| Popis: |
Sea surface temperatures (SSTs) in the tropics can act as a source of boundary-forced predictability for the atmosphere in the extratropics, which is characterized by its large internal variability and poor predictability. However, if the SST forcing is strong and persistent enough to establish the boundary-forced circulation and overcome this chaotic intrinsic variability of the extratropical atmosphere, potential predictability can be increased. Among the strongest events with such an influence on the climate variability throughout the world is the El Niño-Southern Oscillation (ENSO). Here, the potential impact of tropical SSTs on the signal of geopotential heights at 200 hPa (GH200) was tested based on the results of four different ensembles of numerical simulations. Using an intermediately complex AGCM (ICTP AGCM) we designed experiments with SST anomalies serving as AGCM lower boundary forcing prescribed globally, in the tropical zone of all oceans, only inside the tropical Atlantic area, and limited to the tropical Pacific. Additionally, the fifth AGCM experiment containing global SST forcing and double the amount of imposed CO2 concentration was added to the analysis for comparison of the results to warmer climate conditions. All of the simulations within each of the experiments extended over the 156-year long period between 1855 and 2010. Signal over the North Atlantic-European region (NAE ; 30° N-60° N ; 50° W-30° E) was selected as the main focus of the analysis. This monthly GH200 signal was calculated based on the difference between the ensemble mean of each experiment and the climatological mean for the considered time period. To detect the extent of the ENSO influence on the signal over the NAE region, ENSO years were selected based on the value of the standardized Niño3.4 index in the late-winter season. Signal in different AGCM experiments was then averaged over the NAE region and over the whole time period for ENSO years, non-ENSO years and all years, respectively. Results have shown that the signal is the strongest in the late-winter months (January- March) in all of the experiments. The AGCM experiment with SST boundary forcing prescribed only in the tropical Atlantic consistently yielded the least amount of signal among the rest of the experiments. Overall, the GH200 signal is more pronounced and persists in strength and pattern throughout the late winter months when only the ENSO years are taken into account. The strongest signal linked to ENSO events is found in the experiment with the SST forcing prescribed only in the tropical Pacific. Comparing the results from the two ensembles forced with global SST anomalies, larger values of signal were found in the experiment with the two times higher CO2 concentration. |
