Resolving a persistent offshore surface temperature maximum in Lake Superior using an autonomous underwater glider Jay Austin Large Lakes Observatory, University of Minnesota-Duluth, Duluth, Minnesota 55812, USA jaustin@d.umn.edu In November 2009, an autonomous underwater glider (AUG) was deployed for a period of 12 days on the Wisconsin Shelf of Lake Superior. During this period, the AUG made repeated cross-shelf transects from 3 km to 13 km offshore, making 26 cross-shelf transects in all, during which time temperature was measured. Each of these transects displayed a mid-shelf temperature maximum roughly 8 km offshore, with cooler waters both inshore and offshore of this. This is hypothesized to be due to a balance of persistent cooling at the surface and vertical mixing of cooler sub-thermocline waters offshore. Keywords: physical limnology, autonomous glider, thermal structure, shelf dynamics, shelf cooling Introduction Thermal structure in lakes plays a primary role in determining transport mechanisms, especially in the coastal zone. Two familiar examples of this are coastal upwelling and downwelling (phenomena better documented on oceanic shelves) and the formation and migration of the thermal bar. In both of these cases, cross-shelf transport of nutrients, plankton, and pollutants are heavily influenced by stratification or the lack thereof. These two examples have roughly opposite effects on cross-shelf transport; upwelling and downwelling facilitates depth-dependent wind-driven cross-shelf pathways (Lentz, 1992; Smith, 1995), whereas the thermal bar is thought to impede cross-shelf transport (Auer and Gatzke, 2004), trapping constituents in the nearshore. Another example of thermal structure significantly influencing cross-shelf transport is surface heat loss, as would be expected to occur in the winter. While this phenomenon has been looked at for oceanic shelves (Pringle, 2001) as a potential driver of cross-shelf transport, as denser water is formed inshore, it has not been seriously considered in freshwater systems. Part of the reason that little work has been done in this area is that there are few if any well-resolved observations upon which to build theory. This is due in part to the fact that intense cooling often occurs during energetic weather conditions (i.e. high winds), when it is difficult to work safely. Developments in technology in the past decade have presented us with a new platform for sampling that bridges the two dominant approaches to making the necessary observations: CTD transects, which are characterized by high vertical resolution, medium to poor lateral resolution and poor temporal resolution, and moored observations, with low vertical resolution, low lateral resolution, and high temporal resolution. Specifically, the advent of the autonomous underwater glider (hereafter AUG; Rudnick et al., 2004) provides a platform which will allow a compromise between temporal and spatial coverage. While these vehicles have been 316 Aquatic Ecosystem Health & Management, 15(3):316 321, 2012. Copyright C 2012 AEHMS. ISSN: 1463-4988 print / 1539-4077 online DOI: 10.1080/14634988.2012.711212
Austin / Aquatic Ecosystem Health and Management 15 (2012) 316 321 317 increasingly common in the oceans for several years now (Glenn et al., 2008; Perry et al., 2008; Hodges et al., 2009), the glider discussed in this manuscript is the first known to be used in a large lake. Gliders move in a zigzag pattern from near the surface to near the bottom, collecting data continuously, and in the case of the Webb Electric glider discussed here, can stay deployed for over a month at a time on a single set of batteries. This extended deployment time sets gliders apart from actively propelled autonomous underwater vehicles (AUVs), which typically have an endurance of 8 24 h. This provides a dataset which has the vertical resolution of a CTD cast, the lateral resolution of a very finely spaced CTD survey, and can provide repeated measurements of the same location, so that the temporal development of the system can be observed for an extended amount of time. AUGs are also capable of operating in all weather conditions, and therefore their data do not suffer from the fair-weather bias that shipboard data invariably contains. This article will discuss data collected by the University of Minnesota, Duluth s Webb Electric Glider during November of 2009. The original purpose of this deployment was to observe the evolution of thermal structure on a sloped shelf during a period of rapid cooling. We document the existence of a persistent feature, a mid-shelf temperature maximum, the persistence of which suggests that cross-shelf transport in this period is weak, and water constituents are likely to be trapped in the nearshore during this period of the year. By extension, much of the same behavior might be expected during periods of warming in the spring when water temperatures are below the temperature of maximum density, roughly 4 C. Methods The data presented here were collected with a Webb Research Electric Glider. The glider is approximately 150 cm in length and has a mass of 52 kg in air. This AUG propels itself through a combination of small changes in its effective density (by changing its displacement volume) and through small changes in the position of the center of gravity relative to the center of buoyancy, changing the dive/climb angle of the vehicle. This results in a lateral speed of roughly 0.35 ms 1. The fixed angle of descent and ascent results in horizontal resolution varying from roughly 50 m (a full undulation every 100 m) in shallow water to roughly 300 m in deeper, offshore water. The glider carried a SeaBird CTD sensor package designed for use on gliders. Data from these repeated dives and climbs are interpolated onto a regular grid for visualization and analysis. Further data was taken from a meteorological buoy deployed approximately 20 km north of the study site (Figure 1). This buoy was recovered on 14 November, a day before the end of the glider deployment; thus the glider deployment and buoy deployment overlapped for ten days. A detailed description of the data collected by this buoy can be found in Austin and Allen (2011). The buoy collected sufficient information to make estimates of surface heat and momentum fluxes using the algorithms of Fairall et al. (1996) to estimate the latent and sensible fluxes. Results The glider was deployed near the survey site, and made 26 shelf crossings in a period of twelve days. Each shelf-crossing took slightly less than 12 h, so that the glider made it across the shelf and back approximately once per day. The glider was programmed to climb to within 5 m of the surface on each climb and to within 5m of the bottom on each dive. Approximately every four hours, the glider would come to the surface to communicate, providing data all the way to the surface. The period of deployment was characterized by periodically strong winds (Figure 2a) which were predominantly oriented in the NE-SW direction (Figure 2b), which was either downwelling favorable (positive values in Figure 2b) or upwelling favorable (negative values in Figure 2b). It was also characterized by a significant net loss of heat from the lake to the atmosphere (Figure 2c). A single transect from 4 November is shown (Figure 3) as an example of the data, and other transects were similar in spatial structure. The nearly vertical dotted lines in Figure 3 represent the downcasts of the glider, which provide an indication of the horizontal resolution achieved. In this transect, a mid-shelf temperature maximum is clearly visible, though the temperatures in this region were only slightly warmer than those onshore and offshore of the maximum. In addition, sub-thermocline water was apparent offshore of the 50 m isobath, where the water temperature was as
318 Austin / Aquatic Ecosystem Health and Management 15 (2012) 316 321 Figure 1. The western arm of Lake Superior, with the positions of the deployment, transect and recovery marked. The location of the meteorological buoy is shown. (Color figure available online.) low as 6 C in earlier surveys, and as low as 4.5 Cin later ones. The extent of the upslope intrusion of this bottom water did not appear to be directly coupled to the alongshore component of wind (Figure 2b), as measured at the nearby buoy, suggesting that most of the fluctuation in the location of the thermocline was due to internal waves. The average temperature between 5 m and 10 m depth was used as a measure of near surface temperature, and was computed as a function of offshore distance for all transects (Figure 4). This showed that the mid-shelf temperature maximum shown in Figure 3 was not an isolated occurrence; in fact, a temperature maximum of some magnitude occurred on all of the sections, typically between 5 km and 7 km offshore. The temperature maximum was not large, and was in the order of 0.1 0.3 K greater than the water onshore or further offshore. The fact that it persisted over a 12-day span during which several weather patterns passed (Figure 2b), suggested that this feature was not due to a transient feature like a lake eddy (Ralph, 2002); rather it appeared to be geographically locked in place. The position of the temperature maximum also appeared to be insensitive to the wind field, as several strong wind events occurred during this period, with several events reaching 20 ms 1 from varying directions. Formation and persistence of this mid-shelf temperature maximum appeared to be due to two competing effects. First, a roughly uniform rate of surface cooling across the shelf will cause temperature in shallower waters to decrease more rapidly than those in deeper waters, resulting in relatively cool water near the coast. While surface heat flux is technically a function of surface water temperature, the dependence is weak and the surface temperatures are fairly uniform. Second, wind-driven vertical mixing is going to cause more rapid decreases in surface water temperature offshore, due to the presence of cool, sub-thermocline water offshore of roughly the 50 m isobath. Given the observed meteorological data at the adjacent buoy, the surface heat flux was estimated to be roughly 90 Wm 2 (Figure 2c) averaged over the 10 day period of glider-buoy overlap (4 14 November). Since cooling water above 4 C results in convective penetration, the entire water column at a
Austin / Aquatic Ecosystem Health and Management 15 (2012) 316 321 319 Figure 2. Time series of surface forcing during the deployment. (a) Surface wind speed at the meteorological buoy. Grey areas represent timing of onshore transits; clear areas timing of offshore transits. Numbers refer to survey numbers shown in Figure 4. (b) The alongshore component (oriented 20 N of E) of the wind field. In this case, positive values represent downwelling-favorable winds. (c) The net surface heat flux estimated using the buoy measurements. given location must cool off roughly uniformly. If this were the case, the cooling rate of 90 ± 10 Wm 2 should result in a rate of temperature decrease of 0.05 ± 0.01 Kd 1 in 40 m of water. This disregards the influence of density-driven crossshelf circulation, some evidence for which could be seen as slightly cooler water appeared to be moving from onshore below the temperature maximum region. The observed rate of cooling of water shallower than 40 m was 0.05 ± 0.01 Kd 1, consistent with the estimate of cooling from the surface flux measurement. Discussion While there was some evidence of weak crossshelf transport in the section shown (Figure 3), it appeared to be overwhelmed by the vigorous convective mixing that was driven by surface cooling. Most of the sections did show a small layer near the bottom which was cooler than the water above, but in general the water column was nearly uniform. Contrary to stratified transport processes like upwelling and downwelling, strong wind events are likely to drive vertical mixing and suppress crossshelf transport, effectively trapping constituents in the nearshore, while simultaneously mobilizing bottom sediment into the water column. Alternately, winds strong enough to drive Ekman circulation, but not strong enough to keep the water column unstratified, could drive restratification offshore of the maximum during upwelling favorable winds (by driving lighter, warmer water over denser, cooler water), and restratification inshore of the maximum during downwelling-favorable winds. The observations presented here were taken during a time of energetic wind events, which kept the water column largely well mixed onshore of the thermocline.
320 Austin / Aquatic Ecosystem Health and Management 15 (2012) 316 321 Figure 3. Contours of water temperature from a transect on 4 November 2009. The light grey lines are the downcast paths of the glider. The cooler water offshore is an indication that subthermocline water is being mixed into the surface mixed layer, likely further increasing property gradients across the shelf. All of this suggested that nearshore trapping of constituents was not limited to the season when the thermal bar was migrating offshore. It is worthwhile to consider observations of the mid-shelf maximum from a technological point of view as well. If the temperature maximum had been observed during a very finely resolved CTD survey of the shelf, or in surface underway data (i.e. data collected continuously at the surface on board a transiting ship) it would likely be attributed to a transient feature such as an eddy. Without multiple repeated surveys over a period of ten days, which would typically be cost-prohibitive, it would be very difficult to make a statement about the persistence of such a feature. Additionally, it is difficult to find ten consecutive days in Lake Superior in November in which weather conditions are favorable for shipboard work. Likewise, an array of moored equipment would have to have very high cross-shelf resolution in order to detect this feature, also cost-prohibitive. This feature may be resolvable in satellite images (Ullman et al., 1998), though they typically do not have the temperature resolution necessary to pick out such a weak feature, and are subject to the vagaries of cloud cover. The glider, by virtue of its high sampling resolution in space and time, coupled with its ability to remain on location for periods of tens of days, was able to resolve the feature repeatedly. Conclusions Observations made with an autonomous glider on the broad, shallow Wisconsin shelf of Lake Superior during a period of intense cooling revealed a persistent offshore temperature maximum, which appears to be due to a balance between uniform surface cooling and the mixing of deep, cool water into the surface layer. This structure was persistent over a period of 12 days, during which several weather systems traversed the area. The nature of the structure suggests that vertical mixing in the region is vigorous, and may play a significant role in restricting cross-shelf transport. The use of a glider allowed the phenomenon to be observed with much higher spatial and temporal resolution than would be possible with more conventional measures, such as moored or shipboard observations. Acknowledgements This research was made possible through funding from the Great Lakes Observing System (GLOS) and the National Science Foundation, Geosciences Directorate Grant 0825633. Figure 4. Average temperature between 5 and 10 m depth, as a function of offshore distance, for all 26 transects. The heavy lines are the first (top) and last (bottom) transect. References Auer, M.T., Gatzke, T.L., 2004. The Spring Runoff Event, Thermal Bar Formation, and Cross Margin Transport in Lake Superior. Journal of Great Lakes Research 30 (Supplement 1), 64 81. Austin, J.A., Allen, J., 2011. Sensitivity of summer Lake Superior thermal structure to meteorological forcing. Limnology and Oceanography 53(3), 2011.
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