Co-variability between the Pacific Pentadecadal Oscillation and Length of Day

Shoshiro Minobe (minobe@ep.sci.hokudai.ac.jp)

Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan

/Frontier Research System for Global Change, Tokyo, Japan

After a review of the Pacific Pentadecadal Oscillation (PPO), I will present a statistical evidence that suggests a co-variability between the PPO and the length of day (LOD). In this abstract the main points are described.

The existence of the PPO or a 50-70 year oscillation over the North Pacific was found from the analyses of instrumentally observed data and air-temperatures reconstructed from tree-rings (Minobe 1999 GRL). Shortly after Mantua et al. (1997 BAMS) proposed a similar concept as Pacific (inter-) Decadal Oscillation (PDO). Although these two concepts were considered to be the same at the beginning, a closer analysis indicated that the PDO is the superposition of the PPO and the globally distributed bidecadal oscillation (BO), and the simultaneous phase transitions of the PPO and BO marked the climatic regime shifts in the 1920s, 1940s and 1970s (Minobe 1999 GRL; Minobe 2000 Progr. in Oceanogr.). The lack of a clear regime shift around the beginning of the 20th century may be related with the century-scale meridional migration of the BO (as presented by the author in a regular session of the accompanied JOS meeting).

The climatic regime shifts or PPO is known to influence the interannual variability of the Aleutian low (Minobe and Mantua 1997 Progr. in Oceanogr.), and ENSO influence to United State (Gershunov and Barnett 1998 BAMS) and to Australia (Power 1999 Clim. Dyn.). Moreover, the significant influence of the climatic regime shifts on marine ecosystem was evident (e.g., Mantua et al., 1997 BAMS; Yasuda et al., 1999 Fish. Oceanogr.). The importance of the regime shifts have attracted large attentions as shown by the recent publication of a special issue for the "North Pacific Climate Regime Shifts" in September 2000 on Progr. Oceanogr.

A possible relation between the decadal LOD and climate has been studied by several authors. However, none of the previous studies did not show whether or not the proposed relationship bears a critical test of significance. In the present paper, it will be shown that the PPO and the LOD has a statistically significant relation with the PPO leading the LOD by nine years, using a PPO proxy and LOD data for the last two centuries. This relation is commonly seen in lag-correlation analysis and in coherency analysis. Because the angular momentum related with the decadal LOD is much larger than that to be explained by angular momentum exchanges between the solid earth and the atmosphere, the cause of the decadal LOD has been assumed to be a core-mantle coupling. However, it is noteworthy that if the core-mantle

coupling has an intrinsic tendency to oscillate with a period of 60 years as suggested several theoretical and numerical papers, the core-mantle system can be stimulated by the disturbances on a similar timescale from an outside system, which can be the atmosphere and ocean.