Mediterranean sea eddy kinetic energy variability from 11 years of altimetric data

Abstract

The eddy kinetic energy (EKE) variability of the Mediterranean Sea over eleven years (1993–2003) was studied using merged (T/P, Jason, ERS and Envisat) altimetric data. Special efforts were made to improve the data selection and the mapping procedure. The introduction of the Mean Dynamic Topography [Rio M.-H., Poulain P.-M., Pascual A., Mauri E., Larnico G., 2005, “A mean dynamic topography of the Mediterranean Sea computed from altimetric data and in situ measurements”, submitted to J. Mar. Syst. (Liège Colloquium special issue).] made it possible to derive the absolute dynamic topography. Thus, an improved and more complete data set was obtained, which enabled previous studies based on altimetric data ([Iudicone D., Santoreli R., Marullo S., Gerosa P., 1998, “Sea level variability and surface eddy statistics in the Mediterranean Sea from TOPEX/POSEIDON data”, J. Geoph. Res. 103 (C2), 2995–3011.]; [Ayoub N., Le Traon P.-Y., De Mey P., 1998, “A description of the Mediterranean surface circulation from combined ERS-1 and TOPEX/POSEIDON altimetric data”, J. Mar. Syst. 18 (1–3), 3–40.]; [Larnicol G., Le Traon P.-Y., Ayoub N., De Mey P., 1995, “Mean sea level and surface circulation variability of the Mediterranean Sea from 2 years of TOPEX/POSEIDON altimetry ”, J. Geoph. Res. 100 (C12), 25,163–25,177.]; [Larnicol G., Ayoub N., Le Traon P.-Y., 2002, “Major changes in Mediterranean Sea level variability from 7 years of TOPES/Poseidon and ERS-1/2 data”, J. Mar. Syst. 33–34, 63–89.] to be enhanced. In particular, for the first time a relatively accurate description of the mean EKE distribution in the Mediterranean Sea could be provided. The mean EKE structure is the consequence of the superposition of different variability components. As previously described in other studies, mesoscale, seasonal and interannual components play a key role in this variability, but the contribution of long-term decadal signals, as well as sporadic events, are also important. In addition, a specific variability pattern is observed in each sub-basin of the Mediterranean Sea (Western, Ionian and Levantine). The observed permanent/recurrent structures, as well as the transient ones, are subject to annual variations that occurred with different phases: permanent/recurrent structures (such as the Alboran Gyres and the Ierapetra Eddy — IE) are at maximum intensity in late summer (October to January) whereas areas of mesoscale activity (southwest of Sardinia, central Ionian and Mersa-Matruh areas) display maximum intensity in winter (January to April). Both annual and interannual variations are more pronounced in the eastern part of the basin. On the western side, mesoscale activity is relatively stable. The appearance of sporadic energetic structures leads to a positive trend in the Alboran Sea and north of the Tyrrhenian, while a negative trend is observed south of Majorca. The seasonal EKE variations are mainly concentrated southwest of Sardinia. They are found to be modulated by interannual variability, with a weakening of both the mean EKE and the amplitude of the seasonal cycle between 1997 and 2000 and between 2001 and 2003. In the Ionian Sea, the EKE variability is affected by the reversal of the circulation in 1997. This phenomenon suggests that the decadal scale of variability should not be overlooked in this basin. A strong positive EKE trend, coupled with an increase in the seasonal cycle amplitude, is observed in 1997 in the central Ionian. The high correlation of wind stress variations and high-frequency EKE in the Ionian Sea, suggests that the seasonal variations are partly due to wind-induced mesoscale activity. In the Levantine Sea, an even stronger positive EKE trend is observed, partly influenced by strong activity around the IE and in the Mersa-Matruh and Shikmona areas. Seasonal variations are present all over the studied period and are either largely influenced by the IE activity at the beginning and end of the period, or are concentrated at the entrance of the basin and in the Mersa-Matruh area. In the latter, larger amplitude are observed after 1997, as in the Ionian Sea. Also, different processes of variability are observed, confirming that the Mersa-Matruh area must be considered as the site of intense mesoscale activity.