Deep-Sea Research I 49 (2002) 1571–1590
Formation rates of subtropical underwater in the Pacific Ocean
Bridgette M. O’Connor, Rana A. Fine*, Kevin A. Maillet, Donald B. Olson
Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, USA
Received 10 December 2001; received in revised form 5 July 2002; accepted 5 July 2002
Abstract
Water mass formation rates were calculated for subtropical underwater (STUW) in the North and South Pacific by
two partially independent methods. One is based on the World Ocean Circulation Experiment (WOCE)/TOGA drifter
array over two periods: 1988–1992, and 1992–1996. Drifter velocities were used to calculate two components of the
subduction rate, lateral induction and vertical pumping. The second method used CFC-12 data (1987–1994) from
WOCE and Pacific Marine Environmental Laboratory to calculate ages on sy surfaces. Subduction rates were
estimated from the inverse age gradient. The two subduction rate methods are independent, but they share a common
identification of STUW formation area based on satellite-derived surface temperature maps. Using both methods, one
can put bounds on the formation rates: 4–5 Sv in the North and 6–7 Sv in the South Pacific. The drifter calculated
STUW subduction rates for 1988–1992 and 1992–1996 are 21 and 13 m/yr in the North Pacific and 25 and 40 m/yr in the
South. The CFC-12 calculated STUW subduction rate in the North Pacific is 26 m/yr, and 32 m/yr in the South. The
South Pacific rates exceed those in the North Pacific. Consistent differences between the two methods support earlier
studies, they conclude that mixing contributes to STUW formation in addition to the larger-scale circulation effects.
The drifter and tracer rates agree well quantitatively, within 22%, except for the second period in the North Pacific and
there are some differences in spatial patterns. Tracer rates integrate over time, and drifters allow analysis of interannual
variability. The decrease in subduction rate between periods in the North Pacific is due to negative lateral induction
entraining STUW into the mixed layer. The increase in the South Pacific rate is due to an increase in the vertical
pumping. Although Ekman pumping is in phase in the North and South, the subduction rate is out of phase. These
results confirm that subduction depends on the large-scale circulation and a combination of the outcrop pattern and
air–sea fluxes. Temporal differences in rates and partitioning between the hemispheres are consistent with interannual
changes in gyre intensity and current positions.
r 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Subduction; Subtropical underwater; Formation; Chlorofluorocarbon; Tracers; Drifters; Pacific
1. Introduction
The subtropical underwater (STUW) salinity
maximum is a component of the Central Waters
*Corresponding author. Tel.: +1-305-361-4722; fax: +1305-361-4917.
E-mail address: rfi[email protected] (R.A. Fine).
formed by subduction within the subtropical
gyres. Recent studies have suggested the potential
importance of anomalies in subduction and
equatorward transport of subtropical water in
the modulation of ENSO (e.g., Gu and Philander,
1997; Zhang et al., 1998). In the Pacific,
subtropical subduction may remotely affect
the Equatorial Undercurrent and influence the
0967-0637/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 9 6 7 - 0 6 3 7 ( 0 2 ) 0 0 0 8 7 - 0
1572
B.M. O’Connor et al. / Deep-Sea Research I 49 (2002) 1571–1590
properties of water upwelled at the equator. These
ideas, proposed by Pedlosky (1987), are an
extension of the ventilated thermocline theory
(Luyten et al., 1983). They are examined in
models, which show a subtropical circulation cell
with poleward flow in the Ekman layer, subtropical subduction and equatorward flow in the
thermocline closed by upwelling at the equator
(e.g., McCreary and Lu, 1994). Observations have
confirmed the existence of this cell in the North
Pacific (e.g., Fine et al., 1981, 1987). For STUW,
the subtropical cell involves a superposition of the
Ekman and geostrophic flows, which results in
subduction of the saline waters formed in the
subtropical gyres under a layer of fresher waters
(Schmitt et al., 1989; Worthington, 1976). In the
Pacific the subducted high-salinity water is carried
equatorward in the Sverdrup circulation, to be
upwelled near the equator on time scales of 5–15 yr
depending on density (e.g., Fine et al., 2001). The
major freshening occurs at the surface under the
Convergence Zones (Quay et al., 1983) as STUW
flows westward and poleward, and some freshening occurs prior to upwelling through mixing with
overlying waters in the western tropical Pacific.
The STUW is one of the least studied water
masses of the Central Waters of the world’s oceans
even though it has been identified since 1925 for
the Atlantic. Defant (1936) described its distribution in the North Atlantic and related it to a
formation region in the central subtropics.
Worthington (1976) argued that the STUW forms
under the influence of the trades. The STUW has
several names in the literature. It is often referred
to as ‘‘Tropical Water’’. Qu et al. (1999) find
STUW in the North Pacific extending westward
between 101 and 251N, with part flowing southward in the Mindanao Current. Kessler (1999)
examines interannual variability of the salinity
maximum tongue extending from the subduction
region in the southeast Pacific, all the way to the
equator at 1651E in the western Pacific. He finds
higher salinities during periods with westward
current maxima. Bingham et al. (in press) observe
both North Pacific and South Pacific STUW at
1371E near the equator.
Although STUW is common to all oceans, there
appears to be no prior estimate of a formation
rate. Huang and Qiu (1994), Marshall et al. (1993),
and Qiu and Huang (1995) calculate subduction
rates over density intervals, which were not
targeted to specific water masses. All of these
methods involved calculation of the vertical
pumping (including the linear vorticity balance)
and the lateral induction terms. An independent
method by Jenkins (1987, 1998) uses tracer ages to
calculate subduction rates for the northeast
Atlantic. Here, a multi-data approach with drifters
and tracers, and the methods of Marshall et al.
(1993) and Jenkins (1987, 1998) were used. This
provides two independent methods for quantifying
the STUW subduction rate in the North and South
Pacific Oceans. Subduction rates from these two
methods were multiplied by a formation area,
from satellite-derived surface temperature maps,
to get formation rates.
2. Data
The Reid and Mantyla (Reid, 1997) historical
hydrographic data were used to define the properties of the STUW in the Pacific Ocean. The Levitus
et al. (1994) and Levitus and Boyer (1994)
climatology (hereafter referred to as Levitus) were
used to estimate mixed layer depths and compute
geostrophic velocities. World Ocean Circulation
Experiment (WOCE) and Pacific Marine Environmental Laboratory (PMEL) hydrographic data
were used.
The chlorofluorocarbon (CFC) data are from
WOCE (Fine et al., 2001) and PMEL (Wisegarver
et al., 1993). The WOCE data consist of stations
from P17N (May–June 1993), P17C (June–July
1991 and July–August 1991), and P21E (March–
May 1994). The PMEL data consist of stations
from CO2-87 (1987), CO2-88 (1988), CO2-89
(1989) and TEW-87 (1987). The WOCE and
PMEL CFCs were measured by procedures developed by Bullister and Weiss (1988), and all the
data were converted to the SIO-1993 scale. The
analytical precision for WOCE CFC-12 samples
>0.1 pmol/kg varied between 0.8% and 5% (Fine
et al., 2001). For PMEL CFC-12 samples with
concentrations greater than 0.015 pmol/kg, the
analytical precision varied between 0.5% and
B.M. O’Connor et al. / Deep-Sea Research I 49 (2002) 1571–1590
2%, with sampling and handling errors on the
same order (Wisegarver et al., 1993).
The annual mean velocity components for two
periods, January 1988–January 1992 and January
1992–January 1996, were calculated from satellite
tracked drifting buoy data from WOCE/TOGA
(Tropical Ocean/Global Atmosphere). Satellite
tracked drifters were drogued to 15 m and were
used to obtain mean fields of u; v and SST. The
drifter data were processed to obtain mean drifter
velocities on a 2.51 2.51 grid. Details of the
estimation of drifter velocities are provided in
Appendix A.
3. Properties of STUW
To calculate the STUW formation rate, the first
step was to find all stations with salinity maximum
(an increase of >0.03 between two successive
depth levels) between the bottom of the mixed
layer (50 m) and lower thermocline (300 m). The
average salinity, potential temperature and potential density of STUW were calculated from all the
stations with salinity maximum. A range of 71
standard deviation from the mean (Table 1) was
chosen for each property. The STUW is bounded
by 24.2 and 25.2sy in the North Pacific, and by
24.6 and 25.4sy in the South Pacific, (see for
example, Tsuchiya and Talley, 1996). The closed
isohaline contours in the South Pacific represent a
larger area than in the North. The salinity
maximum in the South Pacific is 1.2 higher than
that in the North, due to the higher E-P in the
South (e.g., Baumgartner and Reichel, 1975). The
high-salinity region in the South extends closer to
Table 1
Mean salinity, potential temperature and potential density, and
the range (71 standard deviation) of the STUW in the North
and South Pacific
Salinity
y (1C)
sy
North Pacific
South Pacific
Average
Range
Average
Range
35.01
21.4
24.3
34.6–35.4
19.0–25.0
24.2–25.2
35.77
21.9
24.8
35.6–36.4
19.0–25.0
24.6–25.4
1573
the equator than in the North Pacific, probably
because the mean location of the ITCZ is at about
7–91N. Another reason for this asymmetry may be
related to an asymmetry in the North and South
Pacific shallow subtropical circulation cells, where
there is a net North Pacific thermocline flow that
feeds the Indonesian Throughflow (e.g., Gordon
and Fine, 1996).
The STUW salinity map for the North Pacific
(Fig. 1a) has two high-salinity cells (S > 35:3),
centered at 241N, 1781W and 241N, 1451W. The
South Pacific (Fig. 1b) also has two high-salinity
cells (S > 36:5) centered at 201S, 1271W and 151S,
1351W. The bimodal distribution in the climatological salinity correlates with a similar distribution in climatological wind stress curl (Hellerman
and Rosenstein, 1983). The South Pacific map of
Johnson and McPhaden (1999) on the neutral
surface 25 kg/m3 similarly shows a bimodal salinity
distribution.
4. Methods
The formation area of the STUW was computed
from satellite SST data and climatological surface
salinity data (Levitus et al., 1994). The formation
area was multiplied by the subduction rate to
obtain a volumetric subduction rate. The latter is
equivalent to a formation rate of the water mass.
The methods for calculating subduction rates from
drifter and tracer data are described below.
4.1. Calculation of the formation area
Under the assumption that properties are
conserved during subduction, the STUW formation area was defined as the area where both SST
and surface salinity were within the potential
temperature and salinity range of the STUW
(Table 1). The satellite SST data consisted of
weekly composites for 1988 and 1989. These years
were chosen because they provided the largest
variations observed in the SST over 9 yr (1988–
1996), that is, the natural variations in the
outcrops. Also they were within the range of the
observational years for the drifter and tracer data
sets. The area difference between years was used
B.M. O’Connor et al. / Deep-Sea Research I 49 (2002) 1571–1590
1574
120°E
150°E
180°
150°W
120°W
40°N
40°N
34.5
34.6
34.7
34.8
34.9
30°N
35.1
35.3
35
.1
35
20°N
30°N
35.2
35
20°N
35.
1
10°N
10°N
35.1
0°
(a)
0°
120°E
150°E
150°E
180°
180°
150°W
150°W
120°W
120°W
0°
0°
36.1
36
10°S
35.7
35.8
36.2
35.9
35.8
36.5
20°S
10°S
35.9
36
36.3
20°S
36.4
36.1
35.7
30°S
35.4
35.6
30°S
35.5
40°S
(b)
40°S
150°E
180°
150°W
120°W
Fig. 1. Map of salinity contoured on the surface of the salinity maximum of STUW in the (a) North Pacific and (b) South Pacific.
(Dots indicate the stations where a salinity maximum is found.)
for evaluating effects of interannual variability on
outcrop regions, and for the uncertainty estimates.
The formation areas were calculated for each
week, and an average and standard deviation was
obtained for each month and season. Fig. 2a
shows the surface area of STUW properties for
each season in 1989. The difference between the
spring and summer areas of STUW properties at
the surface was defined as Adiff and was used to
obtain a volumetric subduction rate (formation
rate). The Adiff values obtained for the North and
South Pacific were 5.8 106 and 6.8 106 km2,
respectively. This spring–summer difference was
used, as not all the water formed in late winter/
early spring is subducted. Some water remains in
the mixed layer after subduction has occurred.
This spring–summer difference represents water
that has been isolated by subduction.
The formation areas used in this work were for
1989. Considering the large year-to-year variations
in SST anomalies [for example, North Pacific
(e.g., Nakamura et al., 1997)], it was difficult to
extrapolate the results to other years. In an
attempt to assess year-to-year variations, comparisons were done between the surface areas of
STUW properties in the eastern part of the region
for 1988 and 1989, since there was poor data
coverage in the west in 1988 due to cloud cover.
* but
The year 1988 represents a strong La Nina,
1989 was ‘‘normal’’. There is a good overlap
between the northeast areas for 1988 and 1989,
with 1989 areas larger by about 20% (Fig. 2b).
The large standard deviations in the northern
summer and autumn were probably due to cloud
interference. The southeast Pacific has an excellent
overlap between the surface areas of STUW
properties during 1988 and 1989 (Fig. 2b), with
only a 4% difference.
4.2. Subduction rate calculation
4.2.1. Drifter method
The process by which fluid passes (subducts)
from the mixed layer into the main thermocline of
B.M. O’Connor et al. / Deep-Sea Research I 49 (2002) 1571–1590
Qiu and Huang, 1995; Williams et al., 1995). Fluid
that is subducted outside of this period enters the
seasonal thermocline, but is subsequently reentrained when the mixed layer deepens during
the following winter. Marshall and Marshall
(1994) define an annual subduction rate at which
fluid irreversibly transfers into the permanent
thermocline or subducts (Sann ): (overbars represent
the means)
2.50E+07
South Pacific
North Pacific
area (km2)
2.00E+07
1.50E+07
1.00E+07
5.00E+06
Sann ¼ ðu% w drhw þ w% w Þ:
0.00E+00
winter
spring
(a)
summer
autumn
season
2.00E+07
1989 SEP
1988 SEP
1989 NEP
1988 NEP
1.80E+07
area (km2)
1.60E+07
1.40E+07
1.20E+07
1.00E+07
8.00E+06
6.00E+06
4.00E+06
2.00E+06
dec
oct
nov
aug
month
sept
jul
jun
apr
may
feb
mar
jan
0.00E+00
(b)
1575
2
Fig. 2. Surface area (km ) of STUW properties in the (a) North
and South Pacific versus season for 1989 and (b) northeast and
southeast Pacific versus month for 1988 and 1989. Bars indicate
1 standard deviation of the mean.
the subtropical gyre is governed by mechanical
and thermodynamic forcing. Restratification, by
warming or freshening, must occur to allow a
portion of fluid in a relatively cold, deep winter
mixed layer to irreversibly enter the stratified
thermocline below. The near zero potential vorticity in the homogenous mixed layer must be reset
by restratification to allow a parcel to enter the
thermocline, where the ambient potential vorticity
is much higher. This restratification period (effective subduction period) occurs during spring, when
the mixed layer shoals due to warming (Marshall
and Marshall, 1994). The heat required to shoal
the mixed layer can be supplied either as a flux
through the sea surface, or by convergence in the
Ekman layer. The effective subduction period is
about 1–2 months over most of the subtropical
gyre (Marshall et al., 1993; Huang and Qiu, 1994;
ð1Þ
In the above equation, u% w and w% w are the annual
mean horizontal and vertical velocity components
at the Eulerian interface z ¼ hw ðx; yÞ; which
represents the base of the deepest winter mixed
layer. Eq. (1) explicitly quantifies the rate of fluid
passing into the permanent thermocline and
consists of lateral induction (first term) and
vertical pumping (second term). The lateral
induction term arises from the sloping interface
of the winter mixed layer, which allows lateral
advection of water from the mixed layer into the
thermocline.
The vertical velocity, w% w ; was calculated from
the Ekman pumping, w% Ek ; and the linear vorticity
balance (Marshall et al., 1993):
w% w ¼ w% Ek ðb=f Þðv%g Þhw ;
ð2Þ
where f is the planetary vorticity, and b is its
meridional gradient. The b term included is the
divergence due to the meridional flow on a sphere.
The mean meridional velocity used in the calculation of the b term is the geostrophic component, v%g
computed by combining drifter and Levitus
dynamic height. Drifter annual mean horizontal
velocities (Fig. 3) were used in calculation of the
lateral induction term instead of the geostrophic
components, because the mean flow causes lateral
advection of water across the mixed layer base.
For example, the Ekman component can drive
geostrophic flow via w% Ek :
There were two assumptions in calculation of
the Ekman velocity (w% Ek ) at the base of the winter
mixed layer:
(1) the Ekman velocity at the sea surface is equal
to zero;
(2) the horizontal divergence of the flow is
constant with depth in the mixed layer.
B.M. O’Connor et al. / Deep-Sea Research I 49 (2002) 1571–1590
1576
120E
150E
180Ê
150W
120W
90W
40N
40N
30N
30N
0.3 m/s
20N
20N
10N
10N
0Ê
0Ê
10S
10S
20S
20S
30S
30S
40S
40S
(a)
120E
150E
180Ê
150W
120W
90W
120E
150E
180Ê
150W
120W
90W
40N
40N
30N
30N
0.3 m/s
20N
20N
10N
10N
0Ê
0Ê
10S
10S
20S
20S
30S
30S
40S
40S
(b)
120E
150E
180Ê
150W
120W
90W
120E
150E
180Ê
150W
120W
90W
40N
40N
30N
30N
0.3 m/s
20N
20N
10N
10N
0Ê
0Ê
10S
10S
20S
20S
30S
30S
40S
40S
(c)
120E
150E
180Ê
150W
120W
90W
Fig. 3. The mean surface velocity field for the Pacific based on 50-day low-passed drifter velocities ensemble averaged onto the grid for
(a) 1988–1992 and (b) 1992–1996 as described in Appendix A. The estimated component of velocity due to geostrophy for (c) 1988–
1992 and (d) 1992–1996. The residual velocity vector (best ageostrophic estimate) not explained from geostrophy for (e) 1988–1992 and
(f) 1992–1996.
B.M. O’Connor et al. / Deep-Sea Research I 49 (2002) 1571–1590
120E
150E
180Ê
150W
120W
1577
90W
40N
40N
30N
30N
0.3 m/s
20N
20N
10N
10N
0Ê
0Ê
10S
10S
20S
20S
30S
30S
40S
40S
(d)
120E
150E
180Ê
150W
120W
90W
120E
150E
180Ê
150W
120W
90W
40N
40N
30N
30N
0.3 m/s
20N
20N
10N
10N
0Ê
0Ê
10S
10S
20S
20S
30S
30S
40S
40S
(e)
120E
150E
180Ê
150W
120W
90W
120E
150E
180Ê
150W
120W
90W
40N
40N
30N
30N
0.3 m/s
20N
20N
10N
10N
0Ê
0Ê
10S
10S
20S
20S
30S
30S
40S
40S
(f )
120E
150E
180Ê
150W
Fig. 3 (continued).
120W
90W
1578
B.M. O’Connor et al. / Deep-Sea Research I 49 (2002) 1571–1590
Ekman pumping was calculated by integrating
the continuity equation from the surface to the
bottom of the winter mixed layer (hw ):
the inverse vertical age gradient corrected for
vortex stretching, as formulated by Jenkins (1987,
1998):
w% Ek ¼ ðqu% a =qx þ qv%a =qyÞhw ;
Sann ¼ fo =½f ðqt=qzÞ;
ð3Þ
where u% a and v%a are the mean zonal and meridional
ageostrophic components calculated for 1988–
1992 and 1992–1996 as described in Appendix A.
Calculation of Ekman pumping in this way is
consistent with slab layer dynamics. This becomes
important when Ekman pumping magnitudes
are compared in the North versus the South
Pacific (Section 6.2). Buoyancy comes into play
indirectly in this computation, because integration
was from the surface to the base of the winter
mixed layer. It follows that much deeper winter
mixed layers may cause higher Ekman pumping
rates. In order to assess the assumptions in the
Ekman pumping calculation, the Ekman pumping
term was calculated from Hellerman and Rosenstein (1983) and ECMWF (Trenberth, 1991) wind
fields.
Both vertical pumping and lateral induction
were calculated on a 2.51 2.51 grid, computed
within the region represented by the closed
salinity contour of the STUW. The winter
mixed layer depth was calculated from Levitus
climatology. It was defined as the depth at which
sigma-t differs from the surface value by 0.125
(Fig. 4). For winter mixed layers the following
months were used in the Northern Hemisphere:
December, January, and February; and Southern
Hemisphere: June, July, and August. The difference in the surface area of STUW properties
between spring and summer (Adiff ) (Fig. 2a) was
multiplied by Sann (m/s) to get a formation rate,
SANN (m3/s):
SANN ¼ Adiff ðu% w drhw þ w% w Þ:
ð4Þ
The analysis used here is ‘‘pseudo-Lagrangian’’
because the drifters are Lagrangian; however, the
analysis involved interpolation onto a fixed grid.
The results are presented in Table 2 and discussed
in Section 6.
4.2.2. Tracer method
The subduction rate (Sann ) for an isopycnal
projected back to its outcrop was calculated from
ð5Þ
where fo and f are the Coriolis parameters at the
outcrop and at the region where the water is
found, respectively. The age of the water is t and z
is the depth.
The formation rate (m3/s) is given by Eq. (5)
multiplied by Adiff
SANN ¼ Adiff fo =½f ðqt=qzÞ:
ð6Þ
The CFC-12 age was calculated from WOCE
and PMEL data at stations within the region of
interest. The CFC-12 apparent ages were mapped
on the upper and lower sy surfaces of the STUW
(Figs. 5a and b).
The CFC-12 apparent age was calculated by
comparing partial pressures to the atmospheric
time histories (Doney and Bullister, 1992; Fine
et al., 1988). CFC-12 was used so ages less than a
decade could be resolved. The CFC-12 atmospheric time histories for the Northern and
Southern Hemispheres were from the ALE/GAGE
network (Walker et al., 2000). The equilibrium
partial pressure (pCFC ) was the mole fraction (ppt)
of CFC in dry air that would be at solubility
equilibrium with the observed CFC seawater
concentration:
pCFC ¼
CFCseawater
:
F ðy; SÞ
ð7Þ
The solubility coefficient (F ) for CFC-12 was
taken from Warner and Weiss (1985).
The CFC-12 concentrations were interpolated
to the STUW upper and lower sy levels and the
depth, age and vertical age gradient calculated.
This approach assumed that the CFC-12 age
profile represented the true advective age of the
isopycnal surface. At most stations in the Pacific,
the STUW salinity maximum was just above the
CFC maximum. The water above the STUW
tended to be a little younger because of its
proximity to the surface. The water below the
STUW salinity maximum was a little older. For
these reasons the effects of mixing on the CFC age
and age gradient was small.
B.M. O’Connor et al. / Deep-Sea Research I 49 (2002) 1571–1590
1579
Table 2
Annual subduction and formation rates for 1988–1992 and 1992–1996 for the North and South Pacific using drifter and tracer methods
North Pacific
South Pacific
Drifters
Tracer
1988–1992
LI
VP
EP
bT
Drifters
1992–1996
Tracer
1988–1992
1992–1996
m/yr
Sv
m/yr
m/yr
Sv
m/yr
Sv
m/yr
m/yr
Sv
21
5
16
19
3
3.7
0.9
2.8
3.4
0.6
13
8
21
24
3
26
4.7
25
12
13
30
17
5.5
2.5
3.0
6.5
3.5
40
10
30
38
8
32
7.0
Note: The drifter method includes lateral induction (LI), vertical pumping (VP), Ekman pumping (EP), and a b term (bT) in the
Sverdrup balance. Formation rates (Sv) are for 1989.
120E
150E
180Ê
150W
120W
90W
40N
40N
50
30N
75
125100
50
50
30N
100
75
25
50
20N
20N
50
25
50
10N
10N
50
0Ê
0Ê
50
50
75
10S
75
10S
20S
100
75
50
100
11025
0
75
30S
75
50 5
100
12
40S
120E
150E
180Ê
150W
120W
125
20
0
15
0
15
0
75
1
20S
30S
75
40S
90W
Fig. 4. A composite of the winter mixed layer depths (hw ) (both Northern Hemisphere winter and Southern Hemisphere winter mixed
layers in the same figure) in the Pacific from Levitus climatology using a sigma-t criteria of 0.125, and mapped on a 11 11 grid.
To estimate the degree to which the CFC-12 age
represents the true advective age, a one-dimensional pipe model was run to 1994 (after Jenkins,
1998). The advective age was contoured as a
function of velocity and the pCFC-12 age. The
pCFC-12 ages are in good agreement with the
advective ages, with pCFC-12 ages younger by
0.4–0.5 yr for ages ranging from 1 to 10 yr. The
pipe model results were used to correct the age
gradient for the subduction rates presented in
Table 2 and Fig. 6. A diffusivity of 1000 m2/s was
used as representative of values within the upper
thermocline (Jenkins, 1991). The model was run
for diffusivities of 500 and 2000 m2/s to test the
sensitivity of the ages. Over the range, the ages
differed very little, with differences on the order of
0.1 yr. This suggests that the pipe model with a
diffusivity of 1000 m2/s was adequate to correct for
mixing effects on pCFC-12 ages in the region. The
meridional geostrophic velocities (0.2–2.0 cm/s) in
the model were calculated (2000 m reference level)
from Levitus dynamic height fields.
B.M. O’Connor et al. / Deep-Sea Research I 49 (2002) 1571–1590
180°
20°N
3
5
30°N
1
4
x
x
x
x
4
20°N
3
2
10°N
90°W
40°N
+
+
+
+
+
+
+
+1
+
+ 2
+
+
30°N
120°W
4
40°N
150°W
3
150°E
10°N
2
5
5
1580
0°
5
5
4
10°S
0°
* ****** **3
20°S
1
2
10°S
1
20°S
WOCE
30°S
tunes
p17n
p21e
^
^
^
30°S
40°S
40°S
150°E
180°
150°W
120°W
90°W
150°E
180°
150°W
120°W
90°W
40°N
4
5
6
1
2
7
4
4
6
7
5
3
x
x
x
x
8
6
8
0°
6
4
3
8
4
8
10°S
5
6
7
* ****** **
20°N
7
5
2
3
PMEL
+ C02-87
10°N
0°
^ C02-89
*x tew-87
C02-88
10°S
4
8
2
4
3
20°S
30°S
3
2
^
^
^
40°S
(b)
30°N
1
5
7
7
10°N
3
3
2
4
5
6
40°N
3
4
4
20°N
2
1
30°N
+
+
+
+
+
+
+
+
+
+
3+
+
5
(a)
^ C02-89
*x tew-87
C02-88
4
2 3
1
4
PMEL
+ C02-87
20°S
WOCE
30°S
tunes
p17n
p21e
40°S
150°E
180°
150°W
120°W
90°W
Fig. 5. Pacific pCFC-12 age (years) contoured on the (a) STUW upper sy level and (b) STUW lower sy level. Superimposed is a curve
representing the subduction area calculated from 1989 SST satellite data. The CFC-12 station locations are indicated by symbols:
WOCE/Miami (1991, 1993 and 1994); and PMEL (1987, 1988 and 1989).
5. Sources of uncertainties in subduction rates
A detailed propagation of errors in the drifter
measurements was not performed, as uncertainties
in the measurements and drifter analysis are much
smaller than the uncertainties in the assumptions.
For example, the uncertainty in the accuracy of the
drifter-measured velocities themselves is 0.2%
(Niiler et al., 1987; Bitterman et al., 1990). As a
result, a detailed propagation of errors was
considered meaningless. Here, uncertainties in
the assumptions underlying the calculations are
quantified.
The largest uncertainties in the assumptions are
in the size of formation areas, which are based on a
constant T=S relationship. For example, mixing
would alter this relationship. However, in a
parallel study diapycnal mixing was estimated to
be negligible post-formation (O’Connor, 2001),
and this lends support to the validity of the
assumptions in the estimation of the formation
regions. In the drifter method, the major assumptions in the calculation of Ekman pumping were:
constant horizontal divergence in the mixed layer
and zero vertical velocity at the surface. These
assumptions were considered independent of each
B.M. O’Connor et al. / Deep-Sea Research I 49 (2002) 1571–1590
other. The drifter calculated Ekman pumping rates
agree well with those from the more standard
calculations using the winds. Ekman pumping
rates calculated from Hellerman and Rosenstein
(1983) winds were 34 m/yr in the North and 35
m/yr in the South Pacific. The ECMWF climatology gave 26 m/yr in the North and 50 m/yr in the
South Pacific. Agreement between drifter and wind
results lends support to the assumptions in the
Ekman pumping calculation. It also suggests that
the mixed layer calculation using Levitus climatology is robust. Finally, the difference (about 20%)
between the two independent methods, drifter and
tracer, can be viewed as an estimate of the
uncertainties. The results below suggest that these
estimates are quite reasonable.
Calculation of SANN is subject to uncertainties
from a variety of sources. Uncertainties can be
divided into two types: random and systematic.
Random uncertainties are assumed to be independent, and the cumulative random uncertainty was
calculated as the square root of the sum of squares
of the individual terms. The common random
uncertainties in both methods include those related
to formation areas. The uncertainties associated
with defining the formation areas were conservatively estimated at 7%. This value was derived by
assuming a very large offset of 51 51 in the
digitization of the areas.
5.1. Drifter method
Random uncertainties in the drifter method
were considered due to errors in the decomposition
into ageostrophic and geostrophic velocities, and
systematic uncertainties, which include averaging,
use of climatology, and assumption of slab
dynamics. These assumptions were considered
independent of each other.
A test of the overall significance of the drifter
momentum measurements (in a process sense)
involves the degree to which the velocity field can
be successfully divided into geostrophic and
ageostrophic flows (see Appendix A). Here, the
uncertainty in the meridional velocity component
was used to quantify the uncertainty in the drifter
calculations. It was computed as the difference in
velocity across the latitude range of the water mass
1581
outcrop, divided into the largest standard error in
the domain. The random uncertainties in the
meridional velocity components were 9% and
17% in the South Pacific region in 1988–1992
and 1992–1996, respectively. In 1992–1996, the
uncertainty in the North Pacific region was 11%.
The largest uncertainty (19%) was encountered in
the central North Pacific during the 1988–1992
period. This is due to the convoluted structure of
the meridional velocity during this period in this
region (particularly between 1401E and 1801, see
Fig. 7). Conservatively, over the entire drifter data
set, the random uncertainties in comparing velocities across the equatorial currents were on the
order of 20% or less. This is consistent with the
difference between the currents compared over
the two time periods. The uncertainty in the
analysis and assumptions in the drifter method
was approximately 20%, resulting in a 21%
uncertainty in the formation rate.
The first systematic uncertainty is due to the
effect of the mesoscale eddy field on drifter
velocities. In the frontal zones across the poleward
side of both subtropical gyres, significant numbers
of drifters move to lower dynamic heights. This
tendency and the appearance of deep reference
levels in the fit to geostrophy in these regions
suggest that the drifter motion was tied to eddy
fluxes. Mesoscale variability has been well documented within the Pacific subtropical regions (e.g.,
Wyrtki et al., 1976). Mesoscale eddies modify the
rate at which subduction occurs (Marshall, 1997).
Subduction may be driven locally by eddies within
the STUW formation regions. The averaging
method discussed in Appendix A was meant to
minimize this problem; however, it caused a bias
towards lower velocities.
Some systematic uncertainty in the subduction
rate arises from the use of the Levitus climatology
in the estimation of mixed layer depths. Use of
climatology caused a systematic bias towards
shallower mixed layers. This is directly proportional to the vertical pumping rate, whereas the
effect on the lateral induction is not as straightforward. This effect was evaluated fully elsewhere
(O’Connor, 2001). However, the fact that the
tracer and drifter methods agree to within 22%
suggests that the mixed layer depth calculations
1582
B.M. O’Connor et al. / Deep-Sea Research I 49 (2002) 1571–1590
were minimally biased. The agreement with the
wind results (discussed below) further supports
using the climatology.
5.2. Tracer method
Random uncertainties were considered due to
analytical precision and accuracy of data, solubility function, atmospheric time history, surface
saturations, interpolation onto isopycnal surfaces,
and tracer data resolution. These uncertainties
were considered independent. Systematic uncertainties are due to the effects of mixing on tracer
ages, and the change of tracer ages with time.
Random uncertainties including the analytical
precision of the PMEL CFC-12 data, and sampling and handling, were estimated at 1–2%
(Wisegarver et al., 1993). The analytical precision
for WOCE CFC-12 samples >0.1 pmol/kg varied
between 0.8% and 5% (Fine et al., 2001). The
estimated precision and accuracy of the CFC-12
solubility function were about 0.7% and 1.5%,
respectively (Warner and Weiss, 1985). Uncertainties in the ALE/GAGE atmospheric time histories
were about 5% (Walker et al., 2000). Sensitivity
tests were conducted where CFC-12 concentrations were varied by 75%, which changed
apparent ages by less than 71 yr, resulting in a
74–6% change in subduction rates. Uncertainties
in estimates of CFC-12 surface saturations were
about 5% (Fine et al., 2001).
Random uncertainties also arose from interpolation of CFC-12 concentrations onto isopycnal
surfaces. The interpolation technique used was the
cubic spline. Uncertainties due to interpolation
(Robbins, 1997) are difficult to estimate but
depend largely on the spacing between data points,
which determines the ability to resolve gradients.
The CFC-12 data used here were well resolved in
the vertical, so any interpolation uncertainties are
expected to be very small.
Biases in the spatial distribution and temporal
resolution of the CFC-12 data can lead to
uncertainties in the ages and resulting subduction
rates. The data in the North Pacific were not as
well distributed spatially as that in the South. It
was difficult to estimate the effect of the spatial
bias on the subduction rate. However, west of
1601E in the North Pacific, drifter and tracer rates
were markedly different (Figs. 6a and b). Lastly,
Doney et al. (1997) derive a CFC age conservation
equation including a non-linear mixing term.
However, they estimated that the magnitude of
this term was extremely small, and the effect on the
uncertainty was not included here.
The largest uncertainty in the tracer method was
due to the effects of mixing on the ages, which
were corrected for to some extent by use of the
pipe model. The pipe model gave advective ages,
which were older than the pCFC-12 ages for
reasons discussed above. On application of the
pipe model, North Pacific ages increased by 13–
20%, which represents a 10% increase in the age
gradient, resulting in a 10% decrease in SANN : In
the South Pacific, the ages increased by 8–20%
resulting in a 6% decrease in SANN : The pipe
model may over or under-compensate for mixing.
Here, another 10% contribution from mixing to
the random uncertainty was assumed. The cumulative uncertainty in the tracer method resulted in
a 15% uncertainty in the subduction rate, and a
17% uncertainty in the formation rate.
Apparent saturations increase monotonically at
a rate of about 5–10% per decade due to nonstationarity in the atmospheric source (Beining
and Roether, 1996). Systematic uncertainties were
estimated at 7%, as the tracer data used here
spanned a 7-yr period. The bias was towards older
ages.
6. Analysis and discussion
6.1. Comparison of tracer and drifter rates
The STUW formation rates (Table 2) for 1989
in the North Pacific are 4 Sv (drifter) and 5 Sv
(tracer). In the South Pacific, STUW formation
Fig. 6. The PSTUW subduction rate (m/yr) calculated from (a) the pCFC-12 ages (1987–1994), where dots indicate CFC-12 station
locations, (b) the drifter data (1988–1992), and (c) the drifter data (1992–1996). Superimposed is a curve representing the subduction
area calculated from 1989 SST satellite data.
B.M. O’Connor et al. / Deep-Sea Research I 49 (2002) 1571–1590
150°E
180°
40°N
150°W
120°W
90°W
40°N
40
30
30°N
30
20°N
20
20
10
30
10°N
20°N
30
30°N
1583
10°N
0°
0°
10
10
10
10°S
10°S
20
20°S
20
20
40
30°S
30
30°S
40°S
(a)
20°S
40°S
150°E
180°
150°W
120°W
90°W
150°E
180°
150°W
120°W
90°W
40°N
40°N
30°N
30°N
10
50
20°N
20
30
40
40
50
10
10
20°N
3200
10°N
10°N
50
30
20
40
10
10
0°
10°S
0°
4050
10
20
10°S
30
10
34200
500
20°S
10
20°S
30°S
30°S
40°S
40°S
(b)
150°E
180°
150°W
120°W
90°W
150°E
180°
150°W
120°W
90°W
40°N
40°N
10
20
30
30°N
20
30°N
20°N
50
20
30
40
10
60
50
20°N
10
3040
40
10°N
3020
40
10°N
50
0°
0°
60
10°S
10
20
30
50
40
30
10°S
40
20
20°S
20°S
50
30°S
60
30
10
30°S
40°S
(c)
40°S
150°E
180°
150°W
120°W
90°W
1584
B.M. O’Connor et al. / Deep-Sea Research I 49 (2002) 1571–1590
rates for 1989 are 6 Sv (drifter) and 7 Sv (tracer).
The drifters gave lower rates than the tracers.
Thus, the combined results from the two methods
can be used to put bounds on the process, e.g. 4–
5 Sv formed in the North, and 6–7 Sv in the South
Pacific. The drifter (1988–1992) STUW subduction
rates are 21 and 25 m/yr in the North and South
Pacific, respectively. During 1992–1996, North
and South Pacific STUW subduction rates are 13
and 40 m/yr. The tracer STUW subduction rates
are 26 and 32 m/yr in the North and South Pacific.
During both periods, STUW subduction rates
are higher in the South than in the North Pacific.
One reason is due to the location of the STUW
region. In the South Pacific, the STUW is closer to
the equator than in the North, resulting in higher
Ekman pumping rates in the South Pacific region
due to smaller f (Eq. (2)). A second reason is due
to the stronger salinity stratification and presumably buoyancy flux in the North. Another reason is
because of the deeper mixed layers in the South.
Both the lateral induction and the vertical pumping terms are consistently higher in the South
Pacific than the North.
It is striking that the drifter and tracer methods
gave subduction and formation rates that agree
well quantitatively, within 22%, though the spatial
patterns show some differences. The two methods
do not agree for the second period in the North
Pacific (Table 2). The good agreement between the
drifter and tracer methods can be partially
attributed to the temporal overlap in the data
sets. The drifter data (1988–1992) spanned one El
* The tracer data are from 1987 to 1988, 1991
Nino.
and 1993 for the North Pacific, and 1987, 1989,
1991 and 1994 for the South. The years 1986–1987
* events, so the
and 1991–1994 include El Nino
* years, as
tracer data are biased towards El Nino
are the second drifter period. However, the tracer
signal is integrated over time. This makes it
difficult to estimate the extent to which the tracer
* years. In
data are representative of El Nino
general, maps of the tracer (1987–1994) (Fig. 6a)
and drifter (Fig. 6b) subduction rates (1988–1992)
show similar features, in spatial pattern and
magnitude. Yet, the drifters consistently show a
wider range of rates, and contours are more
densely crowded.
Although the rates from the two methods agree
well, the tracer rates are higher than the drifters,
except for South Pacific during the period 1992–
1996 for several reasons. First, the drifter data
represent an annual average over a 5-yr time
period, and this results in smoothing of the
velocity and mesoscale field (see error section).
The second reason the tracer rates are higher than
the drifter rates may be due to the non-stationarity
in the source for the tracer data, which could result
in a 7% overestimate of the tracer rate. A third
reason is that vertical pumping, from the drifter
method, may be underestimated because of the use
of Levitus climatology (see Section 5.1). The
difference between tracer and drifter rates would
require an increase in mixed layer depth in the
North by 20% and 50% for the first and second
period, respectively, and 22% in the South Pacific
during the first period. These percentages are
acceptable underestimates of mixed layers by the
climatology.
A final issue to consider in explaining the
difference between the drifters and the tracers
may be related to the effects of mixing during
formation. For example, Tomczak and Godfrey
(1994) point out that over a large part of the
STUW formation area in the South, there is
uniform salinity at depths far exceeding Ekman
layer depths. This could be due to mixing. The
tracer rates integrate the effects of diapycnal
mixing at the base of the mixed layer during
STUW formation. However, the drifters do not
take into account diapycnal mixing. Mixing would
cause the drifters to underestimate the rate. It is
expected that this is more of an issue in the South
Pacific where there are deeper mixed layers.
Mixing can enhance the subduction process by at
most 10%, so the drifters may underestimate the
rate by that much. Yet in general, the annual cycle
of the mixed layer deepening is much less for
STUW than the rest of the Central Waters. If the
tracer rates are higher than the drifters because of
mixing (not measured by drifters), then this
supports the idea that mixing (Schmitt, 1981;
Kadko and Olson, 1996) contributes to STUW
formation, in addition to the large-scale poleward
Ekman flux of freshwater as suggested by
Worthington (1976). The difference between the
B.M. O’Connor et al. / Deep-Sea Research I 49 (2002) 1571–1590
drifter and tracer calculation may involve smallscale mixing, and the fact that the drifter calculation did not include the mesoscale thickness fluxes
(bolus subduction, Spall. 1995; Spall and Chapman, 1998). Non-stationarity in the tracer source
may also bias tracer rates towards higher values.
Still, it is not clear why the drifter rate is higher
than the tracer rate during the second period.
Examination of the individual terms (vertical
pumping and lateral induction) helps in understanding the differences between the two periods
and hemispheres.
6.2. Comparison of drifter rates between the two
periods
Between the two periods, 1988–1992 and 1992–
1996, there is a spatial expansion and shift to
higher rates in the South Pacific (Figs. 6b and c).
However, subduction rates decrease in the North
Pacific. The overall decrease in the North Pacific
rate is consistent with what Lukas (2001) finds at
HOTS at the density of the STUW; salinity is
decreased during the second period.
The partitioning between vertical pumping and
lateral induction is quite different between periods
(Table 2). The decrease in the North Pacific
subduction rate between the first and second
period is largely due to the lateral induction
becoming negative, entraining STUW into the
mixed layer. In the North Pacific, lateral induction
is smaller than vertical pumping during both
periods. In the South Pacific, lateral induction
and vertical pumping contribute nearly equally to
the subduction rate during the first period.
However, vertical pumping greatly exceeds lateral
induction during the second period, because the b
term becomes a smaller negative number.
During both periods there was higher Ekman
pumping in the South than in the North Pacific.
Yet, the higher Ekman pumping in the South
Pacific is a little unexpected, because of the higher
wind stress curl in the North. However, there is an
additional contribution to the Ekman pumping
from the much deeper mixed layer in the South
Pacific, and the closer location of STUW to the
equator. In both hemispheres, there was an
increase in Ekman pumping during the second
1585
period. The increase is consistent with wind stress
curl anomalies in the central/eastern parts of the
two subtropical gyres being in phase (White and
Cayan, 1998). For example, local wind stress curl
anomalies during 1988–1992 in the North Pacific
and South Pacific subtropical gyres were weak.
The wind stress curl is only part of what is
influencing differences in subduction rates. There
are additional processes operating here, in particular those affecting lateral induction. This is
evident by the out of phase relationship in the
subduction rates. For example, the North Pacific
rate is higher in the first period, but the South is
higher in the second. One of the reasons for the
difference is negative lateral induction during the
second period in the North. The role of lateral
induction, in addition to vertical pumping, confirms that the subduction process depends implicitly on the large-scale circulation, and the
combination of the winter outcrop pattern and
the air–sea fluxes (Sarmiento, 1983; Jenkins, 1987).
Differences in subduction rates and partitioning
between terms are consistent with changes in
intensity and position of the current axes during
both periods. In the North Pacific between 1401E
and 1401W, v% decreased between the first and
second period (Fig. 7), and u% decreased markedly.
The decrease in u% and v% in turn decreased the
lateral induction term (Eq. (1)). An increase and
southward shift in the North Equatorial Current
(NEC) and North Equatorial Counter Current
* (second period dominated
(NECC) during El Nino
by mature phase) are consistent with observations
of Qiu and Joyce (1992).
There are changes in current intensity and
position in the South Pacific. From the first period
to the second, v%g decreased, which gave a smaller
negative b term (Eq. (2)). The term dv=dy increased, and this in turn increased the Ekman term
(Eq. (3)). These suggest an increase in the South
Equatorial Current (SEC) transport. In addition,
there was a poleward displacement during 1995–
1996 of the SEC, suggesting gyre wobble (Fig. 7).
The increase in the Ekman term in the second
period (at least until 1994) is consistent with
observations from Morris et al. (1996). Using XBT
data from 1987 to 1994, they find a relatively
weaker South Pacific subtropical gyre during
B.M. O’Connor et al. / Deep-Sea Research I 49 (2002) 1571–1590
1586
(a) PANPAC
140 -180¡
1988-92
1992-96
40
30
v
20
10
-.20
-.15
-.10
-.05
(b)180 - 220¡
1988-92
1992-96
.05
.10
.15
.20
40
30
v
20
10
-.15
-.10
-.05
(c) 140 -180¡
1991-93
1995-96
.05
.10
.15
.05
.10
.15
40
30
20
10
-.20
-.15
-.10
-.05
.20
u (m/s)
(d) 260 - 220¡
1991-93
1995-96
-.20
-.15
-.10
-.05
v
40
30
.05
.10
.15
.20
u (m/s)
20
10
Fig. 7. Drifter velocities (m/s) averaged in 21 81 bins versus
latitude for areas in the western, central and eastern Pacific:
(a) 1401E–1801, (b) 1801–1401W, (c) 1401E–1801, and (d)
1001W–1401W. Terms u and v are the average zonal and
meridional velocity components, respectively.
1988–1989. After 1991, they find more intense gyre
transport with stronger Ekman pumping.
Comparison of the two drifter periods suggests a
connection between interannual variability of
ENSO and perhaps longer time scale with the
subduction process. This is consistent with conclusions of Yamagata et al. (1985), who find a gyre
scale adjustment to interannual variability. The
drifter results indicate that the differences in
subduction rates are related to changes in gyre
intensity and possibly gyre wobble (e.g., Armi and
Stommel, 1983; Garzoli et al., 1997). This out of
phase fluctuation in the subduction rates in the
North and South Pacific between the two time
periods agrees with results from Wyrtki and Wenzel
(1984). They find out of phase fluctuations in the
two subtropical gyres with a period of about 4 yr.
ENSO related differences in the subduction
rates over the two periods are to be expected.
The period 1988–1992 encompassed an early phase
*
of El Nino,
and the second period included a
mature phase. The first period included a strong
* during 1988. It is possible that STUW
La Nina
*
formation decreases during El Nino’s
onset as the
trades weaken. However, during the peak phase,
the intensification of the trades (Qiu and Lukas,
1996; Mitchum, 1987) and increased transport of
the NEC and NECC (Qiu and Joyce, 1992;
Wyrtki, 1979) would seem to favor STUW
formation. The southward shift in the boundary
of the NEC will affect lateral induction by
changing the slope of the density surfaces. This
effect may be quite complex, as it is accompanied
by changes in the Kuroshio Current and its
recirculation system. The southward shift in the
NEC (seen from u;
% Fig. 7) during 1992–1996
affected the sign of the lateral induction term,
and the increase in magnitude is consistent with
NEC intensification.
ENSO’s effect on STUW formation and vice
versa is complex, and cannot be evaluated without
a better understanding of the causes of longer-term
variability in ENSO. This will require observations
spanning a much longer time period. For example,
the subduction rate decrease in the North Pacific
during the second period may be related to decadal
variability involving changes in the magnitude and
position of the Aleutian low pressure. During
1988–1992, the Aleutian low was very strong
(Overland et al., 1999) and may have contributed
to the stronger subduction rate at that time.
6.3. Comparison with previous studies
This is probably the first estimate of subduction
rates targeting STUW. Comparisons are made
with previous subduction rate estimates, though
they are not specifically for STUW. The North
Pacific STUW subduction rate presented here
(13–21 m/yr) is lower than that of Huang and
Qiu (1994). They obtain a subduction rate of
about 25 m/yr over the subtropical thermocline of
the North Pacific. The difference suggests that
B.M. O’Connor et al. / Deep-Sea Research I 49 (2002) 1571–1590
denser thermocline waters have a higher subduction rate. Over 24.8–25.0sy ; Huang and Qiu (1994)
obtain a maximum of 5 Sv, which agrees well with
4–5 Sv obtained here. However, their calculation
area covered nearly the entire subtropical region.
For the South Pacific, Huang and Qiu (1998) use a
model to obtain a subduction rate for the
subtropical thermocline of 22 m/yr, which is a
little lower than the subduction rate of 25–40 m/yr
obtained here over both periods. It is difficult to
draw conclusions from these large-scale comparisons with the targeted STUW.
6.4. Implications for understanding temporal
variability
Comparison of the two drifter periods suggests
that interannual variability, of ENSO and perhaps
longer time scale, may be affecting the subduction
rate and partitioning of the terms. This agrees with
results from Yamagata et al. (1985) suggesting
gyre scale adjustment to interannual variability.
Understanding ENSO and decadal variability
effects on the subduction rates and vice versa will
require observations spanning a much longer time
period. The results presented here suggest that
gyre intensity and wobble (Armi and Stommel,
1983; Garzoli et al., 1997) need to be considered in
evaluating temporal changes in subduction rates,
and they raise questions for future studies. Is more
water formed in one hemisphere corresponding to
an intensification of the gyre as it is displaced/
wobbles? Does the subtropical/tropical exchange
increase in one hemisphere and decrease in the
other? Are such changes related to interannual
variability or longer time scale variability? And
what triggers these changes? The next step will be
to extend the calculations to other oceans to get an
understanding of the variability in the formation
of STUW under various geographical and other
climatic conditions. Additionally, there are implications for interannual variability of subtropical
circulation cells, since STUW makes up a significant portion of the water subducted in the
upper thermocline of the subtropics and tropics.
Finally, these results can be used to validate
numerical models, in terms of realistic subduction
rates and pathways for this water mass.
1587
Acknowledgements
The authors gratefully acknowledge CFC analyses under the direction of Kevin Sullivan. We
acknowledge the chief scientists on the legs of the
WOCE cruises: Mizuki Tsuchiya, Jim Swift and
Michael McCartney. We are very grateful to John
Bullister and David Wisegarver for sharing their
CFC data. The authors acknowledge the contributions through discussions with Kevin Leaman,
William Johns, Eric Chassignet and Claes Rooth.
The manuscript was substantially improved in the
review process, for this we thank Roger Lukas, an
anonymous reviewer, and the Editor Michael
Bacon. This work was funded through grants
from the National Science Foundation (OCE9207237, OCE-9413222, OCE 9529847, OCE9811535), and by the National Oceanic and
Atmospheric Administration under an agreement
with the Cooperative Institute for Marine and
Atmospheric Studies.
Appendix A. Estimate of drifter velocities
The Pan Pacific Drifter data gathered as part of
the WOCE and TOGA programs were one of the
more ambitious direct velocity measurement efforts attempted. The drifters were all of standard
design with three-dimensional drogues at 15 m.
The units all had drogue sensors that detected the
loss of the drogue. Only units with intact drogues
were used in the current study (see Niiler and
Paduan, 1995). The drifters gave Lagrangian
trajectories that follow the flow between 10 and
20 m. For the period 1988–1992 there were 284,080
drifter days, and for the period 1992–1996 there
were 538,993 drifter days.
Various schemes were used to produce a gridded
velocity set needed for the subduction calculations.
Earlier studies calculated velocities from the drifter
trajectories, and treated the resulting set of
velocities as an Eulerian data set (Richardson,
1983; Hansen and Paul, 1987). In order to understand the relationship of the drifter velocities to
dynamic height and wind stress fields in the region,
the initial analysis was completed in a Lagrangian
frame. Along each drifter’s trajectory, dynamic
1588
B.M. O’Connor et al. / Deep-Sea Research I 49 (2002) 1571–1590
Fig. 8. For the North and South Pacific, total drifter velocity, VD ; and its residual, Vr ; as compared to best fit to geostrophy, Vg : Wind
stress (t) for the North and South Pacific is shown.
heights were derived from Levitus climatology at
reference level increments of 200 m, from 200 m to
the bottom. Winds were similarly compiled along
each drifter’s trajectory from the ECMWF climatology. Dynamic heights following trajectories
showed fairly smooth increases in the area of
interest after 30–50 days. This time scale was
interpreted as that associated with drifter motion
in the mesoscale eddy field. The along trajectory
velocities, dynamic height gradients, and wind
stresses were averaged with a 50 day Gaussian
low-pass filter. The velocities from individual
drifters were combined in an ensemble average
onto an Eulerian grid of 2.51 2.51.
To provide a better understanding of the
subduction process, the WOCE/TOGA drifter
data were used to decompose the surface circulation into geostrophic and ageostrophic components (Figs. 3a–f). The geostrophic component was
computed from Levitus dynamic heights. The
geostrophic component was subtracted from the
drifter velocity, and the ageostrophic residual was
compared to the Ekman component expected from
the regional wind stress. The residuals were in
agreement with an Ekman component over the
area of interest here. Fig. 8 shows the relationship
between the ageostrophic residual and the wind
stress. The residual is 1051 to the right of the wind
stress in the North. It is 501 to the left of the
wind stress in the South Pacific.
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