Joint Seminar: Connections between CSL and atmospheric circulation
A link between Caspian Sea level (CSL) and Caspian Sea surface temperature (SST) was proposed based on idealized model simulations. According to models, the summer SST in the southern Caspian Sea (CS) basin is less warm when the CS has a larger extend (higher CSL) and opposite in winter. Also, observations (reanalysis ERA5) seem to show some signs of this, especially around 1995 when the CSL was at a maximum. Here we are investigating observational data to check if they confirm the dependency of the Caspian Sea SST on CSL equivalent to CS size variability. Moreover, so far, the process. responsible was explained by budget calculations of the energy in the CS. The energy budget from observational data of the CS is investigated although the reanalysis (ERA5) did not include varying CSL and with it the size of the CS, especially in the northern basin. Nevertheless, this was done with the hope that the real atmosphere was exposed to the varying CS and some impact might be seen. The energy budget turns out to explain the variability of the SST. But this was not due to evaporation as assumed previously, as evaporation plays only a minor role in the energy budget. Here we found that, in observational data, the signal comes from the net solar radiation at the ground. Reduced solar radiation at the ground is expected when many clouds are shielding the solar insolation. In fact, the time before 1995 is known to have a steep increase of the CSL due to an increase of El Niño events as well as to strong NAO anomalies shifting the position of the jet stream together with a shift of baroclinicity and enhanced precipitation over the CS catchment area, which is shown to have caused a reduced solar insolation. Change of the SST might just be another impact from ENSO and NAO and not due to a larger CS. ENSO and NAO have a large impact not only on the CSL and the CS size, but also on the CS SST and therefore they go hand in hand and do not force each other. Although the study was intended to validate some results of model simulations it ended with a hypothesis, how the CS can influence the general atmospheric circulation by an impact of the size of the CS on the ENSO variability with its impact on the atmospheric circulation. Links are suggested, how the size of the CS might have an impact on the occurrence of El Niño events. It has been tried to validate the hypothesis with observations of the CSL and occurrence of El Niño events, which seem to proof its validity
Date
07.09.2021
Time
15:15–16:15 h
Place
- Virtual Seminar
- Hamburg
Speaker
Klaus Arpe
Organizers
- Shih-Wei Fang