Seasonal mixed layer salt budget of the tropical
Atlantic Ocean
Gregory R. Foltz, Semyon A. Grodsky*, James A. Carton, and Michael J. McPhaden1
Submitted to Journal of Geophysical Research - Oceans
April 8, 2003
Department of Meteorology
University of Maryland
College Park, MD 20742
1
NOAA/Pacific Marine Environmental Laboratory
7600 Sand Point Way NE
Seattle, WA 98115
* corresponding author: senya@atmos.umd.edu
Abstract
This paper addresses the atmospheric and oceanic causes of the seasonal cycle of mixed
layer salinity in the tropical Atlantic based on direct observations and model data. Data sets
include up to five years (September 1997 - December 2002) of measurements from moored
buoys of the Pilot Research Array in the Tropical Atlantic (PIRATA), near-surface drifting
buoys, and a numerical ocean model reanalysis. We analyze the mixed layer salt balance at nine
PIRATA mooring locations and find that the seasonal cycles of evaporation, precipitation,
entrainment, and mean horizontal salt advection all contribute to seasonal mixed layer salinity
variability in the northwest (4º - 15ºN along 38ºW). The balance is similarly complex along the
equator. Here precipitation decreases eastward (between 35ºW and 10ºW), while freshening from
zonal advection increases eastward. Horizontal eddy advection provides an important source of
freshening along the equator during boreal summer and fall, when tropical instability waves are
present. Meridional advection, combined with entrainment and vertical turbulent diffusion (we
suspect), opposes the freshening effects of precipitation, mean zonal advection, and eddy
advection, resulting in a weak seasonal cycle of mixed layer salinity. The balance in the
southeast (6 - 10ºS along 10ºW) includes significant contributions from mean horizontal
advection. Here our estimates are highly uncertain due to a lack of knowledge of horizontal
salinity transport.
1. Introduction
Salinity affects the density of seawater, spatial gradients of which affect buoyancy,
turbulent mixing, and ocean currents. Recent numerical modeling studies indicate that realistic
three-dimensional salinity fields are required in order to accurately represent tropical ocean
2
dynamics and thermodynamics (Cooper, 1988; Carton, 1991; Murtugudde and Busalacchi,
1998; Vialard and Delecluse, 1998). In support of these modeling results, a number of
observational studies show that changes in mixed layer salinity can dramatically affect currents
and temperature in the tropics through the formation of barrier layers (Lukas and Lindstrom,
1991; Roemmich et al., 1994; Pailler et al., 1999). In this study we use in situ, satellite, and
model data to determine the oceanic and atmospheric causes of the seasonal cycle of mixed layer
salinity in the tropical Atlantic.
The flux of moisture at the ocean’s surface is given by the difference between
precipitation and evaporation. The annual mean surface moisture flux in the tropical Atlantic
reflects the mean position of the Intertropical Convergence Zone (ITCZ) and the subtropical
high-pressure systems. In the southern (south of 5S) and northern (north of 10N) tropical
Atlantic, annual mean evaporation exceeds precipitation, while in the latitude band of the ITCZ
precipitation dominates, with a (zonally averaged) peak of 20 cm mo-1 near 5N (Yoo and
Carton, 1990). Seasonal changes in the surface moisture flux result mainly from latitudinal
movements of the ITCZ.
The seasonal cycle of evaporation is dominated by the annual harmonic throughout most
of the tropical Atlantic, with increasing amplitude toward the subtropics. In the northern tropics,
evaporation peaks in boreal winter. In this season, the ITCZ is situated close to its southernmost
latitude, with increasing northeast trade winds and decreasing relative humidity northward from
the equator. In boreal summer and fall, the ITCZ is located near its northernmost latitude (10 12N), resulting in higher relative humidity, lower wind speed, and less evaporation north of
5N. The seasonal cycle of evaporation in the southern tropics is similar in amplitude, but
opposite in phase, to that in the north; maximum evaporation occurs in boreal summer, when the
3
southeast trade winds are strong and relative humidity is low. Along the equator in the eastern
basin, wind speed and relative humidity change very little on a seasonal basis, resulting in weak
seasonal variations of evaporation, while in the west a significant annual harmonic appears, with
maximum in late boreal summer. Between the equator and 8N the double passage of the ITCZ
results in a significant semiannual harmonic, with highest amplitude in the west (da Silva et al.,
1994).
The seasonal cycle of precipitation in the tropical Atlantic results from meridional
movements of the ITCZ and zonal shifts of convergence within it. In April, a band of high
precipitation (> 20 cm mo-1) is situated west of 30W between 5S and 5N, increasing westward
toward the coast of South America. Precipitation is also high (> 15 cm mo-1) at this time in the
Gulf of Guinea, increasing eastward along the equator toward the African coast. During May
through August the zonal band of high rainfall (> 20 cm mo-1) shifts northward and increases in
intensity to up to 60 cm mo-1 near the African coast. From September through March, the ITCZ
drifts southward from its northernmost latitude (~ 10-12N during August), while the region of
high rainfall decreases in intensity and becomes increasingly concentrated in the western basin.
As a result, the seasonal cycle of precipitation acquires a significant semiannual harmonic near
5N, with maxima in June and December.
In addition to the surface moisture flux, ocean dynamics can affect mixed layer salinity.
Near-surface currents in the tropical Atlantic are dominated by the westward South Equatorial
Current south of 5N and the eastward North Equatorial Countercurrent centered near 5N (see
Fig. 1). The intensity of these zonal currents fluctuates seasonally in response to the seasonal
movement of the ITCZ and associated changes in near-surface wind stress. Near the equator, the
South Equatorial Current is strongest in boreal summer, with speeds exceeding 50 cm s-1 in mid
4
basin. On and just north of the equator are found strong velocity and temperature fluctuations
associated with tropical instability waves, which we anticipate are also important in mixing
salinity. Near the western boundary, the South Equatorial Current feeds the North Brazil Current,
which flows continuously northward toward the Caribbean during boreal winter and spring. In
boreal summer and fall, the North Brazil Current curves back upon itself near 8N, then flows
eastward to feed the North Equatorial Countercurrent. In the process, strong eddies form in the
western basin between the equator and 10N (Richardson and Reverdin, 1987). These eddies
have little effect on sea surface temperature (SST) due to weak gradients of SST in the western
basin. However, we anticipate that they may have a significant effect on mixed layer salinity.
The annual mean spatial distribution of sea surface salinity (SSS) in the tropical Atlantic
(see Fig. 1) to a large extent reflects the annual mean surface moisture flux and river outflow.
SSS is highest in the subtropics (> 36 psu), where evaporation exceeds precipitation, and
decreases to a minimum near 5N (< 35 psu in the east), where precipitation reaches a maximum
(> 17 cm mo-1). In the eastern basin, salinity is low (< 35 psu) in the Gulf of Guinea, where
annual mean precipitation and river outflow are high, and increases southwestward toward the
subtropics.
Although the surface moisture flux contributes significantly to the annual mixed layer
salinity budget in the tropical Atlantic, horizontal advection also plays an important role. Indeed,
Johnson et al. (2002) found that a substantial portion of the annual mean freshwater flux in the
tropical oceans is balanced by oceanic transports within the surface layer (upper 32.5 m).
They found large contributions from both horizontal and vertical advection near the equator in
the eastern tropical Atlantic and from meridional advection in the northwest, where meridional
currents and salinity gradients are strong (see Fig. 1). Observational studies in the tropical
5
Pacific have also stressed the importance of horizontal advection in the mixed layer salinity
budget on seasonal to interannual time scales (e.g., Delcroix and Henin, 1991; Cronin and
McPhaden, 1998; Delcroix and Picaut, 1998; Henin et al., 1998).
Several observational studies have addressed the seasonal cycle of surface salinity in the
tropical Atlantic. They have revealed strong seasonal variability near the mouths of major rivers
(Amazon and Congo) and important contributions from horizontal advection both in the annual
mean and on seasonal time scales. From ship-of-opportunity data, Dessier and Donguy (1994)
found that the seasonal cycle of surface salinity is dominated by the annual and semiannual
harmonics, with amplitudes that are largest in the far western basin (west of 40W, 0 - 20N)
and along the African coast (5N - 15N and 0 - 15S). They show that the seasonal cycle in the
western basin is dominated by the annual harmonic and is strongly influenced by advection from
the Amazon River. In boreal winter a region of fresh water (< 36 psu) extends northwestward
along the coast of South America from the mouth of the Amazon to 15N. In boreal spring, as
outflow from the Amazon increases, the fresh pool expands outward from the coast to 45W and
northward into the Caribbean. By late boreal summer and fall, coincident with the arrival of the
ITCZ and the strengthening of the North Equatorial Countercurrent, a band of fresh water
extends eastward across the basin between 4 and 10N. Associated with this fresh pool is a
significant barrier layer that is maintained at the surface by eastward advection from the Amazon
and at depth by southwestward advection of high-salinity water subducted in the subtropics
(Sprintall and Tomczak, 1992; Pailler et al. 1999).
The seasonal cycle of surface salinity acquires a significant semiannual harmonic in the
central basin between the equator and 5N, with minima corresponding approximately to the
northward and southward crossings of the ITCZ. Farther east, SSS again is dominated by the
6
annual harmonic, with a minimum in boreal winter along the African coast (0 - 5S). Here the
Congo River reaches maximum outflow in January, nearly four times weaker than that of the
Amazon. Southward and westward from the Gulf of Guinea, the minimum in SSS occurs
progressively later in the year, occurring in boreal fall near 10S, 10W (Dessier and Donguy,
1994).
Based on a limited number of observational studies in the tropical Atlantic, it is clear that
evaporation, precipitation, and horizontal advection all contribute to the observed seasonal cycle
of mixed layer salinity. In addition, analyses of the mixed layer heat budget suggest that vertical
advection and/or turbulent diffusion are also important along the equator (Molinari et al., 1985;
Weingartner and Weisberg, 1991; Carton and Zhou, 1997; Foltz et al., 2003). The need to
understand the seasonal cycle of mixed layer salinity has been supported by modeling studies,
which reveal that mixed layer salinity contributes significantly to upper ocean dynamic height
and potentially SST, through the formation of barrier layers. In this study we examine the
seasonal mixed layer salinity budget using new comprehensive oceanic and surface
meteorological data sets from nine PIRATA mooring locations in the tropical Atlantic. Our study
complements an earlier analysis of the mixed layer heat balance using similar data sets at these
same mooring sites (Foltz et al., 2003).
2. Data and Methods
Following the methodology of Stevenson and Niiler (1983) and Delcroix and Henin
(1991), the mixed layer salt balance can be written as





S
h
 h v  S  v   S   S we     vˆ Sˆdz  ( E  P) S  Fh
t
h
0
7
(1a)

 h
we  H     hv 
 t

(1b)
The terms in (1a) represent, from left to right, local salt storage, horizontal advection (separated
into monthly mean and eddy terms), entrainment, vertical salinity/velocity covariance, surface
moisture flux, and turbulent diffusion at the base of the mixed layer. Entrainment velocity (1b) is
associated with a mass flux that crosses an isopycnal surface (H is the Heaviside unit function).

Here h is the depth of the mixed layer, S and v are salinity and velocity, respectively, vertically

averaged from the surface to a depth of –h, S  and v  are deviations from their monthly means

(the overbar represents a monthy mean), Ŝ and v̂ represent deviations from the vertical average,
S  S  S h , E is evaporation, and P is precipitation. We have found that the vertical
salinity/velocity covariance term in (1a) is relatively small (< 0.7 mg m-2 s-1 on a monthly basis)
at all locations considered in this study (based on climatological subsurface data from the SODA
reanalysis, Carton et al., 2000). We therefore proceed to neglect this term. We also neglect
vertical turbulent diffusion (last term on the right), although we anticipate that this term may
contribute significantly along the equator, especially in the eastern Atlantic cold tongue, based
on studies in the equatorial Pacific (e.g., Hayes et al., 1991; Lien et al., 2002).
The PIRATA mooring array (Servain et al., 1998) consists of 12 buoys (see Fig. 1). We
focus on nine with record lengths exceeding two years. Deployed in 1997 to study oceanatmosphere interactions, these Next Generation Autonomous Temperature Line Acquisition
System (ATLAS) buoys measure subsurface ocean temperature and salinity, rainfall, and surface
air temperature, relative humidity, and wind velocity. Ocean temperature is measured at 11
depths between 1 and 500 m with 20 m spacing in the upper 140 m, while salinity is measured
only at 1, 20, 40, and 120 m. The 1 m temperature and salinity sensors provide bulk measures of
8
SST and SSS, respectively. Air temperature and relative humidity are measured at a height of 3
m above sea level, while rainfall and wind velocity are measured at 3.5 and 4 m, respectively.
The sampling interval is ten minutes for all variables except rainfall, which is sampled at oneminute intervals. The instrument accuracies are: water temperature within  0.01C, salinity
within  0.02 psu, wind speed  0.3 m s-1 or 3% (whichever is greater), air temperature  0.2C,
relative humidity  3%, and rainfall  0.4 mm hr-1 (Freitag et al., 1994, 1999, 2001; Serra et al.,
2001, Lake et al., 2003). Here we use both 10-minute and daily averaged data, which are
transmitted in near-real time via satellite by Service Argos.
All 10-minute PIRATA data used in this study (precipitation, air temperature, relative
humidity, SST, and wind speed) have been analyzed by PMEL for quality control purposes
(Freitag et al., 1999). The quantity of 10-minute quality-controlled subsurface temperature and
salinity data is low. For these data we use daily averaged values based on real-time data streams,
which are available with minimal quality control (H. P. Freitag, personal communication, 2003).
We eliminate questionable values based on visual inspection and only when it is clear that a
sensor was malfunctioning.
Daily salinity measurements required substantial quality control at some locations. Our
quality control procedure begins with a subjective identification of uncharacteristic highfrequency oscillations (we believe to result from a significant difference between response times
of temperature and conductivity measurements). We next remove data with a significant bias (>
1 psu) with respect to the mean of the remaining time series at that location. Finally, we
eliminate data at multiple depth levels if the difference in salinity between the levels begins to
rapidly increase with time, which generally indicates instrument drift. The resulting time series
are gappy. In order to fill in some of these gaps we use data from an adjacent depth level when
9
the time series at the two levels are visually indistinguishable, as frequently occurs in the mixed
layer (see Fig. 2). In some cases, even after gap filling, there are not enough data to estimate
monthly S and S . In these situations, when there are fewer than 15 daily estimates for a given
climatological month (this generally occurs for between two and four months at each location),
we use World Ocean Atlas (WOA) 2001 (Boyer et al., 2002) subsurface salinity to estimate S
and S since we have found reasonable agreement between WOA 2001 and PIRATA monthly
salinity values. With the exception of 1 m and 20 m measurements at 0, 23W and 4N, 38W,
seasonal variations of WOA 2001 and PIRATA subsurface salinity are positively correlated at all
locations. Highest correlation coefficients (> 0.5 at most depths) occur between 8N and 15N
along 38W. Here we also find strong annual mean biases (PIRATA values are lower by up to
0.4 psu except at 15N, where PIRATA are higher by 0.3 psu). We therefore remove the annual
means from WOA 2001 salinity data before using them to replace missing PIRATA values (i.e.,
seasonal variations from WOA 2001 are added to PIRATA annual means).
The data records at 0, 10W are among the shortest of all PIRATA mooring records
used in this study, yet a seasonal cycle is discernable for all surface variables with the possible
exception of wind speed (see Fig. 3). The records at 8N, 38W are somewhat longer and reveal
a stronger seasonal cycle for all surface variables. Precipitation is available directly from the
moorings. We use ten-minute measurements of precipitation and correct for wind effects
following the methodology of Serra et al. (2001). Maximum corrections are in the western basin
during months of high rainfall (corrections are ~ 2 mg m-2 s-1 (0.2 mm hr-1)) during May at 4N,
38W and during November at 8N, 38W). We estimate the remaining terms in (1a) from a
combination of observational and model data.
10
Evaporation is estimated using the bulk parameterization, E   a CeW (q s  q) , where E
is evaporation rate,  a is air density, C e is the transfer coefficient, W is wind speed, q is the
water vapor mixing ratio, and q s  0.98q sat Ts  is the interfacial water vapor mixing ratio, which
is assumed to be proportional to the saturation water vapor mixing ratio (the factor of 0.98
accounts for salinity effects). Tests of this algorithm on data from the Coupled OceanAtmosphere Response Experiment (COARE) in the tropical west Pacific have revealed a bias of
0.16 cm mo-1 (COARE estimates are lower) (Fairall et al., 1996). We use ten-minute
measurements of air temperature, SST, wind speed, and relative humidity to estimate
evaporation and neglect both cool skin and warm layer effects (see Foltz et al., 2003 for an
estimate of their magnitudes).
We estimate the mixed layer depth as the depth at which temperature is 0.5C below
SST. Throughout most of the tropical Atlantic temperature controls density so that our
temperature-based estimates of mixed layer depth agree well with those based on density. An
exception is in the northwestern basin, where persistent barrier layers have been observed
(Sprintall and Tomczak, 1992; Pailler et al., 1999). We have compared temperature-based
(0.5C criterion) and density-based (0.15   criterion, corresponding to a 0.5C criterion if
vertical salinity variations are zero) mixed layer depth estimates, based on daily temperature and
salinity, and found barrier layers of up to 20 m at 8N, 38W (see Fig. 4). Here the barrier layer
is largest during boreal fall, when the low-salinity Amazon plume extends farthest eastward. At
all other locations, we found no evidence of significant barrier layer formation (a moderate
barrier layer (~ 10 m) is present at 12N, 38W during boreal fall). Unfortunately, the number of
simultaneous daily temperature and salinity measurements at most locations is small, and since
there are no salinity measurements between 40 and 120 m, mixed layer depth estimates in this
11
range would be uncertain if based on density. In addition, we have found that accounting for the
seasonal barrier layer at 8N, 38W results in adjustments of less than 1 mg m-2 s-1 to each term
in (1a) (zonal advection and salt storage are the only terms significantly affected). For these
reasons, we estimate mixed layer depth from temperature alone.
To calculate horizontal salt advection and entrainment velocity we first estimate the
seasonal cycle of near-surface (~ 15 m) horizontal velocity following the procedure of Grodsky
and Carton (2001). We anticipate that meridional velocity in the mixed layer is primarily the
result of Ekman drift (since the meridional component of geostrophic velocity is weak) that
decreases with increasing depth. We therefore apply a correction that assumes a linear decrease
in meridional velocity from the observed value at 15 m to zero at -h, following Foltz et al.
(2003). No correction is applied for h < 15 m since we cannot accurately estimate surface
velocity needed for interpolation from 15 m to the surface. This correction leads to annual mean
mixed layer salt advection adjustments (with respect to values obtained from constant vertical
profiles of meridional velocity) of less than 1.2 mg m-2 s-1 at all locations. However, on a
monthly basis, adjustments are more than 2 mg m-2 s-1 at 4N, 38W during August and also at
15N, 38W during January as the result of deep (> 80 m) mixed layers. Along the equator
maximum adjustments are in boreal fall (~ 1 mg m-2 s-1). Since we cannot estimate the vertical
distribution of zonal velocity (we anticipate that it depends strongly on horizontal pressure
gradients, which we cannot calculate), we have not applied a correction to zonal velocity
estimates.
The climatological monthly velocity is multiplied by reanalysis climatological monthly
SSS gradients (Carton et al., 2000) in order to estimate horizontal mixed layer salt advection.
We also use divergence of these velocity estimates, as well as estimates of the time derivative of
12
mixed layer depth based on PIRATA subsurface temperature, to calculate we (see (1b)).
Horizontal gradients of mixed layer depth are estimated from a monthly climatology based on
bathythermograph temperature profiles (White, 1995).
In comparison to monthly mean salt advection, eddy advection is more difficult to
estimate, since in situ velocity measurements from the PIRATA buoys are not yet available. We
anticipate that eddy advection may contribute significantly near the equator based on previous
temperature balance analyses. It is known that tropical instability waves heat the equatorial
mixed layer during boreal summer and fall (Weisberg and Weingartner, 1988), and we anticipate
that they also provide a source of freshening in the presence of a mean southward salinity
gradient. Since we are unable to estimate horizontal eddy salt advection directly we begin by
estimating eddy temperature advection following the method of Baturin and Niiler (1997) and
Swenson and Hansen (1999), which relies on quasi-lagrangian SST and velocity data from
drifting buoys and then assume a constant eddy diffusion coefficient Kh for both heat and salt:
 v T 
2T
 Kh 2
y
y
(2)
 v S 
2 S
 Kh 2
y
y
(3)
We further assume that meridional eddy advection dominates zonal eddy advection on the
equator due to strong meridional temperature/salinity gradients. Combining (2) and (3) gives:
 2 S 


 v S   v T   y 2   v T  S


y
y   2 T 
y T
 2 
 y 
(4)
Here S ( T ) is the salinity (temperature) change between the equator and 5N (estimated from
the SODA reanalysis). The validity of (4) decreases as T  0. We therefore apply (4) only
13
when T  0.3C and fill missing values using a temporal smoothing based on the annual and
semiannual Fourier harmonics.
We use (4) to estimate eddy salt advection at the three equatorial mooring locations (see
Fig. 1). Although we anticipate that eddy advection may also be important off the equator,
especially at 4N, 38W and 8N, 38W where both eddy activity and horizontal salinity
gradients are strong during boreal summer and fall, we cannot apply (4) here since zonal eddy
advection may contribute significantly.
Our analysis procedure begins with the formation of monthly mean climatological cycles
of h, S, S , E and P. We use these PIRATA-based estimates, along with the climatological
velocity, salinity gradients, and eddy advection estimates described previously, to estimate each
term in (1a). Since we are interested primarily in the seasonal cycle, we eliminate high-frequency
variability by fitting each term to annual and semiannual harmonics using least squares (Fourier)
analysis. We use the standard deviation from each harmonic fit as an estimate of the uncertainty
associated with each term in (1). These error estimates account for high-frequency variability
(period < 6 months) that our data cannot accurately reproduce. We also anticipate errors resulting
from the combination of missing PIRATA data and interannual variability. In particular, it is
possible for climatologies of different PIRATA variables to incorporate data from different time
periods (see Fig. 3). For this reason we display the number of daily PIRATA measurements that
go into each climatological monthly estimate at each location. Monthly estimates of each term in
(1) use a maximum of about 150 individual daily measurements (since most PIRATA moorings
have been operational for about five years). Low counts (<< 150) indicate high uncertainty.
3. Results
14
In this section we examine the balance of terms in (1a) at nine PIRATA mooring
locations (see Fig. 1).
a) 15N, 38W and 12N, 38W
At these sites, both surface fluxes and ocean dynamics contribute significantly to
seasonal salt storage (Fig. 5). Seasonal variability of the surface moisture flux is connected to the
seasonal movement of the ITCZ. Rainfall is significant during August – October, when the ITCZ
is near its northernmost position, while evaporation at this time is at a minimum. As a result,
surface salinity decreases at both locations during late boreal summer and early fall. In boreal
winter strong northeast trade winds and low relative humidity lead to high rates of evaporation,
causing surface salinity to increase at 12N. At 15N, ocean dynamics counteract the effects of
evaporation, leading to weak freshening.
Meridional advection has a strong seasonal cycle at both 12N and 15N (Fig. 6).
Meridional velocity reaches a maximum during boreal winter, when the westward northeast trade
winds and resultant northward Ekman drift are strong. Seasonal variations of S / y result from
meridional movements of the zonal subtropical surface salinity front. At 15N, the northward
surface salinity gradient reaches a maximum in boreal winter, when evaporation rates are high to
the north and rainfall is abundant to the south. At this time, strong northward velocity combines
with the strong northward salinity gradient, providing a significant source of freshening at 15N.
The seasonal cycle of meridional advection at 12N is weaker, although seasonal variations of
meridional velocity and S / y here are actually larger. The maximum northward gradient
occurs in boreal fall at 12N, coincident with strong eastward advection of the Amazon
freshwater plume to the south. Although the phase of the seasonal cycle of meridional advection
is similar at 12N and 15N, due to similar seasonal cycles of meridional velocity, the annual
15
mean and seasonal amplitude at 12N are significantly lower since meridional velocity and
S / y are in quadrature. In comparison to meridional advection, the seasonal cycle of zonal
advection is weak (see Fig. 6) due to weaker zonal gradients of surface salinity. Seasonal
changes in zonal advection at both locations reflect seasonal variations of S / x .
Entrainment undergoes significant seasonal changes at 15N. The mixed layer deepens
rapidly from 40 m in September to 80 m in January without compensating horizontal mass
convergence. The resultant entrainment velocity (~ 0.7 m dy-1), combined with a large S (~ 0.3 psu), tends to an increase mixed layer salinity. Entrainment is negligible during July through
September, when mixed layer depth decreases and S is close to zero. Entrainment velocity at
12N is similar, both in phase and amplitude, to that at 15N, but S is significantly weaker at
12N, resulting in much weaker entrainment at this location.
During March through July mixed layer salinity increases at 15N, while the sum of
terms results in a slightly negative tendency. The large discrepancy indicates a missing source of
salt that most likely results from an overestimate of meridional advection. Horizontal salinity
gradients provide the largest source of uncertainty at most locations we consider in this study,
since we must rely on a combination of poorly sampled observations and a numerical model.
Estimates of meridional SSS gradients at 15N are particularly difficult to make due to its close
proximity to the strong zonal subtropical salinity front. Small errors in the meridional placement
of this front result in large errors in meridional advection estimates.
b) 8N, 38W and 4N, 38W
We next consider the mixed layer salinity balance at two locations farther south and
within the latitudinal range of the ITCZ (8N, 38W and 4N, 38W). As expected, precipitation
here is more significant, in terms of both annual mean and seasonal variations (Fig. 7). Rainfall
16
at 8N is dominated by the annual harmonic, with a maximum in September when the ITCZ is
farthest north. At 4N a significant semiannual harmonic appears, associated with the double
passage of the ITCZ in June and December. Seasonal variations of evaporation at these locations
are weaker than those to the north (12N and 15N) due to in phase relationships between wind
speed and relative humidity (relative humidity is low when wind speed is low at 4N and 8N, in
contrast to the out of phase relationship at 12N and 15N). Although precipitation is the most
important factor affecting seasonal mixed layer salinity variability at both 4N and 8N, ocean
dynamics also play an important role.
The seasonal cycles of both meridional and zonal advection differ substantially between
8N and 4N (see Fig. 8). Seasonal variations of S / y at 8N are out of phase with those of
meridional velocity. S / y is northward throughout the year, and is strongest during May –
September, when rainfall is well developed to the south, while V is significant only during boreal
winter. As a result, meridional advection provides only minor freshening during boreal spring.
S / y and V also vary out of phase at 4N. Here S / y is southward throughout the year,
reaching a maximum in August, when both the ITCZ and the Amazon freshwater plume are
situated to the north. At this time, meridional velocity also reaches a maximum at 4N, resulting
in strong northward salt advection during boreal summer and fall.
Zonal advection also contributes significantly at 4N and 8N (see Fig. 8). In contrast to
the conditions at 12N and 15N, the seasonal cycle of zonal velocity at both 4N and 8N is the
dominant factor controlling seasonal variability of zonal advection. At 8N zonal advection is
limited to August – October, when the eastward velocity of the North Equatorial Countercurrent
reaches a maximum. At 4N S / x is small and westward throughout the year, while U
17
alternates between strong westward flow during the first half of the year (associated with the
South Equatorial Current) and eastward flow during the latter half (due to the North Equatorial
Countercurrent). A strong annual harmonic results at 4N, with zonal advection freshening the
mixed layer during boreal spring and increasing salinity during boreal fall.
Entrainment at 4N is strongest during boreal spring, when zonal mass divergence within
the mixed layer is strong, while at 8N meridional mass convergence balances mixed layer
deepening in boreal winter, resulting in zero entrainment throughout the year. At both locations,
discrepancies between the sum of terms and local salt storage rate indicate that all terms have not
been properly represented. At both 8N and 4N we anticipate that horizontal eddy advection
may account for at least part the large discrepancies present during August through December at
8N and throughout the year at 4N.
c) Along the equator
We next consider the salt balance at three locations along the equator (35W, 23W, and
10W). In contrast to the conditions at the off-equatorial sites (4 - 15N along 38W) here we
find that seasonal variations of mixed layer salinity are low, while rainfall is limited to the first
half of the year, when the ITCZ is closest to the equator (see Fig. 9). Precipitation is lowest in
the eastern basin (0, 10W), where the ITCZ fails to reach the equator during boreal winter and
spring. In comparison to precipitation, evaporation is relatively constant throughout the year,
since both relative humidity and wind speed vary weakly on a seasonal basis.
In contrast to the surface moisture flux, which decreases in magnitude eastward along the
equator, horizontal advection increases from 35W to 10W (see Fig. 10). Seasonal variability of
horizontal advection is closely tied to the seasonal cycle of horizontal velocity. At all equatorial
locations, both northward velocity and meridional advection reach their peaks during boreal fall,
18
when the southeast trade winds are strongest. Seasonal variations of both meridional velocity and
meridional advection are weakest at 10W, where V is northward throughout the year and varies
in phase with S / y (V is strongest in boreal fall, when southward S / y is weakest). At 23W
the combination of a weaker seasonal cycle of S / y and stronger variations of V results in a
stronger seasonal cycle of meridional advection. Seasonal variations of S / y are again strong
at 35W. Here the southward SSS gradient peaks in boreal fall, when the Amazon plume extends
farthest eastward and precipitation is high to the north. Although V and S / y vary out of phase
at 35W (tending to increase the amplitude of the seasonal cycle), the annual mean southward
surface salinity gradient here is much weaker than at 23W, leading to similar seasonal cycles of
meridional advection.
Whereas meridional advection tends to increase mixed layer salinity along the equator,
zonal advection provides a source of freshening. At all equatorial locations, seasonal variations
in the intensity of the westward South Equatorial Current have a large impact on the seasonal
cycle of zonal mixed layer salinity advection. At 35W westward velocity is strongest during
boreal fall, and S / x is very weak throughout the year (see Fig. 10). As a result, zonal
advection is important only during the second half of the year and has weak annual mean and
seasonal variations. At 23W zonal advection is more significant as a result of a stronger
westward surface salinity gradient. Here westward velocity reaches maxima in boreal summer
and winter, producing a seasonal cycle with a strong semiannual harmonic. The annual mean and
seasonal cycles of both zonal velocity and S / x reach a maximum at 10W. Here the westward
surface salinity gradient peaks in boreal winter, when rainfall and Congo River outflow freshen
the mixed layer to the east. The amplitude of the seasonal cycle of U here is almost twice that at
23W, resulting in a very strong seasonal cycle of zonal advection at 10W.
19
Meridional eddy advection opposes the effects of meridional advection, contributing a
source of freshening at the three equatorial locations during late boreal summer. At this time the
mean southward surface salinity gradient is strong at all locations and tropical instability waves
are well developed in the central and eastern basin. Entrainment tends to increase mixed layer
salinity at 35W, where horizontal mass divergence peaks in boreal spring and fall, but is much
weaker throughout the year at 23W and 10W.
The sum of terms at 35W results in a period of strong freshening during February
through May, when the sum of evaporation, entrainment, and eddy advection fails to balance
precipitation. During this time the actual mixed layer tendency is less negative or even positive,
indicating that we have either neglected or misrepresented the magnitude one or more terms in
the balance. One possible explanation is that we have underestimated entrainment during this
time period. Weingartner and Weisberg (1991) analyzed one year of in situ upper ocean
horizontal velocity data near the equator at 28W and found strong upwelling near the base of the
mixed layer during mid-April through mid-May. It is thus possible that the coarse horizontal
resolution of our mean velocity data (2-lat x 3-lon) has led to an underestimate of horizontal
mass divergence and associated entrainment velocity. Additional entrainment of ~ 3 to 5 mg m-2
s-1 (equivalent to an entrainment velocity of ~ 2.5 to 4.3 m dy-1) is required to explain the
discrepancy during February – May at 35W. It therefore seems unlikely, based on the upwelling
estimates of Weingartner and Weisberg (1991), that unresolved entrainment alone could account
for the additional increase in mixed layer salinity needed at 0, 35W.
Another possibility is that vertical turbulent diffusion is important at 35W (we have
completely neglected this term). The largest discrepancy between the sum of terms and salt
storage occurs during February through May, when the mixed layer at 35W is shallowest (~ 35
20
to 45 m). At this time velocity is weak near the surface, but is strong and eastward below the
mixed layer (based on climatological data from the SODA reanalysis, Carton et al., 2000). Using
the turbulent diffusion parameterization of Pacanowski and Philander (1981) with U / z =
0.012 s-1, T / z = 0.1 C m-1 (based on PIRATA subsurface temperature), and S / z = 0.02
psu m-1 (assuming the PIRATA-based S is distributed over 10 m) leads to a maximum increase
in salt content of ~ 0.5 mg m-2 s-1 during boreal spring. It therefore seems unlikely that the
discrepancy during boreal spring at 0, 35W can be accounted for by turbulent diffusion alone.
In contrast, the discrepancy between the sum of terms and salt storage during October –
December at 35W requires an additional source of freshening that cannot be provided by
entrainment or turbulent diffusion. One possibility is that we have underestimated horizontal
eddy advection (these estimates are very uncertain on the equator due to a lack of near-surface
drifter data). It is also possible that we have overestimated entrainment, since Weingartner and
Weisberg (1991) found that upwelling was most significant during boreal spring near 28W.
The sum of terms at 0, 23W indicates excess freshening of 2 to 4 mg m-2 s-1 during
January – May and an excess increase in mixed layer salinity of ~ 3 mg m-2 s-1 during September
– November. Explanations for the excess freshening include an underestimate of entrainment and
failure to account for vertical turbulent diffusion. An additional entrainment velocity of ~ 1.7 to
3.4 m dy-1 is required during January – May to balance the excess freshening predicted by the
sum of terms. On the other hand, vertical turbulent diffusion accounts for no more than ~ 0.5 mg
m-2 s-1, with a peak in boreal spring. It therefore seems, as was the case at 35W, that unresolved
entrainment accounts for the bulk of the excess freshening at 0, 23W. The excess increase in
salt content during the end of the year is possibly due to an underestimate of horizontal eddy
21
advection (these estimates are highly uncertain due to a lack of drifter data and the indirect
method used).
The sum of terms at 0, 10W predicts excess freshening during April – August, with a
peak of ~ 5 mg m-2 s-1 in June. If unresolved entrainment alone were to account for this
discrepancy, an entrainment velocity of more than 4 m dy-1 would be required. We therefore
anticipate that vertical turbulent diffusion plays an important role at 0, 10W during May –
June. At this time the mixed layer is very shallow (< 20 m) and near-surface westward velocity
approaches 1 m s-1. Assuming U / z = 0.022 s-1, vertical turbulent diffusion accounts for up to
2.5 mg m-2 s-1 during May – June at 0, 10W. Thus, in contrast to the situations along the
equator at 23W and 35W, here it appears that turbulent diffusion probably plays a major role.
d) 6S, 10W and 10S, 10W
The seasonal mixed layer salt balance south of the equator (along 10W) is highly
uncertain (see Fig. 11). Much of this uncertainty is due to poor estimates of horizontal surface
salinity gradients and hence horizontal advection. The number of in situ surface salinity
measurements assimilated into the SODA reanalysis (used to estimate horizontal gradients) at
these locations is low in comparison to most of the other locations considered in this study. For
example, at 6S, 10W there are no surface salinity measurements during eight months of the
year. As a result, discrepancies between the sum of terms and salt storage are significant (~ 2 to 5
mg m-2 s-1) throughout most of the year at both 6S and 10S.
At 6S, 10W the surface moisture flux is dominated by evaporation. Precipitation is
nearly zero throughout the year, while evaporation peaks in early boreal summer, when the
southeast trade winds are strong and relative humidity is low. In contrast to the conditions along
the equator and along 38W, where S < 0 throughout the year, here S > 0 (salinity decreases
22
with depth in the mixed layer) during the second half of the year. As a result, entrainment
freshens the mixed layer during July – December due to a combination of zonal and meridional
mass divergence. In contrast, entrainment increases SSS during April – June, when S < 0 and
the mixed layer deepens from 40 to 60 m without compensating horizontal mass convergence.
Our results show that both meridional velocity and the meridional SSS gradient are
southward throughout the year at 6S, 10W, indicating that meridional advection provides a
year-round source of freshening. Zonal advection seems to be weaker in the annual mean, with a
seasonal amplitude that is similar to that of meridional advection. As discussed previously, our
estimates of horizontal advection at this location are highly uncertain. Error bars associated with
these terms are possibly twice as large as the signal, due to poorly known horizontal SSS
gradients.
With the exception of entrainment, the balance at 10S, 10W seems to be similar to that
at 6S. Evaporation peaks during boreal summer, while meridional advection provides a yearround source of freshening. Entrainment here is much less important than at 6S due to smaller
S at 10S.
4. Summary
This paper examines the mixed layer salt budget in the tropical Atlantic, based on
measurements from nine PIRATA moorings and a variety of other in situ, satellite, and model
data, in an attempt to explain seasonal cycle of SSS. This study is similar in spirit to that of
Cronin and McPhaden (1998) in the western equatorial Pacific, which uses in situ near-surface
atmospheric and subsurface oceanographic measurements to directly estimate as many terms as
possible in the mixed layer salt budget. Our main results are as follows:
23

At 15ºN, 38ºW and 12ºN, 38ºW changes in mixed layer salinity are balanced
primarily by precipitation, evaporation, and meridional advection, with the
exception that entrainment contributes significantly during boreal winter at 15ºN
(due to a rapidly increasing mixed layer depth and large S ). We anticipate that
uncertainty in our estimate of the meridional SSS gradient may be responsible for
the excess freshening during boreal spring at 15ºN.

The mixed layer salt balances at 8ºN, 38ºW and 4ºN, 38ºW include significant
contributions from precipitation and horizontal advection. Horizontal advection
(both zonal and meridional components) is most important at 4ºN, while
entrainment is additionally important at this location during boreal spring. We
expect that unresolved horizontal eddy salt advection may be at least partially
responsible for discrepancies between the sum of terms and salt storage at both
locations.

The mixed layer salt budgets along the equator (0º, 10ºW; 0º, 23ºW; and 0º,
35ºW) represent a balance of precipitation, horizontal advection (both mean and
eddy), and entrainment. Contributions from zonal advection increase eastward
from 35ºW to 10ºW, while precipitation decreases eastward. Horizontal eddy
advection provides a significant source of freshening at all locations during boreal
summer and fall, while entrainment contributes significantly only at 35ºW. We
believe that unresolved entrainment and vertical turbulent diffusion may explain
24
most of the excess freshening at all locations and that uncertainties in our
estimates of horizontal eddy advection may be the cause of excess increases in
mixed layer salinity during boreal fall.

The mixed layer salinity balance in the eastern equatorial Atlantic (6ºS, 10ºW and
10ºS, 10ºW) includes contributions from evaporation, entrainment, and horizontal
advection. Large discrepancies between the sum of terms and salt storage at both
locations indicate that our estimates of horizontal SSS gradients contain a high
degree of uncertainty.
This study provides the first comprehensive observation-based analysis of the mixed
layer salt budget in the tropical Atlantic. On the equator we have found many similarities with
the mixed layer heat balance (Foltz et al., 2003). In both cases horizontal mean and eddy
advection contribute significantly, and we suspect that unresolved entrainment and vertical
turbulent diffusion account for anomalous heating/freshening. We find that the salt balance
contains large contributions from mean horizontal advection at all off-equatorial locations, and
we anticipate that many of the discrepancies between the sum of terms and salt storage along
38ºW are due to unresolved eddy salt advection. This is in contrast to the mixed layer heat
balance, where off the equator horizontal advection makes minimal contributions and eddy
advection is insignificant. In comparison to the heat advection estimates of Foltz et al. (2003),
our estimates of mean horizontal salt advection are highly uncertain. Additional uncertainties
arise because the seasonal cycle of horizontal SSS gradients in the tropical Atlantic is very
poorly known (due to a lack of in situ and satellite observations) in comparison to that of SST.
25
As a result, our estimates of horizontal SSS gradients, and hence mean horizontal salt advection,
are uncertain.
Not surprisingly, the largest errors in our analysis are at the two locations in the southern
hemisphere (6ºS, 10ºW and 10ºS, 10ºW), where the seasonal cycle of SSS and its gradient are
poorly known. There are generally less than 100 total surface salinity observations (summed over
the entire historical record from WOA 2001) in each 2-lat  3-lon box along 10W. In contrast,
there are more than 400 observations along the equator between 23W and 35W. We therefore
expect that discrepancies at 6ºS, 10ºW and 10ºS, 10ºW are associated with uncertainties in our
estimates of horizontal salt advection. We have also found large discrepancies north of the
equator, particularly at 4ºN, 38ºW and 8ºN, 38ºW. We expect that unresolved horizontal eddy
advection may account for most of these discrepancies. We have found that the sum of terms at
15ºN results in excess freshening (up to 2.5 mg m-2 s-1) during boreal spring and summer. At this
time the meridional SSS gradient is strong, and we anticipate that errors in the placement of this
zonal front may account for the discrepancies at this location. In this region there are generally
less than 100 total surface salinity observations.
Along the equator disagreements between the sum of terms and salt storage are related to
uncertainties in our estimates of horizontal eddy salt advection as well as unresolved entrainment
and vertical turbulent diffusion. At 0º, 23ºW and 0º, 35ºW the sum of terms results in excess
freshening during the first half of the year. We anticipate that these discrepancies are mainly due
to unresolved entrainment (due to the 2º-latitude resolution of our velocity estimates). At 0º,
10ºW excess freshening is ~ 4 - 5 mg m-2 s-1 during May – July. Here we anticipate that vertical
turbulent diffusion contributes significantly (up to 2.5 mg m-2 s-1) in response to a shallow mixed
layer (< 20 m) and strong westward surface currents (~ 1 m s-1). Discrepancies during boreal fall
26
at 0º, 23ºW and 0º, 35ºW, requiring an additional source of freshening, are most likely due to
underestimates of horizontal eddy advection. These estimates are highly uncertain due to a lack
of drifter data on the equator and the indirect method used.
Despite these limitations and the relatively short mooring records, we have been able to
show that horizontal advection is an important component of the mixed layer salt balance at all
nine locations we considered. To complete our understanding at seasonal periods additional work
is needed to quantify the roles of horizontal advection, entrainment, and vertical turbulent
diffusion. In particular, the addition of current meters to the PIRATA buoys would provide better
knowledge of both mean and eddy advection. In addition, satellite-based measurements of
surface salinity from the NASA Aquarius mission will greatly improve estimates of horizontal
salt advection. Though subject to considerable uncertainties because of data limitations, our
results nonetheless provide a basis for evaluating interannual and decadal variability, which are
both linked to the annual cycle.
Acknowledgements
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Figure captions
Figure 1 Locations of the PIRATA moored buoys (solid and open circles). Solid circles indicate buoys
used in this study. Background contours and arrows are climatological annual mean near-surface salinity
from the SODA reanalysis (Carton et al., 2000) and near-surface ocean velocity (Grodsky and Carton,
2001). Bold contour lines represent 1 psu intervals.
Figure 2 Example of the quality control and gap filling procedure for PIRATA subsurface salinity
records. Original PIRATA 1 m (top) and 20 m (middle) salinity, and the final 1 m salinity record (bottom)
after quality control and gap filling. Questionable data during 1998, early 2001, and early 2002 in the
original 1 m time series have been removed, and gaps in the 1 m record in 2002 have been filled with data
from 20 m.
Figure 3 Daily PIRATA near-surface atmospheric and oceanic measurements at (left) 0, 10W and
(right) 8N, 38W during 1998 – 2002. Solid gray lines represent monthly mean World Ocean Atlas 2001
climatological surface salinity, TRMM Microwave Imager rainfall, QuikSCAT near-surface wind speed,
and NCEP/NCAR Reanalysis relative humidity.
Figure 4 Mixed layer depth at 8N, 38W calculated from PIRATA subsurface temperature (0.5 C
criterion) and from PIRATA subsurface temperature and salinity (0.15 ). Note the significant barrier
layer during boreal summer and fall.
Figure 5 Left panels show individual contributions to the salt balance equation (1) in the form of
evaporation, precipitation, entrainment, and mean zonal and meridional heat advection. Plots in lefthand
panels show least squares fits of mean + annual and semiannual harmonics to monthly data. Righthand
panels show the sum of the terms in the lefthand panel and the actual mixed layer salt storage rate.
Shading and cross-hatching in righthand panels indicate error estimates based on standard deviations of
monthly data from least squares harmonic fits. Bars in righthand panels indicate number of days in each
month for which all PIRATA-based terms in the lefthand panel are available (maximum of ~ 150 days for
each month, corresponding to ~ 5 years of data: September 1997 – December 2002).
Figure 6 Zonal and meridional climatological near-surface ocean velocity and surface salinity gradients
at 15N, 38W and 12N, 38W.
Figure 7 As in Figure 5, but for 8N, 38W and 4N, 38W.
Figure 8 As in Figure 6, but for 8N, 38W and 4N, 38W.
Figure 9 As in Figure 5, but for locations along the equator. Horizontal eddy salt advection (defined as
total advection minus climatological monthly advection) is included in the left panels.
Figure 10 As in Figure 6, but for locations along the equator.
Figure 11 As in Figure 5, but for 6S, 10W and 10S, 10W.
32
Table 1 Amplitude and phase of annual and semiannual cosine harmonics (with respect to Jan. 1) and annual mean
of terms in the mixed layer salt balance at 15N, 38W and 12N, 38W. Underlined terms are those that explain the
largest amount of variance at each location.
Annual
mean
(mg m-2 s-1)
15ºN 38ºW
Evaporation
Precipitation
Entrainment
Zonal adv.
Meridional adv.
Salt storage
Mixed layer depth
12ºN 38ºW
Evaporation
Precipitation
Entrainment
Zonal adv.
Meridional adv.
Salt storage
Mixed layer depth
Annual
amplitude
(mg m-2 s-1)
Annual
phase
(months)
Semiannual
amplitude
(mg m-2 s-1)
Semiannual
phase
(months)
2.0
-0.5
0.5
-0.5
-2.0
0.0
60 m
0.5
1.0
1.0
0.5
1.0
1.0
20 m
2
3
0
9
8
5
3
0.0
0.5
0.5
0.5
0.5
1.0
10 m
0
6
0
6
4
5
1
2.0
-1.0
0.0
0.5
-1.0
0.5
40 m
0.5
1.5
0.0
0.5
0.5
2.0
20 m
2
3
0
0
8
3
4
0.5
0.5
0.0
0.5
0.5
1.0
0m
2
1
0
0
1
3
2
Table 2 As in Table 1, but for 8N, 38W and 4N, 38W.
Annual
mean
(mg m-2 s-1)
8ºN 38ºW
Evaporation
Precipitation
Entrainment
Zonal adv.
Meridional adv.
Salt storage
Mixed layer depth
4ºN 38ºW
Evaporation
Precipitation
Entrainment
Zonal adv.
Meridional adv.
Salt storage
Mixed layer depth
Annual
amplitude
(mg m-2 s-1)
Annual
phase
(months)
Semiannual
amplitude
(mg m-2 s-1)
Semiannual
phase
(months)
2.0
-3.5
0.0
0.0
-0.5
0.0
40 m
0.5
4.0
0.0
1.0
0.5
3.5
20 m
2
3
3
3
10
1
4
0.0
0.5
0.0
1.0
0.0
1.5
0m
2
3
2
6
1
5
3
1.0
-4.0
0.5
0.0
1.5
0.0
70 m
0.0
2.5
1.0
1.0
1.5
3.0
30 m
10
9
4
10
9
7
11
0.0
1.5
0.0
0.0
0.5
2.0
10 m
2
2
3
5
1
1
2
33
Table 3 As in Table 1, but for the three equatorial locations.
Annual
mean
(mg m-2 s-1)
0º 35ºW
Evaporation
Precipitation
Entrainment
Zonal adv.
Meridional adv.
Eddy adv.
Salt storage
Mixed layer depth
0º 23ºW
Evaporation
Precipitation
Entrainment
Zonal adv.
Meridional adv.
Eddy adv.
Salt storage
Mixed layer depth
0º 10ºW
Evaporation
Precipitation
Entrainment
Zonal adv.
Meridional adv.
Eddy adv.
Salt storage
Mixed layer depth
Annual
amplitude
(mg m-2 s-1)
Annual
phase
(months)
Semiannual
amplitude
(mg m-2 s-1)
Semiannual
phase
(months)
1.5
-2.5
1.0
-0.5
0.5
-0.5
0.0
60 m
0.5
3.0
0.5
1.0
1.0
1.0
0.5
20 m
9
10
10
4
9
2
9
10
0.0
1.0
1.0
0.0
0.0
1.5
1.0
0m
1
0
4
6
4
6
5
2
1.0
-1.5
0.5
-1.0
1.0
-0.5
0.0
30 m
0.5
2.0
0.5
0.0
1.0
0.5
1.0
10 m
8
10
10
2
10
4
1
10
0.0
1.0
0.0
1.0
0.5
1.5
1.0
0m
4
1
1
3
3
5
4
1
1.0
-0.5
0.5
-2.5
1.5
-1.0
0.0
20 m
0.5
1.0
0.5
1.0
0.5
1.5
1.5
10 m
2
9
3
11
11
4
6
11
0.0
0.5
0.5
2.0
0.5
0.5
1.0
0m
4
0
2
3
4
5
3
4
34
Table 4 As in Table 1, but for 6S, 10W and 10S, 10W.
Annual
mean
(mg m-2 s-1)
6ºS 10ºW
Evaporation
Precipitation
Entrainment
Zonal adv.
Meridional adv.
Salt storage
Mixed layer depth
10ºS 10ºW
Evaporation
Precipitation
Entrainment
Zonal adv.
Meridional adv.
Salt storage
Mixed layer depth
Annual
amplitude
(mg m-2 s-1)
Annual
phase
(months)
Semiannual
amplitude
(mg m-2 s-1)
Semiannual
phase
(months)
2.0
0.0
0.0
0.0
-1.5
0.5
50 m
0.5
0.0
1.0
0.5
0.5
1.5
10 m
6
11
3
0
0
10
9
0.0
0.0
0.5
0.5
0.5
3.0
10 m
5
1
4
4
1
0
6
2.0
0.0
0.0
0.0
-1.0
0.0
50 m
0.5
0.0
0.0
0.5
0.5
1.5
20 m
7
10
4
0
2
0
9
0.0
0.0
0.0
0.0
0.0
1.5
0m
1
0
0
3
0
3
1
35
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