The NAO, low-frequency variability, and the CCSM

The North Atlantic Oscillation (NAO) influences much of the climate over the northern hemisphere. Associated with the NAO are large-scale anomalous sea level pressure oscillations between the Azores High and the Icelandic Low. Multi-decadal variability in the NAO has been documented in several observational records, but recent questions in climatology have speculated upon the effects that anthropogenic forcing and polar amplification might have upon the NAO. For better comprehension of North Atlantic and Arctic multi-decadal variability, it is necessary to look outside of the present day climatological context provided by 50 to 100 year observational records. This study uses data from the NCAR Community Climate Systems Model (CCSM) 2.0 to inspect the low-frequency variability of the NAO. The CCSM 2.0 data is from a fully coupled model run, including components from the ocean, atmosphere, land, and sea ice.

Sea level pressures (PSL) over 290 years are used to create a monthly index for the NAO and are examined along with reference height temperatures (TREFHT) from over the Arctic Ocean. The time series for the NAO exhibits cyclic activity approximately every 30 years. Arctic temperatures show a similar low-frequency variability also. However, the two time series vary in and out of phase with one another throughout different decades over the time period. The nature of this changing relationship and the low-frequency variability seen in the CCSM 2.0 NAO as compared to observational records elicits coupled processes at work in the NAO and addresses the nature of recent climatological trends.