Contribution of the sea surface temperature over the Mediterranean-Black Sea to the decadal shift of the summer North Atlantic Oscillation

Recent observational study has shown that the southern center of the summer North AtlanticOscillation (SNAO) was located farther eastward after the late 1970s compared to before. In this study,the cause for this phenomenon is explored. The result shows that the eastward shift of the SNAO southerncenter after the late 1970s is related to the variability of the Mediterranean-Black Sea (MBS) SST. Awarm MBS SST can heat and moisten its overlying atmosphere, consequently producing a negative sea levelpressure (SLP) departure over the MBS region. Because the MBS SST is negatively correlated with the SNAO,the negative SLP departure can enhance the eastern part of the negative-phase of the SNAO southern center,consequently producing an eastward SNAO southern center shift. Similarly, a cold MBS SST produces an eastwardpositive-phase SNAO southern center shift.The reason for why the MBS SST has an impact on the SNAO after the late 1970s but why it is not the casebeforehand is also discussed. It is found that this instable relationship is likely to be attributed tothe change of the variability of the MBS SST on the decadal time-scale. In 1951--1975, the variability ofthe MBS SST is quite weak, but in 1978--2002, it becomes more active. The active SST can enhance theinteraction between the sea and its overlying atmosphere, thus strengthening the connection between theMBS SST and the SNAO after the late 1970s. The above observational analysis results are further confirmedby sensitivity experiments.