Subsurface tidal currents in Glacier Bay, Alaska

Byron Kilbourne

School of Oceanography
University of Washington
Seattle, Washington 98195-7940

Abstract

Subsurface currents are not easily measured, but it is these currents that make up the bulk of physical oceanographic phenomena. In Glacier Bay, Alaska mixing of ocean water into the bay via energetic tidal currents is the fundamental force driving the physical and ecological dynamics of the bay. Strong tides bring in dense and nutrient rich deepwater. This fertilizes the ecosystem, and forces the less dense fresh water surface layer out to sea. As these strong tides cross the many shallow sills in Glacier Bay, internal waves are created and result in turbulence, thoroughly mixing the water column. Specific details regarding these tidal currents, such as the source of the deepwater and the average current through the mouth of the bay, are still unknown. The focus of this project is to measure subsurface currents and corresponding water properties. This data will be used to determine tidal currents, mean currents, and transport through the mouth of the Glacier Bay.

Research

Subsurface currents are difficult to measure. Surface currents can be made from any vessel; a current profiler or moored current gauge is required to measure subsurface currents. Due to this difficulty, the local source of seawater entering the bay is unknown. One important question is whether Cross Sound, to the southwest, or Chatam Strait, to the southeast, is the primary avenue for transport of ocean deepwater into the bay. As strong tides move water across the entrance sill, the water becomes well mixed. This mixing homogenizes the water column, making it difficult to pinpoint specific tracer signals from a particular region. In addition, very little subsurface current information is available. As a result the net transport of deepwater into the Glacier Bay region is unknown.

Glacier Bay is well known for its high biological productivity; this ecosystem relies on both terrestrial and marine nutrient sources. Understanding how currents flow into and out of the bay will help determine which nutrients are provided from the ocean, how much oxygen is transported with the deepwater, and the role of tidal energy in the ecosystem through mixing across sills (Cokelet and Etherington 2007). This project will measure subsurface currents and corresponding water properties. This data will be used to determine tidal currents, mean currents, and transport through the mouth of the bay. Surface currents are quite strong in Glacier Bay; however, it is unknown to what extent these currents represent the flow at depth. Over the long entrance sill, CTD casts confirm that the water mass is homogeneous (Matthews 1981). Theoretical prediction states that water rich in oxygen and nutrients, originating from the Pacific Ocean, is mixed into the bay across the entrance sill. This mixing makes the water mass over the sill denser than the surface water within the bay. Northward of the entrance sill, the water column is more highly stratified. At flood tide the denser water from the sill flows northward underneath the less dense surface layer into the bay’s deep interior basins (Matthews 1981). This restoration of nutrients by ocean deepwater contributes to the productive benthic ecosystem of Glacier Bay.

Net current is understood to be a combination of constituent tidal and other currents. This research will result in an estimate of volume transport in m³s^-1 through the mouth of the bay. Tidal currents will be modeled as a time dependent sinusoidal wave, and this signal will be filtered from the data to resolve the mean current. The results from the one tidal cycle sampled will be considered typical for the late winter period. Predictive estimates of annual currents will be made based on the seasonal flow dynamics predicted by Matthews (1981). Volume transport will be combined with CTD data to calculate the transport of nutrients and oxygen by one tidal cycle into the bay. Annual volume transport estimates will be used in to make predictive estimates for oxygen and nutrient transport into the bay.

Equipment

The R/V Thompon’s ship mounted acoustic Doppler current profiler (ADCP) will be used to measure currents. The R/V Thompson uses a Teledyne RD Instruments Ocean Surveyor 75KHz ADCP mounted in the keel of the ship, which is 6m below the water surface. This ADCP model has a measurement range of 10m to 1000m, meaning that measurements within 10m of the ADCP cannot be resolved. The combination of these factors limits the upper current measurement to a depth of 16m below the surface. ADCPs measurements are limited at bottom boundaries by a 6% of water depth plus one depth bin (4m to 16m). The bottom boundary data gap for a 60m (the approximate depth for this study) deep area would be 7.6m. This limits the ADCP’s ability to measure currents in waters less than 24m deep. The data is grouped into data bins, 4m to 16m depth averaged cells. The ADCP can record up to 128 cells, so it will be possible to maintain 4m resolution throughout Glacier Bay. Using larger cells, the ADCP can “see” currents to 1000m. The R/V Thompson uses University of Hawaii data acquisition system (UHDAS) to interface with the ADCP. This is the R/V Thompson’s default software and operates within the UNIX operating system.

The R/V Thompson uses a Sea Bird SBE-911+ CTD package. The CTD measures conductivity, in situ temperature, and pressure along a vertical profile. Salinity is calculated from the conductivity measurement, temperature and pressure are measured directly. The CTD is equipped with the full range of optional sensors to measure dissolved oxygen, pH, fluorescence, light (PAR), and turbidity. The CTD will be used to collect this data at three stations near the mouth of the bay over one complete tidal period, as well as all other stations planned for fellow researchers.

Tidal and seasonal phase considerations

Measurements of tidal currents during this cruise are a snapshot in time of the larger seasonal dynamics of the system. The flow patterns at Glacier Bay are strongly influenced by temperature, and thus are annually cyclic. In the winter the air temperature is low, which lowers the water surface temperature. Freshwater sources freeze and precipitation falls as snow. The decrease in freshwater input and lower temperatures reduce density stability in the water column. This lower stability allows greater deepwater flow over the sill. Wind driven mixing is enhanced by this as well, mixing nutrients into the surface layer in the deep basins of the bay. In the spring freshwater runoff increases and surface temperature rises, increasing stratification. This trend continues into the late summer, when stratification is highest. Thus in the summer both tidal and wind mixing are inhibited. As expected, cooler fall temperatures weaken stratification and mixing increases (Matthews 1981). This cruise will likely sample winter cycle dynamics, thus the current measurements made can only be considered indicative of the coolest period of the annual cycle.

In addition to seasonal cycles, the tidal phase at which each measurement is made must be considered. As mentioned earlier, the mixed water mass over the entrance sill moves with the tidal phase. For the purposes of this cruise, this means that CTD casts for a given station could yield highly variable results depending on the tidal phase at which they are made, particularly stations which are near a sill. Estimates of mean values for each station can be made be examining the data with respect to the tidal phase in which it was measured. Tides are predicted with good accuracy, so this will help eliminate bias in estimating average water column properties based on CTD casts made on this cruise.

Ship Time

To measure currents at the mouth of the bay as accurately as possible, it is essential to make several measurements of the current at a specified location over at least one full tidal period. One tidal period in Glacier Bay lasts approximately thirteen hours. To satisfy this requirement I propose a circuit of three stations near the mouth of Glacier Bay on the first full day the ship is on site. On March 19 sunrise and sunset, 0606 and 1817 respectively, fall very close to the predicted low tides at the mouth. If this schedule is not practical, a circuit of these five stations can be conducted over any tidal period. Included is a figure (fig.1) illustrating this circuit. A proposed schedule is attached in table 1. In addition to this circuit, the ship will conduct a transect of Glacier Bay from the mouth to Tarr Inlet to satisfy the requirements of fellow researchers. During this transect, current data will be collected to produce a current timeseries similar to that by Cokelet (2007).

Analysis

Following the cruise, data from ADCP and CTD casts will be compiled into one dataset. A current vector timeseries will be constructed. This timeseries will show a typical profile of subsurface currents for early spring conditions at Glacier Bay. Additionally, current measurements will be used to determine a mean subsurface inflow and outflow at the mouth. These flows will be used to calculate the volume transport, which will be used in conjunction with CTD data to estimate the transport of oxygen, nutrients, and salinity into and out of the bay. The ADCP data will be used to determine the direction of inflow at the eastern and western edges of the mouth. This will show where the influx of deepwater is strongest, and provide evidence to determine the source of this deepwater.

Budget

The budget for this cruise is represented by two sections. There is a $600 allotment for each student’s project provided by the School of Oceanography. This is the operational budget for consumables and equipment not owned by the R/V Thompson. The large cost of the ship time and scientific equipment will be covered by the University of Washington, and is represented in the budget as the proposed cost. A summary of these costs is shown in table 2.

References

Cokelet, E., Jenkins, A., Etherington, L. 2007. A Transect of Glacier Bay ocean currents measured by acoustic Doppler current profiler (ADCP), in Piatt, J.F., and Gender, S.M.,eds, Proceedings of the Fourth Glacier Bay Science Symposium, October 26-28, 2004: U.S. Geological Survey Scientific Investigations Report 2007-5047, p 80-83

Etherington, L., Hooge, P., Hooge, E., Hill, D. 2007. Oceanography of Glacier Bay Alaska: Implications for Biological Patterns in a Glacial Fjord Estuary. Estuaries and Coasts. 30: 927-944.

Matthews, J. 1981. The Seasonal Circulation of the Glacier Bay, Alaska Fjord System. Estuarine, Coastal, and Shelf Science. 12: 679-700

Table 1: Coordinates and time schedule for current measurements.

Station	Latitude	Longitude	Equipment	Time	Depth (m)	Comments
C1	58° 22’53.74”N	136° 1’36.06”W	ADCP, CTD	606	60.39	Sunrise, Low tide @ 0626
C2	58° 22’15.72”N	135° 58’30.86”W	ADCP, CTD	636	69.54
C3	58° 23’1.27”N	135° 55’49.32”W	ADCP, CTD	706	58.56
C2	58° 22’15.72”N	135° 58’30.86”W	ADCP, CTD	736	69.54
C1	58° 22’53.74”N	136° 1’36.06”W	ADCP, CTD	806	60.39
C2	58° 22’15.72”N	135° 58’30.86”W	ADCP, CTD	836	69.54
C3	58° 23’1.27”N	135° 55’49.32”W	ADCP, CTD	906	58.56
C2	58° 22’15.72”N	135° 58’30.86”W	ADCP, CTD	936	69.54
C1	58° 22’53.74”N	136° 1’36.06”W	ADCP, CTD	1006	60.39
C2	58° 22’15.72”N	135° 58’30.86”W	ADCP, CTD	1036	69.54
C3	58° 23’1.27”N	135° 55’49.32”W	ADCP, CTD	1106	58.56
C2	58° 22’15.72”N	135° 58’30.86”W	ADCP, CTD	1136	69.54
C1	58° 22’53.74”N	136° 1’36.06”W	ADCP, CTD	1206	60.39	High Tide @1222
C2	58° 22’15.72”N	135° 58’30.86”W	ADCP, CTD	1236	69.54
C3	58° 23’1.27”N	135° 55’49.32”W	ADCP, CTD	1306	58.56
C2	58° 22’15.72”N	135° 58’30.86”W	ADCP, CTD	1336	69.54
C1	58° 22’53.74”N	136° 1’36.06”W	ADCP, CTD	1406	60.39
C2	58° 22’15.72”N	135° 58’30.86”W	ADCP, CTD	1436	69.54
C3	58° 23’1.27”N	135° 55’49.32”W	ADCP, CTD	1506	58.56
C2	58° 22’15.72”N	135° 58’30.86”W	ADCP, CTD	1536	69.54
C1	58° 22’53.74”N	136° 1’36.06”W	ADCP, CTD	1606	60.39
C2	58° 22’15.72”N	135° 58’30.86”W	ADCP, CTD	1636	69.54
C3	58° 23’1.27”N	135° 55’49.32”W	ADCP, CTD	1706	58.56
C2	58° 22’15.72”N	135° 58’30.86”W	ADCP, CTD	1736	69.54
C1	58° 22’53.74”N	136° 1’36.06”W	ADCP, CTD	1806	60.39	Sunset @ 1817
C2	58° 22’15.72”N	135° 58’30.86”W	ADCP, CTD	1836	69.54	Low Tide @1850
C3	58° 23’1.27”N	135° 55’49.32”W	ADCP, CTD	1906	58.56

Table 2: Cruise Budget

Individual $600 Budget
Consumables for CTD calibration	$0
Subtotal (actual request)	$0
Estimate of provided Services
12 Days Aboard the R/V Thomas G. Thompson	$264000.00
Operational Budget	$264100.00
Overhead (56% of Total Budget)	$336127.27
Total (Proposed Request)