Interbasin exchanges and their roles in global ocean circulation: A study based on years spin up of MOM4p1

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1 Acta Oceanol. Sin., 2014, Vol. 33, No. 1, P DOI: /s Interbasin exchanges and their roles in global ocean circulation: A study based on years spin up of MOM4p1 ZHU Yaohua 1, WEI Zexun 1, FANG Guohong 1, WANG Yonggang 1, GUAN Yuping 2 1 Key Laboratory of Marine Science and Numerical Modeling, First Institute of Oceanography, State Oceanic Administration, Qingdao , China 2 State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou , China Received 16 May 2013; accepted 18 September 2013 The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2014 Abstract A global prognostic model based on MOM4p1, which is a primitive equation nonboussinesq numerical model, has been integrated with years from the state of rest based on the realistic topography to study the long-term pattern of combined wind-driven and thermodynamically-driven general circulation. The model is driven by monthly climatological mean forces and includes horizontal grids and 31 pressure-based vertical levels. The main objective is to investigate the mass and heat transports at interbasin passages and their compensations and roles in the global ocean circulation under equilibrium state of long-term spin up. The kinetic energy analysis divides the spin up process into three stages: the quasi-stable state of wind driven current, the growing phase of thermodynamical circulation and the equilibrium state of thermohaline circulation. It is essential to spin up over a thousand years in order to reach the thermohaline equilibrium state from a state of rest. The Arctic Throughflow from the Bering Strait to the Greenland Sea and the Indonesian Throughflow (ITF) are captured and examined with their compensations and existing data. Analysis reveals that the slope structures of sea surface height are the dynamical driving mechanism of the Pacific-Arctic-Atlantic throughflow and ITF. The analysis denotes, in spite of O ( m 3 /s) of the southward volume transport in the northern Atlantic, that there is still O (1 PW) of heat transported northward since the northward currents in the upper layer carry much higher temperature water than the southward flowing northern Atlantic deep water (NADW). Meridional volume and heat transports are focused on the contributions to NADW renewals and Atlantic meridional overturning circulation (AMOC). Quantitative descriptions of the interbasin exchanges are explained by meridional compensations and supported by previous observations and numerical modeling results. Analysis indicates that the volume and heat exchanges on the interbasin passages proposed in this article manifest their hub roles in the Great Ocean Conveyor System. Key words: numerical modeling, global ocean, interbasin exchange, meridional transport, meridional overturning circulation Citation: Zhu Yaohua, Wei Zexun, Fang Guohong, Wang Yonggang, Guan Yuping Interbasin exchanges and their roles in global ocean circulation: A study based on 1400 years spin up of MOM4p1. Acta Oceanologica Sinica, 33(1): 11 23, doi: /s Introduction The global-scale circulation has long been one of the oceanography s most challenging and exciting research topics. A century ago, Pillsbury (1912) pointed out that global ocean circulation transports heat poleward from the equator. But oceanographers did not focus on the heat transport rate until several decades ago. There has been a developing focus on the world oceanic thermohaline circulation since it is immediately related to the global climate change. Broecker (1987, 1991) introduced the ocean conveyor belt terminology and twolayer thermohaline flow scheme to study the deep layer circulation and upper layer compensation currents. Schmitz (1995) summarized updated research achievements and observational results and proposed a four-layer thermohaline flow scheme based on interbasin water exchange. He illustrated his thermohaline scheme, including bottom water, deep water, intermediate water and upper layer compensation water, and estimated the volume transport rates. Both Broecker s two-layer scheme and Schmitz s four-layer scheme presented canonical pictures for global ocean thermohaline circulation. Huisman et al. (2009) and Marotzke and Willebrand (1991) employed GFDL s (geophysical fluid dynamic laboratory) modular ocean model (MOM2) with idealized rectangle Atlantic and Pacific Ocean and 4 4 coarse grid to integrate thousands of years to study the multiple equilibria of thermohaline circulation. In their studies, coarse grids and large steps were applied Foundation item: The National Basic Research Program Grant of China under contact No. 2011CB403502; the International Cooperation Program Grant of China under contact No. 2010DFB23580; the International Cooperation Program of State Oceanic Administration of China under contract No. QY ; project supported by the First Institute of Oceanography, the State Oceanic Administration of China under contract No. 2010G06; author Guan Yuping is supported by The National Natural Science Foundation of China under contact Nos and *Corresponding author, weizx@fio.org.cn 1

2 12 ZHU Yaohua et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 1, P to save computational resources. Analyses were focused to find the equilibrium states and shift criterion rather than the real thermohaline circulation structure. Wei et al. (2000) integrated MOM2 for 11 years with 1 global resolution and ( 1 / 3 ) high resolution nested grid to study both wind driven and thermodynamically driven general circulation. Typical zonal and meridional cross sections were selected to analyze climatological mass, heat and salt transport rates in the global ocean. The net northward mean through flow in the Bering Strait and the net southward mass transport in the Atlantic, thus Pacific- Arctic-Atlantic interbasin exchange, were not indicated. Dong et al. (2011) employed a global circulation model for the earth simulator (OFES), in which the model code was based on the MOM3 and the spin up run was 50 years from an initial condition at rest with observed mean hydrographic data. Detailed analyses were focused on the role of interocean exchanges on decadal variations of the meridional heat transport in the South Atlantic. However, their numerical results would be different if the spin up run lasted for a thousand years more. Therefore, it becomes necessary and useful to simulate the sufficiently developed state of the general ocean circulation without initial background constraint whereby under real topography, relatively fine resolution and realistic climatological forces can be applied. Along with the development of computer technology and thanks to GFLD s endeavor, it becomes possible to integrate the realistic topography global ocean for thousands of years with relatively high resolution grids. This article presents an integration of years from the state of rest to the equilibrium state in order to investigate the long-term pattern of general circulation system, based on the combined wind-driven and thermodynamically-driven mechanism. The nonboussinesq mass conserving MOM4p1 includes conservative temperature and pressure based vertical coordinates. The analyses in this article indicate that the interbasin exchanges are always closely related to meridional transports. The interbasin exchange results of this article reveal a Pacific-Arctic-Atlantic- Indian Ocean-Pacific circle transport pattern which composes a worldwide ocean circulation. There are six interbasin exchange passages in the world ocean two in the Arctic region, three in the Antarctic Circumpolar Current regime (ACCR) and one in the tropical area. The Greenland Sea and the Norwegian Sea connect the Atlantic and Arctic Ocean, therefore is an Atlantic-Arctic exchange passage (AA section). The Bering Strait is another interbasin passage connecting Arctic and Pacific Ocean (AP section). ACCR links the Southern Pacific, Atlantic and Indian Ocean, therefore is a unique linkage for the great oceans to exchange bottom, deep, intermediate and upper layer water. There are three interbasin exchange passages in ACCR, including Southern Atlantic- Indian Ocean passage (SAI), Southern Indian Ocean-Pacific passage (SIP) and Southern Pacific-Atlantic passage (SPA). The Indonesian Archipelago passage is the sole passage which connects the Pacific and Indian Ocean in the tropical region, thus is the only express way for mass, heat and salt transports in the world ocean. Obviously, it is of great importance to focus on these passages transports because of their hub roles in the global ocean circulation and their relations to meridional transports and Great Ocean Conveyor as well. 2 Model configuration The MOM4p1 is one of the most recent versions of the GFDL ocean model, featured by its generalized vertical coordinates. In this global spherical model, the horizontal resolution of ( grids) and 31 vertical levels (Table 1) are employed. Pressure-based vertical coordinate p has been applied for the mass conserving, nonboussinesq, free surface ocean primitive equations. Lateral boundary conditions are cyclic in zonal direction, and solid walls in meridional direction. Table 1. Mean depths and thickness of vertical levels under p coordinate Level Depth range/10 4 Pa Thickness/10 4 Pa Central depth/10 4 Pa Level Depth range/10 4 Pa Thickness/10 4 Pa Central depth/10 4 Pa The constant horizontal viscosity and diffusivity coefficients are taken as A h = m 2 /s and K h = m 2 /s and the vertical ones are computed in the model with chen scheme. Time steps used for the internal mode (baroclinic) and external mode (barotropic) are and 40 s respectively. For more details about the model description, readers are directed to Elements of MOM4p1 by Griffies (2010). The topography data set is based on NOAA s (1988) ETOPO5 and the International bathymetric chart of the Arctic Ocean (IBCAO). The surface boundary conditions are provided by NOAA National Oceanographic Data Center (NODC) World Ocean Atlas 1994 (WOA), including monthly mean water flux,

3 ZHU Yaohua et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 1, P heat flux, sea surface temperature (SST) and sea surface salinity (SSS). The spin up run is started from a state of rest and has been integrated for years to the equilibrium state. The sea surface is forced by the monthly climatological wind stress, taken from Hellerman and Rosenstein (1983), the water flux and the heat flux with the surface temperature and salinity restoring towards above SST and SSS. Long-term fluctuations in flow patterns are an important component of ocean climate and thus are the focus of this article. Except the kinetic energy stability analysis, the time variability is not considered in this article, but not because of its lack of importance or interest. Different from Schmitz (1995), this article is focused on interbasin mass and heat transports and their relations to meridional transports and NADW, rather than on vertical layer schemes and NADW renewals path. Analyses in this article are based on 50 years mean computational result of the equilibrium state. 3 Result analysis and comparison 3.1 Three developing stages of the total kinetic energy Figure 1 shows the time variation of total kinetic energy of the global ocean along with the years of integration. The most significant feature is the three different stages of kinetic energy s developing process. The total kinetic energy approaches its quasi-stable state within the first three months and lasts for 60 years. This first stage is the quasi-stable state of wind-driven current. As we have recorded, the kinetic energy series are , , , , (unit: J) corresponding to the integration length of 3, 6, 9 months, 10 and 60 years. The second stage is the growing phase of the thermodynamically-driven current in which the total kinetic energy has been growing dramatically until a thousand integrated years with total kinetic energy of J. The third stage is the equilibrium state for thermohaline circulation where the kinetic energy stably fluctuates in a very narrow range to J, with the relative deviation less than within 200 years. Therefore, it is considered as equilibrium state. Even though there are some criteria for the definition of equilibrium state, sometimes scientists do use their own judgment. For example, Marotzke and Willebrand (1991) employed the basin-averaged surface heat flux under O(0.1) W/m 2 as an equilibrium judgment. If just for the purpose of wind driven current or shallow marginal circulation, it could be enough to integrate the model for a decade or so, since it can approach equilibrium state much quicker. 3.2 Sea surface height distribution and its slope structures Figure 2 depicts the global sea surface height (SSH) distribution. The Pacific Ocean has the highest SSH and the Atlantic Ocean has the lowest one. The SSH reaches its peak in the subtropical convergent zone and decreases poleward. Both the Pacific and Atlantic have their higher SSH in the western boundary current (WBC) area and lower values at the eastern ends. SSH in the Northern Pacific is higher than that in the Arctic Ocean, and the latter is higher than in the Nordic Sea. This constitutes a driving force inducing a through flow into the Arctic Ocean from Pacific and eventually flowing into the northern Atlantic. Fig.2. The global ocean circulation model MOM4p1 produced global SSH distribution. Fig.1. Total kinetic energy variation along with integrated years. Therefore, in order to obtain the equilibrium state from the state of rest, it is necessary to integrate more than a thousand years. Otherwise, the underdeveloped thermodynamically-driven circulation causes unreasonable vertical temperature and salinity structures and underestimated transports for the deep ocean thermohaline circulation. This is one of the important conclusions of the article, which is of significance on the global ocean circulation modeling. From the integration year of to 1 400, the total kinetic energy varies from Figure 3a shows the detailed SSH distribution around the Arctic region. In the Bering Strait region, the SSH at the south side is approximately 0.5 m higher than the north side within the span of two latitudinal degrees (typical values are 0.8 m in the south and 0.3 m in the north side of the Strait). The SSH of 0.46 m in the Bering Strait and 0.25 m at 70 N of Chukchi Sea, slightly north of Bering Strait, are significantly higher than the zonal mean SSH of 0.36 m at 70 N cross section between Greenland and Norway. In the Arctic Ocean, the SSH on the Canada basin is higher than in Svalbard-Barents Sea side. At 80 N, the SSH peak occurs on the Canada basin with a value between 0.8 and 1.0 m, while the trough occurs in Svalbard- Barents Sea side with a value between 0.8 and 1.0 m. This slope structure of SSH constructs the dynamical mechanism to drive a Pacific-Arctic-Atlantic interbasin transport. This Pacific- Arctic-Atlantic transport is also an important component of the Great Ocean Conveyor. It needs to be noted that this model is not an ocean-ice coupled model, thus the SSH and the upper layer currents in the Arctic region could be exaggerated, since without ice covering the wind stress effect is exaggerated.

4 14 ZHU Yaohua et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 1, P Fig.3. The SSH distribution in the Arctic region (a) and near the Indonesian Archipelago region (b). Figure 3b illustrates the SSH distribution around the Indonesian Archipelago region. SSH in the east side of the Indonesian Archipelago with a typical value of 1.6 m is significantly higher than in the west side, meanwhile the north side (west Pacific region) is higher than that in the south side (south Indian Ocean). Particularly, 0.6 m of the typical magnitude of SSH at 30 S Indian Ocean and 1.6 m of the SSH in the west Pacific region construct a northeast-southwestward SSH slope. It is this SSH slope that drives ITF from northeast to southwest of Indonesian Archipelago. In the subantarctic frontal zone (SFZ), the SSH decreases dramatically poleward. The Antarctic coastal area has the lowest SSH in the global ocean, with a typical magnitude of 2.5 m and an extreme magnitude of 3 m in the Weddell Sea. Table 2. Mass, heat and salt transports through specific transoceanic sections and interbasin passages Section Latitude Longitude Volume transport/10 6 m 3 s 1 Heat transport/pw Salt transport/10 9 kg s 1 1A 0 N 55 W 15 E B 0 N E C 0 N 100 E 75 W 0.605( from 133 E) A 30 N 90 W 0 E C 30 N 120 E 110 W A 30 S 50 W 17 E B 30 S E C 30 S 150 E 70 W AP BERING ST. 66 N W AA G & N 70 N 25 W 20 E SAI S 20 E SIP S 135 E SPA S 67 W IAP 7 S E Notes: Positive values denote eastward or northward net transport. Abbreviation are AA G&N, cross section at 70 N between Greenland and Norway dividing Atlantic and Arctic; AP, Bering Strait section dividing Arctic and Pacific Ocean; 4, Southern Atlantic-Indian Ocean passage; 5, Southern Indian Ocean-Pacific passage; 6, Southern Pacific-Atlantic passage; 7, Indonesian Archipelago passage. 3.3 Volume transport and its compensation Figure 4 depicts the typical cross sections and interbasin passages in the global ocean. The volume, heat and salt trans- Fig.4. The specific transoceanic sections and interbasin passages. ports through these cross sections have been calculated and listed in Table 2. In the Atlantic, the volume transports through zonal section 1A ( m 3 /s ), 2A ( m 3 /s) and 3A ( m 3 /s) are relatively close since there are no significant straits and passages at both eastern and western boundaries. This net southward volume transport comes from the Arctic Ocean. Although the Norwegian Current flows northward, the southward East Greenland Current (EGC) is apparently stronger, therefore making a net m 3 /s (southward) of volume transport at AA, the cross section of 70 N between Greenland and Norway. This amount of mass loss in the Arctic Ocean is compensated by the northward through flow in the Bering Strait ( m 3 /s) and net P-E (precipitation minus evaporation) into the Arctic Ocean. As we have found, the net P-E is m 3 /s in the Arctic Ocean, which exactly

5 ZHU Yaohua et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 1, P fills the gap between m 3 /s and m 3 /s. Figure 5a depicts the surface currents in the Arctic region, showing clearly that there exist northward currents in the Bering Strait, clockwise circulation in the Canadian basin and anticlockwise in the Barents Sea, Greenland Sea and Norwegian Sea area around Svalbard. The current speed can reach 0.5 m/s in the Bering Strait, with an averaged value of 0.2 m/s in the Strait region. Compared with previous observational data and numerical models, this article presents quite reasonable results on the mean current speed, volume transport and sea level slope structure in the Bering Strait. Overland and Roach (1987) established a two-dimensional barotropic numerical model for the Bering Sea and Chukchi Sea, obtained 0.4 m sea level difference between Bering Sea and Chukchi Sea and m 3 /s northward through flow in the Bering Strait. Li et al. (2005) analyzed current data based on the mooring stations during the Second National Arctic Research Expedition of China in 2003 and showed 20 cm/s mean northward current in the Bering Strait, which is slightly less than 24.5 cm/s of one-year moored measurements from the autumn of by Woodgate et al. (2005). Fig.5. The surface currents in the Arctic region (a), and near the Indonesian Archipelago (b). However, the volume transports among the zonal sections in the Pacific and the Indian Ocean are not balanced because of the existence of the Indonesian Archipelago passage and the South China Sea (SCS). Obviously there is neither volume transport balance between cross section 1B and 3B in the Indian Ocean nor in the Pacific between cross section 1C and 3C. In the Indian Ocean, the southward volume transport of m 3 /s at 3B comes from section 7 ( m 3 /s) and the Torres Strait ( m 3 /s) since only m 3 /s of contribution from the equatorial Indian Ocean section 1B. In the Pacific Ocean, the northward volume transport at 1C ( m 3 /s) is close to 2C ( m 3 /s), but hugely different from 3C ( m 3 /s). But if we calculate the volume transport through the equator from the crucial point E (east of Sorong, the northernmost point of the New Guinea Island) to the eastern end of the Pacific, then the volume transport is m 3 /s. That means the volume transport from 100 to E through the equator is m 3 /s (again, negative value indicates southward transport), which is decomposed to m 3 /s from the Karimata Strait and m 3 /s from the Makassar Strait and the Maluku Sea. Thus, the northward m 3 /s volume transport through cross section 3C is roughly balanced by m 3 /s from the Torres Strait, and above m 3 /s through the equator, the latter joins the North equatorial Current (NEC) and then turns westward. The NEC is divided into the south branch and north branch before it approaches Philippines Peninsula. The south branch mostly joins the Equatorial Counter Current (EC- C). Meanwhile, the north branch becomes strengthened at the western boundary. Some of the latter enters the Luzon Strait westward and its majority forms the Kuroshio and flows northward from the area east of Taiwan. Figure 5b shows the surface current distribution in the Indonesian Archipelago region. A part of NEC south branch, flows into the Sulawesi Sea and the Maluku Sea together with the SCS branch from the Sulu Archipelago. All the southward branches from the Karimata Strait, the Makassar Strait and the Maluku Sea converge in the Banda Sea, with an average rate of m 3 /s, and further flow out toward the Indian Ocean through the Timor Sea together with m 3 /s westward current from the Torres Strait. This above m 3 /s of the interbasin volume transport carried by the ITF through section 7, from the Pacific to the Indian Ocean, is of great significance. As the sole interbasin passage in the tropical region, the Indonesian Archipelago passage acts an express way to fulfill the global mass, heat and salt transport balance quickly. Gordon (2010) calculated the average strength of the ITF based on 3 years observed data from the INSTANT project. His m 3 /s of the observational result is in good agreement with our m 3 /s of the numerical modeling result. To some extent, this proves the reliability of the numerical result of this article. There are indeed some important water exchanges from the Luzon Strait, the Taiwan Strait, the Karimata Strait, the Sulu Sea and the Lombok Strait and even other straits around the SCS, but the grid resolution of the numerical model in this article is not fine enough to reproduce their accurate values. In order to integrate the global ocean over a thousand years from the state of rest to equilibrium state, some scientists (Huisman et al., 2009; Marotzke and Willebrand, 1991) even used 4 4 grid resolution to reduce the computational time. In the southern ocean, the eastward volume transport through meridional section 4 ( m 3 /s) and the southward transport through the zonal section 3B ( m 3 /s) is

6 16 ZHU Yaohua et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 1, P equal to the volume transport through the meridional section 5 ( m 3 /s), meanwhile the latter is decomposed into m 3 /s eastward on the meridional section 6 and m 3 /s northward on zonal section 3C. The m 3 /s on the section 6 and m 3 /s southward transport on the section 3A matches m 3 /s on section 5. Hence, the interbasin mass transports at the Southern Atlantic-Indian Ocean, the Southern Indian Ocean-Pacific and the Southern Pacific-Atlantic are directly balanced by the Southern Ocean meridional transports. On the north Pacific zonal section 2C, the northward volume transport m 3 /s is less than the Pacific-Arctic exchange ( m 3 /s) by m 3 /s, which is just compensated by the net P-E rate. As we have calculated, the net P-E rate over the N northern Pacific Ocean is m 3 /s. Similarly, there is a small volume transport difference between the northern Atlantic sections 2A ( m 3 /s) and AA ( m 3 /s), which is mainly caused by the net P-E rate m 3 /s into N northern Atlantic Ocean and partly induced by the southward flow ( m 3 /s) from the Baffin Bay and the Davis Strait which joins the Labrador current afterwards. For the same reason, it is easy to understand the variation of the volume transports at the zonal section 1A ( m 3 /s), 2A ( m 3 /s) and 3A ( m 3 /s). The annual mean P-E rate is illustrated in Fig. 6. The most notable feature is the intensive P-E at the equatorial Pacific, up to 20 mm/d at the eastern end of the Pacific. The Indonesian Archipelago area together with its vicinity of the western Pacific, the eastern Indian Ocean, gains extensive rainfall of up to 15 mm/d since it s in between the Pacific warm pool and Indian Ocean warm pool. Northern Pacific, northern Atlantic, Arctic Ocean and ACCR gain net P-E as well. However, the subtropical zones at both hemispheres show negative P-E up to 5 mm/d, with a maximum value up to 10 mm/d off the Western Australian coast. As mentioned above, precipitation is explained to be an important compensation of the mass transport balance. Fig.6. Annual mean P-E (precipitation minus evaporation) rate into the ocean. 3.4 Heat transport and its compensation Figure 7 depicts the vertically integrated meridional heat transport rate on zonal grids. The meridional heat transport rate is distributed between 0.5 PW (northward) and 0.5 PW (southward) in most of the world ocean, but is intensified in the WBC areas. It reaches 2 PW at 35 N of Gulf Stream region, 1.5 PW in the Kuroshio region and northeast of Madagascar. It approaches 1 PW to the east of Tanzania, Papua New Guinea and northeast of the Drake Passage. All of the above intensified positive meridional heat transport rates are accompanied with the strong northward WBC. Therefore it is easy to find the intensified negative rates in the strong southward WBC area, especially in the Agulhas current, East Australia current and Brazil current region. Fig.7. Vertically integrated meridional heat transport on zonal grids, which is computed from z -integral of c p ρdx v t/10 15 where dx,v and t are the zonal grid width (m), the north component of current velocity and temperature respectively. In the northern Atlantic and Pacific (north of 40 N), the positive heat transport rate covers a majority area and thus causes a net northward heat transport. In the subtropical zones of the Atlantic and the Pacific (20 to 40 N), the intensified positive meridional heat transports of the Gulf Stream and Kuroshio dominate the northward heat transport. It clearly shows that the Agulhas Current carries a vast amount of heat from the subtropical zone to the ACCR to fulfill equatorial Pacific-Indonesian Archipelago-Indian Ocean-ACCR heat circulation. It is worth mentioning that as strong WBCs, the Brazil Current carries a large amount of heat southward from 35 to 40 S, meanwhile the Malvinas Current (i.e., Falkland Current) carries a huge amount of heat northward from northern sector of Drake Passage from 50 to 40 S, which forms the Brazil- Malvinas confluence. The remarkable features of heat transports listed in Table 2 are the positive values through sections 1A (0.239 PW), 2A (0.687 PW), 2C (0.474 PW) and negative values through sections 3A ( PW), 3B ( PW), 3C ( PW). They show that the heat is transported poleward from the tropical zone. In the Atlantic, however, it is transported northward across the equator even though the mass transport is southward there. Unlike the volume transports in the Atlantic that are all toward the south and vary little as shown in Table 2 for cross sections 1A, 2A and 3A, the heat transports change significantly. Figure 8a indicates that the meridional heat transport is northward in the Atlantic and increases to its peak of 0.75 PW near 35 N, and decreases all the way back to 0 in the Arctic region. These features agree with Hall and Bryden s (1982) result in Fig. 8b, except that the model produced heat transport is southward between 20 and 35 S. The peak value of 0.75 PW is 20% less than Hall and Bryden s result whereas the model produced result is 0.2 PW higher than Hall and Bryden s at 70 N. As shown in Fig. 8b, the magnitudes of heat transports vary signif-

7 ZHU Yaohua et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 1, P Fig.8. Model produced meridional heat transport in Atlantic (a), in PW and published results adopted from Hall and Bryden(1982) derived from integrating BONKEWS (1980) air-sea exchange values(b). Single points are values obtained by the authors using the direct method: Be, BENNETT; Br, BRYAN; BH, BRYDEN and HALL; HB, HALL and BRYDEN; R, ROEMMICH; W, WUNSCH. icantly from models and our result in this article presents a smoother tendency. Figure 9a is the northward heat transport across each parallel of latitude in the global ocean, clearly showing that heat is transported poleward from the equator, with a peak of 1.2 PW near 35 N and a trough of 2.0 PW around 30 S. This result is consistent with the result of Ganachaud and Wunsch (2002) (Fig. 9b) and estimates of Trenberth et al. (2001). Fig.9. Model produced northward heat transport by the global ocean across each circle of latitude (a), and published results adopted from Ganachaud and Wunsch (2002) (b). Figure 10 illustrates the annual mean net heat flux into the global ocean. The peaks of the net heat flux into the ocean occur in the equatorial area with their maximum values up to 100 W/m 2 at both eastern and western ends of the Pacific. The o- cean gains net heat flux in subtropical area, northern ACCR, northeast Pacific and eastern Atlantic coast. In the area between 40 and 50 S in Atlantic, the strong heat flux of up to 80 W/m 2 according to Fig. 10, is superimposed to the Malvinas Current region, therefore enlarges the northward meridional heat transport. The troughs occur at WBC regions in the northern hemisphere, where the Gulf Stream and Kuroshio release heat up to 200 and 150 W/m 2 respectively. The ocean loses net heat flux in the middle latitude areas (both northern and southern hemisphere), in the Arctic Ocean and the southern ACCR. There are still a few other areas notably losing net heat flux, e.g., in the Norwegian Sea, the western coast of Australia and south of Cape Agulhas. Around 45 N of western Atlantic, i.e., east of Nova Sco- Fig.10. Annual mean surface heat flux (positive downward).

8 18 ZHU Yaohua et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 1, P tia, seems an exception. Over there the ocean gains net heat flux up to 80 W/m 2. Thus for the basin-scale oceanic region, the heat transport rates are not always balanced by incoming and outgoing values on the specified cross sections due to a large amount of heat flux involved. Only for certain regions with around zero total heat flux, the heat transport rates can be approximately balanced. Figure 11 illustrates the zonally averaged heat flux shown in Fig. 10. Huge net heat flux (over 60 W/m 2 ) is gained near the equator and decreases dramatically in the subtropical region. It seems almost symmetric at both hemispheres, with fluctuations under negative range. Fig.11. Zonal mean surface heat flux into the global ocean(positive downward). Unlike the volume transports which are roughly balanced on zonal Atlantic sections, the heat transports on 1A (0.239 PW), 2A (0.687 PW) and 3A ( PW) have no way to approach a balance, neither on section 5 (4.181 PW), section 6 (3.509 PW) and 3C ( PW). Again, it indicates that the surface heat flux must be considered while balancing the heat transport rates in basin-scale oceans. Heat transport analysis denotes the heat is transported northward in the north Atlantic even though the volume transport is southward. That maintains the anomalously warm winter air temperatures enjoyed by northern Europe. In the global ocean, the heat is meridionally transported poleward from the tropical region. Therefore, the ocean circulation system acts a conveyor belt to fulfill meridional heat transport, thus changing the world climate. It is of outstanding significance for the ITF to carry PW of heat from the Pacific Ocean to the Indian Ocean, which is roughly equivalent to the amount of the northern Atlantic meridional heat transport. This express way of the interbasin heat transport at low latitudes outlines the ITF s important effect on global climate change. The southward Agulhas Current system (ACS) demonstrates an important role to transport the heat from ITF to the Southern Ocean, even to the northern Atlantic afterwards, thus keeps the efficient heat circulation in the global ocean. The following is going to explain how the MOC fulfills the positive meridional heat transport in the northern Atlantic. Figure 12 shows the vertical profile of zonal mean north velocity component v and potential water temperature t respectively at 35 N in the Atlantic, where the heat transport peak occurs. According to the zonal mean v, it is simply a three-layer vertical scheme. The relatively stronger (up to m/s) northward component v occurs in the upper m, corresponding to the potential temperature from 10 to 24 C, meanwhile the weaker ( m/s) southward component v occurs in the depths from m to m, corresponding to the potential temperature of 3 10 C. Obviously, the northward heat transport in the upper layer is larger than the southward heat transport in the deep layer, the latter performing the NADW s southward movement the lower limb of the great ocean conveyor. This is the meridional heat transport mechanism, i.e., the Gulf Stream carries more heat than the deep layer southward NAD- W, that causes the net northward heat transport in the northern Atlantic and thus brings warm winter air temperatures to northern Europe. At the depths from m to the seabed, there is a slightly northward water movement, which is recognized as the Antarctic Bottom Water (AABW) originating from the vicinity of the Antarctic. Figure 13a is a model produced SST distribution chart in the Atlantic, simply showing how the temperature decreases poleward from the equator, which is very consistent with Levitus and Boyer (1994) SST (Fig. 13b). Poleward surface currents always act as warm current whereas equatorward surface currents always act as cold current. 3.5 AMOC and its strength Figure 14 shows the meridional overturning circulation Fig.12. Vertical profile of zonal mean north component velocity (a) and potential temperature (b) at 35 N in the Atlantic Ocean.

9 ZHU Yaohua et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 1, P Fig.13. Model produced (a) and Levitus annual mean SST (Levitus and Boyer, 1994) (b) in Atlantic. (MOC) pattern of the global ocean. The solid line contours indicate positive stream functions (clockwise circulation) and the dashed line contours indicate negative stream functions (anticlockwise). The zero stream function occurs in the equatorial area from ocean surface down to m. From the equator to 60 N, its depth goes down slightly from m, and down to the bottom around 60 N. In the northern Hemisphere, the MOC is relatively weak. The two positive peaks (corresponding to clockwise sinking) occur at 20 N with the depth of m, and at 50 N with the depth of m. The former happens in the subtropical convergent zone, whereas the latter denotes the formation of the NADW. In the southern Hemisphere the MOC affects to a much larger extent and deeper since the ACCR links the major water exchange passages of all the Southern Pacific, Atlantic and Indian Oceans. The strongest anticlockwise circulation (south sinking) occurs at 60 S and down to the bottom. It can spread to the northern Hemisphere. Figure 15 shows the Atlantic (a) and Pacific (b) MOC separately. The biggest difference between them is that the Atlantic has a strong north sinking in the high latitude area to form the NADW and the Pacific Ocean does not form deep water in the Fig.14. MOC stream functions of the global ocean. north hemisphere, but forms south sinking instead. This phenomenon is coincided with the existing ocean observations and described as the on state of the Great Ocean Conveyor, discussed in detail by Huisman et al. (2009) and Marotzke and Willebrand (1991). Fig.15. MOC stream functions of the Atlantic (a) and the Pacific Ocean (b). In the north Atlantic, when the Gulf Stream flows northward, it releases its heat and gets colder. While this saltier water becomes colder, it becomes heavier and starts sinking. Together with the even colder East Greenland current, it forms low temperature and high salinity NADW. It spreads between 40 N and 65 N with a depth of up to m. The Atlantic MOC stream function shows a strength up to m 3 /s in this article, just supported by Gordon s (1986) estimation of the NADW volume. Broecker s (1991) revised radiocarbon-based estimate of the flux for the NADW from m 3 /s to m 3 /s. He emphasized that it is difficult to assess the error in this estimate but it is probably on the order of 25% (i.e.,± m 3 /s). Plenty of previous studies estimated the Atlantic MOC strength by different approaches. For example, Roemmich and Wunsch

10 20 ZHU Yaohua et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 1, P (1985) estimated m 3 /s of volume transport at the 24 N Atlantic by hydrographic section data; Talley et al. (2003) estimated (18±5) 10 6 m 3 /s of the NADW formation; Ganachaud and Wunsch (2000) obtained (15±2) 10 6 m 3 /s of the NADW overturning in the high latitudes by the inverse model. Underneath the NADW is the Atlantic bottom water. It clearly comes from the Antarctic region with its MOC stream function strength of m 3 /s in the central Atlantic. The peak of the stream function occurs at the depth m and with its meridional span from 10 N to 30 S. According to Fig. 12, the zonal mean northward velocity component v at 35 N is divided into three vertical layers:the upper 800 m of the northward compensating current; the intermediate/deep layer from 800 to m of southward deep current of NADW; the northward bottom current, from m to the seabed. To calculate the mass transport rates of each stages and compare with Gordon and Broecker s MOC strength, 50 N is chosen as the typical latitude of NADW, with the following findings: m 3 /s northward mass transport rate for the upper layer (0 876 m); m 3 /s mass transport rate (southward) for the intermediate/deep water ( m); m 3 /s northward mass transport rate for the bottom water ( m). This is highly consistent with Schmitz s three layer thermohaline Great Ocean Conveyor scheme, not only on the stage depths, but also on the mass transport rates and the extent of NADW. The meridional mass transport rate of m 3 /s also coincides with m 3 /s of the MOC strength. In the north Pacific, however, the Kuroshio is not as cold as the Gulf Stream when it flows northward, since the Pacific-Arctic passage (the Bering Strait) is much narrower than Atlantic-Arctic passage (the Greenland Sea and the Norwegian Sea). Furthermore, in the northmost Pacific, it is not the same mechanism as in the North Atlantic where strong northerly gale drives the cold water from the Arctic Ocean convectively mixing. Thus less cold water forms sinking to the depth of m and only with the stream function strength of m 3 /s. This is the so-called formation of the north Pacific intermediate water. Underneath the Pacific intermediate water is the Pacific deep water and bottom water, which come from the Atlantic deep water and bottom water. The Pacific does not form its own deep water and bottom water. 3.6 Three renewals of NADW and their relations to the interbasin exchanges In this section, the model-produced result shows more detailed descriptions to explain how the NADW is related to the interbasin exchanges and meridional transports, thus being part of the Great Ocean Conveyor. According to Schmitz (1995) and Gordon (1986), about m 3 /s of the intermediate water from ACCR flows through the northern sector of Drake Passage, becomes involved in a Malvinas Current-Brazil Current-Subtropical gyre interaction, and then joins the Benguela current regime (BCR)- North Equatorial Current (NEC) and the Gulf Stream and eventually becomes the primarily renewal of the NADW. Figure 16a depicts the vertical profile of the zonal mean v (north component of current speed) at the section of east of Cape Horn. The positive v component indicates net northward mass transport rate from the northern sector of the Drake Passage m 3 /s has been integrated at 54 S, from 65 to 54 W. It provides the flow to become the above mentioned m 3 /s of the primarily renewal of the NADW. Both Schmitz and Gordon estimated O ( m 3 /s) was the magnitude to become the NAD- W renewal from this origin. Even m 3 /s is comparatively close to their O ( m 3 /s) but there could be still some leaking out of it. When, where and how it leaks? To answer these questions, water properties need be analyzed in detail. As mentioned earlier, this article will not focus on the vertical scheme and paths of the NADW renewals. Fig.16. Vertical profile of zonally averaged v (north) component at section 54 S, W (east of Cape Horn) (a) and 32 S, 8 18 E (west of Cape of Good Hope) (b) respectively. The secondary renewal originates from the O ( to m 3 /s) leak from the ITF according to Schmitz (1995) and Gordon (1986), but this amount could be up to m 3 /s according to our result. It bypasses the Cape Agulhas into the Atlantic Ocean and joins the BCR, then the NEC and the Gulf Stream, becoming the secondary renewal of the NADW. Figure 17 shows the vertical profile of the meridional mean u (east component of current speed) at the section of south of Cape Agulhas. The negative u component denotes net westward mass transport rate from the southern Indian Ocean to the southern Atlantic. A transport of m 3 /s has been integrated at the section of S, 25 E. It looks much bigger than the ITF; however, only a small part of it will turn northward and join the BCR. Most of it will travel back to the ACCR. Figure

11 ZHU Yaohua et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 1, P b shows the vertical profile of the zonal mean v at the section of west of Cape of Good Hope. The positive v component shows the part from the Southern Indian Ocean and turning northward to BCR system. Only m 3 /s has been integrated at 32 S, 8 18 E (coast). This is somewhat close to the result of Schmitz, which is m 3 /s of the ITF leaking to the BCR system and becomes the secondary NADW renewal. However, it could be up to m 3 /s according to our numerical result. It is worth emphasizing again that this part of renewal carries a large amount of heat from the ITF, thus from the tropical area to the northern Atlantic. Fig.17. Vertical profile of meridionally averaged u (east) component at section S, 25 E (South of Cape Agulhas). The third renewal is relatively weak ( m 3 /s) but it s also an interbasin exchange from the Pacific-Arctic Ocean to the Atlantic. This amount of the Arctic through flow is just in between the result of Schmitz (1995) ( m 3 /s) and Broecker (1991) ( m 3 /s). It joins the Arctic circulation system and eventually joins the EGC and becomes the cold part of the NADW renewal. One should notice that this m 3 /s of Atlantic-Arctic exchange rate does not mean only this amount of cold water is coming from the Arctic Ocean. It has been integrated at 70 N section separately, from Greenland to Jan Mayen (10 W) for the EGC, from Jan Mayen to Norway for the Norwegian Current. Therefore the m 3 /s southward mass transport rate breaks down to m 3 /s of the southward EGC and m 3 /s of the northward Norwegian Current. The cold EGC passes the Denmark Strait and flows into the northern Atlantic Ocean. This cold part plays an important role to cool down the other two high salinity renewals, and along with the deep convection forced by atmospheric cooling there, form the cold and high salinity, thus high density water known as the NADW. The NADW can sink to the abyss and flow southward with the rate of m 3 /s at 50 N, forming the conveyor s lower limb. Figure 18a is the model-produced vertical profile of the zonal mean salinity in the Atlantic. It illustrates the upper layer high salinity water from the Gulf Stream sinking gradually during its convergence with the cold EGC. It highly agrees with Levitus (1994) result of the high salinity between 20 and 30 N, with the salinity 35 sinking to the depth of m. These three renewals make the total convergent rate of m 3 /s, including m 3 /s of southward Atlantic- Arctic exchange and northward transport rate of m 3 /s from the upper layer m at 50 N Atlantic, which is slightly different from the result of Schmitz (1995) estimation of the intermediate layer compensation ( m 3 /s). However, according to the stream function in Fig. 15, it is easy to understand that the magnitudes of the upper layer renewals ( m 3 /s) and deep layer returning current ( m 3 /s) vary along with the latitude, and are not always a constant number. Fig.18. Model produced (a) and Levitus (1994, b) vertical profile of zonal mean salinity in the Atlantic. A large transport of approximately m 3 /s of AMOC in section 3.5 is in good agreement with the analysis of the three renewal components of the NADW O ( m 3 /s) from the ACCR, O ( m 3 /s) from the ITF (up to m 3 /s in our results), m 3 /s from Atlantic-Arctic ocean exchange in this section. The vertical structure of the zonal mean north component v in the north Atlantic is highly consistent with Schmitz s three layer thermohaline Great Ocean Conveyor scheme, not only at the layer depths but also at the mass transport rates ( , and m 3 /s of Schmitz s renewals respectively). 4 Conclusions A nonboussinesq mass conserving model based on Mom-

12 22 ZHU Yaohua et al. Acta Oceanol. Sin., 2014, Vol. 33, No. 1, P p1 has been spun up over years from the state of rest to s- tudy the long-term pattern of the general ocean circulation and to investigate the mass and heat transports through the typical cross-sections, especially the six interbasin exchange passages. Analysis reveals the interbasin exchanges and their relations to the meridional transports, as well as the renewals of the NADW quantitatively. (1) The southward net volume transports at the zonal Atlantic sections require a compensation of m 3 /s from the Atlantic-Arctic interbasin exchange. Thus, the southward EGC is m 3 /s stronger than the northward Norwegian current on volume transport. The former provides colder water to stimulate the formation of NADW along with the effect of deep convection forced by atmospheric cooling there. (2) The 0.5 m higher of the SSH at the south side of the Bering Strait compared with the north side induces a northward through flow in the Bering Strait to fulfill a Pacific-Arctic interbasin exchange of m 3 /s. This current joins the Arctic Ocean circulation system and eventually flows out from the Greenland Sea and the Denmark Strait, therefore completing the Pacific-Arctic-Atlantic interbasin exchange. This interbasin through flow has been driven by the SSH slope among the northern Pacific, the Arctic and the northern Atlantic according to the authors. As calculated, the P-E rate in the Arctic region (70 to 90 N) is m 3 /s, which exactly fills the gap of m 3 /s of Atlantic-Arctic exchange and m 3 /s Pacific-Arctic exchange. (3) The Indonesian Archipelago passage is the only interbasin passage in the tropical region, thus acts an express way for mass, heat and salt transports from the Pacific to the Indian Ocean. It is of outstanding significance for the ITF to carry m 3 /s of water, PW of heat and kg/s of salt to fulfill the Pacific-Indian Ocean through flow. By this tropical short-cut, the mass, heat and salt balances in the global ocean can be reached quickly and efficiently, thus greatly affect global climate change. The SSH slope structure of 1.6 m in the west Pacific and 0.6 m at 30 S Indian Ocean constructs ITF s driving force m 3 /s of this article is quite close to Gordon s (2010) m 3 /s of the observational ITF strength, which is based on 3 years observed data from the INSTANT project. (4) As a unique linkage of the southern Pacific, Atlantic and Indian Ocean passage, the ACCR acts an irreplaceable traffic hub and provides all the upper, intermediate, deep and bottom layers of water to exchange. About m 3 /s of water, 4 PW of heat and 1.0 kg/s of salt transports take place here. Almost all the levels of water masses originate here and return here. The difference of interbasin volume transports between southern Atlantic-Indian Ocean passage ( m 3 /s) and southern Indian Ocean-Pacific passage ( m 3 /s) is balanced by the southern Indian Ocean meridional transport ( m 3 /s). Similarly, the difference between southern Indian Ocean-Pacific passage and southern Pacific-Atlantic passage is balanced by the southern Pacific meridional volume transport, meanwhile the difference between southern Pacific- Atlantic passage and southern Atlantic-Indian Ocean passage is balanced by the southern Atlantic meridional volume transport. If P-E rate is taken into account, the above balances would match better. However, for the heat transport balance, the surface heat flux needs be considered. Overall, all the interbasin exchanges are directly related to the meridional transports. (5) The origins and strengths of the three NADW renewals have been analyzed, specifically, O ( m 3 /s) from the intermediate layer of the northern sector of the Drake Passage, O ( m 3 /s) from the Pacific (up to m 3 /s in this model) through the Indonesian Archipelago passage (ITF) and O ( m 3 /s) from the Pacific-Arctic-Atlantic interbasin exchange. The O ( m 3 /s) of the northward upper layer volume transport at 50 N zonal Atlantic section gives an explanatory notes on the consistency with the total-sum of above the NADW renewals. The three vertical layer structure of the zonal mean v component in the north Atlantic is highly consistent with three layer thermohaline Great Ocean Conveyor scheme (Schmitz, 1995), not only on the stage depths but also on the volume transports. (6) Even with m 3 /s southward volume transport at 30 N Atlantic zonal cross-section, there is still PW of heat transported northward since the upper/intermediate layer water has much higher temperature than the southward deep returning current of the NADW. It is this mechanism of Atlantic MOC and meridional heat transport that causes the warm winter air temperatures in northern Europe. (7) It takes only a few years for the wind-driven current or shallow marginal circulation to reach a stable state, but at least a thousand years of integration is needed for the thermodynamically-driven circulation to approach an equilibrium state from the state of rest, otherwise the underdeveloped thermodynamically-driven circulation causes unreasonable vertical temperature and salinity structures and underestimated transport rates for deep ocean thermohaline circulation. Therefore the grid resolution is limited since years of integration with a fine grid resolution is a tremendous computational work. The SSH and surface current in the Arctic region could be exaggerated due to the exaggerated wind stress effect without the consideration of ocean-ice coupling. (8) To authors experience, such a long time scale of integration with a relative coarse resolution but without form drag induced by mesoscale eddies, could cause over- estimated zonal transport in the ACCR unless unrealistically high coefficients for the horizonal viscosity are used. The horizontal viscosity coefficient of used in this article is significantly less than of Bryan (1986) and Marotzke and Willebrand (1991). This is the main reason causing the ACC transport rate larger than the observational data. However, according to Marotzke and Willebrand s (1991) scale analysis taking the no-slip boundary conditions into account, an idealized world ocean model of a design similar to his would lead one to expect an ACC transport of m 3 /s. Ac k now l e d g e m e n t s The authors would like to thank the two anonymous reviewers for their valuable suggestions, and Shu Qi and Song Zhenya for their help on computational environment setup. References Broecker W S The biggest chill. Nat Hist Mag, 97: Broecker W S The great ocean conveyor. Oceanogr, 4: Bryan F High-latitude salinity affects and interhemispheric thermohaline circulation. Nature, 323: Dong S, Garzoli S, Baringer M The role of interocean exchanges on decadal variations of the meridional heat transport in the South Atlantic. J Phys Oceanogr, 41: Gordon A L Interocean exchange of thermocline water. J Geophys Res, 91: Gordon A L, Sprintall J, Van Aken H M, et al The Indonesian Throughflow during as observed by the INSTANT