First Long-Term Measurements on Kazakhstan Shelf of the Caspian Sea Reveal Alternating Currents and Energetic Temperature Variability

Abstract

Moored near-bottom current velocity and water temperature measurements were performed during a period of 194 days (from October 2022 through April 2023) with a 15-min sampling rate at two locations on the shelf of the Kazakhstan sector of the Caspian Sea in its Middle Caspian basin. The area has not been covered by in situ measurements over several decades. The two stations were separated by a distance of 22 km along the coast. The velocity and temperature data collected at 14 m depth were analyzed together with the wind data from the local meteorological station, NCEP/NCAR reanalysis of wind curl data over the Caspian Sea, as well as multi-mission satellite imagery. The analysis revealed that the currents were predominantly along-shore and highly variable in direction, with nearly zero average over the observation period. The along-shore and cross-shore components of velocity exhibited rather high correlation with the along-shore wind stress with the maximum (r = 0.68 and r = 0.53, respectively) at a time lag of about 9.5 h. The velocity series were not significantly correlated with the wind curl averaged over the entire Caspian Sea at any temporal lag, while there were weak but significant correlations between the along-shore current velocity and the curl of the wind fields over the Middle Caspian and Northern Caspian basins with time lags from one to nine days. The along-shore current velocities at the two stations were highly correlated (r = 0.78) with each other at no temporal lag. The temperature at both stations demonstrated nearly identical seasonal march, but a higher frequency variability superimposed on the latter was also evident with amplitudes as high as 2.79 °C. Somewhat surprisingly, the series of these anomalies at the two stations were not correlated either with each other or with surface wind forcing. However, there is evidence pointing to their connection with the cross-shore component of near bottom velocity, i.e., the cross-shore, up or down the bottom slope excursions of water from deeper or shallower depths, retaining a different temperature. During intense winter cooling of the surface layer, this effect is manifested as «warm upwelling» creating strong positive temperature anomalies or the opposite «cold downwelling» and negative anomalies.

Keywords

• Caspian Sea
• Middle Caspian
• Shelf of Kazakhstan
• coastal currents
• temperature variability
• in situ measurements