ACCP NOTES

The Newsletter of the Atlantic Climate Change Program September 1997 Vol. IV No. 2
address e-mail to: wilburn@aoml.noaa.gov

North Atlantic Sector Tree-Ring Records and SST Variability

Rosanne D. D'Arrigo, Edward R. Cook and Heidi Cullen
Tree-Ring Laboratory
Lamont-Doherty Earth Observatory
Palisades, New York 10964

Introduction

A more detailed understanding of decadal and longer term atmosphere-ocean climate variability is hindered by the limited length of instrumental record. Tree rings and other proxy data can help resolve the nature of such variability over centuries to mill ennia, much longer than is possible using instrumental data. These extended records can also aid in model validation. The usefulness of tree rings in reconstructing indices of atmosphere-ocean variability has been demonstrated for a number of areas of t he globe, including the Gulf of Alaska (Wiles et al., in press), the subantarctic (Villalba et al., in press), and the equatorial Pacific (Stahle and Cleaveland 1993). For the North Atlantic sector, this research includes reconstructions of the North Atl antic Oscillation (NAO) (Cook et al., in press) and of Grand Banks sea-surface temperature (SST) (D'Arrigo et al., 1996)

In this article we present spatial correlation fields which compare a 5° x 5° SST data set derived using reduced space optimal analysis (Kaplan et al., 1996) with selected tree-ring records from the adjacent continents surrounding the North Atlanti c. The results confirm previously-observed linkages between tree growth, the NAO, SST and other key features of North Atlantic climate.

Results

The NAO is the principal mode of North Atlantic annual to decadal climate variability (McCartney 1996, Rogers 1990). The NAO sea-level pressure (SLP) index of Rogers (1990) is based on rotated principal components scores of Northern Hemisphere SLP. A r econstruction of the winter NAO index has been derived using tree-ring width records from eastern North America and northwestern Europe (Cook et al., in press). The NAO link with tree rings is a consequence of the significant relationships between the NA O and temperature, precipitation and storm track trajectories over the land areas surrounding the North Atlantic (e.g., Rogers and Van Loon 1979, Kushnir 1996, Cullen and DeMenocal submitted). Figure 1 (left) compares the winter (Dec-Jan-Feb) SST field t o both the instrumental winter NAO index of Rogers (1990) and to the NAO reconstruction (Cook et al., in press). The close agreement in the two correlation patterns strongly suggests that the trees are capturing the essence of the relationship between th e NAO and SST. This pattern of NAO-SST correlations is also very similar to that of the second dominant mode (empirical orthogonal function or EOF) of winter Atlantic SST, which displays the characteristics of the NAO (Kushnir 1996), and with that of an NAO SST index (Cullen and DeMenocal submitted). In Figure 1, the strongest positive correlations for both the NAO index and reconstruction are observed off eastern North America and northwestern Europe, the locations of the tree-ring records used to reco nstruct the NAO. The similarity in pattern provides an independent confirmation of the validity of this reconstruction.

Figure 1 (left): Correlation field of winter (Dec-Jan-Feb) North Atlantic SST with: (top) winter NAO index and (bottom) winter NAO reconstruction for 1900 to 1980.

Figure 1 (right): As in Figure 1 (left) but for the SENA mode.

SENA (Southern Europe-Northeast Atlantic) is another mode of variation in North Atlantic Sector SLP (Rogers 1990). SENA has centers of action over southern Europe and the northeast Atlantic, and accounts for a significant percentage of the SLP variabili ty in the Northern Hemisphere (Rogers 1990). In its positive mode these centers of action are displaced eastward relative to those of the NAO, with the subtropical high situated over the western Mediterranean and the Icelandic Low extending northeast tow ards the Barents Sea (Rogers 1990). Figure 1 (right) compares the winter SENA index (Rogers 1990) with a reconstruction of SENA, based on tree-ring records form eastern North America, Europe and Morocco (D'Arrigo et al., 1995). The SENA pattern (which i s similar to the NAO) is associated with an opposition in winter temperature anomalies between the regions of Greenland and northern Europe, as indicated in both the index and reconstruction plots in Figure 1.

Figure 2 (left) shows a comparison between summer (Jun-Jul-Aug) SST and a millennial tree-ring maximum latewood density series from northern Scandinavia, one of the centers of action of the NAO (Briffa and Schweingruber 1992, Van Loon and Rogers 1978). The density parameter tends to correlate strongly with warm season conditions, while ring-width series can reflect climate during the colder months in addition to summer (e.g., Schweingruber et al., 1993, D'Arrigo et al., 1992). The density correlation p attern with SST is similar to that observed for the summer NAO index (Figure 2, left). 30-40 year oscillations are present in both the tree-ring series from northern Scandinavia and a North Atlantic thermohaline circulation index derived from a coupled o cean-atmosphere model (Delworth et al., 1993, Stocker 1994). The model index could suggest a natural mechanism for such cycles through links with SST and surface air temperature variability in northern Europe (Delworth et al., 1993, Kushnir 1994 & 1996).

Figure 2 (left): Correlation field of summer (Jun-Jul-Aug) North Atlantic SST with (top) summer NAO index and (bottom) Scandinavia density record for 1900-1980.

Figure 2(right): Correlation field of summer (Jun-Jul-Aug) SST and tree-ring density record for southeastern Labrador from 1900-1988.

In the northwest Atlantic, the Labrador Sea and the Grand Banks area southeast of Newfoundland are key regions of North Atlantic climate variability, closely linked to the behavior of the NAO (ACCP 1990, Dickson 1997). The Grand Banks is also a s ite of pronounced variability in SST (Deser 1996). Tree-ring chronologies from Labrador (Schweingruber et al., 1993) were used to reconstruct summer Grand Banks SST back to the early 1700s (D'Arrigo et al., 1996). Figure 2 (right) compares the summer (J un-Jul-Aug) SST field with a tree-ring series used in reconstruction, from southeastern Labrador. A strong positive correlation is found between this tree-ring series and SST south and east of Labrador, extending into the Grand Banks. The overall correl ation pattern is similar to that of the first EOF of Atlantic SST, which shows the greatest loadings in the Grand Banks region (as shown in ACCP 1990).

Summary

Spatial correlation fields between North Atlantic SST and tree-ring records form surrounding land areas have been presented . Results confirm previous studies documenting relationships between North Atlantic sector tree rings and climate. The similar c orrelation fields between SST, and the NAO and SENA sea-level pressure indices and reconstructions provide independent validation of the tree-ring estimates of climate. The correlation field for the tree-ring record for Scandinavia shows a similar patter n to that of the summer NAO, while the pattern found using the tree-ring data from Labrador confirms the strong link with SST in the Grand Banks region near Newfoundland.

References

Acknowledgements

This research was funded by NOAA grand NA36GPO230-01. We thank J. Rogers for contributing the NAO indices, A. Kaplan for providing the SST data set, and K. Briffa and F. Schweingruber for providing tree-ring data.

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