Black Sea Basinscale and Mesoscale Dynamics
and their Dependence on Wind forcing and Bottom Topography

 

ABSTRACT 

 

 The general element of the classical theory of the Black Sea basin-scale circulation is the Rim current (RC) flowing cyclonically along the continental slope. Broad oceanographic application of the satellite data during the past decades has also revealed energetic mesoscale eddy-like structures that provide an effective cross-basin exchange. The aim of this report is to describe and discuss physical mechanisms of interaction between the RC and mesoscale eddies and their dependences on wind forcing and bottom topography. The study was based on the analysis of field observations and results of laboratory modeling. It was revealed by observations that in the northeastern Black Sea, having a very narrow continental slope, the topographic control of RC is weaker. The position and stability of RC is governed by the Ekman pumping. Under the strong (positive) Ekman pumping, which is more typical for the winter period, RC is a coherent and strong jet located over the continental slope. In this case, the lateral, cross-jet, exchange is relatively weak as the coastal and deep waters are separated by the dynamical front. In the alternative case of weak Ekman pumping, which is more typical for the Spring-Summer time, RC becomes unstable, meanders and breaks up into eddies producing strong lateral exchange. The effects of the wind forcing and bottom topography on the along-shore current, dynamically similar to RC, were studied in the laboratory experiment with the two-layer fluid in circular tank placed on the rotating table. It was shown that in case, when the width L of the continental slope is smaller or equal to the baroclinic Rossby deformation radius R, the influence of the bottom topography on the stability and structure of the along-shore current was negligible. After the decline or termination of the wind forcing, the along-shore current shifted in the off-shore direction, became unstable and disintegrated into eddies, providing an intensive water exchange between the near-shore zone and deep regions. The timescale of the disintegration process was approximately 20-30 laboratory days - similar to that observed in natural conditions. In the case when L was equal to or larger than 2R, the instability was rather weak and had a wave-like nature. No energetic eddies were formed in this regime, and exchange between the near-shore and deep regions was reduced. The results of this study allow us to conclude that in the northeastern Black Sea, having a very narrow continental slope, the topographic control of RC is weak. The position and stability of the along-shore current are controlled by the Ekman pumping. The general mechanism of the mesoscale eddy formation is the instability of RC after the decrease of the Ekman pumping. In the northwestern Black sea, where the slope is wide (L is close to 4R), both the RC and mesoscale eddies are topographically controlled.