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VOL. 16, NO.23 1DECEMBER2003JOURNAL OF CLIMATEq 2003 American Meteorological Society 3853LettersENSO-Forced Variability of the Pacific Decadal OscillationMATTHEWNEWMAN,GILBERTP. COMPO,ANDMICHAELA. ALEXANDERNOAA–CIRES Climate Diagnostics Center, University of Colorado, Boulder, Colorado5 March 2003 and 12 June 2003ABSTRACTVariability of the Pacific decadal oscillation (PDO), on both interannual and decadal timescales, is well modeledas the sum of direct forcing by El Nin˜o–Southern Oscillation (ENSO), the ‘‘reemergence’’ of North Pacific seasurface temperature anomalies in subsequent winters, and white noise atmospheric forcing. This simple modelmay be taken as a null hypothesis for the PDO, and may also be relevant for other climate integrators that havebeen previously related to the PDO.1. IntroductionStudies of sea surface temperatures (SST) in the NorthPacific have focused on the Pacific decadal oscillation(PDO) as the leading mode of variability, particularlyon decadal timescales (Mantua et al. 1997). There isconsiderable uncertainty, however, about whether thePDO is truly independent of the leading mode of tropicalvariability, El Nin˜o–Southern Oscillation (ENSO;Zhang et al. 1997; Evans et al. 2001). On the one hand,simultaneous correlations of the November–Marchmean PDO index with various indices of ENSO are low(Mantua et al. 1997). In addition, the spatial patterns ofSST variability in the Pacific on interannual and decadaltimescales are different; interannual variability of SSTexhibits the pronounced ENSO maximum in the tropicaleast Pacific and a weaker center of opposite sign in theNorth Pacific, while on decadal timescales the relativestrength of these centers is reversed so that the amplitudeof the tropical maximum is about 75% that of the NorthPacific center (Zhang et al. 1997).On the other hand, weak simultaneous correlation be-tween ENSO and the PDO may be misleading. Anom-alous tropical convection induced by ENSO influencesglobal atmospheric circulation and hence alters surfacefluxes over the North Pacific, forcing SST anomaliesthat peak a few months after the ENSO maximum intropical east Pacific SSTs (Trenberth and Hurrell 1994;Alexander et al. 2002). This ‘‘atmospheric bridge’’ ex-plains as much as half of the variance of January–MarchCorresponding author address: Matthew Newman, NOAA–CIRESClimate Diagnostics Center, Mail Code R/CDC, 325 Broadway, Boul-der, CO 80303-3328.E-mail: [email protected] mean anomalies of SST in the central NorthPacific (Alexander et al. 2002). Furthermore, North Pa-cific SSTs have a multiyear memory during the coldseason. Deep oceanic mixed layer temperature anoma-lies from one winter become decoupled from the surfaceduring summer and then ‘‘reemerge’’ through entrain-ment into the mixed layer as it deepens the followingwinter (Alexander et al. 1999). Thus, over the courseof years, at least during winter and spring, the NorthPacific integrates the effects of ENSO.The prevailing null hypothesis of midlatitude SSTvariability posits that the ocean integrates forcing byunpredictable and unrelated weather, approximated aswhite noise, resulting in ‘‘reddened’’ noise with in-creased power at low frequencies and decreased powerat high frequencies (e.g., Frankignoul and Hasselmann1977). In this paper, we propose an expanded null hy-pothesis for the PDO: variability in North Pacific SSTon seasonal to decadal timescales results not only fromred noise but also from reddening of the ENSO signal.2. Data and resultsThe analysis described in this paper is based on sim-ple indices of SST and atmospheric variability for theyears 1900–2001. SSTs are from the Hadley Sea Iceand Sea Surface Temperature analysis (Rayner et al.2003) for the years 1900–99 and from the National Oce-anic and Atmospheric Administration (NOAA) recon-structed SST (Smith et al. 1996) dataset for 2000–01.Both datasets are on a 18318 grid. Monthly meananomalies were determined by removing the 1950–2001climatological monthly means; empirical orthogonalfunctions (EOFs) are determined for this period. Very3854 VOLUME16JOURNAL OF CLIMATEFIG. 1. (a) Annual cycle of PDO autocorrelation, plotted as a func-tion of month. Heavy dashed lines indicate correlation with Mar PDO;e.g., the cross represents correlation of the PDO in Feb of year 0 andMar of year 1 (i.e., lag 113). (b) Same as (a) but for the NPI. (c)Annual cycle of cross correlation between the PDO and the NPI. TheNPI leads the PDO for positive lags; the PDO leads the NPI fornegative lags. The month ordinate refers to the NPI; e.g., the crossrepresents correlation between Mar NPI and Dec (lag 19) PDO. (d)Annual cycle of cross correlation between ENSO and the PDO. ThePDO leads ENSO for positive lags; ENSO leads the PDO for negativelags. The month ordinate refers to the PDO; e.g., the cross representscorrelation between Mar PDO and Oct (lag 25) ENSO. The thinwhite line is the 0.58 contour. Correlations are for the period 1950–2001, and contour (fill) interval is 0.2 (0.1). Only values that are atleast 90% significant are shaded.similar results were obtained by instead using theNOAA dataset for the 1950–2001 period, and/or bycomputing the EOFs separately for each month.The PDO index is determined by projecting SST onthe leading EOF of monthly SST in the Pacific northof 208N (Mantua et al. 1997). The ENSO index is de-termined by projecting SST on the leading EOF ofmonthly SST in the region 208N–208S, 1208E–608W;similar results are obtained with other ENSO indices.Atmospheric variability is represented by the North Pa-cific index (NPI; Trenberth and Hurrell 1994), an av-erage of sea level pressure (SLP) in the region 308–658N, 1608E–1408W. The NPI measures the strength ofthe Aleutian low during the Northern Hemisphere coldseason, and it encompasses the area of maximum SLPvariance in the North Pacific for all months. All indicesare departures from the annual cycle, subjected to athree-month running mean, detrended over the 102-yrrecord, and normalized to have unit variance. Prior to1950, the general paucity of data may make month-to-month variation of the SST indices unreliable. Thus, theseasonal cycle of correlation is only calculated for the1950–2001 period, while all other analyses, which focuson timescales of 1 yr and greater, use the entire record.Detrending the 1950–2001 period (not shown) some-what reduces the autocorrelation of the PDO and itspower for


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