The hydrology and biochemistry of the Cretan Straits (Antikithira and Kassos Straits) revisited in the period June 1997–May 1998

Three CTD surveys (June 1997, January 1998, May 1998) were undertaken at the Antikithira and the Kassos Straits of the Cretan Arc in the Eastern Mediterranean. In these cruises, water samples were also collected for oxygen, nutrients and chlorophyll-a analyses, while current-meter moorings were deployed in the channels of the Cretan Deep Water (CDW) outflow from June 1997 to July 1998. The same sub-basin-scale circulation features as in 1994–1995 govern the upper circulation: The Mirtoan/West Cretan Cyclone prevails throughout the study period in the Antikithira Strait, while in the Kassos Strait the East Cretan Cyclone in the north part of the Strait interacts either with the Ierapetra Gyre or the Rhodes Gyre in the southern part of the Strait, thus creating varying flow regimes. The Transitional Mediterranean Water (TMW) appears denser (σθ 29.17–29.18) in comparison to its density values in 1994–1995 and its core occupies a deeper layer ( 500–700 m). The CDW occupies a near-bottom layer in the outflow channels less than 200 m thick. An overall decrease in the CDW volume is observed from June 1997 to May 1998 which is correlated with a weakening in the presence of TMW in each Strait. The CDW and TMW distributions and variability are also illustrated through their chemical signal of, correspondingly, high-oxygen, low-nutrient and low-oxygen, high-nutrient values. The existence of a cyclonic/upwelling circulation structure in the Antikithira Strait, as opposed to a more variable hydrodynamic regime in the Kassos Strait, results in higher chlorophyll-a concentrations and shallower chlorophyll-a maxima in the Antikithira Strait during summer in comparison to the corresponding phytoplankton parameters in the Kassos Strait. The CDW outflowing speeds are on the order of 20 cm/s and the mean CDW transport from both straits during the deployment period is 0.15 Sv, which is substantially reduced with respect to the total mean value of 0.6 Sv of the period 1994–1995. The total CDW transport during spring and early summer of 1998 for both straits is 0.05 Sv. This decrease denotes an inter-annual weakening in the evolution of the East Mediterranean Transient.