A Hybrid Atmosphere-Ocean Coupling Approach on the. Simulation of Tropical Asian-Pacific Climate

Transcription

1 A Hybrid Atmosphere-Ocean Coupling Approach on the Simulation of Tropical Asian-Pacific Climate Xiouhua Fu a, Bin Wang a,b, Tim Li a,b, and Fei-fei Jin c a IPRC, SOEST, University of Hawaii, Honolulu, Hawaii b Department of Meteorology, SOEST, University of Hawaii, Honolulu, Hawaii c Department of Meteorology, Florida State University, 404 Love Building, Tallahassee, Florida Manuscript Corresponding author address: Dr. Joshua Xiouhua Fu, International Pacific Research Center, SOEST, University of Hawaii at Manoa, 1680 East West Road, POST Bldg. 4 th Floor, Honolulu, HI 96822

2 ABSTRACT A unique Hybrid coupled General Circulation Model (HcGCM), that first exercises full coupling, has been developed. This coupled model combines an ECHAM AGCM and an intermediate tropical ocean model. In this study, the first 40-year (after 5- year spin-up) model output has been analyzed and validated with available long-term observations (and analysis products). Overall, the model simulations of the climatology and variability in the tropical Asia-Pacific sector (including Indo-Pacific mean SST and its annual cycle, Asian Summer monsoon, tropical intraseasonal oscillation (TISO) and ENSO) are fairly realistic and comparable with (some even better than) those state-of-the-art coupled GCMs. The model bias of mean SST is within 1 o C in most of the ocean domain except in the southeast Pacific, where the model suffers a warm-bias syndrome present in most coupled GCMs. The simulated TISO exhibits significant seasonality as in the observations with dominant eastward-propagating MJO in boreal winter and northwardpropagating mode over the Indian and western Pacific regions in boreal summer. The model ENSO has two spectral peaks with periods about 2 years and 5 years. It also shows significant annual phase locking with minimum (maximum) variance in boreal spring (in late fall). The encouraging results from this hybrid coupled model indicate that in representing the present-day climatology and its variability (with time scales from intraseasonal to interannual) over the tropical Asia-Pacific sector, a hybrid coupled model is as good as fully coupled GCMs. Use of a hybrid coupled model also saves considerable computational resources compared to a fully coupled GCM. 2

3 1. Introduction In the past two decades, under the auspices of the TOGA (Tropical Ocean-Global Atmosphere) program and the follow-on CLIVAR (Climate Variability and Predictability) program, many atmosphere-ocean coupled models have been developed to improve our understanding of the nature and predictability of tropical Pacific and global climate variability (Meehl et al. 2000; Latif et al. 2001; Davey et al. 2002). In the TOGA decade ( ), the coupled models are primarily designed to simulate and predict the El Nino-Southern Oscillation (ENSO) and the associated tropical and extra-tropical climate variability. Neelin et al. (1992) made the first inter-comparison of tropical Pacific behaviors of coupled models, including both coupled general circulation models (CGCM) and intermediate coupled models. Many of the CGCMs had considerable errors in the annual-mean temperature of the equatorial Pacific and its zonal gradient. Some of them cannot produce a correct warm-pool/cold tongue configuration in the equatorial Pacific. Interannual variability ranged from very weak to moderate. Among these models, ENSO simulated by the Cane-Zebiak anomaly coupled model is most realistic (Zebiak and Cane 1987). Even today, the Cane-Zebiak model is still one of the best coupled models (including coupled GCMs) in terms of the ENSO simulation and prediction (Latif et al. 2001; Chen 2004). A few years later, Mechoso et al. (1995) conducted another CGCM inter-comparison focused on the mean state and seasonal cycle in the tropical Pacific. Their results showed large improvements compared to those models evaluated by Neelin et al. (1992). At least, all participating models produce realistic warm-pool/cold tongue configuration and reasonable zonal SST gradient along the equatorial central Pacific. Nevertheless, the CGCMs still had substantial biases from the observed state. The 3

4 simulated equatorial cold tongue generally tends to be too strong, too narrow, and extends too far west. SSTs in the southeast Pacific are generally too warm, which is accompanied by a fictitious double inter-tropical convergence zone (ITCZ). The CGCMs also have a variety of problems in simulating the seasonal cycle of the equatorial SST in the eastern Pacific (e. g., a too-weak annual harmonic but a too-strong semiannual harmonic). To summarize the development of coupled models during the TOGA decade, Delecluse et al. (1998) concluded that substantial progress was made in representation of the tropical mean state and climate variability through the synergic efforts of the observational, theoretical, and hierarchal modeling studies (McPhaden et al. 1998; Neelin et al. 1998; Latif et al. 1998). However, several general systematic errors (e.g., in the mean state and seasonal cycle) have yet to be eliminated, especially in the east Pacific. Two recent CGCM inter-comparison projects, ENSIP (the El Nino simulation intercomparison project, Latif et al. 2001) and STOIC (a study of coupled model climatology and variability in tropical ocean regions, Davey et al. 2002), revealed that there still is substantial potential for further model improvement. Latif et al. (2001), through comparing the performance of 24 CGCMs in the tropical Pacific, indicated that almost all models (even those employing flux correction) still have considerable problems in simulating the SST climatology (e.g., cold bias in the equatorial Pacific and a too-weak annual cycle) although some improvements are found relative to earlier intercomparison studies. Only a few of the coupled models realistically simulate the ENSO in terms of gross equatorial SST anomalies (e.g., amplitude and annual phase locking). In particular, many models overestimate the variability in the western equatorial Pacific and underestimate the SST variability in the east, which may be associated with the extension 4

5 of the model cold tongue too far westward. Davey et al. (2002) further found that the interannual variability (both SST and zonal wind stress) is commonly too weak in the models. Most models have difficulty in reproducing the observed Pacific horseshoe pattern of negative SST correlations with interannual Nino-3 SST anomalies, and the observed Indo-Pacific lag correlations. Both inter-comparison projects confirm that improving the simulations of the tropical Pacific climatology and ENSO remains a continuing challenge for the coupled-model community. In this paper, we present a hybrid coupled GCM (HcGCM) newly developed at the International Pacific Research Center (IPRC), University of Hawaii (UH). This model couples the ECHAM-4 AGCM (Roeckner et al. 1996) with an intermediate ocean model (Fu and Wang 2001). Active air-sea coupling is in the tropical Indo-Pacific region only 1. In contrast to other anomalous hybrid coupled GCMs (e.g., Alexander 1992; Kirtman and Zebiak 1997; Yeh et al. 2004), this hybrid coupled model exercises full coupling. The model was designed to simulate the annual mean, annual cycle, and interannual variability within one framework. The model simulations of the climate and variability in the tropical Asia-Pacific sector are very encouraging even compared to those wellknown, state-of-the-art coupled GCMs (e.g., NCAR CCSM, Meehl and Arblaster 1998; SINTEX CGCM, Gualdi et al. 2003). The main objective of this study is to validate the model simulations of the climatology and intraseasonal-to-interannual variability in the tropical Asia-Pacific sector with available observations, thus establishing a baseline for evaluating future improvements and for comparison with other models. 1 IPRC s mission is to understand the nature and predictability of climate variability and regional aspects of global environmental change in the Asia-Pacific sector ( 5

6 It is worthwhile to mention that there are three other coupled GCMs that also used ECHAM-4 as their atmospheric component. Two coupled versions were developed at Max Planck Institute, Germany: 1) ECHO-2, which coupled ECHAM-4 with HOPE ocean general circulation model (Frey et al. 1997); 2) ECHAM-4 is coupled to OPYC3 general circulation model with annual-mean heat flux correction (Bacher et al. 1998); 3) SINTEX CGCM (Gualdi et al. 2003) couples the ECHAM-4 with the ORCA ocean general circulation model. Through comparing the results from different atmospheric GCMs (or from the same GCM with different resolutions) coupled to one ocean model, Guilyardi et al. (2004) have suggested the important role of the atmospheric component in setting up ENSO characteristics. On the other hand, coupling one AGCM to different ocean models may give us a clue about how the ocean component will affect the behaviors of coupled systems. In our following analyses, we will compare some of our results, in which we have used an intermediate ocean model rather than an ocean general circulation model, with other ECHAM-4 family CGCMs. Because our focus is the tropical Asia-Pacific climate, we will validate the model simulations of tropical Pacific climate along with the Asian-Australian monsoon and tropical intraseasonal oscillations (TISO). In literature, a few AGCM inter-comparison projects have been conducted to focus on the Asian summer monsoon (Gadgil and Sajani 1998; Kang et al. 2002) and the TISO (Slingo et al. 1996; Waliser et al. 2003). We will also refer to some results from these inter-comparison projects when we validate those relevant simulations in this hybrid-coupled model. The remaining parts of the paper are organized as follows. The model and the data used to validate the model are given in section 2. In section 3, we validate the model 6

7 climatology (both annual-mean and annual cycle) in the tropical Asia-Pacific sector. In section 4, the seasonality of the simulated TISO, both the northward-propagating ISO in boreal summer and eastward-propagating MJO in boreal winter, is compared to the available observations. The interannual variability of the tropical Pacific is evaluated in section 5. Finally, we summarize our main results and discuss the pathways to further improve the model in section The Hybrid Coupled GCM (HcGCM) a. The atmospheric component ECHAM-4 The atmospheric model used in this study is the ECHAM-4 general circulation model, which has been documented in detail by Roeckner et al. (1996). A brief description is given here for the convenience of readers. We used the T30 version (the corresponding horizontal resolution is about 3.75 o ) in this study instead of the standard ECHAM-4 T42 version, because it produces similar results as the higher-resolution versions but requires fewer computational resources (Stendel and Roeckner 1998). The model has 19 layers extending from the surface to 10 hpa. Its land surface scheme is a modified bucket model with improved parameterization of rainfall-runoff (Dumenil and Todini 1992). The surface fluxes of momentum, heat, water vapor, and cloud water are based on the Monin-Obukhov similarity theory. The vertical diffusion in the model is computed with a high-order closure scheme depending on the turbulent kinetic energy. Gravity wave drag associated with orographic gravity waves is simulated after Miller et al. (1989). The mass flux scheme of Tiedtke (1989) for deep, shallow, and mid-level convection has been used with the modified closure schemes for penetrative convection 7

8 and the formation of organized entrainment and detrainment (Nordeng 1994). The radiation scheme is a modified version of the European Center for Medium-Range Weather Forecasts (ECMWF) scheme. Two- and six-band intervals are used in the solar and terrestrial part of the spectrum, respectively. b. The updated intermediate ocean model The ocean component of this hybrid coupled model is a tropical upper ocean model with intermediate complexity. It was originally developed by Wang et al. (1995) for the tropical Pacific and improved by Fu and Wang (2001). The ocean model comprises a mixed layer, in which the temperature and velocity are vertically uniform, and a thermocline layer in which temperature decreases linearly from the mixed layer base to the thermocline base. Both layers have variable depths. The deep ocean beneath the thermocline base is motionless with a constant reference temperature. This ocean model combines the mixed-layer thermodynamics of Gaspar (1988) and the upper ocean dynamics of McCreary and Yu (1992). It well reproduces the annual cycles of sea surface temperature, upper ocean currents, and mixed layer depth in the tropical Pacific (Fu and Wang 2001; Wang and Fu 2001). In this study, the parameterization of the entrained water temperature has been updated. In our previous studies, the entrained water temperature is parameterized in terms of the vertical temperature gradient between the mixed layer and the deep inert layer (Wang et al. 1995). The weakness of this parameterization is that the mixed-layer temperature is not sensitive to the changes of thermocline depth. Therefore, the SST interannual variability that is strongly associated with thermocline feedback (Zebiak and Cane 1987) is very weak in our early versions of coupled models (e.g., Fu and Wang 8

9 2001; Fu et al. 2002). Following the footprints of other intermediate ocean models (Zebiak and Cane 1987; Seager et al. 1988; McCreary et al. 1993; Jin 1996, 1998), we have parameterized the entrained water temperature as an explicit function of thermocline depth, very similar to the one used in Jin (1998). In this study, the ocean model is active in both the tropical Indian and Pacific Oceans (from 30 o S to 30 o N) with realistic but simplified coastal boundaries of the oceans. It is feasible to further extend the ocean domain to the tropical Atlantic Ocean and mid-latitude region (Lu et al. 1998). The horizontal resolution of the model is 0.5 o longitude by 0.5 o latitude, which requires an approximate time interval of 3 h. No-flux conditions for temperature and free-slip conditions for velocities are applied at the coastal boundaries. c. The coupling procedure The ECHAM-4 was coupled with the intermediate ocean model in the tropical Indian and Pacific Oceans without heat flux correction (except that the SSTs in the northsouth open boundaries have been relaxed to the observations, Fu and Wang 2001). Beyond the coupling regions, the underlying sea surface temperature is specified as the climatological monthly mean of the 16-year ( ) SST dataset used as the boundary conditions in the AMIP II experiments (Taylor et al. 2000). In the active air-sea coupling domain, atmospheric component exchanges information with ocean component once per day. The atmosphere provides daily mean surface winds and heat fluxes to the ocean model. The latter send daily mean SST back to the former. The coupled model is integrated with seasonally varying solar radiation forcing. The initial atmospheric field is a restart file from previous long-term integration on January 1. The initial ocean field is the steady state in January after a ten-year integration of the ocean model forced with 9

10 observed climatological surface winds and heat fluxes. The spin-up period for the coupled model is 5 years. The output from the next 40 years integration was used in the following analyses. d. The data used to validate the model Several long-term datasets either from the observations or from the analysis and reanalysis have been used in this study to validate the model simulations. The datasets include the Hadley Center monthly-mean SST from 1901 to 2000 (GISST, Rayner et al. 1998), Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) pentad-mean rainfall from 1979 to 2000 (Xie and Arkin 1997), daily mean winds of ECMWF analysis from 1991 to 2000, and monthly-mean sea level pressure from NCEP reanalysis from 1961 to 2000 (Kalnay et al. 1996). 3. The Model Climatology a. In the tropical Asia-Pacific sector During the 40-year integration, the coupled model exhibits no apparent climate drift though no heat flux correction is applied. The simulated annual-mean SST (averaged in 40-years) agrees with the observations (averaged in 100-years) quite well (Fig. 1). The model bias is within 1 o C in most tropical Indo-Pacific regions except in the southeast Pacific, particularly close to the Peruvian coast, where the model SST is too high (Fig. 1c). This warm bias is associated with the fictitious eastward extension of the south Pacific convergence zone (SPCZ) warm water. In fact, this problem is a common syndrome for almost all state-of-the-art CGCMs (Davey et al. 2002; Frey et al. 1997; Gordon et al. 2000; Meehl et al. 2001; Guilyardi et al. 2003; Wang et al. 2004, among 10

11 others). It is believed that this problem originates primarily from deficiencies in atmospheric models caused by the lack of stratocumulus or too-weak along-shore winds (Philander et al. 1996; Schneider et al. 1997; Mechoso et al. 2000), and probably amplified by local air-sea coupling (Li and Philander 1996). Figure 2a-d shows the climatological zonal wind shear (200 hpa-850 hpa) and precipitation from the observations and the coupled model. We assessed the simulated wind shear here, because it is suggested to be an important factor to initiate and steer the intraseasonal variability in the Indo-western Pacific region (Wang and Xie 1996; Jiang et al. 2004). In boreal summer, the major observed rainfall systems (Fig. 2a) are associated with the Asian summer monsoon, ITCZ and eastern North Pacific summer monsoon (ENPSM, Murakami et al. 1992). Strong easterly shear is observed in the northern Indian Ocean and the western North Pacific (WNP) associated with the monsoon rainbelt around 15 o N (Fig. 2a). The coupled model well captures this easterly shear (Fig. 2b) with a maximum of about 30 m s -1 located in the northwest Indian Ocean. In the eastern Pacific, the model tends to overestimate the easterly shear associated with the ENPSM even though the rainfall is a little underestimated. The ITCZ rainfall in the central Pacific (between 160 o W and 120 o W) is weaker than the observed. This bias also exists in the stand-alone ECHAM-4 GCM (Roeckner et al. 1996). This rainfall bias may be associated with the westerly bias in the upper troposphere (Fig. 4b). The erroneous westerly duct in the model favors the intrusion of the subtropical dry (and cold) air into the equatorial region and tends to suppress the model ITCZ convection in the central Pacific (Mapes and Zuidema 1996; Yoneyama and Parsons, 1999; Tompkins 2001). Increasing the model horizontal resolution mitigates this bias (Gualdi et al. 2003). 11

12 In the Asia-western Pacific region, the model captures the major rainfall systems, for example, strong rainfall at the eastern Arabian Sea and the Bay of Bengal (Figs. 2a-b). The simulated maximum rainfall in the Arabian Sea shifts too far away from the Indian western coast probably owing to the coarse resolution (see also Gualdi et al and Rajendran et al. 2004). The observed rainbelt just south of the equatorial Indian Ocean is reproduced but with a weaker intensity. This rainbelt may play an essential role in initiating the dominant northward propagating ISO in the Indian Ocean (Waliser et al. 2003; Fu and Wang 2004b), but was missed by many AGCMs (Kang et al. 2002). The observed rainbelt in the South China Sea and the WNP is captured but with a quite different orientation compared to the observations (Figs. 2a-b). A rainy center is observed in the SPCZ but the model only shows a hint there. Overall, the simulated rainfall pattern associated with Asian summer monsoon is very similar with that in the SINTEX CGCM (Gualdi et al. 2003) and is much better than the simulations with stand-alone ECHAM-4 AGCM (Roeckner et al. 1996) and many other AGCMs (Kang et al. 2002). This result supports our previous finding that warm-pool air-sea coupling significantly improves the simulation of mean Asian summer monsoon (Fu et al. 2002). In boreal winter, major convective zones move to the Southern Hemisphere (Figs. 2c-d) following the seasonal migration of solar radiation. The major rainbelt extends from the southern Indian Ocean to the SPCZ, with a tail (the Pacific ITCZ) remaining in the Northern Hemisphere. The observed easterly shear centers over the maritime continent. The overall rainfall and wind shear patterns are well simulated by the model (Fig. 2d). However, there are two systematic errors in the simulation, the northern ITCZ 12

13 is too weak and the SPCZ extends too far eastward. Most likely, both errors are associated with the warm bias in the southeast Pacific (Fig. 1c). Figures 3a-d compare the climatology of 1000-hPa winds from the ECMWF analysis and the model. In boreal summer (Figs. 3a-b), the monsoonal flows associated with the Asian summer monsoon in the Indian Ocean are reasonably captured. In the tropical Pacific, the simulated northeast trades are more realistic than the southeast trades. The latter are too strong in the western South Pacific (between 150 o E and 150 o W), which may be the cause of weaker SPCZ rainfall in the model (Fig. 2a). In boreal winter (Figs. 3c-d), the overall flow patterns are similar between the model and the observations except that the strong northeast trades in the model penetrate too far equatorward and the south Pacific convergence zone extends too far eastward (Fig. 3d). The upper troposphere (200-hPa) zonal winds from the ECMWF analysis and coupled model are compared in Figs. 4a-d. In boreal summer, the easterlies associated with the Asia-western Pacific summer monsoon are reproduced but with a smaller amplitude, particularly in the northern Indian Ocean (Fig. 4b). On the other hand, the easterlies in the eastern equatorial Pacific are slightly too strong. As mentioned before, an erroneous westerly duct is produced in the equatorial central Pacific. In boreal winter, the observed easterlies associated with the Australian monsoon (Fig. 4c) are much weaker than their summer counterparts (Fig. 4a). The simulated easterly center locates over the maritime continent as in the observations but with slightly smaller amplitude (Fig. 4d). b. In the equatorial Indo-Pacific region Davey et al. (2002) evaluated the global equatorial SSTs and zonal wind stresses 13

14 simulated in 15 CGCMs (without heat flux correction). They found that most CGCMs (11 of 15) have significant cold bias in the western-central equatorial Pacific. At the same time, easterly winds are considerably overestimated in the western Pacific, but underestimated in the central Pacific (figures 1-2 in Davey et al. 2002). The causes of these systematic errors are probably associated with the misrepresentations of oceanic mixing (Li et al. 2001), adjacent monsoon systems (Fu et al. 2004), and local atmosphereocean feedback (Jin 1998). We have compared the mean SSTs and zonal winds in the equatorial Indo-Pacific Oceans from this hybrid coupled model with the observations (Figs. 5a-b). The model SST bias (Fig. 5a) is very small in most equatorial regions except in the eastern end of the Pacific (east of 120 o W), where simulated SST is too warm with a bias about 2 o C as in most coupled GCMs (figure 1 in Davey et al. 2002). For this hybrid coupled model, the warm bias primarily originates from the underestimated stratocumulus in the atmospheric component (figure not shown), which is one of our targets for further model improvement. The simulated zonal winds (Fig. 5b) also show a reasonable agreement with the observations except in the western Pacific, where the model easterlies are slightly too strong. In the deep tropics, the downward solar radiation at the top of atmosphere has a dominant semiannual cycle. However, the observed SST in the equatorial eastern-central Pacific shows a peculiar annual cycle (Fig. 6a) with highest and lowest SST, respectively, in spring and fall. Many state-of-the-art CGCMs have various problems in reasonably simulating this feature (Mechoso et al. 1995; Latif et al. 2001). Some models (e.g., NCAR CCSM and SINTEX CGCM) tend to produce a dominant semiannual cycle in the 14

15 equatorial eastern Pacific (Meehl and Arblaster 1998; Guilyardi et al. 2004). The failure to simulate a reasonable annual cycle may result in an unrealistic annual phase locking of the model El Nino (e.g., Guilyardi et al. 2004). This hybrid coupled model produces a significant SST annual cycle in the eastern Pacific (Fig. 6b) even though the simulated phase lagged the observations by about 1 month and the amplitude is somewhat reduced. Compared to other ECHAM-4 family CGCMs (e.g., Frey et al. 1997; Gualdi et al. 2003), the annual cycle seems better represented in this hybrid coupled model. The reason is likely related to the introduction of an explicit mixed-layer in our intermediate ocean model (Fu and Wang 2001). In the western Pacific, the observed semiannual cycle is also well reproduced by the model. In the Indian Ocean, the model captures the annual cycle in the eastern basin and the semiannual cycle in the western basin. However, the observed strong summer cooling near the Somali coast is underestimated, suggesting the coastal upwelling is not well represented. Higher horizontal resolutions in both the atmosphere and ocean models are probably needed to mitigate this bias. Figures 7a-b compare the annual cycles of the equatorial zonal winds with the observations and the model. The major observed features are captured in the model: the annual cycle in the eastern Pacific, the winter (and spring) westerly and summer easterlies in the western Pacific, and semiannual cycle in the Indian Ocean. The simulation, however, shows a few obvious biases, particularly in the western Pacific, such as a stronger westerly in boreal spring and a weaker westerly in boreal winter. 4. The Tropical Intraseasonal Oscillation (TISO) The atmosphere-ocean variability (e.g., precipitation, low-level winds, surface 15

16 heat fluxes and SST) associated with the TISO has its strongest signal in the tropical Indo-Pacific sector even though its impacts could spread around the world. The TISO significantly regulates the onset and retreat of Asian-Australian monsoons (Yasunari 1980), tropical storm activity (Maloney and Hartmann 2000), and ENSO evolution (McPhaden and Yu 1999), even the subseasonal rainfall variability over Americas (Mo 2000; Jones 2000). This intraseasonal mode is first revealed by Madden and Julian (1971) as an eastward propagating planetary-scale zonal wind oscillation with a period of about days (popularly termed as Madden-Julian Oscillation or MJO). Many follow-up studies have found that the eastward propagating MJO prevails primarily in boreal winter. In boreal summer, the dominant intraseasonal mode propagates northeastward from the equatorial Indian Ocean to East Asia (Yasunari 1979; Lau and Chan 1986; Wang and Rui 1990). As revealed by several model inter-comparison projects (Slingo et al. 1996; Sperber et al. 1997; Waliser et al. 2003), most current GCMs have considerable problems in realistically representing the TISO. Many recent studies have suggested that air-sea coupling significantly improves the simulations of the TISO in terms of its intensity, convection-sst phase relationship, propagation and seasonality (Flatau et al. 1997; Waliser et al. 1999; Inness and Slingo 2003; Fu et al. 2003). Fu and Wang (2004a, b) further demonstrated that two different TISO solutions, respectively, exist in an air-sea coupled system and a forced atmosphere-only system. The solution from the coupled system resembles the observations more than that from the atmosphere-only system. In this section, the TISO simulated in this hybrid coupled GCM is assessed briefly. We focus on the seasonal variations of the TISO, such as the changes of spatial pattern of 16

17 rainfall variability, eastward-propagating MJO in boreal winter and northwardpropagating ISO in boreal summer. Figures 8a-d show the standard deviations of filtered rainfall (with period of days retained), which are used to represent the intensity of the TISO, from the CMAP observations and the coupled model. In boreal winter (Fig. 8a), major rainfall variances associated with TISO activities locate in the southern Indian Ocean, SPCZ, ITCZs and South America. The model captures almost all these activities with the intensity slightly overestimated in the maritime continent, southern Indian Ocean and SPCZ (Fig. 8b). The simulation in the ITCZ is slightly weaker than the observations, probably a consequence of the underestimated mean rainfall in this area (Figs. 2c-d). During boreal summer (Fig. 8c), major rainfall variability shifts to the Northern Hemisphere following the seasonal march of mean rainfall. The action centers appear in the equatorial and northern Indian Ocean, South China Sea, the WNP and ITCZs. They are well collocated with the pattern of mean summer rainfall (Fig. 2a). As in the model mean (Fig. 2b), the orientation of maximum TISO intensity in the western North Pacific is not in line with the observations (Figs. 8c-d). In the eastern Pacific ITCZ, a relatively isolated TISO center occurs in association with the ENPSM (Maloney and Esbensen 2003). In general, the coupled model captures all the action centers that appeared in the observations but with slightly larger amplitude (Fig. 8d). Compared to the simulations of all 10 AGCMs that participated in the CLIVAR/Asian-Australian monsoon inter-comparison project (Waliser et al. 2003), the simulation of this hybrid coupled model is much better in terms of both the spatial pattern and amplitude of the TISO rainfall variability. 17

18 A limited-domain wavenumber-frequency spectral analysis is used to summarize the spatio-temporal characteristics of the TISO in the Asia-Pacific region. The advantages and usefulness of this method have been substantiated by several previous studies (Teng and Wang 2003; Fu et al. 2003; Fu and Wang 2004a, b). In boreal winter (NDJFMA), this wavenumber-frequency analysis is applied between 40 o E and 140 o W to extract the eastward-westward propagating modes. In boreal summer (MJJASO), the analysis is limited between 10 o S and 30 o N, focusing on the northward-southward propagating modes. Figures 9a-b compare the wavenumber-frequency spectra of rainfall (averaged between 10 o S and 5 o N) associated with the eastward-westward propagating intraseasonal modes in boreal winter from the CMAP observations and the coupled model. The results from the observations and the model are, respectively, 22-year ( ) mean and 39- year mean. In the observations (Fig. 9a), eastward propagating disturbances overwhelmingly dominate their westward counterparts. The maximum spectrum corresponds to the MJO mode discovered by Madden and Julian (1971) with a period of 50 days and a wavelength of 200 degrees in longitude. The associated eastward propagating speed is about 5 m s -1. The major characteristics of the observed MJO (e.g., period, wavelength (or speed), and intensity) seem well simulated by this model (Fig. 9b). On the other hand, there are also several biases in the simulation, for example, too strong westward propagating disturbances and too much variance with shorter periods and smaller spatial scales. During boreal summer, northward propagating TISO dominates in the Indian and western Pacific Oceans (Lau and Chan 1986; Wang and Rui 1990; Fu and Wang 18

19 2004b). Our previous studies have focused over the Indian Ocean (Fu et al. 2003; Fu and Wang 2004a). Here, we shift our attention to the western Pacific. Figures 10a, b compare the wavenumber-frequency spectra of rainfall variability averaged between 120 o E and 150 o E from the CMAP observations and the coupled model. The TISO characteristics in the simulation resemble those in the observations. The observed maximum spectrum corresponds to an oscillation of a period about days and a wavelength of 40 degrees in latitude. The corresponding northward propagating speed is about 1 m s -1. In the simulation (Fig. 10b), the dominant period is about days with both northward and southward variances slightly larger than the observations. The model also tends to produce too much variability in shorter time scales and smaller spatial scales. The above analyses indicate that the observed seasonal variations of the TISO have been largely captured by this hybrid coupled model. The spatial patterns of the TISO intensity (Fig. 8) and mean rainfall (Fig. 2) are highly correlated with each other in both the observations and the simulation. This coincidence probably suggests that the better simulation of mean rainfall is the pre-requirement for the better simulation of TISO. Additional analyses of 10 AGCMs outputs from the CLIVAR/monsoon intercomparison project (Kang et al. 2002) showed that none of them are able to reasonably capture both the dominant northward propagation in summer and eastward propagation in winter (figure not shown). It is very encouraging to see that this hybrid coupled model is capable of representing this significant seasonality of TISO with some realism. 5. The ENSO Variability The most significant interannual variability in the tropical Asia-Pacific region is 19

20 the variability associated with ENSO. Though considerable improvements of the simulation and prediction of ENSO have been made during the past twenty years (Latif et al. 1998; Chen et al. 2004), current coupled models still need to be improved with regard to realistically representing ENSO (Latif et al. 2001; Davey et al. 2002; Wang et al. 2004). In this section, the ENSO variability simulated by this hybrid coupled GCM will be evaluated. First, we compare the spatial patterns of SST standard deviation in the tropical Asia-Pacific sector between the observations and the model (Figs. 11a-b). The coupled model produces significant SST variations in the central-eastern equatorial Pacific as in the observations. The observations have two maximum variance centers: one near the Peruvian coast and the other in the eastern equatorial Pacific (~110 o W). The model well locates the first maximum center, but the second center is shifted 40 degrees west of the observed one. This is a common error presented in many other coupled GCMs (e.g., Frey et al. 1997; Knutson and Manabe 1998; Meehl et al. 2001), probably associated with too much westward extension of the cold tongue in mean state. The simulated time series of SST anomaly in the Nino-3.4 region (Fig. 12b) has a somewhat similar evolution as in the observations during the period of (Fig. 12a). As in the observations, the simulated SST time series indicates considerable irregularity. For example, before the warm event that peaks in 1930 in the observations (Fig. 12a), there is no significant cold event preceding it. Similar warm events (years 34 and 37) appear in the simulation. The model also produces significant biennial variations during years as in the observations during the period of The elongated warm event (years 27-30) with embedded annual variability in the simulation also finds a 20

21 resemblance in the observations from 1939 to With a longer simulation (~ 85 years), a power spectrum analysis indicates that the time series of SST anomaly in the Nino-3.4 region has two peaks with periods of about 2 years and 5 years, respectively (figure not shown). The model ENSO shows reasonable phase locking with annual cycle (Fig. 13). The observed SST standard deviation in the Nino-3.4 region is minimum (~ 0.45 o C) in late spring and maximum (~ 0.65 o C) in winter. The simulated SST standard deviation has considerable annual variations as in the observations. The minimum (maximum) in the model occurs one month (two months) earlier than that in the observations, with the annual range slightly larger. Compared to the results from those coupled GCMs participating in the ENSIP project (Latif et al. 2001), the simulated ENSO annual phase locking in this model is better than most of them. The reason is most likely due to the reasonable simulation of annual cycle in the central-eastern Pacific. For example, the ENSO simulated in SINTEX CGCM, which also used ECHAM-4 as its atmospheric component, has no apparent annual phase locking due to inappropriate representation (a weaker annual harmonics and a stronger semiannual harmonic) of annual cycle (Guilyardi et al. 2004). Although the strongest SST signal associated with ENSO is in the equatorial eastern Pacific, its impacts actually spread around the world. Figures 14a-b compare the global sea-level-pressure (SLP) teleconnection patterns correlated with the Nino-3.4 indices from the NCEP reanalysis (surrogate of the observations) and the coupled model. The model captures almost all the large-scale teleconnection features presented in the observations, particularly the see-saw patterns between the tropical Pacific Ocean and 21

22 Indian Ocean. Compared to the observations (Fig. 14a), the simulated pattern shifts a bit westward due to the flaw in SST anomaly pattern (Fig. 11). The teleconnection with North American continent is also reproduced with the simulated positive SLP center locating slightly westward compared to the reanalysis. El Nino in the Pacific also acts to increase the SLP in the equatorial Atlantic (Fig. 14a). The model tends to exaggerate this connection. 6. Summary and Discussions a. Summary We have successfully developed a unique Hybrid coupled GCM (HcGCM) that is able to reasonably simulate both climatology (annual-mean and annual cycle) and variability in the tropical Asia-Pacific region. This hybrid coupled GCM combined the ECHAM-4 AGCM (Roeckner et al. 1996) with an intermediate ocean model developed at University of Hawaii (McCreary and Yu 1992; Wang et al. 1995; Fu and Wang 2001). In this study, the first 40-year (after 5-year spin-up) output from this coupled model has been analyzed and compared to available observations and other state-of-theart coupled GCMs. Overall, the model simulations of the climatology and variability in the tropical Asia-Pacific sector (e.g., Pacific mean SST and its annual cycle, Asian Summer monsoon, tropical intraseasonal oscillation (TISO) and ENSO) are quite reasonable and comparable with those sophisticated CGCMs. The mean SST difference between the model and observations is within 1 o C in most of the tropical Indo-Pacific Oceans (Fig. 1c). The cold bias problem of the equatorial western-central Pacific troubled most coupled GCMs (Figure 1 in Davey et al. 22

23 2002) is only minor in this hybrid coupled model. This indicates that the climatological Bjerknes atmosphere-ocean feedback mechanism (Neelin and Dijkstra 1995; Jin 1996), which is critical to configure the warm-pool/cold tongue along the equatorial Pacific, has been reasonably represented. However, the model also suffers a warm-bias syndrome in the southeast Pacific like most other coupled GCMs (Davey et al. 2002). The primary reason is that the atmospheric model considerably underestimates the stratocumulus in this region. The simulated seasonal cycle in the eastern Pacific quite resembles the observations with a dominant annual harmonic (Fig. 6). However, the amplitude is slightly underestimated with a phase delayed about 1-2 months. Even so, the simulation is still much better than those of many fully coupled GCMs (e.g., NCAR CCSM, Meehl and Arblaster 1998; SINTEX CGCM, Guiyardi et al. 2004). The Asian summer monsoon and tropical intraseasonal variability are also well simulated in this hybrid coupled model as in most other ECHAM-4 family coupled models (e.g., Gualdi et al. 2003; Sperber et al. 2003). The spatial patterns of summermean rainfall (Fig. 2) and its intraseasonal standard deviation (Fig. 8) in the model are much closer to the observations than all 10 AGCMs participating in the CLIVAR/Asian- Australian monsoon inter-comparison project (Kang et al. 2002; Waliser et al. 2003). The simulated tropical intraseasonal oscillations (TISO) exhibit significant seasonality as in the observations. In boreal winter (NDJFMA), the TISO action centers are primarily located in the southeast Indian Ocean and Pacific SPCZ region (Fig. 8b). The dominant ISO mode is the eastward-propagating MJO (Fig. 9). In boreal summer (MJJASO), major TISO activities shift to the Northern Hemisphere (Fig. 8d), with dominant northwardpropagating mode in the Indian and western Pacific regions (Fig. 10). 23

24 The simulated ENSO is comparable to the observed one in terms of the variance and frequency. The model ENSO has two spectral peaks with periods about 2 years and 5 years. The ENSO variance shows enough meridional expansion, but the location of the maximum shifts a bit too westward. The simulated ENSO indicates reasonable annual phase locking with minimum (maximum) variance in boreal spring (in late fall). This ENSO characteristic was not captured by the SINTEX CGCM, probably due to the misrepresentation of annual cycle in that model (Guilyardi et al. 2003). This hybrid coupled model also reasonably captures the global teleconnection of ENSO, including the remote impacts to the Indian Ocean sector and North America (Fig. 15). b. Discussions The results presented in this paper clearly indicate that with regards to representing the present-day climatology and its variability (with time scales from intraseasonal to interannual) in the tropical Asia-Pacific sector, a hybrid coupled model is as good as fully coupled GCMs. Use of a hybrid coupled model also saves a lot of computational resources compared to a fully coupled GCM in terms of both running time (this model is up to 2-3 times faster than a fully coupled model with similar temporal and spatial resolution) and data storage. Although very encouraging results have been obtained with this hybrid coupled model, we believe that there is still plenty of room to improve this model for both the atmospheric component and oceanic component. Further model improvements will focus on following two aspects. 24

25 First, we will try to improve the stratocumulus simulation in the atmospheric model, which probably leads to significant mitigation of the warm bias near the Peruvian coast in the coupled model (Fig. 1c). Recent numerical studies with regional atmospheric models (Wang et al. 2004; McCaa and Bretherton 2004) have shown that the stratocumulus in the southeastern Pacific is very sensitive to the cumulus parameterization schemes and model resolutions. Because both studies have proven that the stratocumulus can be reasonably represented with a mass flux scheme, it is optimistic that ECHAM-4 AGCM (also using a mass flux scheme) can be improved through better validation of the cumulus parameterization scheme and increase of model resolution. Second, the parameterization of entrained water temperature can be further improved. Because of the coarse vertical resolution of intermediate ocean models, they can t produce an entrainment temperature the way as in ocean general circulation models (Gent and Cane 1989). The success of intermediate ocean models largely depends on the parameterization of entrained water temperature. The usefulness of this approach has been supported by a lot of previous studies (Zebiak and Cane 1987; Seager et al. 1988; McCreary et al. 1993; Wang et al. 1995; Jin 1998; Fu and Wang 2001). The results presented in this study further support that this framework is a pragmatic approach to represent the thermodynamics and dynamics of upper tropical oceans. As suggested by previous studies (Perigaud and Dewitte 1996; Zhang and Zebiak 2003), the scheme of entrained water temperature can be further improved through validating it with available ocean observational data or reanalysis products. We are also aware that ultimate improvement of fully coupled GCMs needs to better represent various mixing processes 25

26 in the ocean GCMs. Our current effort to develop a hybrid coupled GCM is probably a complementary approach of fully CGCMs. Acknowledgements. This work was supported by NOAA PACS Program, NSF Climate Dynamics Program, NASA Earth Science Program and by the Japan Agency for Marine- Earth Science and Technology (JAMSTEC) through its sponsorship of the IPRC. XF likes to thank Diane Henderson for editing the manuscript. This paper is SOEST contribution number xxxx and IPRC contribution number yyyy. 26

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37 Figure Captions Figure1, Annual-mean SST from the observations (100-year mean from GISST) (a), from the hybrid coupled model (40-year mean) (b), and model bias ((b)-(a)). Contour interval is 1 o C. Figure 2, Seasonal-mean zonal wind vertical shear (200 hpa-850 hpa) and rainfall in boreal summer (JJAS) from the observations (a), from the model (b); and in boreal winter (DJFM) from the observations (c) and the model (d). Shadings are for rainfall (mm day -1 ) and contours are for vertical shear (m s -1 ). Figure 3, 1000-hPa wind vector and wind speed (contours) in boreal summer (JJAS) from the observations (a) and the model (b); and in boreal winter (DJFM) from the observations (c) and the model (d). Contour interval is 2 m s -1 (larger than 6 m s -1 are shaded). Figure 4, 200-hPa zonal wind speed in boreal summer (JJAS) from the observations (a) and the model (b); and in boreal winter (DJFM) from the observations (c) and the model (d). Contour interval is 5 m s -1 (larger than 40 m s -1 are shaded). Figure 5, Annual means of (a) SSTs ( o C) and (b) surface zonal winds (m s -1 ) from the observations and the model along the equatorial Indo-Pacific Oceans. Figure 6, Annual cycles of SSTs along the equatorial Indo-Pacific Oceans from the observations (a) and the model (b). Contour interval is 0.5 o C (positive values are shaded). Figure 7, Annual cycles of surface zonal winds along the equatorial Indo-Pacific Oceans from the observations (a) and the model (b). Contour interval is 0.5 m s -1 (positive values are shaded). 37

38 Figure 8, Rainfall standard deviations associated with tropical intraseasonal oscillations (with periods between days) in boreal winter (NDJFMA) from the observations (a) and the model (b); and in boreal summer (MJJASO) from the observations (c) and the model (d). Contour interval is 1 mm day -1 (larger than 2 are shaded). Figure 9, Wavenumber-frequency spectra of rainfall associated with west-eastward propagating disturbances in boreal winter (NDJFMA) averaged between 10 o S and 5 o N from the observations (a) and the model (b). Contour interval is 3 (mm day -1 ) 2. Figure 10, Wavenumber-frequency spectra of rainfall associated with south-northward propagating disturbances in boreal summer (MJJASO) averaged between 120 o E and 150 o E from the observations (a) and the model (b). Contour interval is 3 (mm day -1 ) 2. Figure 11, Standard deviation of SST anomalies in Indo-Pacific sector from the observations (a) and the model (b). Contour interval is 0.1 o C (larger than 0.2 are shaded). Figure 12, Time series of Nino-3.4 SST anomalies ( o C) from the observations (a) and the model (b). Figure 13, Seasonal cycles of SST internannual standard deviations ( o C) at Nino-3.4 region from the observations (a) and the model (b). Figure 14, Maps of correlation of the Nino-3.4 SST anomaly time series with global sealevel pressure anomaly from the NCEP reanalysis (a) and the model (b). Correlation coefficients larger than 0.6 or smaller than -0.6 are shaded. 38

39 Figure1, Annual-mean SST from the observations (100-year mean from GISST) (a), from the hybrid coupled model (40-year mean) (b), and model bias ((b)-(a)). Contour interval is 1 o C. 39

40 Figure 2, Seasonal-mean zonal wind vertical shear (200 hpa-850 hpa) and rainfall in boreal summer (JJAS) from the observations (a), from the model (b); and in boreal winter (DJFM) from the observations (c) and the model (d). Shadings are for rainfall (mm day -1 ) and contours are for vertical shear (m s -1 ). 40

41 Figure 3, 1000-hPa wind vector and wind speed (contours) in boreal summer (JJAS) from the observations (a) and the model (b); and in boreal winter (DJFM) from the observations (c) and the model (d). Contour interval is 2 m s-1 (larger than 6 m s-1 are shaded). 41

42 Figure 4, 200-hPa zonal wind speed in boreal summer (JJAS) from the observations (a) and the model (b); and in boreal winter (DJFM) from the observations (c) and the model (d). Contour interval is 5 m s -1 (larger than 40 m s -1 are shaded). 42

43 Figure 5, Annual means of (a) SSTs ( o C) and (b) surface zonal winds (m s -1 ) from the observations and the model along the equatorial Indo-Pacific Oceans. 43

44 Figure 6, Annual cycles of SSTs along the equatorial Indo-Pacific Oceans from the observations (a) and the model (b). Contour interval is 0.5 o C (positive values are shaded). 44

45 Figure 7, Annual cycles of surface zonal winds along the equatorial Indo-Pacific Oceans from the observations (a) and the model (b). Contour interval is 0.5 m s -1 (positive values are shaded). 45

46 Figure 8, Rainfall standard deviations associated with tropical intraseasonal oscillations (with periods between days) in boreal winter (NDJFMA) from the observations (a) and the model (b); and in boreal summer (MJJASO) from the observations (c) and the model (d). Contour interval is 1 mm day -1 (larger than 2 are shaded). 46

47 Figure 9, Wavenumber-frequency spectra of rainfall associated with west-eastward propagating disturbances in boreal winter (NDJFMA) averaged between 10 o S and 5 o N from the observations (a) and the model (b). Contour interval is 3 (mm day -1 ) 2. 47

48 Figure 10, Wavenumber-frequency spectra of rainfall associated with south-northward propagating disturbances in boreal summer (MJJASO) averaged between 120 o E and 150 o E from the observations (a) and the model (b). Contour interval is 3 (mm day -1 ) 2. 48

49 Figure 11, Standard deviation of SST anomalies in Indo-Pacific sector from the observations (a) and the model (b). Contour interval is 0.1 o C (larger than 0.2 are shaded). 49

50 Figure 12, Time series of Nino-3.4 SST anomalies ( o C) from the observations (a) and the model (b). 50

51 Figure 13, Seasonal cycles of SST internannual standard deviations ( o C) at Nino-3.4 region from the observations (a) and the model (b). 51

52 Figure 14, Maps of correlation of the Nino-3.4 SST anomaly time series with global sealevel pressure anomaly from the NCEP reanalysis (a) and the model (b). Correlation coefficients larger than 0.6 or smaller than -0.6 are shaded. 52