Characterization of the Atlantic Water and Levantine Intermediate Water in the Mediterranean Sea using Argo Float Data

Abstract. The Atlantic Water (AW) and Levantine Intermediate Water (LIW) are important water masses that play a crucial role in the internal variability of the Mediterranean thermohaline circulation. In particular, their variability and interaction, along with other water masses that characterize the Mediterranean basin, such as the Western Mediterranean Deep Water (WMDW), contribute to modify the Mediterranean Outflow through the Gibraltar Strait and hence may influence the stability of the global thermohaline circulation. This work aims to characterize the AW and LIW in the Mediterranean Sea, taking advantage of the large observational dataset provided by Argo floats from 2001 to 2019. Using different diagnostics, the AW and LIW were identified, highlighting the inter-basin variability and the strong zonal gradient that characterize the two water masses in this marginal sea. Their temporal variability was also investigated focusing on trends and spectral features which constitute an important starting point to understand the mechanisms that are behind their variability. A clear salinification and warming trend have characterized the AW and LIW in the last two decades (~0.007 and 0.008 yr−1; 0.018 and 0.007 °C yr−1, respectively). The salinity and temperature trends found at subbasin scale are in good agreement with previous results. The strongest trends are found in the Adriatic basin in both the AW and LIW properties. A subbasin dependent spectral variability emerges in the AW and LIW salinity timeseries with peaks between 2 and 10 years.



Methods 177
As discussed in the introduction, many indicators/characteristics have been adopted in 178 literature to track the AW and LIW in the Mediterranean Sea. Most of them consider, as best After the post-processing, we proceeded with the search of the minima/maxima salinity peaks 191 for the AW/LIW between 0-100/100-700 m for each profile. We define a peak as a data point that 192 is smaller/larger than its two neighboring samples for the AW/LIW, imposing a minimum 193 difference value of 0.01. If no peaks are found due to high vertical mixing, the profile is excluded.

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The peak with the maximum prominence in the salinity field identifies the AW/LIW in each 195 profile and is used to identify the respective depth and temperature of the water mass core.

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Moreover, if the minimum/maximum salinity is located at the profile endpoints between 0-197 100/100-700 m, it is associated to the AW/LIW only if its closest value satisfies the minimum 198 difference condition.

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The number of profiles counted in each subbasin therefore not only depends on the data 200 sampling over that region, but also on external processes acting on the layer of interest such as 201 the mixing activity due to eddies and/or air-sea interaction: the stronger is the mixing, the lower 202 is the number of profiles considered.

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Once the AW and LIW core are identified in each profile from 2001 to 2019, the AW and LIW 204 timeseries were computed in each subbasin to analyze the low frequency variability (LFV) and 205 trends at interannual to decadal timescale over the available timeseries. In this respect, the high 206 frequency variability was filtered out, first by subtracting the mean seasonal cycle to the raw 207 timeseries, and then applying a median yearly average filter. This last step is needed since the 208 data are not homogeneous in time in every subbasin from 2001 to 2019, and therefore without it, 209 the seasonal variability can contaminate the estimation of the trends. The latter have been 210 computed using the linear least-squares method to fit a linear regression model to the data.

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The AW and LIW inter-basin variabilities were analyzed taking advantage of the boxplot 212 approach applied to each parameter and region (Fig. 2). Inside each grey box, the black bold line

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null hypothesis (that states that the population is normally distributed) is rejected with a 5% level 220 of statistical significance. This method is also applied to the timeseries trends.

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The AW temperature is highly variable, ranging between ~5 and ~30 °C, with a wider range year. This episode is also captured by our analyses (Fig. 3). As observed for the AW salinity

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The depths of the AW core oscillate between 0 and 90 m with the main mode sinking eastward 283 (Fig. 2c). The distributions are all skewed toward lower depths, with the maximum PDF near

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In the other sub-basins, more than the ~80% of profiles is considered, suggesting that 297 identifying the LIW is much easier than the AW, which is highly perturbed by external forcings.

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Flowing away from the region of formation, the LIW interacts with the surrounding water 299 masses and becomes less salty; the salinity gradually drops from ~39.2 to ~38.4 from the variability decreases flowing westward maybe because the LIW becomes deeper, sinking from 303 ~100 to ~650 m (Fig. 2g). The highest salinity is reached in the Cretan basin, where the formation 304 of salty and warm Cretan Intermediate Water, caused by strong wind-induced evaporation and 305 heat loss during winter, influences the LIW properties and detection (Schroeder, 2019).

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The LIW temperature decreases westward from ~18 to ~12.8 °C. The range is higher in the The AW salinity temporal evolution is shown in Fig. 3, where significant trends (at 5% level of 330 significance) are found in each region (Table 1)     The wavelet power spectra show that the AW salinity (Fig. 9), during the observed periods, is