Studies of Climate Variability Using General Circulation Models

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1 Present and Future of Modeling Global Environmental Change: Toward Integrated Modeling, Eds., T. Matsuno and H. Kida, pp by TERRAPUB, Studies of Climate Variability Using General Circulation Models Masahide Kimoto Center for Climate System Research, University of Tokyo, Meguro, Tokyo , Japan Abstract Some studies on interannual to decadal climate variability using atmospheric and coupled ocean-atmosphere general circulation models (GCMs) developed at the Center for Climate System Research, University of Tokyo, are introduced. The AGCM has been used for numerical experiments to quantify the role of land-surface feedback in the interannual variability of the Indian Monsoon and remote impacts of sea surface temperature anomalies in the Indian and equatorial eastern Pacific Oceans on an anomalous East Asian summer monsoon. A coupled upper-ocean-atmosphere GCM has been used to investigate coupled interannual and decadal variability in the tropical Pacific and extratropical Pacific and Atlantic Oceans. The coupled model shows decadal, quasi-quadrennial, and quasi-biennial sea surface temperature variability in the Pacific basin. Active involvement of extratropical-subtropical winds and ocean heat content in the simulated decadal mode is conspicuous. The coupled GCM also exhibits a decadal ocean-atmosphere mode over the North Atlantic. An experiment using a coupled atmosphere-mixed-layer ocean model reveals that a positive feedback exists in the North Atlantic atmosphere-ocean system. INTRODUCTION General circulation models (GCMs) of the atmosphere and oceans serve as a powerful tool for studying the global climate and its variability. They have been under continuous development since the pioneering age of the 1960s. Though far from perfect, carefully validated GCMs offer us a valuable opportunity to conduct experiments on the global climate. The Center for Climate System Research (CCSR) of the University of Tokyo works on the development of global climate models. In this article, some of the studies on interannual to decadal climate variability using atmospheric and coupled ocean-atmosphere general circulation models are introduced. MODELS The atmospheric GCM (AGCM) has been developed cooperatively by CCSR and the National Institute for Environmental Studies (NIES) and is called the CCSR/NIES AGCM. It is a global spectral model with sigma vertical coordinate and includes full physics. 49

2 50 M. KIMOTO A two-stream k-distribution radiation transfer code developed by Nakajima et al. (2000) is incorporated in the AGCM. It enables efficient computation of the effects of trace gases and aerosols as evidenced by the global warming study of Nozawa et al. (2001). Other physical processes included are: a simplified Arakawa-Schubert (1974) scheme for cumulus convection, the Le Treut and Li (1991) prognostic cloud water scheme, the McFarlane (1987) orographic gravity wave drag (GWD), the Mellor-Yamada (1974) level 2.0 turbulence closure scheme, a bulk scheme for surface fluxes (Louis, 1979; Uno et al., 1995), a multilayer land surface energy budget treatment, a bucket ground hydrology, and a river runoff routine model. Details of the AGCM can be found in Numaguti et al. (1997). A new version of the AGCM currently under development includes a new land-surface model, the Kim-Arakawa GWD scheme, an optional flux-form semi-lagrangian advection scheme for tracers, a non-local turbulent mixing scheme, direct and indirect effects of aerosols, a better tuned large-scale and convective cloud models, etc. The ocean general circulation model employs Boussinesq and rigid-lid approximations. In some studies introduced in the following, simplified forms of the ocean part are used. Fig. 1. (a) Time series of June July August mean anomalies of the Webster-Yang (1992) broadscale monsoon index between the years 1979 and The AGCM result is compared with ECMWF and NCEP data. The index is defined as the zonal wind shear between 850 and 200 hpa levels averaged over 40 E 110 E, 5 N 20 N region. (b) Anomalies of March April May soil wetness averaged over 3 simulated weak monsoon years. The contour interval is with zero lines suppressed. Shaded areas represent regions where the difference between weak and strong monsoon years is larger than the interannual standard deviation. The unit of soil wetness is m m 1. (c) Anomalies of the broad-scale monsoon index for the composite weak and strong monsoons in the control integration (soild lines) and the ensemble results (dashed) of the experiment where the land surface initial conditions are changed. The unit of monsoon index is m s 1.

3 Studies of Climate Variability Using General Circulation Models 51 MONSOON VARIABILITY The Asian monsoon system exhibits considerable year-to-year variability that affects significantly the life and economy of the inhabitants. The AGCM has been used to study the interannual variability of the Indian and East Asian monsoon systems. Role of land-surface feedback in the interannual variability of Indian monsoons A statistical correlation between Indian monsoon rainfall and Eurasian snow cover of earlier seasons, originally pointed out by Blanford (1884), was revisited by Hahn and Shukla (1976) which called much attention to the role of land surface processes in the interannual variability of Indian monsoons. Some earlier GCM studies showed a positive impact, but sometimes the experimental setup suffered Fig. 1. (continued).

4 52 M. KIMOTO from arbitrariness in specifying anomalous land surface parameters due to the lack of observational information. Interannual variability in the Indian summer monsoon and related landsurface processes over the Eurasian continent has been investigated by Shen et al. (1998) in a ten-year integration of the CCSR/NIES AGCM. A version with triangular 21 truncation and 20 vertical levels (T21L20) was integrated with observed sea surface temperatures (SSTs) as a bottom boundary condition. It is found that the simulated interannual variability in the broad-scale summer monsoon during this decade shows good correlation with observations (Fig. 1a). Furthermore, distinct precursory signals over the Eurasian landmass have been found in the simulation; excessive snow and increased soil moisture over Eurasia south of 50 N in the pre-monsoon winter and spring are followed by weaker than normal monsoons (Fig. 1b) and vice versa. More snow and wet ground conditions tend to suppress the monsoon development keeping the land surface cool. Thus, the model results support an active role of land surface processes in the monsoon variability. However, the decade was also characterized by a conspicuous swing in the El Niño Southern Oscillation (ENSO) in the equatorial Pacific, which is known statistically to have a relation to the strength of Indian monsoon. In order to assess the role of land surface processes more quantitatively, a numerical experiment has been carried out by exchanging the spring-time Eurasian land surface conditions between weak and strong monsoon years while keeping the atmospheric initial conditions the same as the control integration (Shen et al., 1998). Note that the initial land conditions were sampled out of the model s natural variability avoiding arbitrary specification. This experiment shows that the land surface feedback does contribute to the simulated interannual variability but is not strong enough to change the sign of the monsoon circulation anomalies (Fig. 1c). It appears that the influence of the ENSO-related SST anomalies plays a more important role in influencing the simulated monsoon. The subtropical western Pacific anticyclone and the East Asian summer monsoon The East Asian countries experienced an extremely wet summer in More than 150% of the normal rainfall has been observed over a large portion of East Asia extending from southern China, and the Korean peninsula to Japan. A record-breaking flood occurred over the Changjiang River basin of China and lasted for almost three months. Heavy rainfalls hit the northern and eastern Japan and the Korean peninsula in July and August. In 1998 summer, the strong 1997/98 El Niño was decaying and a new La Niña was developing. Concurrently, a warm event similar to that in the Pacific basin occurred in the Indian Ocean starting from June 1997 through 1998 summer (Webster et al., 1999). Sea surface temperature anomalies (SSTAs) near Sumatra exceeded 2 C. A T42L20 version of the CCSR/NIES AGCM has been integrated with observed SSTAs in order to understand the anomalies of the 1998 East Asian summer monsoon.

5 Studies of Climate Variability Using General Circulation Models 53 Fig. 2. (a) Observed JJA-mean anomalies of precipitation (shadings; mm day 1 ) and 850 hpa winds (vectors; ms 1 ). (b) Same as (a), but for the ensemble simulation with global SSTAs. Figure 2a shows the observed anomalies of the June July August (JJA) mean precipitation and winds at 850 hpa for It is found that the most conspicuous feature of the 850 hpa circulation was the southwestward intrusion of the western Pacific subtropical high. The resultant anomaly pattern of 850 hpa winds exhibited a strong anticyclonic circulation located over a region extending from the South China Sea to the western Pacific. The low-level circulation functions to enhance the moisture transportation from the South China Sea and the western Pacific to the East Asian countries, thereby activating, strengthening and maintaining the Baiu/Meiyu front that governs the rainy season of East Asia.

6 54 M. KIMOTO Corresponding to the circulation anomalies, precipitation was deficient over the tropical and subtropical western Pacific and excessive over the East Asian landmass to its north. In the Pacific Ocean, inactive ITCZ and resultant rainfall deficiency extended from the tropical western to the eastern Pacific. This, together with the positive rainfall anomalies over the equatorial eastern Pacific and the tropical southeastern Pacific, was likely to be influenced by both the 1997/98 El Niño and the developing 1998/99 La Niña conditions. In addition, unprecedented warming near the island of Sumatra appeared in the summer months of Accompanied by the SSTA to the southwest of Sumatra, excessive rainfall anomalies were found. An ensemble experiment consisting of 10 independent integrations of about 4 months from 00UTC of April 21, 22,..., 30 to the end of August was conducted giving the observed global monthly SST as the lower boundary condition. Figure 2b shows the ensemble mean anomalies of the 850 hpa winds and precipitation for JJA. The main features of the 1998 East Asian summer monsoon are well reproduced, except for those in the mid-high latitudes. The low-level anticyclonic circulation anomaly over the subtropical western Pacific and South China Sea is captured well by the model. This low-level anticyclonic anomaly was important in transporting a vast amount of moisture from the southern ocean to the East Asian landmass during the 1998 summer according to the moisture budget analysis. Corresponding to this circulation anomaly, the anomaly pattern of precipitation simulated by model also shows general similarities to the observation. Deficient precipitation, i.e., suppressed convective activities, over the subtropical western Pacific and equatorial Pacific are captured well, although the sign of the anomalies in a small region over the South China Sea is reversed. Excessive rainfall is found over an oceanic area to the west of Sumatra and over the East Asian landmass. In order to clarify which part of the SSTA was most influential in the East Asian monsoon, additional ensemble integrations have been carried out retaining various regional SSTAs (Shen et al., 2001). In summary, two key regions have been identified as most efficient in inducing the western Pacific subsidence and associated low-level anticyclonic anomalies: the southeastern part of Indian Ocean (80 E 135 E, 30 S EQ), and the tropical Pacific Ocean (135 E 75 W, 20 S 20 N). Positive SSTAs over these regions enhanced convection aloft and the remote subsidence over the western Pacific through modifications of Walker and local Hadley circulations, respectively. The positive SSTA over the former region was the remains of the 1997/98 El Niño, while that over the latter was a part of the Indian Ocean Dipole mode (Saji et al., 1999). The experiment indicates that the dual effect caused the persistent and strong low-level anticyclonic anomaly in the subtropical western Pacific and thereby was responsible for the anomalous 1998 East Asian summer monsoon. Positive SSTAs over the North Indian Ocean and the South China Sea regions tend to enhance convection aloft, but this was not supported by observations. SSTAs over these regions may have been forced by atmospheric anomalies.

7 Studies of Climate Variability Using General Circulation Models 55 COUPLED INTERANNUAL DECADAL VARIABILITY In much of the interannual and longer variability, interactions between the atmosphere and oceans should play important roles. Coupled upper-oceanatmosphere GCMs have been used to investigate interannual and decadal variability in the tropical Pacific and extratropical Pacific and Atlantic Oceans. Interannual to decadal variability in the Pacific basin A coupled upper-ocean-atmosphere GCM has been integrated for about 100 years to investigate interannual to decadal variability in the Pacific basin. The atmospheric part of the coupled model employs a T21L20 version. The horizontal resolution of the ocean GCM (OGCM; Kimoto et al., 1997) is 2.0 latitude 2.5 longitude except in the 20 N 20 S equatorial band, where the latitudinal spacing is 0.5 within 10 from the equator and is increased outside to 20 latitudes. The OGCM has 20 vertical levels, of which 13 lie in the upper 300 meters. Convective adjustment and a level 2.5 turbulence closure scheme are used for the vertical mixing. The computational domain is global but excludes the Arctic Ocean. The model does not include sea ice, and the SST is relaxed to the observed climatology in latitudes higher than 50. Otherwise, the model employs no flux correction. Starting from the resting isothermal atmosphere and Levitus temperature and salinity profile with no currents in the ocean, the model has been integrated for a total of 105 years. The first 10 years were discarded and the resultant 96-year record was analyzed. The SST is one of the key variables in the large-scale ocean-atmosphere variability. Here, the multi-channel singular spectrum analysis (MSSA; Kimoto et al., 1991; Plaut and Vautard, 1994) is used to extract spatio-temporal patterns of the SST variability. The MSSA is an extension of a more conventional empirical orthogonal function (EOF) analysis. Out of the sequence of anomaly maps, each of which consists of L grid-point values, the ordinary EOF extracts spatial patterns of dominant variability, usually in the order of decreasing associated variance. An anomaly map of a one-time level is regarded as the vector of length L to be analyzed. MSSA regards an M-temporally-lagged sequence of anomaly maps as the vector, of length M L, to be analyzed. The analyzed modes, therefore, include not only spatial patterns, but also their temporal evolutions. Both EOF and MSSA extract principal patterns of variability as eigenvectors of a data covariance matrix. As explained by Vautard et al. (1992), SSA can detect an oscillatory mode as a pair with degenerate eigenvalues. They behave as generalized sines and cosines in the time domain. Details of SSA or MSSA can be found in Vautard et al. (1992) and Allen and Robertson (1996). In order to avoid making the size of the covariance matrix too big, it is convenient to apply conventional EOF to the SST field to reduce the spatial dimension by truncation. The EOF analysis was first applied to the Pacific basin monthly SST anomalies between 36 S and 60 N. Retaining the first 27 amounts to accounting for 80.5% of the domain integrated variance in the original field. The temporal length of the maps, or window width, is taken to be 121 months,

8 56 M. KIMOTO Fig. 3. Power spectra of the 1st EOF mode of the Pacific SSTA (solid), its 95% confidence level (dashed), and contributions by the oscillatory MSSA modes (shades). long enough to enable detection of decadal modes. Slight changes in the window width do not affect the results. Three oscillatory pairs have been detected: a decadal mode (DC) with a 9- year period (MSSA modes 2 and 3; 7.8% of the total variance), a quasiquadrennial or ENSO mode (QQ) with a 56-month period (modes 4 and 5; 4.9%), and a quasi-biennial mode (QB) with a 24-month period (modes 6 and 7; 4.1%). Figure 3 shows the contributions of these MSSA modes to the power spectrum of the 1st (ordinary) EOF, which accounts for 23.3% of variance and is well separated from the following mode with only 6.9%. The shaded spectra indicate the contributions by the MSSA modes (cf. Vautard et al., 1992). The QQ and QB modes have also been identified in observational analyses of equatorial SSTA by Jiang et al. (1995) and of the Pacific basin SSTA similar to the present study by Zhang et al. (1998). The periodicity of the DC mode does not correspond exactly to the observation, which is not very well-defined, but is generally thought to be longer (e.g., Zhang et al., 1998). However, the spatial pattern of SSTA of the DC mode (Fig. 4a for the phase with maximal tropical and extratropical SSTAs) closely resembles the observed interdecadal mode; the opposite polarity of anomalies are seen in the equatorial central Pacific and extratropical North Pacific, the former having a horse-shoe shape in the tropical eastern Pacific and the latter with an elliptical shape surrounded by anomalies of reversed sign in the eastern half of the ellipse. This corresponds well to the observed climatic shift occurring in the mid-1970s (e.g., Nitta and Yamada, 1989; Trenberth, 1990).

9 Studies of Climate Variability Using General Circulation Models 57 Fig. 4. (a) SSTA (contour) and wind stress (vector) anomalies of the MSSA DC mode, obtained by regressing weakly bandpass filtered monthly anomalies onto the temporal coefficient of MSSA mode 2. (b) Regression pattern similar to (a), but for OHCA onto MSSA mode 2. Labeled lines denote sections along which temporal evolutions of anomalies are shown in Fig. 5. The aspects of ocean dynamical adjustment can be better visualized by examining the upper-ocean heat content (OHC). Figure 4b shows temperature anomalies averaged over the upper 300 m in the ocean in the same phase of the oscillation as Fig. 4a. Inspecting the movie of OHC associated with the DC mode evolution, several characteristics can be noted; a clockwise movement of OHC anomalies (OHCAs) in the extratropical to subtropical North Pacific. This is reminiscent of the interdecadal mode simulated by Latif and Barnett s (1994) coupled GCM. The subtropical OHC anomalies between 10 and 20 N propagate westward, then northeastward along the coasts of Taiwan and Japan, then turn

10 58 M. KIMOTO Fig. 5. Temporal evolutions of anomalies associated with MSSA mode 2 along the sections labeled in Fig. 4b. The ordinate denotes time in lagged years with respect to an arbitrary origin. (a) SSTA along 32 N, (b) OHCA along 170 W, (c) OHCA along 18 N (east-west direction reversed), and (d) OHCA averaged between 130 E and 150 E. (e) OHCA and (f) SSTA on the equator. eastward to occupy the central North Pacific centered about 35 N. On the other hand, the equatorial anomalies have a Kelvin-wave-like eastward propagating component, similar to that associated with ENSO, but much slower. Interestingly, near the western boundary of the subtropical Pacific near the Phillipines, a part of the subtropical OHC leaks toward the tropics. These features are conveniently seen in Fig. 5 which shows the time evolutions of SST and OHC anomalies (regressed with the temporal coefficient of the decadal MSSA mode) along the sections marked in Fig. 4b. Time goes upward and the panel (a) of Fig. 5 is the longitude-time section of the SSTA along 32 N. Panel (b) is the latitude-time section of OHCA along 170 W in the central Pacific evidencing subducted midlatitude anomalies propagating southward (latitude is decreasing to the right). Panel (c) follows the OHCA on the 18 N latitude line, the east-west direction being reversed. Panel (d) follows the equatorward movement of the OHCA between 22 N and the equator in the western Pacific averaged between 130 E and 150 E. Panels (e) and (f) are longitude-time sections of OHCA and SSTA, respectively, on the equator. Figure 5 appears to show the existence of a signal pathway from the extratropical ocean surface, through the subsurface of the subtropical and tropical western Pacific oceans, all the way to the surface of the eastern equatorial Pacific (Jin et al., 2001). The return of the signal back to the extratropics may be fairly easily accomplished by an atmospheric bridge associated with the equatorial SSTA (Lau and Nath, 1996).

11 Studies of Climate Variability Using General Circulation Models 59 Figure 4a also shows wind stress anomalies. Comparing with Fig. 4b, one notes that the east-west oriented subtropical OHCA is sandwiched by the extratropical westerly and off-equatorial and appears to lie on the maximal negative curl. The associated Sverdrup transport is southward and is consistent with the subsequent equatorward leak of a part of the subtropical OHCA mentioned earlier. The decadal mode simulated by the coupled GCM appears to show active involvement of both tropical and extratropical ocean-atmosphere systems. Although it bears resemblance to the observed interdecadal mode, the period appears much shorter (cf. Knutson and Manabe, 1998). The dynamics of the mode should be better clarified with carefuly designed experiments. The details of the QQ and QB modes are not shown here, but their spatial patterns do have substantial resemblance. They seem to be confined more to the tropics than the DC mode. Clarifying the similarity and dissimilarity of the patterns and dynamics of the three modes of variability is left as a future challenge. Air-sea coupling in the North Atlantic Watanabe et al. (1999) reported that the coupled GCM described above has another air-sea coupled mode over the North Atlantic with a period of about 10 years. Spatial patterns of atmospheric and SST anomalies resemble those of an observed counterpart: the well-known North Atlantic Oscillation (NAO) and a tripole SST pattern (Deser and Blackmon, 1993). In midlatitudes, it is generally believed that a considerable part of SST anomaly is forced by atmospheric anomalies, in sharp contrast to tropical situations. However, in considering the dynamics of midlatitude decadal ocean-atmosphere variability, Watanabe and Kimoto (2000a) pointed out that, in order for an oscillatory mode to survive various damping mechanisms, a positive feedback between the ocean and atmosphere is necessary; therefore, a component should exist in which the ocean forces the atmosphere. However, such a component is overwhelmed by the atmosphere-to-ocean forcing in midlatitude; therefore, its detection in observational data is awkward. Watanabe and Kimoto (2000b) conducted a series of coupled atmospheremixed-layer ocean model experiments to see whether a positive atmosphereocean feedback is found over the North Atlantic. A 60-yr integration of a T21L11 version of the AGCM coupled with a 50-m deep motionless ocean was carried out. This coupled run (called CTL) was compared with two other uncoupled runs: one with prescribed midlatitude sea surface temperatures (SSTs) at the climatology (called PS1) and another with daily SSTs derived from the coupled experiment (PS2), respectively. The uncoupled atmosphere in turn forces the slab ocean to obtain SST responses in the PS1 and PS2 runs. The patterns of maximum atmosphere-ocean covariability show the NAO and tripole SST anomalies both in the coupled and uncoupled fields, indicating a dominant role of the atmosphere in generating the SST anomalies. On the other hand, analyses of the temporal variability in the three runs suggest an active role

12 60 M. KIMOTO Fig. 6. Heterogeneous regression maps for the leading mode of a singular value decomposition analysis between 500 hpa height and combined forcing and response SST fields of the PS2 run. (a) 500 hpa height, (b) prescribed forcing SST anomalies which are obtained from CTL, and (c) response SST anomalies calculated in PS2. Contour intervals for SST and height regressions are 0.1 K and 10 m while the negative contours are dashed. The statistically significant area at the 95 (99) % level is indicated by the light (dark) shadings. of SST anomalies in determining the polarity of the air-sea coupled variability longer than the interannual time scale. A combined analysis of the forcing SST, upper-air height, and the response SST anomalies in the PS2 run identified that a patch of positive SST anomalies in the midlatitude band around 40 N effectively excite the positive phase of the NAO, which in turn reinforces the tripole SST anomalies (Fig. 6). This relationship has further been confirmed by a 9-member ensemble of an AGCM experiment forced by the SST patch. Because the forcing and response SST anomalies patterns bear a resemblance, these results manifest the positive feedback at work in the coupled atmosphere-ocean patterns. The processes responsible for this positive feedback have been elaborated by a series of linear model experiments. The model is a linearized primitive equation with respect to an arbitrary three-dimensionally varying basic state. This linear model has been used (i) to obtain a stationary linear response to a given forcing (Branstator, 1990), and (ii) to simulate feedback due to high-frequency transients

13 Studies of Climate Variability Using General Circulation Models 61 onto a large-scale flow (a storm track model; Branstator, 1995). In the GCM, the local thermal adjustment of the atmosphere to the SSTAs results in increased precipitation over the Gulf stream region. Associated diabatic heating given to the linear model yields a positive height response to the east, which highly projects onto the southern part of the NAO. This stationary response in turn induces a northward deflection in the storm track activity, leading to an eddy vorticity feedback that tends to force the positive phase of the NAO. CONCLUDING REMARKS There is no doubt that GCMs provide a powerful experimental tool to study the climate and its variability. They are comprehensive but far from complete. Therefore, continuous efforts for improvement and validation are necessary. A model with mediocre performance may not propose a credible mechanism. At the same time, however, the output of the state-of-the-art GCMs is as complicated as nature itself. Carefully designed experiments and analyses are necessary to make intelligent use of them. Acknowledgments The work presented here is the result of pleasant collaboration with many colleagues. Especially, the author thanks Dr. A. Numaguti for developing a substantial part of the AGCM, and Drs. X. Shen and M. Watanabe for sharing the fun of the jigsaw puzzles with him. REFERENCES Allen, M. R. and A. W. Robertson, 1996: Distinguishing modulated oscillations from coloured noise in multivariate datasets. Climate Dyn., 12, Arakawa, A. and W. H. Schubert, 1974: Interactions of cumulus cloud ensemble with the large-scale environment, Part I. J. Atmos. Sci., 31, Blanford, H. F., 1884: On the connexion of Himalayan snowfall and seasons of drought in India. Proc. Roy. Soc. London, 37, Branstator, G., 1990: Low-frequency patterns induced by stationary waves. J. Atmos. Sci., 47, Branstator, G., 1995: Organization of storm strack anomalies by recurring low-frequency circulation anomalies. J. Atmos. Sci., 52, Deser, C. and M. Blackmon, 1993: Surface climate variations over the North Atlantic Ocean during winter: J. Climate, 6, Hahn, D. J. and J. Shukla, 1976: An apparent relationship between Eurasian snow cover and Indian monsoon rainfall. J. Atmos. Sci., 33, Jiang, N., J. D. Neelin, and M. Ghil, 1995: Quasi-quadrennial and quasi-biennial variability in the equatorial Pacific. Climate Dyn., 12, Jin, F.-F., M. Kimoto, and X. A. Wang, 2001: A model of decadal ocean-atmosphere interaction in the North Pacific basin. Geophys. Res. Lett., 28, Kimoto, M., M. Ghil, and K.-C. Mo, 1991: Spatial structure of the 40-day oscillation in the Northern Hemisphere extratropics. Proc. 8th Conf. Atmos. & Oceanic Waves and Stability, Amer. Meteor. Soc., Boston, pp Kimoto, M., I. Yoshikawa, and M. Ishii, 1997: An ocean data assimilation system for climate monitoring. J. Meteor. Soc. Japan, 75, Knutson, T. R. and S. Manabe, 1998: Model assessment of decadal variability and trends in the tropical Pacific Ocean. J. Climate, 11, Latif, M. and T. P. Barnett, 1994: Causes of decadal climate variability over the North Pacific and

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