Shayne McGregor
Interdecadal sea surface temperature variability in the tropical Pacific Ocean: the role of equatorial and off-equatorial wind stress forcing, oceanic Rossby waves, and the coupled system
The mechanisms forcing interdecadal sea surface temperature (SST) variability of the Pacific Ocean are investigated with the help of the Australian Bureau of Meteorology Research Centre coupled general circulation model (CGCM) and a first baroclinic mode ocean (“shallow-water”) model (SWM). An empirical orthogonal function analysis was performed on the anomalous lowpass filtered SST and vertically averaged temperature (VAT) in the upper 300m generated in a 100-year CGCM simulation. The dominant mode SST spatial pattern and interdecadal-scale time variability from the CGCM simulation is consistent with the Interdecadal Pacific Oscillation (IPO). In a separate control simulation, the SWM was forced with wind stresses generated in the CGCM 100-year integration to identify if the fundamental processes of interdecadal SST variability generated in the CGCM are captured by the ocean dynamics of the SWM (through Rossby and Kelvin wave propagation). The SWM control simulation produces a dominant interdecadal mode of thermocline depth variability that is very similar spatially and temporally to the first interdecadal SST and vertically averaged temperature (VAT) modes from the CGCM, indicating that large-scale wind forced upper ocean dynamics could play a dominant role in producing interdecadal SST variations observed in the CGCM’s tropical Pacific.
A series of SWM experiments are then used to identify the role of “uncoupled” equatorial wind stress forcing, off-equatorial wind stress forcing, the coupled ocean-atmosphere response and the western boundary reflection of Rossby waves on the equatorial Pacific Ocean’s thermocline depth variability. The coupled atmospheric response, in the equatorial region, is found to amplify both the “uncoupled” equatorial wind stress forced equatorial thermocline depth anomalies and the off-equatorial wind stress forced equatorial thermocline depth anomalies. The equatorial Pacific Ocean’s overlying wind stresses explains the majority of the interdecadal time scale equatorial thermocline depth variability and off-equatorial wind stress forcing is responsible for producing the remaining small but significant fraction of the tropical Pacific Ocean’s interdecadal thermocline depth variability. We further show that Rossby wave reflection at the western Pacific boundary underpins the majority of this off-equatorially forced equatorial thermocline depth variability and that the response of the equatorial thermocline to Rossby wave reflection may be predicted years in advance with the aid of satellite altimetry observations and the 1-½ layer approximation. Overall, these results could have significant implications for the predictability of tropical Pacific Ocean SST and climate, as it suggests that the interdecadal variability of the tropical Pacific Ocean is made up of a relatively unpredictable equatorially forced component and a predictable off-equatorially forced component.