Interannual and Seasonal Variability of the Caspian Coastal Upwelling Offshore Aktau (Kazakhstan)

Upwelling is a well-known feature of water dynamics off the eastern coast of the Caspian Sea during the warm season. Frequent synoptic upwelling events lasting several days lead to the formation of a stable seasonal anomaly of the sea surface temperature (SST) of the Caspian Sea. The frequency and intensity of synoptic upwelling events against the background of seasonal upwelling in different months of the warm period (May–September) in different areas along the eastern coast of the Caspian Sea and their interannual variability have been little studied to date. The present work is devoted to the study of this issue for the Aktau region (Kazakhstan) in the northern part of the Middle Caspian (43.64° N, 51.17° E). The study is based on an analysis of daily in situ measurements of water temperature at a coastal hydrometeorological station in Aktau for the period from May to September in 2021–2024, satellite-origin SSTs at points located in the Aktau coastal zone at 51.17° E and outside the upwelling zone at 50° E (at the same latitude) based on nighttime measurements of the MODIS-Aqua radiometer with a 4-km spatial resolution and 8-day time averaging, and data on wind speed and direction with a three-hour interval at the Aktau weather station, Kazakhstan. The analysis performed showed that the highest intensity of seasonal upwelling in the Aktau area in all the years under consideration occurred in July–August. Against the background of seasonal upwelling, from 7 to 13 synoptic upwelling events happened in different years, and these events were preceded in most cases by an increase in upwelling-favorable wind. The largest annual number of days with synoptic upwelling events occurred in 2023 and 2024 (105 and 81 days, respectively). The largest differences (upwelling intensity) between the minimum in situ temperatures in the coastal zone during synoptic upwelling events and the corresponding satellite-derived temperatures in the open sea were also observed in 2023 and 2024 (11.2 °C and 10.4 °C, respectively). These years (2023–2024) coincided with one of the five strongest El Niño events on record.