Estuaries and Coasts (2010) 33:1164–1175
DOI 10.1007/s12237-010-9319-9
Wind Modulation of Dissolved Oxygen in Chesapeake Bay
Malcolm E. Scully
Received: 17 June 2009 / Revised: 21 November 2009 / Accepted: 13 June 2010 / Published online: 29 June 2010
# Coastal and Estuarine Research Federation 2010
Abstract A numerical circulation model with a simplified
dissolved oxygen module is used to examine the importance of wind-driven ventilation of hypoxic waters in
Chesapeake Bay. The model demonstrates that the interaction between wind-driven lateral circulation and enhanced
vertical mixing over shoal regions is the dominant mechanism for providing oxygen to hypoxic sub-pycnocline
waters. The effectiveness of this mechanism is strongly
influenced by the direction of the wind forcing. Winds from
the south are most effective at supplying oxygen to hypoxic
regions, and winds from the west are shown to be least
effective. Simple numerical simulations demonstrate that
the volume of hypoxia in the bay is nearly 2.5 times bigger
when the mean wind is from the southwest as compared to
the southeast. These results provide support for a recent
analysis that suggests much of the long-term variability of
hypoxia in Chesapeake Bay can be explained by variations
in the summertime wind direction.
Keywords Hypoxia . Chesapeake Bay . Dissolved oxygen .
Wind . Vertical mixing
Introduction
It is largely accepted that anthropogenic eutrophication has
increased the extent and severity of hypoxia in coastal and
estuarine waters (Diaz 2001). Yet, assessing human impacts
and predicting the potential benefit of nutrient reductions
M. E. Scully (*)
Center for Coastal Physical Oceanography,
Old Dominion University,
Norfolk, VA 23508, USA
e-mail: [email protected]
are often confounded by the natural complexity and
variability of these systems. Dissolved oxygen is modulated
by both physical and biological processes that vary over
time scales ranging from minutes to decades. These
physical and biological processes often are tightly coupled,
and distinguishing the importance of different mechanisms
can be extremely difficult. A complete understanding of the
physical processes that modulate hypoxia remains an
obstacle to quantifying the impact of eutrophication on
estuarine systems.
The traditional view is that hypoxia in Chesapeake Bay
develops during the summertime when oxygen utilization
exceeds replenishment by vertical mixing (Officer et al.
1984). This view essentially assumes a one-dimensional
oxygen balance in the vertical. However, Malone et al.
(1986) suggested that the rotational response to winddriven forcing may provide an important mechanism for
exchanging both nutrients and dissolved oxygen between
the surface and sub-pycnocline waters in Chesapeake Bay.
Sanford et al. (1990) also noted significant synoptic scale
variability in near bottom oxygen measurements, which
were attributed to wind-driven lateral oscillations of the
pycnocline. Wind straining of the density field can exert a
first-order control on the vertical density stratification and
thus play an important role in modulating vertical mixing
(Scully et al. 2005; Chen and Sanford 2009). Both
O'Donnell et al. (2008) and Wilson et al. (2008) suggested
that wind straining of the density field plays an important
role in controlling hypoxia in Western Long Island Sound
by modulating the vertical mixing.
More recently, Scully (2010) demonstrated that the interannual variations of the summertime volume of hypoxic
water in Chesapeake Bay are positively correlated with the
duration of westerly winds and negatively correlated with
the duration of southeasterly winds. This study found that
Estuaries and Coasts (2010) 33:1164–1175
more of the inter-annual variation in the volume of hypoxic
water was explained by wind direction than any other
variable considered. Scully (2010) further suggested that
increases in hypoxic conditions in the bay over the past
several decades may have been enhanced by shifts in the
summertime wind forcing caused by decadal scale climate
variability. While these results highlight the importance of
wind forcing to the oxygen dynamics of Chesapeake Bay,
they do not provide any insight into the physical mechanisms driving this behavior. Despite the overall importance
of wind to this system, observations conclusively demonstrating the importance of wind to the oxygen dynamics are
lacking. While such observations are possible, the spatial
resolution necessary to accurately capture the oxygen budget
would be challenging, making numerical approaches an attractive alternative. In this paper, a realistic three-dimensional
circulation model is used to conduct a detailed study of the
physical response of dissolved oxygen in Chesapeake Bay to
wind-driven forcing. The model is used to assess the
importance of wind direction and provide an oxygen budget
for the sub-pycnocline hypoxic zone in the bay.
Methods
Model Simulations
The numerical experiments in this study were conducted
utilizing the Regional Ocean Modeling System (ROMS;
Shchepetkin and McWilliams 2005). Simulations employ
the numerical grid developed for the open source Chesapeake
Bay ROMS Community Model. This is a 150×100 curvilinear grid with 20 vertical terrain-following coordinates. It
includes all of the main tributaries to the bay as well as the
shelf region immediately adjacent to the bay mouth. The
model was run in an idealized manner, using only M2 tidal
forcing at the oceanic boundary and constant river discharge
for all of the tributaries. The discharge for each tributary was
calculated from the annual mean from the seaward-most
gauging station maintained by the US Geological Survey.
Turbulence closure is achieved using the k–ε model with the
stability functions of Kantha and Clayson (1994). The background diffusivity for both momentum and scalars is set to
10−5 m2/s consistent with value used by Li et al. (2005).
A very idealized dissolved oxygen model was implemented. The goal was to use as simple a model as possible,
so that the physical processes that modulate dissolved
oxygen could be isolated from biological variability. To do
this, oxygen is introduced into the model as an additional
tracer. A constant water column respiration rate of 0.8 gO2/
m3/day was applied everywhere inside the estuarine portion
of the model domain. This rate is within the range of
measured values for Chesapeake Bay (Kemp et al. 1992). A
1165
gradient flux condition was imposed at the sea surface
following the formulation proposed by Marino and
Howarth (1993), assuming a constant piston velocity of
3 cm/h. This is consistent with a wind speed of roughly
4 m/s, the approximate summertime average over Chesapeake Bay. Oxygen at both the oceanic and river
boundaries was fixed to saturation values. Oxygen concentrations were not allowed to become negative, essentially
imposing a respiration rate of zero for anoxic conditions.
While this ignores the potential influence of sulfide
reduction, it is in keeping with the goal of using the
simplest possible model for oxygen dynamics.
To account for wind forcing, a spatially uniform surface
momentum flux was applied to the model domain. Values
of wind stress were calculated using the drag formulation of
Large and Pond (1981). The sea-level response of the Bay
to wind forcing consists of both a local response to the
surface wind stress as well as a remote effect driven by
coastal upwelling/downwelling (Wang 1979). With the
limited coastal domain, the model does not fully capture
this remote barotropic process that is driven by large-scale
shelf dynamics. To include this effect, an empirically
derived subtidal setup/set down based on wind strength
and direction was added to the tidal boundary forcing. This
empirical formulation was derived using 20 years of
observed water levels at the Chesapeake Bay Bridge tunnel
and winds observed at Chesapeake Light. The observed
water levels were de-tided, and the residual was regressed
against the north and east components of the wind. This
analysis gave the following relationship for the subtidal
water level fluctuations at the mouth of the Bay:
h ¼ 0:12 N þ 0:12 E
ð1Þ
where N and E are the north and east components of the
wind velocity, respectively (in meters per second, positive
for winds from north and from east).
To examine the role of wind forcing, a series of numerical experiments (see Table 1) were conducted using the
model configuration described above. For the base model
run (A1), no wind forcing was used. The model was run
until both the salinity field and oxygen distributions were
steady at subtidal time scales. To examine the importance of
individual wind events from different directions, simulations using an idealized 3-day wind event were conducted
(B1–B4). For these runs, the idealized forcing consists of a
wind that increased from 0 to 6 m/s and back to zero
sinusoidally over a period of 3 days. This forcing was
chosen to be largely representative of the typical wind
events observed in summertime over Chesapeake Bay. The
wind event begins on day 3 of the simulations, ends on
day 6, and is followed by 4 days without wind. Model runs
were conducted with wind forcing from the north (B1), east
(B2), south (B3), and west (B4).
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Table 1 Description of model
runs
Estuaries and Coasts (2010) 33:1164–1175
Model ID
Model description
A1
B1
B2
B3
B4
C1
C2
C3
D1
D2
D3
Base mode run with no wind forcing
3-day sinusoidal wind event from north (max 6 m/s)
3-day sinusoidal wind event from east (max 6 m/s)
3-day sinusoidal wind event from south (max 6 m/s)
3-day sinusoidal wind event from west (max 6 m/s)
Constant wind speed (4 m/s) with oscillatory direction (W to E with mean S)
Constant wind speed (4 m/s) with oscillatory direction (SW to NE with mean SE)
Constant wind speed (4 m/s) with oscillatory direction (NW to SE with mean SW)
Observed winds at Thomas Point Light for May–June 1996
Observed winds at Thomas Point Light for May–June 1998
Observed winds at Thomas Point Light for May–June 1996, magnitude reduced by 10%
In reality, wind direction over Chesapeake Bay is rarely
constant for 3 days. More typically, wind direction varies at
synoptic time scales associated with the passage of weather
systems. A series of additional simulations were conducted
with winds that varied in direction in an idealized way (C1–
C3). For these runs, wind speed was held constant at 4 m/s
while the wind direction oscillated back and forth through a
180° arc centered on the mean wind direction. Consistent
with the typical summer time conditions, mean wind
direction for these three runs was south (C1), southeast
(C2), and southwest (C3). Wind direction oscillated through
a 180° arc with a period of 6 days.
Finally, a series of model runs were conducted using
observed winds (D1–D3). These runs focused on the
summers of 1996 and 1998—two years with very similar
river discharge and estimated nitrogen loading but very
different amounts of observed hypoxia (Hagy et al. 2004).
For these runs, a spatially uniform surface wind stress was
calculated using the observed wind speed and direction
measured at the Thomas Point Light for the summer of
1996 (D1) and 1998 (D2). The Thomas Point Light is
located along the central portion of the main stem of the
Bay (Fig. 1) and is assumed to largely represent the wind
forcing over the entire system. To highlight only the role of
wind, no other forcing was changed (tides and river
discharge were held constant). The model was started using
the equilibrated fields from the model run with the
oscillatory southerly wind (C1). Because the model results
will be compared to the annual surveys of hypoxia reported by Hagy et al. (2004), which were generally
conducted in early July, the model was run for a 2-month
period beginning on May 1st of each year. This assumes
that the wind forcing over the months of May and June is
most relevant to the extent of hypoxia observed in early
July. Because the mean wind speed in 1996 was slightly
greater than 1998, an additional run was conducted
where the magnitude of the wind in 1996 was reduced
by 10% (D3).
Model Analysis
There are several definitions and criteria for hypoxia and
anoxia in estuarine waters. For the purposes of this paper,
hypoxic conditions are defined when water column concentrations of dissolved oxygen fall below 1 mg/L. Using
this criteria, the total volume of hypoxic water in the model
domain can be calculated at any given time. While the total
volumetric extent of hypoxia is of interest, in order to
synthesize and compare results from different model runs, it
is useful to define a fixed volume that represents the region
most susceptible to hypoxia in the bay, but that does not
change between different model runs. This fixed volume is
defined based on the location of the 1-mg/L oxygen contour
from the base model run (A1). This volume is largely
located below the pycnocline over regions where the water
column depth is greater than 10 m. In the absence of wind
forcing, unrealistically large volumes of the bay are
predicted to be hypoxic particularly in the lower bay, so
this volume is limited to the region north of 37.5° N
latitude. This fixed volume is then used for several
calculations including evaluation of the terms in the subpycnocline oxygen budget.
The equation for the conservation of oxygen can be
presented as:
@O2
@
@O2
Kz
þ r O2 ¼
R
@z
@t
@z
ð2Þ
where the first term on the left-hand side represents the time
rate of change of oxygen and the second term represents the
divergence in advective flux. The two terms on the righthand side represent the flux divergence due to vertical
mixing and loss by respiration, respectively. For the
purpose of this analysis, the advective fluxes are separated
into a longitudinal and lateral component. It should be
noted that this distinction is somewhat arbitrary for a
estuarine systems with complex bathymetry as the two
fluxes are strongly related via the continuity constraint. For
Estuaries and Coasts (2010) 33:1164–1175
1167
Fig. 1 Modeled average
dissolved oxygen concentration
(run C1). a Average bottom
oxygen concentration; b average
oxygen concentration along the
thalweg of the main stem of
Chesapeake Bay. Magenta lines
in a denote the locations of the
upper and lower Bay cross sections. Magenta star indicates
location of the Thomas Point
Light NDBC station
39.5o
Thomas Point Light
39o
Upper Bay
Cross-Section
38.5o
38o
Lower Bay
Cross-Section
37.5o
37o
o
76.6o
77
0
1
2
3
76.2o
4
5
mg/L
75.8o
6
7
8
0
-5
-10
-15
-20
-25
-30
o
39
simplicity, the longitudinal flux is calculated as the
divergence between the flux into and out of the two ends
of the volume. The remaining flux is attributed to lateral
processes and includes both lateral and vertical advection.
Each term is averaged spatially over the entire fixed subpycnocline volume defined above.
Results
Distribution of Dissolved Oxygen
Despite the simplified oxygen dynamics, the model
reproduces an overall distribution of dissolved oxygen
concentration that is consistent with typical summertime
observations. The model predicts hypoxic conditions below
the pycnocline for significant portions of the mid and upper
o
o
38.5
38
Degrees North
37.5
o
bay. The hypoxic zone is generally confined to the deep
central portion of the bay extending roughly from the
mouth of the York River in Virginia to the region parallel to
Baltimore, MD. For the period from 1950 through 2001,
Hagy et al. (2004) report that the volume of water in the
Bay with concentration less than 1 mg/L varied from 0.8 to
9.6 km3. In comparison, for the case with constant wind
speed of 4 m/s and a mean direction from the south (C1),
the model predicts 7.6 km3. While the overall distribution is
largely consistent with observations, it should be noted that
using this relatively simple approach does generally overpredict the southern extent of the hypoxic region.
Idealized 3-Day Wind Events
As described in “Methods” section, simulations of individual wind events were run for winds from four different
1168
Estuaries and Coasts (2010) 33:1164–1175
a) Modeled wind stress
0.08
Pascals
0.06
0.04
0.02
0
0
5
10
15
b) Modeled volume of hypoxic water (< 1 mg/L)
hypoxic volume (km3)
14
12
10
N
S
E
W
8
6
4
0
5
10
15
model day
Fig. 2 a Surface wind stress magnitude for idealized model runs; b
time series of predicted hypoxia (defined as the volume of water with
oxygen concentration less than 1 mg/L north of latitude 37° N) for
model runs with wind from the north (solid black line), south (solid
gray line), east (dashed black line), and west (dashed gray line)
directions: north, south, east, and west (model runs B1–
B4). The time series of wind stress magnitude is shown in
Fig. 2a. For all wind directions, there is a significant
reduction in the volume of hypoxic water during the wind
event, followed by a gradual return to pre-wind conditions
once the wind subsides (Fig. 2b). However, there are
marked differences between the different wind directions.
The wind event from the south has the greatest reduction in
the volume of hypoxic water followed by the slowest return
b) South Wind
a) North Wind
1.5
lat. adv.
mixing
0.5
0
-0.5
-1
1.5
long. adv.
respiration
1
gO2/m3/day
gO2/m3/day
1
0.5
0
-0.5
0
5
10
model day
-1
15
c) East Wind
1.5
1
1
0.5
0
5
10
model day
15
0.5
0
-0.5
-0.5
-1
0
d) West Wind
1.5
gO2/m3/day
gO2/m3/day
Fig. 3 Time series of terms in
the sub-pycnocline oxygen budget for the modeled a north, b
south, c east, and d west wind
events. Solid black line denotes
respiration term, solid gray line
denotes longitudinal advection,
dashed black line denotes vertical mixing, and dashed gray line
denotes lateral advection
to pre-wind conditions. In contrast, the rapid decrease in the
volume of hypoxic water associated with the northerly wind
is followed by the most rapid return. The smallest reduction
in the volume of hypoxic water is associated with the wind
event from the west. It is noteworthy that the minimum volume
of hypoxic water predicted during the south wind event
(5.5 km3) is almost a factor of two smaller than the minimum
volume predicted during the west wind event (9.3 km3).
Using the numerical model, the sub-pycnocline oxygen
budget can be evaluated for the idealized 3-day wind events
(Fig. 3). There are several notable results from this budget.
First, for all wind directions, the magnitude of the lateral
advective term is either equal to or greater than the flux due
to vertical mixing (Table 2). For the wind event from the
south, which has the greatest overall increase in subpycnocline oxygen content, the lateral advective flux
exceeds vertical mixing by over a factor of 4. The greatest
lateral advective flux is predicted in response to the southerly wind event. The west wind simulation has the smallest
increase in sub-pycnocline oxygen content and the smallest
flux associated with lateral advection. Despite the fact that
the wind from the south has the greatest reduction in the
volume of hypoxic water, it has the lowest overall flux due
to vertical mixing. In all cases, longitudinal advection of
oxygen into the hypoxic zone is minimal. Because the
model does not allow negative concentrations of oxygen,
the respiration rate must approach zero for anoxic conditions. As a result, the volume averaged respiration rate
goes down when a large percentage of the volume goes
anoxic, and differences in the calculated respiration rate
simply reflect differences in the overall extent of anoxic
0
5
10
model day
15
-1
0
5
10
model day
15
Estuaries and Coasts (2010) 33:1164–1175
1169
∂O2/∂t
∂/∂z Kz∂O2/∂z
v∂O2/∂y+w∂O2/∂z
u∂O2/∂x
Respiration
North
South
East
West
0.397
0.470
0.452
0.019
−0.543
0.447
0.181
0.800
−0.032
−0.502
0.285
0.244
0.505
−0.031
−0.433
0.154
0.283
0.253
0.022
−0.403
All values are reported as gO2/m3 /day. Values are averaged over a fixed
volume defined by the 1-mg/L contour from base model run (A1) without
wind forcing. Values are averaged over the duration of the 3-day wind
event
conditions between model runs. It is important to point out
that the vertical mixing term only accounts for a direct flux
of oxygen into the fixed volume. Mixing that occurs
elsewhere and is advected in appears as an advective flux
in this analysis. As will be discussed below, this is an
important mechanism for providing oxygen to the subpycnocline waters.
Model Runs with Constant Wind Speed
and Variable Direction
The results from model runs B1-B4 demonstrate that the
oxygen dynamics in Chesapeake Bay are sensitive to the
direction of the wind forcing. However, the response to
wind forcing presented above suggests that neither the
oxygen nor salinity dynamics are in steady state at
synoptic time scales. This is particularly apparent for
the southerly wind case where neither the oxygen nor the
stratification has returned to their original values by the
end of the model run. While significant insight can be
gained from the simple numerical simulations presented
above, the model runs with constant wind speed and
variable wind direction allow a more realistic examination of this non-steady-state behavior. Figure 4 shows
the predicted volume of hypoxic water for simulations C1,
C2, and C3. The model starts from the equilibrated run
without wind forcing (A1), which predicts large volumes of
water with oxygen concentration below 1 mg/L. With the
addition of wind forcing, the volume of hypoxic water is
reduced rapidly. While there are oscillations associated with
the shifting wind direction, the oxygen field becomes
approximately steady over longer time scales. The total
volume of hypoxic water predicted by the model is strongly
dependent upon the overall mean wind direction. The
greatest extent of hypoxia is predicted for the case with a
mean wind direction from the southwest (C3) while the
least hypoxia is predicted for the run with a mean wind
from the southeast (C2). In fact, the model predicts that a
shift in mean wind direction from southeast to southwest
results in a nearly 2.5-fold increase in the volume of
hypoxic water in the bay. While this non-steady response at
time scales less than several days makes evaluating the
importance of the instantaneous wind direction more
difficult, the overall dependence on the mean direction is
noteworthy.
Model Runs with Observed Wind Forcing (1996 and 1998)
The sensitivity to wind direction is highlighted further by
comparing the two years with very similar river discharge,
nutrient loading, and wind speed but with different wind
direction. The average Susquehanna River discharge between January and May was 2,355 and 2,363 m3/s for 1996
and 1998, respectively. Estimated nitrogen loading also was
very similar with average spring loading rates of 3.84×105
and 3.80×105 kg/day for 1996 and 1998, respectively
(Hagy et al. 2004). Despite these similarities, Hagy et al.
(2004) report that the estimated volume of water with
oxygen concentration less than 1 mg/L was over twice as
large in 1998 (9.61 km3) as in 1996 (4.54 km3). The early
summer wind magnitudes observed in these two years were
very similar. In contrast, there were significant differences
in the observed wind direction (Fig. 5). Based on the
Thomas Point Light data, the wind speeds averaged over
May and June are 4.9 and 4.8 m/s, for 1996 and 1998,
respectively. During 1996, the wind was directed primarily
from the south/southeast—the direction found to result in
the greatest wind-driven ventilation. In contrast, the
histogram from 1998 shows two distinct peaks, with a
significant duration of summer winds from the west/
northwest—the direction least conducive to bottom water
ventilation.
The time series of the predicted volume of hypoxic water
for the two summers is shown in Fig. 6. While there is
considerable variability in the time series for each year, the
18
hypoxic volume (km3)
Table 2 Oxygen budget for idealized wind runs
SW
S
SE
16
14
12
10
8
6
4
2
0
5
10
15 20 25
model day
30
35
40
Fig. 4 Time series of predicted hypoxic volume for model runs with
constant wind speed and oscillatory wind direction (runs C1, C2, and
C3). Wind direction oscillates through a 180° arc with a period of
6 days, centered on a mean direction from the southwest (dashed gray
line), south (solid black line), and southeast (dashed black line)
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Estuaries and Coasts (2010) 33:1164–1175
Fig. 5 Frequency histograms
for wind observed at the
Thomas Point Light NDBC
station (38°53′54″ N,
76°26′12″ W) for the period of
May through June. a Wind
speed 1996; b wind speed 1998;
c wind direction 1996; d wind
direction 1998
a) Wind Speed 1996
0.10
b) Wind Speed 1998
0.10
mean = 4.9 m/s
0.08
0.08
0.06
0.06
0.04
0.04
0.02
0.02
0
5
10
15
mean = 4.8 m/s
0
5
m/s
c) Wind Direction 1996
0.15
0.10
0.10
0.05
0.05
E
model predicts a greater average volume of hypoxic water
for the summer of 1998. Averaged over the month of June,
the predicted volumes of hypoxic water (<1 mg/L) were 5.2
and 9.4 km3 for 1996 and 1998, respectively. These values
are consistent with the observations reported by Hagy et al.
(2004) for these two summers and demonstrate that
relatively subtle changes in wind direction can have a
significant impact on the extent of hypoxia in Chesapeake
Bay. To confirm that these results are not actually the result
of slight differences in the wind magnitude, the summer of
1996 was rerun with a 10% reduction in wind speed (D3).
This reduced the mean wind speed for the summer of 1996
from 4.9 to 4.4 m/s, a reduction to roughly 2 standard
deviations below the long-term (1986–2007) May–June
mean at this location. Despite this reduction, the average
S
direction
15
d) Wind Direction 1998
0.15
N
10
m/s
W
N
N
E
S
direction
W
N
volume of hypoxic water predicted by the model (7.4 km3)
was still over 40% greater than predicted for 1998, when
averaged over the month of June.
The terms in the sub-pycnocline oxygen budget highlight the differences between these two years (Table 3). In
both 1996 and 1998, both lateral advection and vertical
mixing balance the consumption of oxygen through
respiration. Lateral advection exceeds vertical mixing by a
factor of 2 and 1.6 for 1996 and 1998, respectively. Thus,
the differences in hypoxia between the two years largely
reflect differences in the magnitude of the lateral advection
term and not differences in vertical mixing. In contrast to
the slight differences in vertical mixing between the two
years (0.204 vs. 0.228 gO2/m3/day), there is significantly
greater ventilation by lateral advection in 1996 (0.463 gO2/
14
hypoxic volume (km3)
12
1996
1998
Table 3 Oxygen budget for runs with observed wind forcing
10
1996
8
1998
6
∂O2/∂t
−0.025
0.001
4
∂/∂z Kz∂O2/∂z
v∂O2/∂y+w∂O2/∂z
u∂O2/∂x
Respiration
0.228
0.463
−0.005
−0.711
0.204
0.323
0.001
−0.527
2
0
May
Jun
model day
Jul
Fig. 6 Time series of predicted hypoxic volume for model runs forced
with observed winds from Thomas Point Light for the summer of
1996 (dashed black line) and 1998 (solid gray line)
All values are reported as gO2/m3 /day. Values are average over a fixed
volume defined by the 1-mg/L contour from base model run (A1) without
wind forcing. Values are average over the entire month of June
Estuaries and Coasts (2010) 33:1164–1175
1171
m3/day) as compared to 1998 (0.323 gO2/m3/day). Given
the consistency in the other forcing for these two runs, the
difference in lateral advection is attributed to differences in
wind direction.
Discussion
Longitudinal Straining
It is instructive to use the results from the numerical
simulations to examine the mechanisms that result in the
asymmetric response of the oxygen dynamics to changes in
wind direction. The traditional view of hypoxia in estuarine
waters is that vertical mixing across the pycnocline controls
the supply of oxygen to sub-pycnocline waters. It is often
assumed that the strength of vertical mixing is directly
related to the overall level of density stratification. Since
the interaction between the axial density gradient and
residual estuarine circulation is the mechanism by which
stratification is created, it follows that wind-driven modulation of the along-channel residual flow may play a key
role in modulating the extent of hypoxia. The model results
for the idealized 3-day wind events (B1–B4) show that the
wind has a clear impact on the strength of the subtidal
residual estuarine velocity (Fig. 7a). Winds from both the
north and west tend to increase the strength of the residual
velocity, while winds from the south and east reduce or
even reverse the near-bed residual flow. This result
demonstrates that the simple along-channel straining mechanism proposed by Scully et al. (2005) is modified by the
earth’s rotation in Chesapeake Bay.
For estuarine systems wider than the external Rossby
radius of deformation, the integrated wind-driven surface
transport is expected to be 90° to the right of the wind (in
the northern hemisphere). The width of the Chesapeake
Bay is smaller than the external Rossby radius of deformation but of the same order as the internal Rossby radius.
a) along-channel velocity
Given the importance of lateral advective processes to the
oxygen budget presented above, it is instructive to examine
what controls the strength of lateral exchange. The
strongest lateral flows are predicted for winds from
the south, where the surface transport is to the right of
c) across-channel velocity
2
2.5
6
1
2
4
cm/s
cm/s
Lateral Circulation
b) top-to-bottom salinity diff.
8
1.5
2
N
S
E
W
0
-2
-4
Therefore, the integrated wind-driven Ekman transport has
a component in both the along-wind and orthogonal
direction. As a result, both north and west winds induce
transport with a component in the down-estuary direction
near the surface. This down-estuary surface transport is
balanced with an increased up-estuary flow near the bed.
The opposite is observed for both the south and east wind
cases (Fig. 7a).
The straining of the density field caused by the windinduced changes in the along-channel residual circulation
plays a key role in the overall changes in stratification.
Winds from the north, south, and east all reduce the overall
degree of salinity stratification during the wind events
(Fig. 7b). In contrast, stratification is observed to increase
during the west wind event before eventually decreasing
slightly. These patterns of stratification are largely consistent
with the rotationally modified wind-driven along-channel
circulation. However, the differences between the north and
west wind events, both of which increase the along-channel
residual circulation, also demonstrate the importance of
lateral circulation in modulating stratification. Winds from
the north induce a strong lateral circulation, while winds
from the west have a much weaker lateral response (Fig. 7c).
The stronger lateral flow in response to winds from the north
significantly tilts the isopycnals toward a more vertical
position, thus reducing the overall degree of stratification.
The lateral tilting of the isopycnals is significantly reduced in
response to winds from the west, and the overall degree of
stratification is more strongly controlled by the alongchannel straining.
0
5
10
model day
1
15
0.5
0
-1
-2
0
5
10
model day
Fig. 7 Predicted a subtidal along-channel velocity, b top-to-bottom
salinity difference, and c subtidal lateral velocity for the four modeled
wind directions: north (solid black line), south (solid gray line), east
(dashed black line), and west (dashed gray line). Values were
15
-3
0
5
10
model day
15
averaged over the entire hypoxic layer in the main stem of Chesapeake
Bay and low-pass filtered. Salinity stratification is reported as the
difference between the hypoxic layer and the overlying water
1172
0
0.50
0.25
0.10
0.05 0.01
0
-5
depth (m)
the wind (toward the east), with a compensatory transport
in the lower hypoxic layer toward the west (negative in
Fig. 7c). The lateral response is weaker and in the opposite
direction (bottom flow to the east) for winds from the
north. For both east and west winds, the lateral flow in the
lower layer is smaller. The strength and direction of lateral
flow is coupled to the along-channel dynamics. Both north
and west winds increase the along-channel residual shear.
For an inviscid flow, this increase in along-channel shear
will act to increase the slope of the pycnocline via the
thermal wind balance (upwelling to the east and downwelling to the west). The tilting of the pycnocline is
accomplished by a lateral flow that is consistent with
Ekman dynamics. However, because the estuary is of finite
width, some component of the surface transport is in the
same direction as the wind stress. For the case of the west
wind, the lateral surface transport induces downwelling on
the east side and upwelling on the west, largely canceling
the thermal wind response. A similar cancelation results
from winds from the east. The result is much weaker lateral
flows for both east and west winds. The greater response
to north and south winds occurs because the transport
orthogonal to the wind stress induces a pattern of
upwelling and downwelling that reinforces the thermal
wind response.
The degree of salinity stratification also impacts the
strength of lateral circulation. The lateral flow tilts isopycnal surfaces, setting up an adverse pressure gradient that
tends to suppress lateral flow (Seim and Gregg 1997; Chant
and Wilson 1997; Lerczak and Geyer 2004). As a result,
stronger density stratification tends to result in weaker
lateral circulation. Because of this interaction, winds from
the south not only induce a lateral flow in the bottom water
that is opposite to the flow observed during north winds but
it is also stronger due to the overall lower levels of stratification. This suggests the somewhat surprising result that,
as long as some level of density stratification is maintained,
the strength of stratification plays a greater role in limiting
the lateral advective exchange of oxygen than in limiting the
direct turbulent flux of oxygen.
This interpretation is supported by calculations of the
gradient Richardson number (Ri). It is generally accepted
that turbulent mixing by shear instability cannot develop
when Ri >0.25 (Miles 1961; Howard 1961). Once the level
of stratification is sufficient for Ri to exceed this threshold,
it is expected that direct turbulent flux through the
pycnocline is effectively reduced to zero. As a result,
further increases in the strength of stratification cannot
decrease the turbulent flux through the pycnocline any
further. The model results suggest that, for typical summer
forcing conditions, significant portions of the estuarine
cross section maintain a value of Ri in the pycnocline that is
never reduced below its critical value (Fig. 8). In contrast,
Estuaries and Coasts (2010) 33:1164–1175
-10
-15
Percentange to time where Ri<0.25
2
4
6
8
10
width (km)
Fig. 8 Contours of the total percentage of time that the gradient
Richardson number (Ri) is below 0.25 for the model run C1
regions over the shallow shoal regions frequently experience conditions of Ri <0.25. Thus, oxygen is readily
mixing downward over the shoals, providing a source for
advective transport into the deeper hypoxic regions.
The Importance of Bathymetry
The lateral differences in the frequency of subcritical value
of Ri highlight the importance of lateral bathymetry to the
oxygen dynamics. The bathymetry in most coastal plain
estuaries is not symmetric about the channel. Typically, the
deepest portion of the channel is located on the right
(looking up-estuary) side of the estuary, with a broad shoal
region to the left. The mid and upper regions of Chesapeake
Bay are generally consistent with this morphology. In
contrast, in parts of the lower Bay, the deepest part of the
channel is located on the left (west)-hand side of the
channel with a more pronounced shoal region to the right
(east). The interactions between wind-driven lateral circulations and this bathymetry play a key role in the observed
asymmetric response of the oxygen dynamics to wind
direction. This can be demonstrated most clearly by looking
at the response of the oxygen field to winds from the north
and south at two locations, one in the upper Bay and one in
the lower Bay.
Figure 9 shows contours of dissolved oxygen at these
two locations averaged over both the north and south wind
events (runs B1 and B3, respectively). Also shown are the
differences in oxygen concentration for each simulation, at
both locations. The three upper panels (Fig. 9a–c) are from
a cross section in the upper bay near the mouth of the
Severn River (see Fig. 1). At this location, there is a broad
gently sloping shoal on the western side of the channel with
Estuaries and Coasts (2010) 33:1164–1175
b) North Wind
c) South minus North
0
0
-5
-5
-5
-10
depth (m)
0
depth (m)
depth (m)
a) South Wind
-10
-10
mg/L
-15
-15
Upper Bay
2
4
6
8
0
10
2
4
mg/L
6
-15
8
4
2
6
8
-5
-5
Lower Bay
0
5
10
width (km)
15
a comparatively narrow and steep shoal region on the
eastern side. During the wind event from the south, the
surface water is deflected toward the eastern side of the Bay
due to the Coriolis force. There is a compensatory flow in
the lower layer that advects hypoxic bottom water laterally
up onto the broad western shoal. On the eastern side of the
channel, oxygenated surface water downwells and is
advected west replacing the hypoxic water below the
pycnocline in the channel. Because of the steep lateral
bathymetry on the eastern side, the oxygenated surface
water only has to travel a relatively short distance to reach
the region in the deep channel where persistent hypoxia is
observed. In contrast, during winds from the north, the
surface water is deflected toward the west and the bottom
waters are advected to the east. The oxygenated surface
water that downwells along the western shore has a
considerably larger distance to travel to reach the hypoxic
zone in the channel.
The difference between the oxygen concentrations for
the north and south wind events is clearly dominated by the
lateral rotational response of the oxygen field. Because of
the bathymetry at the upper Bay location, there is a much
greater lateral flux of oxygen into the hypoxic region in
response to the wind from the south. This is in contrast to
the contours shown for the cross section in the lower bay
north of the mouth of the York River (Fig. 9d, e). While the
general response to the wind forcing is the same at this
location, the bathymetry here is quite different. The broad
shoal region is on the eastern side of the channel, with the
steeper bathymetry to the west. In contrast to the upper Bay,
Fig. 9f now shows that there is a much greater lateral flux
of oxygen into the hypoxic region in response to the wind
from the north.
depth (m)
-5
depth (m)
0
-20
-10
-15
mg/L
-20
0
0
5
2
4
4
6
8
10
f) South minus North
0
-15
0
2
0
-10
-2
10
e) North Wind
d) South Wind
depth (m)
Fig. 9 Contours of dissolved
oxygen concentration averaged
over 3-day wind event: a south
wind upper bay, b north wind
upper bay, c difference between
north and south winds in upper
bay (positive values indicate
greater oxygen concentration
resulting from south winds), d
south wind lower bay, e north
wind lower bay, and f difference
between north and south winds
in lower bay (positive values
indicate greater oxygen concentration resulting from south
winds). Solid black line denotes
the 1-mg/L oxygen contour that
serves as the vertical boundary
of the volume used for budget
calculations
1173
2
10
width (km)
4
-10
-15
mg/L
-20
6
15
-2
8
0
5
0
10
width (km)
2
4
15
It is important to note that, in the absence of turbulent
mixing, the lateral advective fluxes shown in Fig. 9 would
not result in a net flux of oxygen into the hypoxic region.
Without mixing, this hypoxic water would simply relax
laterally back into the channel as the wind subsides.
However, significant turbulent oxygen flux occurs when
the hypoxic water is laterally advected onto the shoals
(Fig. 10). Oxygen enters the water column through the air–
water interface. This surface flux of oxygen is directly
proportional to the concentration gradient between the
surface water and the atmosphere. When water with low
oxygen concentration is advected from the deeper channel
up onto the shoals, it brings water with low oxygen
concentration to the surface, enhancing the surface flux of
oxygen into the water column. When this water relaxes
back into the channel at the end of the wind event, the
enhanced surface flux results in a net influx of oxygen to
the hypoxic layer.
The lateral bathymetry in the mid to upper bay enhances
the vertical flux of oxygen in response to winds from the
south. In this region, southerly winds drive hypoxic water
from the channel onto the broader western shoal, bringing
water with lower oxygen concentration to the surface. This
enhances the surface oxygen flux over the western shoal
region. This broad shoal region also typically has large
areas where Ri <0.25, allowing significant vertical oxygen
flux to occur through turbulent mixing within the water
column (Fig. 10). This newly oxygenated water is then
advected back into the channel as the winds relaxes. Under
northerly winds, the lateral flow advects hypoxic water
from the channel onto the narrower eastern shoal. Here,
both the surface area of water with low oxygen concentration and total area where Ri <0.25 are reduced, resulting in
1174
b) North Wind--Upper Bay
a) South Wind--Upper Bay
0
0
-5
depth (m)
5
1
-10
-15
-5
1
-10
-15
2
4
6
0
8
2
10
4
6
8
10
g O2 m-2 day-1
13
2
4
6
12
14
16
0
9
depth (m)
depth (m)
-5
1
-10
-15
-20
Model Sensitivity
One of the conclusions from the analysis presented above is
that lateral advective processes play a dominant role in the
ventilation of hypoxic regions in Chesapeake Bay. However, the relative importance of lateral advection to direct
vertical mixing is dependent upon the choice of background
diffusivity. Li et al. (2005) showed that model predictions
of the strength of the residual circulation, stratification, and
9
5
1
-10
-15
-20
5
less vertical oxygen flux. The net result is significantly
greater oxygen flux in response to south winds in the upper
and mid bay because of the interactions between the lateral
flow and the bathymetry. The opposite appears to be true in
the lower bay, where the larger shoal is located on the
eastern side of the channel. Here, winds from the north
drive water with low concentrations of oxygen onto the
broader eastern shoal, where significant oxygen flux can
occur. The effectiveness of winds from the south is limited
in the lower bay because of the relatively smaller amount of
shoal region located to the west. The total extent of hypoxia
in the bay appears to be reduced the most by southerly
winds because hypoxia is generally more severe in the mid
to upper portions of Chesapeake Bay, where the large shoal
areas are found to the west of the channel. However, these
results suggest that southerly winds are less effective at
ventilating regions of lower Chesapeake Bay.
10
13
5
-5
8
d) North Wind--Lower Bay
c) South Wind--Lower Bay
0
13
9
5
9
depth (m)
Fig. 10 Cross-sectionally
averaged contours of vertical
turbulent oxygen flux for a
south winds upper bay, b north
winds upper bay, c south winds
lower bay, and d north winds
lower bay. Contour interval is
2 gO2m−2 day−1. Solid black line
denotes the 1-mg/L oxygen
contour that serves as the vertical boundary of the volume used
in budget calculations. Locations for cross sections are
shown in Fig. 1
Estuaries and Coasts (2010) 33:1164–1175
10
width (km)
15
5
10
width (km)
15
salt content of Chesapeake Bay were all sensitive to the
choice of background diffusivity. In their work, a value of
10−5 m2/s was found to match observations best. This is the
value used in this study as well, but it is important to note
that both the total extent of hypoxia predicted and the
relative importance of vertical mixing are sensitive to the
value that is used. To test the sensitivity of the model to
the value of background diffusivity, the model was run
with the value set to 10−4 and 10−6 m2/s. Increasing the
background diffusivity decreased the predicted extent of
hypoxia, while reducing this value increased hypoxia. Even
though the overall extent of hypoxia was sensitive to the
value of background diffusivity, the asymmetric response to
wind direction was robust. Regardless of the value of
background diffusivity, southerly winds always resulted in
the smallest amounts of hypoxia and westerly winds always
resulted in the most. Further, the interactions between the
lateral flow, bathymetry, and vertical mixing described
above remained an important mechanism for supplying
oxygen to the hypoxic region. Similarly, varying the value
for the respiration rate and piston velocity changed the
extent of hypoxia predicted by the model. However, it is
important to stress that the conclusions of this paper are
generally not impacted when the background diffusivity is
changed by 2 orders of magnitude, or values for the
respiration rate and piston velocity are varied within the
range of values reported in the literature.
Estuaries and Coasts (2010) 33:1164–1175
Biological Feedbacks
In order to isolate the physical processes that modulate
hypoxia, an extremely simplified oxygen model was used
in this study. This model is clearly a gross simplification of
the biological processes that impact oxygen concentration
in the bay. Biological processes in Chesapeake Bay may be
strongly influenced by the estuarine dynamics described in
this study. For instance, Malone et al. (1986) suggest that
wind-driven lateral circulations may play an important role
in supplying nutrients from the sub-pycnocline waters to
the surface layer. As a result, strong lateral exchange may
enhance surface productivity by phytoplankton. Clearly,
these biological feedbacks could significantly impact the
oxygen dynamics described above. Capturing such complex interactions is extremely challenging in either an
observational or modeling study. Such interactions were not
captured by this study and are clearly an area that requires
further study.
Conclusions
A series of numerical experiments were conducted to
examine the dependence of the oxygen dynamics in
Chesapeake Bay on wind direction. Model results demonstrate that the greatest ventilation of hypoxic bottom waters
occurs in response to winds from the south with the least
ventilation in response to winds from the west. Even slight
changes in the mean wind direction appear to have
significant consequences for oxygen dynamics. A simple
numerical experiment demonstrates that a shift in mean
wind direction from the southeast to southwest can result in
a nearly 2.5-fold increase in the volume of hypoxic water in
the bay. The sensitivity to wind direction is largely driven
by the interactions between lateral wind-driven circulation,
bathymetry, and vertical mixing. Large parts of the upper
and mid Chesapeake Bay have an asymmetric distribution
of lateral bathymetry with broad shoal regions to the west.
During wind events from the south, bottom hypoxic water is
laterally advected onto the broad western shoal where significant vertical oxygen flux occurs. This oxygenated water
relaxes back into the channel as the wind subsides resulting
in a net flux of oxygen. Winds from the west have limited
lateral exchange, which prevents significant ventilation.
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