Search

The statistical method, Average Predictability Time (APT) decomposition, is used in the present paper to estimate the decadal predictability of upper ocean heat content over the global ocean, North Pacific and North Atlantic, respectively. The twentieth century simulations from CMIP5 outputs are the main data sources in this study. On global scale, the leading predictable component is characterized by a warming trend over the majority of oceans, which is related to the anthropogenic forced response. The second predictable component has significant loadings in the North Atlantic, especially in the subtropical region, which originates from the Atlantic Multidecadal Oscillation (AMO) predictability. To separate interactions among different ocean basins, we further maximize APT in individual North Pacific and North Atlantic oceans. It is found that the second and the third predictable component in North Pacific are significantly correlated with the well-known North Pacific Gyre Oscillation mode and the Pacific Decadal Oscillation respectively. Upper limit prediction skill of these two components are on the order of 6 years. In contrast, the most predictable component derived from the North Atlantic features an AMO-like spatial structure with its prediction skill up to 18 years, while the basin mode due to global warming only exists as the third component. This indicates the interdecadal variability in the North Atlantic is strong enough to mask the anthropogenic climate signals. Furthermore, predictability in the real world is also investigated and compared with model results by using observation-based data. [ABSTRACT FROM AUTHOR]

The western North Pacific Subtropical Countercurrent (WSTCC), anchored by the western subtropical front (WSTF), varies from seasonal to multidecadal time scales and affects the climate variability in the North Pacific. However, conflicting views exist for physical mechanisms of the decadal to multidecadal variability of the WSTF/WSTCC. While many previous studies attribute the WSTF/WSTCC decadal variability to the anomalous wind forcing by the Pacific Decadal Oscillation, by synthesizing observations, reanalysis products and the pre‐industrial model simulation experiment, the present study demonstrates that they are controlled by the Atlantic Multidecadal Oscillation (AMO) through the Subtropical Mode Water (STMW) variability. The AMO induced decadal temperature and volume anomalies of the STMW are stored in the subsurface ocean through the subduction process south of the Kuroshio Extension. Then these anomalies propagate southwestward along the thermocline circulation and reaches the northern flank of the WSTCC in about 5 years. During the AMO‐positive phase, the anomalous warm and thin STMW flattens the upper thermocline, reduces the meridional temperature gradient and weakens the WSTF intensity, lagging 5 years behind the AMO index. Subsequently, the WSTCC decreases and moves southward, and vice versa for the AMO‐negative phase. Our results provide a potential predictability for the WSTF and WSTCC decadal to multidecadal variability and are important for the North Pacific climate change and the biogeochemical processes. Plain Language Summary: The North Pacific subtropical countercurrent (STCC) is a shallow eastward jet stretching from the western Pacific to the north of Hawaii, accompanied by the subtropical front (STF) in the upper 200 m. Specifically, the western branch (western North Pacific Subtropical Countercurrent [WSTCC] and western subtropical front [WSTF]) is much stronger and broader than the eastern part. Decadal to multidecadal variability of the WSTCC and WSTF is known to have significant effects on the North Pacific climate change and biogeochemical processes. However, conflicting views exist for physical mechanisms of the decadal to multidecadal variability of the WSTF/WSTCC. While many previous studies attribute the WSTF/WSTCC decadal to multidecadal variability to the anomalous wind forcing by the Pacific Decadal Oscillation, the present study demonstrates that they are controlled by the Atlantic Multidecadal Oscillation (AMO) through the Subtropical Mode Water (STMW) variability. The STMW anomalies caused by the AMO are stored in the subsurface ocean and would propagate southwestward along the circulation, reach the WSTF/WSTCC region in about 5 years and influence the current's decadal to multidecadal variations. Our results provide a potential predictability for the WSTF and WSTCC variability on decadal to multidecadal time scales and are important for the North Pacific climate change and the biogeochemical processes. Key Points: The decadal‐multidecadal variation of the western North Pacific subtropical front and countercurrent is controlled by the Atlantic Multidecadal Oscillation (AMO)The AMO modulates the Subtropical Mode Water (STMW) variability, which controls the upper pycnocline slope and dominates the intensity of the countercurrentIt takes ∼5 years for the STMW to reach the countercurrent region, corresponding to the time lag of the frontal changes with the AMO [ABSTRACT FROM AUTHOR]