Chinese Journal of Oceanology and Limnology   2016, Vol. 34 issue(6): 1347-1357     PDF       
http://dx.doi.org/10.1007/s00343-016-5125-y
Institute of Oceanology, Chinese Academy of Sciences
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Article Information

YANG Lina(杨丽娜), YUAN Dongliang(袁东亮)
Heat and salt transport throughout the North Pacific Ocean
Chinese Journal of Oceanology and Limnology, 34(6): 1347-1357
http://dx.doi.org/10.1007/s00343-016-5125-y

Article History

Received Apr. 16, 2015
accepted for publication Jun. 15, 2015
accepted in principle Aug. 29, 2015
Heat and salt transport throughout the North Pacific Ocean
YANG Lina(杨丽娜)1,2,3, YUAN Dongliang(袁东亮)1,3,4        
1 Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China;
2 University of Chinese Academy of Sciences, Beijing 100049, China;
3 Key Laboratory of Ocean Circulation and Waves, Chinese Academy of Sciences, Qingdao 266071, China;
4 Qingdao Collaborative Innovation Center of Marine Science and Technology, Qingdao 266003, China
ABSTRACT: Absolute geostrophic currents in the North Pacific Ocean are calculated using the P-vector method and gridded Argo profiling data from January 2004 to December 2012. Three-dimensional structures and seasonal variability of meridional heat transport (MHT) and meridional salt transport (MST) are analyzed. The results show that geostrophic and Ekman components are generally opposite in sign, with the southward geostrophic component dominating in the subtropics and the northward Ekman component dominating in the tropics. In combination with the net surface heat flux and the MST through the Bering Strait, the MHT and MST of the western boundary currents (WBCs) are estimated for the first time. The results suggest that the WBCs are of great importance in maintaining the heat and salt balance of the North Pacific. The total interior MHT and MST in the tropics show nearly the same seasonal variability as that of the Ekman components, consistent with the variability of zonal wind stress. The geostrophic MHT in the tropics is mainly concentrated in the upper layers, while MST with large amplitude and annual variation can extend much deeper. This suggests that shallow processes dominate MHT in the North Pacific, while MST can be affected by deep ocean circulation. In the extratropical ocean, both MHT and MST are weak. However, there is relatively large and irregular seasonal variability of geostrophic MST, suggesting the importance of the geostrophic circulation in the MST of that area.
Key words: absolute geostrophic current     P-vector     meridional heat transport (MHT)     meridional salt transport (MST)    
1 INTRODUCTION

The global ocean circulation is crucially important in regulating the Earth's climate through its ability to transport mass, heat, salt, and nutrients. Redistribution of heat from regions of surplus to heat deficient regions directly affects global climate patterns and variations. Unlike the transport of heat, salt transport does not feed back directly on the atmosphere, but does play a dynamic role in the thermohaline circulation through the effect of salinity stratification. Therefore, the transport of salt is related to the distribution of mass and heat (Delcroix and Hénin, 1991 ; Talley, 2008). For this reason, a correct representation of both heat and salt transport is necessary for a credible ocean model.

Traditionally, there are four approaches used to estimate oceanic meridional heat transport (MHT): a direct method using oceanographic observations (Hall and Bryden, 1982 ; Bryden et al., 1991 ; Macdonald, 1998 ; Roemmich et al., 2001 ; Zhang et al., 2002 ; Ganachaud and Wunsch, 2003 ; Talley, 2003 ; Uehara et al., 2008 ; Douglass et al., 2010); an indirect method based on surface energy fluxes (Lamb and Bunker, 1982 ; Hastenrath, 1982 ; Hsiung, 1985 ; Hsiung et al., 1989); an indirect method based on the residual term in the global air-sea-land-ice energy balance (Vonder Haar and Oort, 1973 ; Trenberth and Caron, 2001); and simulations conducted using numerical models (Marsh et al., 1996 ; Klinger and Marotzke, 2000 ; Msadek et al., 2013).

The findings of previous studies of heat transport in the North Pacific are inconsistent, both in direction and in magnitude. The reported heat transport across 30°N ranged between 1.14 petawatt (PW; 1 PW=1015 W) northward and 1.17 PW southward, as summarized by Bryden et al.(1991). Recent direct estimates of MHT at or near 24°N in the Pacific Ocean based on hydrographic data ranged between 0.5 PW and 1.0 PW, with uncertainties of between 0.1 PW and 0.3 PW (Bryden et al., 1991 ; Macdonald, 1998 ; Roemmich et al., 2001 ; Zhang et al., 2002 ; Ganachaud and Wunsch, 2003 ; Talley, 2003 ; Uehara et al., 2008 ; Douglass et al., 2010). The greatest problem with the energy budget method is uncertainty in the net radiation at the top of the atmosphere, especially at low latitudes where the heat balance is difficult to determine because of the small difference between heat gained and heat lost. Poor agreement among heat transport calculated using the energy budget method is related to temporal and spatial differences in sampling, random and systematic errors resulting from the use of different satellite sensing systems, and differences in parameter selections for the calculations, for example, the calculation of shortwave radiation. Hastenrath (1982) pointed out that imbalances in net radiation at the top of the atmosphere taken from Vonderhaar and Ellis (1974) and Gruber (1977) were about 10 W/m2 ; a systematic error of 10 W/m2 in Q v, o (Q, v, and o represent heat transport, divergence of heat transport, and ocean-land transport, respectively) would lead to an error of the order of 1 PW in oceanic heat transport around 30°N. Variability among results calculated using the direct method could arise from temporal and spatial differences in ship-based measurements, and from different methods used to calculate geostrophic currents. There is no doubt that the quality of energy heat flux data has improved in recent years, and this improvement is likely to continue with further advances in equipment for direct hydrographic measurements.

Previous studies (Bryden et al., 1991 ; Roemmich et al., 2001 ; Zhang et al., 2002) suggest that, in contrast to all other oceans, shallow processes dominate heat transport in the North Pacific. The northward MHT across 24°N in the Pacific is due half to a zonally averaged, vertical meridional circulation cell and half to a horizontal circulation cell, both of which are within the upper 700 m of the water column (Bryden et al., 1991 ; Zhang et al., 2002).

Because of the paucity of data, there are few studies of the seasonal variability of MHT in the North Pacific. Using the direct method, Bryden et al.(1991) suggested that the annual variation of poleward heat transport across 24°N is 0.2 PW. Zhang et al.(2002) found a minimum heat flux of ~0 PW in January and February and a broad maximum in the second half of the year, with a peak of 1.1 PW in July and a secondary maximum of 1.0 PW in November. Based on a numerical model, the dominant factor for this seasonal variation was attributed to the vertical cell.

Hsiung et al.(1989) used net surface energy flux data to investigate MHT in the Pacific and reported that seasonal variability exhibits two regimes. Transport north of 20°N was mostly northward with a maximum of about 1.5 PW at 25°N in July-September. Transport between 20°N and 20°S was evenly divided between northward (with a maximum of about 2.1 PW at 10°N in March) and southward (with a maximum of 1.5 PW at 10°S in August).

Using Simple Ocean Data Assimilation (SODA) data, Wang and Carton (2002) found that seasonal MHT is in phase with the annual migration of solar radiation and is primarily associated with annual changes in Ekman transport. This association leads to an annual variation of MHT of 1.4 PW with a maximum in November-December and a minimum in September. Apart from the contribution from Ekman transport, the geostrophic MHT in the central-eastern Pacific was also considered to contribute to the strong seasonal variability (Li et al., 2011). In addition, Kraus and Levitus (1986) investigated the contribution of the Ekman flux to MHT and found that Ekman flows are the dominant influence in the tropical Pacific. As a result of the meridional migration of the trade wind belt and simultaneous seasonal changes in mixed layer temperature, the Ekman MHT component tends to be larger in summer than in winter.

Historically, there are fewer salinity data than temperature data, so there are very few estimates of salt transport. Ganachaud and Wunsch (2003) calculated a mean (error) freshwater convergence of 0.14(0.26)×109 kg/s in the North Pacific (24°N-47°N) based on hydrographic sections from the World Ocean Circulation Experiment (WOCE). Using a time series of expendable bathythermograph (XBT) and expendable conductivity-temperature-depth (XCTD) data, Uehara et al.(2008) found a freshwater divergence of -0.26(0.11) Sv in the northern part of the North Pacific and a freshwater convergence of 0.08(0.07) Sv in the western Pacific. Douglass et al.(2010) estimated a freshwater transport of -0.1(0.06) Sv across the XBT track (at approximately 29°N).

Using the air-sea exchange of freshwater and coastal runoffand a meridional integration starting from the Bering Strait (0.77 Sv), Wijffels et al.(1992) found that freshwater transport is northward throughout the Pacific. Salt transport in the North Pacific is equal to that through the Bering Strait (26.7[3.3]×106 kg/s; Coachman and Aagaard (1988)). Stammer et al.(2003) analyzed mean freshwater transport using a general circulation model constrained by the WOCE data and found that global average transport is southward north of 20°S, except around 10°N where it approaches zero. Based on SODA data, Li et al.(2012) suggested a strong seasonal variability of meridional salt transport (MST) in the North Pacific south of 14°N. The MST is southward during May to November and northward at other times, reflecting seasonal oscillation of Ekman MST and geostrophic MST in the central-eastern Pacific.

Despite the informative results discussed above, many aspects of MHT and MST remain to be clarified. Heat transport calculated using the direct method is mostly based on one-time transects and as a result, sectional transport data across the entire ocean basin lack synchronicity. Estimates based on the integration of surface fluxes generally lack the point-wise structure of global ocean heat transport. Therefore, it can be difficult to determine whether numerical results from model simulations are consistent with observations. There are less salt transport data than temperature transport data and indirect estimates of MST introduce large errors resulting from uncertainties in precipitation, evaporation, and runoff. To date, there has been no observation-based analysis of point-wise ocean heat and salt transport in the North Pacific. In the present study, a picture of the North Pacific gyre is obtained based on observations, followed by a detailed study of annual mean and seasonal cycles of both heat and salt transport. The study uses Argo profiling data, which are available for most of the world oceans and lack seasonal bias, and the findings will complement previous results.

In the following section, the data and methods used to calculate absolute geostrophic currents (AGCs), MHT, and MST are described. In Section 3, the annual mean and the seasonal cycle of MHT and MST are analyzed. Key results are summarized in Section 4.

2 DATA AND METHOD 2.1 Data

This study is based primarily on Argo profiles and atmospheric wind products. Temperature and salinity profiles from Argo floats for January 2004 to December 2012 are gridded monthly onto a 1° longitude×1° latitude mesh with 58 vertical layers in the top 2 000 m of the ocean, provided by the Scripps Institution of Oceanography (Roemmich and Gilson 2009). These gridded data are used to calculate the P-vector AGCs, the geostrophic MHT, and the geostrophic MST.

Surface wind data (2004-2012) from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis, with a spatial resolution of 2.5° longitude by 2.5° latitude (Kalnay et al., 1996) are used to calculate the Ekman MHT and the Ekman MST.

2.2 P-vector AGCs

The AGCs are calculated using the P-vector method based on the above-mentioned gridded Argo data. Briefly, the intersection of the constant potential density and potential vorticity surfaces is denoted as the direction of the geostrophic currents, which is called the P-vector. The thermal wind relation is then used to calculate the magnitude of the geostrophic currents (Chu, 1995, 2006). There are two requirements of the P-vector method. One is that the constant potential density surface is not parallel to the constant potential vorticity surface, and the other is that a β spiral exists among the horizontal velocities in different vertical layers. In practice, least-squares fitting in the vertical is used to better satisfy the conditions. In addition, the conservation of potential density and potential vorticity does not meet in the upper mixed layer, so the P-vector method is applied only in the intermediate layers. Details of this approach can be found in Yuan et al.(2014).

2.3 Estimates of MHT and MST

Using AGCs, Argo temperature and salinity profiles, and surface wind data, MHT is estimated as the sum of the geostrophic MHT and the Ekman MHT, as shown in Eq.1:

    (1)

and the MST (sum of the geostrophic MST and the Ekman MST) is calculated based on Eq.2:

    (2)

where xE is the most easterly of the Argo data scope; x represents every single point west of xE ; H =2 000 m; ρ, c p, θ, and s denote the density, specific heat capacity, potential temperature, and salinity, respectively. The four variables in the Ekman layer are estimated as averages of the top 50 m, and v g is the meridional AGC. Vek =- τ x /(ρ0 f) represents the Ekman volume transport where τ x indicates zonal wind stress calculated based on NCEP/NCAR wind velocity data and the drag coefficient of Garratt (1977), ρ0 =1 025.0 kg/m3 represents the average density of seawater, and f is the Coriolis parameter.

It should be noted that the western boundary currents (WBCs) are not resolved in the gridded Argo data, so the above calculation only represents MHT and MST in the interior of the North Pacific. However, the contribution of the WBCs will be estimated indirectly in the next section.

2.4 Standard error (SE) assessment

The SE of a time average as an unbiased estimate of climatology will decrease with an increase in sampling number. Therefore, the SE is estimated using the method of Leith (1973) : where σ is the standard deviation, and v is the number of degrees of freedom (DOF). Generally, the sampling number is not the effective number of DOF because of autocorrelation of the time series. Following Bretherton et al.(1999), the DOF is estimated by ν =[1- r (Δt)2 ]/ [1+ r (Δt)2 ] N, where Δt is the sampling interval, r is the lagged autocorrelation coefficient, and N is the sampling number. The SEs of the mean MHT and MST in this paper are estimated based on the standard deviation of the monthly transport and the number of DOF.

3 RESULT AND DISCUSSION

The P-vector AGCs at the surface of the North Pacific Ocean are shown in Fig. 1. An obvious subtropical gyre is seen in the figure. The North Equatorial Current (7°-20°N), North Equatorial Countercurrent (5°-7°N), Subtropical Counter Current (roughly 20°-25°N), and the Kuroshio extension (east of Japan) are clearly identified. The western equatorial Pacific is a region of very high sea surface temperature and relatively low sea surface salinity, and as a result the area is referred to as the “warm pool”(warmer than 28.5℃) and the “fresh pool”(salinity<34.5). In contrast, the central Pacific where evaporation dominates is characterized by high salinity (>35). The temperature and salinity decrease poleward from the subtropics. These features are consistent with historical understanding.

Figure 1 AGCs at the surface of the North Pacific Ocean The white lines represent isotherms (℃), the background colors indicate salinity, and the bold black line indicates salinity of 34.25.
3.1 Annual mean meridional heat and salt transport

The MHT and MST are calculated using Eqs.1 and 2. The results of each step during the integral are kept, in analogy with the calculation of stream function. Therefore, the transport systems shown in Fig. 2 are denoted as the zonally accumulated MHT and MST. The geostrophic MHT and MST (Fig. 2a, d) above 2 000 m are mainly to the south, and the Ekman component (Fig. 2b, e), with a large magnitude south of 30°N, is generally in the opposite direction to the geostrophic component. From Fig. 2c and f, the total interior MHT (MST) shows poleward heat (salt) transport in the tropical oceans and equator-ward heat (salt) transport in the subtropical oceans, suggesting significant heat gain (excessive precipitation) in the tropics and heat loss (strong evaporation) in the subtropics. Poleward of 40°N, the MHT (MST) of the ocean currents is small, suggesting weak oceanic heat (salt) exchange at high latitudes.

Figure 2 Zonally accumulated meridional heat (a-c) and salt (d-f) transport of AGCs (a, d), surface Ekman currents (b, e), and the sum of the two currents (c, f) The contour interval is 0.5 PW for heat transport, and 0.2×109 kg/s for salt transport. The shading denotes southward transport. The dot-dashed lines denote -0.1 and 0.1 PW (a-c), and -0.05 and 0.05×109 kg/s (d-f).

The zonally integrated MHT and MST of AGCs, surface Ekman currents, and the sum of the two currents over the North Pacific Ocean are shown in Fig. 3, with the SE (calculated according to Section 2.4). The SE of the geostrophic component is negligible compared to the relatively larger errors in the Ekman component and the total interior transport south of 15°N, with a maximum of about 0.5 PW for MHT and 2×108 kg/s for MST. The time-mean geostrophic and Ekman MHT (MST) are generally opposite in sign, with the Ekman MHT (MST) dominating in the tropics and the geostrophic MHT (MST) dominating in the subtropics. Therefore, the total interior MHT (MST) is poleward in the tropics and equator-ward in the subtropics. In the subtropics near 27°N the southward total interior MHT and MST have a maximum of about 2 PW and >1×109 kg/s, respectively, and the northward total interior MHT and MST reach a maximum of 1.8 PW and 0.6×109 kg/s, respectively, at around 10°N. In the present study, the geostrophic MHT and MST in the subtropics are generally opposite to the existing estimates summarized by Ganachaud and Wunsch (2000, 2003). This discrepancy is because, in general, the Argo gridded data do not include the WBC transport. However, the WBC MHT and MST can be estimated. Based on the net surface heat flux data, the total MHT (surface-heat-flux MHT) across the entire ocean basin is calculated using meridional integration, assuming that heat transport is zero at 65°N. Neglecting upwelling and vertical mixing, the difference between the surface-heat-flux MHT and the total interior MHT largely represents the MHT of the WBCs, which is generally opposite to the total interior MHT and equally large in magnitude. However, salt cannot be directly exchanged between the ocean and the atmosphere. The salinity varies with precipitation, evaporation, and run-off, but salt itself is conserved. As a result, net salt transport across each latitude of the North Pacific Ocean equals that across the Bering Strait (~26.7×106 kg/s; Coachman and Aagaard, 1988 ; Wijffels et al., 1992). Therefore, the WBC MST can be obtained by subtracting the total interior MST from the MST through the Bering Strait, and shows features analogous to those of the WBC MHT. The direction of the estimated WBC MHT (MST) has a sign-changing point at about 12°N (13°N). This is consistent with previous studies on the bifurcation latitude of the North Equatorial Current (Toole et al., 1988 ; Qu and Lukas, 2003 ; Kim et al., 2004 ; Wang and Hu, 2006), supporting the correctness of the calculation and suggesting that the WBCs play an important role in maintaining the heat and salt balance of the North Pacific Ocean.

Figure 3 Zonally integrated meridional heat transport (a; PW) and salt transport (b; 109 kg/s) of AGCs (black dashed line), surface Ekman currents (gray solid line with circles), and the sum of the two currents (gray solid line with asterisks) The gray solid line in (a) represents the MHT calculated based on the net surface heat flux data of Da Silva et al.(1994). Black solid lines in (a) and (b), respectively, represent the WBC MHT and the WBC MST.

From Fig. 4a, the interior geostrophic MHT for each vertical layer (thickness is the difference between two adjacent layers) indicates that southward geostrophic heat transport concentrates mainly in the thermocline of the tropics and the subtropics. For the geostrophic MST (Fig. 4b), the maximum depth of salt transport with a magnitude of 1×107 kg/s can reach to 1 200 m in the vicinity of tropical/subtropical boundary, where there is a significant core of northward MST beneath 1 400 m. The results suggest that shallow processes dominate MHT in the North Pacific, while MST can be greatly affected by much deeper ocean circulation. Generally, heat and salt transport at high latitudes is small.

Figure 4 Interior geostrophic MHT (a) and MST (b) on each layer The contour interval is 2×1013 W for MHT and 1×107 kg/s for MST. Gray shading indicates southward transport.
3.2 Seasonal variability of meridional heat and salt transport

Figure 5 shows the seasonal anomaly (the residuals when the climatology is subtracted from the monthly means) of MHT (a-c) and MST (d-f) of AGCs (a, d), Ekman currents (b, e), and the sum of the two currents (c, f). In the tropics, the total interior MHT (MST) shows significant seasonal variability, which is consistent with that of the Ekman MHT (MST). Both the Ekman and the total interior transport become weaker in summer and autumn and stronger in winter and spring. In contrast, the seasonal variability of the geostrophic MHT and MST is much weaker, with a maximum variation of about 0.5 PW and 0.2×109 kg/s, respectively. In the subtropics, heat and salt transport shows no obvious seasonal variation. However, relatively large and irregular annual variation is found in the geostrophic MST and the total interior MST. This suggests that the annual variation of the geostrophic circulation dominates in the extratropics. Furthermore, because of the absence of WBCs in the calculations in the present study, it is not unexpected that the results are not consistent with previous studies. For example, based on net surface energy flux data Hsiung et al.(1989) found that northward heat transport across 25°N had a maximum of about 1.5 PW in July-September. The seasonal variability of the total MHT can be estimated using the monthly net surface heat flux data, but in the present paper Argo data is the focus. Furthermore, clarifying seasonal variability of the total interior MHT and MST is important to better understand the climate dynamics of the North Pacific Ocean and to test numerical models.

Figure 5 Seasonal anomaly of net meridional heat (a-c) and salt (d-f) transport of geostrophic currents (a, d), surface Ekman currents (b, e), and the sum of the two currents (c, f) The solid contour in a-c is 0.5 PW; the dot-dashed line denotes -0.25 PW and 0.25 PW. The solid contour interval in d-f is 0.2×109 kg/s; the dotdashed line denotes -0.1×109 kg/s and 0.1×109 kg/s, and the bold black line marks a zero value. The shading denotes negative anomalies.

From Fig. 6, the annual variation of the zonally averaged zonal wind stress in the tropics shows structure consistent with that of the total interior MHT and MST. This indicates that zonal wind stress is the dominant factor influencing seasonal variability of the MHT and MST in the tropical North Pacific Ocean. The seasonal variability of zonal wind stress is also important outside the tropics. However, the corresponding Ekman MHT and MST show weak annual variation, meaning that zonal wind stress has little to do with seasonal variability of the MHT and MST in the extratropical North Pacific, where the Coriolis parameter becomes larger.

Figure 6 Annual mean of zonally averaged zonal wind stress (a); seasonal anomaly of zonally averaged zonal wind stress (b) The shading denotes the negative anomalies. The unit is dyn/cm2.

Figure 7 shows the seasonal anomaly of the zonally integrated geostrophic MHT (a-f) and MST (g-l) on 58 vertical layers along different latitudes (5°, 10°, 15°, 20°, 25°, and 30°N). The MHT shows large annual variation only in the upper layers, approximately above the thermocline. However, the large seasonal changes in MST can extend much deeper, even to 2 000 m. This suggests a more significant effect of the deep ocean circulation on salt transport than on heat transport but further studies are needed to confirm this.

Figure 7 Seasonal anomaly of zonally integrated meridional heat (a-f) and salt (g-l) transport on each vertical layer at different latitudes The contour interval is 0.5×1013 W for the MHT and 0.25×107 kg/s for the MST. The areas with negative values are shaded.

Previous studies (e.g., Stammer, 1998 ; Wunsch, 1999) have shown that the effects of mesoscale eddies are of major importance in the WBCs. The distribution of temperature, salinity, and velocity are reconstructed by eddies (Kamenkovich et al., 2011), thus affecting the meridional heat and salt transport. Chen et al.(2012) estimated meridional eddy heat and salt transport in the South China Sea and explored the dynamics of seasonal modulation of eddy transport using a 2 1 / 2 -layer reduced-gravity model. Their results showed that baroclinic instability is critical to the seasonal variation of eddy kinetic energy, and consequently, eddy transport. Accordingly, the seasonal variability of MHT and MST in WBCs and the North Equatorial Counter Current of the North Pacific Ocean could be further investigated using altimetry data, Argo profiles, and numerical models.

4 CONCLUSION

In this study, the P-vector method is used to calculate AGCs in the North Pacific Ocean based on gridded Argo profiling data from January 2004 to December 2012, and point-wise geostrophic MHT and MST are analyzed. Zonally integrated geostrophic and Ekman MHT (MST) are generally opposite in sign, with the southward geostrophic MHT (MST) dominating in the subtropics and the northward Ekman MHT (MST) dominating in the tropics. There are relatively large SEs in the Ekman component and the total interior transport south of 15°N with a maximum of about 0.5 PW and 2×108 kg/s for MHT and MST, respectively. The geostrophic MHT in the tropics and subtropics is concentrated in the upper layers, while the large-magnitude MST can extend much deeper, even to 2 000 m near the tropical/ subtropical boundary where there is a significant northward MST beneath 1 400 m. Using net surface heat flux data and MST through the Bering Strait, the MHT and MST of the WBCs are estimated, to the best of our knowledge, for the first time. The WBC MHT (MST) is generally opposite to the total interior MHT (MST) and equally large in magnitude.

The total interior MHT and MST in the tropics show nearly the same seasonal variability (weaker in summer and autumn, and stronger in winter and spring) as that of the corresponding Ekman MHT and MST, which is in accordance with the variation of zonal wind stress. In the extratropical North Pacific, annual variation of heat and salt transport is much weaker than in the tropics. However, relatively large and irregular annual variation is found in the geostrophic MST and the total interior MST, suggesting the dominance of the geostrophic circulation at high latitudes. Additionally, the MHT shows large annual variation only in the upper layers, while the MST changes throughout the water column. This finding suggests that shallow processes dominate MHT in the North Pacific, while MST can be greatly affected by much deeper ocean circulation.

References
Bretherton C S, Widmann M, Dymnikov V P, et al, 1999. The effective number of spatial degrees of freedom of a timevarying field. Journal of Climate, 12 (7) : 1 990 –2 009. Doi: 10.1175/1520-0442(1999)012<1990:TENOSD>2.0.CO;2
Bryden H L, Roemmich D H, Church J A, 1991. Ocean heat transport across 24°N in the Pacific. Deep Sea Research Part A. Oceanographic Research Papers, 38 (3) : 297 –324. Doi: 10.1016/0198-0149(91)90070-V
Chen G X, Gan J P, Xie Q, Chu X Q, Wang D X, Hou Y J, 2012. Eddy heat and salt transports in the South China Sea and their seasonal modulations. J. Geophys. Res., 117 (C5) . Doi: 10.1029/2011JC007724
Chu P C, 1995. P-vector method for determining absolute velocity from hydrographic data. Mar. Technol. Soc. J., 29 (2) : 3 –14.
Chu P C. 2006. P-Vector Inverse Method. Springer-Verlag, Berlin, Heidelberg, Germany. 605p.
Coachman L K, Aagaard K, 1988. Transports through Bering Strait:Annual and interannual variability. J. Geophys.Res., 93 (C12) : 15 535 –15 539. Doi: 10.1029/JC093iC12p15535
Da Silva A M, Young C C, Levitus S. 1994. Atlas of surface marine data 1994, volume 1:algorithms and procedures.NOAA Atlas NESDIS6. U.S. Department of Conmerce, NOAA, NESDIS:83p.
Delcroix T, Hénin C, 1991. Seasonal and interannual variations of sea surface salinity in the tropical Pacific Ocean. J.Geophys. Res., 96 (C12) : 22 135 –22 150. Doi: 10.1029/91JC02124
Douglass E, Roemmich D, Stammer D, 2010. Interannual variability in North Pacific heat and freshwater budgets. Deep Sea Research Part II:Topical Studies in Oceanography, 57 (13-14) : 1 127 –1 140. Doi: 10.1016/j.dsr2.2010.01.001
Ganachaud A, Wunsch C, 2000. Improved estimates of global ocean circulation, heat transport and mixing from hydrographic data. Nature, 408 (6811) : 453 –457. Doi: 10.1038/35044048
Ganachaud A, Wunsch C, 2003. Large-scale ocean heat and freshwater transports during the world ocean circulation experiment. Journal of Climate, 16 (4) : 696 –705. Doi: 10.1175/1520-0442(2003)016<0696:LSOHAF>2.0.CO;2
Garratt J R, 1977. Review of drag coefficients over oceans and continents. Mon. Wea. Rev., 105 (7) : 915 –929. Doi: 10.1175/1520-0493(1977)105<0915:RODCOO>2.0.CO;2
Gruber A. 1977. Determination of the earth-atmosphere radiation budget from NOAA satellite data. NOAA Technical Report NESS 76, U. S. Department of Commerce, National Oceanic and Atmospheric Administration, National Environmental Satellite Service, Washington D C, UK. 28p.
Hall M M, Bryden H L, 1982. Direct estimates and mechanisms of ocean heat transport. Deep Sea Research Part A.Oceanographic Research Papers, 29 (3) : 339 –359. Doi: 10.1016/0198-0149(82)90099-1
Hastenrath S, 1982. On meridional heat transports in the world ocean. J. Phys. Oceanogr., 12 (8) : 922 –927. Doi: 10.1175/1520-0485(1982)012<0922:OMHTIT>2.0.CO;2
Hsiung J, Newell R E, Houghtby T, 1989. The annual cycle of oceanic heat storage and oceanic meridional heat transport. Quart. J. Roy. Meteor. Soc., 115 (485) : 1 –28. Doi: 10.1002/(ISSN)1477-870X
Hsiung J, 1985. Estimates of global oceanic meridional heat transport. J. Phys. Oceanogr., 15 (11) : 1 405 –1 413. Doi: 10.1175/1520-0485(1985)015<1405:EOGOMH>2.0.CO;2
Kalnay E, Kanamitsu M, Kistler R, Collins W, Deaven D, Gandin L, Iredell M, Saha S, White G, Woollen J, Zhu Y, Leetmaa A, Reynolds B, Chelliah M, Ebisuzaki W, Higgins W, Janowiak J, Mo K C, Ropelewski C, Wang J, Jenne R, Joseph D, 1996. The NCEP/NCAR 40-Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77 (3) : 437 –471. Doi: 10.1175/1520-0477(1996)077<0437:TNYRP>2.0.CO;2
Kamenkovich I, Cheng W, Schmid C, Harrison D E, 2011. Effects of eddies on an ocean observing system with profiling floats:idealized simulations of the Argo array. J.Geophys. Res., 116 (C6) . Doi: 10.1029/2010JC006910
Kim Y Y, Qu T D, Jensen T, et al, 2004. Seasonal and interannual variations of the North Equatorial Current bifurcation in a high-resolution OGCM. J. Geophys. Res., 109 (C3) : C03040. .
Klinger B A, Marotzke J, 2000. Meridional heat transport by the subtropical cell. J. Phys. Oceanogr., 30 (4) : 696 –705. Doi: 10.1175/1520-0485(2000)030<0696:MHTBTS>2.0.CO;2
Kraus E B, Levitus S, 1986. Annual heat flux variations across the tropic circles. J. Phys. Oceanogr., 16 (8) : 1 479 –1 486. Doi: 10.1175/1520-0485(1986)016<1479:AHFVAT>2.0.CO;2
Lamb P J, Bunker A F, 1982. The annual march of the heat budget of the north and tropical Atlantic Oceans. J. Phys.Oceanogr., 12 (12) : 1 388 –1 410. Doi: 10.1175/1520-0485(1982)012<1388:TAMOTH>2.0.CO;2
Leith C E, 1973. The standard error of time-average estimates of climatic means. J. Appl. Meteor., 12 (6) : 1 066 –1 069. Doi: 10.1175/1520-0450(1973)012<1066:TSEOTA>2.0.CO;2
Li P, Zhang Q L, Liu H W, Xu J P, 2011. Seasonal variation of the North Pacific meridional net heat transport. Advances in Marine Science, 29 (3) : 275 –284.
Li P, Zhang Q P, Liu H W, Xu J P, 2012. Seasonal variation of meridional salt transport in the North Pacific Ocean. Journal of Tropical Oceanography, 31 (4) : 28 –34.
Macdonald A M, 1998. The global ocean circulation:a hydrographic estimate and regional analysis. Progress in Oceanography, 41 (3) : 281 –382. Doi: 10.1016/S0079-6611(98)00020-2
Marsh R, New A L, Roberts M J, Wood R A, 1996. An intercomparison of a Bryan-Cox-type ocean model and an isopycnic ocean model. Part II:The subtropical gyre and meridional heat transport. J. Phys. Oceanogr., 26 (8) : 1 528 –1 551.
Msadek R, Johns W E, Yeager S G, Danabasoglu G, Delworth T L, Rosati A, 2013. The Atlantic meridional heat transport at 26. 5°N and its relationship with the MOC in the RAPID array and the GFDL and NCAR coupled models. Journal of Climate, 26 (12) : 4 335 –4 356.
Qu T D, Lukas R, 2003. The Bifurcation of the North Equatorial Current in the Pacific. J. Phys. Oceanogr., 33 (1) : 5 –18. Doi: 10.1175/1520-0485(2003)033<0005:TBOTNE>2.0.CO;2
Roemmich D, Gilson J, Cornuelle B, Weller R, 2001. Mean and time-varying meridional transport of heat at the tropical/subtropical boundary of the North Pacific Ocean. J. Geophys. Res., 106 (C5) : 8 957 –8 970. Doi: 10.1029/1999JC000150
Roemmich D, Gilson J, 2009. The 2004-2008 mean and annual cycle of temperature, salinity, and steric height in the global ocean from the Argo Program. Progress in Oceanography, 82 (2) : 81 –100. Doi: 10.1016/j.pocean.2009.03.004
Stammer D, Wunsch C, Giering R, Eckert C, Heimbach P, Marotzke J, Adcroft A, Hill C N, Marshall J, 2003. Volume, heat, and freshwater transports of the global ocean circulation 1993-2000, estimated from a general circulation model constrained by World Ocean Circulation Experiment (WOCE) data. J. Geophys. Res., 108 (C1) : 3 007 . Doi: 10.1029/2001JC001115
Stammer D, 1998. On eddy characteristics, eddy transports, and mean flow properties. J. Phys. Oceanogr., 28 (4) : 727 –739. Doi: 10.1175/1520-0485(1998)028<0727:OECETA>2.0.CO;2
Talley L D, 2003. Shallow, intermediate, and deep overturning components of the global heat budget. J. Phys. Oceanogr., 33 (3) : 530 –560. Doi: 10.1175/1520-0485(2003)033<0530:SIADOC>2.0.CO;2
Talley L D, 2008. Freshwater transport estimates and the global overturning circulation:shallow, deep and throughflow components. Progress in Oceanography, 78 (4) : 257 –303. Doi: 10.1016/j.pocean.2008.05.001
Toole J M, Zou E, Millard R C, 1988. On the circulation of the upper waters in the western equatorial Pacific Ocean. Deep Sea Research Part A. Oceanographic Research Papers, 35 (9) : 1 451 –1 482. Doi: 10.1016/0198-0149(88)90097-0
Trenberth K E, Caron J M, 2001. Estimates of meridional atmosphere and ocean heat transports. Journal of Climate, 14 (16) : 3 433 –3 443. Doi: 10.1175/1520-0442(2001)014<3433:EOMAAO>2.0.CO;2
Uehara H, Kizu S, Hanawa S, Yoshikawa Y, Roemmich D, 2008. Estimation of heat and freshwater transports in the North Pacific using high-resolution expendable bathythermograph data. J. Geophys. Res., 113 (C2) : C02014 .
Vonder Haar T H, Oort A H, 1973. New estimate of annual poleward energy transport by northern hemisphere oceans. J. Phys. Oceanogr., 3 (2) : 169 –172. Doi: 10.1175/1520-0485(1973)003<0169:NEOAPE>2.0.CO;2
Vonderhaar T H, Ellis J S. 1974. Atlas of radiation budget measurements from satellites, 1962-1970. Atmospheric Science Technical Report no. 231, Colorado State University, Fort Collins, CO, United States. 180p.
Wang J D, Carton J A, 2002. Seasonal heat budgets of the North Pacific and North Atlantic Oceans. J. Phys.Oceanogr., 32 (12) : 3 474 –3 489. Doi: 10.1175/1520-0485(2002)032<3474:SHBOTN>2.0.CO;2
Wang Q Y, Hu D X, 2006. Bifurcation of the North Equatorial Current derived from altimetry in the Pacific Ocean. Journal of Hydrodynamics, Series B, 18 (5) : 620 –626. Doi: 10.1016/S1001-6058(06)60144-3
Wijffels S E, Schmitt R W, Bryden H L, Stigebrandt A, 1992. Transport of freshwater by the oceans. J. Phys. Oceanogr., 22 (2) : 155 –162. Doi: 10.1175/1520-0485(1992)022<0155:TOFBTO>2.0.CO;2
Wunsch C, 1999. Where do ocean eddy heat fluxes matter? J. Geophys. Res., 104 (C6) : 13 235 –13 249. Doi: 10.1029/1999JC900062
Yuan D L, Zhang Z C, Chu P C, Dewar W K, 2014. Geostrophic circulation in the Tropical North Pacific Ocean based on Argo Profiles. J. Phys. Oceanogr., 44 (2) : 558 –575. Doi: 10.1175/JPO-D-12-0230.1
Zhang D, Johns W E, Lee T N, 2002. The seasonal cycle of meridional heat transport at 24°N in the North Pacific and in the global ocean. J. Geophys. Res., 107 (C7) : 20-1 –20-24.