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Novel Observations of Full-Depth Advection in Late Summer 
Lake Superior

by Kaelan Weiss

Introduction
 Within the study of physical processes in large 
lakes, much focus is given to dynamics at or near 
the surface of lakes. !ese shallow processes are 
relatively easy to observe when compared to deep-
lake, or hypolimnion, processes. Near-surface 
dynamics are also generally regarded as more 
energetic, more important for particle transport, 
and more impactful on human communities than 
their deep-lake counterparts. !erefore, shallow 
dynamics represent a signi"cant portion of literature 
on physical processes in large lakes. Lake Superior, 
the largest of the Laurentian Great Lakes, is no 
exception to this trend. To help address the limited 
research on hypolimnion processes, this paper 
presents observations of rapid and drastic changes 
throughout the full water column in Lake Superior 
during late summer based on moored observations 
of temperature and pressure. !ese drastic changes 
occurred annually to varying degrees at a site in the 
southeastern basin of the lake over the duration of the 
2009-2012 mooring deployment. Although similar 
dynamics are observed annually, the 2009 record 
is given primary focus due to its greater rapidity 
of changes to the water column compared to other 
years. !e observations suggest that the deepest 
parts of the lake (>300 m) can see large #uctuations 
in temperature and thermal structure during stable 
strati"cation regimes and are generally more active 
than assumed.
 !ere is no clear set of dynamics which can 
easily explain the observations, which underscores a 
lack of understanding of how water moves in Lake 
Superior. !is ignorance has broader rami"cations. 
Understanding the transport of water, and thus heat, 
is fundamental to understanding how large lakes will 
respond to changing climate. Surface temperatures 
of Lake Superior have been shown to be warming 
faster than regional air temperatures, and the 
duration of ice cover is likewise shrinking (Austin 
and Colman 2007; Zhong et al. 2016). Growing 
surface temperatures and shorter winters have both 

increased primary productivity in the lake (O’Beirne 
et al. 2017). To understand how anthropogenic 
climate change will a$ect Lake Superior further, 
more must be known about how heat is distributed 
and moves across and within the lake.
 Although shallow dynamics are heavily studied 
over hypolimnion dynamics, studies concerning 
the latter are present and represent a wide array of 
physical processes that cause water motion in the 
deep regions of lakes. In 2002, Ralph showed that 
eddies, or circular currents, are present in Lake 
Superior. Usually on the scale of 10-100 km across, 
these currents exist at all depths and contribute to 
horizontal variability of lake surface temperature 
(Ralph 2002). In addition to circular currents, 
studies of general patterns of circulation in Lake 
Superior show that coastal water can propagate 
o$shore, mixing and interacting with the water 
in the open lake. !ese studies were conducted 
by observation (Beletsky et al. 1999) and through 
numerical modeling (Bai et al. 2013; McKinney 
et al. 2018). !e study of cold intrusions, or cold 
water swi%ly descending from the lake surface to 
the bottom, has been applied to large lakes as well, 
namely Crater Lake (Crawford and Collier 2007) 
and Lake Baikal (Wüest et al. 2005). However, these 
phenomena occur during winter in lakes much 
deeper and steeper than Lake Superior. !e changes 
in temperature due to cold intrusions are also quite 
small, generally on the order of a tenth of a degree 
Celsius.
 Another dominant process in the interior of 
large lakes is internal waves, or waves that propagate 
between layers of di$erent density (i.e. di$erent 
temperature). Internal waves are either linear or 
nonlinear. In a study of Lake Biwa, Japan, Shimizu 
et al. (2007) showed that, while linear internal 
waves appear in the hypolimnion, they generally do 
not change the characteristics of the water column 
except for mixing along density layers. On the other 
hand, nonlinear internal waves can theoretically 
trap masses of water in their interior while they 



Figure 1: Lake-wide mooring locations (from Titze and 
Austin 2014).

Figure 2: Bathymetry surrounding the SM site (red dot). 
!e southeast region of Lake Superior is characterized by 
highly irregular bathymetry. Data from National Centers 
for Environmental Information.

Novel Observations of Full-Depth Advection in Late Summer Lake Superior

Aisthesis      Volume 11,  202033

travel and therefore cause advection—or horizontal 
transport—of water (Ostrovsky and Stepanyants 
1989). A later study by Preusse et al. (2012) showed 
that fully developed stable cores likely do not exist in 
nature.
 In addition to circulation and internal waves, 
vertical mixing due to turbulence can also a$ect the 
hypolimnion. Michalski and Lemmin (1995) showed 
that, while vertical mixing plays a large role in the 
upper hypolimnion (<90 m), it plays a lesser role 
in the deep hypolimnion. While many mechanisms 
that a$ect the deepest parts of large lakes have been 
explored and revealed by years of research, the set 
of observations presented here represent natural 
phenomena that are not readily attributed to any 
single mechanism within the list above.

Methods
 From the summer of 2009 to the summer of 
2012, seven full-depth temperature moorings were 
deployed across the extent of Lake Superior (Figure 
1). !e depths of the mooring locations ranged from 
170 m at the Far Western Mooring (FWM) to 380 m 
at the Southern Mooring (SM). !e moorings were 
equipped with 10-16 thermistors (i.e. temperature 
sensors) spaced closely near the surface and farther 
apart toward the bottom. Since the moorings have no 
surface signature, Coast Guard regulations stipulate 
that the top of the mooring—and therefore shallowest 
thermistor—be located 10 m below the surface of the 
lake. !e deepest thermistor on each mooring was 
located just above the release mechanism at about 5 
m above the lakebed.
 Since a variety of thermistor models were 
used across mooring deployments, not all data 
were measured at the same frequency. To account 
for individual instrument storage capacity, some 

thermistors took measurements every 10 min while 
others took measurements every 1 min. In both 
cases, the measurement period was greater than 
the response time of the sensor to ensure maximal 
accuracy of ~ 5 mK. !e mooring at the SM site was 
equipped with Brancker Research (RBR) TR-1000 
sensors that measured temperature every 30 min and 
TR-1050 sensors that measured temperature every 
10 min. Two TR-2050 pressure and temperature 
sensors were also included at the 10 m and 40 m 
depths. Pressure was measured to determine the 
actual depth of the mooring once it was deployed 
which, upon review of the pressure record, was about 
10 m deeper than originally intended.
      !e seven moorings were deployed in regions 
of Lake Superior characterized by di$erent basin 
size, depth, distance from shore, and surrounding 
bathymetry (underwater topography). Bathymetric 
data were taken from the National Centers for 
Environmental Information (NCEI) Great Lakes 
Bathymetry repository. A three arc-second (~90 m) 
grid was available for Lake Superior from this source 
at the time of this study. Notably, the SM site is located 
in a region of intense bathymetric irregularity; the 
deepest point of the lake (~400 m) is located directly 
to the north, and a seamount that extends upward 
to 25 m below the lake surface is positioned directly 
to the southeast. !e mooring was also deployed 
in a deep submarine trench that runs north-south 
(Figure 2). Compared to the other mooring sites, the 
SM site is surrounded by areas with more variable 
bathymetry.



Figure 3: (a) Water temperature as a function of depth 
and time at the SM site during the second half of 2009. 
!e 4.25°C and 8°C isotherms are plotted in black. (b) 
Heat content per unit area relative to 4°C at the SM site. 
!e sudden spike in heat content and plunging of both 
isotherms marks the onset of the advective event. A 16-hr 
running average "lter was applied in order to remove the 
inertial signal. Both plots share the same time axis.

Novel Observations of Full-Depth Advection in Late Summer Lake Superior

Aisthesis      Volume 11,  202034

      During most of the ice-free season, meteorological 
conditions over the surface of Lake Superior are 
recorded by three National Data Buoy Center 
(NDBC) buoys. Buoy 45004 is the nearest to the SM 
site and is located within 2 km of the EM site (see 
Figure 1). For the purposes of this analysis, hourly 
wind speed and direction from this buoy were taken 
to describe the wind "eld at the SM location 74 km 
to the south.
      !e relative heat content at mooring locations was 
found by modeling the water column as a sequence 
of vertically stacked, discrete layers of water centered 
on each thermistor, and then summing the heat 
content of all layers. Accordingly, heat content was 
estimated as

where H is the heat content per unit area of the water 
column in J m-2, &w is the density of water assumed 
to be a constant 1000 kg m-3, cP is the speci"c heat 
of water taken as 4180 J kg-1 C-1, Ti is the measured 
temperature of the ith layer in C, TMD is the temperature 
of maximum density of fresh water, and 'zi is the 
thickness of the ith layer in m. By de"ning the heat 
content relative to the temperature of maximum 
density, the zero-crossing becomes a phenological 
indicator for switching between positive (summer) 
and negative (winter) thermal strati"cation regimes 
(Titze and Austin 2014). Although the temperature 
of maximum density is a function of pressure (and 
therefore depth), it is assumed to be constant, and 
assigned an approximate value of 4°C (Chen and 
Millero 1986). !e constant approximation is valid 
because this study is concerned with changes in 
relative heat content on the order of 109 J m-2, and 
a nonconstant temperature of maximum density 
engenders #uctuations two orders of magnitude 
smaller (Chen and Millero 1986).
 In addition to relative heat content, the stability 
of the water column as a function of depth was 
calculated as a proxy for strength of strati"cation. 
!e de"nition for stability (Boehrer and Schultze 
2008)

was discretized for calculation using the moored 
data such that

where g is the local gravitational acceleration (10 
m s-2), &0 is a reference density (1000 kg m-3), &pot is 
the potential density—a function of temperature—
in kg m-3 (Chen and Millero 1986), and z is depth 
in m. During periods of su(cient strati"cation, 
the thermocline depth is taken to be the depth of 
maximum stability.

Results
 Leading up to September 29, 2009, the southern 
mooring thermistor record exempli"es characteristic 
late summer thermal structure (Boehrer and Schultze 
2008; Titze and Austin 2014). Figure 3a shows the 
SM temperature at depth from August through 
December 2009. !e thermocline is situated at or 
above the shallowest thermistor, so most (if not all) of 
the mooring is in the hypolimnion. !e hypolimnion 
makes up a majority of the water column and ranges 



Figure 4: (a) !e magnitude of the meridional (north-
south) wind component is plotted in blue on the le% 
vertical axis. !e temperature at 250 m depth from the SM 
site advanced in time by 48 hr is plotted in dotted-black 
on the right vertical axis. (b) !e same as above, but for 
the zonal (east-west) wind component in orange. A 4-hr 
running average "lter was applied to all data and both 
plots share the same time axis.

Novel Observations of Full-Depth Advection in Late Summer Lake Superior

Aisthesis      Volume 11,  202035

in temperature from 10°C down to 4°C at the bottom, 
with only a 0.5°C change occurring from 50 m to 
380 m. !e 4.25°C and 8°C isotherms are plotted 
(Figure 3a), and the former remains stable at 50 m 
depth except for a small de#ection at the beginning 
of September. During the month of September, the 
maximal stability of the water column was generally 
between 2x10-4 and 8x10-4 m2 s-2. Since the shallowest 
two thermistors were likely at or below the base of 
the thermocline, these values probably represent an 
underestimation of the actual stability. 
 From September 29 to 30, the water column 
at the SM site underwent a signi"cant change in 
character. !e 10 thermistors in the top 100 m 
reported increases in temperature ranging from 
0.5°C to 5°C, with larger changes corresponding 
roughly linearly with less depth. !e "ve thermistors 
located 150 m to 374 m below the surface all 
recorded increases in temperature of about 0.5°C as 
well. Within the period of one day, the entire 380 m 
water column shi%ed upward in temperature by at 
least a half a degree Celsius, with greater shi%s near 
the surface. During this same time, the thermocline 
steadily deepened to a maximal depth of 60 m before 
rebounding upward to 20 m. !e deepening of the 
thermocline was matched with a slight decrease in 
maximal stability from about 1.5x10-4 to 1x10-4 m2 
s-2, suggesting a general weakening of strati"cation 
and increase in mixed-layer depth.
 By October 1, the temperature of the entire water 
column peaked above 4.25°C, and temperature at all 
depths continued to #uctuate both positively and 
negatively over the course of the next two days. 
Notably, there were roughly day-long periods when 
the shallowest 100 m experienced thermal changes in 
direct opposition to changes below 100 m. By October 
3, the depth of the 4.25°C isotherm climbed and then 
remained generally between 100 m and 200 m depth, 
and no signi"cant changes in maximal stability 
occurred. Following the rapid temperature changes 
at all depths between September 29 and October 3, 
2009, the water column at the SM site entered an 
altered, but semi-stable strati"cation regime for the 
remainder of the year. As the lake moved toward 
winter homogenization, or “overturn,” maximal 
stability and surface temperatures continued to 
decrease as expected from seasonal changes.
 Concurrent with the large temperature 
#uctuations was a strong wind event over Lake 
Superior: a northerly wind peaked at 18 m s-1 at 

midnight on September 29 and gradually backed 
o$ over the course of the next three days (Figure 
4). When the meridional (north-south) wind 
component is compared to the temperature at 250 
m (chosen as an indicator for signals propagating 
into the deep lake), a striking relationship is 
revealed. Figure 4a shows the meridional wind 
component and the water temperature at 250 m 
advanced forward in time by 48 hr. A%er the initial 
event, there is a strong correspondence between 
episodes of increased windspeed from the north and 
temperature variability in the deep part of the SM 
site. For comparison, the same temperature signal is 
plotted similarly against the zonal (east-west) wind 
component in Figure 4b. !e temperature variability 
during such events is always of the same nature: a 
warming at depth that is proportional to windspeed 
and then subsequent cooling a%er the wind ceases. 
High wind episodes before the September 29 event 
do not appear in the deep-water signal, and similarly, 
winds blowing from the south have little to no e$ect 
either. !e thermal response at the SM site is highly 
asymmetric in regard to wind forcing.
 !e moored pressure record also provides insight 
into the dynamics of the SM event (Figure 5). Directly 
preceding October 1, the pressure quickly jumped 
by 1-1.5 dbar with an hour-long maximal excursion 
upwards of 2.5 dbar. A%er two days of heightened 
values, the pressure at both sensors returned to pre-



Figure 5: (a) Pressure as a function of time from the 
shallower pressure sensor at the SM site. !e advective 
event is apparent in the large spike in pressure directly 
preceding October 1st. !e ambient pressure suggests the 
mooring was deployed about 10 m deeper than originally 
intended. (b) !e same as above, but for the deeper 
pressure sensor. Both plots share the same time axis.

Novel Observations of Full-Depth Advection in Late Summer Lake Superior

Aisthesis      Volume 11,  202036

event ambient levels. !e jump in pressure could be 
the result of a strong current “blowing” the mooring 
over slightly and causing the sensors to descend in 
the water column. For every meter of depth, pressure 
increases approximately 1 dbar, so it’s possible that 
the current blew the mooring over enough to lower 
the sensors 1.5 m in the water column. While nothing 
can be said about the direction of the current and 
very little can be determined about its magnitude, the 
pressure signals suggest that the large temperature 
#uctuations during the event were accompanied by 
a rapid increase in local circulation.
 !is energetic period at the SM site can be viewed 
from a lake-wide perspective as well. At the time of 
the event, seven total moorings were deployed and 
recorded the evolution of the temperature "eld 
across the extent of Lake Superior. Intriguingly, no 
other mooring site showed signi"cant changes in the 
deep part of the lake following the storm. Changes in 
thermal structure at the other sites are apparent but 
limited to the upper water column. !e response is 
isolated at the SM location, suggesting that a speci"c 
set of parameters, perhaps a combination of local 
circulation and lakebed geometry, caused northerly 
winds to stir up the deep lake.
 Yet another description of the event is provided 
by the heat content at the SM location. Figure 3b 
shows that the heat content remained relatively 

stable within 300 MJ m-2 of the zero-crossing with a 
few excursions upward to 500 MJ m-2 during August 
and September leading up to the event. !en on 
September 29, the heat content increased rapidly by 
over 1500 MJ m-2 in about two days before settling to a 
new average value of 1250 MJ m-2. !is rapid increase 
in heat content is equivalent to the entire water 
column warming by 1°C and happened as a result 
of the full-depth upward temperature shi% described 
above.  A maximal heating rate of 2500 W m-2 was 
observed over a four-hour period during the event. 
Given that heat #ux from the lakebed is negligible 
and the heat #ux at the lake surface is bounded above 
by about 1000 W m-2 at Lake Superior’s latitude, the 
observed heating cannot be explained by energy 
being absorbed at the lake surface alone. Instead, 
the rapid change in heat content suggests that the 
event is advective in nature. In other words, a region 
of water substantially warmer than its surroundings 
was transported onto the mooring location. For the 
advection of thermal energy on this scale to occur 
in the relatively short time frame of four hours, a 
combination of two things likely occurred: a strong 
current persistent throughout the water column and 
the existence of a sharp front between water masses 
of signi"cantly di$ering thermal character. It is also 
worthy of note that the SM location did not begin to 
lose heat until well into December.
 Although the results presented above occurred 
in 2009, qualitatively similar observations of full-
depth temperature variability were also recorded in 
2010 and 2011 at the SM site. However, the changes 
observed in the water column during 2009 represent 
a more drastic and rapid transition out of summer 
strati"cation than the observations from the two 
subsequent years.

Discussion
 From the available observations, a general picture 
of dynamics a$ecting the full water column captured 
at the Southern Mooring can be drawn. Perhaps most 
noticeably, a rapid, drastic, and full-depth change in 
thermal structure at the site is apparent. !e event 
occurred during stable late-summer strati"cation 
and marked the start of a period with persistent 
increased heat content, a deeper mixed layer, 
and lower stability. !e event followed sustained 
northerly winds, and a%erwards, similar northerly 
winds were seen propagating into the temperature 



Novel Observations of Full-Depth Advection in Late Summer Lake Superior

Aisthesis      Volume 11,  202037

observations throughout the deepest portion 
of the water column. !is relationship between 
wind and deep-lake temperature persisted until 
winter homogenization. Notably, the magnitude of 
increased temperature at depth corresponds well 
with northerly and northwesterly winds but does not 
seem to be related to winds from any other direction. 
!e pressure record suggests that the mooring was 
“blown” over for two days following the high winds, 
presumably by a signi"cant current. Unfortunately, 
neither direction nor magnitude of the current can 
be determined with the given observations. !is 
argument for increased circulation is echoed by 
the large and rapid increase in local heat content, 
the only reasonable explanation for which is the 
advection of a mass of warmer water onto the 
mooring. Additionally, the event was isolated at the 
SM site; similar events were not recorded at any of 
the other six mooring locations across the lake. !e 
Southern Mooring site is located 40 km o$shore 
which is su(ciently far to be removed from coastal 
downwelling, or warm surface water “piling up” 
along the coast. !e site is also marked by highly 
irregular bathymetry. Trenches, pits, and seamounts 
all contribute to the high variability around the site. 
Lastly, these types of events that happen during late 
summer strati"cation and drastically change the 
character of the water column at this speci"c location 
appear to happen annually to varying degrees.
 Despite the diverse set of observations, the 
underlying mechanisms that cause these events in 
Lake Superior are unknown. General assumptions 
tend to mark the hypolimnion of large lakes as rather 
quiescent during summer strati"cation, especially 
when compared to the upper layer of the lake. 
However, these "ndings suggest that the hypolimnion 
is more energetic than is generally thought. !is 
highlights an incomplete understanding of the 
dynamics governing the deepest regions of large 
lakes and the interactions and boundaries between 
shallow and deep layers.
 It is likely that these types of events are caused 
by a combination of known dynamics rather than 
an entirely novel mechanism. !e localization of 
the observed event at the SM site suggests that a set 
of parameters unique to that location are at play. 
!e most notable di$erence between the SM site 
and the other mooring sites is the highly irregular 
bathymetry. Additionally, the coincidence of high 

windspeed and deep-lake temperature variability 
points to wind forcing as a principal cause. Sustained 
periods of wind drive currents, the presence of which 
is strongly suggested by the pressure and heat content 
observations. !erefore, a possible explanation 
for the rapid change in thermal structure is that 
circulation caused speci"cally by northerly winds 
encountered unique bathymetric features at the SM 
site, and a concentration of warm water cascaded 
into the deep layer of the lake. !e speci"cs of such 
an event are unknown and beyond the scope of this 
study.
 Since the set of observations made at the SM 
site in 2009 suggests that the event was advective 
in nature, two circumstances must have been true 
within the lake: horizontal currents were present, 
and regions of water with di$ering thermal character 
(i.e. heat content) existed in close proximity. !e 
rapid change in character of the water column can be 
explained by a transition between the two di$erent 
water masses being transported over the stationary 
mooring by local circulation. Although these 
conditions may be assumed from the observations, 
neither were directly observed due to the limitations 
of the mooring. Velocity is not known because 
only temperature and pressure were recorded, and 
the spatial variability of temperature is not known 
because there was only one mooring at the location. 
!e observations are also limited by slow recording 
speed; most thermistors logged every 30 min.
 To amend these shortcomings and further 
characterize the events at the SM site, a new set of 
moorings could be deployed. By adding an acoustic 
doppler current pro"ler (ADCP) to each mooring, 
water velocity at di$erent depths in the water column 
would be recorded in addition to temperature and 
pressure. In many applications, ADCPs are placed 
near the top of the mooring to provide high vertical 
resolution to the horizontal velocity data. However, 
this deployment would require full-depth velocity 
measurements since the observed events occur 
throughout the entire water column. !erefore, the 
ADCP would be placed near the bottom of the lake. 
In the years since 2009, instrumentation has become 
faster and able to store signi"cantly more data, so a 
temperature and pressure sampling rate of at least 
once per second could be achieved. !is represents 
an 1800x increase in temporal resolution, meaning 
the precise transition between thermal regimes 



Novel Observations of Full-Depth Advection in Late Summer Lake Superior

Aisthesis      Volume 11,  2020

could be determined during an event. Finally, at 
least one additional mooring could be deployed 
in close proximity to the SM site to form a cluster 
of moorings. Preferably, two additional moorings 
would be deployed. One would be placed 5 km to 
the south of the SM site in the same trench and at a 
similar depth. !e second mooring would be placed 
5 km to the east of the SM site outside of the trench at 
a depth of 100-200 m. !is second mooring location 
is on a steeper slope and nearer to the seamount that 
sits to the southeast of the SM site. By deploying 
the three moorings orthogonally and at di$erent 
depths, one could determine the horizontal scales 
that support drastic di$erences in thermal character 
and the interactions that water masses have with 
bathymetric features in the region.
 In addition to a more advanced deployment, 
more analysis must be conducted on the data that is 
already available. Although the deep advective events 
occur to a lesser degree in 2010 and 2011, these years 
still o$er insight into the dynamics that govern the 
SM site. Speci"cally, interannual variability of the 
relationship between northerly wind and deep-lake 
temperature #uctuation could be examined.
 !is set of moored temperature and pressure 
observations is a salient example of how little is 
known about the deep-water dynamics of large 
lakes despite the body of research dedicated to their 
study. !rough further observations and analysis 
of the previously unwitnessed events recorded in 
the southeastern basin of Lake Superior, important 
governing mechanisms may come into focus. !e 
world’s large lakes hold much of the planet’s available 
fresh water, and understanding how these resources 
continue to respond to a changing climate has the 
potential for broad physical, ecological, and human 
implications.

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Novel Observations of Full-Depth Advection in Late Summer Lake Superior

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