Prediction of the Caspian Sea level using ECMWF seasonal forecasts and reanalysis

Abstract

The hydrological budget of the Caspian Sea (CS) is investigated using the European Centre for MediumRange Weather Forecasts interim reanalysis (ERAi) and seasonal forecast (FCST) data with the aim of predicting the Caspian Sea Level (CSL) some months ahead. Precipitation and evaporation are used. After precipitation events over the Volga River, the discharge (Volga River discharge (VRD)) follows with delays, which are parameterized. The components of the water budget from ERAi and FCSTs are integrated to obtain time series of the CSL. Observations of the CSL and the VRD are used for comparison and tuning. The quality of ERAi data is sufficiently good to calculate the time variability of the CSL with a satisfactory accuracy. Already the storage of water within the Volga Basin allows forecasts of the CSL a few months ahead, and using the FCSTs of precipitation improves the CSL forecasts. The evaporation in the seasonal forecasts is deficient due to unrealistic sea surface temperatures over the CS. Impacts of different water budget terms on the CSL variability are shown by a variety of validation tools. The importance of precipitation anomalies over the catchment of the Volga River is confirmed, but also impacts from the two southern rivers (Sefidrud and Kura River) and the evaporation over the CS become obvious for some periods. When pushing the FCSTs beyond the limits of the seasonal FCSTs to 1 year, considerable forecast skill can still be found. Validating only FCSTs by the present approach, which show the same trend as one based on a statistical method, significantly enhances the skill scores.

Long‐term Caspian Sea level change

“…We note that a previous study [Arpe et al, 2014] found good agreement between observed CSL change and ECM P-E flux estimates, plus Volga River R. In that study average annual PER flux was removed, including its mean value. The effect of this after integration over time would be to exclude linear trends, which are a dominant feature at longer time scales.…”

“…The effect of this after integration over time would be to exclude linear trends, which are a dominant feature at longer time scales. Removing PER flux means (or using PER flux anomalies) appears to be a valid approach when focusing in nonlinear CSL changes [Arpe et al, 2014]. If the PER flux mean was not removed, the PER-predicted, long-term CSL changes (based on ECM P-E, plus Volga River R) would be significantly larger than those observed.…”

Precipitation and temperature of the southwest Caspian Sea region during the last 55 years: their trends and teleconnections with large‐scale atmospheric phenomena

“…In 1995, the CSL reached its peak. Before that year the Volga had excessive precipitation and after that reduced precipitation (Arpe et al , , ). The direct connection between T2m and ENSO is weak.…”

Past and future impact of the winter North Atlantic Oscillation in the Caspian Sea catchment area

“…The NAO, EA and SCA indices in this study are defined as standardized first, second and third principal component time series calculated from seasonal-mean SLP anomalies in the region [70 W-60 E, 20 -80 N], which includes the CS basin. We focus on the winter season, Dec-Jan-Feb (DJF), as the NAO and other modes display their largest climate impact in the boreal winter months (Arpe et al, 2000;Arpe et al, 2014;Hurrell et al, 2003;Hurrell et al, 2013). All data was detrended before performing the EOF analysis.…”