Scripps Institution of Oceanography, La Jolla, California. (Manuscript received 3 March 2009, in final form 15 June 2009) ABSTRACT

Transcription

1 800 J O U R N A L O F C L I M A T E VOLUME 23 Convection Parameterization, Tropical Pacific Double ITCZ, and Upper-Ocean Biases in the NCAR CCSM3. Part II: Coupled Feedback and the Role of Ocean Heat Transport GUANG J. ZHANG AND XIAOLIANG SONG Scripps Institution of Oceanography, La Jolla, California (Manuscript received 3 March 2009, in final form 15 June 2009) ABSTRACT This study investigates the coupled atmosphere ocean feedback and the role of ocean dynamic heat transport in the formation of double ITCZ over the tropical Pacific in the NCAR Community Climate System Model, version 3 (CCSM3) and its alleviation when a revised Zhang McFarlane (ZM) convection scheme is used. A hierarchy of coupling strategy is employed for this purpose. A slab ocean model is coupled with the atmospheric component of the Community Atmosphere Model, version 3 (CAM3) to investigate the local feedback between the atmosphere and the ocean. It is shown that the net surface energy flux differences in the southern ITCZ region between the revised and original ZM scheme seen in the stand-alone CAM3 simulations can cool the SST by up to 1.58C. However, the simulated SST distribution is very sensitive to the prescribed ocean heat transport required in the slab ocean model. To understand the role of ocean heat transport, the fully coupled CCSM3 model is used. The analysis of CCSM3 simulations shows that the altered ocean dynamic heat transport when the revised ZM scheme is used is largely responsible for the reduction of SST bias in the southern ITCZ region, although surface energy flux also helps to cool the SST in the first few months of the year in seasonal variation. The results, together with those from Part I, suggest that the unrealistic simulation of convection over the southern ITCZ region in the standard CCSM3 leads to the double-itcz bias through complex coupled interactions between atmospheric convection, surface winds, latent heat flux, cloud radiative forcing, SST, and upper-ocean circulations. The mitigation of the double-itcz bias using the revised ZM scheme is achieved by altering this chain of interactions. 1. Introduction This study is the second of a two-part investigation on the coupled interaction between convection, largescale atmospheric circulation, sea surface temperature (SST), and ocean circulation in the formation of double ITCZ and associated upper-ocean biases in the National Center for Atmospheric Research (NCAR) Community Climate System Model, version 3 (CCSM3), and its mitigation with a revised Zhang McFarlane (ZM) convection scheme (Zhang and McFarlane 1995; Zhang 2002), reported in Zhang and Wang (2006). Double ITCZ remains a thorny issue in coupled ocean atmosphere global climate models (Dai 2006; Lin 2007) in spite of more than a decade of research efforts (Mechoso et al. Corresponding author address: Guang J. Zhang, Scripps Institution of Oceanography, La Jolla, CA gzhang@ucsd.edu 1995). In Song and Zhang (2009, hereafter Part I) we presented a 10-yr model climatology of the CCSM3 simulations using two versions of the ZM scheme and analyzed the atmospheric feedback. It showed that while the simulation with the original ZM scheme produces a double-itcz bias, use of the revised ZM scheme dramatically reduced all biases related to the spurious double ITCZ and overly strong cold tongue in precipitation, SST, surface wind stress, ocean thermocline, upper-ocean currents, temperature, and salinity. To understand the physical mechanisms through which modifications of the convection scheme in the atmospheric model alleviate the double-itcz bias in the CCSM3, the paper investigated the impacts of convection schemes on the atmospheric forcing and potential feedback in the uncoupled atmospheric model, the Community Atmosphere Model, version 3 (CAM3). It was shown that the CAM3 with the revised ZM scheme produced less convection in the southern ITCZ region than that with the DOI: /2009JCLI Ó 2010 American Meteorological Society

2 1FEBRUARY 2010 Z H A N G A N D S O N G 801 original ZM scheme. Despite this, the stronger negative shortwave flux SST feedback and evaporation SST feedback induced by the revised ZM scheme produced less net surface energy flux into the ocean, which can cool SST if coupled with an ocean model and suppress convection in the central and eastern Pacific, thereby lessening the double-itcz bias in the coupled simulation. One should recognize, though, that the atmospheric feedback only measures the response of atmospheric surface energy flux to SST changes in uncoupled runs driven by prescribed SST. In a coupled model, with heat input and wind stress from the atmospheric feedback to the ocean, the ocean circulation and SST will change in response, which in turn may affect the atmosphere. For instance, shortwave cloud forcing from the atmosphere and surface latent heat flux can change the local SST. Surface wind stress and its curl can change the ocean circulation. Both can affect the ocean heat advection. Such feedback processes do not materialize in uncoupled atmospheric simulations. Furthermore, the feedback strengths of the net surface energy flux and its components may be different in the coupled model from that in the stand-alone atmospheric model (Sun et al. 2009). Therefore, this paper will investigate the roles of coupled atmosphere ocean feedback and the ocean dynamic transport in the formation of double ITCZ. The paper is organized as follows: Section 2 briefly describes the model, the convection schemes, the simulations, and the observational data used to compare with model results; section 3 investigates the role of local coupling between the atmosphere and the ocean using a slab ocean model (SOM) coupled with the CAM3; section 4 examines the role of ocean heat transport in the improved ITCZ simulation; section 5 identifies a mechanism of coupled feedback; and section 6 summarizes the results. 2. Model, convection schemes, and data A detailed description of the coupled global climate system model CCSM3, convection schemes, and experimental design were given in Part I. Two convection schemes the original ZM scheme and the revised ZM scheme are used in this study. The original ZM convection scheme uses convective available potential energy (CAPE)-based closure, which assumes that convection consumes the CAPE with a relaxation time of 2 h, while the revised ZM scheme is closed using the large-scale generation of CAPE in the free troposphere. The two primary experiments analyzed here are referred to as CTL and RZM, respectively. In the CTL, the standard CCSM3 configuration (i.e., the original ZM convection scheme) is used, whereas in the RZM the revised ZM convection scheme is used. The same 10-yr averages for years 3 12 of the 12-yr simulations with the CCSM3 that were presented in Part I are used here to examine the double-itcz bias. The slab ocean model experiments will be described in next section when the need arises. To identify the double-itcz bias in model simulations, the optimum interpolation SST (OISST) analysis of Reynolds et al. (2002) from 1971 to 2000, and the ocean temperature and current velocity data of the National Centers for Environmental Prediction (NCEP) Global Ocean Data Assimilation System (GODAS; see Ji et al. 1995; Behringer et al. 1998) from 1980 to 1999 are used in this study. The surface wind stress data of the NCEP Ocean Data Assimilation System (ODAS) tropical Pacific Ocean monthly analyses (Ji et al. 1995; Derber and Rosati 1989) from 1980 to 1999, and the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40; see Uppala et al. 2005) from 1980 to 2001 are also used to evaluate the model results. 3. Local coupled atmosphere ocean feedback Convection affects the atmospheric state and circulation, which in turn affect the ocean through input of heat, water, and wind stress. In Part I, it was shown that there was more heat flux going into the ocean in the spurious southern ITCZ region when the original ZM convection scheme was used than when the revised ZM scheme was used in the stand-alone atmospheric model CAM3. To illustrate this, Fig. 1 shows the annually averaged horizontal distribution of the difference in net surface energy flux into the ocean between the simulation using the revised ZM scheme and that using the original ZM scheme in the tropical central and eastern Pacific. Also shown are differences of the surface shortwave cloud forcing (SWCF) and latent heat flux, the two largest contributors to the net energy flux differences. In both ITCZ regions, particularly the southern one, there is less net heat input into the ocean when the revised ZM scheme is used. The SWCF difference shows that clouds in the simulation with the revised ZM scheme cause a greater reduction of shortwave radiative flux into the ocean than in the simulation with the original ZM scheme. This effect accounts for more than 50% of the net energy flux difference between the two simulations in the southern ITCZ region. The latent heat flux difference is comparable in magnitude to the SWCF difference, with negative values in the ITCZ regions meaning that there is more latent heat flux out of the ocean in the simulation with the revised ZM scheme.

3 802 J O U R N A L O F C L I M A T E VOLUME 23 FIG. 1. Net surface energy flux difference in CAM3 between the revised and original Zhang McFarlane convection schemes and contributions from shortwave cloud radiative forcing and latent heat flux at the surface. Positive values indicate that the ocean receives more energy with the revised ZM scheme. Contour intervals are 10 W m 22. The net energy flux difference in the subtropics mostly comes from that in the latent heat flux. In stand-alone atmospheric model simulations, the surface energy flux is not felt by the ocean because SSTs are prescribed. To understand the coupled feedback between the atmosphere and the ocean and its effect on SST and ITCZ simulation, we couple the CAM3 to a SOM available from the CAM3.0 release (Collins et al. 2004) and carry out two sets of simulations. In each set, two simulations are performed: one with the original ZM scheme and the other with the revised ZM scheme. In the SOM, the ocean temperature is a prognostic variable, which is allowed to fully interact with the surface energy flux. The geographic structure of ocean mixed layer depths is specified from Levitus et al. (1998). Because of the lack of ocean dynamics (current), an internal ocean mixed layer heat flux (Q flux), emulating the horizontal ocean heat transport and deep-water exchange, is specified. In the first set of simulations, the seasonally varying, monthly mean internal ocean mixed layer heat flux is specified from the observations of Large and Yeager (2004) from 1984 to 2004 to represent the dynamic ocean heat transport. The model is integrated for 10 yr in each simulation using both the original and revised ZM schemes. We will refer to these simulations as CTLSOM_obs for the original ZM scheme and RZMSOM_obs for the revised ZM scheme, respectively. In the second set of model integrations, referred to as CTLSOM_cpl and RZMSOM_cpl for the original and revised ZM schemes, respectively, the seasonally varying internal ocean mixed layer heat flux is specified from a 5-yr simulation of the CCSM3 CTL run, which represents the average ocean dynamic heat transport in the CTL run. The difference between the two sets of simulations demonstrates the sensitivity of the intermediately coupled model-to-ocean heat transport. Figure 2 shows the annual mean SST and precipitation distributions in the tropical Pacific for CTLSOM_obs and RZMSOM_obs and their differences. In both simulations the SSTs are qualitatively similar to those in the observations [e.g., warm SSTs in the warm pool and along the ITCZ and South Pacific convergence zone (SPCZ)], with those from the RZMSOM_obs colder in the southern ITCZ region by 1.58C. The SST differences between the simulations with the original and revised ZM schemes are comparable to those in the fully coupled model simulations shown in Part I. Thus, surface energy flux feedback can indeed help cool the SST in the southern ITCZ region when locally coupled to the ocean model. The precipitation distributions show that the original ZM scheme has a tendency to produce more precipitation south of the equator in the central Pacific than the revised ZM scheme, more so than in the standalone atmospheric model presented in Part I. When the simulations use prescribed ocean heat transport from the CCSM3 CTL run, as shown in Fig. 3, the SST and precipitation distributions are much more severely distorted from the observations than those shown in Fig. 2. The SSTs in the central and eastern Pacific south of the equator are much warmer than observed, and the precipitation distributions show strong double-itcz bias in both runs. This indicates that ocean dynamic heat transport plays a much more important role in causing the double-itcz biases than local surface energy flux.

4 1FEBRUARY 2010 Z H A N G A N D S O N G 803 FIG. 2. Annual mean (left) SST (8C) and (right) precipitation (mm day 21 ) from (a),(b) CTLSOM_obs, (c),(d) RZMSOM_obs, and (e),(f) their respective differences (RZMSOM_obs 2 CTLSOM_obs). Shadings are for SST. 298C, precipitation.7 mm day 21, and positive contours in the difference plots. Nevertheless, the differences between the simulations using the original and revised ZM schemes show, again, that the revised ZM scheme helps to reduce the positive SST and precipitation biases in the southern ITCZ region, regardless of the prescribed ocean dynamic heat transport. Figure 4 shows the seasonal variation of the differences in SST and net surface energy flux into the ocean between RZMSOM_cpl and CTLSOM_cpl, averaged over the southern ITCZ region (58 108S, W). On the annual average, SST with the revised ZM scheme is colder by 0.88C than that with the original ZM scheme.

5 804 J O U R N A L O F C L I M A T E VOLUME 23 FIG. 3. Same as Fig. 2, but the internal ocean mixed layer heat flux is specified from a 5-yr simulation of the CCSM3 instead of observations. Because there is no difference in ocean heat transport in the two runs, the SST differences are entirely due to differences in surface energy fluxes. The SST variation lags the surface flux variation by about 2 months. There is as much as 50 W m 22 less heat input into the ocean in February and March in the RZMSOM_cpl, which leads to a sharp relative decrease in SST, by 1.28C from January to April. This is similar to that seen in Zhang and Wang (2006), who attributed the early cooling in the fully coupled CCSM3 integration to less surface energy flux with the revised ZM scheme. Afterward, the net energy flux difference becomes positive from May to December, leading to a gradual increase in SST during this period. Most of the energy flux differences from

6 1FEBRUARY 2010 Z H A N G A N D S O N G 805 FIG. 4. Seasonal variation of differences in SST, shortwave cloud forcing, latent heat flux, and net surface energy flux into the ocean between RZMSOM_cpl and CTLSOM_cpl, averaged over the southern ITCZ region (58 108S, W). January to April are due to shortwave cloud forcing. From May to December, both shortwave cloud forcing and latent heat flux differences contribute about equally. Other components (differences in clear-sky shortwave flux, longwave flux, and sensible heat flux) in the net energy flux differences are small and thus are not shown in the plot. Note that on annual average, the net surface energy flux difference is zero, and the SST difference is negative between the two runs. This is because at equilibrium in each simulation, as in the case of the annual average, surface energy flux must balance the prescribed mixed layer ocean heat transport, which is identical in both runs. On the other hand, the difference in equilibrium SSTs between the two slab ocean coupled runs depends on the net surface energy flux difference of the uncoupled runs and the feedback of the net surface energy flux. Because in the stand-alone CAM3 runs the revised ZM scheme produces W m 22 less surface energy flux into the ocean than the original ZM scheme in the region (Fig. 1), the slab ocean responds with a colder SST; and because the net surface energy flux feedback to SST is negative (see Part I), cooling in SST will give rise to increased surface energy flux into the ocean, thus lessening the cooling. In the end, when the equilibrium is reached, the SST is colder in the run with the revised ZM scheme than with the original ZM scheme, as seen in Fig Role of ocean heat transport In the last section, it was shown that with reduced net energy flux into the ocean using the revised ZM scheme, the local coupled feedback between surface energy flux and SST can reduce the warm SST bias in the southern ITCZ region by up to 1.58C. However, the SST simulation is very sensitive to the prescribed ocean heat transport, indicating the important role of ocean dynamic transport in fully coupled model simulation of SST and ITCZ. Thus, in this section, we analyze the ocean mixed layer heat budget with a focus on ocean heat transport. Because most significant improvement in double-itcz and SST biases in the fully coupled CCSM3 simulation using the revised ZM convection is in boreal summer, we will show the results for June August (JJA) only, unless stated otherwise. Figure 5 shows the JJA mean SST distribution from OISST observations, and CTL and RZM simulations. Both observed and RZM-simulated SST show that the warmest SST is located in the western Pacific warm pool and extends out along the ITCZ and SPCZ, with the 288C SST contour located west of 1508W in the Southern Hemisphere. The CTL (Fig. 5b) produces large warm biases in the tropical Pacific south of the equator, where the warm pool extends eastward zonally between 58 and 108S to the eastern Pacific such that an anomalous warm-water tongue with maximum SST exceeding 298C appears in the central and eastern Pacific. In addition, the cold tongue in the CTL is also stronger and is displaced approximately 208 west of the observed position. The SST distribution in the RZM run is closer to that observed, although it is slightly colder west of the date line. Compared to the CTL, the SST simulated by the RZM is colder by about 18C in the southern ITCZ

7 806 J O U R N A L O F C L I M A T E VOLUME 23 FIG. 6. (a) Difference of net surface energy flux (thick solid line), sensible (dash dotted line), latent (dotted line), longwave (dashed line), and shortwave (thin solid line) heat fluxes between RZM and CTL, averaged over S; and (b) difference of temperature forcing from net surface energy flux (dash dotted line), entrainment (dotted line), zonal advection (solid line), and meridional advection (dashed line) between RZM and CTL, averaged over S for JJA. FIG. 5. JJA mean sea surface temperature (8C) from (a) OISST, (b) CTL, and (c) RZM. SSTs. 288C are shaded. The two dashed lines show the latitude bounds (58 108S) of the double-itcz region. region (58 108S, W) in boreal summer, which is the main reason for the alleviation of spurious precipitation there in the RZM run. How is this reduction of SST over the southern ITCZ region in the RZM simulation related to the net surface energy flux and ocean heat transport? Figure 6a shows the JJA differences of the net surface energy flux, shortwave, longwave, and sensible and latent heat fluxes between RZM and CTL averaged over S. The sensible heat flux difference shows small positive values, indicating less loss of sensible heat from ocean in the RZM resulting from the colder SST. This is to some extent compensated by the longwave radiative flux difference. The latent heat flux difference is mostly negative in the central and eastern Pacific east of 1608W, indicating that more latent heat is lost from the ocean (resulting from stronger surface winds, as will be seen later) in the RZM. The shortwave flux differences show strong positive values east of the date line, which are associated with a decrease in clouds resulting from reduced convection in the RZM. To sum up, there is more net surface energy flux (up to 25 W m 22 ) into the ocean in the RZM than in the CTL between the date line and 1208W. Thus, the surface energy flux differences in the fully coupled CCSM3 runs cannot explain the colder SSTs in the southern ITCZ region when the revised ZM scheme is used. The ocean dynamic heat transport must have played an important role here. To quantify the ocean dynamic processes involved in SST bias over the southern ITCZ region, an analysis of ocean mixed layer heat budget is conducted over the S latitude band. The equation for the mixed layer temperature can be written as (Swenson and Hansen 1999)

8 1FEBRUARY 2010 Z H A N G A N D S O N G 807 T t 5 Q Q s ( h) u T T y r 0 c p h x y w e DT, (1) h where T is the mixed layer temperature (a proxy of SST), h is the mixed layer depth, w e is the entrainment velocity (the vertical velocity at the mixed layer base), DT is the temperature difference between the mixed layer and just below the mixed layer, r 0 is the reference density of seawater, and c p is the specific heat of seawater at constant pressure. Here Q denotes the net surface energy flux, which is positive into the ocean, and Q s (2h) is the downward radiative energy flux at the bottom of the mixed layer. Respectively, u and y are the velocities of zonal and meridional currents. Horizontal diffusion has been omitted. Thus, the four terms on the right-hand side of Eq. (1) represent the net energy flux into the mixed layer, zonal advection, meridional advection, and entrainment, respectively. Each of the terms is computed and output from the ocean component model of the CCSM3, and averaged over the simulated mixed layer depth h, which varies spatially. Figure 6b shows the differences in the four terms between RZM and CTL, averaged over the S latitude band. West of the date line, surface energy flux difference causes relative cooling, and horizontal temperature advection causes relative warming in the RZM. In the central and eastern Pacific (east of the date line), the differences between temperature forcing from the meridional advection and entrainment are mostly out of phase; although significant, they largely cancel each other. This leaves the zonal advection to be the dominant relative cooling term, while the dominant relative warming is produced by surface energy flux. Further examination of the 3D ocean circulation (not shown) in the region between the two simulations finds that relative to the CTL, the RZM has downward water motion at the base of the mixed layer, mass divergence in the zonal direction, and convergence in the meridional direction. The meridional convergence of cold water relative to the warm water in S reflects the negative meridional temperature advection seen in Fig. 6b. Clearly, the relative cooling of SST in the RZM, or equivalently the warming of SST in CTL over the southern ITCZ region and thereby the double-itcz bias, is mainly caused by ocean heat transport. This does not mean that the surface energy flux difference has no contribution to the alleviation of SST biases in other seasons. Figure 7 shows the seasonal variation of differences in SST, net surface energy flux, and ocean heat transport. West of 1208W, the SST difference is mostly negative throughout the year except near the date line, where it is positive from September to December. The largest SST differences are west of FIG. 7. Seasonal variation of difference in (a) SST (8C), (b) surface energy flux (W m 22 ), and (c) ocean temperature advection (8C month 21 ) between RZM and CTL averaged over S.

9 808 J O U R N A L O F C L I M A T E VOLUME W from February to July, and between 1608 and 1408W from August to October. The surface energy flux differences are negative from January to April and positive from May to December in the central Pacific; in the eastern Pacific east of 1208W, it is mostly negative except a brief period around May; and near the date line, it is positive from July to October and negative from November to next June. Clearly, these surface flux differences are responsible for the SST differences in the first half of the years in most of the region west of 1208W as well as (or at least partly) the SST difference near the date line in the second half of the year. The ocean heat advection difference is small and positive in January March in the central and eastern Pacific except near the date line, where positive advection is more significant. From April to December in the central Pacific, particularly between 1708 and 1408W, there is strong negative temperature forcing through heat transport. It is solely responsible for the cooling of SST seen in the second half of the year in the central Pacific. The results from Figs. 6 and 7 suggest that for JJA ocean heat transport plays an important role in SST simulation in the southern ITCZ region. On seasonal variation, however, the role of surface energy flux dominates from January to March, and the ocean heat transport dominates from April to December. 5. Mechanism of coupled feedback The net surface energy flux into the ocean and surface wind stress are two of the most important atmospheric factors that can affect the ocean simulation. The importance of ocean heat transport implies that the revised ZM convection scheme alleviates the warm bias in SST by modifying the surface wind stress, which drives the ocean circulation. How does the revised ZM convection scheme affect the surface wind stress, which in turn affects the temperature advection? To answer this question, the longitudinal cross sections of the atmospheric and ocean circulations and zonal surface wind stress from CTL, RZM, and the reanalyses for JJA are shown in Fig. 8. Convection and the rising branch of the Walker circulation in the CTL are located in the western and central Pacific between 1608E and 1508W (Fig. 8a). The associated convergent flow in the lower troposphere superposed to the background climatological easterly winds leads to weak easterly trades in the convection region and strong easterly trade winds to its east (1408W and eastward). The RZM simulation (Fig. 8b) shows that the rising branch of the Walker circulation is located west of 1808E over the warmest SST and the sinking branch is located east of 1508W. The easterly trade winds in the boundary layer corresponding to this Walker circulation bring moist surface air to the west to fuel further convection. ERA-40 (Fig. 8c) shows a similar Walker circulation pattern to that in the RZM, with the rising branch located west of the date line. Corresponding to the weak easterly winds in the central and western Pacific, the easterly surface wind stress in the CTL is about half of that in the RZM between 1808E and 1408W (Figs. 8d,e). This extremely weak easterly wind stress leads to the eastward retreat of the observed South Equatorial Current (SEC). In its place a strong eastward South Equatorial Countercurrent (SECC) between 1608E and 1308W is developed (Fig. 8g), which transports warm water from the western Pacific warm pool to the east, resulting in warm temperature advection in the western and central Pacific. On the other hand, the RZM simulation produces much stronger easterly wind stress, comparable to that in ERA-40 (Fig. 8f). This stronger surface wind stress drives a westward SEC across the Pacific (Fig. 8h), transporting cold water to the west, similar to the observation-based assimilation product (Fig. 8i). This partly explains why the RZM run produces a relatively cold zonal temperature advection in the southern ITCZ region. Another factor, probably a more important one, is the difference in wind stress curl. As will be shown below, the wind stress curl induced by the enhanced easterly wind stress in the RZM acts to weaken the eastward SECC, thereby contributing to the relatively cold ocean temperature advection in the southern ITCZ region. The observation- and reanalysisbased surface wind stress, ocean circulation, and temperature are all close to those in the RZM. Because the SECC in the western and central Pacific directly contributes to the warm SST biases by advecting the warm pool water eastward in the CTL run, it is of great importance to examine the mechanism of the formation of SECC in the coupled simulation. The surface countercurrent is a direct response of the ocean to the curl of surface wind stress (Sverdrup 1947; Wyrtki and Meyers 1975) through Ekman pumping, which is proportional to the surface wind stress curl. A negative (positive) wind stress curl induces upwelling of colder ocean water and hence the ridging of the thermocline in the Southern (Northern) Hemisphere. The sea level gradient and hence pressure gradient induced by the thermocline ridge then drives the current. Noting that over the tropical open ocean the wind stress curl is mainly determined by the meridional change of zonal wind stress, in Fig. 9 we show the wind stress curl and zonal wind stress, averaged over W from the CTL, RZM, and reanalyses, respectively. The ocean zonal current and temperature cross sections are also presented. In the CTL (Fig. 9a), corresponding to the decrease of easterly trade winds over the spurious southern

10 1FEBRUARY 2010 Z H A N G A N D S O N G 809 FIG. 8. JJA mean zonal wind (m s 21, shaded) and (top) wind vectors in the longitude height plane with vertical velocity in 0.5 mb day 21, (middle) zonal wind stress, and (bottom) longitude depth cross section of ocean potential temperature (shaded, 8C) and zonal current (white contours, eastward in solid and westward in dashed lines, cm s 21 ), averaged over S for (left) CTL, (middle) RZM, and (right) reanalysis products [(c) ERA-40, (f) ODAS, and (i) GODAS]. ITCZ, the zonal wind stress decreases from 0.9 to 0.2 dyne cm 22 from 138 to 58S, and then increases to 0.4 dyne cm 22 from 58S totheequator(redline),resulting in a large negative curl in the former and a positive curl in the latter belt (blue line), respectively. Responding to the wind stress curl, the upwelling and downwelling occur from 138 to 58S and from 58S to the equator, resulting in a thermocline ridge at 88S and a trough at 48S, respectively. The corresponding sea level trough and ridge at 88 and 48S (black line), and thus the resulting meridional sea level gradient, drives an eastward SECC between 88 and 48S. This process is similar to a feedback mechanism proposed by Zhang et al. (2007), although they emphasized the latitudinal change of wind direction in their wind stress curl estimate. By contrast, the zonal wind stress simulated by the RZM (Fig. 9b) decreases from 0.75 to 0.3 dyne cm 22 between 108S and 58N. However, it decreases fast south of 48S and more gradually north of it, resulting in a weak negative wind stress curl minimum and hence a weak thermocline ridge at around 78S. This leads to a weak SECC between 48 and 78S below the sea surface. The zonal wind stress in the ODAS reanalyses decreases gradually from 0.75 to 0.45 dyne cm 22 between 138Sand58N (Fig. 9c), resulting in a very weak wind stress curl. Therefore, no significant thermocline ridge or SECC develops between 158 and 48S (Fig. 9f). Comparison between the simulations and reanalyses again shows that the RZM simulation is more realistic in coupling the atmosphere and the ocean. The above analyses demonstrate that the decrease of easterly trade winds in the southern ITCZ region in the CTL resulting from convection in the central Pacific can lead to spurious SECC and hence anomalous warming of SST, which in turn can result in more convection, and

11 810 J O U R N A L O F C L I M A T E VOLUME 23 FIG. 9. JJA mean wind stress curl (10 29 dyne cm 23 ) and zonal wind stress (dyne cm 22 ) averaged over W for (a) CTL, (b) RZM, and (c) ODAS, with sea surface height (m) also plotted for (a) and (b). Latitude depth cross section of JJA mean ocean potential temperature (shaded, 8C) and zonal current (white contours, eastward in solid and westward in dashed lines, cm s 21 ), averaged over W for (d) CTL, (e) RZM, and (f) GODAS. thus a further decrease of easterly trade winds in the central Pacific. Additionally, the decrease of easterly trade winds west of 1408W leads to less latent heat flux into the atmosphere in the CTL (as seen in Fig. 6a), which can also increase the SST in the western and central Pacific. This positive coupled feedback loop among convection, surface evaporation, surface wind stress, upper-ocean current, and SST leads to a warm SST bias and spurious southern ITCZ. When the revised ZM scheme is used, convection in the southern ITCZ region is suppressed because of the more stringent criteria for deep convection (Song and Zhang 2009). The moisture is transported westward from the central Pacific to the SPCZ over the western Pacific warm pool by the easterly trade winds (Fig. 8b). The increased moisture convergence over the warm pool then can produce more convection, leading to a stronger Walker circulation and thus stronger easterly winds in the lower troposphere. The intensified easterly trade winds lead to more evaporation and the associated wind stress curl weakens the SECC, thereby breaking the positive feedback loop among convection, surface evaporation, surface wind stress, upper-ocean current, and SST so that the double-itcz bias is alleviated in the RZM run. It is worth noting that the results shown in Figs. 6 9 represent the differences of two equilibriums states, because they are averages over the last 10 yr of the 12-yr-long simulations. The transition from the beginning of the coupled simulations, when the atmospheric and ocean states are identical, to the different equilibrium states is fast within the first year or so (Zhang and Wang 2006). During the transition period, the relative importance of surface energy flux and ocean heat transport may be different from what is shown above. 6. Summary In Part I of this two-part investigation, it was shown that the following three factors from the atmospheric side of the coupled model CAM3 contributed to the differences in the model simulations: 1) sensitivity of convection to SST for different SST ranges, 2) convection shortwave flux SST feedback, and 3) convection wind evaporation feedback. Convection is favored in the double-itcz region when the original ZM convection scheme is used. Both lack of strong negative shortwave flux SST feedback and positive evaporation SST feedback further amplify this tendency. As the second part of this study, we analyzed the coupled atmosphere ocean feedback and the role of ocean dynamic heat transport in the formation of double ITCZ in the Pacific in the standard CCSM3 simulation when the original Zhang McFarlane convection scheme is used, and in its alleviation when the revised ZM scheme is used. To investigate

12 1FEBRUARY 2010 Z H A N G A N D S O N G 811 the local feedback between the atmosphere and ocean, CAM3 is coupled to a slab ocean model. Simulations using this intermediately coupled model show that the net surface energy flux differences in the southern ITCZ region between the revised and original ZM scheme, as seen in the stand-alone CAM3 simulations, can cool the SST by up to 1.58C, regardless of the prescribed ocean mixed layer heat transport required by the slab ocean model approach. However, the simulated SST distribution is very sensitive to the prescribed ocean heat transport, indicating the important role of the ocean dynamic process. To further understand the coupled atmosphere ocean interaction, we analyzed the ocean mixed layer heat budget in the fully coupled CCSM3 simulations for boreal summer months, when the double-itcz difference is the largest in the simulations using the original and revised ZM scheme. The SST cooling in the RZM simulation relative to the CTL simulation is found to be due to cold temperature advection, instead of surface energy flux, as in the case of the slab ocean model. Nevertheless, seasonal variation of surface energy flux and ocean heat transport indicates that surface energy flux cools the SST in the first few months of the year and ocean heat transport is responsible for the cooling from June to December. Further analysis shows that a convection wind ocean current SST feedback mechanism, which is summarized below, is responsible for the double-itcz bias. The atmospheric interaction loop identified in Part I favors convection over the southern ITCZ region in the central Pacific when the original ZM scheme is used. The convergence of surface winds induced by convection reduces easterly trade winds west of the initial convection region. The reduced easterly has two effects on ocean circulation. First, it can weaken the westward South Equatorial Current (SEC). Second, by modifying the wind stress curl, it can produce eastward South Equatorial Countercurrent (SECC). Both of these effects help transport warm water eastward to the central Pacific to increase the SST and further strengthen convection there. When the revised ZM convection scheme is used, the convection in the southern ITCZ region is suppressed resulting from the more stringent trigger condition for deep convection. The moisture is transported westward from the central Pacific to the western Pacific warm pool by the easterly trade winds. The increased moisture convergence over the warm pool then produces more convection, leading to a stronger Walker circulation and thus stronger easterly winds in the lower troposphere. The intensified easterly trade winds suppress (or break) the positive convection wind current SST feedback, leading to the mitigation of the double- ITCZ bias in the RZM run. Acknowledgments. This research was supported by the Biological and Environmental Research Program (BER), U.S. Department of Energy Grant DE-FG02-09ER64736, and by the U.S. National Science Foundation Grant ATM The computational support was provided by the Scientific Computing Division of the National Center for Atmospheric Research. The authors thank Professor Xuehong Zhang and an anonymous reviewer for their valuable comments. REFERENCES Behringer, D. W., M. Ji, and A. Leetmaa, 1998: An improved coupled model for ENSO prediction and implications for ocean initialization. Part I: The ocean data assimilation system. Mon. Wea. Rev., 126, Collins, W. D., and Coauthors, 2004: Description of the NCAR Community Atmosphere Model (CAM3.0). NCAR Tech. Note NCAR/TN-4641STR, 226 pp. [Available online at Dai, A., 2006: Precipitation characteristics in eighteen coupled climate models. J. Climate, 19, Derber, J. C., and A. Rosati, 1989: A global oceanic data assimilation system. J. Phys. Oceanogr., 19, Ji, M., A. 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