Role of the tropical Atlantic sea surface temperature in the decadal change of the summer North Atlantic Oscillation

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1 Click Here for Full Article JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114,, doi: /2009jd012395, 2009 Role of the tropical Atlantic sea surface temperature in the decadal change of the summer North Atlantic Oscillation Jianqi Sun, 1,2 Huijun Wang, 1,2 and Wei Yuan 2,3 Received 1 May 2009; revised 11 July 2009; accepted 13 August 2009; published 24 October [1] Recent observational studies have shown that the southern center of the summer North Atlantic Oscillation (SNAO) shifted eastward after the late 1970s. In this study, this phenomenon and its causes are further explored. It is found that the decadal spatial shift of the SNAO southern center is related to the decadal variability of the tropical Atlantic sea surface temperature (TASST). In the past half century, the TASST experienced an abrupt change around the late 1970s, with a rapid warming in the recent 2 decades. Thus in the period after the late 1970s when the TASST is relatively warmer, the TASST released more energy into the atmosphere, then stimulated strong convection over the tropical Atlantic, and further excited anomalous wave-like patterns. This strengthened the circulation in the region of the Mediterranean Sea, namely the east part of the SNAO southern center, consequently leading to an eastward shift of the SNAO southern center. Meanwhile, in the period before the late 1970s when the TASST is relatively cooler, the TASST released less energy into the atmosphere, so its impact on the overlying atmosphere was significantly weakened and confined to the lower-level atmosphere of the tropical Atlantic. Thus the TASST possibly did not influence the variability of the SNAO southern center, and the SNAO pattern exhibited a traditional distribution with two centers over the North Atlantic. Citation: Sun, J., H. Wang, and W. Yuan (2009), Role of the tropical Atlantic sea surface temperature in the decadal change of the summer North Atlantic Oscillation, J. Geophys. Res., 114,, doi: /2009jd Introduction [2] The North Atlantic Oscillation (NAO) is the most active interannual and decadal variability teleconnection pattern in the Northern Hemisphere. Due to its importance in the climate variability of the Northern Hemisphere, the NAO has long been a topic of interest [e.g., Walker and Bliss, 1932; van Loon and Rogers, 1978; Wallace and Gutzler, 1981; Barnston and Livezey, 1987; Hurrell, 1995; Hurrell and van Loon, 1997; Chang et al., 2001; Li et al., 2003; Li, 2004; Yang et al., 2004; Yu and Zhou, 2004; Furevik and Nilsen, 2005]. [3] Recently, a new feature of the NAO has been revealed. Hilmer and Jung [2000] and Jung et al. [2003] found that there is a decadal change in the relationship of the NAO with the North Atlantic and surrounding regions climate components, such as surface air temperature, sea ice volume exports, cyclone activity, and heat flux. Further analysis indicated that such decadal changes may result from the decadal shift of the NAO spatial pattern: the action centers of the NAO shifted eastward after the late 1970s. 1 Nansen-Zhu International Research Centre, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China. 2 Climate Change Research Center, Chinese Academy of Sciences, Beijing, China. 3 China Meteorological Administration Training Centre, Beijing, China. Copyright 2009 by the American Geophysical Union /09/2009JD012395$09.00 Hence, it is important to understand the physical mechanism for the observed eastward shift of the NAO action centers that occurred around the late 1970s. Peterson et al. [2003] indicated that eddy fluxes caused by transient eddies and anomalous advection caused by the changed mean flow may have played important roles in establishing the eastward shift of the NAO action centers. Lu and Greatbatch [2002] argued that the NAO shift may be related to North Atlantic storm activity. Ulbrich and Christoph [1999] performed an integration of the coupled ECHAM/OPYC3 model under increasing greenhouse gas concentrations, and found an eastward shift of the NAO action centers in the simulation, which closely resembles the observed shift, thus implying that anthropogenic climate change may also be an important external forcing variable for the NAO pattern shift. [4] It is well known that the NAO is strong in winter, so most previous studies focused on the NAO in the winter time. However, actually the NAO is also one of the teleconnection patterns that have a year-round presence. Some studies have indicated that the summer North Atlantic Oscillation (SNAO) is a dominant pattern over the North Atlantic region and it plays an important role in the Northern Hemispheric climate [Hurrell and Folland, 2002; Hurrell et al., 2003; Linderholm et al., 2007; Sun et al., 2008; Yuan and Sun, 2009]. In studying SNAO variability, Sun et al. [2008] and Yuan and Sun [2009] found that, similar to the winter NAO pattern, the SNAO southern center showed an eastward shift around the late 1970s. Such 1of9

2 change of the SNAO variation is further analyzed using the ERA-40 reanalysis data. The relationships of the Atlantic SST with the SNAO and atmospheric circulations are investigated in sections 4 and 5. Sections 6 and 7 contain the discussion and conclusion, respectively. Figure 1. Linear regression patterns of summer SLP based on the normalized SNAO index for the periods of (a) and (b) The contour interval is 0.3 hpa. a decadal spatial shift altered the relationship of the SNAO with the simultaneous Northern Hemispheric surface air temperature, with a low correlation before the late 1970s and a high correlation after the late 1970s. However, why was there an eastward shift of the SNAO southern action center after the late 1970s? This is still an open question. [5] Considering that the atmospheric internal variability exhibits a very short time scale, the decadal variability of the SNAO may be attributed to the variability of some lowvarying external forcings. Here, we propose the possible role of sea surface temperature (SST). By analyzing the observational data and performing a sensitivity experiment with the atmospheric general circulation model, Sun and Yuan [2009] found that the decadal variability of the SST over the Mediterranean Black Sea (MBS) contributed to the spatial shift of the SNAO pattern, indicating that the SST variability may play an important role in this process. On the other hand, such a large-scale atmospheric circulation change could not be caused by a single factor. Some other regions SST could also have impact on the SNAO pattern shift, which needs further exploration. Previous studies have revealed that the decadal variability of the Atlantic SST has an effect on the winter NAO decadal change [Peng et al., 2003; Frankignoul and Kestenare, 2005; Li et al., 2007]. In this study, we attempt to explore the connection between the SNAO and the Atlantic SST to determine whether this relationship can shed any light on the eastward shift of the SNAO southern center. [6] This paper is divided into seven sections. The data sets are introduced in section 2. In section 3, the decadal 2. Data Description [7] The atmospheric data set applied in this study is the reanalysis data set (ERA-40) produced by the European Centre for Medium-Range Weather Forecasts [Uppala et al., 2005]. The variables analyzed include geopotential heights, sea level pressure (SLP), air temperature, and vertical and meridional velocities. All of these variables are gridded at a 2.5 latitude by 2.5 longitude resolution. [8] The outgoing long-wave radiation (OLR) data, used to infer tropical convection, is provided by the NOAA/ OAR/ESRL PSD, Boulder, Colorado, United States, and is available from June 1974 onward with a period of missing data between March and December in [9] The needed SST data are the Met Office Hadley Centre s SST data set (HadISST1), which is a unique combination of globally complete monthly fields of SST at a 1 latitude by 1 longitude resolution [Rayner et al., 2003]. This SST data set is taken from the Met Office Marine Data Bank, which from 1982 onward, also includes data received through the Global Telecommunications System. In order to enhance data coverage, monthly median SST data for from the Comprehensive Ocean- Atmosphere Data Set were also used for caused in which there were no Marine Data Bank data. HadISST1 is reconstructed using a two stage reduced-space optimal interpolation procedure, followed by a superposition of quality-improved gridded observations onto the reconstructions to restore local detail. SSTs values near sea ice are estimated using statistical relationships between the SST and sea ice concentration. [10] The NAO index used is the difference between the normalized SLPs over Gibraltar and over Reykjavik, Iceland ( 3. Decadal Change of SNAO Variation [11] To describe the spatial position of the SNAO centers before and after the late 1970s, the SNAO-related atmospheric circulations for these two subperiods ( and ) are depicted in Figure 1. However, in contrast to the study of Sun et al. [2008] using the NCEP/ NCAR reanalysis data, our results are based on the ERA-40 reanalysis data. As shown in Figure 1, for the former subperiod, the two SNAO centers exhibit a meridional dipole pattern over the North Atlantic. For the latter subperiod, although the two centers still show a meridional dipole pattern, there is a remarkable shift in location. The southern center of the SNAO is now dominant over the Mediterranean Sea region, located more eastward than in the former subperiod. This result is quite similar to the results of Sun et al. [2008], indicating that the recent spatial shift of the SNAO centers is robust and not an artifact of the NCEP/NCAR reanalysis. [12] From Figure 1, we can see that the most remarkable changes in the SNAO-related circulation pattern occur over 2of9

3 Figure 2. Time series of the MSC index (solid line) and its corresponding linear trends for the periods of (dashed line with solid circles) and (dashed line with open circles). The MSC index is defined as the averaged SLP over the region 35 N 50 N, 0 30 E. the Mediterranean Sea region. This implies that the circulation over this region experienced a significant decadal change before and after the late 1970s. In order to confirm this hypothesis, we define the mean SLP over the region (0 30 E, 35 N 50 N) as an index to represent the Mediterranean Sea circulation variability and designate it as the Mediterranean Sea circulation (MSC) index. As shown in Figure 2, the MSC index exhibits a decadal change around the late 1970s. During the period before the late 1970s, the MSC index shows a relatively weak variability. Its standard deviation is However, after the late 1970s, the variability of the MSC index is increased, with the standard deviation increasing significantly to In addition, for the former subperiod, the MSC exhibits a very weak increasing trend, while for the latter subperiod, it shows a significant decreasing trend. [13] To investigate the role of the decadal change of the MSC in the spatial shift of the SNAO pattern, the regressed SLPs based on the SNAO index before and after removing the MSC information by linear regression are plotted. As shown in Figure 3, for the former subperiod, the SNAOrelated patterns are consistent before and after removing the MSC information, implying that there is no signal of the MSC in the variability of the SNAO at this time. However, for the latter subperiod, the SNAO-related pattern does not show a spatial shift after removing the MSC information. The resultant southern center is located over the North Atlantic, which is quite similar to those in the former subperiod before and after removing the MSC signal, indicating that a spatial shift would not occur without a decadal change of the MSC. The strong variability of the MSC can enhance the eastern part of the southern center after the late 1970s, thus leading to the eastward shift of the SNAO southern center. Thus, the question of why there is a decadal shift of the SNAO southern center around the late 1970s, which was mentioned in the Introduction, is now converted to the question of why there is a decadal variability of the MSC around the late 1970s. 4. Decadal Change of the Relationship Between the MSC and the Atlantic SST [14] Figure 4 shows the correlations between the MSC index and the Atlantic SST for the two subperiods. It suggests that there are almost no large-scale significant correlations for the former subperiod (Figure 4a), indicating that the connection between the MSC and the Atlantic SST is weak at this time. For the latter subperiod, the situation is changed. There are significant negative correlations over the tropical Atlantic and northwestern North Atlantic (Figure 4b), indicating that the connection between the MSC and the Atlantic SST becomes strong in the latter subperiod. Considering the influence of the significantly decreasing trend of the MSC index in the latter subperiod (Figure 2), the detrended correlation between the Atlantic SST and MSC index is further calculated (figure omitted). The correlation pattern is consistent with that obtained before detrending, indicating that the SSTs of these two Figure 3. Linear regression patterns of summer SLP based on the normalized SNAO index after removing the MSC information by the linear regression method for the periods of (a) and (b) The contour interval is 0.3 hpa. 3of9

4 Figure 4. Correlation patterns of summer SST with the MSC index for the periods of (a) and (b) Areas with correlations that are significant at the 95% confidence level are shaded. regions are related to the MSC on both interannual and decadal variability in the latter subperiod. [15] Similar results are also obtained from an additional index analysis. The tropical Atlantic (northwestern North Atlantic) SST index is defined as the mean SST over the tropical Atlantic (10 S 10 N, 60 W 0 ) (northwestern North Atlantic (40 N 50 N, 60 W 30 W)). Running correlations with a 22 year window width for the period of are shown in Figure 5. The correlations between the MSC and the Atlantic SST indices show a rapid rise from insignificant before the late 1970s to a much high correlation that jumps above the 5% significance level around The analysis of the running correlation further confirms that the relationship between the MSC and the Atlantic SST varies with time, and also shows that the decadal abrupt change point of this relationship is really around the late 1970s, indicating that it is reasonable and objective to divide the period into above two subperiods ( and ) in analyzing these two climatic systems relationship. [16] However, there is one point that must be noted: the SSTs over these two regions generally have a good consistent variability. The correlation coefficient between their indices is 0.62 for the period of after detrending. Thus, the significant correlation of the SST over one region might result from its consistent variability with another region s SST, which has a real connection with the MSC variability. In order to isolate the SST that has a real connection with the MSC, partial correlations are calculated. The partial correlation between the MSC and the northwestern North Atlantic SST indices, holding the tropical Atlantic SST (TASST) index fixed, is 0.28 for the period of , which is quite low. However, the partial correlation between the MSC and the TASST indices, holding the northwestern North Atlantic SST index fixed, is 0.45, which is still significant at the 95% confidence level. Figure 5. Running correlations between the MSC index and the tropical Atlantic SST index (solid line) as well as the northwestern North Atlantic SST index (solid line with circles) with a 22-year window width for the period of The dashed line indicates the 95% significance level. 4of9

5 This indicates that the tropical Atlantic may have an impact on the MSC, and the high correlation between the MSC and the northwestern North Atlantic SST may be strongly dependent on the variability of the TASST. 5of9 5. TASST-Related Atmospheric Circulation [17] In this section, the TASST-related large-scale circulation anomalies are analyzed in order to investigate the manner in which the TASST exerts an impact on the MSC during the latter subperiod and to determine why it has no impact on the MSC during the former subperiod. Here, the SNAO-related atmospheric circulation is also investigated for the two subperiods of and [18] Figure 6a shows the linear regression pattern of the 250 hpa geopotential height associated with the TASST over the period of It suggests that the atmospheric circulations show a wave-like response to the anomalous tropical Atlantic heating. As shown in Figure 6a, the TASST-related anomalous circulations exhibit two wavelike patterns residing on each side of the equator. The Northern Hemispheric wave train shows a tripole pattern with a southeast-northwest tilt. Such meridional wave train patterns are quite similar to those associated with the MSC and the SNAO (Figures 6b and 6c), which both also show a tripole pattern northeastward from northern Africa to the North Pole region, indicating that this meridional pattern may serve as a bridge connecting the TASST and MSC as well as the SNAO in the latter subperiod. [19] It is well known that the warming SST in the tropics can significantly increase its overlying geopotential height. But it is not the case for the tropical Atlantic in this study. We hypothesize that the insignificant geopotential height anomalies over tropical Atlantic might be resulted from the balance between the tropical Atlantic warming induced geopotential height anomalies and the anomalies induced by other factors, for example the ENSO event. If we remove the signal of the first principal component of the tropical Pacific basin SST (which has been used to represent the variability of the ENSO event) using the linear regression method, the geopotential height over the tropical Atlantic is enhanced (figure omitted). This result indicates that the tropical eastern Pacific SST induced geopotential height anomalies may offset the response of the geopotential height to the tropical Atlantic, thus resulting in insignificant geopotential height anomalies over the tropical Atlantic. Some other factors may also contribute to this phenomenon, which needs further exploration. [20] The wave-like response to the anomalous tropical Atlantic heating is also well reflected in the latitude-pressure cross section of the meridional circulations. As shown in Figure 7, the TASST-related circulation pattern shows a symmetrical feature about the equator, although the significant areas are asymmetrical. The tropical vertical circula- Figure 6. Linear regression patterns of summer 250 hpa geopotential height against the (a) tropical Atlantic SST index, (b) MSC index, and (c) SNAO index for the period of The correlations that are positively (negatively) significant at the 95% confidence level are darkly (lightly) shaded.

6 significant 250 hpa geopotential height anomalies merely show two small significant areas located over the eastern tropical South Atlantic and the eastern middle North Atlantic (Figure 9a). The anomalous circulations over most of the remaining regions are weak and insignificant. At the lower Figure 7. Linear regression pattern of latitude-pressure cross section of geopotential height averaged along E against the tropical Atlantic SST index for the period of The correlations that are positively (negatively) significant at the 95% confidence level are darkly (lightly) shaded. tions (30 S 30 N) are primarily baroclinic. This suggests that the atmospheric circulation over the tropical Atlantic is likely stimulated by anomalous convective latent heating of the region. This point is well reflected in the OLR pattern. As shown in Figure 8, corresponding to a warm TASST, there is a significantly enhanced convection along the tropical Atlantic. Thus, the anomalous TASST can lead to anomalous convection activity over the tropical Atlantic. The anomalous convection activity can then stimulate anomalous atmospheric circulations emanating from the tropical Atlantic poleward to the high latitudes of both hemispheres, consequently exerting an impact on the MSC and further on the SNAO. [21] However, the above physical process is invalid for the first period. Differing from the situation in the latter subperiod, the TASST impact on the overlying atmosphere is quite weak in the former period. At the upper level, the Figure 8. Linear regression pattern of summer OLR against the tropical Atlantic SST index for the period of The correlations that are positively (negatively) significant at the 95% confidence level are darkly (lightly) shaded. Figure 9. Linear regression patterns of summer (a) 250 hpa geopotential height and (b) SLP against the tropical Atlantic SST index for the period of The correlations that are positively (negatively) significant at the 95% confidence level are darkly (lightly) shaded. 6of9

7 Figure 10. Composite difference of the summer SST over the tropical Atlantic between the two subperiods ( minus ). Areas with composite differences that are significant at the 95% confidence level are shaded. level, the major significant correlation is located over the tropical Atlantic (Figure 9b), indicating that the connection of the TASST with the overlying upper-level tropical and extratropical atmospheric circulations is significantly weakened in the former subperiod, as compared to the latter subperiod. [22] Since there are such remarkable differences in the relationship between the TASST and the overlying atmospheric circulations, the connection of the TASST with the MSC and SNAO is changeable, with a close connection after the late 1970s and a weak connection before. region, resulting in strengthened local Hadley circulation over the tropical Atlantic region (Figure 12). Hence, in the latter subperiod, the significant TASST-related heating of the atmosphere can reach to the upper level and the significant areas show two warm centers residing on each side of the equator at the upper level (Figure 13a). Such patterns result from the adiabatic descent, compensating the equatorial ascent caused by the sea surface warming (Figure 12). Meanwhile, for the former subperiod, the heated atmosphere is confined to the lower-level tropical Atlantic, and the anomalous air temperature magnitude is decreased along the upward direction (Figure 13b), indicating that the convection activity is weak and that the turbulent mixing in the boundary layer is very effective over that region during this time. [25] Thus, the TASST transfers more energy to the overlying atmosphere in the latter subperiod than in the former subperiod. Consequently, the TASST exerts a more significant influence on the atmosphere, as discussed in section 5. This is one possible mechanism that may be 6. Discussions [23] The above analyses indicate that the relationship between the TASST and the overlying atmospheric circulation has bee unstable during the past half century. It is natural to ask why the relationship is instable. In this section, a possible reason is discussed. [24] Figure 10 presents the spatial pattern of the SST difference over the tropical Atlantic before and after the late 1970s. It suggests that the TASST exhibits a significant warming during the past 2 decades. As the SST warms over the tropical Atlantic, more latent heat flux is transferred from the ocean to the overlying atmosphere. Thus, in the latter subperiod, the released latent heat flux from the tropical Atlantic is significantly increased, as compared to the former subperiod (Figure 11). The increased released latent heat flux then enhances the convection activity of the Figure 11. Composite difference of the summer latent heat flux over the tropical Atlantic between the two subperiods ( minus ). Areas with composite differences that are positively (negatively) significant at the 95% confidence level are lightly (darkly) shaded. Figure 12. (a) Composite differences of latitude-pressure cross section of meridional circulations averaged along 40 W 10 E between the two subperiods ( minus ) and (b) climatology of the latitudepressure cross section of meridional circulations averaged along 40 W 10 E for the period The units are ms 1 for the meridional wind and hpa s 1 for the vertical motion. The values of the vertical velocity are multiplied by 200. Areas with composite differences of upward (downward) motion velocity that are significant at the 95% confidence level are darkly (lightly) shaded. 7of9

8 be simply described as follows. Over the period of , when the TASST released more energy into the atmosphere, the warm TASST strengthened the convection activity over the tropical Atlantic, further exciting wave-like patterns propagating to the extratropics. Under the impact of the wave-like pattern, the variability of the MSC is changed, which further activates the variability of the eastern part of the SNAO southern center, consequently leading to an eastward shift of the SNAO southern center. Meanwhile over the period of , when the TASST released less energy into the atmosphere, the impact of the TASST was confined to the lower-level atmosphere over the tropical Atlantic. The TASST potentially did not exert an influence on the climate over the extratropics and the SNAO pattern showed a traditional distribution with two centers over the North Atlantic. [27] On the other hand, this study also raises further some questions. First, the above analysis indicates that the decadal shift of the SNAO southern center is related to the decadal variability of the TASST. What factors lead to the TASST decadal variability? This question is important for understanding the variability of the TASST and further for predicting shifts in the SNAO pattern. Second, in this study, the statistical analysis is merely based on a linear relationship. In the coupling process between the SST and the SNAO spatial shift, some nonlinear processes may also play a role. Thus, in the future work, the use of numerical models for further investigating the impact of these SST variabilities on the MSC and SNAO is also of importance. In addition, other processes may be related to the spatial shift of the SNAO pattern, which also requires further investigation. Figure 13. Linear regression patterns of latitude-pressure cross section of air temperature averaged along 40 W 10 E against the tropical Atlantic SST index for the periods of (a) and (b) The correlations that are positively (negatively) significant at the 95% confidence level are darkly (lightly) shaded. responsible for the unstable relationship between the TASST and the overlying atmospheric circulation in the last half century. 7. Conclusion [26] This study is performed to explore the cause of a spatial shift of the SNAO southern center that occurred in the late 1970s. Here, we focus on the role of the SST. It is found that in the past half century, the TASST experienced an abrupt decadal change around the late 1970s, with a rapid warming in recent two decades. Such decadal variability of the TASST contributes to the eastward shift of the SNAO pattern after the late 1970s. A process that may be responsible for the influence of the SST on the SNAO pattern can [28] Acknowledgments. The authors are grateful to Shuanglin Li, Yongqi Gao, and two anonymous reviewers for their valuable comments and helpful advices. This research was jointly supported by the National Basic Research Program of China (grant 2009CB421406), the Innovation Key Program (grants KZCX2-YW-Q1-02 and KZCX2-YW-BR-14) of the Chinese Academy of Sciences, and the project (grants and ) supported by the National Natural Science Foundation of China. References Barnston, A. G., and R. E. Livezey (1987), Classification, seasonality and persistence of low-frequency atmospheric circulation patterns, Mon. Weather Rev., 115, , doi: / (1987)115< 1083:CSAPOL>2.0.CO;2. Chang, C. P., P. Harr, and J. Ju (2001), Possible roles of Atlantic circulations on the weakening Indian monsoon rainfall-enso relationship, J. Clim., 14, , doi: / (2001)014<2376: PROACO>2.0.CO;2. Frankignoul, C., and E. Kestenare (2005), Observed Atlantic SST anomaly impact on the NAO: An update, J. Clim., 18, , doi: / JCLI Furevik, T., and J. E. O. Nilsen (2005), Large-scale atmospheric circulation variability and its impacts on the Nordic Seas Ocean climate-a review, in The Nordic Seas: An Integrated Perspective, Geophys. Monogr. Ser., vol. 158, edited by H. Drange et al., pp , AGU, Washington, D. C. Hilmer, M., and T. Jung (2000), Evidence for a recent change in the link between the North Atlantic Oscillation and Arctic sea ice export, Geophys. Res. Lett., 27, , doi: /1999gl Hurrell, J. W. (1995), Decadal trends in the North Atlantic Oscillation: Regional temperatures and precipitation, Science, 269, , doi: /science Hurrell, J. W., and C. K. Folland (2002), A change in the summer atmospheric circulation over the North Atlantic, CLIVAR Exchanges, 25, 1 3. Hurrell, J. W., and H. van Loon (1997), Decadal variations in climate associated with the North Atlantic Oscillation, Clim. Change, 36, , doi: /a: of9

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