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GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO. 24, 2221, doi:10.1029/2002GL015743, 2002

Midlatitude circulation patterns associated with decadal and interannual Pacific Ocean variability Oliver W. Frauenfeld and Robert E. Davis Department of Environmental Sciences, University of Virginia, Charlottesville, Virginia, USA Received 25 June 2002; revised 9 October 2002; accepted 10 October 2002; published 28 December 2002.

[1] The exact nature of the interaction between the Pacific Ocean and the climate of the Northern Hemisphere is still in large part uncertain. To investigate these interactions, canonical correlation analysis is applied to relate midlatitude atmospheric circulation variability with Pacific Ocean SST variability. The leading mode corresponds closely to decadal PDO-like variability and the second mode is comparable to ENSO. Consequently, we identified different atmospheric circulation patterns related to the spatially similar, but temporally different modes of SST variability. Circulation variability over Eurasia is linked with the decadal SST variability, while the interannual SST variability is linked to circulation variability over the eastern Pacific. This suggests that decadal SST variability, in part, could be forced by the atmosphere. Furthermore, while the PDO switched back into its negative phase in the late 1990s, the decadal SST variability as related to circulation did not. Instead, the interannual mode has shifted into a negative phase, indicating a change in the nature of ocean-atmosphere interaction in the late INDEX TERMS: 3339 Meteorology and Atmospheric 1990s. Dynamics: Ocean/atmosphere interactions (0312, 4504); 3319 Meteorology and Atmospheric Dynamics: General circulation; 3309 Meteorology and Atmospheric Dynamics: Climatology (1620). Citation: Frauenfeld, O. W., and R. E. Davis, Midlatitude circulation patterns associated with decadal and interannual Pacific Ocean variability, Geophys. Res. Lett., 29(24), 2221, doi:10.1029/2002GL015743, 2002.

1. Introduction [2] It is likely that the Pacific Ocean plays a key mechanistic role in hemispheric and global scale climate changes, both gradual and abrupt. For example, the tropical Pacific shift into a warm mode during the winter of 1976 – 1977 coincided with a global climate shift toward a warmer regime. Many studies have found that tropical sea surface temperatures (SSTs) induce responses in the midlatitude North Pacific via an altered atmospheric circulation [e.g., Trenberth and Hurrell, 1994]. Given the widespread notion that tropical Pacific anomalies, such as El Ninõ-Southern Oscillation (ENSO) variability, drive midlatitude circulation, most studies have focused on that scenario. Comparatively less emphasis has been placed on how changing atmospheric conditions at annual to decadal time scales might influence the oceans. This is the case especially in the extratropics, where atmospheric forcing may induce SST anomalies via altered surface energy fluxes, vertical turbulent mixing, and wind-driven vertical and horizontal Copyright 2002 by the American Geophysical Union. 0094-8276/02/2002GL015743

motions [Alexander, 1992]. Empirical studies have shown that changes in atmospheric circulation can lead to the formation of such SST anomalies in the North Pacific [Emery and Hamilton, 1985]. [3] There remains a debate about the interaction between Pacific SSTs and midlatitude circulation. Is the climate of the Northern Hemisphere (NH) midlatitudes primarily influenced by variations in the tropical Pacific Ocean, the extratropical Pacific, or some combination? The goal of this investigation is to determine the patterns of atmospheric circulation variability associated with the patterns of Pacific SST variability—specifically, we wish to identify those regions in the Pacific that are related to specific features of the northern circumpolar vortex. [4] A unique characteristic of this research is the vortexbased approach to characterize midlatitude flow. Using the vortex allows for a complete representation of extratropical circulation variability while preserving the waveform, size, and shape of the hemispheric vortex. The vortex-based approach fully captures both the location and amplitude of all midlatitude atmospheric teleconnections. In some circulation studies, the circumpolar vortex has been quantified at a large spatial scale such that the size of the entire circumpolar vortex or vortex quadrants was examined [e.g., Angell, 1992]. Our research uses a spatial resolution of 5° longitude ‘‘slices’’ that provide a much more detailed characterization of the shape and size of the vortex on a regional scale while still providing a complete representation of hemispheric circulation [Burnett, 1993]. [5] This study is among the first to provide a detailed investigation of the midlatitude circulation of the entire NH, at multiple levels in the atmosphere, and the entire tropical and NH extratropical Pacific SST. Most prior research has focused on interactions between circulation and either the tropical or extratropical Pacific Ocean, therefore potentially not capturing basin-wide patterns of variability. Similarly, many studies have focused on the relationship between Pacific SST and downstream effects on circulation over the Pacific/North America (PNA) region. However, evidence also exists for upstream anomalies, over Eurasia, that affect Pacific SST and atmospheric circulation [Barnett et al., 1989]. Therefore, investigating the circulation of the entire NH in conjunction with the entire Pacific Ocean (north of 25°S) provides a greatly enhanced depiction of ocean-atmosphere interactions.

2. Data and Methods [6] The SST data used in this analysis are the monthly Kaplan Extended SST anomalies. [Kaplan et al., 1998]. Data for 25°S – 60°N were extracted for 1949– 2000. The circumpolar vortex data consist of mean monthly NH 700

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FRAUENFELD AND DAVIS: DECADAL AND INTERANNUAL PACIFIC OCEAN VARIABILITY

correlation patterns is thus obtained. Finally, the canonical patterns are further examined by computing composites of the loadings patterns of the corresponding SST and vortex fields by averaging the original data for the 31 months (5th percentile) with the highest and lowest scores.

3. Results

Figure 1. SST loadings for CV 1. Contours represent correlations between CV 1 and Pacific SST. hPa, 500 hPa, and 300 hPa geopotential heights for 1949 – 2000 derived from the NCEP/NCAR reanalysis [Kalnay et al., 1996]. A mean representative geopotential height contour for 700 hPa, 500 hPa, and 300 hPa is selected for each month of the year that consistently falls within the primary baroclinic zone of the 700, 500, and 300 hPa vortex, respectively, for each month. The representative contour is converted into a series of 72 latitudinal and longitudinal intersections along each 5° meridian, thus the vortex is characterized by a series of 72 straight-line segments connecting the intersections [Burnett, 1993]. This contour at each level is considered to be the ‘‘center contour.’’ To capture more completely the geometry of the circulation field, a ‘‘southern contour’’ and a ‘‘northern contour’’ was also selected at each pressure level. These additional contours were chosen to be 2 – 3 standard contours south and north of the center contour, thereby capturing the circulation at lower and higher latitudes. [7] The SST and circulation fields are analyzed using canonical correlation analysis (CCA). CCA is a form of multivariate correlation whereby two sets of variables are related [Hotelling, 1936]. A linear combination of variables from one data set is formed that is associated optimally with a linear combination formed of the variables from the other data set—the strength of their association being the canonical correlation. Each canonical variable (CV) is orthogonal to and thus independent of the previous one. In comparing many variables of one data set to many variables of another, CCA is very susceptible to multicollinearity and overfitting. One way to limit the number of input variables, insure independent variables, and prevent overfitting is to preprocess the data using principal components analysis (PCA). The PC time series, normalized to allow each component to contribute equally, then serve as input for CCA. Employing this orthogonalization and normalization, however, has the drawback that while the CCA accounts for the greatest variance between two data sets, it does not necessarily account for sizeable portions of the variance in the original data [Barnston and Ropelewski, 1992]. Careful consideration must therefore be given to ensure that the canonical patterns are physically meaningful. The vortex and SST data are first standardized to remove any seasonal cycle and PCA is then performed on the 488 grid point SST data, and on the 648 variable (3 vertical levels 3 contours 72 longitude slices) vortex data. The normalized PC scores are used as input for the CCA, and the spatial (loadings) and temporal (scores) variability of the canonical

[8] Using a variety of guidance rules (Rule N test, Guttman criterion, and scree test), 18 SST PCs and 24 vortex PCs were retained as input for the CCA. The CCA output consists of 18 CVs, the first 10 of which are significant according to Wilk’s lambda and a chi-square test. [9] The first pair of CVs has a canonical correlation of 0.71, indicating that the two patterns contain 51% overlapping variance. The SST pattern accounts for 7.5% of the variance of its original data, and the vortex pattern 6.3% variance of its original data. While the canonical correlation may seem low in terms of a maximal association between two data sets, the strength of the canonical correlation is a function of how many PCs are used for input. The number of input variables used here is conservative, optimizing the trade-off between strength in canonical correlation and variance accounted for by the spatial patterns. [10] The spatial SST pattern of the first CV displays a dipole with opposite anomalies in the eastern tropical South Pacific and the central North Pacific (Figure 1). A zonal anomaly region of the same sign as the southeast Pacific anomaly is found in the vicinity of the Philippine Sea, which extends towards the central Pacific around 30°N. With the exception of this latter feature, the SST pattern is similar to the Pacific Decadal Oscillation (PDO) [Mantua et al., 1997]. The associated vortex pattern exhibits variability at all levels and for all three contours over Eurasia and to a lesser degree the eastern Pacific (Figure 2). [11] For the ‘‘positive’’ composite, when the SST anomalies in the southeastern tropical Pacific and the Philippine Sea-central Pacific are positive and the anomaly in the central North Pacific is negative, there is an amplified ridge in the vortex over Asia and the vortex is predominantly contracted throughout the hemisphere (Figure 3). In the opposite ‘‘negative’’ phase, a deep trough is observed over Asia (Figure 3). There are virtually no differences in the vortex between the positive and negative phase over the

Figure 2. Vortex loadings for CV 1; lines represent correlations between CV 1 and the vortex.


Figure 3. Positive phase (+) and negative phase ( ) composites relative to the 1949– 2000 mean vortex position for the 700 hPa southern contour of CV 1. remainder of the NH, i.e. this first CV pattern exhibits variability primarily over Eurasia. [12] The time series of these spatial patterns show a very distinct decadal cycle (Figure 4, black line). The first half of the record—from 1949 to 1977—is almost exclusively characterized by the negative phase. Around the time of the Pacific Climate Shift of 1977 a regime shift occurred and the last 24 years of the record have been dominated by the positive phase. The time series are similar to those of the PDO (Figure 4, grey line), and both the monthly time series of the SST CV and vortex CV are significantly correlated with the PDO at 0.45 and 0.36 respectively. An interesting feature in the CV time series is that while the PDO shifted from its positive phase back into its negative phase in the late 1990s, the CV time series did not. In fact, both the SST and vortex CV time series exhibit their biggest positive departures at the approximate timing of the PDO’s shift back into its negative phase. [13] The second pair of CVs has a canonical correlation of 0.64 and therefore 40% of overlapping variance. The spatial SST patterns associated with this CV pair are ENSOlike and account for 13% of the variance of the original data (Figure 5). A zonal anomaly region spanning the tropical equatorial Pacific extends from the west coast of South America to the western Pacific and northward along the coast of North America. An anomaly of opposite sign covers the central Pacific. The dipole in this pattern, however, is not just between the equatorial Pacific and the central Pacific, but also between the extratropical central Pacific and the extratropical east Pacific, off the coast of California. Therefore, while the general SST pattern is

Figure 4. Time series of SST CV 1 (black line) and the PDO index (grey line) [Mantua et al., 1997].

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Figure 5. Same as Figure 1 but for CV 2. ENSO-like, this second SST CV seems to characterize extratropical SST variability as well. The vortex variability related to this SST pattern is PNA-like. Areas of strong variability are found over the eastern Pacific and the west and east coasts of North America. [14] The composite patterns indicate that during the positive phase, when SST anomalies are positive in the tropical equatorial Pacific and off the coast of the United States and negative in the north central Pacific, the vortex is expanded over the central-eastern Pacific and over North America (Figure 6). In contrast, during the negative phase, there is a strong ridge over the eastern Pacific. This second CV vortex pattern is therefore characterized primarily by central-eastern Pacific variability. [15] The time series of the second pair of CVs confirm that the patterns are essentially depicting ENSO variability (Figure 7). The SST time series is significantly correlated with the Ninõ region 3 SST index (R = 0.52), and the vortex time series are also significantly correlated with this index. All El Ninõ and La Ninã events during the 1949 – 2000 period are captured well by the CV time series. The biggest negative departure in the time series occurs in the late 1990s. This feature is not very pronounced in the Ninõ 3 time series. In fact, this unprecedented negative departure seems to match the PDO’s switch back into its prior, negative phase.

4. Discussion and Conclusions [16] The strongest association between Pacific SST and NH circulation is one of decadal variability. The SST patterns are similar to the PDO; however, a zonal anomaly extending from the Philippine Sea eastward at 30°N into the central Pacific is also evident. The decadal SST configuration is associated with upstream atmospheric variability over Asia, indicating that perhaps this decadal variability is

Figure 6. Same as Figure 3 but for CV 2.

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Figure 7. Same as Figure 4 but for CV 2 and Ninõ 3 index.

linked with the atmosphere upstream. Upstream effects on the circulation of the NH, such as variability of Eurasian snow cover, have been established both in model simulations [Barnett et al., 1989] and empirical studies [Cohen and Entekhabi, 1999]. An enhanced ridge over Asia is associated with positive SST departures in the Philippine Sea-central Pacific zonal anomaly as well as in the equatorial tropical Pacific and a negative SST anomaly in the central north Pacific. Further, the vortex is predominantly contracted across the entire NH, indicating that the shift around 1976 – 77 in both the atmosphere and the ocean has coincided with hemispheric warming. A deep trough over Asia is associated with the opposite SST pattern. Both the positive and negative phase are characterized by variability exclusively over Eurasia. [17] The second-strongest interaction between the Pacific Ocean and the midlatitude atmosphere is characteristic of interannual ENSO-like SST variability. The associated vortex patterns are PNA-like. When the Pacific Ocean SST pattern is similar to a warm ENSO event, the NH vortex is expanded across the Pacific, supporting the findings of Frauenfeld and Davis [2000]. During a cold ENSO event, the vortex is strongly contracted over the eastern Pacific. The atmospheric variability associated with the ENSO-like SST variability is thus primarily characterized by changes over the eastern Pacific. [18] Using CCA on Pacific SSTs and the NH circumpolar vortex at three levels in the atmosphere, we were able to partition Pacific SST variability into decadal PDO-like variability and interannual ENSO variability. Specifically, we decomposed the leading mode of Pacific SST variability, which accounts for roughly 23% of SST variability, into its decadal and interannual variability which explain 7.5% and 13.1%, respectively. This result not only indicates that the midlatitude atmospheric circulation has a different response/ forcing for similar spatial SST patterns at different temporal scales, but it allows us to isolate the different atmospheric circulation patterns associated with these SST patterns. The key difference in the decadal versus the interannual variability seems to be in the atmospheric circulation over the Eurasian land mass and the zonal Philippine Sea-central Pacific SST region. Although the standard definition of the

PDO does not incorporate any variability in this area of the Pacific, it also does not take into account atmospheric circulation. Similarly, as the PDO is the leading mode of SST variability in the Pacific, it contains both decadal and interannual variability. The approach taken here produces different temporal-scale patterns of SST variability as they relate to atmospheric variability, or vice versa, and hence treats the ocean and the atmosphere as two interacting media. [19] It is intriguing that while our decadal SST pattern is related to the PDO, the PDO apparently switched back into its negative phase in the late 1990s. The decadal SST pattern uncovered here did not. Instead, the interannual ENSO-like pattern has exhibited an unprecedented period of negative anomalies since the late 1990s. This could suggest that the decadal Pacific variability has in fact not shifted back, but instead the observed shift is related to shorter-scale Pacific SST variability. Given the nature of this analysis, however, it is also possible that the decadal variability did shift back, but that it is the atmosphere that has not switched according to its analogous decadal variability. Nevertheless, the manner in which the NH midlatitude atmospheric circulation interacts with the Pacific Ocean has changed since the late 1990s.

[20] Acknowledgments. We thank Adam Burnett for providing the vortex data and Chip Knappenberger for his insights. We also thank two anonymous reviewers for their helpful comments.

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O. W. Frauenfeld and R. E. Davis, Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22904, USA.