The persistent winds (Fig. 2) out of the north during winter of 1994-95 resulted in one of the most extensive ice years in decades. Although the horizontal extent of the ice cover was very large, the ice was thin due to its sudden formation and rapid advection southward. The rapid disappearance of the ice in April supports the inference that, although extensive, the ice was not particularly thick. The presence of first-year ice does not prevent transferral of mixing energy to the water column. Simulations from a model (Overland et al., 1984) designed to investigate the relationships among wind, ice motion, and mixed layer development showed that sufficient turbulence was generated by wind-driven ice to mix the upper 50 m of the water column. This, coupled with tidal mixing, could thus mix the entire water column over the middle shelf.
b. Shipboard measurements
In mid-March, the density structure at mooring 2 consisted of a well-mixed layer capped by a shallow lens of less saline water (Fig. 3a). By late April, the water column exhibited its characteristic structure of two layers separated by a sharp pycnocline at ~37 m. Concentrations of chlorophyll-a were low in the late winter. By spring the concentration and distribution of chlorophyll-a were similar to previous observations (Sambrotto et al., 1986; Niebauer et al., 1995), with high concentrations of chlorophyll-a in the upper layer and relatively low, but non-zero, concentrations in the lower layer.
Figure 3. At mooring site 2, (a) concentration of chlorophyll-a as measured from CTD casts taken on deployment (circle) and recovery (triangle) and (b) concentration of dissolved nitrate upon deployment (circle) and recovery (triangle). The density structure upon deployment (dashed) and recovery (solid) is also shown in each panel.
The concentration of dissolved nitrate was the inverse of the chlorophyll-a distribution (Fig. 3b). In March, a high concentration of nitrate occurred from the surface to the bottom of the water column. By April 30, nutrients had been largely depleted from the upper 30 m (the upper mixed layer), but remained high at 60 and 76 m. A week later, nitrate was undetectable in the upper 30 m of the water column.
At mooring site 1 concentrations of dissolved nitrate were high throughout the water column on March 14 and April 23 (Fig. 4). By May 5 a slight draw-down of nutrients had occurred in the upper 20 m. Seven days later on May 12 concentrations of nitrate had been significantly reduced throughout the water column and depleted at the surface. Concentrations of chlorophyll were low or undetectable until May 12.
Figure 4. At mooring site 1, (a) concentration of chlorophyll-a as measured from CTD casts on March 14 (open circle), April 23 (closed circle), May 5 (open triangle), and May 17 (closed triangle); (b) concentration of dissolved nitrate for same days as in (a).
The structure and properties of the water around mooring 3 differed markedly from those around moorings 1 and 2 (Fig. 4a). On deployment there was a deep (60 m) mixed layer. In spring the remnant of this mixed layer was still evident, with a shallow active mixed layer in the upper ~15 m. In the winter the single chlorophyll measurement at 7 m was low, and in spring a high concentration of chlorophyll occurred uniformly in the upper 45 m. Although maximum concentration was less than measured at mooring 2, concentrations of >20 mg m-3 of chlorophyll occurred deeper in the water column, an effect of the mixed layers at mooring 3 being deeper than at mooring 2.
In March at mooring 3, dissolved inorganic nitrate concentration was uniform in the upper mixed layer (Fig. 4b) and slightly higher at 76 m. In early May, when mooring 3 was recovered, concentration had decreased in the upper layer, but remained high below the pycnocline at 81 m. Unlike at mooring 2, the ice edge bloom did not completely exhaust the nutrients in the upper layer on the outer shelf.
From the chlorophyll-a observations it is evident that a phytoplankton bloom occurred at sites 2 and 3 during April, while at site 1 the bloom began much later in mid May. The time-series observations at the moorings permit us to examine the evolution of the early under-ice phytoplankton bloom.
c. Middle Shelf: Mooring 2
Temperature was measured at eight depths on the mooring and at the nearby TRAP
(Table 1; Fig. 5a).
Initially, wind and tidal mixing resulted in uniform temperature from top to
bottom (~0.5°C). As the ice was blown southward shortly after deployment
and melted by the warmer water, the water temperature decreased. On March 27,
at its coldest, the water columnisothermal at <-1.5°C. The melting ice
decreased initial salinity (~32.25 psu) by ~0.5 psu at 44 m (Fig.
5b). Although surface salinity likely began decreasing with the arrival
and melting of sea ice, the decrease in salinity at this depth resulted from
deepening of the mixed layer, which occurred several days after the arrival
of ice. The shallow (<5 m) surface layer evident in the CTD data taken during
deployment has been mixed away. At these low temperatures the thermal expansion
coefficient of water is small and any change in density is controlled by changes
in salinity. Thus, the uniformity in temperature was not due to convective mixing.
The energy required for mixing the water column resulted from the four days
(beginning March 23) of strong winds (>10.0 m s
).
Figure 5. As in Figure 3, except for mooring site 3.
To account for the change in temperature, we use a simple box model (Azumaya
and Ohtani, 1995). The change in temperature (
T)
produced by the latent heat from melting ice can be calculated as
| T
= (Sc-Si)/(Sc-Sb) (Lh/C) |
(1) |
where Sc is salinity of the water beneath the ice, Sb is the
salinity of the ice, and Si is salinity of water after ice melt. Lh
is the latent heat (3.4 × 10
J kg)
and C the specific heat of the 76-m water column (4.2 × 10
J kg °C).
The salinity measured at the moorings was Sc = 32.25 psu before mixing
occurred and Si = 31.8 psu afterward. Using Sb = 10.0 psu, since
this is new ice, yields T = 1.6°C,
which is consistent with the observed change of temperature form 0.0°C
to -1.7°C. If Sb = 5 psu, a typical salinity of old ice (Azumaya
and Ohtani, 1995), then T = 1.3°C.
From March 30 to April 1, strong currents (~15 cm s)
from the southwest resulted in the temperature below ~25 m rapidly increasing
by ~1.5°C. At this location the two-layer structure was not initiated by
freshwater resulting from ice melt, but rather by advection of warmer, more
saline water into the lower layer. For the remaining 30 days of this deployment
the temperature in the bottom layer (the cold pool) remained relatively constant.
The upper layer, however, underwent significant warming starting on April 15,
when winds out of the south caused the ice to quickly disappear via melting
and/or advection. The warming of the upper layer enhanced the two-layer structure.
The low-frequency currents were not uniform with depth (Fig.
6a). The net speed ranged from 3.7 cm s
near the surface to 1.0 cm s at the bottom.
Also evident were pulses of currents >10 cm s
lasting 2-3 days, which are common over the middle shelf (Schumacher
and Kinder, 1983).
Figure 6. (a) Temperature measured at mooring 2. The vertical location of each instrument is indicated by an arrow along the depth axis. Plotted over the contours of temperature at the appropriate depths are 6 hourly low-pass filtered current vectors (upward indicates northward flow). (b) Chlorophyll-a as determined by an A-3 (7 m) and as inferred from a fluorometer (44 m). The estimated period when ice covered the mooring is indicated by the blue line. Salinity at 44 m (the shallowest SEACAT that gave data) is shown in red.
Vertical shear was calculated using velocity from the 4-m bins from the ADCP (Fig. 7). The mixed layer depth was defined as the depth at which the temperature differed by 0.1°C from that measured at 1 m. Because of the sparseness of temperature sensors, the depth is only an estimate. There is a close relationship between mixed-layer depth and shear, with maximum shear usually occurring below the mixed layer. There were two periods of large shear. The first coincided with the storm which forced ice southward over the mooring, indicating active mixing during ~5 days. Enhanced shear is evident to bottom of the water column. The second and largest shear occurred after the establishment of a two-layered system. (Note that the upper layer is not always uniform, but at times is weakly stratified.) From April 8-25, the mixed layer did not deepen below 20 m, even with the presence of strong shear beneath the mixed layer. The storm of April 24-25 deepened the mixed layer by ~5 m. It is not known whether this change in depth is also related to the removal of the ice, which occurred a few days earlier.
Figure 7. (a) Wind speed derived from atmospheric pressure. (b) Daily averaged
shear (s) determined from hourly ADCP
records at mooring 2. Velocity was measured in 4-m bins. The 2-hourly depth
of the mixed layer (defined by differing by 0.1° from the temperature at
1 m) at 6-hr intervals is indicated by +.
Four days after the arrival of ice and ensuing melting, the chlorophyll-a
concentration at 7 m began increasing (Fig. 6b).
It continued a steady increase of ~0.6 mg m
d until the ice began to retreat, after
which the concentration remained relatively constant. The sharp decrease in
chlorophyll-a on April 15 coincided with a storm which increased the
shear beneath ~30 m. The deepening of the upper layer on April 25 also resulted
in a slight decrease in chlorophyll-a. The concentration of chlorophyll
(determined by fluorometry) at 44 m remained low except when the water column
actively mixed to below 60 m. The salinity, temperature, and shear all indicate
that an active mixing occurred to below 50 m during ~5 days in late March. The
chlorophyll at 44 m increased to that observed at 7 m as a result of the mixing
of the water column. It is interesting that the chlorophyll at 7 m continued
to increase during this time.
The high-frequency variability evident in both the temperature and chlorophyll-a
series is tidal in nature (M
and
K
are the dominant tidal constituents
at this site) and provide an indication of the horizontal variability in temperature,
salinity, and chlorophyll-a in the vicinity of mooring 2. The tidal ellipse
at mooring 2 was ~5 km in diameter, indicating that the horizontal gradients
in the immediate vicinity of the mooring were weak.
d. Outer shelf: Mooring 3
The temperature structure at mooring 3 (Fig. 8a),
which was characteristic of the outer shelf, differed markedly from that at
mooring 2 (Fig. 6a). A surface mixed layer (60
m) existed over a stratified lower layer. The lowest temperatures occurred with
the onset of ice, but were evident only in the upper 40 m. Once again the changes
in salinity determined the changes in density. If the newly melted ice is mixed
over 45 m, Sc = 31.9 psu, Si = 31.55 psu, and applying Eq. 1 with
Sb = 10 psu (5 psu) yields a temperature loss of T
= 2.2°C (1.8°C). This compares well with the observed change of temperature
from 0.5°C to -1.5°C. Currents at mooring 3 were stronger than those
observed at mooring 2, where the mean flow was very weak. These stronger currents
would result in greater advection of water, thus making 1-dimensional models
less reliable.
Figure 8. (a) Temperature measured at mooring 3. The vertical locations of the measured temperatures are indicated by arrows. Plotted over the contours of temperature at the appropriate depths are 6 hourly low pass filtered current vectors. (b) chlorophyll-a as measured by an A-3 (7 m) is shown in green. The period when ice covered the mooring is indicated by the blue line. Salinity at 10 m is shown in red.
As at mooring 2, the chlorophyll-a concentration increased at site 3 upon advent of the ice and accompanying ice melt (Fig. 8b). The maximum concentration, however, was less than observed at mooring 2, and there was also more variability in chlorophyll-a. The sharp decreases which occurred at mooring 3 on April 15 and 24-25 also occurred at mooring 2, likely a result of atmospheric forcing which is similar at both stations. The tides once again caused the diurnal variability evident in the time series.
The chlorophyll estimates from water samples were approximately twice those estimated from the moored absorbance meter at mooring 3. A smaller discrepancy occurred at mooring 2. These discrepancies may have resulted from horizontal variability, but more likely resulted from use of an average chlorophyll-specific absorbance coefficient obtained from literature rather than specifically tuned for the types and physiological state of the phytoplankton present at the moorings (Bricaud and Stramski, 1990).
e. Mixed layer depth
Time series of chlorophyll-a and mixed layer depth at each mooring (Fig. 9) showed that chlorophyll-a concentrations increased during periods of stability. These mixed-layer depths were defined using temperature data. Mixed-layer depths determined from the salinity data at moorings 1 and 3 are consistent with depths determined from temperature. It is interesting that at mooring 1 the stable water column in March did not support a bloom, even though it did at mooring 2.
Figure 9. The 2-hourly depth of the mixed layer and hourly concentration of chlorophyll-a estimated from A-3 for (a) mooring 1, (b) mooring 2, and (c) mooring 3.
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