Group Meeting R98229014 Kirsten Feng

3.6.3 Pacific Decadal Variability AL pressure system North Pacific SST in PDO Inter-decadal: IPO Variability indices: NPI PDO/IPO

3.6.3 Pacific Decadal Variability Why not PDO: Schneider and Cornuelle (2005) suggested that the PDO is not itself a mode of variability but is a blend of three phenomena. They showed that the observed PDO pattern and evolution can be recovered from a reconstruction of North Pacific SST anomalies based on a first order autoregressive model and forcing by variability of the Aleutian low, ENSO and oceanic zonal advection in the Kuroshio-Oyashio Extension. For measuring Pacific climate variability: ENSO and NPI

3.6.3 Pacific Decadal Variability There is substantial low-frequency variability, with extended periods of predominantly high values indicative of a weakened circulation (1900–1924 and 1947–1976) and predominantly low values indicative of a strengthened circulation (1925–1946 and 1977–2005).

3.6.4 The North Atlantic Oscillation and Northern Annular Mode NAO: The only teleconnection pattern prominent throughout the year in the NH is the NAO (Barnston and Livezey, 1987). It is primarily a north-south dipole in sea level pressure characterised by simultaneous out-of-phase pressure and height anomalies between temperate and high latitudes over the Atlantic sector, and therefore corresponds to changes in the westerlies across the North Atlantic into Europe.

3.6.4 The North Atlantic Oscillation and Northern Annular Mode Figure 3.31. Normalised indices (units of standard deviation) of the mean winter (December–March) NAO developed from sea level pressure data. In the top panel, the index is based on the difference of normalised sea level pressure between Lisbon, Portugal and Stykkisholmur/Reykjavik, Iceland from 1864 to 2005. The average winter sea level pressure data at each station were normalised by dividing each seasonal pressure anomaly by the long-term (1864 to 1983) standard deviation. In the middle panel, the index is the principal component time series of the leading EOF of Atlantic-sector sea level pressure. In the lower panel, the index is the principal component time series of the leading EOF of NH sea level pressure. The smooth black curves show decadal variations (see Appendix 3.A). The individual bar corresponds to the January of the winter season (e.g., 1990 is the winter of 1989/1990). Updated from Hurrell et al. (2003); see http://www.cgd.ucar.edu/cas/jhurrell/indices. html for updated time series.

3.6.5 The Southern Hemisphere and the Southern Annular Mode Figure 3.32. (Bottom) Seasonal values of the SAM index calculated from station data (updated from Marshall, 2003). The smooth black curve shows decadal variations (see Appendix 3.A). (Top) The SAM geopotential height pattern as a regression based on the SAM time series for seasonal anomalies at 850 hPa (see also Thompson and Wallace, 2000). (Middle) The regression of changes in surface temperature (°C) over the 23-year period (1982 to 2004) corresponding to a unit change in the SAM index, plotted south of 60°S. Values exceeding about 0.4°C in magnitude are signifi cant at the 1% signifi cance level (adapted from Kwok and Comiso, 2002b).

Thank you!