Response of North Pacific and North Atlantic decadal variability to weak global warming

The Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Variability (AMV) are the two dominant low-frequency modes in the climate system. This research focused on the response of the two leading low-frequency modes under weak global warming. The observational data is taken from Hadley Centre Sea Ice and Sea Surface Temperature data set (HadISST) and coupled model outputs from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Four models were selected (bcc-csm1-1, CCSM4, IPSL-CM5A-LR, and MPI-ESM-LR) which have long weak global warming scenarios (RCP2.6) to examine changes in PDO and AMV. Results showed that these models can capture the two low-frequency modes in both pre-industrial run and RCP2.6 run. Under the weak global warming, the time scales of PDO and AMV have a significant decrease while the amplitude only have a little decrease. The interesting thing is that under the weak global warming, the North Pacific sea surface temperature anomaly (SSTA) standard deviation decrease only in decadal time scale and the North Atlantic SSTA standard deviation decrease both in interannual and decadal time scales. The coupled system consists of a slow ocean component, which has a deadal time scale, and a fast atmospheric component, which is calculated by subtracting the decadal from the total. It suggests that under the global warming, the change of PDO is dominant by ocean dynamic and the change of AMV is dominant by ocean dynamic and stochastic atmosphere forcing.