SCIENCE CHINA Earth Sciences. Changes of the tropical Pacific Walker circulation simulated by two versions of FGOALS model

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1 SCIENCE CHINA Earth Sciences RESEARCH PAPER doi: /s Changes of the tropical Pacific Walker circulation simulated by two versions of FGOALS model MA ShuangMei 1,2 & ZHOU TianJun 1,3* 1 LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing , China; 2 University of Chinese Academy of Sciences, Beijing , China; 3 Climate Change Research Center, Chinese Academy of Sciences, Beijing , China Received September 24, 2013; accepted January 23, 2014 Here we assessed the performances of IAP/LASG climate system model FGOALS-g2 and FGOAS-s2 in the simulation of the tropical Pacific Walker circulation (WC). Both models reasonably reproduce the climatological spatial distribution features of the tropical Pacific WC. We also investigated the changes of WC simulated by two versions of FGOALS model and discussed the mechanism responsible for WC changes. Observed Indo-Pacific sea level pressure (SLP) reveals a reduction of WC during and , and an enhancement of WC during During the three different time spans, the WC in FGOALS-g2 shows a weakening trend. In FGOALS-s2, tropical Pacific atmospheric circulation shows no significant change over the past century, but the WC strengthens during and The simulated bias of the WC change may be related to the phase of the multi-decadal mode in coupled models, which is not in sync with that in the observations. The change of WC is explained by the hydrological cycle constraints that precipitation must be balanced with the moisture transporting from the atmospheric boundary layer to the free troposphere. In FGOALS-g2, the increasing amplitude of the relative variability of precipitation ( P/P) is smaller (larger) than the relative variability of moisture ( q/q) over the tropical western (eastern) Pacific over the three time spans, and thus leads to a weakened WC. In FGOALS-s2, the convective mass exchange fluxes increase (decrease) over the tropical western (eastern) Pacific over the past 53 a ( ) and the last 23 a ( ), and thus leads to a strengthened WC. The distributions of sea surface temperature (SST) trends dominate the change of WC. Over the past 55 a and 23 a, tropical Pacific SST shows an El Niño-like (a La Niña-like) trend pattern in FGOALS-g2 (FGOALS-s2), which drives the weakening (strengthening) of WC. Therefore, a successful simulation of the tropical Pacific SST change pattern is necessary for a reasonable simulation of WC change in climate system models. This idea is further supported by the diagnosis of historical sea surface temperature driven AGCM-simulations. Walker circulation, FGOALS-g2, FGOALS-s2, historical climate simulation, hydrological cycle constraint, SST change pattern, AMIP simulation Citation: Ma S M, Zhou T J Changes of the tropical Pacific Walker circulation simulated by two versions of FGOALS model. Science China: Earth Sciences, doi: /s The Walker circulation is one of the important atmospheric systems. It is a large-scale zonal overturning of air across the equatorial Pacific Ocean. Based on the zonal SST contrasts across the equatorial Pacific and corresponding atmospheric circulation features, this vertical circulation was *Corresponding author ( zhoutj@lasg.iap.ac.cn) first found by Bjerkness (Bjerknes, 1969). The air rises over the western Pacific warm pool and descends over the eastern Pacific cold tongue, with the winds blowing from east to west and reversed in high-level. This thermal driven circulation is closely connected with the Southern Oscillation (SO), which is found by Walker and thus called the Walker circulation (Bjerknes, 1969). Its interannual variability is closely linked to ENSO (Allan et al., 1996). During an El Science China Press and Springer-Verlag Berlin Heidelberg 2014 earth.scichina.com link.springer.com

2 2 Ma S M, et al. Sci China Earth Sci January (2014) Vol.57 No.1 Niño event, SST gets warmer and upwelling current gets weaker over the equatorial central and eastern Pacific, while SST gets cooler over the western Pacific warm pool. For the atmospheric circulation, the sinking over the equatorial eastern Pacific and ascending over the western Pacific weaken, leading to a weakened WC. During La Niña event, WC is enhanced (Julian et al., 1978). WC has significant impacts on the global climate change through teleconnections (Deser et al., 1990). The interaction between WC and monsoon circulation has great impacts on the Indo-Pacific regional climate (Webster et al., 1998). The strength of equatorial Pacific zonal wind stress, associated with WC, is important to equatorial Pacific Ocean circulation and biogeochemical processes (Cane et al., 1977; Barber et al., 1983). The change of WC under global warming has been a hot topic of climate change community. During the 20th century, especially the past six decades ( ), analyses of multi-variables have shown physically consistent trends, which include a weakening of the central tropical Pacific trade winds, zonal mass exchange and the zonal SLP gradient between the eastern and western Pacific, accompanied by a decreasing of precipitation over the Maritime Continent, indicative of a slowdown of WC (Vecchi et al., 2006; Power et al., 2007, 2011; Desert et al., 2010; DiNezio et al., 2013; Zhang et al., 2006; Tokinaga et al., 2012b; Yu et al., 2010). Over the past three decades ( ), the tropical Pacific easterly has increased. The eastern Pacific has undergone a cooling trend, while the western Pacific has undergone a warming trend, which resembles a La Niña-like pattern. Precipitation decreased over the western and increased over the eastern Pacific. Increased (decreased) SLP over the eastern (western) Pacific caused an increase in the east-west SLP gradient, supporting the intensification of WC (Luo et al., 2012; Li et al., 2012; Sohn et al., 2012). Most simulations of CMIP3 and CMIP5 climate models show that WC weakens during the 20th century, consistent with the observed SLP gradient change. The WC is projected to be weakened in the twenty-first century (Vecchi et al., 2006, 2007; Power et al., 2011; DiNezio et al., 2013; Held et al., 2006). The mechanism of WC change has been discussed in many previous studies but remains inconclusive. Previous studies indicate that both external forcing (global warming) and internal variability (ENSO, interdecadal Pacific Oscillation (IPO), and so on) contribute to the observed weakening of WC during the 20th century, but the contribution of external forcing is larger than that of internal variability (Vecchi et al., 2006; Power et al., 2011; DiNezio et al., 2013; Yu et al., 2012; Collins et al., 2010). Knutson and Manabe (1995) pointed out that the radiative cooling should be balanced by the adiabatic warming associated with the subsidence in nonconvecting regions. In a warmer climate, the dry static stability increases at a faster rate than the radiative cooling of the troposphere, implying a weakening of WC. Meanwhile, the weakening of WC in response to warming can be understood in term of mass balance. Because the increase in strength of the global-averaged precipitation (1% K 1 3% K 1 ) is constrained by the moisture transport from the boundary layer to the free troposphere, it cannot keep up with the rapid increase in troposphere vapor (7.5% K 1 ). It indicates that the exchange of mass between the boundary layer and troposphere must decrease, and then the atmospheric circulation also must slow down (Held et al., 2006). Moreover, it is suggested that global warming is largely responsible for the observed weakening of WC during the 20th century, because El Niño events become more frequent in response to global warming (Power et al., 2011). In conclusion, the weakening of WC over the 20th century is closely connected with global warming. Many studies have been devoted to the relationship between tropical Pacific SST change and WC change. Under global warming, the atmospheric thermodynamic constraints lead to an El Niño-like tropical Pacific warming structure, while the ocean dynamical processes favor a La Niña-like pattern (Vecchi et al., 2008; DiNezio et al., 2009, 2010). In fact, both the tropical ocean dynamical process mechanism and the atmospheric Bjerknes feedback mechanism could contribute to SST change pattern. A weakening of WC is expected to occur in response to global warming, even with a zonally uniform warming of the tropical Pacific Oceans (Vecchi et al., 2008; DiNezio et al., 2009, 2010). It is also suggested that tropical Pacific warming pattern is a good indicator of WC change (Meng et al., 2012; Gastineau et al., 2009). Liu et al. (2013) noted that the global surface temperature in the late 20th century is warmer than in the Medieval Warm Period but precipitation is less. The reason is that the tropical Pacific SST gradient increases when it is due to increased solar radiation and decrease when it is due to increased greenhouse-gas forcing, and accompanied by a stronger and weaker WC, respectively. In addition, Tokinaga et al. (2012a, 2012b) pointed out that El Niño-like tropical Pacific warming pattern is the main cause of the weakening of WC over the past six decades ( ). The enhanced tropical Indian Ocean warming in recent decades ( ) favors stronger trade winds of the western Pacific and hence is likely to have contributed to the La Niña-like state of tropical Pacific through the oceanatmosphere interactions. Increased SST gradient favors a stronger WC and hence leads to a stronger global monsoon precipitation (Luo et al., 2012; Wang et al., 2012). The previous studies have enriched our understanding of both observed and simulated WC changes. But the WC changes over the three different time spans (including , , and ) and in particular responsible mechanisms remain inconclusive. The current study aims to understand the responses of WC to specified external forcings in the historical climate simulation of two versions of LASG/IAP s FGOALS model. The following questions will be addressed: (1) how well do the two ver-

3 Ma S M, et al. Sci China Earth Sci January (2014) Vol.57 No.1 3 sions of FGOALS simulate the mean state of WC? (2) What are the characteristics of WC change over the three different time spans? (3) What are the mechanisms responsible for the WC change? 1 Model, data, and analysis method 1.1 Model, experiments, and data description FGOALS-g2 and FGOALS-s2 are the two new versions of the Flexible Global Ocean-Atmosphere-Land System (FGOALS) models developed by LASG/IAP. Both models have participated in the Coupled Model Inter-comparison Project (phase 5) (CMIP5). Both versions of FGOALS consist of four interactive component models, including atmospheric, oceanic, land, and sea ice models that are coupled together by NCARs flux coupler module, version 6 (CPL6). These two versions of FGOALS share the same ocean and land component models, but differ in their atmospheric and sea ice components. For FGOALS-s2, the atmospheric component is the Spectral Atmospheric Model of the IAP/LASG s version 2 (SAMIL2), with a horizontal resolution of about 2.81 (lon.) 1.66 (lat.); the sea ice component is the Community Sea Ice Model version 5 (CSIM5). In FGOALS-g2, the atmospheric component is the Grid-point Atmospheric Model of the IAP/LASG s version 2 (GAMIL2), with a horizontal resolution of about ; the sea ice component is the Los Alamos Sea Ice Model (CICE). The ocean component of FGOALS is the LASG/IAP s Climate System Ocean Model version 2 (LICOM2). For more detailed information about these two model versions, the readers are referred to Li et al. (2013) and Bao et al. (2013). To reveal the characteristics of WC change and understand the related mechanisms, the outputs of the 20th century historical climate simulations (20C3M) are used in this study. 20C3M was made with identical forcing agents, including greenhouse gases, sulfate aerosols, ozone, volcanic aerosols, and solar variability, recommended by CMIP5. The initial condition of the FGOALS 20C3M experiments comes from Preindustrial (PI) control run and the outputs of this experiment cover the period of Both models 20C3M experiments have three different realizations. The performances of FGOALS-s2 and FGOALS-g2 in the simulation of the 20th century global and regional surface air temperature changes have been assessed by Zhou et al. (2013). PI control run of FGOALS-g2 (FGOALS-s2) integrated 900 (600) years, starting from the equilibrium state of a stand-alone 500-a spin-up integration of LICOM2. In the PI control run, the values of external forcing agents were fixed at the level of the year 1850, in which the solar constant was 1365 W m 2, the greenhouse gas mixing ratios including CO 2, CH 4, and N 2 O were 284 ppm, 790 ppb, and 275 ppb, respectively. The ozone mixing ratio and the concentrations of aerosols including sulphate, black carbon, and organic carbon, and sea salt were prescribed as the values in the PI period. To understand the driver of WC, both models outputs of AMIP experiment are analyzed. AMIP experiments were performed using the monthly-mean observed SST to force the atmospheric model. FGOALS-g2 has one realization covering the period of FGOALS-s2 has three realizations covering the period of For the comparison between models and observations, we use the monthly mean observational and reanalysis data including: (1) NCEP/NCAR reanalysis data (for convenience, also called observation in this article) of (Kalnay et al., 1996); (2) Hadley centre Sea Level Pressure dataset (HadSLP2) of (Allan et al., 2006); (3) Observational SST dataset (ERSST-v3) of (Smith et al., 2008); (4) precipitation of GPCP over the period of (Xie et al., 1997). 1.2 Analysis method Following previous studies, in order to objectively measure the strength of WC, we define two WC indices. One is based on zonal mass streamfunction (Yu et al., 2010, 2012), the other is based on zonal SLP gradient (Vecchi et al., 2006; Power et al., 2011; DiNezio et al., 2013). They are separately expressed as p a uddp g, (1) SLP SLP( W, 5 S 5 N) SLP( E, 5 S 5 N), (2) where ψ denotes the zonal streamfunction, a is the radius of the Earth, ΔΦ is the width of the band 5 S 5 N along the equator in radians, g is the gravity acceleration, u D is the divergent component of the zonal wind, p is the pressure, and ΔSLP is the zonal SLP gradient. To estimate changes in WC, the equatorial atmospheric mass flux ψ is averaged between W. The larger the value of ψ and ΔSLP is, the stronger the WC is. Based on the mass conservation, global-averaged precipitation is balanced with the moisture transport from the atmospheric boundary layer to the free troposphere. Held and Soden (2006) made the following hydrological cycle constraints: P M q, (3) where P is precipitation, M is mass flux from the boundary layer to the free troposphere, and q is a measure of boundary layer specific humidity. If we ignore the effect of horizontal advection, eq. (3) is a good approximation of hydrological cycle in the Tropics. To discuss how the above constraints affect the change of WC, following Held and Soden, we explain WC change using the convection mass fluxes 0

4 4 Ma S M, et al. Sci China Earth Sci January (2014) Vol.57 No.1 ( M/M) which are the difference between local fractional change in precipitation ( P/P) and column-integrated water vapor ( q/q). It can be simply set as M P q, (4) M P q where denotes the departure relative to the mean of Results Usually, the characteristics of WC change are described using the change of SLP, SST, low-level wind field, 500-hPa vertical velocity, 200-hPa velocity potential, precipitation and zonal streamfunction. Thus, in this study, we first examine the performance of the two versions of FGOALS in reproducing the climatological spatial distributions of the tropical Pacific atmospheric circulation. Meanwhile, we investigate the WC change over different timespans in two models by comparing it with the observation. Finally, the mechanism of WC change is discussed. 2.1 Climatology of the Walker circulation The climatological spatial distributions of physical variables associated with the tropical Pacific WC are shown in Figure 1. The main features of the observed mean state over the tropical Pacific circulation are as follows (Figure 1(a), (d), (g), (j)). Tropical Pacific SST distribution has strong meridional asymmetry, with relative warm SST in the western tropical Pacific, and relative cold SST in the eastern tropical Pacific. Large-scale tropical circulation is basically the Figure 1 Climatology ( ) of the tropical Pacific circulation. (a) (c) SST ( C, color shaded), SLP (hpa, contours); (d) (f) 500-hPa vertical velocity (10 2 Pa s 1, positive are downward, color shaded), 200-hPa velocity potential (10 6 m 2 s 1, contours) and divergent wind (m s 1, vectors); (g) (i) precipitation (mm d 1, color shaded), surface winds (m s 1, vectors); (j) (l) zonal mass streamfunction (10 9 kg s 1 ). Left, central and right panels correspond to the results of observation, FGOALS-g2 ensemble mean and FGOALS-s2 ensemble mean, respectively.

5 Ma S M, et al. Sci China Earth Sci January (2014) Vol.57 No.1 5 response to ocean heating. Over warm pool, atmospheric heating is driven by the sensible heating of the equatorial land masses of Indonesia and the latent heat release associated with deep, moist convection. This heating promotes ascent, in conjunction with the lower-tropospheric convergence, upper-tropospheric divergence, weak surface wind, low SLP, and rich precipitation. Over cold tongue, SLP is higher than that over warm pool, and surface winds are strong and accompanied by obvious divergence in lower troposphere and convergence in the upper troposphere. At the same time, descending motion is unfavorable to rain, and thus precipitation is weak over cold tongue. Zonal mass and energy is exchanged continuously under the strong interactions of the tropical Pacific zonal thermal contrasts and dynamic contrasts. Zonal mass exchange is the strongest in regions from the Date Line to 100 W. As the tropical Pacific circulation climatology in each realization (figure not shown) is similar with that in the ensemble mean (EM), we only give the results of the EM in this study. The climatology of SLP, SST, wind field, precipitation, and zonal mass exchange fluxes that are simulated separately by FGOALS-g2 and FGOALS-s2 are shown in the center and right panels of Figure 1. In both models, the maximums of SST, upper-tropospheric divergence, vertical velocity, and precipitation are located in the western Pacific. The Western Pacific SLP is lower than that over the eastern Pacific. Easterly trades cover the whole equatorial Pacific basin. To quantitatively evaluate the models performance in simulating the climatology of tropical Pacific WC, the weighted pattern correlation coefficient (PCC) and the standard deviation ration (SDR) between models and observation are calculated and results are given in form of Taylor diagram (Figure 2). Models which can reproduce the observed results well would have SDR and PCC close to unity. In both models, PCCs of related climate elements are larger than 0.65, and are statistically significant at the 1% level. SDRs of most climate elements are larger than 0.75 and smaller than 1.25, suggesting that the WC climatology simulated by the two versions of FGOALS is reasonable in both the spatial structure and amplitude. However, the biases are also evident and are concentrated mainly on the following aspects. If we use the simple criterion of SST warmer than 28 C to define the scope of the western Pacific warm pool, the warm pool SST simulated by FGOALS-g2 is colder than observation, making the corresponding SLP larger than that in observation (Figure 1(a) (c)). SST and SLP in both models have more homogeneous distributions than those in the observation, with related SDR smaller than 1.0. Meanwhile, in both models, descending motions over the equatorial eastern Pacific extend westward excessively (Figure 1(d) (f)), with relative small PCC and relative large SDR; the South Pacific Convergence Zone (SPCZ) expands eastward excessively, and nearly parallels to ITCZ in FGOALS-g2 (Figure 1(g) (i)). Furthermore, both models cannot well reproduce the precip- Figure 2 Taylor diagrams of the climatology of tropical Pacific atmospheric circulation in FGOALS-g2 ensemble mean ( ) and FGOALSs2 ensemble mean ( ). The angular coordinate is weighted pattern correlation coefficient between model results and the observations. The radial coordinate is the standard deviation of model results divided by the standard deviation of the observations. Color numerical value 1 to 7 matches SST, SLP, 500-hPa vertical velocity, 1000-hPa horizonal wind field, 200-hPa velocity potential, zonal mass streamfunction, precipitation, respectively. itation maximum to the southwest of Sumatra. Over the equatorial Pacific, the zonal mass exchange fluxes are weaker in both models than in the observations (Figure 1(j) (l)). For the simulations of tropical Pacific SST, SLP and wind field in the lower levels, the performances of FGOALS-s2 are better than FGOALS-g2; for the simulations of 200-hPa velocity potential, it is conversed. For the simulation of 500-hPa vertical velocity and equatorial Pacific zonal mass exchange climatology, PCC of FGOALSg2 is larger than that of FGOALS-s2, but SDR of FGOALSs2 is closer to unite. Thermal factors and dynamical factors interact with each other over the tropical Pacific. SST, SLP, precipitation, horizontal wind, and vertical wind together affect the WC change. On the whole, both models perform well in the simulation of WC climatology. Therefore, simulations of the two versions of FGOALS can be used to study the change of WC and further to analyze the responsible mechanism. 2.2 The change of tropical Pacific Walker circulation Figure 3 shows the trends of annual WC indices as a function of the start date of the detection period and time series of SLP anomaly. The detection periods begin at different years from 1850 to 1985 and end in We first calculate the WC trend over the period , then for the period , sliding backward year by year, finally to 1985 and stop. Interannual variability of WC is evident in the observations. The correlation coefficient between SLP

6 6 Ma S M, et al. Sci China Earth Sci January (2014) Vol.57 No.1 Figure 3 The evolution of the tropical Pacific Walker circulation (WC) trends as a function of detection periods. Black dots denote the observed tropical Pacific SLP gradient anomaly (ralative to the mean over , unite is hpa). Lines denote the evolution of WC indices trends. (a) WC trend (hpa (100 a) 1 ) based on the zonal SLP gradient. (b) WC trend (10 9 kg s 1 (100 a) 1 ) based on zonal mass streamfunction averaged over the equatorial Pacific (5 S 5 N, 150 E 120 W). The solid black line represents observations, and the solid red (blue) line represents the ensemble mean of FGOALS-g2 (FGOALS-s2). The thin dash lines are for the different realizations of FGOALS-g2 (red) and FGOALS-s2 (blue). and El Niño3.4 index is -0.86, which passed the 95% statistical confidence level of a two-tailed t-test. This suggests that WC is intensified (weakened) during a La Niña (El Niño) event. This study focuses on the long term changes in WC. From the evolution of the observed SLP trends, during the detection period longer than 85 a beginning from 1920 to 1850, the SLP trends are negative, indicating WC slowdown. The fluctuation of the trend magnitude is small and the long term trends are basically stabilized around 0.4 hpa (100 a) 1. For the detection periods beginning after 1920, the fluctuations of trend and weakening amplitudes become larger. Over , WC trend reaches the minimum ( 0.8 hpa (100 a) 1 ), which is statistically significant at the 5% level. Beginning at the late 1970s, WC trend shifts to positive and reaches a maximum of 2.2 hpa (100 a) 1 during , but is statistically insignificant at the 5% level. As can be seen, the observed WC change over the different detection periods is different. For ensemble mean (EM) of FGOALS-g2, during the detection periods longer than 30 years, the evolution of SLP trends is similar to observed result. For time-spans beginning before 1920, the trends of SLP are negative and have small fluctuations. Since 1920, weakening trends of SLP become stronger and reach minimum in After 1950, weakening trends of SLP weaken and shift to positive in the late 1970s. However, the trends of SLP are negative in recent three decades. SLP trend evolutions among different realizations are basically consistent with that of EM. However, due to the internal noise, biases still exist among different realizations. For example, during the detection periods longer than 55 a, weakening trends of WC in first realization are larger than those in other two realizations. Only the first realization reproduces the observed enhanced WC in recent two decades. During the detection periods shorter than 30 a, the spreads of SLP trend among different realizations are larger. In FGOALS-s2, over the periods longer than 105a beginning before 1900, SLP trends are negative only in first realization, and positive in other two realizations. The fluctuations of SLP trends are small during the detection periods beginning before After 1920, the fluctuations of SLP trends and the spreads among realizations become larger. In terms of ME, positive and negative trends of SLP take place alternately. During the periods longer than 105 a, SLP trends approach zero. For the periods longer than 65 a and shorter than 105 a, SLP has relatively weak negative tendency. For the periods longer than 45 a and shorter than 65 a, the tendency of SLP is positive and reaches maximum around During the periods longer than 25 a and shorter than 45 a, SLP trends are basically negative and shift to the intensification of WC in the 1980s, with the strongest intensification during The evolution of SLP trends over the periods shorter than 45 a is similar to that of the observations. Although the equatorial Pacific SLP gradient is not a direct measure of the strength of Walker circulation, the trend evolution of WC index defined by area-averaged zonal mass exchange fluxes (zonal stream-function) over the equatorial Pacific (5 S 5 N, 150 E 120 W) is consistent with that of SLP (Figure 3(b)). For the ensemble mean and different realizations of FGOALS-g2 (FGOALS-s2), correlation coefficients between two WC indices are larger than 0.83 (0.9) and are statistically significant at the 1% level. Therefore, the long term change of WC is not dependent on the choice of indices. 2.3 Trend distributions of tropical Pacific Walker circulation Previous studies confirmed that WC slowed down during the 20th century ( ) and the last six decades ( ) (Deser et al., 2010; Tokinaga et al., 2012a, 2012b), and strengthened in the recent two decades (Luo et al., 2012; Sohn et al., 2012), based on a synthesis of precipitation, cloud, SLP, and surface wind observations. On the basis of the above analysis of WC indices evolution, we found the tendency of WC change is negative and its fluctuation is relatively weak for the detection periods longer than 85 a, whereas the slowdown (intensification) rate of WC reaches its maximum over ( ). Thus, this study focuses on the WC change over the above three different time spans past century ( ), last 55 years ( ), and recent 23 years ( ), and the changes of tropical Pacific WC simulated by the two versions of FGOALS are investigated in detail.

7 Ma S M, et al. Sci China Earth Sci January (2014) Vol.57 No.1 7 The distributions of WC trend for the past century are described in Figure 4. The observed SLP decreases over the central and eastern Pacific and increases over the Maritime Continent, indicating a slowdown of WC. Changes of WC in FGOALS-g2 are as follows: (1) SLP decreases over the eastern Pacific and increases over the western Pacific, along with the reduction of SLP and slowdown of WC (Figure 4(b)). As a whole, SLP change characteristics are close to the observed result (Figure 4(a)), with PCC of 0.63 with the observation statistically significant at the 1% level. (2) In respond to the change in SLP, westerly wind trends are obvious in the lower level of central and eastern Pacific (Figure 4(d)). Changes in 200 hpa velocity potential exhibit significant increase over the western Pacific and the Maritime Continent and decrease over the central and eastern Pacific, suggesting that upper-tropospheric divergence is reduced over the Maritime Continent and the western Pacific and intensified over the central and eastern Pacific with an eastward shift of upper divergence center (Figure 4(f)). (3) The ascending air over the Maritime Continent and coasts of Central America and descending air over the cold tongue is weakened, with the intensifying of convection over the central Pacific and an eastward shift of rainfall center (Figure 4(e), (j)). All these consistent changes support a reduction of WC. Above simulated WC changes agree with the observed results (Deser et al., 2010), indicating that FGOALS-g2 can reasonably simulate the WC change over the past century. The tendency of the two WC indices in FGOALS-s2 is not obvious over the past century (Figure 3). It is also Figue 4 Tropical Pacific climate trend over (a) (c) SLP (hpa (105 a) 1 ); (d),(e) surface zonal wind (m s 1 (105 a) 1 ); (f),(g) 200-hPa velocity potential (10 6 m 2 s 1 (105 a) 1 ); (h), (i) 500-hPa vertical velocity (10 2 Pa s 1 (105 a) 1 ); (j), (k) precipitation (mm d 1 (105 a) 1 ). Left and right panels correspond to the results of FGOALS-g2 ensemble mean and FGOALS-s2 ensemble mean, respectively. The trends that passed the 95% statistical confidence level are stippled.

8 8 Ma S M, et al. Sci China Earth Sci January (2014) Vol.57 No.1 reflected in the trend distribution. (1) Except for the specific regions such as southwest of Indonesia, Galapagos Island and limited scope to the northwest of central Pacific, the tendency of SLP is almost positive over the whole tropical Pacific ocean basin. Equatorial Pacific SLP gradient exhibits a weakening trend (Figure 4(c)). Except for the equatorial Pacific, tropical Pacific SLP change is inconsistent with the observation (PCC, 0.35). (2) Westerly anomaly over the western Pacific and easterly anomaly over the central and eastern Pacific and northwest Pacific are evident (Figure 4(e)). (3) Apart from the regions near Data Line, 200 hpa velocity potential exhibits an intensified tendency over the tropical Pacific (Figure 4(g)). (4) The weakening of ascending over Indonesia reduces corresponding precipitation, and the strengthening of ascending over the equatorial central Pacific, north central and southwest of Pacific increases corresponding precipitation. To west of South America, descending motion weakens and precipitation increases (Figure 4(i), (k)). Above tropical circulation changes imply that the equatorial Pacific circulation in FGOALS-s2 was weakened during , but the tropical pacific circulation showed no basin-wide weakening trend. The changes of tropical circulation over are shown in Figure 5. For the observations, the distributions of SLP trend are similar to results of the past century (Figure 5(a)), with PCC being 0.81 and statistically significant at the 1% level. SLP changes in FGOALS-g2 show similar features with observations, with PCC being 0.77 and statistically significant at the 1% level (Figure 5(b)). Meanwhile, the trend distributions of tropical SLP, lower zonal wind, 200 hpa velocity potential, 500-hPa vertical velocity, and precipitation are similar to those in the past century (Figure 5(d), (f), (h), (j)), with PCC being 0.83, 0.79, 0.91, 0.68, 0.60, respectively, and all passing the 99% statistical confidence level. In addition, the WC change structures in FGOALS-g2 agree with previous results derived from the observations (Zhang et al., 2006; Tokinaga et al., 2012b; Yu et al., 2010). The above changes of tropical Pacific WC Figure 5 Same as in Figure 4, but for the trends over

9 Ma S M, et al. Sci China Earth Sci January (2014) Vol.57 No.1 9 Figure 6 Same as in Figure 4, but for the trends over suggest that WC simulated by FGOALS-g2 slowed down over the past 55 a. Conversely, WC simulated by FGOALSs2 strengthened over the past 55 years, inconsistent with the observed results. In FGOALS-s2, the main features are as follows: (1) Changes in SLP exhibit a significant increase over the central to eastern Pacific and a decrease over the western Pacific with a negative correlation with observations (PCC, 0.70) (Figure 5(c)). (2) A significantly intensified easterly occurs over the equatorial central Pacific (Figure 5(e)). (3) Intensity of the upper divergence over the western Pacific and convergence over the eastern Pacific is enhanced significantly, enhancing corresponding vertical movement (Figure 5(g), (i)). (4) Precipitation increases over the western to central Pacific and decreases over the eastern Pacific (Figure 5(k)). The trend distributions of tropical circulation in the observations and simulations over the period are shown in Figure 6. In the observation, WC enhances with SLP increased over the eastern Pacific and decreases over the western Pacific. In the FGOALS-g2, SLP trend pattern has a negative correlation with that of observations, with PCC of 0.49, and a positive (negative) SLP tendency over the eastern (western) Pacific. Moreover, westerly trend is evident over the tropical Pacific. Upper divergence and ascending motion (convergence and descending) exhibit a weakening trend over the western (eastern) Pacific. Accordingly, precipitation decreases (increases) over the western (eastern) Pacific with an eastward shift of convection center (Figure 6(b), (d), (f), (h), (j)). All these changes support a reduction of WC simulated in FGOALS-g2 in the recent two decades, inconsistent with the observations. But in FGOALS-s2, changes of SLP, surface zonal wind, 200 hpa velocity potential, 500 hpa vertical velocity, and precipitation are similar to those in the last five decades, with PCC reaching 0.92, 0.87, 0.95, 0.84, 0.84, respectively, and all passing the 99% statistical confidence level. Additionally,

10 10 Ma S M, et al. Sci China Earth Sci January (2014) Vol.57 No.1 the SLP trend correlation coefficient between FGOALS-s2 and observations is 0.83 and statistically significant at the 0.1% level. In the last 23 years, WC enhances and the characteristics of tropical circulation change are similar to those of the observations (Luo et al., 2012; Sohn et al., 2012). The above detailed analysis indicates that WC in FGOALS-g2 slows down over , which is consistent with the observations. In FGOALS-s2, change of WC is not obvious over For the period , WC still shows a weakening trend in both the observations and FGOALS-g2, but a strengthening trend in FGOALS-s2. For the detection period , WC enhances in both the observations and FGOALS-s2, and still slows down in FGOALS-g2. For the tropical Pacific WC change over the three different time-spans, some biases between the two versions of FGOALS and the observations are still evident. Here the possible causes are discussed. Global warming induced by increasing greenhouse gas forcing and internal climate variability (such as the low-frequency variability of ENSO, IPO, and PDO) combined to affect WC change (Power et al., 2011; DiNezio et al., 2013). The weakening of WC during the 20th century is thought to be due partly to global warming (Vecchi et al., 2006; Power et al., 2011; Held et al., 2006) and partly to the increase (decrease) of internally generated El Niño (La Niña) events (Power et al., 2011). Vecchi et al. (2006) pointed out that, for the periods shorter than 100 a, it is very likely that the multi-decadal internal variability dominates the long term trend of SLP. As a result, the records longer than 100 a are required to detect changes in WC that are caused by global warming. It is also suggested that, for longer detection periods, global warming has larger effects on WC change than internal variability, and thus WC trends remain relatively steady in different time-spans. For the detection periods shorter than 100 a, internal variability such as ENSO, PDO, and IPO play a more important role in WC change as the detection periods shorten. Thus internal variability can either promote or cancel out the weakening trend resulting from external forcing and make fluctuation of WC trend become larger (DiNezio et al., 2013). Furthermore, Meng et al. (2012) suggested that strong internal variability of WC still exists on the centennial timescales based on control integrations with the CMIP models. Thus, the radiative forcing signal in 20th century WC may have been too weak to be detectable. On the basis of above research results, we examine the SLP change in the PI control runs of two models over the three different time-spans. In PI control runs, sliding trends of SLP gradient on the centennial timescales are not obvious and cannot pass the 80% statistical confidence level of t-test (Figure 7). But it is possible that internal variability acts to enhance or weaken the WC. Over 55 a time-spans, the probability that internal variability acts to enhance or weaken the WC would increase and further increase over the 23 a time-spans. In Figure 7 The linear trends of SLP gradient in PI control run of FGOALS-g2 (black) and FGOALS-s2 (gray) for 100 a ruuning (a), for 55 a runing (b) and for 23 a running (c). The abscissa is model year. Ordinate values represent the 100 a (55 a and 23 a) trend ((hpa (100 a 1 ) (hpa (55 a) 1 and hpa (23 a) 1 )) beginning at the corresponding model year. Asterisks denote that trends are statistically significant at the 20% level. other words, internal variability exerts an increasing role in the change of WC as the detection periods shorten. The phase of the multi-decadal mode in coupled models is not in sync with that in the observations. We can only analyze the forced WC change to external forcing using coupled climate models. Thus, over , internally generated natural variability superimposes on the effect of external forcing, making the weakening trend of WC in FGOALS-g2 smaller than the observed value and the tropical circulation change in FGOALS-s2 not obvious. Over and , WC change may be dominated by internal variability in the observations, which cannot be reproduced in models due to their different phase evolution of internal variability. Consequently, the intensification of WC simulated by FGOALS-g2 (FGOALS-s2) over ( ) may be model-dependent. Meanwhile, the reduction of WC over the recent 23 a in FGOALS-g2 and the enhancement of WC over the past 55 a in FGOALS-s2 also might be related closely to internal variability of model. Furthermore, WC tendency in FGOALS over the detection periods longer than 55 a is weaker than the observed value. Possible causes may be related to the following aspects: (1) There are some spurious signals in reconstruction data as the observation data are sparse before 1950 (L'Heureux et al., 2013) which can lead to a spuriously larger observed trend. (2) Most climate models underestimate the observed weakening tendency of WC, and thus

11 Ma S M, et al. Sci China Earth Sci January (2014) Vol.57 No.1 11 biases between models and observations may come from models themselves. 2.4 Mechanism of tropical Pacific Walker circulation change To understand WC change mechanism, based on the assumption proposed by Held and Soden (2006), fractional trend distributions of convective mass fluxes ( M/M, the term on the left of eq. (4) are shown in Figure 8. Over , fractional change in M simulated by FGOALSg2 exhibits a significant reduction over the western Pacific, Maritime Continent, and southwestern coasts of North America and exhibits an increase over the central Pacific. However, the percentage changes in M over cold tongue are not obvious (Figure 8(a)), indicating an enhancement of WC. In FGOALS-s2, except for the east coast of Australia and the equatorial central Pacific, convective mass fluxes are increased throughout the tropical Pacific (Figure 8(b)), leading to a slowdown of equatorial Pacific atmospheric circulation and an unobvious change of tropical Pacific circulation. Over and , in FGOALS-g2, M/M shows a significant decreasing tendency over the western Pacific; M is strengthened over the equatorial Pacific and cold tongue, with larger strengthened areas (Figure 8(c), (e)). Accordingly, WC slows down. In FGOALS-s2, M is enhanced over the equatorial central Pacific, northwestern and southwestern Pacific, and weakened over the eastern Pacific (Figure 8(d)), corresponding to the enhancement of WC. A comparison of Figure 8 with Figures 4 6 shows that fractional change patterns of convective mass fluxes derived from hydrological cycle constraints are similar to those of 500-hPa vertical velocity. Over three time-spans, PCCs between change patterns of M and vertical velocity are larger than 0.77 and statistically significant at the 1% level. Thus, hydrological cycle constraints can be used to explain the change in the tropical Pacific WC. As fractional change in precipitation is larger than that in column-integrated water vapor, convective mass exchange must increase in order to maintain the balance between precipitation and transport of water vapor, and vice versa. When M decreases over the tropical western Pacific and increases over the eastern Pacific, WC slows down, and vice versa (Figure 10). To reveal the impacts of SST change pattern on WC, the trend distributions of tropical SST over the three different time-spans are described in Figure 9. In the observations, the tropical Pacific exhibits a basin-wide warming and increased SST gradient (Figure 10(a)) during , with warming center located over cold tongue (Figure 9(a)). Figure 8 The fractional linear trend distributions of convective mass fluxes in FGOALS-g2 ensemble mean (left panels) and FGOALS-s2 ensemble mean (right panels) over the three different time spans. (a), (b) For the detection period (% (105 a) 1 ); (c), (d) for the detection period (% (55 a) 1 ); (e), (f) for the detection period (% (23 a) 1 ). Trends that passed the 95% statistical confidence level are stippled.

12 12 Ma S M, et al. Sci China Earth Sci January (2014) Vol.57 No.1 Figure 9 The linear trend distributions of SST in the observations (top panels), ensemble mean of FGOALS-g2 (middle panels) and ensenmble mean of FGOALS-s2 (bottom panels) over the three different time spans. (a), (d), (g) For the detection period (K (105 a) 1 ); (b), (e), (h) for the detection period (K (55 a) 1 ); (c), (f), (i) for the detection period (K (23 a) 1 ). The dotted areas indicate that SST trends are statistically significant at the 5% level. However, the uncertainty of SST change over the tropical Pacific during the past century is large, as data cover over the tropical Pacific are sparse before 1920 (Deser et al., 2010; Tokinaga et al., 2012b, Vecchi et al., 2007; Meng et al., 2012; Solomon et al., 2013). During , warming over the cold tongue is stronger than that over the warm pool (Figure 9(b)), causing a reduced zonal SST gradient (Figure 10(b)). Over , the tropical Pacific SST change exhibits a La Niña-like pattern with warming over the western Pacific and cooling over the eastern Pacific. In FGOALS-g2, during three detection periods, warming center is located to the east of Data Line. Wherein, the change of SST gradient between warm pool and cold tongue is unobvious over the past century. Tropical SST exhibits an El Niño-like change pattern (Figure 9(d) (f)) with a induced zonal SST gradient (Figure 10). In FGOALS-s2, over the past century, the eastern equatorial Pacific shows a stronger warming than the western equatorial Pacific, but the warning over the eastern extra-equatorial Pacific is weaker than the warming over the western Pacific. Over the past 55 a and 23 a, tropical Pacific SST change exhibits a La Niñalike pattern (Figure 9(g) (i)) with an increased zonal SST gradient. Based on the above discussion about tropical circulation and SST change over the three different periods, it is suggested that WC slows down (strengthens) when the tropical Pacific shows an El Niño-like (La Niña-like) change pattern. SST change pattern may dominate the change of WC. There are some lines of evidence in previous studies supporting that SST change patterns drive the long term change of WC. Meng et al. (2012) performed atmospheric general circulation model (AGCM) by using observed SST (originate from HadISST) with La Niña-like change pattern over The results show a weakened WC responding to this SST forcing. They also chose the strongest positive and negative 100 a SST trends from PI control runs and use them to force AGCM. In response to an increasing (decreasing) of tropical zonal SST gradient, WC is enhanced (weakened). Tokinaga et al. (2012a, 2012b) suggested that WC slows down over based on a synthesis of related climate variables in the observations. However, the uncertainty in observed SST warming patterns is large. The merged surface temperature (MST) trend of the bucket SST and nighttime marine surface air temperature features an El Niño-like pattern over HadISST trend features a La Niña-like pattern. Change pattern in ERSST is neither El Niño-like nor La Niña-like. AGCM is separately forced by the above three SST trend patterns, as well as a spatially uniform SST increase (SUSI) pattern. MST-forced experiments produce an enhanced WC. In contrast, the HadISST-

13 Ma S M, et al. Sci China Earth Sci January (2014) Vol.57 No.1 13 forced experiments have a strengthened WC, whereas the ERSST-forced experiments show no significant change in WC. The SUSI-forced experiments simulate a slight weakening of WC. Their study suggested that the SST trend patterns are the key to WC change (Tokinaga et al., 2012a). To further understand the driving effect of SST on the change of WC, we investigate the response of tropical Pacific circulation to a given SST forcing (Figure 11). In recent three decades, the given tropical Pacific SST in AMIP experiments of two versions of FGOALS shows a warming tendency over the western Pacific and a cooling tendency over the eastern Pacific, with La Niña-like change pattern (Figure 11(a), (b)). Although AMIP experiment of FGOALSs2 has three realizations, responses of tropical Pacific atmospheric circulation to the given SST forcing are similar among realizations (figure not shown). For AMIP experiments of FGOALS-s2, PCCs of circulation variables between EM and different realizations are larger than 0.86 and statistically significant at the 1% level; this study only shows the result of EM. For La Niña-like SST forcing, tropical Pacific circulation changes are similar in both models. Change features are as follows: increased SLP over the eastern Pacific contrasts to the decreased SLP over the western Pacific, causing an increase in the zonal SLP gradient (Figure 11(c), (d)); enhanced easterly wind is apparent in response to SLP change (Figure 11(e), (f)); strengthened ascending and associated upper divergence over the western Pacific are accompanied by increased precipitation; precipitation decreases over the eastern Pacific in response to the reduction of descending and associated upper convergence (Figure 11(g) (l)). Above results indicate that intensified WC is driven by tropical Pacific La Niña-like SST change pattern. 3 Summary and discussion 3.1 Summary Figure 10 Linear trends of tropical Pacific zonal gradient of SLP, SST and convective mass fluxes (M) over the three different time spans. SLP = SLP ( W, 5 S 5 N) SLP ( E, 5 S 5 N), SST=SST ( E, 15 S 15 N) SST ( W, 20 S 5 N), M = M ( E, 20 S 20 N) M ( W, 20 S 5 N). (a) Over , unit of SLP, SST and M is hpa (105 a) 1, K (105 a) 1 and 1, respectively. (b) Over , unit of ΔSLP, ΔSST and M is hpa (55 a) 1, K (55 a) 1 and 1, respectively. (c) Over , unit of ΔSLP, ΔSST, and M is hpa (23 a) 1, K (23 a) 1 and 1, respectively. The error bars show the minimum-maximum range of trends simulated by different realizations. In this study, to examine the performance of two versions of FGOALS in reproducing the observed mean state and changes of tropical Pacific Walker circulation, the results of FGOALS 20th century historical climate simulations were analyzed. We also investigated the WC change over three different time spans simulated by two versions of FGOALS model. Based on the results of 20C3M and AMIP experiments, the mechanisms responsible for the WC changes are investigated. The major findings are summarized below: (1) Both versions of FGOALS reasonably reproduced the climatology features of the tropical Pacific Walker circulation. In the observations, the evolution of WC during the 20th century features the following characteristics: if we focus on the changes of WC prior to 2004, the WC tendency is significantly negative over the periods longer than 30as and a weakening trend over is the most significant; in recent two decades ( ), the WC change exhibits a significant positive trend. Over , FGOALS-g2 simulates a slowdown of WC with a smaller magnitude than the observations, while the FGOALS-s2 shows no significant change in WC. Over , a slowdown of WC is evident in FGOALS-g2 simulation, while FGOALS-s2 reproduces an enhanced WC. Over , an enhanced WC is evident in FGOALS-s2, while a weakening tendency of WC stands out in FGOALSg2 simulation. The discrepancy between the simulations and observations is dominated by internal variability of the coupled system, since we should not hope the simulated internal variability is in phase with the real world in time evolutions. (2) The changes of tropical Pacific WC can be explained by the hydrological cycle constraints. In FGOALS-g2, over the three different time spans, the fractional change in column-integrated water vapor is smaller (larger) than that in precipitation over the western (eastern) Pacific, causing a decrease (increase) of corresponding convective mass fluxes and hence a slowdown of WC. In FGOALS-s2, over the past century, except for the regions such as the east coast of