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

Hyun-Su Jo,1 Sang-Wook Yeh,1 and Cheol-Ho Kim2

Received 12 June 2013; revised 21 July 2013; accepted 29 July 2013; published 19 August 2013.

[1] This study examines a possible mechanism for theNorth Pacific regime shift in the winter of 1998/1999.Since the 1976/1977 climate regime shift, the sea surfacetemperature (SST) in the North Pacific has exhibited along-term warming trend in the western and centralregions, which is mainly due to the two regime shifts thatoccurred in the winter of 1988/1989 and 1998/1999,respectively. In particular, the 1998/1999 regime shift ischaracterized by a dipole-like structure along 40°N where asignificant warming is prominent in the southwestern andcentral North Pacific. The slow dynamic adjustments of SSTand zonal wind to the meridional heat exchange throughthermal advection may be responsible for the 1998/1999regime shift. We also assert that an intrinsic multidecadalSST oscillation in the North Pacific contributes to the1998/1999 regime shift. Furthermore, another possibility,which is associated with the oceanic teleconnection fromthe tropics to the midlatitude, is also briefly discussed.Citation: Jo, H.-S., S.-W. Yeh, and C.-H. Kim (2013), A possiblemechanism for the North Pacific regime shift in winter of 1998/1999, Geophys. Res. Lett., 40, 4380–4385, doi:10.1002/grl.50798.

1. Introduction

[2] It is important to understand the origins of the NorthPacific sea surface temperature (SST) variability on decadal-to-multidecadal time scales because they largely influencethe weather and climate variability around the Pacific[Mantua et al., 1997; Di Lorenzo et al., 2008; Alexanderet al., 2010]. In addition, the North Pacific is one of the mostproductive marine resources in the world; therefore, under-standing these mechanisms is very crucial to predict thechanges in the marine ecosystem in the North Pacific[Mantua et al., 1997; Hollowed et al., 2002; Chavez et al.,2003; Peterson and Schwing, 2003]. So far, there exists awealth of studies to examine the origin of the North Pacificclimate variability on decadal-to-multidecadal time scales[Nitta and Yamada, 1989; Latif and Barnett, 1994; Lau andNath, 1994; Miller et al., 1994; Trenberth and Hurrell,1994; Mantua et al., 1997; Minobe, 1997; Giese andCarton, 1999; Miller and Schneider, 2000; Deser et al.,

2004; Wu et al., 2005; Mestas-Nunez and Miller, 2006; Yehet al., 2011].[3] Among them, a number of studies on the North Pacific

climate regime shift in the winter of 1976/1977 indicate thatits origin can be attributed to changes in the mean stateof the SST in the tropical Pacific through atmosphericteleconnections from the tropics to the midlatitudes[Graham, 1994; Trenberth and Hurrell, 1994; Zhanget al., 1997; Deser et al., 2004; Deser and Phillips, 2006].In addition, some studies have examined the origin of theNorth Pacific regime shift in the winter of 1988/1989,arguing that this regime shift in the North Pacific is not veryrelated to the tropical SST forcing, unlike the 1976/1977climate 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; Yasunakaand Hanawa, 2002; Yeh et al., 2011]. To be specific, themean SST change in the North Pacific in the winter of1988/1989 regime shift was not accompanied with a changein the mean SST in the tropical Pacific. In addition, thisregime shift was more influenced by the atmospheric circu-lation originating from the high latitudes, including thepolar region, and the interactions between these atmosphericcirculations and the SST in the North Pacific.[4] Basically, these results reflect the two lines of thought

on the origin of North Pacific regime shifts of mean SST,i.e., one is originated from the tropical-midlatitude interactionsincluding atmospheric teleconnections from the tropics andthe other is mainly due to the air-sea interactions in the midlat-itudes. Therefore, a detailed analysis of each regime shift inthe North Pacific is necessary in order to understand the ori-gins and mechanisms. After the 1988/1989 regime shift inthe North Pacific, another regime shift occurred again in thewinter of 1998/1999 [Hare and Mantua, 2000; Minobe,2000; Schwing et al., 2000; Minobe, 2002]. It is known thatthe marine ecosystem largely changed off the west coast ofthe 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, includingthe 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 1998/1999regime shift. For example, Minobe [2002] argued that thecooling and sea level declines in the late 1990s in the BeringSea might be related to a regime shift in 1998/1999; however,the mechanisms behind the 1998/1999 regime shift are still notcompletely understood. In this study, we focus on the origin ofthe North Pacific climate regime shift in the winter of 1998/1999. In particular, we study the multidecadal oscillation inthe North Pacific SST to determine the origin of the 1998/1999 regime shift.

1Department of Marine Sciences and Convergent Technology, HanyangUniversity, Ansan, South Korea.

2Korea Institute of Ocean Science & Technology, Ansan, South Korea.

Corresponding author: S.-W. Yeh, Department of Marine Sciences andConvergent Technology, Hanyang University, 1271 Sa3-dong, Ansan426-791, South Korea. (swyeh@hanyang.ac.kr)

©2013. American Geophysical Union. All Rights Reserved.0094-8276/13/10.1002/grl.50798

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GEOPHYSICAL RESEARCH LETTERS, VOL. 40, 4380–4385, doi:10.1002/grl.50798, 2013

2. Data and Methodology

[5] We use multiple observational data sets for the periodfrom December 1976 to February 2012. The monthly SST istaken from the monthly mean 2° × 2° gridded ExtendedReconstruction SST, version 3 (ERSST.v3) [Smith et al.,2008]. For the same period, we use surface wind data fromthe National Centers for Environmental Prediction–NationalCenter for Atmospheric Research Reanalysis 1 [Kalnayet al., 1996]. We compute a meridional current from theSimple Ocean Data Assimilation (SODA) [Carton andGiese, 2008]. This study uses the seasonal mean data duringthe boreal winter (December-January-February, hereafterDJF), where 1976/1977 denotes 1976D/1977JF. The seasonalmeans are calculated from the monthly data during the winter,and seasonal anomalies are obtained by subtracting theseasonal means from the total winter mean field. Unless other-wise 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 meanstate changes between the two periods. The effectivedegrees of freedom to calculate the statistical significanceare obtained by the methodology described in Livezey andChen [1983].

3. Results

[7] Spatial pattern of the SST linear trend in the NorthPacific in the period of 1976/1977 to 2011/2012 is displayedin Figure 1. The linear trend is obtained by the linear regres-sion coefficients at each grid point using the time series ofanomalous SST in the period of 1976/1977 to 2011/2012.The entire analyzed period is purposely limited after the1976/1977 climate regime shift to suppress its effect on theSST anomalies, and the analyzed period (i.e., 1976/1977 to2011/2012) is adequate to examine the SST variability onthe low-frequency scales.[8] The spatial pattern of the SST trend is characterized by a

significant warming in the southwestern and central NorthPacific including the East Asia marginal sea such as the EastChina Sea, the Yellow Sea, the East Sea, and the OkhotskSea. In contrast, a cooling trend is confined in the easternNorth Pacific around the west coast of North America. It iswell known that the North Pacific SST experiences a signifi-cant cooling with an elliptical shape in the western and centralNorth Pacific and a warming in the eastern North Pacific frombefore 1976/1977 to after 1976/1977 (i.e., 1976/1977 climate

regime shift). However, Figure 1 indicates that the SSTbecame 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 betweenthe periods of 1976/1977 and 1997/1998 (Figure 2a) and be-tween the periods of 1988/1989 and 2011/2012 (Figure 2b).These figures demonstrate that there are some differences inthe 1988/1989 and 1998/1999 North Pacific regime shifts interms of the spatial patterns. Namely, the 1988/1989 regimeshift is characterized by a basin-scale warming in the NorthPacific, where the center of the maximum SST is observedFigure 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.

Figure 2. Difference in North Pacific (20°N–60°N, 120°E–120°W) during winter mean SST (°C) (a) between 1976–1987 and 1988–1997, and (b) between 1988–1997 and1998–2011. Dots denote the region where the statisticalsignificance is above the 95% confidence level based on aStudent’s t test. (c) Time series of SST (°C) anomaly inthe western and central North Pacific (20°N–44°N, 120°E–150°W) during winter for the period 1976–2011 and the epochaverage divided by the decadal abrupt changes. The decadalabrupt changes exceed the 95% confidence level.

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in the central North Pacific along 45°N (Figure 2a). On theother hand, the spatial pattern of the SST changes in the1998/1999 regime shift shows a dipole-like structure along40°N where a significant warming is prominent in the south-western and central North Pacific and a cooling is prominentin the northwestern and eastern North Pacific, which is consis-tent with previous studies on the 1998/1999 and 1976/1977regime 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 1988/1989and the 1998/1999 regime shifts.[10] More specifically, a similar spatial pattern between the

SST trend (Figure 1) and the SST composite differencebefore and after 1998/1999 (Figure 2b) indicates that the1998/1999 regime shift contributes to the North PacificSST trend since 1976/1977. The correlation betweenFigures 1 and 2b is 0.79, which is larger than the correlationcoefficient between Figures 1 and 2a (i.e., 0.48). This is alsoconfirmed by the time series of anomalous SST averaged in

Figure 3. (a) Anomalous zonal-mean zonal wind in the period 1988/1989 to 1997/1998. (b) Anomalous zonal meanmeridional surface current for the period 1998/1999 to 2007/2008. (c–g) Same as Figure 3b except that the anomalous zonalmean meridional current is averaged from the surface to the depth of 45, 100, 200, 300, and 3000m, respectively. The zonalmean is taken over 120°E–120°W.

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the southwestern and central North Pacific (20°N–44°N,120°E–150°W) where the warming trend of SST since1976/1977 is quite significant as shown in Figure 1. It isevident that a pronounced change of mean SST occursaround 1998/1999 (Figure 2c). Such a change of meanSST is also observed during other seasons (figure notshown). While the anomalous low SST is dominant in thesouthwestern and central North Pacific before 1998/1999,such anomalous low SST abruptly changes to the highSST after 1998/1999. Hereafter, we examine the mecha-nisms associated with the 1998/1999 regime shift in thesouthwestern and central North Pacific (Figures 2b and 2c)by analyzing atmosphere and ocean variables in the periodof 1988/1989 to 2011/2012.[11] To examine the 1998/1999 regime shift, we hypothe-

size that slow dynamic adjustments of the SST and zonalwind in association with the meridional heat exchangethrough ocean thermal advection play a dominant role inthe 1998/1999 regime shift in the North Pacific. In otherwords, our hypothesis is based on a slow adjustment betweenthe atmospheric wind stress forcings and the ocean circula-tion (i.e., ocean thermal advection) with a lagged time, whichcan produce the interdecadal variability (about 10–20 years)[Latif and Barnett, 1994; Robertson, 1996; Jin, 1997; An,2008]. To support this hypothesis, we calculate the meridio-nal 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 Pacificbasin is dominant before the regime shift of 1998/1999.Note that a negative value in Figure 3a means weaker thanthe climatological westerly wind. The basin-scale anomalouseasterly wind is largely due to the anomalous warm SST overthe whole North Pacific before the regime shift of 1998/1999as shown in Figure 2a. As pointed out by An [2008], basin-scale anomalous warm (cool) SST in the North Pacific is ableto provide a favorable condition for the easterly (westerly)zonal wind under the assumption that the atmosphereresponds effectively and quickly adjusts to the SST changesin the North Pacific.[12] Such surface easterlies can induce negative wind

stress curl in the northern part of the North Pacific and posi-tive wind stress curl in the southern part of the North Pacific(see the schematic diagram in Figure 4). Concurrently, thepositive and negative wind stress curls are able to induce ananomalous northward meridional current in the southern partand an anomalous southward current in the northern part ofthe North Pacific, respectively (Figure 3b). This is consistentwith the Sverdrup balance [Sverdrup, 1947].

[13] Figure 3b shows the meridional structure of anoma-lous zonal mean (120°E ~ 120°W) meridional surface currentafter 1998/1999 (i.e., 1998/1999 to 2007/2008), andFigures 3c–3f show the averaged meridional currents fromthe surface to the depth of 45, 100, 200, 300, and 3000m, re-spectively. Those surface currents are obtained from theSODA data. The meridional structure of surface current anddepth-averaged currents is quite similar except their magni-tudes. We argue that the anomalous northward meridionalcurrent in the southern part and the anomalous southwardcurrent in the northern part of the North Pacific across 40°Nafter the regime shift of 1998/1999 are mainly due to theanomalous wind stress forcings after the regime shift of1988/1989. Here we argue that the adjustment time scale ofthe oceanic meridional current to a given wind stress forcingis around 10 years. Consequently, the meridional heat advec-tion by meridional currents induces the anomalous low SSTin the northern part and the anomalous high SST in thesouthern part of the North Pacific as shown in Figure 3b.[14] Figure 4 displays the schematic diagram of the evolu-

tion feature of SST, surface zonal wind, wind stress curl, andupper layer meridional current anomalies, which explains theregime shift of 1998/1999 in the North Pacific. These resultssuggest that the slow dynamic adjustment of the ocean due tobasin-scale anomalous atmospheric wind stress forcings inthe North Pacific before 1998/1999 is able to explain the1998/1999 regime shift with a meridional structure of meanSST change through the meridional thermal advection.

4. Discussion and Concluding Remarks

[15] Since the 1976/1977 climate regime shift, the NorthPacific SST exhibits a long-term trend, including signaturesof regime shifts. Based on the SST composite analysis, weconfirmed the spatial pattern of the SST in relation to thetwo regime shifts in the North Pacific for the period of thewinter of 1976–2011. One is the 1988/1989 regime shift,which is characterized by a basin-scale warming in theNorth Pacific. The other is the 1998/1999 regime shift, whichis characterized by a dipole-like structure along 40°Nwhere a

Figure 4. Schematic diagram of the evolution feature ofSST, surface zonal wind, wind stress curl, and upper layer me-ridional current anomalies indicating the multidecadal climateoscillation. The positive and negative wind stress curls aremarked with a plus sign and minus sign, respectively.

Figure 5. Time series of SST (°C) anomaly in NorthEquatorial Current bifurcation region (10°N–20°N, 120°E–170°E) during winter for the period 1976–2011 and theepoch average divided by the decadal abrupt changes. Thedecadal abrupt changes exceed the 95% confidence level.

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significant warming is prominent in the southwestern andcentral North Pacific and a cooling is prominent in the north-western and eastern North Pacific.[16] We suggested a possible mechanism for the 1998/

1999 regime shift as follows. Before 1998/1999, anomalouseasterly zonal wind prevailed in the North Pacific basin.Such anomalous surface easterly wind forcings accompaniedan anomalous negative (positive) wind stress curl in thenorthern (southern) part of North Pacific. Subsequently, theanomalous wind stress curl forcings induced the anomaloussouthward and northward meridional currents in upper layersin the northern and southern North Pacific, respectively,since 1998/1999. Those meridional currents led to a dipole-like structure of warming and cooling along 40°N, whichwas associated with the 1998/1999 regime shift in theNorth Pacific SST. We argue that a multidecadal oscillation,which is associated with slow dynamic adjustments of theSST and a zonal wind with the meridional heat exchangethrough 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 regimeshift, although we did not examine them in the present study.In particular, there is a possibility that the 1998/1999 regimeshift is associated with the tropical-midlatitude interactionsthrough oceanic teleconnections. This is because the south-western and central North Pacific region where the 1998/1999 regime shift occurred corresponds to the KuroshioExtension region. According to recent studies [Chen andWu, 2012; Wu et al., 2012], the Kuroshio currents have car-ried more warm water and heat to the western North Pacificafter the late 1990s. We also briefly analyzed a time seriesof the anomalous SST averaged in the North EquatorialCurrent bifurcation region (10°N–20°N, 120°E–170°E),where the Kuroshio currents bifurcate from the western trop-ical Pacific [Qiu, 2001;Qu and Lukas, 2003]. Interestingly, itis remarkable that the anomalous SST averaged in the NorthEquatorial Current bifurcation region shows a prominentwarming since 1998/1999, which is similar to the time seriesof the anomalous SST averaged in the southwestern andcentral North Pacific (Figure 2c). A simultaneous correlationcoefficient between the two time series (i.e., Figures 2c and 5)is 0.67, which is statistically significant at the 99% confidencelevel. These results seem to support the notion that an en-hancement of warm water transport by the Kuroshio currentsfrom the tropics to the southwestern may have contributed tothe 1998/1999 regime shift in the North Pacific. On the otherhand, the changes in air-sea heat flux, which is primarilythrough the sensible and latent heat fluxes associated withthe changing wind field over the North Pacific [Cayan,1992], may explain the observed changes in SST in the1988/1999 regime shift. Furthermore, the subduction andsubsurface westward advection of heat where it outcropsnear the Kuroshio Extension [Schneider et al., 1999]could be another possible mechanism. These topics will befurther explored in future research, and it is useful to exam-ine which mechanisms are more important to explain the1998/1999 regime shift.[18] Finally, it is noteworthy that the SST and wind anom-

aly spatial patterns after the 1998/1999 regime shift presentedin this study are strikingly similar to those prior to the 1976/1977 regime shift [Peterson and Schwing, 2003]. In particu-lar, the mechanism to explain the 1998–1999 shift in this

study is remarkably similar to that shown by Parrish et al.[2000] for the 1976–1977 regime shift, indicating that the1998/1999 regime shift could be considered as a return tothe conditions prior to 1976/1977.

[19] Acknowledgments. This work was supported by the NationalResearch Foundation of Korea grant funded by the Korean Government(MEST) (NRF-2009-C1AAA001-2009-0093042). The authors are gratefulto the two anonymous reviewers who gave very constructive comments.[20] The Editor thanks two anonymous reviewers for their assistance in

evaluating this paper.

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