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PUBLICATIONS Journal of Advances in Modeling Earth Systems RESEARCH ARTICLE 10.1002/2014MS000415 Key Points: Climate models has cold SST biases in Northern basins The cold biases in the Atlantic bias have remote impacts on the Pacific The atmospheric bridge is the main pathway for connecting biases in both basins

Correspondence to: C. Zhao, [email protected] Citation: Zhang, L., and C. Zhao (2015), Processes and mechanisms for the model SST biases in the North Atlantic and North Pacific: A link with the Atlantic meridional overturning circulation, J. Adv. Model. Earth Syst., 7, 739–758, doi:10.1002/2014MS000415. Received 9 DEC 2014 Accepted 6 APR 2015 Accepted article online 6 APR 2015 Published online 25 MAY 2015

Processes and mechanisms for the model SST biases in the North Atlantic and North Pacific: A link with the Atlantic meridional overturning circulation Liping Zhang1,2 and Chuanhu Zhao3 1

Atmospheric and Oceanic Sciences Program, Princeton University, Princeton, New Jersey, USA, 2NOAA Geophysical Fluids Dynamics Laboratory, Princeton, New Jersey, USA, 3Physical Oceanography Laboratory, Key Laboratory of OceanAtmosphere Interaction and Climate in Universities of Shandong, Ocean University of China, Qingdao, China

Abstract Almost all of CMIP5 climate models show cold SST biases in the extratropical North Atlantic (ENA) and tropical North Atlantic (TNA) as well as in the North Pacific which are commonly linked with the weak simulated Atlantic meridional overturning circulation (AMOC). A weak AMOC and its associated reduced northward oceanic heat transport are associated with a cooling of the ENA Ocean, whereas the TNA cooling is attributable to both weak AMOC and surface heat flux. The cold biases in the ENA and TNA have remote impacts on the SST bias in the North Pacific. Here we use coupled ocean-atmosphere model experiments to show the mechanisms and pathways by which the ENA and TNA affect the North Pacific. The model simulations demonstrate that the cooling SST bias in the North Pacific is largely due to the remote effect of the cooling SST bias in the ENA, while the remote impact of the TNA cooling SST bias is of secondary importance. The ENA cooling bias triggers the circumglobal teleconnection via the Northern Hemisphere annular mode, producing a strengthening of the Aleutian low, an enhancement of the southward Ekman and Oyashio cold advection, and thus a cooling SST in the North Pacific. In contrast, the TNA cooling produces a surface high extending to the eastern tropical North Pacific, inducing the northeasterly wind anomalies north, northerly cross-equatorial wind anomalies, and northwesterly wind anomalies south of the equator. This C-shape wind anomaly pattern generates an SST warming in the tropical southeastern Pacific, which eventually leads to an SST warming in the tropical central and western Pacific by the windevaporation-SST feedback. The tropical Pacific warming in turn leads to an SST cooling in the North Pacific by the Pacific North American teleconnection pattern.

1. Introduction Large-scale climate variations in the North Pacific and North Atlantic, such as the Pacific decadal oscillation (PDO) and Atlantic multidecadal oscillation (AMO), have been observed [e.g., Mantua et al., 1997; Enfield et al., 2001]. The PDO and AMO are associated with changes of climate and extreme weather events such as drought and hurricanes [e.g., Mantua and Hare, 2002; Hessl et al., 2004; Enfield et al., 2001; McCabe et al., 2004; Goldenberg et al., 2001; Wang et al., 2013; Zhang and Wang, 2012], and thus play an important role in economic, ecology, society, and people’s life.

C 2015. The Authors. V

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

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The observed large-scale AMO pattern resembles the simulated SST anomaly pattern induced by fluctuations of the Atlantic meridional overturning circulation (AMOC) [e.g., Delworth and Mann, 2000]. Both paleo data and coupled model simulations suggest that the AMO can influence the North Pacific variability such as the PDO. For example, higher oxygen levels off the California coast (indicating reduced upwelling and reduced California current) were synchronous with Greenland cooling stadials [Behl and Kennett, 1996], which are in turn hypothesized to link to the AMOC weakening. Using a hybrid version of the GFDL-CM2.1 climate model, Zhang and Delworth [2007] demonstrated that the AMO provides a source of multidecadal variability to the North Pacific. These research studies suggest that there is a link between the AMOC, Atlantic, and Pacific climate variations. The linkage between the AMOC, Atlantic, and Pacific climate variations can be also seen from the North Atlantic waterhosing experiments [e.g., Stouffer et al., 2006]. In response to a 1 Sv freshwater forcing in the subarctic North Atlantic, the AMOC slows down rapidly in all of the models. The associated climatic

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anomalies exhibit a large degree of similarity, especially in the Atlantic basin: a cooling of surface temperature and southward displacement of the ITCZ. The oceanic heat transport is fundamental for this response [e.g., Yang, 1999; Knutti et al., 2004; Wu et al., 2008]. Beyond the Atlantic basin, coupled models also exhibit a robust response over the North Pacific [e.g., Timmermann et al., 2007]. When the AMOC nearly shuts down, the North Pacific cools throughout the basin with maximum amplitude of 3–5 C along the oceanic frontal zone, and the westerly winds intensify, associated with deepening of the wintertime Aleutian low. Wang et al. [2014] analyzed 22 CMIP5 climate models and found a common pattern of global SST biases (defined as the SST difference between the model and observed SSTs). In the Northern Hemisphere, almost all of climate models show cold SST biases in the extratropical North Atlantic (ENA) and tropical North Atlantic (TNA) as well as in the North Pacific. These cold SST biases are linked with a weak simulated AMOC. However, the detailed mechanisms and processes for causing these cold biases need to be studied. In particular, it is not clear how the cold biases in the ENA and TNA affect the cold bias in the North Pacific. In this paper, we use both CMIP5 model outputs and coupled model experiments to show the potential cause of the SST biases in the Atlantic and their impacts on the state of the coupled system bias in the North Pacific. The results of these models suggest that the cause of the large systematic error in the Atlantic appears to be the systematic error in the Atlantic AMOC and its associated oceanic heat transport in the region, and these large cold biases can have a remote impact on the SST bias in the North Pacific. The cold bias in the ENA plays a more important role in the cold bias in the North Pacific than the cold bias in the TNA. In section 2, we describe the CMIP5 model outputs, dataset and coupled ocean-atmosphere model used in this study. Section 3 briefly summarizes and examines the SST biases in the Atlantic and North Pacific, their relationship with the AMOC in CMIP5 models. Section 4 investigates processes and sources of these SST biases. Section 5 uses coupled model experiments to show the possible pathways through which the ENA and TNA SST biases affect the North Pacific SST bias. Finally, section 6 discusses the implication of this study to coupled model bias in general and gives a brief summary.

2. CMIP5 Model Outputs, SST Data, and Coupled Model This study uses 22 coupled GCMs output data of the ‘‘historical’’ simulations provided to the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC-AR5). The model data can be downloaded from the website of the Coupled Model Intercomparison Project phase 5 (CMIP5; Taylor et al., 2012) (http:// cmip-pcmdi.llnl.gov/cmip5/). The historical run is forced by observed atmospheric composition changes which reflect both anthropogenic (greenhouse gases, aerosols) and natural sources (volcanic influences, solar forcing and emissions of short-lived species and their precursors) and, for the first time, including time-evolving land cover. These historical runs cover much of the industrial period from the mid-19th century to the near present and are sometimes referred to as ‘‘twentieth century’’ simulations. The CMIP5 model sponsor, country, name, and letter denotation are shown in Table 1. Observational data set is used to validate the variability of coupled GCM simulations. SST data are from the NOAA Extended Reconstruction Sea Surface Temperature version 3 (ERSST v3) [Smith and Reynolds, 2004]. The temporal coverage is from January 1854 to the present and it has a spatial resolution on a 2 32 grid. The data can be obtained from http://www.ncdc.noaa.gov/oa/climate/research/sst/ersstv3.php. Because we are interested in only large-scale features, unless otherwise specified, all model outputs and ERSST data are interpolated to a 1o31o grid. To investigate the physical processes and mechanisms connecting the North Atlantic and North Pacific biases, we conduct three groups of partial coupling experiments (see section 5 for details) by using the fully coupled NCAR Community Earth System Model (CESM1.0.4, B_1850 case) [Meehl et al., 2012]. The atmospheric component in this version of CESM is the Community Atmospheric Model version 4 (CAM4) having 96 latitudes and 144 longitudes (1.9o32.5o) on a finite volume grid and 26 hybrid levels in the vertical. The ocean model is a version of the Parallel Ocean Program (POP) developed at Los Alamos National Lab with 1 horizontal resolution and enhanced meridional resolution (1/3 ) in the equatorial tropics and the North Atlantic and with 60 vertical levels. The fully coupled control simulation has been integrated for 1500 years without apparent climate shifts.

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Table 1. The 22 Climate Models Used in This Study and Their Sponsor, Country, Name, and Letter Denotation Sponsor, Country Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia Canadian Center for Climate Modeling and Analysis, Canada National Center for Atmospheric Research (NCAR), USA M et eo-France/Centre National de Recherches M et eorologiques, France Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia European Earth System Model, EU U.S. Department of Commerce/National Oceanic and Atmospheric Administration (NOAA)/Geophysical Fluid Dynamics Laboratory (GFDL), USA National Aeronautics and Space Administration (NASA)/Goddard Institute for Space Studies (GISS), USA Met office Hadley Centre, UK

Institute Pierre Simon Laplace, France

Center for Climate System Research (University of Tokyo), National Institute for Environmental Studies, and Frontier Research Center for Global Change (JAMSTEC), Japan Max Planck Institute for Meteorology, Germany Meteorological Research Institute, Japan Norwegian Climate Centre, Norway

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3. North Atlantic and Pacific SST Biases and the AMOC in CMIP5 Models Wang et al. [2014] found common patterns of global model SST biases in CMIP5 climate models. They also showed that regional model SST biases of the global patterns are commonly linked with AMOC simulations. Here we briefly summarize these results and perform some further analyses. As shown in Figures 1a–1d, both the North Atlantic and North Pacific show the cold SST biases with a magnitude of several degrees Celsius in all seasons. The largest model SST bias occurs in the ENA, with the cooling pattern resembling the cold phase of the AMO and featuring by a comma-like distribution with the largest amplitude over the subpolar region and an extension along the east side of the basin and into the tropics. If an AMOC index is defined by the maximum value of its streamfunction in the latitude band 20 –60 N, CMIP5 models show that the cold SST bias in the ENA is stronger when the AMOC is weaker, and vice versa, with an intermodel correlation of 0.85 between the SST bias and the AMOC index (Figure 2a). The relationship is consistent with the model SST response to the North Atlantic freshwater forcing [e.g., Zhang and Delworth, 2005; Wu et al., 2008], suggesting that the cold SST biases are due to the weak simulated AMOC. The North Pacific SST bias is also positively correlated with the ENA SST bias, with an intermodel correlation coefficient up to 0.61 (Figure 2b). This approximately linear relationship has a strong seasonality, with maximum (0.70) and minimum (0.48) correlations in the winter and summer, respectively (Figures 3a–3d). Additionally, the cold SST biases in the TNA positively covary with those in the North Pacific and ENA (Figures 2c and 2d). These correlations imply that the model SST biases in the North Atlantic and the North Pacific bias are connected with each other. To further verify the relationship of the AMOC with the model SST biases in the North Atlantic and North Pacific, we perform an intermodel singular value decomposition (SVD) analysis for the SST bias and the AMOC of 22 CMIP5 models over the Northern Hemisphere oceans. The first intermodel SVD mode (SVD1) explains 45% of total variance, and is characterized by a cold bias in the North Pacific and Atlantic Oceans associated with a weakened AMOC as manifested by the negative value of the AMOC stream function (Figures 4a and 4b). The two associated principal components have a positive correlation of 0.80 (Figure 4c). The maximum SST amplitudes occur in the North Atlantic subpolar region, which highly resembles the ensemble mean SST bias displayed in Figure 1. All of these results indicate that the North Pacific and Atlantic SST biases are connected with each other, both of which are associated with the AMOC. That is, if the

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model cannot reasonably simulate the AMOC mean state, they will generate at least partially SST bias in the North Atlantic. Then, the North Atlantic bias can induce the model biases in the TNA and North Pacific by the ocean and atmosphere bridges. In the next sections, we will examine the possible sources of biases in the Atlantic and North Pacific Oceans.

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4. Processes Related to the SST Biases in the North Atlantic and Pacific To further examine the processes or sources of the model SST biases, we choose three models (M15, M17, and M21) with the coldest SST biases in the North Atlantic and Pacific (CNPAC-CATL) and three models (M3, M10, and M22) with the relatively warm SST biases in the North Pacific and Atlantic (WNPAC-WATL). This selection can be easily done from the SST principal components in Figure 4c or from the scatter plot of North Atlantic and North Pacific SST biases (Figure 2b). As expected, the annual mean SST difference between the CNPAC-CATL and WNPAC-WATL models is characterized by cooling SST anomalies in the North Atlantic and North Pacific (Figure 5a). For long-term mean SST, the mixed-layer heat budget can be expressed as follows: Q 1 Do 5 0, where Q denotes the net heat flux (positive downward) and Do represents ocean dynamical effects due to the three-dimensional advection and mixing. Figure 5b shows that the cooling SST over the ENA Ocean is damped by the surface net heat flux, implying the important role of ocean dynamics. As expected, the AMOC and its associated northward heat transport are significantly reduced in the CNPACCATL models compared to the WNPAC-WATL models (Figures 6a and 6b), which in turn produces the cooling SST bias in the ENA region (Figure 5a). The subpolar gyre over the North Atlantic also shows a strengthening as indicated by the negative sea surface height (SSH) anomaly (Figure 5b), which is a typical fingerprint of the weakened AMOC [Zhang, 2008]. This spin up of subpolar gyre further favors the cooling SST anomaly over the ENA due to the anomalous cold advection from the high latitude and the anomalous vertical cold advection and mixing. In contrast, the TNA cooling SST bias is not only attributed to the decrease of the northward heat transport, but also contributed by the heat flux as a result of the wind changes (Figures 5a and 5b).

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As presented in Figure 5c, we perform a detailed heat budget analysis in the western and central North Pacific Ocean. The cooling SST bias is largely associated with the temperature advection by the anomalous meridional current, while the contribution from the surface heat flux is negative. This suggests that there is an anomalous southward current in the western and central midlatitudes, which can be also implied from the surface wind and SSH anomaly (Figures 5a and 5b). The surface wind in the North Pacific is characterized by a westerly anomaly in the midlatitude (Figure 5a), which is in favor of generating a southward cold Ekman transport. Moreover, the subpolar gyre over the North Pacific is significantly strengthened and shifts southward in response to the cyclonic wind (Figures 5a and 5b), which leads to a strengthening and southward movement of the Oyashio and thus generates a cooling SST anomaly in the western North Pacific. In addition, the temperature advections by the anomalous zonal and vertical currents also play a positive role. This is because the cyclonic wind strengthens the subpolar gyre and the associated eastward current in the southern branch, which in turn brings more western subpolar cold water to the central North Pacific. One the other hand, the cyclonic wind strengthens the Ekman pumping, which induces an anomalous upward current and therefore a cooling SST over the North Pacific. Overall, we can find that the cooling SST bias in the ENA is attributed to the reduced northward heat transport associated with the weak simulated AMOC. While in the TNA, both the northward heat transport and the reduced surface heat flux contribute to the cooling SST bias. Over the North Pacific, the cooling SST bias is primarily determined by the temperature advection by the anomalous southward current as a result of the surface cyclonic wind. However, the potential pathway by which the North Atlantic SST bias affects the North Pacific is still unknown. To investigate the possible mechanisms, we calculate the geopotential height differences at low (850 mb), middle (500 mb), and high (250 mb) heights, as exhibited in Figures 7a–7c. The anomalous atmosphere circulation displays an equivalent barotropic structure, with a low pressure belt in

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the midlatitudes to the subtropics and a high pressure cap over the entire Arctic, resembling the negative phase of the Northern Hemisphere annular mode [Thompson and Wallace, 1998]. The low pressure belt in the North Atlantic and North Pacific is also seen in the surface wind (Figure 5a), which shows a reinforcement of the Aleutian low and Iceland low. This suggests that the North Atlantic cooling SST bias can be transmitted to the North Pacific through the annular mode and vice versa. Therefore, it is expected that if the model fails in simulating the AMOC, it will generate at least partially cooling biases in the ENA and TNA Oceans. These North Atlantic cooling SST biases can be further propagated to the North Pacific Ocean and then can be strengthened by the local air-sea coupling, which ultimately may positively feedback to the Atlantic Ocean. In other words, the Northern Hemisphere SST biases in coupled models are largely due to the AMOC simulation. Previous studies argue that the physical processes involved in the impacts of the North Atlantic SST on the North Pacific SST can be through two possible paths: the midlatitude atmosphere [e.g., Zhang and Delworth, 2007; Wu et al., 2008; Kang et al., 2014] and the tropical atmosphere and oceans [e.g., Timmermann et al., 2005; Dong et al., 2006; Xie et al., 2008; Wu et al., 2005, 2007; Zhang et al., 2011b; Okumura et al., 2009; Ham et al., 2013; Zhang et al., 2014]. Zhang and Delworth [2007] suggested that a warm (cold) phase of the AMO reduces (enhances) the meridional SST gradients in the midlatitude, which further reduces (increases) the surface atmosphere eddy heat transport and upper level vorticity flux. These cause northward (southward) shifts of midlatitude westerly winds both over the North Atlantic and North Pacific. The northward (southward) shift of the westerly wind over the North Pacific leads to a

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Figure 5. Shown are the (a) SST ( C, shading) and surface wind stress (N/m2, vectors), (b) net surface heat flux (positive downward, W/m2, shading) and sea surface height (SSH, contour interval is 0.01 m) differences between the CATL-CNPAC and WATL-WNPAC models, and (c) heat budget analysis over the North Pacific region (see box in Figure 5a). Ta denotes the mixed layer temperature. Here, the mixed layer depth is defined as the depth where the potential density is larger than that in the surface by 0.125 kg/m3. Mix denotes the mixing which is estimated as the residual. HFLX denotes the surface net heat flux. The rest of the terms describe advection, with capital (U, V, W, T) and lower case (u, v, w, t) letters denoting mean and anomaly, respectively. -uTx, -vTy, -Utx, -Vty, -wTz, and -Wtz represent term 2U’ @T =@x, 2V ’ @T =@y, 2U@T ’ =@x, 2V @T ’ =@y, 2W ’ @T =@z, and 2W @T ’ =@z, respectively. Units for the temperature Ta and the heat budget terms are C and 10 W/m2, respectively.

weakness (strengthening) of the Aleutian low, which further causes the warm (cold) SST anomalies in the North Pacific. Wu et al. [2008] argued that the strong North Atlantic cooling (warming) accelerates (decelerates) the westerly winds over the North Pacific by triggering an annular mode, and thus a cooling (warming) SST in the North Pacific. Both of these arguments are referred to the midlatitude pathway, although the former focuses on the effect of transient eddies on the Northern Hemisphere westerly, while the latter focuses on the atmosphere teleconnection. This midlatitude pathway is similar to our composite analysis from CMIP5 models to some degree. Wu et al. [2005, 2007] and Zhang et al. [2011b] further demonstrated that a warming of the TNA could lead to a La Nina-like SST response in the tropical Pacific, which in turn produces a North Pacific warming through the Pacific North American (PNA) teleconnection as suggested by Alexander et al. [2002]. Okumura et al. [2009] also showed a direct atmospheric pathway for the TNA to affect the Aleutian low and North Pacific SST. Although our analysis suggests that the North Pacific cooling SST bias can be remotely or at least partially affected by the North Atlantic SST bias through the annular mode, it is still not clear which path is more important and which path favors triggering the annular mode: the midlatitude path or the tropical path? Therefore, in the next section, we attempt to conduct a series of numerical experiments to examine which path is dominant to the North Pacific cooling SST anomaly.

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5. Pathways Connecting the North Atlantic and Pacific 5.1. Midlatitude Pathway To examine the potential pathways connecting the model SST biases in the North Atlantic and North Pacific, we conduct two groups of partial coupling experiments by using the fully coupled NCAR Community Earth System Model (CESM1.0.4), with each group consisting of two parallel runs. The first group of partial coupling experiments is designed to test the midlatitude path from the ENA to the North Pacific, which is named as the PC_ENA. The first group includes a control restoring run and an ENA SST bias run. In the control restoring run, the entire tropical (20 S–20 N, 0oE–0oW) and ENA (20 N–60 N, east coast to west coast) SSTs are restored to their climatological SST seasonal cycle at every time integration step. The ENA SST bias run is configured as the same as the control restoring run but with the ENA region restoring to the CESM1.0.4 climatological SST seasonal cycle added with the monthly SST bias in CMIP5 models (Figure 1). Both the two experiments are integrated for 100 years. The difference averaged in the last 30 years is taken as the response. Overall, in the first group experiments, the tropical regions are decoupled, and thus the forced SST bias in the ENA can only transmit to the North Pacific region through the midlatitude path. Figure 8a shows the SST response in the PC_ENA run. The cooling over the North Pacific Ocean is a prominent feature in this experiment. The area-averaged cooling anomaly over the North Pacific is about 1.12 C, which accounts for 62% of the cooling bias in the coupled model (Figure 1). The cooling over the western North Pacific also exhibits a seasonal dependence with the maximum and minimum in late summer and winter, respectively (not shown). This implies that a large amount of the cooling bias in the North Pacific Ocean can be remotely impacted by the ENA bias. Over the North Pacific Ocean, the

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atmospheric response tends to be dominated by the equivalent barotropic low (Figures 9a–9c). This circumglobal teleconnection is a reminiscent of the annular mode, the leading mode of Northern Hemispheric atmosphere variability in the winter [Thompson and Wallace, 1998]. In response to the barotropic low, the surface wind anomaly is featured by a cyclone wind over the North Pacific Ocean and an intensified westerly in the midlatitude (Figure 10a). The midlatitude westerly anomaly first cools the ocean by reinforcing the southward Ekman cold advection and the surface turbulent heat flux cooling (not shown). The anomalous positive wind stress curl (cyclonic wind) then drives a strengthening and southward shift of the subpolar gyre after the adjustment of westward Rossby waves as indicated by the SSH anomaly (contours in Figure 10a), which in turn generates a SST cooling in the western North Pacific by the anomalous cold advection from the high latitude to the low latitude. The strengthening and southward shift of the subpolar gyre in the North Pacific is further seen from the barotropic streamfunction response (Figure 10b). The barotropic component of the ocean circulation response is predominantly due to changes of wind stress in response to the cooling bias. To demonstrate that, we calculate the Sverdrup transport based on the wind stress anomalies (Figure 10c). The differences between the Sverdrup transport and the simulated barotropic transport are less significant, except in the western boundary regions as expected.

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In the North Pacific Ocean, the SST and geopotential height responses suggest that there is a local positive feedback, which is also pro−20 posed by the previous studies [Zhang and Delworth, 2005; Wu et al., 2008; Zhang et al., −40 2011a, 2011b; Zhang and Wu, 2012]. On one 15 hand, the extratropical atmospheric teleconW 0E −60 50 1 nection can transmit the effects of the North 180W Atlantic cooling to the midlatitude North Figure 7. Shown are the (a) 850 mb, (b) 500 mb, and (c) 250 mb geopoPacific by the global nature of the annular tential height (m) differences between the CATL-CNPAC and WATLmode. On the other hand, the cold SST in the WNPAC models. Kuroshio-Oyashio extension (KOE) region can feedback to the atmosphere, leading to an even stronger strengthening of the Aleutian Low. Therefore, once the North Pacific cold SST is initiated by the North Atlantic bias, then it can be amplified by the local positive air-sea feedback. In consequence, we conclude that the North Pacific bias can be remotely impacted by the North Atlantic bias by the midlatitude path, which can explain nearly 62% of the SST cooling bias in the North Pacific.

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5.2. Tropical Pathway We conduct the second group of partial coupling experiments to examine the relationship between the cooling biases in the TNA and North Pacific, which is named as the PC_TNA. The first experiment is also the control restoring run. The ocean and atmosphere are fully coupled except in the TNA region where we restore the model-produced SST to the climatological SST seasonal cycle at every integration step. The second experiment is configured as the same as the first experiment but with the TNA region restoring to the CESM1.0.4 climatological SST added with the monthly CMIP5 model SST cooling bias in the region of 0 – 20 N and from east to west coast (Figure 1). The difference averaged in the last 30 years is taken as the response. As shown in Figure 8b, the SST response in the North Pacific is characterized by a horseshoe-like SST pattern, with a cooling in the northwestern and central North Pacific and a warming in the east extending northwest into the subpolar ocean and southwest into the tropics. The magnitude of the area averaged cooling SST over the North Pacific Ocean is about 0.6 C, which accounts for 33% of the North Pacific cooling bias in CMIP5 models presented in Figure 1. This suggests that the SST bias in the North Pacific Ocean is partially attributed to the remote effect of the SST biases in the TNA Ocean. The North Pacific cooling anomaly triggered by the TNA bias is mainly through the tropical Pacific region. As seen in Figure 8b, the tropical Pacific is characterized by a warm SST extending from the southeastern Pacific (SEP) region to the western tropical Pacific. According to Alexander et al. [2002], the tropical Pacific warming can produce a cooling of the North Pacific Ocean by triggering the PNA teleconnection pattern [Horel and Wallace, 1981]. This can be seen from the geopotential response (Figures 9d– 9f). Figures 9d–9f show that the structure of the middle latitude geopotential response is similar to the PNA pattern, with a low pressure in the North Pacific, a high pressure in North America, and a low pressure in the south. The tropical Pacific warming is generated as follows: The cold TNA bias suppresses convection and rainfall over the AWP region (Figure 11b), producing a surface high extending to the eastern tropical North Pacific (Figure 11c). Over the eastern tropical Pacific, this anomalous high induces northeasterly wind anomalies north, northerly cross-equatorial winds, and northwesterly winds south of the equator (Figure 11a). This

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C-shape wind anomaly generates a dipole SST anomaly through changes in evaporation. This coupled WES feedback, which has been extensively studied in relation to tropical Atlantic variability [e.g., Xie and Carton, 2004], acts to amplify the SST dipole. The formation of crossing interbasin wind is primarily due to the weakening of the Hadley-type circulation from the AWP region to the SEP as a result of the cooling bias imposed over the TNA region. As exhibited in Figures 12a and 12b, the TNA cooling is associated with a divergent

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circulation in the low troposphere that crosses the equator into the South Pacific and vice versa for the upper troposphere. This meridional circulation reduces the South Pacific subtropical anticyclone (Figure 11c) and the associated subsidence, which in turn leads to a reduction of the low clouds, the easterly trade wind, and thus an increase of the SST. The warming of the SEP Ocean is further transmitted to the western tropical Pacific Ocean by the WES mechanism [e.g., Xie, 1996]. As exhibited in Figure. 13, the SEP warming induces a northwesterly wind on its west side. These northwesterly wind anomalies decelerate the mean southeasterly winds, reducing oceanic evaporative heat loss to induce warming on the northwestward flank of the original SST warming and causing the coupled SST-wind pattern to propagate equatorward. Eventually, the tropical Pacific Ocean is occupied by the warming SST in the boreal winter, which triggers the PNA teleconnection. The PNA teleconnection can be also seen from the barotropic streamfunction response (Figure 12c). Here, the barotropic and baroclinic streamfunctions are calculated as wt 5ðw850mb 1w250mb Þ=2 and wc 5ðw850mb 2w250mb Þ=2, respectively. In response to the PNA teleconnection, the surface wind in the North ZHANG AND ZHAO


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Pacific Ocean is cyclonic, which generates a horseshoe-like SST pattern, with a cooling in the northwestern and central North Pacific and a warming in the east extending northwest into the subpolar ocean and the southwest into the subtropics (Figure 8b). This SST response is mainly due to the surface heat flux and temperature advection by the anomalous meridional current (not shown). The eastern warming is also propagated to the western tropical Pacific by the positive WES feedback [e.g., Vimont et al., 2009]. To demonstrate this process, we examine the seasonal development of SST, surface wind, and latent heat flux (Figure 14). In March and April, the warming due to the warm northward advection is primarily limited to the north of 10 N and east of 180 W (Figures 14a and 14b). Associated with the warm anomalies, anomalous cyclonic winds develop in the east, which decelerate the northeast trades and thus reduce the oceanic latent heat loss (Figures 14f and 14g). Note that both wind and downward heat flux anomalies have extended to the lower latitude and more westward, which favor an equatorward extension of the warm anomalies in the following months (Figure 14c). At the same time, westerly wind anomalies develop in the western tropical Pacific and reduce the latent heat loss (Figure 14h). This can further substantiate equatorward progression of the warm anomalies (Figures 14d and 14e). Therefore, the northeastern Pacific warming can be eventually transmitted to the tropical Pacific Ocean, which in turn reinforces the subsequent PNA teleconncetion. Therefore, the air-sea coupling in the tropical-extratropical Pacific acts as a positive feedback to amplify the North Pacific cooling. In addition to the barotropic response, the atmosphere response also has the baroclinic component. The baroclinic streamfunction response shows a pair of anticyclones: one in the TNA and northeastern Pacific and the other in the SEP (Figure 12d). This model response is largely consistent with Gill’s [1980] solution to

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a cooling anomaly slightly north of the equator [Heckley and Gill, 1984]. There is also a pair of cyclones in the western Pacific. This can be interpreted as follows: The northeastern Pacific warming anomalies can gradually propagate to the western tropical Pacific due to the positive WES feedbacks (Figure 14), which induces a large amount of precipitation there (Figure 11b). The heating anomaly over the western tropical Pacific eventually induces a baroclinic response represented by a pair of cyclones. Note that the TNA cooling bias may influence the North Pacific SST through the TNA-North Atlantic-North Pacific path by exciting the barotropic Rossby waves [e.g., Hoskins and Karoly, 1981; Okumura et al., 2009]. As presented in Figure 12c, the barotropic component also shows a pattern of alternating high and low centers from the TNA to high latitudes, which is consistent with the pure atmosphere model response [e.g., Wang et al., 2010; Okumura et al., 2009]. To examine the importance of this path, we perform the third group experiment named PC_TNAxTPAC run, which is configured as the same as the second group experiment, but with the tropical Pacific Ocean (20oS–20oN, west coast to east coast) decoupled by restoring the model-generated SST to the climatological SST cycle. Figure 15 shows the SST response in this third group experiment. Although the North Pacific Ocean features a cooling SST anomaly, its magnitude is too small, particularly in the western midlatitude of the North Pacific. Therefore, the North Pacific SST cooling response to the TNA cold bias is primarily through the tropical Pacific path.

6. Discussion and Summary In this paper, we report the results from 22 coupled model simulations provided from the Coupled Model Intercomparison Projection (CMIP) phase 5 multimodel ensembles. Our intercomparison study of

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coupled climate models shows that virtually all the current generation of coupled models without flux correction have significant, but similar biases in the Northern Hemisphere Oceans. These biases manifest themselves as the SST in the North Pacific, TNA and ENA Oceans being too cold. The magnitude of these biases can be as large as 3oC or more, particularly in the ENA Ocean, resulting in a significant distortion of the semiannual cycle of the northern hemisphere SST in these models. We explored the potential cause of the large biases in the Atlantic and its impact on the state of the coupled system bias in the North Pacific. The results of these models suggest that the cause of the large systematic error in the Atlantic appears to be the systematic error in the AMOC and its associated oceanic heat transport in the region, and these large cold biases can have a remote impact on the SST bias in the North Pacific Ocean. They contribute positively to the cold bias in the North Pacific Ocean. The study suggests that effort of

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reducing coupled model biases in the North Hemisphere Oceans should take into the consideration not only the local processes, but also the remote influence.

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The detailed mechanism through which the −2 −1.5 −1 −0.5 0 0.5 1 1.5 2 Atlantic SST biases Figure 15. Annual mean SST ( C) response in the PC_TNAxTPAC run. affect the North Pacific is currently under investigation. A preliminary analysis of CMIP5 models indicates that the Arctic oscillation or annular mode plays an important role in transmitting the Atlantic bias to the North Pacific Ocean. The strong Atlantic cooling reinforces the Aleutian low over the North Pacific by triggering an annular mode. The surface wind in the North Pacific is anomalously cyclonic, which is in favor of generating a southward Ekman cold advection. In addition, the subpolar gyre is significantly strengthened in response to the cyclonic wind, which leads to a strengthening of the Oyashio and thus favors a cooling anomaly in the North Pacific midlatitudes. The CESM1.0.4 simulations further demonstrate that the North Pacific cooling bias is largely due to the remote effect of the cooling bias in the North Atlantic Ocean, while the remote impact of the TNA cooling bias is of secondary importance. Similar to the CMIP5 model analyses, the North Atlantic cooling bias triggers the circumglobal teleconnection (Arctic Oscillation), producing a strengthening of the Aleutian low, an enhancement of the southward Ekman and Oyashio cold advection, and thus a cooling SST in the North Pacific Ocean. In contrast, the North Pacific cooling anomaly triggered by the TNA bias is mainly through the tropical Pacific region. The TNA cooling bias suppresses local convection and rainfall, producing a surface high extending to the eastern tropical North Pacific. Over the eastern tropical Pacific, this anomalous high induces northeasterly wind anomalies north, northerly cross-equatorial winds, and northwesterly winds south of the equator. This C-shape wind anomaly generates a warming in the SEP Ocean, which eventually leads to a warming in the central and western Pacific by the positive WES feedbacks. The tropical Pacific warming favors a cooling of the North Pacific Ocean by triggering the PNA teleconnection pattern. While emphasizing the role of ocean heat transport in the Atlantic sector, we do not wish to down play the potential importance of local feedbacks in causing the large systematic error in oceans such as the tropical Atlantic. It is very likely that regional feedbacks between stratocumulus clouds, surface winds, upwelling, coastal currents and ocean eddies are very important contributors to the large warm SST bias off the coast of Angola and Namibia and Peru. Future investigations are clearly needed to understand the causes of the large biases in the tropical Atlantic and Pacific Oceans. Our study here has some implications for the classic double intertropical convergence zone (ITCZ) problem in the coupled models. Recent studies suggested that the ITCZ problem is tied to bias in the Southern Hemisphere (SH) extratropics [e.g., Hwang and Frierson, 2013]. They demonstrated that the warm SST bias in the Southern Ocean as a result of unreasonable cloud parameterization and radiative forcing, leads to a warmer temperature anomaly in the SH than in the Northern Hemisphere. This interhemispheric temperature contrast favors an anomalous Hadley cell with a deep convection center in the warmer SH due to energy balance and thus produces an excess of precipitation in the SH in climate models. In the current paper, we found that the North Atlantic and North Pacific exhibit significant cooling SST bias in the coupled model, which is suggested to be related to the weak simulated AMOC. This cooling SST bias in the Northern Hemisphere extratropics could also induce a temperature contrast between two hemispheres, which is then expected to drive a southward shift of the Hadley circulation and thus an excessive of precipitation in the SH. Thereby, the relative roles of Southern Ocean warm bias and Northern Hemisphere cooling bias on the classic double ITCZ problem need to be addressed in the future.

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Journal of Advances in Modeling Earth Systems Acknowledgments The CMIP5 data for this paper are available through the Earth System Grid — Center for Enabling Technologies (ESG-CET), on the page http://pcmdi9.llnl.gov/. The ERSST data are available at NOAA’s Earth System Research Laboratory, on the page http://www.esrl.noaa.gov/psd/data/ gridded/. Dataset: NOAA Extended Reconstructed SST. Dataset name: sea surface temperature. The model output used in this paper is available upon request (email: [email protected]). Acknowledgement is made to the Chinese science foundation (NSFC, 41106009).

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