An unusual coupled mode in the tropical Pacific during 2004

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1 An unusual coupled mode in the tropical Pacific during 2004 Karumuri Ashok 1, Swadhin K. Behera 1, Suryachandra A. Rao 1, Hengyi Weng 1, and Toshio Yamagata 1,2,* 1 FRCGC/JAMSTEC, , Showamachi, Kanazawa-Ku, Yokohama, Kanagawa, , Japan 2 Also at Department of Earth & Planetary Science, Graduate School of Science, University of Tokyo, Japan * yamagata@eps.s.u-tokyo.ac.jp ABSTRACT The sea surface temperature anomalies (SSTA) of tropical Pacific in the boreal summer of 2004 show a tripolar pattern with warm SSTA in the central tropical Pacific flanked on both sides by cold SSTA. It is found that the phenomenon involves equatorial coupled ocean-atmosphere dynamics that are different from the conventional El Niño; the peak warming of the SSTA is confined to the central equatorial Pacific until boreal winter instead of propagating to the east. Anomalous twin Walker circulation cells associated with the SSTA give rise to rainfall anomalies that also form a tripole. Because of this unique nature, we distinguish this condition as El Niño Modoki (Pseudo El Niño) - a new coupled phenomenon. Similar strong events are also observed in 1986, 1990, 1991, 1992, 1994 and 2002 since

2 1. Introduction Several parts of the globe experienced anomalous climate conditions during the summer (June-August; JJA) of 2004 (Figure 1). Many areas of Japan and the Far East region experienced severe drought and heat waves whereas Philippine Islands received surplus rainfall due to active cumulus activities. The Maritime Continent, and southern Mexico and Ecuador suffered from drought whereas central tropical Pacific received surplus rainfall. Peninsular India experienced weaker than normal monsoon conditions and Queensland in Australia also received less than normal austral winter rainfall. These anomalous summer conditions are seen to be associated with an unusual distribution of sea surface temperature anomalies (SSTA), which is composed of anomalous warming in the tropical central Pacific flanked by colder than normal SST in east and west (Figure 2a). Interestingly, the peak warming did not migrate to the eastern tropical Pacific in course of time as in case of El Niño events. In fact, the central warming maximum persisted through the following winter. The evidence of similar anomalous SSTA is also seen in studies such as Donguy and Dessier [1983] and McPhaden [2004]. Interestingly, some earlier EOF analyses also hint such non-enso type SSTA patterns in the tropical Pacific [see EOF3 pattern of Meyers et al. 1999; also Weare at al. 1976]. Because of the unique behavior of this phenomenon that is to be discussed in section 3, we classify the tropical Pacific SSTA pattern during 2004 as part of a new phenomenon and we named it as the El Niño Modoki (Pseudo-El Niño). Modoki is a classical Japanese word which means similar but different. This terminology was introduced by the corresponding author in 2004 to explain a probable cause behind the abnormal summer conditions in Japan. Readers are referred to the following websites: 2

3 ( In this article, we describe salient features of this new phenomenon by depicting the associated ocean-atmosphere conditions. 2. Datasets used The Hadley Centre Global Sea Ice and Sea Surface Temperature (HadISST) dataset [Rayner et al., 2003] from January 1979 to April 2005 is used in this study. Also used are Climate Prediction Center Merged Analysis of Precipitation (CMAP) data [Xie and Arkin, 1996] and NCEP/NCAR reanalysis products [Kalnay et al., 1996] for the period after Merged sea surface height (SSH) data from Aviso, France, and satellite derived winds from Quick Scat (after 2000) are also used.. 3. Features of El Niño Modoki The El Niño Modoki seen in boreal summer of 2004 is characterized by a tripolar pattern in SSTA (Figure 2a). The central anomalous SST warming was caused from April by a series of intraseasonal downwelling Kelvin waves in response to anomalous westerlies over the western tropical Pacific. In response to anomalous westerlies in the western Pacific, the thermocline became anomalously shallow there, causing colder than normal SST in that region. Due to the excited Kelvin waves, the SSH in the central tropical Pacific was anomalously raised (Figure 2b) along with subsurface warming. However, as the intraseasonal westerly wind bursts were not as strong as during strong El 3

4 Niño years such as 1997 (figure not shown), the maximum in the Kelvin wave-induced downwelling was rarely seen in the eastern tropical Pacific in most of the period despite some weak and intermittent eastward propagation beyond the central tropical Pacific. The central warming was further aided by the presence of anomalous tropical easterlies over the tropical eastern Pacific. The same wind anomalies enhanced upwelling in the eastern equatorial Pacific; this may be a major difference of the El Niño Modoki from a conventional El Niño. Further, maximum temperature anomalies up to 1.5~2.5ºC along the equator in summer and fall of 2004 are also observed at thermocline depth in the central tropical Pacific. It is interesting to note that the warmest SSTA is located between 155ºW and 160ºE until February This occurs despite the propagation of positive SSTA to the east between September and December also along with some spreading of the anomalous warm waters to the west during the boreal winter (Figure 2c); also peculiar is that the SSTA did not amplify during the eastward propagation. By the end of January 2005, anomalously cooler SSTA is again seen in both the eastern and western equatorial Pacific flanking the warmer central equatorial Pacific SSTA. All this is different from the post-1977 El Niños that are characterized by propagation of the warmest SSTA from the central tropical Pacific to the coast of Peru. The interesting patterns of SSTA (Figure 2a) in the tropical Pacific are associated with unusual patterns of wind and rainfall anomalies during boreal summer of The anomalous summer SST cooling in the east and west that flanks the anomalously warm central tropical Pacific are overlaid with anomalous low level easterlies and westerlies (Figure 2b). This is related to an above normal rainfall in the central equatorial Pacific flanked on both sides by the negative rainfall anomalies (Figure 1). The SSTA gives rise 4

5 to a pair of Walker cells (Figure 3) with a common ascending limb overlying the warmer central Pacific. This double-cell pattern shows a marked difference from the single-cell Walker circulation in the tropical Pacific during a typical El Niño case. The surface easterlies and westerlies associated with the Walker cells then reinforce the tripolar SSTA through ocean dynamics. This is evidenced from the persistence of SSTA and subsurface temperature anomalies as discussed in the previous paragraph. The El Niño Modoki phenomenon appears to involve ocean-atmosphere coupled dynamics, as evidenced by the interaction among SSTA, subsurface temperature, wind and rainfall anomalies. The atmospheric condition associated with the descending branch of the western Walker cell (Figure 3) located over the western Pacific caused the drought over the Maritime Continent (Figure 1) and its influence extended northwest till Bangladesh, south India and Sri Lanka. The deficit rainfall in the western Pacific region also extended southward to southeastern Australia in the Southern Hemisphere. In contrast, positive rainfall anomalies were observed in the Philippines, Thailand, Myanmar and northern India near the anomalous updraft region of the northern Hadley cell. Further north, countries including southern part of Japan suffered drought during this summer owing to the Pacific-Japan teleconnection pattern [see Nitta, 1987]. We note that this impact in East Asia is opposite to that of the conventional El Niño [cf. Ropelewski et al., 1987; Diaz et al., 2001; Saji and Yamagata 2003]. Negative rainfall anomalies over the equatorial eastern Pacific in response to the descending branch of the eastern Walker cell extend to Mexico, whereas surplus rain is seen further north up to 40ºN over North America. This is again different from the characteristic El Niño influence of surplus rainfall over the west coast of U. S. Further details of boreal summer (and winter) global 5

6 teleconnections associated with the tropical Pacific SSTA during 2004, along with discussion on the relevant mechanisms that explain the teleconnection in tropics and higher latitudes, are discussed in Ashok et al. [2006; available at By carrying out an EOF analysis of the tropical Pacific (20ºS-20ºN) SSTA from January 1979 to February 2005, we have identified that the SSTA shown in Figure 2a largely corresponds to the EOF2 of the tropical Pacific SST variability that explains 12% of variance (see Figure 2b of Ashok et al., 2006). The principal component (Figure 4) of the EOF2 (PC2 hereafter) is positive almost throughout 2004, and its amplitude exceeds 0.8 of its standard deviation during boreal summer of 2004 and 0.7 during the second half of the following boreal winter. As demonstrated in PC2, there are several other years such as 1986, 1990, 1991, 1992, 1994 and 2002 in which the SST conditions in the tropical Pacific and their relevant teleconnections resemble those of 2004 from boreal summer through winter [Ashok et al., 2006]; incidentally, we notice that the amplitudes of the SSTA during many of these events are stronger than that of the 2004 El Niño Modoki event. Interestingly, PC3 was also stronger than normal during Hence the tripolar SSTA during 2004 was amplified (Figure A1 in auxiliary section) in the tropics [see Figure 3 of Ashok et al., 2006]. The robustness of the present EOF analysis is also verified by rotation of first 10 EOF modes, Also, the composite tropical Pacific SSTA during El Niño Modoki years confirms the warmest SSTA being restricted to the central tropical Pacific from boreal summer through next winter [see Figure 5 of Ashok et al., 2006], demonstrating that these events are not a part of the ENSO evolution. 6

7 The weak amplitude and changing phases of NINO3 and NINO3.4 indices also show that the anomalous conditions in 2004 are quite distinct from those of the so-called protracted El Niño [Allan, 2003]. The event is also not adequately captured by the definition of dateline El Niño [Larkin and Harrison, 2005a, b]. This is because the definition is based only on the SST warming in the NINO3.4 region. Larkin and Harrison [2005a] have mentioned correctly that there is no need of any significant warming in the cold tongue region, but they did not distinguish the existence of even significant cooling in the eastern and western tropical Pacific, which is associated with tripolar SSTA. We note that the magnitude of the cooling in the eastern tropical Pacific during 2004 summer is comparable to that of the central tropical Pacific warming (Figure 2a). Based on lead and lag correlation analyses, Ashok et al. [2006] found that the maximum lead/lag only explains 30% of either of the EOF1 or the EOF2 type phenomena. This further supports that the 2004 El Niño Modoki event is not a part of ENSO evolution. Rather, it should be classified into another coupled mode in the tropical Pacific. The characteristics of ENSO as defined by Trans-Nino index (TNI) by Trenberth and Stepaniak [2001] (henceforth TS01) is also examined here in the context of El Niño Modoki. The TNI is a measure of zonal SSTA gradient between the central equatorial Pacific and the eastern equatorial Pacific (Table 1). As reported by Trenberth et al., [2002], the TNI is highly correlated with the principal component of SVD2 of SSTA and several atmospheric fields such as precipitation. Further, they find that NINO3.4 and TNI are significantly correlated at different leads/lags. Based on this, they claim that the TNI explains a pattern, which is a part of ENSO evolution. We notice, however, that the lag/lead correlations between TNI and NINO3.4 not only weakened after 1976 but also 7

8 changed signs (see Figure 2 of TS01). This suggests that, depending on the background state, the TNI is composed of several different modes of variability. We find that the principal component of EOF2 is positive for most of the time after March 2002 (Figure 4). In contrast, the phase of NINO3 and NINO3.4 indices change sign several times. The amplitude of the EOF1, which is basically related ENSO, is also weak after 2003 summer. Conversely, the SSTA over the central pole of the El Niño Modoki is consistently warm from March 2002 through February 2005(Figure 4). This indicates that the tropical Pacific has been dominated by the EOF2 mode since 2002 spring. Thus, the El Niño Modoki in summer 2004 is neither preceded by La Niña nor succeeded by El Niño; this situation is different from the hypothesis of TS01. In fact, out of the three major El Niño events after 1977, only the event fits to the condition stipulated by TS01. All these indicate that the anomalous tropical Pacific condition in 2004, which largely resembles the EOF2 pattern, is not a manifestation of ENSO evolution. More detailed analyses to be reported elsewhere [Ashok et al., 2006] also suggest that similar events represented by the EOF2 can well be described as a distinct class of coupled phenomenon in tropical Pacific. The El Niño Modoki events seem to have become more prominent statistically because of the changes in background conditions of the tropical Pacific [Ashok et al., 2006]. 4. Conclusions In boreal spring of 2004, the equatorial central Pacific SSTA was anomalously warm with colder than normal SSTA on its both sides in the tropical eastern and western Pacific. This tripolar SSTA configuration with the warmest anomaly in the tropical Pacific lasted through boreal winter without extending to the Peruvian coast. This 8

9 situation was much different from that of typical El Niño events. Because of this distinction, we have described this tripolar SSTA structure in the equatorial Pacific during 2004 as a new class of phenomenon called El Niño Modoki [see Ashok et al., 2006 for details]. This classification as another mode in the tropical Pacific is strongly supported by the fact that this event involves unique coupled ocean-atmosphere dynamics and has its own teleconnections over the globe. In particular, its impacts on summer conditions in the Far East are just opposite from those of El Niño. The SSTA pattern is captured basically by the EOF2 pattern of the tropical Pacific SST variability for the period from Januray 1979 to February Some aspects in the spatial structure and temporal evolution of El Niño Modoki were captured in earlier works by Meyers et al. [1999] and Weare et al. [1976]. However, they did not deal with the variance contributions from important El Niño Modoki events in 1994, 2002, 2004 and a major La Niña Modoki event in We have also demonstrated that the 2004 El Niño Modoki event is not a part of the evolution of El Niño. Since changes in characteristics of the tropical Pacific coupled system may have vast impact on global teleconnection patterns [Navarra et al., 1999; Meyers et al., 2006], we believe that the present topic is of considerable importance even from a societal viewpoint. In a companion paper [Ashok et al., 2006], we have examined general properties and teleconnection features of the El Niño Modoki event using ocean/atmosphere data since Acknowledgements The authors were encouraged by discussions with Prof. R. Lukas. Useful comments from the reviewers of an earlier version are acknowledged. Data from Aviso is acknowledged. Figures in this paper have been prepared using the GrADs/COLA and FERRET softwares. 9

10 Table 1: Different SSTA indices used in this study Index NINO3.4 NINO3 NINO1+2 NINO4 Trans-Niño index Definition The area-averaged SSTA over 5ºN-5ºS, 170ºW-120ºW. The area-averaged SSTA over 5ºN-5ºS, 150ºW-90ºW. The area-averaged SSTA over equator-10ºs, 90ºW-80ºW. The area-averaged SSTA over 5ºN-5ºS, 160ºE-150ºW. difference of normalized SSTA between NINO4 and NINO1+2 10

11 REFERENCES Ashok, K., S. K. Behera, S. A. Rao, H. Weng, and T. Yamagata (2006), El Niño Modoki and its teleconnection, submitted to J. Geophys. Res. Allan, R. J., C. J. C. Reason, J. A. Lindesay, T. J. Ansell (2003), 'Protracted' ENSO episodes and their impacts in the Indian Ocean region. Deep-Sea Research II, 50, Donguy J-R., and A. Dessier (1983), El Niño-like events observed in the tropical Pacific, Mon. Wea. Rev., 111, Diaz, H. F., M. P. Hoerling, and J. K. Eischeid (2001), ENSO variability, teleconnections and climate change. Int. J. Climatol., 21, Kalnay, E., and coauthors (1996), The NCEP/NCAR 40-year reanalysis project. Bull. Amer. Meteor. Soc., 77, McPhaden, M. J. (2004), Evolution of the 2002/03 El Niño Bulletin of the American Meteorological Society, 85, Meyers, S. D., J. J. O Brien, and E. Thelin (1999), Reconstruction of monthly SST in the tropical Pacific Ocean during using adaptive climate basis functions. Mon. Wea. Rev., 127, Meyers, G., P. Mcintosh, L. Pigot, and M. Pook (2006), The years of El Niño, La Niña and interactions with the tropical Indian Ocean, J. Climate (in press). Navarra et al.. A., M. N. Ward, and K. Miyakoda (1999), Tropical-wide teleconnections and oscillation. I: Teleconnection indices and type I/II states, Q. J. R. Meteorol. Soc., 125,

12 Larkin N. K., and D. E. Harrison (2005), On the definition of El Niño and associated seasonal average U.S. weather anomalies. Geophys. Res. Lett., 32, L16705, doi: /2005gl Larkin N. K., and D. E. Harrison, (2005) Global seasonal temperature and precipitation anomalies during El Niño autumn and winter. Geophys. Res. Lett., 32, L13705, doi: /2005gl Nitta, T., (1987), Convective activities in the tropical Pacific and their impacts on the northern hemisphere summer circulation. J. Meteor. Soc. Japan, 65, Rasmusson, E. M., and T. H. Carpenter (1982), Variation in tropical sea surface temperature and surface wind fields associated with Southern Oscillation/El Niño, Mon. Wea. Rev., 110, Rayner, N. A., and co-authors (2003), Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century, J. Geophys. Res., 108, No. D14, 4407 doi: /2002jd Ropelewski, C.F., and M.S. Halpert (1987), North American precipitation and temperature patterns associated with the El Niño/Southern Oscillation (ENSO). Mon. Wea. Rev., 114, ,. Saji, N. H., and T. Yamagata (2003), Possible impacts of Indian Ocean Dipole events on global climate, Climate Res., 25, Trenberth, K., and D. P. Stepaniak (2001), Indices of El Niño Evolution, J. Climate, 14, Trenberth, K.E., D.P. Stepaniak, and J.M. Caron (2002), Interannual variations in the atmospheric heat budget. J. Geophys. Res., 107, 4066, /2000JD

13 Xie, P., and P. A. Arkin (1996), Analyses of global monthly precipitation using rain gauge observations, satellite estimates and numerical model predictions. J. Climate, 9, Weare, B.C., A.R. Navato, R. E. Newell (1976), Empirical orthogonal analysis of Pacific sea surface temperatures, Journal of Physical Oceanography, 6,

14 Figure 1 June-August rainfall anomalies (mm.day -1 ) for

15 (a) (b) (c) Figure 2 Abnormal conditions during JJA in 2004: (a) SSTA in ºC, (b) evolution of SSHA in cm (shaded) and wind anomalies(m.s -1 ; vectors), averaged between 2ºS-2ºN, and (c) evolution of SSTA averaged between 5ºS-5º N.. 15

16 Figure 3 Anomalous Walker circulation averaged over 10ºS to 10ºN during JJA in Figure 4 Normalized time series of the PC2of tropical Pacific SST variability (Bar) and PC3 (yellow), NINO3 SSTA (blue), and NINO4 SSTA (red). 16

El Niño Modoki and its possible teleconnection

El Niño Modoki and its possible teleconnection KARUMURI ASHOK, SWADHIN K. BEHERA, SURYACHANDRA A. RAO, HENGYI WENG Frontier Research Center for Global Change/JAMSTEC, 3173-25, Showamachi, Kanazawa-Ku,