Multidecadal Connection Between Key Variables and Jet Stream
To investigate the potential relationship between JS position and MPP, we analyzed key variables including geopotential height (HGT) to obtain the JS position, northern wind stress (NWS), sea surface temperature (SST), and surface chlorophyll (CHL) concentration, the latter serving as a proxy for MPP (cf. Methods).
The time-series of the different variables averaged across the EGoL subregion alongside their correlation coefficients show the strongest correlation to occur between SST and CHL concentration (-0.82), followed by SST and JS position (0.67), and JS position and CHL concentration (-0.54) (Fig. 1). Moreover, the variables under consideration show a strong seasonal pattern (Fig. 1).
A deseasoning and detrending process of JS position and CHL concentration (monthly means for both) highlights to which extent the relationship between these two variables is influenced by the seasonality of the driving processes, namely atmospheric dynamics and MPP (Fig. 2A-C), which results in a correlation coefficient of -0.91 for the seasonal component (Fig. 2B). The residual component of JS position and CHL concentrations (Fig. 2C) shows a correlation coefficient of 0.31, which indicates some degree of connection beyond seasonality, though rather weak, amongst these two variables.
Yearly Variability of Coupling Amongst Variables
A year-by-year examination of the data reveals that in specific years the coupling between variables is tighter than in other years (Fig. 3). While the relationships between JS position and SST, and amongst CHL concentration and SST, remain consistent throughout the 2000–2023 study period, this is not the case for JS position and NWS, and for JS position and CHL concentration. For the latter relationship, some years (i.e. 2000, 2003, 2004, 2007, 2009, 2012, 2014, 2015, 2016, and 2019) show a stronger inverse link (≤ -0.6) than the rest of the years, with 5 years ≤ -0,7 (i.e. 2003, 2012, 2014, 2015 and 2016). Other years have much weaker inverse links, with values amongst − 0.36 and − 0.15 (Fig. 3).
The detrended and deseasonalized time series again reveal some remarkable, year-specific correlations amongst variables (Fig. 3). Namely, in some years where the JS position correlates well with NWS (e.g. 2005, 2012, and 2020), a good correlation with CHL concentration is also observed, whereas no equivalent pattern is found with SST. Therefore, these two variables —CHL concentration and SST— do not respond similarly to JS position.
In the test area, 2012 is one of the most well-known years for intense dense shelf-water cascading and open-sea convection events17. Our analysis reveals, for this particular year, a high correlation with key variables, both before and after deseasoning and detrending (Fig. 3). Moreover, upon deseasonalizing and detrending, we detect a robust connection among JS position-related variables (NWS, CHL and SST), whereas CHL and SST exhibit a lower cross-correlation (Fig. 3).
Dominant Frequencies and Variance Patterns
Fast Fourier Transforms (FFTs) of JS position, NWS and CHL concentration primary variables demonstrate highly similar spectra amongst them with nearly identical frequencies, as illustrated by an intraseasonal oscillation or cyclicity of about 10–12 days (Supp. Figure 2).
Computing time-lagged cross-correlations between the position of the JS and CHL concentrations shows that the highest correlation involves a 10–12 day-lag (Supp. Figure 3). Therefore, the CHL concentration response to oscillations in the position of the JS is about 10–12 days, which aligns with the frequencies observed in the FFT analysis (Supp. Figure 2).
The variance patterns of JS position and CHL concentration beyond seasonal cyclicity was further investigated by means of an Empirical Orthogonal Functions (EOFs) analysis of the deseasoned and detrended HGT and NWS time series. This method decomposes the data into independent modes (or dominant patterns) that are ranked according to the amount of variance in the dataset, therefore revealing underlying spatial structures within the relationships between different variables.
The EOFs modes for both HGT and NWS highlight the distinctive behaviour of these two variables in the NW Mediterranean Sea (Fig. 4). In particular, the maximum latitudinal gradient of the HGT EOFs reveals that Mode 1 (55% covariance) crosses the EGoL subregion, while Mode 3 (7% covariance) is located quite closely to the south (Fig. 4A).
The Principal Component Analysis (PCA) of the monthly average JS position and monthly average CHL concentration (Supp. Figure 4A-C) yields a 0.84 correlation coefficient amongst these variables for the first mode (PCA Mode 1 in Supp. Figure 4A), thus underscoring the pronounced seasonal pattern behind their relationship while also suggesting a potential link between HGT and CHL concentration dynamics. The cycles in PCA modes 2 and 3 do not show any correlation although they exhibit some visual coherence (Supp. Figure 4B and C). Consequently, we performed another FFT analysis, which results highlight the near-perfect correlation (r = 0.97) between the FFTs of the first PCA mode for both variables, alongside with a correlation of 0.43 in the second mode (FFT PCA modes 1 and 2 in Supp. Figure 5).
Temporal trends and Anomalies
A remarkable result from our analysis is the striking contrast in the trends of JS position and CHL concentration through time. Whereas during the investigated period, and despite interannual variability, the JS position exhibits a consistent overall northward trend, corresponding to a poleward shift of 75 km in the two last decades or so (Fig. 5A), also coinciding with an increase of 1.41ºC of SST in the test area (Fig. 5B), CHL concentration conversely displays an equally consistent overall declining trend within the NW Mediterranean Sea region (Fig. 5C). Notably, these two trends display a perfect negative correlation, therefore underlining the inverse relationship between JS position and CHL concentration in the study area.
Subsequent statistical analysis of the time series of JS position, SST, and CHL concentration further illustrates that the rolling standard deviation (STD) of the subtropical JS position displays a significant positive trend too (Fig. 6A). This points —in addition to its consistent northward shift— to an increase in the variability of the latitudinal position of the JS. Conversely, SST and CHL concentration do not show such behaviour in the test area (Fig. 6B-C).
An analysis of the JS position anomaly relative to the climatology during the study period was also conducted, with no particularly clear patterns emerging beyond (i) a high concentration of positive anomalies, and (ii) the two strongest positive anomalies (exceeding 6º) occurring in the last five years of the time series (Supp. Figure 6).