Spatiotemporal variation of the southern Caspian Sea surface temperature during 1982–2016

Dominant features of spatiotemporal variability patterns of Sea Surface Temperature (SST) in the southern Caspian Sea were characterized based on 35 years (1982–2016) of satellite-derived fine-scale (0.25 degree) data. The average rate of increase in SST for the whole southern Caspian Sea (0.029 ± 0.009 °C year⁻¹ region-averaged) was faster than the average warming rate of world's ocean (in comparison to 0.011 °C year⁻¹ measured for 1971–2010). The long-term SST warming was evident in both maximum and minimum data series of SST. Monthly SST trend analysis results suggested that the warming was clearly stronger over the period between May and October. Also, the results of our study revealed that the annual warming trend was not spatially uniform in the southern Caspian Sea as the highest warming was detected over the northwest part. To better perceive the thermal condition variability over the southern Caspian Sea, not only the conventional warming trend (at monthly and annual scales) but also other dynamic and crucial features of SST variation (e.g. timing patterns and duration of warm season as well as frequency of extreme hot SST days) were taken into account and characterized. We observed a remarkable inter-annual variation in timing and the extent of the warm season during the 35 years of the study in the southern Caspian Sea (19 days difference between the longest and the shortest warm periods). Also, we found marked spatial heterogeneity in timing patterns and duration of the warm season in the study area where the longest periods were found especially in the deeper central and northern regions. However, the spatiotemporal variability of the number of extreme hot SST days was less pronounced in the southern Caspian Sea as we found no spatial heterogeneity in the frequency of extreme hot SST days.