1. Introduction
The global meridional overturning circulation (MOC) exchanges water between the surface and deep ocean and between the major ocean basins (Marshall and Speer 2012; Talley 2013; Cessi 2019). The MOC transports heat, freshwater, and biogeochemical tracers, thereby influencing climate and the cycling of carbon and nutrients in the ocean (Talley 2003; Sarmiento and Toggweiler 1984; Galbraith and de Lavergne 2019). The Atlantic MOC (AMOC) is associated with a northward transport of upper-ocean water toward northern sites of deep sinking, and a southward transport of deep water (Wunsch and Heimbach 2013; Cessi 2019). A striking interbasin asymmetry of the MOC is the absence of a strong Pacific MOC and of deep sinking in the North Pacific.
A fundamental and yet unresolved question is why there is an AMOC but no Pacific MOC (PMOC) in the present climate (Huisman et al. 2012; Ferreira et al. 2018; Weijer et al. 2019). It is well established that it is the contrast in surface salinity between the Pacific and the Atlantic that prevents deep sinking in the North Pacific (Weyl 1968; Warren 1983). In the North Pacific, surface water is fresher and lighter than the deep water, which is close to the mean deep-water salinity of the World Ocean. However, the salinity contrast in itself provides no satisfying process-based explanation, and there are diverging ideas of why this contrast arises. Several hypotheses have been proposed to explain the asymmetry in circulation and salinity between the two basin. These hypotheses fall into two main categories [see Ferreira et al. (2018) for a review]:
- H1: The salinity contrast is set by differences in net evaporation over the basins. Here, the Atlantic–Pacific difference in the surface freshwater balance is primarily viewed to be created by zonal asymmetries of the atmospheric circulation and the drainage basins (Weyl 1968; Emile-Geay et al. 2003; Ferreira et al. 2010; Wills and Schneider 2015). To the extent that the atmospheric circulation is not modified by changes in the MOC, a single equilibrium state of the MOC is expected.
- H2: The salinity contrast is set by differences in oceanic salt transports. Asymmetries in basin geometry and wind forcing as well as the oceanic salt–advection feedback contribute to elevate the Atlantic salinity (Reid 1961; Stommel 1961; Warren 1983; Nilsson et al. 2013; Jones and Cessi 2017; Weijer et al. 2019). The MOC may have multiple equilibrium states.
The asymmetry in salinity likely results from a combination of these atmospheric and oceanic processes, but their relative importance remains uncertain. Several asymmetries in mountain range distributions and ocean basin geometry have been identified that act to increase Atlantic surface salinities relative to the Pacific, either by affecting the net evaporation or the oceanic salt transports (Seager et al. 2002; Maffre et al. 2018; Reid 1961; Nilsson et al. 2013; Jones and Cessi 2017). However, progress has been limited in quantifying the numerous proposed processes and in determining their relative importance. A quantitative understanding of the geographical and climatic factors that determine the sinking locations in the World Ocean is of fundamental significance. First, when developing present-day climate models, or even upgrading existing ones, some models can yield a PMOC rather than an AMOC, or a strongly reduced AMOC compensated by increased Southern Ocean sinking (see Mecking et al. 2016; Ferreira et al. 2018, and references therein). This may indicate that the geographical features assumed to favor Atlantic sinking are rather weak or that their impacts are inadequately represented in some climate models. The AMOC “problem” is usually addressed by tuning of model parameters and drainage pathways until a realistic AMOC is obtained: an approach that may yield a model AMOC with incorrect stability features and sensitivity to global warming (Stouffer et al. 2006; Cimatoribus et al. 2012; Weijer et al. 2019; Cael and Jansen 2020). Second, the locations of the deep sinking and associated MOC pathways in past epochs of Earth can have a strong influence on carbon cycling and climate (DeConto and Pollard 2003; Ferrari et al. 2014; Galbraith and de Lavergne 2019). Thus, knowledge of which aspects of the basin geometry and climatic conditions control the MOC is crucial for understanding the ocean’s role in past as well as future climate transitions.
Motivated by these broader questions concerning the ocean salinity distribution and the MOC, we here explore and develop a diagnostic concept introduced by Ferreira et al. (2018): to analyze zonally averaged observations in evaporation–salinity diagrams. This representation encapsulates the forcing (net evaporation) and the response (surface salinity). Specifically, we extend the work of Ferreira et al. (2018) to analyze zonally averaged observations with higher latitudinal resolution in evaporation–salinity diagrams and to interpret the results using a conceptual advective–diffusive model. We begin by briefly examining observations of zonal-mean net evaporation and surface salinity. Next, we introduce and analyze the conceptual model, and then return to the observations and discuss what they can tell us about the relative importance of atmospheric and oceanic processes in setting the present-day Atlantic–Pacific salinity asymmetry.
2. The observed relationship between zonal-mean net evaporation and surface salinity
Here, we analyze net evaporation data from ERA-Interim reanalysis for the period 1979–2012 (Dee et al. 2011), with treatment of continental runoff as described in Wills and Schneider (2015), and climatological surface salinity from the World Ocean Atlas 2013 (Zweng et al. 2013). The climatological salinity is based on observations taken between 1955 and 2012, but by construction it is more influenced by the data-rich later part of the period. We have also calculated and analyzed a time-mean salinity based on the individual decadal data from 1975 to 2012 in the World Ocean Atlas 2013. For the time-mean relationship between zonal-mean net evaporation and surface salinity, which is our focus, the difference in using the 1975–2012 mean and the climatological salinity is small enough that we for simplicity have chosen to use the standard climatological salinity in the World Ocean Atlas 2013.

The zonal-mean net evaporation adjusted for (a) runoff [
Citation: Journal of Physical Oceanography 51, 3; 10.1175/JPO-D-20-0126.1
Figure 1 also reveals some deviations from a simple one-to-one relation between
- Their slopes are smaller in the tropics than in the extratropics.
- The curves tend to turn and loop near the subtropical salinity maxima: progressing poleward the curves turn anticlockwise.
- In the Indo-Pacific, the
–S relation is more equatorially asymmetric and indicates a higher salinity sensitivity to variations in net evaporation than in the Atlantic. (Progressing away from the black markers in Fig. 2, the curves are approximately parallel in the Atlantic, but not in the Indo-Pacific, where the equatorial asymmetry is larger).

A representation of the zonal-mean data in Fig. 1 in a diagram with net evaporation (adjusted for river runoff) on the x axis and sea surface salinity on the y axis for the (a) Atlantic and (b) Indo-Pacific basins from 65°S to 65°N. Note in the calculation of the zonal-mean salinity marginal seas are excluded for the Atlantic but included for the Indo-Pacific. The color scale indicates the latitude and the black marker shows the equator. Dashed lines show regression least squares fits [Eq. (2)] to the data. The slope in the Atlantic (Indo-Pacific) corresponds to a salinity change of 0.7 (1.3) psu m−1 yr−1.
Citation: Journal of Physical Oceanography 51, 3; 10.1175/JPO-D-20-0126.1
Regional details in Fig. 2 can be removed by calculating more coarse-grained

The
Citation: Journal of Physical Oceanography 51, 3; 10.1175/JPO-D-20-0126.1
This preliminary analysis of the
3. Relationship between net evaporation and surface salinity: A conceptual advective–diffusive model
The zonal-mean surface salinity is affected by meridional advection and diffusion as well as vertical salt fluxes (Ponte and Vinogradova 2016). The zonal-mean near-surface meridional flow is dominated by wind-driven Ekman transports and is generally directed poleward in the tropics and equatorward in the extratropics (Schott et al. 2013; Gordon et al. 2015). Hence, the near-surface zonal-mean flow has meridional structure, which implies vertical motion. The wind-driven gyres have only a small impact on the zonal-mean meridional flow. However, zonal shears of the gyres and vertical shears of shallow subtropical cells (McCreary and Lu 1994; Nilsson and Körnich 2008; Schott et al. 2013) as well as their seasonal and interannual variations, increase the effective meridional diffusivity on the zonally averaged salinity in the near-surface ocean (Rhines and Young 1983; Young and Jones 1991; Wang et al. 1995; Rose and Marshall 2009; Jones and Cessi 2018).

Sketch of the conceptual model of the zonal-mean salinity S(y) in a surface layer of depth h. The salinity is forced by the net evaporation
Citation: Journal of Physical Oceanography 51, 3; 10.1175/JPO-D-20-0126.1
a. A simple harmonic net evaporation field
- A diffusive limit (υ = 0), where ϕ = 0 and the ellipse reduces to a straight-line segment. For fixed values of υ and κ, this limit is approached as the wavenumber l becomes large.
- An advective limit (κ = 0) where ϕ = π/2 and the salinity is shifted 90° downstream relative to the net evaporation. Here, Eq. (14) describes a closed ellipse. For fixed values of υ and κ, this limit is approached as the wavenumber l becomes small compared to υ/κ.
Figure 5a shows evaporation–salinity relations Eq. (14) for phases given by ϕ = 0 (Pe = 0) and ϕ = π/7 (Pe ≈ 3). For nonzero advection, the relation between

Relations between net evaporation and salinity for a cosine evaporation field [Eq. (8)]. The results are presented in nondimensional form. (a) Harmonic solutions, given by Eq. (14), in the diffusive limit (ϕ = 0 or Pe = 0, dashed line) and for an advective–diffusive case (ϕ = π/7 or Pe ≈ 3, solid line). The color indicates the latitude (−1 < y/L < 1), and the diamond and square markers indicate the equator (y = 0) and the subtropical evaporation maximum (|y/L| = 0.5), respectively. (b) The
Citation: Journal of Physical Oceanography 51, 3; 10.1175/JPO-D-20-0126.1
Figure 5b shows the evaporation–salinity relation for ϕ = π/7 (corresponding to Pe ≈ 3) where the homogeneous salinity solution Eq. (16) has been added to satisfy the northern boundary condition of zero diffusive flux [Eq. (6)]. This increases the strength of the advection relative to diffusion near the northern boundary and elevates the salinity. However, there is no salt–advection feedback (Stommel 1961) as the velocity is prescribed and independent of the salinity in the model. The homogenous solution increases the salinity going northward, and the resulting
The simple cosine evaporation field illustrates how advection can shift the salinity extrema relative to the net evaporation extrema, causing a multivalued
b. Solutions for equatorially symmetric net evaporation fields
There are two equatorially symmetric features of the real net evaporation distribution (Fig. 1a) that differ from the simple single-wavenumber cosine field [Eq. (8), Fig. 4]. First, the peak in net evaporation is located closer to the equator than to the pole. Second, the amplitude of the wet equatorial extremum is larger than the amplitudes of the dry subtropical and wet subpolar extrema. Primarily, this reflects the narrow ascending regions of the Hadley circulation that confine the net precipitation in the intertropical convergence zones. These features cannot be represented by a single wavenumber cosine function and additional higher wavenumber must be included in a Fourier series expansion of
Here, we use an

Salinity solutions obtained from Eq. (7) for a net evaporation field that resembles the equatorially symmetric Atlantic net evaporation; see the appendix for computational details. (a) The symmetrized evaporation and salinity solutions for two Peclet numbers that satisfy zero diffusive salt flux at the northern boundary. The results are presented in nondimensional form. (b) An
Citation: Journal of Physical Oceanography 51, 3; 10.1175/JPO-D-20-0126.1
Figure 7 shows the

Relations between evaporation and salinity for the solutions shown in Fig. 6a, with an equatorially symmetric net evaporation field that resembles the Atlantic one. (a) A purely diffusive solution, which is equatorially symmetric (Pe = 0), and (b) an advective–diffusive solution (Pe = 2) are shown. The color indicates the latitude (y/L), and the black diamond marks the equator. Note that (a) shows only the Northern Hemisphere (0 < y/L < 1), whereas (b) shows both hemispheres (−1 < y/L < 1).
Citation: Journal of Physical Oceanography 51, 3; 10.1175/JPO-D-20-0126.1

(a) Zonal widths of the Atlantic, Indo-Pacific, and Pacific basin sectors obtained using the basin masks of Zweng et al. (2013). The dashed lines show widths of basin sectors with a constant longitudinal extent. (b) Freshwater transports per unit width G(y) [see Eq. (19)] for the Atlantic and Indo-Pacific basin sectors, calculated from the ERA-Interim reanalysis for 1979–2012 (Dee et al. 2011) as described in the text. Dashed lines show the equatorially symmetric parts of G(y), which is tied to the equatorial asymmetry of the net evaporation fields. Note that the meridional freshwater transport [F = BG, see Eq. (18)] is greater in the wider Indo-Pacific sector than in the Atlantic sector.
Citation: Journal of Physical Oceanography 51, 3; 10.1175/JPO-D-20-0126.1
Figure 7b shows how advection modifies the diffusive
Figure 6b shows the model
c. Solutions for equatorially asymmetric net evaporation fields
We now consider how hemispheric asymmetries of the

(a) Diffusive salinity solutions (Pe = 0) calculated from Eq. (20) using the data shown in Fig. 8. The dashed/dashed–dotted lines show the salinity field obtained by using the equatorially symmetric/equatorially antisymmetric parts of the freshwater transport G(y). (b) Atlantic advective–diffusive salinity fields calculated using the same data as in (a); see the text and the appendix for details. Dashed–dotted lines show the salinity field obtained by using the equatorially antisymmetric parts of G(y). [The red lines duplicate the Atlantic diffusive Pe = 0 solutions in (a)]. Note that as the Atlantic basin width varies slightly, the Peclet numbers vary with latitude, but these variations are modest; see Eq. (21). In both panels the parameter κh, which controls the amplitude of the salinity variation, is 1.5 × 106 m3 s−1.
Citation: Journal of Physical Oceanography 51, 3; 10.1175/JPO-D-20-0126.1
It is relevant to note that κ, which in the model represents an effective diffusivity associated with mesoscale eddies and wind-driven gyres, is in reality expected to have latitudinal variations. Scaling arguments suggest that diffusivity due to wind-driven gyres is proportional to the square of the wind stress curl (Wang et al. 1995; Rose and Marshall 2009), and hence has peaks at the latitudes where the transports of the tropical, subtropical, and subpolar gyres have their maxima. Further, mesoscale eddy diffusivity tends generally to decline poleward and has a local minimum near the equator (Abernathey and Marshall 2013). For simplicity, we will here take the diffusivity κ to be constant in our calculations. However, we can qualitatively infer how latitudinal variations in κ would affect our results in the diffusive limit. By inspecting Eq. (17), we see that a locally higher/lower κ gives a lower/higher salinity gradient. We also note that in the diffusive limit, variations in κ cannot shift the extrema of the salinity field, which locations occur where F(y) = 0.
In Fig. 9a the Atlantic solution is broadly similar to the observations, whereas the Indo-Pacific solution has a too pronounced northward decrease in salinity. In the calculation, the ocean physics (i.e., κh) is identical in the two “basins” implying that the differences in the salinity fields are caused only by the difference in freshwater forcing. Figure 9a also shows the salinity solutions associated with the antisymmetric and symmetric parts of G, respectively. It is the stronger interhemispheric freshwater transport per basin width (related to the equatorially symmetric part of G) in the Indo-Pacific that creates its greater south-to-north salinity difference. Physically, this results from interhemispheric moisture transport, in part associated with the Asian monsoon system (Emile-Geay et al. 2003; Wills and Schneider 2015; Craig et al. 2020). The symmetric salinity fields are fairly similar in the two basin sectors, reflecting that the equatorially symmetric parts of the net evaporation fields are roughly similar but somewhat stronger in the Atlantic.
The difference in salinity between the northern and southern ends of the basin is proportional to the integral of −G(y) over the entire basin [Eq. (20)]. Essentially, this integral measures the equatorial asymmetry of the
Figure 10 shows the

(a) Atlantic and (b) Indo-Pacific relations between net evaporation and salinity from the diffusive solutions (Pe = 0) defined by Eq. (20); shown in Fig. 9a. The dashed lines show straight line least squares fits to the data [Eq. (2)]: the slope in the Atlantic (Indo-Pacific) corresponds to a salinity change of 0.8 (1.2) psu m−1 yr−1.
Citation: Journal of Physical Oceanography 51, 3; 10.1175/JPO-D-20-0126.1

Atlantic relations between net evaporation and salinity from the advective–diffusive salinity solutions with (a) Pe ≈ 1 and (b) Pe ≈ 2, shown in Fig. 9b. The dashed lines show straight line least squares fits to the data [Eq. (2)]; the slopes in (a) and (b) correspond to a salinity change of 0.6 and 0.5 psu m−1 yr−1, respectively.
Citation: Journal of Physical Oceanography 51, 3; 10.1175/JPO-D-20-0126.1
We underline that the Atlantic basin has a fairly uniform zonal width. In the Atlantic, the simpler model with constant basin width [Eq. (7), Fig. 7] gives advective–diffusive solutions that are very similar to the ones of the model that accounts for varying basin width [Eq. (A9), Fig. 9]. In the Indo-Pacific, on the other hand, a constant northward volume transport affects the model salinity field more strongly in the northern extra tropics, where the basin is narrower and the local Peclet number higher (not shown). Furthermore, since the Indo-Pacific is wider than the Atlantic, the same northward overturning volume transport would correspond to a smaller Peclet number in the Indo-Pacific: the associated weaker northward salt advection is one factor that should favor northern sinking in the narrower Atlantic over northern sinking in the wider Indo-Pacific (Jones and Cessi 2017).
Summarizing some key results of the conceptual model analyses, we note that the limit of diffusive salt transports yields
4. Understanding observations based on the conceptual model
We now go on to further discuss the observed
a. Is the salt transport in the near-surface ocean diffusive?
The purely diffusive model calculations with realistic forcing reproduce two salient features of the observed
In the diffusive model calculation (Fig. 10), we use κh = 1.5 × 106 m3 s−1 to get a realistic salinity range. In the tropics, surface salinities are representative of the vertical-mean salinity in a relatively thin upper layer of about 100 m (see Fig. 12), which would imply an effective diffusivity κ of about 1.5 × 104 m2 s−1 in the surface ocean. This magnitude of κ is about a factor of 3 larger than the zonal-mean of the estimated mesoscale eddy diffusivities in the tropics (Abernathey and Marshall 2013), but similar to estimated local peak values in eddy diffusivities (Zhurbas and Oh 2004; Abernathey and Marshall 2013). Zonal shears associated with the wind-driven gyres serve to enhance the meridional diffusivity acting on the zonal-mean salinity (Rhines and Young 1983; Young and Jones 1991; Wang et al. 1995; Rose and Marshall 2009), which may partly rationalize the high value of κ used in the conceptual model.5 It is also possible that the large model diffusivity compensates for salinity damping processes such as vertical mixing that are not included in the model.

Zonal-mean salinity in the (a) Atlantic and (b) Indo-Pacific. The blue (black) line shows the salinity vertically averaged from the surface down to 50 (1000) m, and the red lines show the salinity in the thermocline at 200 m. Data are from the World Ocean Atlas 2013 (Zweng et al. 2013).
Citation: Journal of Physical Oceanography 51, 3; 10.1175/JPO-D-20-0126.1
Advection is another mechanism that can shift salinity extrema downstream of net evaporation extrema, irrespective of the details of the net evaporation field (Fig. 5). Gordon et al. (2015) proposed that the poleward shifts of the salinity maxima relative to those in net evaporation are primarily caused by the wind-driven surface Ekman flows, which are directed poleward in the trade-wind belt equatorward of about 30° latitude. However from the zonal-mean
b. Effects of vertical mixing
The damping due to horizontal advection and diffusion decreases with increasing spatial scales. These scale-dependent damping processes are likely too weak to control the surface salinity variations at the largest spatial scales, where vertical mixing should become more important. This is indicated by the diffusive calculation (Fig. 9a), where the Indo-Pacific solution has a north–south salinity difference that is too large compared to observed salinity variations. This reflects the weak diffusive damping of forcing at low wavenumbers.
In the diffusive calculation (Fig. 9a), the spatial-mean net evaporation over the basin sectors was removed. If the basin-mean net evaporation is retained in the calculations, there will be a corresponding uniform diffusive salinity divergence and salt export at the boundaries to balance the freshwater loss. As this spatially uniform forcing has virtually an infinite length scale, the diffusive response entails basin-scale gradients associated with large salinity variations. Specifically, including the mean Atlantic freshwater loss of 0.17 m yr−1 in the calculation, the north–South Atlantic salinity difference grows from 1 to 12 psu. This further indicates that the forcing of the surface salinity due to variations in the surface freshwater flux on interhemispheric to interbasin scales are countered by vertical mixing rather than horizontal diffusion or advection.
c. Signatures of the AMOC
The Atlantic surface salinity is fairly symmetric with respect to the equator, but as shown in Fig. 12, the Atlantic salinity is more equatorially asymmetric at depth. Presumably, this reflects the vertical structure of the meridional flow in the Atlantic. Near the surface, the zonal-mean meridional flow is roughly symmetric around the equator and primarily controlled by wind-driven Ekman transport (Gordon et al. 2015). The Atlantic meridional overturning circulation (AMOC), on the other hand, has a relatively weak impact on the near-surface flow but yields a vertical-mean northward flow in the upper kilometer of the basin (Wunsch and Heimbach 2013; Cessi 2019). Near the surface, the latitude bands with alternating meridional flow directions and enhanced zonal-mean diffusivity due to wind-driven gyres and shallow overturning cells are likely to reduce the advective signature of the AMOC on the salinity field.
Figure 13 shows the Atlantic and Indo-Pacific

A diagram with zonal-mean net evaporation (adjusted for river runoff) on the x axis and the zonal-mean salinity averaged over the upper 1000 m on the y axis. Climatological salinity from Zweng et al. (2013) is used. The (a) Atlantic and (b) Indo-Pacific basins from 40°S to 55°N; the northern limit is chosen to exclude parts in the North Pacific where large areas are shallower than 1000 m. The color scale indicates the latitude and the black marker shows the equator. Dashed lines show regression least squares fits [Eq. (2)] to the data. The slope in the Atlantic (Indo-Pacific) corresponds to a salinity change of 0.17 (0.25) psu m−1 yr−1. When the data are area averaged in subpolar and subtropical latitude bands, the Atlantic
Citation: Journal of Physical Oceanography 51, 3; 10.1175/JPO-D-20-0126.1
We note that the largest poleward shift of the surface salinity maximum relative to the net evaporation maximum is found in the subtropical North Atlantic. In Fig. 2 this manifested in a more pronounced loop of the
5. Discussion and conclusions
We used diagrams relating net evaporation and salinity to examine how atmospheric and oceanic processes shape the zonal-mean salinity in the Atlantic and Indo-Pacific. Diagrams based on observations yield curves in the
- The zonal-mean salinity field in the upper ocean (~100–150 m) appears be primarily controlled by meridional diffusive transport created by mesoscale and gyre-scale ocean eddies as well as shallow subtropical overturning cells. The effective meridional diffusivity inferred from the conceptual model is on the order of 104 m2 s−1.
- The poleward shift of the surface salinity maxima relative to the net evaporation maxima in the subtropics can be caused by either diffusive or advective transport; the
–S diagram alone cannot determine which process dominates. - The larger spatial scales associated with the interhemispheric asymmetry in the Indo-Pacific net evaporation field may be as important for creating the low surface salinities in the northern basin as the local net evaporation rate.
- The Atlantic depth-averaged
–S relation (Fig. 13) shows a greater signature of advection than the Atlantic surface relation, which appears to be shaped by diffusive transport (point 1 above).
The asymmetry in net evaporation between the Atlantic and the Pacific (and also the Indo-Pacific) is clearly important for the northern subpolar basin difference in surface salinity. Modeling studies indicate that if the present-day surface freshwater forcing pattern is amplified, the salinity difference between the North Atlantic and the North Pacific increases, and so does the AMOC (Cael and Jansen 2020). Some studies on the role of the net evaporation have emphasized local differences in subpolar regions (Warren 1983; Emile-Geay et al. 2003), whereas others have emphasized basin-integrated differences (Weyl 1968; Rahmstorf 1996). The present idealized diffusive model calculations show that, even in a basin sector with zero mean net evaporation, hemispheric asymmetries in the net evaporation field can cause a significant north–south salinity gradient. The fact that the center of mass of the net evaporation is shifted south of the equator in the Indo-Pacific sector acts to lower surface salinities in the north relative to the south, where the Antarctic Circumpolar Current serves to keep the Southern Ocean surface salinities almost zonally uniform (see Fig. 1b and Marshall and Speer 2012). Notably, Emile-Geay et al. (2003) argued that atmospheric freshwater transport due to the Asian Monsoon is crucial for creating subpolar net precipitation rates that are higher in the North Pacific than in the North Atlantic (Craig et al. 2017, 2020). With a scale-dependent damping of the surface salinity, a larger meridional fetch of the subpolar precipitation will depress the local surface salinity more. This underlines that it is not only the local precipitation rates that matter: surface freshwater forcing with low meridional wavenumber, for example, due to the Asian monsoon and other large-scale atmospheric circulation patterns in the Indo-Pacific sector (Wills and Schneider 2015; Craig et al. 2020), are a significant factor for the low surface salinities in subpolar North Pacific. Ultimately, the importance of the low wavenumber evaporative forcing on the surface salinity is determined by the relative strengths of horizontal advective–diffusive transports and vertical mixing [see Eq. (23)].
It is relevant to ask if the effective meridional diffusivities are different in the North Atlantic and North Pacific and hence may contribute to the basin asymmetry in surface salinity. In fact, estimated subpolar mesoscale-eddy diffusivities are higher in the North Atlantic, particularly when comparing the central and eastern subtropical gyres: eddy diffusivities are typically a factor of 2 larger in the North Atlantic (Zhurbas and Oh 2004; Abernathey and Marshall 2013). Simple models of meridional diffusive transport due to wind-driven gyres suggest that the effective diffusivity increases with basin width (Wang et al. 1995; Rose and Marshall 2009), which in turn suggests that the gyres should accomplish a larger meridional salt transport in the wider Indo-Pacific than in the narrower Atlantic. However, the North Pacific narrows significantly northward and is as narrow as the North Atlantic at 55°N (Fig. 8a). Thus, the widths of the northern subpolar gyres are fairly similar in the two basins. Furthermore, the tilted zero wind stress curl line and its temporal migrations in the North Atlantic are two factors that serve to enhance meridional salt transport carried by wind-driven gyres (Warren 1983; Seager et al. 2002; Czaja 2009); these features may be more important than a relatively small difference in basin widths for the surface salinity difference.
Ferreira et al. (2018) attempted to assess the relative importance of atmospheric and oceanic processes in setting the subpolar surface salinity difference of ~2 psu between the North Atlantic and North Pacific by analyzing a
We thank Paola Cessi for interesting discussions and valuable comments on this work. We also thank two anonymous reviewers for providing constructive suggestions.
Data availability statement
All data used here are available from the references given in the text.
APPENDIX
Mathematical and Physical Aspects of the Model Solutions
Here, we provide details on how solutions to the conceptual model can be obtained. We also discuss the boundary condition of zero diffusive flux at the northern model boundary: how it affects the salt flux at the southern model boundary and how this can be interpreted physically.
The solutions with nonzero advection in Figs. 6 and 9 have higher salinities in the north than the south. As the diffusive salt flux at the northern boundary is taken to be zero, Eq. (A4) implies that the diffusive flux (−κdS/dy) is positive at the southern boundary: salt conservation demands a diffusive flux at the southern boundary balancing the advective salt export from the “upper ocean” model domain; see Fig. 4. In a more complete model with vertical structure (and in reality), salt is carried from the surface to the interior ocean with the northern sinking, and is returned to the surface with the upwelling in the south. In the upwelling region near the southern boundary, processes such as vertical diffusion and advection are presumably important in the salinity balance. Thus, in the conceptual model, the lateral diffusive salt flux across the southern boundary can be viewed as a crude substitute for vertical advective–diffusive transports in a model with an active lower layer.
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In a steady state, it is freshwater and not salt that is transported, but the freshwater transport multiplied by a mean ocean salinity can be viewed as a virtual salt transport (Craig et al. 2017).
We exclude marginal seas in the zonal-mean surface salinity but include them in the zonally averaged
If
This is consistent with Eq. (15) if l−1 is viewed as a measure of the local distance between adjacent extrema in
In diffusive energy balance models, thermal ocean diffusivities, which accounts for wind-driven gyres, are typically on the order of 105 m2 s−1 (Rose and Marshall 2009).