1. Introduction
The ocean–atmosphere system displays large zonal and meridional variations over the tropical Pacific. The equatorial easterly wind stress generates upwelling of cold water, resulting in the equatorial Pacific cold tongue that suppresses atmospheric convection/precipitation. Positive ocean–atmosphere feedback proposed by Bjerknes (1969) is important for the equatorial cold tongue. The tropical Pacific climatology also exhibits pronounced north–south asymmetry with intense precipitation and warm sea surface temperature (SST) in the Northern Hemisphere (NH) intertropical convergence zone (ITCZ). Ocean–atmosphere interactions, such as wind–evaporation–SST (WES) (Xie and Philander 1994) and stratus–SST (Philander et al. 1996) feedbacks, are important for maintaining the northward-displaced ITCZ.
Generations of coupled general circulation models (CGCMs) used for climate predictions and projections suffer from large errors in simulating the equatorial Pacific cold tongue and ITCZ for nearly two decades (Mechoso et al. 1995; Yu and Mechoso 1999; Davey et al. 2002; Meehl et al. 2005; Dai 2006; Lin 2007; Randall et al. 2007; de Szoeke and Xie 2008). Most CGCMs suffer from a cool SST error and excessive westward extension of the equatorial Pacific cold tongue in comparison to observations. In a recent heat budget analysis, Zheng et al. (2012) examined 15 coupled simulations from phase 3 of the Coupled Model Intercomparison Project (CMIP3) and attributed the equatorial Pacific cold tongue error to too strong zonal currents as well as excessive oceanic upwelling and the vertical temperature gradient associated with overly strong surface zonal winds. However, whether the surface wind errors in the coupled models are due to biases of the atmospheric models or come from ocean–atmosphere feedback is still unclear.
Most of the state-of-the-art CGCMs suffer from the so-called double ITCZ problem (Lin 2007; Hirota et al. 2011; Hwang and Frierson 2013). Excessive precipitation in the Southern Hemisphere (SH) corresponds to a SH counterpart to the strong NH ITCZ, despite decades of hard work by modeling centers; for example, by modifying the convection parameterization scheme (Hess et al. 1993; Frey et al. 1997; Zhang and Wang 2006; Song and Zhang 2009; Chikira 2010; Zhang and Song 2010), changing the surface wind stress formulation (Luo et al. 2005), or increasing the model resolution (Mechoso 2006). The cause of the double ITCZ problem in CGCMs is complex. Many studies attribute this error to tropical origins, including a poor representation of ocean–atmosphere feedbacks (Lin 2007), unrealistic winds in the eastern Pacific warm pool because of the complex Central American orography along with improper southern tropical low-level clouds (de Szoeke and Xie 2008), unrealistic SST threshold leading to the onset of deep convection (Belluci et al. 2010), improper entrainment effect (Hirota et al. 2011), and/or unrealistic alongshore winds at Chile and Peru associated with the orography of the Andean cordillera (Zheng et al. 2011).
The recent work of Hwang and Frierson (2013) argues that cloud biases outside the tropics contribute to the double ITCZ problem, consistent with the theoretical prediction that the tropical precipitation shifts toward the hemisphere receiving extra heat from ocean or sun (Kang et al. 2008, 2009). Although Hwang and Frierson’s (2013) work reveals a robust statistical relationship between the tropical double ITCZ problem and extratropical cloud biases in CGCMs, many issues remain unresolved. For example, whether the extratropical cloud biases are of atmospheric origin or as a result of ocean–atmosphere feedback is still unclear. Once the interhemispheric asymmetry biases in the tropics are initiated by the extratropical cloud biases, how do tropical ocean–atmosphere feedbacks amplify and maintain them? While previous studies focus separately on the Pacific (e.g., de Szoeke and Xie 2008) and Atlantic (e.g., Breugem et al. 2006; Richter and Xie 2008) double ITCZ biases, it is unclear whether the basin-scale precipitation errors are limited to individual tropical basins or coherently connected to global biases Hwang and Frierson (2013) identified. There are physical reasons for developing global-scale errors in the tropics. For example, tropospheric temperature is nearly uniform in space within the tropics because of fast equatorial waves (Sobel and Bretherton 2000; Sobel et al. 2002; Johnson and Xie 2010). In addition, the seasonal cycle of extratropical cloud radiative forcing and tropical double ITCZ biases has not been studied.
Forced with observed SST, Atmospheric Model Intercomparison Project (AMIP) simulations can be used to isolate biases in atmospheric models and relate them to those of coupled models. By comparing pairs of CMIP and AMIP runs, Li and Xie (2012) show that tropical mean SST biases in CGCMs originate from cloud biases in atmospheric models. The present study adopts this method to isolate the sources of major model biases in the tropical Pacific; that is, in the cold tongue and ITCZ.
The present study examines the nature and sources of tropical Pacific precipitation pattern biases in 18 CGCMs from phase 5 of CMIP (CMIP5). We show that precipitation errors in the equatorial Pacific cold tongue and ITCZ remain large in CMIP5 climate models. By comparing pairs of CMIP and AMIP simulations, we examine to what extent these errors can be traced back to biases of atmospheric models. Regarding the equatorial Pacific cold tongue, our study reveals that the deficient precipitation and easterly wind biases are absent in AMIP simulations and result from the interaction with SST/thermocline. We find that errors of excessive shortwave radiation resulting from insufficient cloud radiative forcing in the SH midlatitudes originate from atmosphere-only models. The double ITCZ biases are of the global scale and most pronounced during the SH rainy season to balance the interhemispheric heating asymmetry. In particular, WES feedback mediates the extratropical cloud radiative forcing to influence the ITCZ.
The CMIP5 CGCMs are becoming widely used for global climate predictions and projections (Taylor et al. 2012). Large errors in simulation of the Pacific cold tongue and ITCZ mean limit the skills of CGCMs in simulating El Niño–Southern Oscillation (ENSO) and its teleconnections (Latif et al. 2001; Guilyardi 2006; AchutaRao and Sperber 2006; Wittenberg et al. 2006; Misra et al. 2008; Ham and Kug 2012). Our study highlights physical processes for the two types of model biases and will help reduce these errors in future model development/improvement efforts by the modeling community.
The rest of this paper is organized as follows: Section 2 describes the models and datasets used in this study. Section 3 isolates the equatorial cold tongue and double ITCZ problems in tropical Pacific precipitation pattern in CMIP5 multimodel ensemble by using an intermodel empirical orthogonal function (EOF) analysis (Li and Xie 2012). Sections 4 and 5 further investigate the equatorial Pacific cold tongue and double ITCZ biases, respectively. Section 6 is a summary with discussion.
2. Models and datasets
We examine the 30-yr (1970–99) climate of the historical runs from the World Climate Research Programme (WCRP) CMIP5 multimodel ensemble (Taylor et al. 2012), together with the available AMIP simulations. Monthly mean outputs from 18 CMIP5 CGCMs are used, including precipitation, surface winds, SST, latent heat flux (LHF), sensible heat flux (SHF), and shortwave and longwave radiation fluxes at the sea surface; ocean temperature; shortwave and outgoing longwave radiation in all sky; and outgoing shortwave/longwave radiation in clear sky at the top of the atmosphere (TOA). Table 1 shows the model names, modeling centers, and whether the AMIP simulations are available. Further information on individual models is available online (at http://www-pcmdi.llnl.gov/; Taylor et al. 2012). Here, we only use one ensemble member (“r1i1p1”) run for each model.
A list of the 18 CMIP5 climate models used in this study and the labels to denote them in the figures and text.
For comparison, we also examine both the observed and reanalyzed (assimilated) datasets (for simplicity referred to as observations). Specifically, the precipitation climatology is obtained from the Global Precipitation Climatology Project (GPCP; Adler et al. 2003) and Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP; Xie and Arkin 1997) datasets for 1979–2008. The results from the two precipitation datasets are very similar, and only the results from the GPCP precipitation dataset are shown except in Fig. 1. The SST climatology is gained from the National Oceanic and Atmospheric Administration (NOAA) optimum interpolation sea surface temperature (OISST) analyses (Reynolds et al. 2002) for 1982–2011. The surface wind observations are the climatology from August 1999 through August 2009 based on monthly Quick Scatterometer (QuikSCAT) satellite retrievals (http://www.remss.com) and from the National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) reanalysis project for 1970–99 (Kalnay et al. 1996). Although the two surface wind products cover different time periods, the climatology is very similar. Only the results from the QuikSCAT retrievals are shown, unless otherwise specified. In addition, the ocean temperature from the Simple Ocean Data Assimilation (SODA) reanalysis (Carton and Giese 2008) from 1970 to 1999 is used. Finally, the observed energy fluxes at the TOA for 2001–11 are from Clouds and the Earth’s Radiant Energy System (CERES) Energy Balance and Filled (EBAF) dataset (Wielicki et al. 1996). Here, we do not use observed surface heat flux datasets because of large uncertainties (Yu and Weller 2007; Zheng et al. 2012). All model outputs and observational datasets are interpolated to a uniform 2° × 2° horizontal grid, unless otherwise specified.
Zonally and annually averaged (a) precipitation and (b) SST in the tropical Pacific (120°E–80°W) in observations (black lines) and 18 CMIP5 CGCMs (colored lines). (c) As in (a), but for observations and 11 AMIP simulations. Here, the precipitation and SST for observations and each model are normalized by their respective tropical means (20°S–20°N).
Citation: Journal of Climate 27, 4; 10.1175/JCLI-D-13-00337.1
3. The leading bias modes
Figures 1a and 1b show the zonally and annually averaged precipitation and SST in the tropical Pacific (120°E–80°W) for observations (black lines) and 18 CMIP5 CGCMs (colored lines), respectively. Here, we focus on the model biases in tropical Pacific precipitation and SST distributions by normalizing with their respective tropical means (20°S–20°N) for observations and each model. Although the CGCMs qualitatively simulate the meridional profiles of annual mean precipitation and SST in the tropical Pacific, there are two important types of biases to note. First, most CMIP5 CGCMs still suffer from the insufficient precipitation and cool SST biases on the equator; that is, an excessive equatorial Pacific cold tongue as in the previous generations of CGCMs. Second, while observations show a strong interhemispheric asymmetry of precipitation and SST with one single NH ITCZ, most CMIP5 CGCMs produce excessive precipitation in the 4°–10°S band and deficient precipitation in the 4°–8°N band in the tropical Pacific. This is the well-known double ITCZ problem. These two types of biases in precipitation are not significant in the AMIP simulations (Fig. 1c).
Although these results are similar to the findings in earlier studies (e.g., Lin 2007), the meridional profiles normalized by the tropical means in Fig. 1 exclude the effects of the tropical mean precipitation and SST offset biases in the CGCMs (Li and Xie 2012) and highlight the model biases in spatial pattern. For instance, while the absolute precipitation values on the equator in CGCMs are symmetrically distributed around observations within a range of about 2 mm day−1 (Fig. 2b of de Szoeke and Xie 2008), equatorial precipitation in Fig. 1a is systematically lower than observations by excluding the systematical overestimation of tropical mean precipitation in models relative to observations. This tropical mean error may be a result of observational uncertainties (Trenberth et al. 2009; Stephens et al. 2012) or biases of deep convection schemes in the atmospheric models (Lin 2007). Similarly, while the absolute model SSTs in the tropical South Pacific are symmetrically distributed around observations within a range of about 2°C (Fig. 2a of de Szoeke and Xie 2008), SSTs in 4°–10°S in Fig. 1b are systematically higher than observations by excluding the tropical mean offset, which may be a result of biases in atmospheric model representation of cloud cover for the tropics (Li and Xie 2012). This reduced SST asymmetry explains why the model precipitation is systematically overestimated in the tropical South Pacific. In fact, the onset of tropical deep convection in CGCMs is not controlled solely by local SST itself, but depends on the difference between the local and tropical mean SSTs (Xie et al. 2010; Johnson and Xie 2010), because the tropical free-tropospheric temperature is more uniform in space than SST, flattened by fast communications by equatorial waves (Sobel et al. 2002).
To isolate the dominant patterns of tropical Pacific precipitation biases, we carry out an intermodel EOF analysis. Instead of temporal variance in conventional EOF analysis, our EOF analysis maximizes the variance of precipitation differences among CMIP5 models. First, the annual mean precipitation for each CMIP5 CGCM is normalized by their respective tropical mean values. Then, the intermodel EOF analysis is applied to the zonally averaged precipitation deviations from the multimodel ensemble (MME) mean over the tropical Pacific. Figure 2 shows the regressed patterns of precipitation, SST, and surface winds onto the first two principal components (PCs) as well as the PCs. When the intermodel EOF analysis is applied to two-dimensional (x–y) normalized precipitation anomalies, the leading EOF modes are very similar (figure not shown).
Regression patterns of normalized precipitation (color shaded), normalized SST (contours), and surface wind velocity (m s−1; wind speed smaller than 0.2 m s−1 has been masked out) onto the (a) first and (b) second PCs of intermodel variability in zonally and annually averaged normalized precipitation over the tropical Pacific in 18 CMIP5 CGCMs. (c) The first two PCs. The explained variances are given in parentheses at the top of (a),(b).
Citation: Journal of Climate 27, 4; 10.1175/JCLI-D-13-00337.1
a. EOF1
The first intermodel EOF mode (EOF1; Fig. 2a), explaining 57.6% of total intermodel variability, captures the deficient precipitation and cool SST in the equatorial Pacific, flanked by the excessive precipitation and warm SST on the flanks. Spatial correlation between precipitation and SST patterns is 0.56, illustrating a strong control of SST pattern on annual mean tropical Pacific precipitation, consistent with the previous studies (Xie et al. 2010; Ma and Xie 2013). In the previous study (Li and Xie 2012), we applied the intermodel EOF analysis to annual mean SST climatology (nonnormalized) over tropical oceans in the CMIP3 multimodel ensemble. While the first mode characterizes the tropical mean SST offset bias, the second mode reflects the equatorial cold tongue SST bias, similar to Fig. 2a that normalizes with the tropical mean to isolate pattern biases. The easterly wind anomalies are found in the western half of the equatorial Pacific, acting to enhance the equatorial eastern Pacific SST cooling by shoaling (deepening) the ocean thermocline in the east (west), suggestive of Bjerknes feedback in the excessive equatorial Pacific cold tongue.
Models with high PC1 values would have a large precipitation deficit in the equatorial Pacific cold tongue. Here, we choose three models of the highest PC1 values as the strong equatorial cold tongue (EQ−) models (M4, M15, and M16) and three models of the lowest PC1 values as the weak equatorial cold tongue (EQ+) models (M1, M8, and M9). Indeed, EQ− models have deficient precipitation on the equator, with excessive precipitation on the sides (Fig. 3a).
Zonally averaged precipitation (mm day−1) (a) for the tropical Pacific and (b) for the global tropics in observations and EQ− and NS− climate models.
Citation: Journal of Climate 27, 4; 10.1175/JCLI-D-13-00337.1
b. EOF2
The second intermodel EOF mode (EOF2; Fig. 2b), explaining 20.1% of total intermodel variability, exhibits a latitudinal asymmetrical precipitation distribution with excessive precipitation in the SH tropics and deficient precipitation in 4°–8°N. The pattern is most pronounced in the central and eastern Pacific and Atlantic. The anomalous cross-equatorial northerly surface winds occur in all three ocean basins, transporting anomalous moisture southward with the excessive precipitation in the SH tropics. Thus, EOF2 represents the double ITCZ problem. We choose three models of the highest PC2 values as the strong double ITCZ problem (NS−) models (M8, M11, and M18) and three models of the lowest PC2 values as the weak or non–double ITCZ problem (NS+) models (M4, M10, and M14). Indeed, NS− models exhibits excessive Pacific precipitation in the SH (Fig. 3a), indicative of an interhemispheric asymmetry bias.
Although EOF2 is only derived from the tropical Pacific precipitation biases, the anomalous interhemispheric asymmetry in precipitation and cross-equatorial northerly surface winds are not limited to the tropical Pacific but are also coherent in most of tropical regions, especially in the Atlantic. This implies that the double ITCZ biases represented by EOF2 are of the global scale. Indeed, the globally averaged meridional precipitation in the NS− models is nearly symmetric about the equator, suggestive of the global nature of the double ITCZ biases (Fig. 3b).
Table 2 shows the intermodel relationship between the global and sectional averaged tropical precipitation asymmetry indices in observations and 18 CMIP5 models. Three sectors are considered: the Pacific (120°E–80°W), South America–Atlantic (80°W–0°), and Africa–Indian Ocean (0°–90°E). The tropical precipitation asymmetry indices are the precipitation difference between the NH (0°–20°N) and SH (0°–20°S) tropics normalized by the respective tropical means. The tropical precipitation asymmetry indices in the three regions are closely related to the global precipitation asymmetry index, with the intermodel correlations of 0.88, 0.79, and 0.63, respectively. Similarly, the sectorial cross-equatorial surface winds in the three ocean basins are highly correlated with the global cross-equatorial surface winds (Table 3), with r = 0.92, 0.82, and 0.79, respectively. These results indicate that the interhemispheric asymmetry biases in tropical precipitation captured by EOF2 are global.
Intermodel correlations between the global and sectional averaged tropical precipitation asymmetry indices in observations and 18 CMIP5 models. See text for details.
Intermodel correlations between the global and sectional averaged cross-equatorial meridional surface winds in observations and CMIP5 models. Note that only 17 CMIP5 CGCMs are employed here since the surface wind data are unavailable in M2.
4. Equatorial Pacific cold tongue bias
Figure 4a shows the scatter diagram for precipitation in the equatorial Pacific (5°S–5°N, 140°E–100°W) between CMIP5 CGCMs and the corresponding AMIP simulations. While almost all AMIP simulations have positive precipitation biases relative to observations, most of their coupled simulations have negative precipitation biases. Most models, except M8 (a model with an extremely warm SST bias in the equatorial Pacific cold tongue; Lin 2007), experience less equatorial Pacific precipitation in CMIP than in AMIP simulations. The intermodel correlation of the equatorial Pacific precipitation between the CMIP and AMIP simulations is only 0.35 (not statistically significant), suggesting that the insufficient precipitation in the equatorial Pacific cold tongue is not an intrinsic error of atmospheric models but rather arises from the interaction with ocean. Indeed, models with a stronger (weaker) decrease in CMIP–AMIP tend to have less (more) precipitation in the equatorial Pacific (Fig. 4b), with a high intermodel correlation of 0.91. In addition, the intermodel correlation between the precipitation and SST in the equatorial Pacific among observations and 18 CMIP5 CGCMs is 0.80 (Fig. 4c), confirming the interaction with the SST biases.
Comparison of precipitation (mm day−1) (a) between CMIP and AMIP; (b) between CMIP and CMIP–AMIP; and (c) with SST (°C) in the equatorial Pacific in observations (asterisk) and CMIP5 models (open circles). Note that only a subset of AMIP simulations is available in (a),(b).
Citation: Journal of Climate 27, 4; 10.1175/JCLI-D-13-00337.1
Figures 5a and 5b compare the Pacific precipitation and SST along the equator among observations and EQ− models, respectively. All the EQ− models represent the negative precipitation and cool SST biases compared to observations, suggestive of an excessive equatorial Pacific cold tongue. While the zonal equatorial winds in the EQ− models agree well with observations in the eastern half of the equatorial Pacific, they feature pronounced easterly wind biases in the western basin (Fig. 5c). This relationship between precipitation, SST, and zonal wind is suggestive of Bjerknes feedback. By contrast, the AMIP simulations have excessive precipitation in the equatorial Pacific, particularly in the western equatorial Pacific (Fig. 5a). Both the insufficient precipitation and easterly wind biases are absent in the AMIP runs (Figs. 5a,c), indicative of the interaction with the ocean. All the EQ− models have too shallow a thermocline (represented by the 20°C isotherm) in the eastern equatorial Pacific (Fig. 5d) that helps bring cold water to the surface, consistent with excessive easterly wind errors in the western basin. While we focus on three models of strong EQ− biases, the shallow thermocline biases in the eastern equatorial Pacific occur in most of other models (figure not shown), albeit weaker in magnitude.
Zonal sections of (a) precipitation (mm day−1), (b) SST (°C), (c) zonal wind (m s−1), and (d) 20°C isotherm depth (m) for the Pacific Ocean along the equator (±2°) in observations (black line) and the CMIP (solid colored lines) and AMIP (dashed colored lines) simulations of the EQ− models.
Citation: Journal of Climate 27, 4; 10.1175/JCLI-D-13-00337.1
5. Double ITCZ bias
a. Cloud radiative forcing effect
Figure 6 examines the relationship of the global–zonally averaged meridional surface winds on the equator with the PC2 values and global tropical asymmetry indices (0°–20°N minus 0°–20°S) in precipitation and SST among CMIP5 CGCMs. The cross-equatorial surface wind anomalies transport anomalous moisture southward, resulting in a global double ITCZ problem. Indeed, models with stronger southward cross-equatorial surface wind anomalies tend to have larger PC2 values and weaker interhemispheric tropical asymmetry in precipitation and SST, with the intermodel correlations of −0.81, 0.88, and 0.76, respectively. Thus, models with stronger southward cross-equatorial surface wind biases tend to suffer from a more severe double ITCZ problem.
Globally averaged meridional surface winds (m s−1) on the equator vs (a) the PC2 and vs the global tropical asymmetry indices (0°–20°N minus 0°–20°S) in (b) precipitation and (c) SST among CMIP5 CGCMs. Here, the precipitation and SST in each CMIP5 CGCM are normalized by their respective tropical means (20°S–20°N). Note that only 17 CMIP5 CGCMs are employed here since the surface wind data are unavailable in M2.
Citation: Journal of Climate 27, 4; 10.1175/JCLI-D-13-00337.1
Figure 6 is consistent with a previous study (Hwang and Frierson 2013). Anomalous southward cross-equatorial surface winds are associated with an anomalous northward cross-equatorial energy transport by the Hadley cell. For annual mean, an energy perturbation into the atmosphere from the sea surface/TOA at one given region will be balanced by an anomalous atmospheric energy transport (e.g., Zelinka and Hartmann 2012; Donohoe and Battisti 2012). If more energy enters into the atmosphere at the surface and TOA in the SH than in the NH, the northward cross-equatorial energy transport by the Hadley cell with a southward displacement of the ITCZ is needed to balance the interhemispheric energy transport (Kang et al. 2008, 2009). Thus, exploring the interhemispheric asymmetry biases of atmospheric energy terms at the surface and TOA helps understand the origins of the anomalous cross-equatorial energy and moisture transports as well as the double ITCZ problem in CGCMs (Hwang and Frierson 2013).
Figure 7a shows the zonal mean distributions of net surface heat flux composited from the NS− and NS+ models. There is no large difference in net surface heat flux between the models. We then focus on the net downward cloud radiative forcing shortwave (CRF-sw) and longwave (CRF-lw) fluxes at the TOA. Figure 8 summarizes the interhemispheric asymmetry of these quantities and their extratropical and tropical contributions. Interhemispheric difference in extratropical CRF-sw (Fig. 8b) is associated with insufficient CRF-sw in the SH midlatitudes (30°–60°S; Fig. 7b; Hwang and Frierson 2013). It is worth noting that the SH midlatitude cloud bias is already present in atmosphere-only models (Fig. 7b) but has little effect on CRF-lw. In the tropics, the interhemispheric difference in CRF-sw exhibits an opposing change from atmospheric to coupled simulations. An increase in convective cloud resulting from a southern ITCZ bias in CMIP reduces shortwave radiation south of the equator. This CRF-sw bias is a negative feedback for the ITCZ bias but largely offset by an opposing change in CRF-lw bias from AMIP to CMIP (Fig. 8c). The increase in convective cloud south of the equator in CMIP reduces outgoing longwave radiation (OLR) there.
Zonal mean of (a) net surface heat flux (W m−2; positive downward) in NS− and NS+ models; (b) biases in TOA CRF-sw and CRF-lw (W m−2; positive downward) in CMIP and AMIP simulations of the NS− models.
Citation: Journal of Climate 27, 4; 10.1175/JCLI-D-13-00337.1
NH minus SH biases of net downward TOA radiative flux, CRF-sw, and CRF-lw in the CMIP and AMIP runs of the NS− models for (a) the whole hemisphere (0°–90°), (b) the extratropics (30°–60°), and (c) the tropics (0°–20°).
Citation: Journal of Climate 27, 4; 10.1175/JCLI-D-13-00337.1
Figure 9a shows the relationship in interhemispheric asymmetry between the midlatitude CRF-sw and tropical precipitation among 18 CMIP5 CGCMs. Models with a weak interhemispheric asymmetry in midlatitude CRF-sw tend to have a weak interhemispheric asymmetry in tropical precipitation and suffer from a severe double ITCZ problem. The intermodel correlation is 0.78, consistent with the results from the NS− composites in Fig. 8. Similarly, there is a strong correlation (r = 0.92) in interhemispheric asymmetry among models between tropical CRF-lw and precipitation (Fig. 9b), suggestive of a positive feedback. Interestingly, the correlation between the interhemispheric asymmetry in tropical CRF-sw and precipitation is low among CMIP5 climate models (figure not shown; r = −0.12). This might be because of two opposing CRF-sw feedback regimes in the tropical southeastern Pacific and Atlantic. At the beginning, an increased SST in these regions breaks up the low-cloud deck and increases the downward CRF-sw (Philander et al. 1996; Ma et al. 1996; Yu and Mechoso 1999). When a double ITCZ develops, however, a further increase in SST will result in increased convection/high cloud and reduce the downward CRF-sw.
Comparison of NH–SH difference in normalized tropical precipitation with that in (a) midlatitude CRF-sw (W m−2) and (b) tropical CRF-lw (W m−2) among CMIP5 CGCMs (open circles). Correlation coefficients in (a),(b) are calculated when excluding IPSL-CM5A-LR (M12) and IPSL-CM5A-MR (M13) models (plus signs) with the large asymmetry biases in net surface heat flux.
Citation: Journal of Climate 27, 4; 10.1175/JCLI-D-13-00337.1
Our analysis suggests a possible mechanism for the double ITCZ problem. The excessive downward shortwave associated with too low CRF-sw in AMIP simulations warms the SH midlatitudes. The interhemispheric asymmetry of extratropical heating is then spread into the tropics, inducing an anomalous Hadley cell with the northward energy transport in the upper branch and southward moisture transport in the lower branch across the equator in order to balance the interhemispheric energy deficit.
b. Tropical ocean–atmosphere feedback
Although there is no large difference in the interhemispheric asymmetry in net surface heat flux between the NS− and NS+ models, this does not necessarily mean that ocean–atmosphere interactions via sea surface flux are not important for developing the double ITCZ biases. In fact, surface heat flux includes distinct and opposing components of atmospheric forcing and ocean response. They adjust to a new equilibrium in response to a perturbation such as an excessive heating in the SH midlatitudes. As illustrated above, while the excessive heating in the SH midlatitudes is spread into the SH tropics by baroclinic eddies (Hwang and Frierson 2013), it forces anomalous southward cross-equatorial surface winds. Further, the reduced southeasterly (intensified northeasterly) trade winds in the tropics could help warm (cool) SST south (north) of the equator, resulting in a reduced interhemispheric asymmetry in tropical SST/precipitation. Indeed, coupled ocean–atmospheric patterns in Fig. 2b are suggestive of WES feedback over the eastern Pacific and Atlantic (Xie and Philander 1994; Xie 1996), a mechanism that could connect the midlatitude cloud bias to the double ITCZ problem within the tropics.
Figure 10 shows the tropical NH minus SH differences in precipitation, SST, and zonal wind, together with the cross-equatorial meridional wind over the tropical eastern Pacific (10°S–10°N, 140°–80°W) and Atlantic (10°S–10°N, 40°W–0°) for observations and CMIP and AMIP simulations of the NS− models. Compared to observations, the NS− coupled models develop too weak a NH–SH asymmetry in tropical precipitation and SST (Fig. 10a). Unlike the coupled simulations, the tropical precipitation asymmetry is quite realistic in the AMIP simulations with prescribed SST (Fig. 11a). The tropical precipitation asymmetry in the coupled runs is highly correlated with that in the CMIP minus AMIP runs (Fig. 11b) as well as with the tropical SST asymmetry (Fig. 11c), with the intermodel correlations of 0.95 and 0.90, respectively. Compared to AMIP, the reduced NH–SH SST/precipitation asymmetry is associated with weakened NH–SH asymmetry in zonal wind (Fig. 10b) and cross-equatorial wind (Fig. 10c) in coupled runs, suggestive of WES feedback in the tropics.
Area-averaged NH (0°–10°N) minus SH (10°S–0°) tropical differences in (a) precipitation and SST and (b) zonal surface wind and (c) area-averaged cross-equatorial meridional surface wind over the eastern Pacific (140°–80°W) and Atlantic (40°W–0°) for observations and for CMIP and AMIP simulations of the NS− models.
Citation: Journal of Climate 27, 4; 10.1175/JCLI-D-13-00337.1
Comparison of tropical precipitation asymmetry indices (a) between CMIP and AMIP; (b) between CMIP and CMIP–AMIP; and (c) with tropical SST asymmetry indices in observations (asterisk) and CMIP5 models (open circles). Note that only a subset of AMIP simulations is available in (a),(b).
Citation: Journal of Climate 27, 4; 10.1175/JCLI-D-13-00337.1
Figure 12 shows the tropical NH minus SH differences in net surface radiation, SST response and WES terms of surface LHF, and ocean heat transport over the tropical eastern Pacific and Atlantic in CMIP minus AMIP simulations of the NS− models. The WES effect is a major component of the surface heat budget for the tropical SST bias of north–south asymmetry. The net surface radiation is a negative feedback for the tropical meridional asymmetry bias in SST because a southern ITCZ in coupled models increases convective cloud and reduces downward shortwave radiation south of the equator. However, the contribution of net surface radiation is relatively small and roughly canceled with that of ocean heat transport. Thus, while the NH–SH asymmetry bias ultimately originates from the SH midlatitude clouds in atmosphere-only models, WES feedback is important for the coupled adjustment within the tropics.
Area-averaged NH (0°–10°N) minus SH (10°S–0°) tropical differences in net surface radiation, SST response and WES terms of surface LHF, and ocean heat transport over the eastern Pacific (140°–80°W) and Atlantic (40°W–0°) for the MME mean and each of CMIP minus AMIP simulations of the NS− models. Error bars for the MME mean are computed from the standard deviation of the NS− models. Note that the positive (negative) signs on the y axis denote a negative (positive) feedback for the NH–SH asymmetry bias in tropical SST.
Citation: Journal of Climate 27, 4; 10.1175/JCLI-D-13-00337.1
c. Seasonal cycle errors
Figure 13a shows the seasonal cycle of the CRF-sw biases in the NH and SH midlatitudes in the CMIP and AMIP simulations of the NS− models relative to observations. Large excessive downward shortwave radiation occurs in the SH midlatitudes (30°–60°S) during austral spring and summer because of AMIP cloud biases. The anomalous heating in the extratropics can be spread into the SH tropics by baroclinic eddies (Hwang and Frierson 2013; Kang et al. 2014), causing anomalously high temperatures. As a result, atmospheric asymmetry develops with the anomalously excessive (insufficient) precipitation and weakened southeasterly (intensified northeasterly) trade winds in the SH (NH) tropics (Fig. 13c). WES feedback amplifies the coupled biases (Figs. 13c,d), corroborating the importance of coupled feedback. In the SH tropics, while the warm SST bias peaks in November–December (the same time as the excessive solar radiation in the SH midlatitudes), atmospheric biases (precipitation and wind) are most pronounced during January–May when the climatological ITCZ displaces south of the equator (Fig. 13b). This displacement in time of atmospheric biases from the SST bias is probably as a result of convective feedback that takes place in the climatological ITCZ (Huang et al. 2013).
(a) Seasonal cycle of the CRF-sw (W m−2) biases in the NH and SH midlatitudes (30°–60°) in CMIP and AMIP simulations of the NS− models. (b) Zonal mean precipitation (color shaded; mm day−1), SST (contours; °C), and surface winds (m s−1; wind speed smaller than 1 m s−1 has been masked out) over the eastern Pacific (140°–80°W) and Atlantic (40°W–0°) for observations. (c),(d) As in (b), but for (c) the biases of the NS− models and (d) their AMIP counterparts.
Citation: Journal of Climate 27, 4; 10.1175/JCLI-D-13-00337.1
6. Summary and discussion
We have examined the biases in tropical Pacific precipitation pattern in the historical climate simulations of 18 CMIP5 CGCMs together with their AMIP runs. The precipitation deficit in the equatorial Pacific cold tongue (i.e., the cold tongue bias) and excessive precipitation in the SH tropics (i.e., the double ITCZ) remain the most prominent biases for the tropical Pacific climate simulation in the current generation of CGCMs. These two types of biases explain more than three-fourths of intermodel variability in tropical Pacific precipitation pattern. Our analysis also shows that the anomalies related to the double ITCZ problem are not confined to the Pacific but rather global in nature.
Based on an intermodel EOF analysis, the first bias mode features precipitation deficit and cool SST biases in the equatorial Pacific cold tongue, along with the easterly wind anomalies in the western half of the equatorial Pacific and too shallow a thermocline in the eastern basin. These anomalies are indicative of Bjerknes feedback. The CMIP–AMIP comparison indicates that the atmospheric biases for the cold tongue bias mode are absent in atmospheric simulations forced with observed SST. These results suggest that the equatorial Pacific cold tongue biases do not come from errors of the atmospheric models but result from the interaction with SST/thermocline via Bjerknes feedback.
Biases of interhemispheric asymmetry in tropical precipitation are of the global scale and closely linked to anomalous northerly surface winds across the equator in all three ocean basins. From an energetics perspective (e.g., Hwang and Frierson 2013), this implies an anomalous northward cross-equatorial energy transport by the Hadley cell, induced by excessive downward solar radiation in the SH midlatitudes. Our analyses suggest a mechanism as follows. The deficient shortwave reflection resulting from AMIP cloud biases during austral spring and summer warms the SH midlatitudes. The double ITCZ biases develop in the tropics in response to the interhemispheric heating asymmetry. In particular, WES feedback mediates the midlatitude cloud radiative forcing to influence ITCZ. In the SH tropics, despite a November–December peak in warm SST biases, the precipitation and other atmospheric biases are most pronounced during the local rainy season (January–May), indicative of the importance of convective feedback.
In observations, the equatorial Pacific cold tongue is partly tied to the latitudinal asymmetries of the eastern Pacific climatology. Xie (1998) indicates that the off-equatorial upwelling induced by the southerly cross-equatorial wind resulting from the northward-displaced NH ITCZ is crucial for the generation of the equatorial Pacific cold tongue. Previous studies (e.g., de Szoeke and Xie 2008) regard the equatorial Pacific cold tongue biases as a part of the double ITCZ problem. Although the meridional asymmetry biases may be important for the equatorial cold tongue problem in some CGCMs as reported by de Szoeke and Xie (2008), the present study suggests that the equatorial Pacific cold tongue biases are not always tied to the double ITCZ problem, for example, in M4 (a strong equatorial Pacific cold tongue model but with a realistic ITCZ) and M8 (a weak equatorial Pacific cold tongue model but with a strong double ITCZ problem), suggesting that different physical processes are involved for the two types of model biases. In fact, a shallow thermocline bias in oceanic models could cause the Pacific SST to cool and develop the equatorial Pacific cold tongue biases via Bjerknes feedback. In contrast to some previous studies (e.g., Frey et al. 1997; Yu and Mechoso 1999; Luo et al. 2005; Zhang and Song 2010) that emphasize the various atmospheric origins of the equatorial Pacific cold tongue problem, the equatorial Pacific cold tongue biases might originate from ocean models; for example, a poor ocean mixing scheme. More work is needed to investigate this possibility by analyzing the Ocean Model Intercomparison Project (OMIP) simulations.
The present study highlights the importance of the SH midlatitude cloud biases in AMIP simulations for triggering the tropical double ITCZ problem. This mechanism does not fully explain variations in the degree of the double ITCZ biases among models. It does not rule out other possible sources of error either. For example, several studies (e.g., Zhang et al. 2007; Chikira 2010) show that the double ITCZ problem of reduced magnitude occurs in some AGCMs forced with prescribed SST.
Acknowledgments
We wish to thank the climate modeling groups (Table 1) for producing and making available their model output; the WCRP’s Working Group on Coupled Modeling (WGCM) for organizing the CMIP5 analysis activity; the Program for Climate Model Diagnostics and Intercomparison (PCMDI) for collecting and archiving the CMIP5 multimodel data; and the Office of Science, U.S. Department of Energy for supporting these datasets in partnership with the Global Organization for Earth System Science Portals. This research is supported by the National Basic Research Program of China (2012CB955603), NSF (ATM-0854365), and NOAA.
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