oceanic circulation variability that leads to quasi-periodic oscillations and/or dominant patterns. Examples include the interannual fluctuations in the eastern equatorial Pacific mean SST, identified with the El Niño Southern Oscillation (ENSO) [
7], a tripole of SST anomalies in the North Atlantic [
8] associated to the North Atlantic Oscillation (NAO) [
9], and the multidecadal SST fluctuation in the North Atlantic domain associated to the Atlantic Multidecadal Oscillation (AMO) [
10]. On the other hand, increased concentration of atmospheric greenhouse gases have altered the Earth’s energy balance, resulting in the accumulation of thermal energy in the climate system [
11]. During the past fifty years more than 90% of this extra energy has been accumulated into the oceans with a primary consequence of raising SSTs [
12]. Indeed, from the beginning of the 1970s, a global mean surface temperature (including land surface temperature) warming trend became evident “beyond the bounds of natural variability” [
11]. SST is thus a key parameter to track climate change, monitor and characterize the state of the global climate system and contribute to the definition of the present state of the oceans, at both global and regional scales.
At regional scale, the Mediterranean Sea plays a role of the sentinel (hotspot) of global warming since it responds to climate change [
13]. Indeed, the Mediterranean was one of the first seas where a warming trend of the deep water temperatures in the western basin was attributed to global warming [
14]. The Mediterranean SST has been experiencing an intensive and continuous warming trend since the mid-1980s and this warming tendency is likely projected to increase throughout the 21st century under present climate scenarios [
15]. Several studies have shown a consistent increase in the mean Mediterranean SST in the last three decades, taking also advantage of the availability of long-term satellite-based SST data. Based on the 4 km Advanced Very High Resolution Radiometer (AVHRR) Pathfinder SST dataset [
16], the author of [
17] estimated a mean warming trend of
C/year in the western Mediterranean Sea and
C/year in the eastern basin from 1985 to 2006. Analogous results were obtained in [
18], where a mean warming trend of
C/year in the western Mediterranean Sea from 1985 to 2007 was found. These authors [
18] also evidenced a significant seasonal variability in the SST trend, observing the highest warming rate in spring, approximately
C/year, and much lower warming rates, approximately
C/year or less, during the other three seasons. Based on Reynolds’ SST reanalysis [
19], a satellite-based gap-free (optimally interpolated) SST dataset on a 0.25
grid, the authors of [
20] found a mean warming trend of
C/year during a 31-year period (1982–2012) over the Mediterranean Sea, ranging from
C/year in winter to
C/year in spring.
More recently, using the same dataset, the authors of both [
21] and [
22] estimated a mean warming trend of ~
C/year for the whole Mediterranean Sea, although looking at different periods: from 1993 to 2017 and from 1982 to 2016, respectively. The authors of [
23] investigated decadal variability in the Mediterranean SST on a 24-year period (1985–2008), making use of a regionally optimized satellite-based SST dataset over the Mediterranean Sea [
24] and in situ-based SST data. These authors [
23] estimated a mean warming trend of ~
C/year for the whole Mediterranean Sea, ~
C/year for the western basin and ~
C/year for the eastern basin. Between 1985 and 2008, these authors [
23] also evidenced a marked seasonal variability in the SST trend, estimating a higher warming rate in spring (~
C/year) and summer (~
C/year) than in autumn (~
C/year) and winter (~
C/year).
The analysis of the spatial patterns of the Mediterranean SST trend has showed a marked difference between the western and eastern Mediterranean. The authors of [
23] evidenced two main periods, before and after 1990, during which the two sub-basins showed opposite trend patterns. They suggest that the 1985–1990 period might be regarded as a transition period, after which the observed trend pattern reversed, with an initial warming trend in the western Mediterranean and a cooling trend in the eastern basin until 1990. The authors of [
25] found a similar pattern of eastward increasing trend in the Mediterranean SST since the early 1990s. In addition, the authors of [
23] pointed out that the shift (from westward to eastward) coincided with a change from a high positive to a low NAO phase, thus suggesting a potential link between NAO and SST variations on decadal timescales. Furthermore, throughout the period 1973–2008, the same authors evidenced a high correlation between AMO and the first temporal mode of the Mediterranean SST anomalies, the latter associated with the long-term SST trend.
Among the natural modes of climate variability, NAO and AMO are particularly relevant for the Mediterranean climate. NAO is the dominant pattern of wintertime atmospheric circulation variability over the extra-tropical North Atlantic domain [
9]. The decadal oscillation of the sea level pressure (SLP) anomalies is followed by wind, air temperature, and precipitation changes, especially in winter and early spring [
26], over the North Atlantic and across Europe, including the Mediterranean Sea. In particular, a positive phase of NAO is associated with warmer conditions over the western Mediterranean and cooler conditions over the eastern basin, whereas the contrary occurs during negative phases.
AMO can be defined in several but similar forms. According to the definition in [
27], the 60
S–60
N SSTs are subtracted from the North Atlantic SSTs while the North Atlantic SSTs (0–70
N) are simply detrended following the [
28] definition. AMO is particularly relevant for the North Atlantic-Mediterranean system [
29]. Indeed, according to the authors of [
23,
30,
31], the Mediterranean SST shows multidecadal AMO-like variability, with over 30% of explained variance. In particular, the authors of [
31] evidenced the presence of a significant oscillation in the Mediterranean SST with a period of approximately 70 years, very close to the AMO index. However, the nature/origin of this Mediterranean AMO variability is still a matter of debate, concerning the existence of a dynamical linkage transmitting the AMO signal from the Atlantic to the Mediterranean (e.g., anomalous heat advection processes or cloud-related processes).
Herein we analyze the variability in the Mediterranean SST and the adjacent Northeastern Atlantic box (west of Gibraltar) from seasonal to decadal timescales. We provide updated trend estimates for the whole Mediterranean Sea, its main sub-basins (i.e., western Mediterranean Sea, Adriatic Sea, Ionian Sea, and Levantine–Aegean Sea), and the Northeastern Atlantic box, making use of the most recent satellite SST dataset over the Mediterranean Sea (1982–2018). In addition, by using two long-term in situ-based SST reconstructions, covering the last 165 years, we analyze the Mediterranean multidecadal SST variability in relation to AMO. Specifically, in the framework of the Copernicus Marine Environment Monitoring Service (CMEMS), the Satellite Oceanography Group of the Italian National Research Council (CNR-GOS) has recently produced an updated version of the Mediterranean optimally interpolated SST dataset [
32]. This is a satellite-based dataset which provides daily (nighttime) 4-km resolution optimally interpolated SST data from 1982 to 2018 over the Mediterranean Sea and the Northeastern Atlantic box. Indeed, even if there is a variety of historical SST datasets available (e.g., Reynolds v.2 reanalysis [
19], Operational SST and Sea Ice Analysis (OSTIA) [
33], and European Space Agency’s Climate Change Initiative (ESA-CCI) SST dataset [
34]), this dataset provides the longest record of foundation temperature (namely, the temperature below the layer affected by the diurnal cycle [
35]) interpolated fields at high resolution (4 km), with relatively high accuracy (root mean square error of ~
C when compared with in situ data) obtained by a regionally optimized processing. Longer time series but at lower spatial and temporal resolution are provided by the Extended Reconstructed SST (ERSST, monthly 2
× 2
degrees spatial resolution, 1854–today) [
36] and by the Hadley Centre Sea Ice and Sea Surface Temperature dataset (HadISST, monthly 1
× 1
degrees spatial resolution, 1870–today) [
37].
4. Discussion
Mapping changes of regional SST distributions in response to ongoing background warming is an important step towards understanding climate variability and its impacts on weather extremes, marine ecosystem, and related services. SST changes may appear as a shift of the entire distribution (thus showing a trend in the mean values), but also as changes in the shape of the distribution [
55]. The CMEMS Mediterranean SST time series, covering the satellite era, is actually displaying both a significant long-term trend and a strengthening of the seasonal cycle. Indeed, comparing the CMEMS satellite-based data to the longer HadISST and ERSST timeseries, we found that the Mediterranean warming trend from 1982 to 2018 is approximately 3.7 times higher than the global ocean warming trend from 1980 to 2005 (=0.011
C/year, [
12]).
Noticeably, although the Mediterranean SST trend shows a continuous warming trend, the decadal and spatial analysis (
Section 3.3.1) of SST variations evidenced that the warming rate is not uniform, neither in time nor in space. Specifically, three main periods have been identified: 1982–1993 (first period), 1994–2005 (second period), and 2006–2018 (third period). During each period, the Mediterranean SST trend has been estimated in
C/year (1982–1993),
C/year (1994–1995) and
C/year (2006–2018). After an initial, short cooling trend (1982 to ca. 1984), the first period is characterized by a warming phase, from 1985 to 1990, followed by a cooling until the beginning of 1993. From 1993, the second period begins with a strong increase in SST until 1995 and, after a short cooling between 1995–1996, it continues with a warming trend till 2003, where a cooling trend begins, lasting until 2005. The third period shows a continuous warming trend, with no cooling or pausing phases. The first two periods are thus characterized by an alternate warming and cooling tendency, while the third one seems to show an overwhelming warming tendency, at least until 2018.
Regarding the Northeastern Atlantic box, the decadal variability is more pronounced than that of the Mediterranean. Indeed, whereas the first period (1982–1993) evidences a strong warming trend ( C/year), the second one (1994–2005) does not present a statistically significant trend (i.e., no trend at ). Finally, the third period (2006–2018) experiences a mild warming trend of C/year, which is more than twice smaller than the first period trend. Overall, the Mediterranean Sea and the adjacent Northeastern Atlantic box warmed at about the same rate from 1982 to the late 1990s. From 1999 onward, although the trend continues to be positive in the Mediterranean Sea, it slows down in the Northeastern Atlantic box.
Our satellite-based analysis has been compared to the reconstructed HadISST and ERSST data and to the AMO index in order to frame the Mediterranean SST variability, as observed during the satellite era, in a larger time window, i.e., the last two centuries. Past studies [
23,
30,
31] have already highlighted the high correlation between AMO and the Mediterranean SST. The authors of [
31] showed a multidecadal AMO-like Mediterranean SST variability throughout the longest currently available period (i.e., 1854 to present). Other works [
56,
57] indicated that AMO is currently in a modest warming phase. Our analysis shows that the pause of the increasing tendency of AMO since 2000 is captured by the slowdown in the Northeastern Atlantic box SST trend while it is not followed by the Mediterranean SST (
Figure 4). The nature/origin of this Mediterranean AMO-like variability is still a matter of debate and investigation. As potential sources of this AMO-like variability, the authors of [
23] suggest the influence of the Atlantic inflow on the Mediterranean SST variability through the advection of heating from Gibraltar, whereas the authors of [
31] hypothesize either an atmospheric origin or an internal variability related to the Mediterranean Thermohaline Circulation (THC), in analogy to what proposed by [
58] for the North Atlantic. Our analysis of the Mediterranean SST variability opens a further question concerning the bifurcation between AMO and Mediterranean SST at the beginning of this last millennium (2000–today).
The spatial pattern of the Mediterranean SST trend also displays a marked difference between its western and eastern basins (
Figure 6). These patterns have been suggested to depend on different teleconnections: the west being under the NAO [
23] and the east under the South Asian Monsoon influence (e.g., [
59]). Regarding mesoscale patterns, the SST trend map (
Figure 6) shows some evidence on the fact that SST trend patterns overlay some mean oceanographic Mediterranean structures [
20] (e.g., West Alboran Gyre, West Cretan Gyre, Ierapetra Gyre, Rhodes Gyre, and the Tyrrhenian Gyre; see
Figure 1 and
Figure 6). Moreover, we find minimum trend values in some areas that are characterized by a thick upper mixed layer or recurrent deep convection (e.g., in the Gulf of Lion; see
Figure 1 and
Figure 6). However, the general SST trend spatial pattern does not overlay with both climatological mixed layer depth and seasonal thermocline depth [
60]. This suggests that spatial differences in the warming cannot be solely explained by the distribution of the excess of heat over a larger vertical layer, but rather it opens to hypothesize an effective role of mean circulation and local mean air–sea heat fluxes on SST variability (see
Figure 1 and
Figure 6).
A relevant seasonal variability and strong positive and negative anomalies, superimposed to the Mediterranean SST trend, are clearly detected in the CMEMS SST dataset (see
Section 3.3.2). Indeed, a change of the seasonal cycle amplitude is identified by a significant positive (
C/year) and negative (
C/year) trend of the seasonal component during summer (JJA) and winter (DJF), respectively, whereas no trend during spring (MAM) and autumn (SON) is observed. This gives rise to an increase of the SST trend in summer (about 0.056
C/year), and to a slowdown in winter (~0.029
C/year). No impact is observed in spring and autumn, where the trend (~0.040
C/year) matches the mean annual trend (0.041
C/year). In previous works [
18,
22,
23], a change in the seasonal cycle was generally deduced from a change in the mean trend when computed over the season months. The authors of [
18], in their analysis of satellite-based SST observations from 1985 to 2007 in the western Mediterranean, evidenced a significant change in the mean annual trend (about 0.03
C/year) from April to June, estimated in about 0.06
C/year. Over the two decades, these authors found an increase of the mean SST of April, May and June by nearly 1
C, suggesting a lengthening of summer and an advance of summer onset in late spring. The authors of [
23] also evidenced a marked seasonal variability in the mean annual trend in the whole Mediterranean (about 0.037
C/year from 1985 to 2008), ranging from 0.054
C/year in spring (MAM) and 0.044
C/year in summer (JJA), to 0.027
C/year in autumn (SON) and 0.023
C/year in winter (DJF).
Changes in seasonal variability affect not only the SST trend during summer and winter seasons (
Figure 9), that is, higher warming in summer than in winter, but could also induce an amplification of marine heatwave events, in terms of frequency, magnitude and duration, which in turn can have a vast range of adverse impacts on marine ecosystems [
51,
52]. Indeed, a marine heatwave has been defined as “a prolonged discrete anomalously warm water event that can be ascribed by its duration, intensity, rate of evolution, and spatial extent” [
52]. Based on our analysis (
Figure 5), from 2000 the Mediterranean Sea featured the highest SSTs, some of which classified as strong marine heatwaves [
51,
52]. The signature of the well-known marine heatwave occurred in 2003 is indeed clearly visible in the western Mediterranean basin, the Ionian Sea, and the Adriatic Sea (
Figure 5). This extreme warming event lasted for the whole month of June and the summer mean SST anomaly was almost 2
C higher than the climatology [
52]. We notice, moreover, that there is no signal of the 2003 warm surface water anomaly in the Northeastern Atlantic Box (
Figure 5).
In general, from the beginning of the 2000s, the Mediterranean Sea experienced the highest temperatures and an overwhelming SST trend (
Figure 5) along with strong marine heatwaves, well described also in terms of their biological impacts in [
51,
52]. In addition, warmer SSTs could facilitate (atmospheric) heat waves events. The authors of [
61] suggest that the strong increase in SSTs along coasts could impact the sea breeze circulation, which is driven by the land-sea thermal contrast, with the consequence of the loss of the sea breeze mitigation effect on coastal land temperatures. Overall, the authors of [
51] evidenced a steep increase in frequency and duration of the total marine heatwave events in the Mediterranean Sea, highlighting how this regime shift follows the increase in extreme warm daily SSTs under current warming trends (also at global scale [
62]).
5. Summary and Conclusions
This work analyzes the long-term SST variations, at interannual and decadal timescales, within the Mediterranean Sea and the adjacent Northeastern Atlantic box (west of the Strait of Gibraltar).
Our analysis is based on the CMEMS Mediterranean SST dataset, a daily (nighttime) optimally interpolated 4-km resolution SST time series from 1982 to 2018, and the X-11 seasonal adjustment procedure, used to decompose the input SST signal into the seasonal, trend and irregular components. The analysis of the CMEMS Mediterranean SST changes over the last (nearly) four decades highlights four main outcomes.
First, the trend analysis gives updated values for the whole Mediterranean Sea (about 0.041 C/year), its main sub-basins, and the adjacent Northeastern Atlantic box (about 0.027 C/year). Second, the seasonal analysis evidences a significant change in the seasonal amplitude of the Mediterranean SST signal. Specifically, summer and winter seasonal components clearly show positive and negative trends, which in turn lead to a strenghtening of the difference between the two seasons, likely reflecting also an increase in the number of extremes events. Third, the spatial distribution of the mean trend evidences an uneven pattern over the whole Mediterranean Sea, with the eastern basin getting warmer more rapidly than the western. Fourth, the CMEMS Mediterranean SST dataset, complemented with the reconstructed in situ data, clearly shows that the Mediterranean SST trend component closely follows the Atlantic Multidecadal Oscillation (AMO) from 1854 to 2007. Afterwards, a “pausing” phase of AMO is observed until the end of the study period and it is captured by the trend in the Northeastern Atlantic box that, though not representative of the whole North Atlantic, shows a warming slowdown from 1999 to 2015. Noticeably, after 2007, while AMO remains constant in the average, the Mediterranean SST continues to increase. This reveals a more complex interaction between the Mediterranean SST variability and climate indexes.
Understanding the reason why the Mediterranean SST and AMO diverge during the last decade opens a challenging question that needs to be further investigated through, e.g., the synergy of observational data and modeling, in a cross-disciplinary and process-based fashion.