Thermohaline and other Catastrophes

Changing the conveyor belt The younger Dryas period Draining lake Agassiz Heinrich oscillations El Niño and La Niña

Indication of shifts in Labrador current The waters of the Labrador Current have a cooling effect on the Atlantic coast from Quebec to Massachusetts. A change of its thermohaline circulation will impact the Gulf stream. This would particularly affect western European countries. There is some evidence for the stability of the Gulf Stream but also evidence for a weakening of the Labrador current.

Water temperature pattern Eckman currents and swirls in the Labrador sea The trend of the velocities (m/s per decade). The trend indicates a weakening of the gyre.

http://www.nasa.gov/centers/goddard/news/topstory/2004/0415gyre.html

Geostrophic gyres, indicating the variability of high altitude regions. The blue range suggests a slowing of the Labrador gyre

Based on NASA observation a weakening of the Labrador gyre flow appeared, possibly influencing water conditions and direction of North Atlantic flow! 30% reduction in Gulf stream observed in 2005. The scientific discussion rages on.

The North Atlantic Conveyor Belt

The last ice age, glacial period lasted from 110,000 to 14,000 years ago (Milankovitch cycle). A thick ice shelf covered most of the present area of Northern Europe, Greenland, Canada, and the United States. During this period the down-welling of the Atlantic conveyor belt was located significantly further south.

With the end of the last glacial period ~14,000 years ago, atmospheric temperatures increased, the ice shelf started melting and the location of the down-welling of the conveyor belt moved again northwards to the present Iceland polar sea region. But subsequent temperature fluctuations seem to indicate a disruption in the thermocline cycle.

Cooler and less salty water environment

Warmer and saltier water environment

The observational evidence

Plant pollen in lake sediments and bog peat

Dryas octopetala

18O abundance distribution in GISP2 ice core

The younger Dryas

Dryas octopetala

ice age

Multiple indications for sudden cold period about 12,000 years ago following the short postglacial warming period.

Several explanations offered by scientists, including volcano eruption and asteroid impact. One explanation was the change of the thermohaline cycle due to post ice age melting processes with a sudden release of fresh water into Northern Atlantic.

Heinrich events

Lake Agassiz - 12,000 years ago

The drainage of Lake Agassiz

Tyrrell Sea

Clearwater-Lower Athabasca Spillway CLAS Through Lake McConnell ad Mackenzie River through Beaufort Gyre into North Atlantic

Dimensions of Lake Agassiz Surface area and volume varied largely over time due to melting and lake merging processes. Rough estimates were obtained by geological analysis of former sea floor level

S≈350,600 km2 V≈38,700km3

(about d=110 m depth)

Estimated release of 21.000 km3 of water which translates into a drop of sea level of about 52 m. The drainage time is t ≥ 500 days (1.5-3 years) with an estimated flood discharge of up to Q=2.16·106 m3/s. [Japan Tsunami (h ≈ 8 m L ≈ 50 km, v ≈ 50 km/h: Q ≈ 5.56·106 m3/s)]

This translates into a total discharge of 21,000 km3 of water into Hudson Bay (V=123,000 km3) over 3 years reducing the average salinity of North Atlantic waters (V≈10,000,000 km3).

Model simulations have difficulties in reproducing the observed rapidity of the cooling and extension of the younger Dryas event.

vAQDischarge :

A: cross sectional area for flow, v: flow velocity

dSVVolume :

Multiple Observational Evidence

Climate changes associated with the Younger Dryas, high-lighed here by the light blue bar, include (from top to bottom): cooling and decreased snow accumulation in Greenland, cooling in the tropical Cariaco Basin, and warming in Antarctica. Also shown is the flux of meltwater from the Laurentide Ice Sheet down the St. Lawrence River. Not shown is the anticipated short term fresh water release from Lake Agassiz which would have occurred at 13000 bp. https://www.ncdc.noaa.gov/paleo/abrupt/data4.html

Heinrich Events Heinrich events are interpreted as melting processes of the Laurentide Ice Sheet producing surges of an icebergs into the Hudson Strait. The melting of icebergs releases coarse-grained sediments that is being deposited far from continental margins. The ice-rafted sedimentation in the North Atlantic shows evidence of sharply defined episodes of high sedimentation.

Ocean Belt Oscillations Oscillation theory predicts several quasi stable modes for ocean belt. Observational evidence are the so-called Heinrich events that are explained as short warm ice-melting periods over the last ice age triggering belt oscillations. Rapid change between modes is predicted by model simulations. Changes can be triggered by large inflow of fresh water supply such as rainfall, rivers, ice melting, which alter the salinity level. The turn-off of the ocean belt by sudden release of fresh water causes an increase in glaciation, which in turn freezes the fresh water and which leads to a gradual increase in salinity, which eventually re-establishes the conveyor belt operation.

.

Box Model of Circulation Pattern

T2

S2

warm

T1

S1

cool

H H

surface flow q>0

abyssal flow q<0

low latitudes high latitudes

Solar heating H in equatorial regions and heat loss in arctic regions defines the temperatures T2 and T1 (T2 > T1), respectively. Subsequent evaporation affects the salinity S of ocean water (S2>S1); q represents the flow of water, which is considered positive in the pole-ward direction.

qkk

q

SqHSSqHdt

dS

SqHSSqHdt

dS

0

12

0

212

121

The change of salinity with time depends on the flow q, the heat H that causes evaporation, (and the mixing with melt water). The flow depends on the density conditions with 0 being the reference density and the friction term k with the surrounding water at rest. .

ST 10

is the thermal expansion coefficient and the haline contraction (since an increase in S increases the density while an increase in T reduces the density.)

0222212

0

SSTkHSqHdt

dS

dt

dS

STkk

q

Temperature and salinity influence flow in opposite directions (sensitive balance)!

Steady state condition for the change of salinity

2

2

4

1

2

1

0022

,0

Tk

HTS

k

HTSSorSSTkH

STqfor

There are two steady state solutions to which the system will return if perturbed and one solution, which would change the steady state situation.

The difference in salinity S depends on the temperature difference and the heating (cooling) term H!

presently existing temperature difference driven flow

The motion is from regions of greater to lesser heating. The temperature difference between equatorial and arctic regions is considered constant. Two new variables are introduced: as non-dimensional salinity difference and E as a dimensionless salinity flux.

E

E

Tk

HE

T

STkH

4112

1

4112

1

4

2

1

2

2

Two temperature driven solutions for quadratic equation for the salinity difference in terms of a salinity flux E, providing a steady thermohaline flow.

E

Tk

HTS

k

HTSS

SSTkH

STq

4112

1

4

1

2

1

0

0

0

3

2

2

Third, presently non existing possibility of a salinity difference driven thermohaline flow!

The solid curve in the -E diagram demonstrates the equilibrium solutions for a steady flow. For < 0.5 and > 1 the steady state re-establishes after a perturbation occurred. For the middle range, 0.5 < < 1 a new equilibrium will establish. For high salinity flux E>0.25 only a salinity driven flow is possible.

In glacial periods T becomes large and E small, giving two solutions depending on salinity difference between pole and equator!

1

2

3

Calculate the three possible salinity differences 1, 2, and 3 for a given salinity flux E=0.1 and E=0.3

2413.009.11.04112

1

3.0887.01.04112

1

3.0113.01.04112

1

113

112

111

.EEE

possiblenotEEE

possiblenotEEE

Mixing large amounts of fresh water into polar ocean regions would increase salinity difference at constant temperature difference switching from 1 to 2 . Depending on the salinity flux E this mode 2 would switch back to temperature driven mode 1 (younger Dryas period) but may also change to salinity driven mode 3 with dramatic consequences.

Salinity driven flow 3 is different from present existing conditions 1! The flow is reversed, salty water sinks already in the warm box (at lower latitudes) and flows as abyssal current towards the poles where up-welling occurs (instead of down-welling), pushing cold water as surface flow towards equatorial regions. This can occur if very huge amounts of fresh water are released by e.g. melting of ice shelves. This would also cause extensive glaciation at lower latitudes – initiating a new ice age.

Rapid melting of Greenland and arctic ice shelf raises concern about impact on the operation of the conveyor belt since the release of melt water with low salinity may push down-welling location further south triggering a new area of cooling and glaciation in the northern hemisphere.

Arctic melting

Retreating Arctic ice, 1979-2003, NASA observations

Long Range Predictions of Future

Rising waters

El Nino Southern Oscillation ENSO ENSO is known to be a natural oscillation of the ocean-atmosphere system with El Nino as the ocean component of the interaction between the upper layers of the tropical Southern Pacific and the atmosphere. ENSO appears as enhanced sea surface temperature in the Easter Pacific near the Peruvian coast. The water temperature is usually dominated by the cold water flow of the Humboldt current. The ENSO effect enhances temperatures and reduces biological productivity. The effect is observed around Christmas about every seven years and was therefore dubbed El Nino (the son) by the local fishermen.

Normal conditions ENSO conditions The Southwestern Pacific hot ocean water region is typically contained by trade winds blowing from the east. These winds change directions every 7 years expanding the warm water zone eastwards.

L corresponds to low atmospheric pressure, H to high atmospheric pressure.

Normally low pressure region over Australia and Indonesia delivering rainfall, in El Nino years, there is high pressure over these regions and the low pressure area moves east and rainfall occurs over the Pacific Ocean.

The Southern Oscillation Index SOI is defined in terms of the Mean Sea Level Pressure P between Tahiti and Darwin, Australia. SOI indicates El Nino episodes:

positiveSOIPPENSONon

negativeSOIPPENSO

PP

PPPPSOI

TahitiDarwin

TahitiDarwin

monthDarwinTahiti

meanDarwinTahitimonthDarwinTahiti

:

:

10

Computed at a monthly basis SOI ranging from -3 to +3

http://www.ncdc.noaa.gov/teleconnections/enso/indicators/soi.php

There is a directly observable correlation between the SOI and the water surface temperature SST of the coast of South America!

La Nina is the name chosen for a situation where strong trade winds confine the hot water condition to the southwestern region of the Pacific.

1. The trigger of the El Nino event are recurrent spontaneous statistical fluctuations sea surface temperature reflecting different oscillation modes in the coupled ocean SST system.

2. This SST fluctuation causes a spontaneous reduction of the the east west pressure gradient in the atmosphere.

3. Changed atmospheric pressure gradient reduces wind (collapse of trade winds as already observed by Darwin on his journey on the Beagle) and wind driven drag of water westwards

4. Reduced drag allows warm water to expand eastwards preventing cold water up-welling from Humboldt current.

The ENSO occurrence has global consequences, it changes rain fall patterns in Africa and North America and causes droughts in Australia. The ENSO event is recorded by tree ring, ice core, and coral reef analysis for millennia.

ENSO driven draught in Australia ENSO driven rainstorm in Southern California

US Tornado Season

There are statistics based arguments that the number of Tornados in the US tornado alley is correlated with El Niño and La Niña conditions in the Southern Pacific Ocean.