Revisited relationship between tropical and North Pacific sea surface ...

GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L02703, doi:10.1029/2011GL050005, 2012

Revisited relationship between tropical and North Pacific sea surface temperature variations Jong-Yeon Park,1 Sang-Wook Yeh,2 and Jong-Seong Kug1 Received 13 October 2011; revised 12 December 2011; accepted 16 December 2011; published 19 January 2012.

[1] The relationship between the tropical and North Pacific sea surface temperature (SST) variations is reexamined following the results of Deser and Blackmon (1995, DB95) based on a much longer period of data (1949–2010). As in DB95, the two leading SST modes, the El Niño-Southern Oscillation (ENSO) mode and the North Pacific mode, represent the SST variations in the Pacific domain before 1992. Considering the period after 1992, however, one needs to consider a new mode of SST variation along with the two modes mentioned to understand the relationship between the tropical and North Pacific SST variations. A new SST mode, known as the Warm Pool mode, exhibits a strong variance in the warm pool region and undergoes a phase shift after the mid-1990s, reflecting a warming in the warm pool region and a cooling in the central and eastern equatorial Pacific. It is found that the Warm Pool mode accompanies the North Pacific Oscillation-like atmospheric variability over the North Pacific. Through this teleconnection, the Warm Pool mode mostly shows a relationship between the warm pool SST and the associated North Pacific SST component and which has some similarities with the North Pacific Gyre Oscillation. Citation: Park, J.-Y., S.-W. Yeh, and J.-S. Kug (2012), Revisited relationship between tropical and North Pacific sea surface temperature variations, Geophys. Res. Lett., 39, L02703, doi:10.1029/2011GL050005.

1. Introduction [2] It is important to understand the teleconnections between tropical and extratropical sea surface temperatures (SST) because their relationship can modify the weather and climate in the mid-latitudes in response to the tropical SST forcings [Alexander et al., 2002; Lau et al., 2004; Ceballos et al., 2009; Lie and Lee, 2010; Yoon and Yeh, 2010]. Such SST teleconnection within the Pacific basin involves various physical processes and intrinsic climatic variabilities, including North Pacific Oscillation (NPO), Pacific Decadal Oscillation (PDO), El Niño-Southern Oscillation (ENSO) teleconnection, and SST-wind coupling [Vimont et al., 2003; Linkin and Nigam, 2008; Di Lorenzo et al., 2010]. For example, Vimont et al. [2003] showed a possible linkage between NPO and ENSO with focusing on the importance of footprinting SST in the subtropics. Moreover, Di Lorenzo et al. [2010] suggested central Pacific Warming may derive changes in North Pacific Gyre Oscillation (NPGO) through 1

Korean Ocean Research Development and Institute, Ansan, South Korea. 2 Department of Environmental Marine Science, Hanyang University, Ansan, South Korea. Copyright 2012 by the American Geophysical Union. 0094-8276/12/2011GL050005

extra-tropical atmospheric teleconnection and wind-driven SST modulation. [3] Among the studies examining the SST teleconnections, Deser and Blackmon [1995] (hereafter, DB95) provided an interesting view of the relationship between the tropical and North Pacific SST anomalies during the boreal winter (November–March). Using an Empirical Orthogonal Function (EOF) analysis, DB95 identified the two modes of SST variability over the Pacific Ocean spanning the latitudes of 20oS to 60oN during winter. The two modes are the El Niño-Southern Oscillation (ENSO) mode and North Pacific mode. The ENSO mode explains 43% of the variance over the Pacific Ocean and is highly correlated with the eastern tropical Pacific SST variability, as well as having a connection to the eastern North Pacific. On the other hand, the North Pacific mode is linearly independent of the ENSO mode and exhibits maximum amplitude and explained variance along 40°N, west of 170°W. The timescale for the North Pacific mode is longer than that for the ENSO mode. Furthermore, DB95 identified the gross features of the 500 hPa patterns associated with the ENSO mode and the North Pacific mode, respectively, and found that they are similar, although the North Pacific mode is more associated with the Pacific/North American (PNA) pattern. [4] So far, the results in DB95 have been widely referenced and become the conventional view in terms of the relationship between tropical and North Pacific SST variations. However, changes in the Pacific SST have been quite dramatic during recent decades, possibly due to both the natural forcing and anthropogenic forcing. Therefore, it is necessary to revisit the relationship between these variations. The purpose of this study is to update the results of DB95 and reexamine the relationship between the tropical and North Pacific SST variations.

2. Data and Methods [5] The SST data used in this study are obtained from different products, including the extended reconstructed SST version 3 (ERSST v3) [Smith et al., 2008], the Hadley Center Sea Surface Temperature data (HadISST) [Rayner et al., 2003], the Kaplan Extended SST version 2 (Kaplan SST v2) [Kaplan et al., 1998], and the Comprehensive Ocean–Atmosphere Data Set (COADS) [Woodruff et al., 1998]. The primary dataset is the ERSST, while the other datasets provide supplementary data to demonstrate further confidence in the presented results. Atmospheric variables, such as geopotential heights and surface winds, are obtained from the National Centers for Environmental PredictionNational Center for Atmospheric Research (NCEP-NCAR) reanalysis 1 [Kalnay et al., 1996]. The analysis period spans from 1949 to the most available recent data.

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PARK ET AL.: SST RELATIONSHIP

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Figure 1. (a) EOF 1 and (b) EOF 2 of the Pacific SST during the period 1949–1992. (c–e) EOF 1, 2, and 3, respectively, for the period 1949–2010. Patterns are shown in normalized form, as they were in DB95. [6] This study uses the seasonal mean data during winter (November-December-January-February-March, hereafter, NDJFM). As in DB95, most analysis is based on statistical methods including Empirical Orthogonal Function (EOF) analysis and linear regression analysis. The EOF is based on the covariance matrix and is shown in normalized form (i.e., correlation coefficients between the time series of the EOF and the original data), which is the same as in DB95. For the significance of the regression analysis, the effective degree of freedom is obtained using the methodology of Livezey and Chen [1983].

3. Results [7] Figures 1a and 1b show the first two EOFs of the winter SST anomalies for the period of 1949–1992, which is the same period as used in DB95. Similar to DB95 (Figure 1 in DB95), the first two leading EOFs before 1992 represent the ENSO mode (Figure 1a) and the North Pacific mode (Figure 1b), respectively. The ENSO mode explains 46% of the variance over the Pacific domain. The strongest correlation appears in the equatorial Pacific, and its opposite sign is found in the western and central North Pacific basin. In contrast, the North Pacific mode is characterized by a strong correlation from the western to the central North Pacific, and its sign is the same in the central and eastern tropical Pacific. Table 1 indicates simultaneous correlation coefficients between the NINO3 SST index and the principal component (PC) time series of each mode, indicating that the North Pacific mode is linearly independent of ENSO. As shown in DB95, in addition, the timescale of the PC time

series for the North Pacific mode is noticeably longer than that of the ENSO mode, which is dominant on an interannual timescale (not shown). Overall, the ENSO mode and the North Pacific mode appear in the first two leadings modes to represent the SST variability in the Pacific domain for the period of 1949–1992, which is consistent with the results of DB95. It is noteworthy that we also obtain the same results using three different SST datasets (i.e., HadISST, Kaplan version 2, and COADS) over the period of 1949–1992 (not shown). [8] Figures 1c–1e represent the same EOFs of the winter SST anomalies but instead use data through 2010 (i.e., 1949–2010). Compared with DB95, the most striking difference is that a new EOF SST mode (Figure 1d) appears in the second EOF mode that was not found in DB95. On the other hand, the ENSO mode and the North Pacific mode appear in the first and the third EOF modes, as shown in Figures 1c and 1e, respectively. Note that the EOFs from 1992 to 2010 are almost similar to those in Figures 1c–1e. In addition, the second mode for the period of 1949–2010 Table 1. Correlations Between the PC Time Series and the Nino3 Index for the Time Periods 1949–1992 and 1949–2010 PC1

PC2

0.95a

Nino3 Index 1949–1992 0.20

0.89a

Nino3 Index 1949–2010 0.17

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a

Values indicate the 95% statistical significance.

PC3

0.19

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Figure 2. Principal components of (a) EOF1, (b) EOF2, and (c) EOF3 using four different SST datasets: ERSST, Kaplan, HadISST, and COADS. does not appear in the high-order EOFs for the period of 1949–1992. The North Pacific mode is linearly independent of ENSO (Table 1), and the timescale of its PC time series is longer than that of the ENSO mode (see Figure 2), which is similar to the results before 1992. [9] The second EOF mode shows the largest correlation in the warm pool region and a zonal contrast of SST anomalies across the tropical Pacific basin. In addition, SST anomalies above 20oN are characterized by a north-south dipole-like structure with a strong meridional gradient along 40°N, which have some similarities with the spatial pattern of the NPGO [Di Lorenzo et al., 2008]. Hereafter, we refer to the second EOF mode as the so-called Warm Pool mode because the maximum correlation is found in the warm pool region. It is noteworthy that such changes in the EOF modes between the two periods (1949–92 versus 1949–2010) are also found when we use three additional different SST datasets (i.e., the HadISST, the Kaplan SST, and the COADS) (not shown). In other words, four different SST datasets consistently indicate that the Warm Pool mode appears in the second EOF mode of SST variability in the Pacific domain during the period of 1949–2010, although it does not appear before 1992. Therefore, one needs to consider a new relationship between the tropical and North Pacific SST variations in the Pacific domain along with the ENSO mode and the North Pacific mode when including data after 1992. In addition, this result indicates that the SST variability, which is represented by the Warm Pool mode, is significantly enhanced during recent decades; therefore, DB95 could not capture this Warm Pool mode of SST variability before 1992.

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[10] In order to further verify the consistencies of the different SST datasets, we display the PC time series of the ENSO mode, the Warm Pool mode, and the North Pacific mode, respectively, for the period of 1949–2010 in four different SST datasets (Figure 2). The PC time series of the modes are well-correlated among the four different SST datasets. In particular, the PC time series of the Warm Pool mode are characterized by a shift from a negative sign to a positive sign around the mid-1990s, reflecting a warming in the warm pool region and a cooling in the central and eastern equatorial Pacific during recent decades. Of course, there are some disagreements between the different datasets for PC2 and PC3. These partial disagreements are may be caused from the characteristics of each SST dataset related to the addition of satellite data. [11] Due to poor data coverage in the equatorial western Pacific in the pre-satellite era, the same analysis has been carried out just using data before 1975. The result shows that the two leading modes are almost the same as the two modes before 1992 (not shown). Moreover, these dominant patterns show the consistency among four different SST datasets, indicating that the results do not significantly change in pre and post satellite era. [12] Lastly, we tested the above result by using different methods of EOFs. Given that EOF analysis is not always stable and sometime is ambiguous, the same analyses using correlation-based matrix and rotated EOFs have been conducted. Moreover, EOF analyses in different regions (using Pacific basin SST and tropical Pacific SST) have been also tested due to a possible dependence of basin-scale EOFs on the relatively strong trend in the mid-latitude. It is found that There is little change in terms of the spatial pattern and corresponding the PC time series (not shown), which is indicative of to some extent the robustness of current result. [13] To obtain large-scale atmospheric patterns associated with the three leading modes, we show the regressed patterns of 500 hPa geopotential height, 850 hPa wind, and SST against the three PC time series for the period of 1949–2010. The 500 hPa geopotential anomaly patterns associated with the ENSO mode (Figure 3a) and with the North Pacific mode (Figure 3c) are similar with those of DB95 (see Figures 3a and 3b in DB95). That is, wave-like structures from the North Pacific to North America are dominant (i.e., the largest height anomalies occur around the Aleutian low, with weaker anomalies of the opposite polarity over northwestern Canada for the ENSO mode) (Figure 3a). On the other hand, the height anomalies associated with the North Pacific mode exhibit a structure that more closely resembles the PNA pattern [Wallace and Jiang, 1987]. Therefore, one may conclude that the atmospheric patterns associated with the ENSO mode and the North Pacific mode have not changed much since 1992. Alternatively, the atmospheric circulation associated with the Warm Pool mode is characterized by a dipole-like structure in the meridional direction over the North Pacific Ocean. A negative center of regressed 500 hPa geopotential height is observed at 55°N, 170°W over the Bering Sea. On the other hand, a positive center is located in the southern region over the North Pacific. Such a dipole-like pattern of regressed 500 hPa geopotential height has a remarkable resemblance to the NPO and the West Pacific pattern, which are two faces of the same variability, constituting prominent modes of mid-latitude wintertime

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Figure 3. (a–c) The 500 hPa geopotential height anomalies, and (d–f) surface wind anomalies regressed onto the PC time series 1 (Figures 3a and 3d), PC time series 2 (Figures 3b and 3e), and PC time series 3 (Figures 3c and 3f). Shaded area represents the 95% confidence region. atmospheric variability, the so-called NPO/WP pattern [Wallace and Gutzler, 1981; Linkin and Nigam, 2008]. [14] Figures 3d–3f show that the North Pacific components of the three SST modes underlie the zonal and meridional surface wind anomalies, such that when the westerlies are stronger, the SSTs are cooler and vice versa. Surface fluxes may be important in forcing the SST anomalies over the North Pacific, as stated in DB95. The surface wind anomalies associated with the three modes are reasonably consistent with the SST-wind relationship in the mid-latitudes [Lau and Nath, 1994]. Westerly wind anomalies enhance the total wind speed, cooling the ocean mixed layer by enhancing the fluxes of sensible and latent heat, strengthening entrainment and cold advection due to Ekman currents. In detail, the spatial pattern of the North Pacific SST anomalies associated with the ENSO mode and the North Pacific mode is characterized by anomalous elliptical shaped regions of cold or warm temperatures in the western and central North Pacific, accompanied by an opposite temperature to the east, north and south. On the other hand, the pattern associated with the Warm Pool mode is characterized by changes in SST

anomalies in the meridional direction, which is in contrast to the ENSO mode and the North Pacific mode. However, one may conclude that the North Pacific SST component is forced in relation to the three modes, at least in part by changes in the atmospheric circulation.

4. Discussion and Concluding Remarks [15] Using a longer period of data, the relationship between the tropical and North Pacific SST variations during winter has been reexamined. In addition to the two dominant modes in the Pacific Ocean (the ENSO mode and the North Pacific mode), the Warm Pool mode appears when the recent period of data is included. The previous two dominant modes, which have been identified by DB95, are ranked as the first and the third modes in the period of 1949–2010 and exhibit similar large-scale patterns in comparison with those before the early 1990s. [16] The Warm Pool mode accompanies the NPO-like atmospheric pattern and the NPGO-like SST pattern in the North Pacific. According to previous studies, the NPGO is dominantly forced by the atmospheric forcing associated

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Figure 4. Schematic figure of physical linkages involved in the Warm Pool mode from the viewpoint of mean difference. with the NPO-like variability. Furthermore, the amplification of the NPGO variance is found in observations during the recent decades [Douglass et al., 2006; Di Lorenzo et al., 2008; Ceballos et al., 2009]. Considering the linear trend in the Warm Pool mode and the result from previous studies, we argue that the Warm Pool mode contributes to the change in the NPGO-like mean SST field through the atmospheric teleconnections. In fact, we obtained the NPO-like dipole structure, which is similar to Figure 3b, by subtracting the mean filed during Warm Pool cooling period (1949–1996) from that during Warm Pool warming period (1997–2010). Since the SST in the warm pool exhibits a simple thermodynamic energy balance between evaporative heat loss and radiative energy input with little ocean dynamics [Newell et al., 1978], the warm pool SST is associated with anomalous radiative forcing, including the forcing associated with anthropogenic greenhouse gas concentration increases. [17] One may argue that the ability of the observational products to estimate the effects of global climate change in the tropics is not so reliable because SST trends in the tropical Pacific are known to be inconsistent among different SST datasets [Vecchi et al., 2008]. However, our supplementary analysis shows that SST trends during the period 1949–2010 are similar in all SST datasets (not shown). Thus, we believe that the warm pool mode is a quite robust feature in the Pacific basin during the last 60 years. [18] Based on these results, the synthesis of a possible physical linkage involved in the Warm Pool mode has been proposed (Figure 4). Global warming, which is mainly due to an increase of anthropogenic greenhouse concentration, projects on the SST in the western Pacific, and the increased Warm Pool SST induces the change in NPO-like atmospheric circulation pattern over the North Pacific. This atmospheric change in turn contributes to the change in the NPGO-like SST. Note that the same analysis with the detrended SST also derives the same pattern with the Warm Pool mode and its time series are strongly correlated with the NPGO index, indicating that indeed the Warm Pool mode projects onto NPGO-like variability (not shown). Consequentially, the SST warming in the western Pacific and the NPGO-like pattern constitute the Warm Pool mode in the Pacific basin. Even though Figure 4 did not explain all-embracing relationship in the tropics-midlatitude teleconnection from interannual to decadal time scales, it highlights connections between warm pool warming and the North Pacific SST

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variability via atmospheric teleconnection on the low-frequency timescales. By comparing the mean geopotential height at 500 hPa between two sub-periods (1949–1996, 1997–2010), one can also find the NPO-like dipole structure which is similar to the regressed pattern in Figure 3b (not shown). Additionally, we have investigated the relationship between warm pool warming and a latitudinal displacement of Aleutian Low pressure and found that warm pool warming tends to move the center of Aleutian Low pressure to the north. Thus, such a low-frequency warming in the warm pool is able to change the atmospheric mean state over the North Pacific into NPO-like dipole pattern. [19] An understanding of how the Warm Pool mode forces the NPO/WP-like atmospheric circulations still needs to be reached. One possible explanation is the Warm Pool warming influenced by external warming signal provides a favorable condition for central Pacific El Niño, and then it drives the decadal modulation of the NPGO through tropically forced atmospheric anomalies [Di Lorenzo et al., 2010]. However, central Pacific warming signal seems to be more reflected in the 3rd mode in Figure 1e, and also there is a possibility that North Pacific atmospheric anomalies can generate central Pacific warming in terms of lowfrequency filter. Therefore, more refined work is clearly needed for this issue. Given the incomplete understanding of the Warm Pool mode, exploring the mode and its associated atmospheric variability using fully coupled models could be a valuable research topic for understanding changes in the tropics-mid-latitude atmospheric and oceanic teleconnection under global warming conditions. [20] Acknowledgments. This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2009-C1AAA001-2009-0093042). [21] The Editor thanks two anonymous reviewers for their assistance in evaluating this paper.

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