Modelling climate change impactson deep water renewal in Lake Baikal

Sebastiano Piccolroaz, Marco Toffolon

Department of Civil and Environmental EngineeringUniversity of Trento (Italy)

European Geosciences Union

8 April 2011Vienna, Austria

marco.toffolon@unitn.it

General Assembly 2011

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!

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)

T [°C]

dep

th [

m]

1 2 3 40

500

1000

1500

weak wind forcing: small depth of downwelling back to initial position

Deep ventilation mechanism: thermobaric instability

temperature of maximumdensity

actual temperature profile

downwelling lighter than ambient water

T [°C]1 2 3 4

0

strong wind forcing: large depth of downwelling down to the bottom

downwelling heavier than ambient water

compensation depth

A simplified 1D model

• a simple way to represent the phenomenon

• suitable to predict long-term dynamics

• just a few input data required

main features

• check for unstable conditions vertical stabilization

densityz

unstable stable

C

z• vertical diffusion equation (temperature,

oxygen, other solutes) with source terms

flux

• simplified downwelling mechanism (based on wind energy input)

wind

downwelling

Calibration and validation of the model

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)

Parameters to be calibrated:

•vertical profile of the “effective” diffusivity

•downwelling volumes per single event (statistical distribution)

•energy associated with wind velocity (statistical distribution)

Thanks to Johny Wüest and Martin Schmid (EAWAG) for the data!

1) Vertical eddy diffusivity•mean annual

•lack of stratification

Model parameters

3) Downwelling volumes: 5÷65 km3/event (uniform distribution)

2) Probability distribution of the energy transmitted by the wind•summer•winter

Result of calibration

temperature

CFC-12

15 February

Long-term asymptotic trend (800 years)

Boundary conditions: present state

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

Main statistics (600 years)

Analysis of downwelling dynamics

15 May

15 June 15 December

15 September

Climate change scenarios

Estimates of single aspects, e.g. temperature (Magnuson et al., 2000; Hampton et al., 2008), lack of data about wind

wind forcing increase/decrease

superficial temperature increase (global warming):

•+4°C (summer), +2°C (autumn)

•reduction of ice-covered period

iceice

iceice

5 days 11 days

Possible effects of climate change

warming (+2/4°C)

weak wind

strong windstrong 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 eco-biology 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.