A possible mechanism for the North Pacific regime shift in winter of 1998/1999

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 40, , doi: /grl.50798, 2013 A possible mechanism for the North Pacific regime shift in winter of 1998/1999 Hyun-Su Jo, 1 Sang-Wook Yeh, 1 and Cheol-Ho Kim 2 Received 12 June 2013; revised 21 July 2013; accepted 29 July 2013; published 19 August [1] This study examines a possible mechanism for the North Pacific regime shift in the winter of 1998/1999. Since the 1976/1977 climate regime shift, the sea surface temperature (SST) in the North Pacific has exhibited a long-term warming trend in the western and central regions, which is mainly due to the two regime shifts that occurred in the winter of 1988/1989 and 1998/1999, respectively. In particular, the 1998/1999 regime shift is characterized by a dipole-like structure along 40 N where a significant warming is prominent in the southwestern and central North Pacific. The slow dynamic adjustments of SST and zonal wind to the meridional heat exchange through thermal advection may be responsible for the 1998/1999 regime shift. We also assert that an intrinsic multidecadal SST oscillation in the North Pacific contributes to the 1998/1999 regime shift. Furthermore, another possibility, which is associated with the oceanic teleconnection from the tropics to the midlatitude, is also briefly discussed. Citation: Jo, H.-S., S.-W. Yeh, and C.-H. Kim (2013), A possible mechanism for the North Pacific regime shift in winter of 1998/ 1999, Geophys. Res. Lett., 40, , doi: /grl Introduction [2] It is important to understand the origins of the North Pacific sea surface temperature (SST) variability on decadalto-multidecadal time scales because they largely influence the weather and climate variability around the Pacific [Mantua et al., 1997; Di Lorenzo et al., 2008; Alexander et al., 2010]. In addition, the North Pacific is one of the most productive marine resources in the world; therefore, understanding these mechanisms is very crucial to predict the changes in the marine ecosystem in the North Pacific [Mantua et al., 1997; Hollowed et al., 2002; Chavez et al., 2003; Peterson and Schwing, 2003]. Sofar, thereexistsa wealth of studies to examine the origin of the North Pacific climate variability on decadal-to-multidecadal time scales [Nitta and Yamada, 1989;Latif and Barnett, 1994;Lau and Nath, 1994; Miller et al., 1994; Trenberth and Hurrell, 1994; Mantua et al., 1997; Minobe, 1997; Giese and Carton, 1999; Miller and Schneider, 2000; Deser et al., 1 Department of Marine Sciences and Convergent Technology, Hanyang University, Ansan, South Korea. 2 Korea Institute of Ocean Science & Technology, Ansan, South Korea. Corresponding author: S.-W. Yeh, Department of Marine Sciences and Convergent Technology, Hanyang University, 1271 Sa3-dong, Ansan , South Korea. (swyeh@hanyang.ac.kr) American Geophysical Union. All Rights Reserved /13/ /grl ; Wu et al., 2005; Mestas-Nunez and Miller, 2006; Yeh et al., 2011]. [3] Among them, a number of studies on the North Pacific climate regime shift in the winter of 1976/1977 indicate that its origin can be attributed to changes in the mean state of the SST in the tropical Pacific through atmospheric teleconnections from the tropics to the midlatitudes [Graham, 1994; Trenberth and Hurrell, 1994; Zhang et al., 1997; Deser et al., 2004; Deser and Phillips, 2006]. In addition, some studies have examined the origin of the North Pacific regime shift in the winter of 1988/1989, arguing that this regime shift in the North Pacific is not very related to the tropical SST forcing, unlike the 1976/1977 climate regime shift [Tanaka et al., 1996; Walsh et al., 1996; Overland et al., 1999; Watanabe and Nitta, 1999; Hare and Mantua, 2000; Hollowed et al., 2001; Yasunaka and Hanawa, 2002; Yeh et al., 2011]. To be specific, the mean SST change in the North Pacific in the winter of 1988/1989 regime shift was not accompanied with a change in the mean SST in the tropical Pacific. In addition, this regime shift was more influenced by the atmospheric circulation originating from the high latitudes, including the polar region, and the interactions between these atmospheric circulations and the SST in the North Pacific. [4] Basically, these results reflect the two lines of thought on the origin of North Pacific regimeshiftsofmeansst, i.e., one is originated from the tropical-midlatitude interactions including atmospheric teleconnections from the tropics and the other is mainly due to the air-sea interactions in the midlatitudes. Therefore, a detailed analysis of each regime shift in the North Pacific is necessary in order to understand the origins and mechanisms. After the 1988/1989 regime shift in the North Pacific, another regime shift occurred again in the winter of 1998/1999 [Hare and Mantua, 2000; Minobe, 2000; Schwing et al., 2000; Minobe, 2002]. It is known that the marine ecosystem largely changed off the west coast of the United States from a warm sardine regime to a cool anchovy regime across 1998/1999 [Chavez et al., 2003]. In addition, the climate state over the North Pacific, including the atmosphere, the upper ocean, and the ecosystem, underwent a striking transition in late 1998 [Bond et al., 2003; Chavez et al., 2003; Peterson and Schwing, 2003]. Several studies have examined the origin of the 1998/1999 regime shift. For example, Minobe [2002] argued that the cooling and sea level declines in the late 1990s in the Bering Sea might be related to a regime shift in 1998/1999; however, the mechanisms behind the 1998/1999 regime shift are still not completely understood. In this study, we focus on the origin of the North Pacific climate regime shift in the winter of 1998/ In particular, we study the multidecadal oscillation in the North Pacific SST to determine the origin of the 1998/ 1999 regime shift. 4380

2 Figure 1. Spatial pattern of the linear trend in sea surface temperature based on the December-January-February (DJF) seasonal mean for the 1976/1977 to 2011/2012 periods. regime shift). However, Figure 1 indicates that the SST became warmer (cooler) in the western and central (eastern) North Pacific after 1976/1977. [9] To examine the details of the SST trends since 1976/ 1977, we show the mean SST composite difference between the periods of 1976/1977 and 1997/1998 (Figure 2a) and between the periods of 1988/1989 and 2011/2012 (Figure 2b). These figures demonstrate that there are some differences in the 1988/1989 and 1998/1999 North Pacific regime shifts in terms of the spatial patterns. Namely, the 1988/1989 regime shift is characterized by a basin-scale warming in the North Pacific, where the center of the maximum SST is observed 2. Data and Methodology [5] We use multiple observational data sets for the period from December 1976 to February The monthly SST is taken from the monthly mean 2 2 gridded Extended Reconstruction SST, version 3 (ERSST.v3) [Smith et al., 2008]. For the same period, we use surface wind data from the National Centers for Environmental Prediction National Center for Atmospheric Research Reanalysis 1 [Kalnay et al., 1996]. We compute a meridional current from the Simple Ocean Data Assimilation (SODA) [Carton and Giese, 2008]. This study uses the seasonal mean data during the boreal winter (December-January-February, hereafter DJF), where 1976/1977 denotes 1976D/1977JF. The seasonal means are calculated from the monthly data during the winter, and seasonal anomalies are obtained by subtracting the seasonal means from the total winter mean field. Unless otherwise stated, these results apply to the winter season only. [6] The statistical significance test used in this study is based on a Student s t test because we compare the mean state changes between the two periods. The effective degrees of freedom to calculate the statistical significance are obtained by the methodology described in Livezey and Chen [1983]. 3. Results [7] Spatial pattern of the SST linear trend in the North Pacific in the period of 1976/1977 to 2011/2012 is displayed in Figure 1. The linear trend is obtained by the linear regression coefficients at each grid point using the time series of anomalous SST in the period of 1976/1977 to 2011/2012. The entire analyzed period is purposely limited after the 1976/1977 climate regime shift to suppress its effect on the SST anomalies, and the analyzed period (i.e., 1976/1977 to 2011/2012) is adequate to examine the SST variability on the low-frequency scales. [8] The spatial pattern of the SST trend is characterized by a significant warming in the southwestern and central North Pacific including the East Asia marginal sea such as the East China Sea, the Yellow Sea, the East Sea, and the Okhotsk Sea. In contrast, a cooling trend is confined in the eastern North Pacific around the west coast of North America. It is well known that the North Pacific SST experiences a significant cooling with an elliptical shape in the western and central North Pacific and a warming in the eastern North Pacificfrom before 1976/1977 to after 1976/1977 (i.e., 1976/1977 climate Figure 2. Difference in North Pacific (20 N 60 N, 120 E 120 W) during winter mean SST ( C) (a) between and , and (b) between and Dots denote the region where the statistical significance is above the 95% confidence level based on a Student s t test. (c) Time series of SST ( C) anomaly in the western and central North Pacific (20 N 44 N, 120 E 150 W) during winter for the period and the epoch average divided by the decadal abrupt changes. The decadal abrupt changes exceed the 95% confidence level. 4381

3 Figure 3. (a) Anomalous zonal-mean zonal wind in the period 1988/1989 to 1997/1998. (b) Anomalous zonal mean meridional surface current for the period 1998/1999 to 2007/2008. (c g) Same as Figure 3b except that the anomalous zonal mean meridional current is averaged from the surface to the depth of 45, 100, 200, 300, and 3000 m, respectively. The zonal mean is taken over 120 E 120 W. in the central North Pacific along 45 N (Figure 2a). On the other hand, the spatial pattern of the SST changes in the 1998/1999 regime shift shows a dipole-like structure along 40 N where a significant warming is prominent in the southwestern and central North Pacific and a cooling is prominent in the northwestern and eastern North Pacific, which is consistent with previous studies on the 1998/1999 and 1976/1977 regime shifts [Parrish et al., 2000; Schwing et al., 2000; Minobe, 2002; Yasunaka and Hanawa, 2002]. Therefore, one can conclude that the SST trend since 1976/1977 (Figure 1) is due to a combined effect of both the 1988/1989 and the 1998/1999 regime shifts. [10] More specifically, a similar spatial pattern between the SST trend (Figure 1) and the SST composite difference before and after 1998/1999 (Figure 2b) indicates that the 1998/1999 regime shift contributes to the North Pacific SST trend since 1976/1977. The correlation between Figures 1 and 2b is 0.79, which is larger than the correlation coefficient between Figures 1 and 2a (i.e., 0.48). This is also confirmed by the time series of anomalous SST averaged in 4382

4 Figure 4. Schematic diagram of the evolution feature of SST, surface zonal wind, wind stress curl, and upper layer meridional current anomalies indicating the multidecadal climate oscillation. The positive and negative wind stress curls are marked with a plus sign and minus sign, respectively. the southwestern and central North Pacific (20 N 44 N, 120 E 150 W) where the warming trend of SST since 1976/1977 is quite significant as shown in Figure 1. It is evident that a pronounced change of mean SST occurs around 1998/1999 (Figure 2c). Such a change of mean SST is also observed during other seasons (figure not shown). While the anomalous low SST is dominant in the southwestern and central North Pacific before 1998/1999, such anomalous low SST abruptly changes to the high SST after 1998/1999. Hereafter, we examine the mechanisms associated with the 1998/1999 regime shift in the southwestern and central North Pacific (Figures 2b and 2c) by analyzing atmosphere and ocean variables in the period of 1988/1989 to 2011/2012. [11] To examine the 1998/1999 regime shift, we hypothesize that slow dynamic adjustments of the SST and zonal wind in association with the meridional heat exchange through ocean thermal advection play a dominant role in the 1998/1999 regime shift in the North Pacific. In other words, our hypothesis is based on a slow adjustment between the atmospheric wind stress forcings and the ocean circulation (i.e., ocean thermal advection) with a lagged time, which can produce the interdecadal variability (about years) [Latif and Barnett, 1994; Robertson, 1996; Jin, 1997; An, 2008]. To support this hypothesis, we calculate the meridional structure of anomalous zonal mean (120 E ~ 120 W) wind before 1998/1999 (i.e., 1988/1989 to 1997/1998) (Figure 3a). The anomalous easterly over the North Pacific basin is dominant before the regime shift of 1998/1999. Note that a negative value in Figure 3a means weaker than the climatological westerly wind. The basin-scale anomalous easterly wind is largely due to the anomalous warm SST over the whole North Pacific before the regime shift of 1998/1999 as shown in Figure 2a. As pointed out by An [2008], basinscale anomalous warm (cool) SST in the North Pacific is able to provide a favorable condition for the easterly (westerly) zonal wind under the assumption that the atmosphere responds effectively and quickly adjusts to the SST changes in the North Pacific. [12] Such surface easterlies can induce negative wind stress curl in the northern part of the North Pacific and positive wind stress curl in the southern part of the North Pacific (see the schematic diagram in Figure 4). Concurrently, the positive and negative wind stress curls are able to induce an anomalous northward meridional current in the southern part and an anomalous southward current in the northern part of the North Pacific, respectively (Figure 3b). This is consistent with the Sverdrup balance [Sverdrup, 1947]. [13] Figure 3b shows the meridional structure of anomalous zonal mean (120 E ~ 120 W) meridional surface current after 1998/1999 (i.e., 1998/1999 to 2007/2008), and Figures 3c 3f show the averaged meridional currents from the surface to the depth of 45, 100, 200, 300, and 3000 m, respectively. Those surface currents are obtained from the SODA data. The meridional structure of surface current and depth-averaged currents is quite similar except their magnitudes. We argue that the anomalous northward meridional current in the southern part and the anomalous southward current in the northern part of the North Pacific across 40 N after the regime shift of 1998/1999 are mainly due to the anomalous wind stress forcings after the regime shift of 1988/1989. Here we argue that the adjustment time scale of the oceanic meridional current to a given wind stress forcing is around 10 years. Consequently, the meridional heat advection by meridional currents induces the anomalous low SST in the northern part and the anomalous high SST in the southern part of the North Pacific as shown in Figure 3b. [14] Figure 4 displays the schematic diagram of the evolution feature of SST, surface zonal wind, wind stress curl, and upper layer meridional current anomalies, which explains the regime shift of 1998/1999 in the North Pacific. These results suggest that the slow dynamic adjustment of the ocean due to basin-scale anomalous atmospheric wind stress forcings in the North Pacific before 1998/1999 is able to explain the 1998/1999 regime shift with a meridional structure of mean SST change through the meridional thermal advection. 4. Discussion and Concluding Remarks [15] Since the 1976/1977 climate regime shift, the North Pacific SST exhibits a long-term trend, including signatures of regime shifts. Based on the SST composite analysis, we confirmed the spatial pattern of the SST in relation to the two regime shifts in the North Pacific for the period of the winter of One is the 1988/1989 regime shift, which is characterized by a basin-scale warming in the North Pacific. The other is the 1998/1999 regime shift, which is characterized by a dipole-like structure along 40 N where a Figure 5. Time series of SST ( C) anomaly in North Equatorial Current bifurcation region (10 N 20 N, 120 E 170 E) during winter for the period and the epoch average divided by the decadal abrupt changes. The decadal abrupt changes exceed the 95% confidence level. 4383

5 significant warming is prominent in the southwestern and central North Pacific and a cooling is prominent in the northwestern and eastern North Pacific. [16] We suggested a possible mechanism for the 1998/ 1999 regime shift as follows. Before 1998/1999, anomalous easterly zonal wind prevailed in the North Pacific basin. Such anomalous surface easterly wind forcings accompanied an anomalous negative (positive) wind stress curl in the northern (southern) part of North Pacific. Subsequently, the anomalous wind stress curl forcings induced the anomalous southward and northward meridional currents in upper layers in the northern and southern North Pacific, respectively, since 1998/1999. Those meridional currents led to a dipolelike structure of warming and cooling along 40 N, which was associated with the 1998/1999 regime shift in the North Pacific SST. We argue that a multidecadal oscillation, which is associated with slow dynamic adjustments of the SST and a zonal wind with the meridional heat exchange through the thermal advection, is responsible for the 1998/ 1999 regime shift in the North Pacific. [17] In addition to this explanation, several other possible mechanisms may be responsible for the 1998/1999 regime shift, although we did not examine them in the present study. In particular, there is a possibility that the 1998/1999 regime shift is associated with the tropical-midlatitude interactions through oceanic teleconnections. This is because the southwestern and central North Pacific region where the 1998/ 1999 regime shift occurred corresponds to the Kuroshio Extension region. According to recent studies [Chen and Wu, 2012; Wu et al., 2012], the Kuroshio currents have carried more warm water and heat to the western North Pacific after the late 1990s. We also briefly analyzed a time series of the anomalous SST averaged in the North Equatorial Current bifurcation region (10 N 20 N, 120 E 170 E), where the Kuroshio currents bifurcate from the western tropical Pacific[Qiu, 2001; Qu and Lukas, 2003]. Interestingly, it is remarkable that the anomalous SST averaged in the North Equatorial Current bifurcation region shows a prominent warming since 1998/1999, which is similar to the time series of the anomalous SST averaged in the southwestern and central North Pacific (Figure 2c). A simultaneous correlation coefficient between the two time series (i.e., Figures 2c and 5) is 0.67, which is statistically significant at the 99% confidence level. These results seem to support the notion that an enhancement of warm water transport by the Kuroshio currents from the tropics to the southwestern may have contributed to the 1998/1999 regime shift in the North Pacific. On the other hand, the changes in air-sea heat flux, which is primarily through the sensible and latent heat fluxes associated with the changing wind field over the North Pacific [Cayan, 1992], may explain the observed changes in SST in the 1988/1999 regime shift. Furthermore, the subduction and subsurface westward advection of heat where it outcrops near the Kuroshio Extension [Schneider et al., 1999] could be another possible mechanism. These topics will be further explored in future research, and it is useful to examine which mechanisms are more important to explain the 1998/1999 regime shift. [18] Finally, it is noteworthy that the SST and wind anomaly spatial patterns after the 1998/1999 regime shift presented in this study are strikingly similar to those prior to the 1976/ 1977 regime shift [Peterson and Schwing, 2003]. In particular, the mechanism to explain the shift in this study is remarkably similar to that shown by Parrish et al. 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