Pacific Decadal Oscillation documented in a coral record of North Pacific winter temperature since 1873

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1 Click Here for Full Article GEOPHYSICAL RESEARCH LETTERS, VOL. 37,, doi: /2010gl043572, 2010 Pacific Decadal Oscillation documented in a coral record of North Pacific winter temperature since 1873 Thomas Felis, 1 Atsushi Suzuki, 2 Henning Kuhnert, 1 Norel Rimbu, 3,4 and Hodaka Kawahata 2,5,6 Received 8 April 2010; revised 3 June 2010; accepted 14 June 2010; published 29 July [1] The Pacific Decadal Oscillation (PDO), the leading mode of sea surface temperature (SST) anomalies in the extratropical North Pacific Ocean, has widespread impacts on precipitation in the Americas and marine fisheries in the North Pacific. However, marine proxy records with a temporal resolution that resolves interannual to interdecadal SST variability in the extratropical North Pacific are extremely rare. Here we demonstrate that the winter Sr/Ca and U/Ca records of an annually banded reef coral from the Ogasawara Islands in the western subtropical North Pacific are significantly correlated with the instrumental winter PDO index over the last century. The reconstruction of the PDO is further improved by combining the coral data with an existing eastern mid latitude North Pacific growth ring record of geoduck clams. The spatial correlations of this combined index with global climate fields suggest that SST proxy records from these locations provide potential for PDO reconstructions further back in time. Citation: Felis, T., A. Suzuki, H. Kuhnert, N. Rimbu, and H. Kawahata (2010), Pacific Decadal Oscillation documented in a coral record of North Pacific winter temperature since 1873, Geophys. Res. Lett., 37,, doi: / 2010GL Introduction [2] The Pacific (inter)decadal Oscillation (PDO) has been described as the leading mode of North Pacific sea surface temperature (SST) anomalies poleward of 20 N. Interdecadal changes in Pacific climate as reflected in the PDO index over the past century had widespread impacts on natural systems, including water resources in the Americas and marine fisheries in the North Pacific [Mantua et al., 1997; Mantua and Hare, 2002]. However, PDO reconstructions from the North Pacific basin that extend beyond the instrumental period are primarily based on data from the adjacent continents, including tree ring chronologies [Biondi et al., 2001; D Arrigo et al., 2001; Gedalof and Smith, 2001; MacDonald and Case, 2005; D Arrigo and Wilson, 2006] and documentary records [Shen et al., 2006]. In the subtropical South 1 MARUM Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany. 2 Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan. 3 Department of Atmospheric Physics, Faculty of Physics, University of Bucharest, Bucharest, Romania. 4 Climed Norad, Bucharest, Romania. 5 Graduate School of Frontier Sciences, University of Tokyo, Tokyo, Japan. 6 Ocean Research Institute, University of Tokyo, Tokyo, Japan. Copyright 2010 by the American Geophysical Union /10/2010GL Pacific, proxy records from annually banded reef corals were shown to capture the basin wide signature of the PDO [Evans et al., 2001; Linsley et al., 2008], which has been also termed the Interdecadal Pacific Oscillation (IPO) [Power et al., 1999]. Gedalof et al. [2002] combined published coral and tree ring records from the Pacific to generate a basin wide reconstruction of the PDO. However, until recently such coral records were not available from the North Pacific, where the occurrence of warm water reefs at subtropical latitudes is relatively rare. [3] Although the PDO index is a measure of SST variations in the extratropical North Pacific, marine proxy records from this region with interannual or better temporal resolution are limited to a geoduck clam reconstruction from the midlatitudes [Strom et al., 2004] and a coralline red algal reconstruction from the subarctic [Halfar et al., 2007]. Boreal winter is the commonly used season for detecting prominent PDO variability and associated large scale teleconnections in instrumental climate data [Mantua et al., 1997; Mantua and Hare, 2002; Deser et al., 2004], but only a limited number of proxy records has been interpreted as documenting PDO variability during this season [e.g., Biondi et al., 2001; Gedalof et al., 2002]. [4] Recently, SST and large scale salinity variations have been inferred from the skeletal composition of an annuallybanded reef coral from the Ogasawara Islands [Felis et al., 2009]. This group of islands is located in the western subtropical North Pacific within the region used to define the PDO. Here, we explore the potential of Sr/Ca and U/Ca from this coral as indicators of winter PDO variations. In addition we combine our data with the SST reconstruction based on geoduck clams from the eastern mid latitude North Pacific [Strom et al., 2004] to test whether this PDO reconstruction improves over those derived from the individual records. At the two sites, the PDO is characterized by SST anomalies of opposite sign, which potentially provides an enhanced signal with respect to reconstructing the PDO. 2. Material and Methods [5] The Ogasawara coral record consists of bimonthly resolved time series of Sr/Ca, U/Ca and d 18 O for the period generated from the aragonitic skeleton of a massive Porites colony [Felis et al., 2009]. This shallowwater coral is growing at Chichijima ( N, E), Ogasawara Islands (Japan), located in the southwestward return flow of the Kuroshio recirculation system. Coral Sr/Ca and U/Ca are established proxies for SST [Corrège et al., 2000; Hendy et al., 2002], and in the case of the Ogasawara coral record are significantly correlated with the instrumental record of local SST on both seasonal and 1of6

2 Figure 1. (a) Winter Sr/Ca and U/Ca time series (November February) of the Ogasawara coral record from the western subtropical North Pacific and corresponding error bars. Coral based sea surface temperature (SST) anomalies using the corresponding proxy SST relationships for annual average data [Felis et al., 2009] are shown. (b) Time series of the Pacific (inter)decadal Oscillation (PDO) index [Mantua et al., 1997] for winter (November February). (c) Time series of the geoduck clam growth index from the mid latitude North Pacific [Strom et al., 2004]. (d) Time series of the combined coralgeoduck index. Bold lines: 3 year running averages. interannual time scales [Felis et al., 2009]. Here we focus on November December January February (NDJF) SST variations estimated from coral Sr/Ca and U/Ca, as PDO variability is strongest during winter time [Mantua et al., 1997; Mantua and Hare, 2002; Deser et al., 2004]. The NDJF coral Sr/Ca and U/Ca time series were calculated from the bimonthly resolved coral record as the average of the corresponding November December (ND) and January February (JF) values. The NDJF value of a given year (e.g., 1901) includes the ND value of the preceding year (1900). For comparison with the coral proxy records of SST we used the established index of the PDO [Mantua et al., 1997] for NDJF, which starts in 1900 and is derived from instrumental North Pacific SST poleward of 20 N ( edu/pdo/pdo.latest). [6] We generated an index of reconstructed PDO variability by combining the NDJF Sr/Ca and U/Ca time series of the Ogasawara coral record with a growth index based on geoduck clams (Panopea abrupta) from the Strait of Juan de Fuca [Strom et al., 2004], a coastal site in the eastern midlatitude North Pacific ( N, W, Washington State, northwest U.S.). The ring width index is based on annual growth rings of 6 to 23 geoduck shells, and has been reported to document local SST and PDO variability [Strom et al., 2004]. All three time series were detrended and normalized to unit variance over their period of overlap ( ). Then, a coral index was calculated as the average of the detrended and normalized NDJF Sr/Ca and U/Ca time series. Consequently, the coral geoduck index was derived as the average of this coral index and the detrended and normalized geoduck growth index. In order to place the PDO variations reconstructed from the coral geoduck index into a large scale climatic context, we used gridded data sets of global SST based on marine observational records (ERSSTv3b, 2 grid) [Smith et al., 2008] and of global land surface temperature and precipitation based on climate observations from meteorological stations (CRU TS 2.1, 0.5 grid) [Mitchell and Jones, 2005]. Prior to field correla- 2of6

3 Figure 2. (a) Results of multitaper method (MTM) spectral analysis with a red noise null hypothesis [Ghil et al., 2002] (number of tapers, 3; bandwidth parameter, 2; 90%, 95%, and 99% significance levels are indicated) of detrended and normalized winter (November February) time series ( ) for coral Sr/Ca (left), coral U/Ca (middle), and the Pacific (inter)decadal Oscillation (PDO) index [Mantua et al., 1997] (right). (b) Results of MTM cross spectral analysis of the detrended and normalized winter (November February) coral Sr/Ca and PDO index time series ( ) (left) and of the corresponding coral U/Ca and PDO index time series (middle). The 95% significance level for coherence is indicated. tion analyses, all data were linearly detrended and then smoothed with a 3 year running average. 3. Results and Discussion [7] The winter (NDJF) Sr/Ca and U/Ca time series of the Ogasawara coral record are significantly correlated with the instrumental PDO index [Mantua et al., 1997] for NDJF over the last century (Figure 1). For the period , the correlations between the NDJF PDO index and the NDJF coral Sr/Ca (r = 0.30) and U/Ca (r = 0.31) time series are statistically significant (99.5% level, 2 sided t test). The correlations remain high when coral Sr/Ca and U/Ca lag the PDO index by 1 to 6 years. The coral Sr/Ca and U/Ca records are proxies for regional SST variability in the western subtropical North Pacific Ocean [Felis et al., 2009], whereas the PDO index represents SST anomalies averaged over the entire North Pacific poleward of 20 N [Mantua et al., 1997]. Therefore, in order to suppress local high frequency variability in the coral based SST reconstructions and to enhance the common signal with the PDO index on decadal to interdecadal time scales, we applied a 3 year running average to all time series. About 60% of the total variance is captured by the smoothed records. The resulting correlations with the NDJF PDO index increase for both NDJF coral Sr/Ca (r = 0.51) and U/Ca (r = 0.54) and are statistically significant even when we take into account the reduction in the number of degrees of freedom that arises from the filtering of the time series (99.5% level, 2 sided t test). The correlations with the PDO index are somewhat lower compared to those obtained from tree ring networks [Biondi et al., 2001; D Arrigo et al., 2001; Gedalof and Smith, 2001; MacDonald and Case, 2005; D Arrigo and Wilson, 2006], which are based on a large number of individual chronologies from multiple sites. However, considering that our results are based on a single proxy record the strength of the correlations is remarkable. [8] The improved correlation of the smoothed time series suggests that primarily the low frequency components of the PDO are reflected in the proxy records. To constrain this further we applied multitaper method (MTM) spectral analysis with a red noise null hypothesis [Ghil et al., 2002] on our reconstructions and the instrumental PDO over the same period of time ( ). This null hypothesis assumes that low frequency energy in the spectrum accumulates from white noise filtered by autocorrelation, but does not distinguish between tropical and extratropical forcing of the PDO [Newman et al., 2003]. In the proxy reconstructions, significant variance is identified at multidecadal periods of 47 years and interannual periods of 3.2 and years in both the NDJF coral Sr/Ca and U/Ca time series. Similarly, enhanced variance at multidecadal time scales and significant variance at interannual periods of 3.4 and 2.4 years is present in the NDJF PDO index (Figure 2a). Coherence between the two coral time series and the PDO index is statistically significant on the multidecadal time scale (Figure 2b). The similarity between the individual spectra at long periods meets our expectation, but the comparison also indicates that some of the high frequency components of PDO variability are captured by the coral. The period of 47 years is at the limit of detection with respect to the length of the considered time series, where oscillatory behavior becomes indistinguishable from a secular trend, however, the coherence at pentadecadal periods is supported by the direct comparison of the records (Figure 1). Prominent pentadecadal variability is a well known characteristic of North Pacific climate and PDO variability [Minobe, 1997; Mantua and Hare, 2002]. The interannual periods likely reflect the signature of quasibiennial and El Niño Southern Oscillation signals in North Pacific SST [Tourre et al., 2001], that also imprint the PDO. 3of6

4 Figure 3. Correlation maps for the coral geoduck index and global sea surface temperature [Smith et al., 2008] ( ) (a), global land surface temperature (b), and precipitation (c) [Mitchell and Jones, 2005] ( ) for winter (November February). (d, e, and f) The same as in Figures 3a, 3b, and 3c, but for the instrumental winter (November February) index of the Pacific (inter)decadal Oscillation (PDO) [Mantua et al., 1997]. Correlation maps were generated for 3 year running averages. Correlations of r = 0.3 and higher are statistically significant (95% level, 1 sided t test), taking into account the reduction in the number of degrees of freedom. The coral (diamond) and geoduck (square) sites are indicated. [9] Combining our coral based reconstructions of western North Pacific SST with a SST reconstruction from the eastern North Pacific that is based on growth rings of geoduck clams [Strom et al., 2004] provides an index that is better correlated with the NDJF PDO index than any of the individual records (Figure 1). The correlation of this coral geoduck index with the NDJF PDO index during the period (r = 0.47) is statistically significant (99.9% level, 2 sided t test), although the geoduck growth index has been originally reported to correlate best with regional March October SST [Strom et al., 2004]. The correlation between the coralgeoduck index and the NDJF PDO index improves for 3 year running averages (r = 0.69; 99.9% level, 2 sided t test). The variability at periods >7 years calculated from the unsmoothed time series explains a similar percentage of variance in both the coral geoduck (63%) and the PDO index (57%). A comparable result is obtained for 3 year running averages of the coral geoduck (70%) and the PDO index (61%). [10] In order to investigate if the coral geoduck index captures the characteristic large scale impact of the PDO on temperature and precipitation throughout the North Pacific basin and adjacent continental areas during boreal winter over 4of6

5 the last century [Mantua et al., 1997; Mantua and Hare, 2002], we performed spatial correlation analyses using global fields of SST [Smith et al., 2008] and land surface temperature and precipitation [Mitchell and Jones, 2005] (Figure 3). During the period 1901/ , the spatial correlations of the coral geoduck index with global fields of SST, land surface temperature and precipitation resemble the typical PDO pattern [Mantua et al., 1997; Mantua and Hare, 2002] for NDJF (Figure 3). For the warm (high index) phase of the PDO, this spatial pattern is characterized by anomalously cool SST in the western to central subtropical to midlatitude North and South Pacific coinciding with anomalously warm SST in the central to eastern tropical to subtropical Pacific, along the west coast of the Americas, in the Bering Sea, in most of the Indian Ocean and areas of the Atlantic Ocean (Figures 3a and 3d). Over land, the warm phase of the PDO is characterized by anomalously warm temperatures in northwestern North America, northeastern Australia and the Indonesian archipelago from Sumatra to New Guinea, central northern Asia, and regions of eastern and southern Africa and northern South America, and anomalously cool temperatures in Japan and eastern Canada (Figures 3b and 3e); and by anomalously dry conditions in eastern Australia, northeastern Asia, southern and northwestern Africa, and central North America, and anomalously wet conditions in the southwest U.S. and northern Mexico, and the coastal Gulf of Alaska (Figures 3c and 3f). [11] The combination of different climate archives from the Pacific basin increases the correlation with the PDO index as demonstrated by Gedalof et al. [2002], who used coral records from the tropical Pacific and the subtropical South Pacific, and tree ring records from the Americas. The PDO reconstruction of Gedalof et al. [2002] is well correlated with the winter PDO index (r = 0.64). However, combining this reconstruction with the coral geoduck index further increases the correlation with the NDJF PDO index (r = 0.68; 99.9% level, 2 sided t test). For 3 year running averages, the correlation increases further (r = 0.80) and is statistically significant (99.9% level, 2 sided t test). This result demonstrates that proxy records of SST such as represented by the coral geoduck index can provide a contribution from the North Pacific Ocean to reconstructions of basin scale PDO variability. 4. Conclusions [12] In the western subtropical North Pacific Ocean, annually banded corals that are growing since centuries can be found [Tsunoda et al., 2008; Felis et al., 2009]. Our results suggest that coral based proxy records of winter SST from open ocean locations of this region provide a promising archive for reconstructing the winter PDO signal in North Pacific SST beyond the period of instrumental observations. Furthermore, we demonstrate that combining proxy records from regions of PDO related SST anomalies of opposite sign improves the PDO reconstruction. In the example presented here, the smoothed (3 year running average) PDO index based on combined western North Pacific coral [Felis et al., 2009] and eastern North Pacific clam data [Strom et al., 2004] explains 47% of the variance in the instrumental winter PDO index. The combined results demonstrate that subseasonally to annually resolved proxy records of SST from both sides of the extratropical North Pacific Ocean can contribute a marine perspective to reconstructions of basinscale PDO variability, which is important as different PDO reconstructions do not agree prior to the 20th century. [13] Acknowledgments. We thank T. Sato and H. Adachi for core drilling, S. Tsukamoto (Tokyo Metropolitan University) for logistic support, A. Strom for providing proxy data, and two anonymous reviewers for constructive comments. 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