Transcript PPNW2008 - Toffolon

European Geosciences Union
General Assembly 2011
Vienna, Austria
8 April 2011
Modelling climate change impacts
on deep water renewal in Lake Baikal
Sebastiano Piccolroaz, Marco Toffolon
Department of Civil and Environmental Engineering
University of Trento (Italy)
[email protected]
Lake Baikal (Siberia): an extraordinary deepwater renewal
•
the world oldest and deepest lake (max depth 1˙642 m)
•
the largest freshwater body by volume (volume 23˙600 km3)
•
length 636 km, max width 79 km
•
unique ecosystem: more than 1˙500 endemic species
Deep ventilation: high oxygen
concentration up to the lake bottom!
Deep ventilation mechanism: thermobaric instability
weak wind forcing: small depth of
downwelling  back to initial position
1
2
3
T [°C]
strong wind forcing: large depth of
downwelling  down to the bottom
4
1
0
0
depth [m]
downwelling lighter
than ambient water
500
1000
1500
compensation
depth
temperature of
maximum
density
actual
temperature
profile
downwelling heavier
than ambient water
Estimates of downwelling volumes:
• 100 km3 per year (Peeters et al., 2000)
• 1÷10 km3 per event (Wϋest et al., 2005)
• 50÷100 km3 per season (Schmid et al., 2008)
2
3
T [°C]
4
A simplified
1D model
• a simple way to represent the phenomenon
• suitable to predict long-term dynamics
• just a few input data required
wind
• simplified downwelling mechanism
(based on wind energy input)
downwelling
density
main
features
• check for unstable conditions
 vertical stabilization
z
unstable
stable
C
• vertical diffusion equation (temperature,
oxygen, other solutes) with source terms
flux
z
Calibration and validation of the model
Parameters to be calibrated:
• vertical profile of the “effective” diffusivity
• downwelling volumes per single event (statistical distribution)
• energy associated with wind velocity (statistical distribution)
Methods for calibration and validation:
• measured vertical profiles of eddy diffusivity (Ravens et al., 2000; Wϋest et al.,
2000)
• estimated volumes per event or per year (Weiss et al. 1991; Killworth et al., 1996;
Peeters et al., 2000; Wϋest et al., 2000; Schmid et al., 2008)
• comparison of short-term simulations with measured temperature profiles
produced by single downwelling events (2006-2007)
• comparison of the present situation with “asymptotic” temperature profiles
resulting from long-term simulations (centuries)
• comparison with the formation of CFC profiles (1942-1994) (Peeters et al., 2000)
Thanks to Johny Wüest and Martin Schmid (EAWAG) for the data!
Model parameters
1) Vertical eddy diffusivity
• mean annual
• lack of stratification
2) Probability distribution of the
energy transmitted by the wind
• summer
• winter
3) Downwelling volumes: 5÷65 km3/event (uniform distribution)
Result of calibration
temperature
CFC-12
15 February
Long-term asymptotic trend (800 years)
Boundary conditions: present state
Main statistics (600 years)
number of events per year
period
deep (>1000m)
total
whole year
1.65
13.3
winter
1.04
8.5
summer
0.61
4.8
features of downwelling events
volume per year
58 km3
max temp.
3.454 °C
mean temp.
3.224 °C
min temp.
2.815 °C
Analysis of
downwelling
dynamics
15 May
15 September
15 June
15 December
Climate change scenarios
Estimates of single aspects,
e.g. temperature (Magnuson et
al., 2000; Hampton et al., 2008),
lack of data about wind
5 days
ice
ice
11 days
ice
ice
superficial temperature increase
(global warming):
• +4°C (summer), +2°C (autumn)
• reduction of ice-covered period
wind forcing increase/decrease
Possible effects of climate change
warming
(+2/4°C)
strong wind
weak wind
strong wind
+ warming
Importance of the shape of temperature variations
steeper temperature variation in the
“right” periods:
 fewer downwelling events
 reduction of deep water cooling
+2°C
Conclusions
Modelling results:
• evaluate long-term scenarios
• analyse downwelling dynamics statistically
• assess the influence of single factors
• estimate the impact of climate change
Physical results:
• downwelling volume is estimated as 58 km3/year
• wind forcing is the most important factor
• surface warming: controversial influence
• the present situation is quite stable
• expected climate change may increase deep ventilation
Ice ring in the southern basin of Lake Baikal
(Nasa Earth Observatory, April 25, 2009; Balkhanov et al., TP 2010)
Potential energy balance: required vs. available
Required 
Available 
Abstract
Deep ventilation in deep, temperate lakes is an interesting physical phenomenon having important implication on the ecobiology of the whole freshwater basin. It is characterized by the sinking of cold and oxygenated surface waters down to
high depths, resulting in a cooling and natural oxygenation of the hypolimnetic waters. The mechanism is triggered by
thermobaric instability (i.e. the decrease of the temperature of maximum density of fresh water, which is about 4°C at the
atmospheric pressure, with increasing depth and pressure) in presence of an external forcing (e.g. surface winds, thermal
bars, river inflows) that is strong enough to move a portion of surface water down to a certain critical depth. Climate
change can cause variations of the lake surface temperature and of the temporal and spatial wind field distribution, thus
affecting the external forcing mechanisms and consequently the long term behaviour of deep water renewal in a
freshwater basin.
Lake Baikal (Siberia), the deepest and largest lake in the world in terms of volume, is characterized by an intense annual
renewal of deep waters that is able to reach the deepest layers up to 1642 m depth. Due to its dimension and the great
amount of wide-ranging physical and eco-biological phenomena occurring in it, Lake Baikal has been greatly studied and
monitored. Notwithstanding, estimates of the extension of the water volumes sinking downward to the bottom of the lake
are often controversial.
In this work, deep mixing and ventilation occurring in the South Basin of Lake Baikal (1461 m deep) is numerically
investigated by means of a simplified one-dimensional vertical model. Numerical simulations on single downwelling
occurrences considering a typical annual temperature evolution have been carried out, comparing the results with the
observational data (by courtesy of Prof. Alfred Wüest, EAWAG) in order to calibrate the model. In this phase, an estimate
of current mean annual intrusion volume and temperature has been performed (with values respectively of 20 km^3 and
of 3.15°C, approximately). Moreover, long time (i.e. centuries) simulations have been run aimed at: (1) understanding
whether the actual profile is in equilibrium with the existing external conditions; and (2) simulating the effects of different
climate change scenarios (e.g. global warming and wind variation). The results suggest that the current deep water
temperature profile of Lake Baikal is an equilibrium configuration achieved during the past centuries. Furthermore, it
seems that the deep water conditions are significantly resistant to climate change, and that possible variations of wind
intensity are more significant than the warming of surface waters in altering the mechanism of downwelling. The resilience
of the current configuration may be considered coherent with the geological age of the lake and of the peculiar ecosystem
that has adapted to such conditions.