Download 9. Oceans and Climate

9. Oceans and Climate
1.
The oceans are divided into two main regions that are of climatic
importance. The surface well-mixed waters of the ocean are known
as the mixed layer (up to depths of 100s of meters), which is
separated from the cold deep ocean by a thin boundary called the
thermocline. The reaction time of the mixed layer to atmospheric
perturbations is on the scale of months, while the deep ocean reacts
on time scales of decades to centuries.
2.
Storage of heat: the atmosphere has a pressure of approximately
1 atm = 1kg/m2. Heat storage in the atmosphere (greenhouse effect)
is primarily due to water vapour. The amount of water vapour in the
atmosphere is equivalent to approximately 2.5cm of water).
The oceans mixed layer thickness is about 100 meters, so for the
water vapour loading of the atmosphere, the top layer of the ocean
has (2.5cm x 4000 = 100 m) 4000 times more heat storage
capability than the atmosphere. Therefore, to raise the temperature
of the ocean mixed layer and the atmosphere both 1oC, the oceans
need a heat input dQ at least 4000 times larger than the atmosphere.
Since re-radiation is a function of temperature, the ocean will lose its
heat by re-radiation slower than the atmosphere – for a 1oC air
temperature change, the oceans will change by 1/4000oC, resulting in a
re-radiation from the oceans of 4x10-15 relative to the air (~T4).
Diffusion of heat from the mixed layer into the deep ocean results
in long delays in the climate system reaching equilibrium.
Precipitation – Evaporation (m/yr)
3.
Transport of heat: The ocean currents are basically caused by the
uneven heating of the earth’s surface. This uneven heating results in
the atmospheric wind patterns that drive the currents, but also results
in temperature gradients in the ocean surface waters. In addition, the
currents in the oceans are limited by the continental boundaries,
which guide the currents north and south. However, the role of
ocean currents is to reduce temperature gradients in the ocean by
mixing cold and warm waters. The atmosphere and oceans are equal
partners in the amount of heat they transport poleward.
Sea Surface Temperature
In the deep oceans circulation patterns are determined by water density
that are determined by temperature and salinity.
: 26 = 1.026 g/cc
The deep ocean circulation has been compared to a giant “conveyor belt”
with timescales of hundreds to thousands of years. The driving force
of the global ocean conveyor belt is thought to be the formation of
North Atlantic Deep Water (NADW) due to large fresh water fluxes
(from precipitation) into the ocean in the northern Atlantic. The fresh
water reduces the surface salinity, resulting in the sinking of the salty
Gulf Stream waters to great depths. Changes in the NADW
patterns, and the strength of the conveyor belt, have direct effects on
European climate, and perhaps global climate. The source regions of the
NADW are around Iceland and the Labrador Sea.
Deep Ocean Circulation
Ocean Conveyor Belt
4.
Ocean-Atmosphere fluxes: vertical fluxes of both heat and moisture
from the ocean into the atmosphere affect the climate by reducing the
static stability. The parameterization for these fluxes is:
Fh =  cp (Tw-Ta) v ch
Fq =  (qw – qa) v cq
T is temperature for water, air
v is wind velocity
q is specific humidity
ch,cq are the drag coefficients
The drag coefficients are themselves functions of the temperature
difference and the wind speed. If the ocean is colder than the air it
indicates high stability, and the drag coefficients are small.
1993-2003 change in ocean heat content
Moisture Flux
Fluxes impact hurricane development
5.
Diffusion effects: gases released to the atmosphere (especially CO2)
may diffuse into the ocean if the ocean is undersaturated with respect
to that gas. CO2 can diffuse into the ocean because the dissolved
CO2 in the sea makes up only a small fraction of the dissolved
inorganic carbon, the major part being bicarbonate (HCO3-) and
carbonate (CO3=) ions. The reactions are:
CO2 + H2O  H+ + HCO3
-
-
HCO3  H+ +CO3=
The equilibrium concentration of CO2 in air is proportional to the
dissolved CO2 (in the form of H2CO3 – carbonic acid).
It takes a 10% increase in the CO2 partial pressure in the atmosphere
in order to increase the CO2 content of the sea water by 1%. In
addition, the temperature of the oceans also determines the amount
of CO2 that can be absorbed from the atmosphere. Warmer water
can hold less CO2.
Ocean Acidification
6.
The atmosphere affects the ocean through a momentum flux as well:
In the tropics winds, and hence ocean currents are generally from the
east to the west (as a result of the earth’s rotation and Corioli’s force).
This results in water being transported to the western boundaries of
the oceans, where the waters are then transported polewards (e.g.
Gulf Stream, Kuroshio Current). Ocean currents such as the Gulf
Stream are responsible for transporting excess heat from the tropics
to higher latitudes, thus maintaining the earth’s thermal equilibrium.
On the eastern side of the
oceans the wind stress on the
ocean surface results in denser
(colder) water rising from the
deep. Thus strong winds are
associated with oceanic
upwelling of cold water (e.g.
upwelling off the coast of Peru,
Ecuador – El Nino). In the
deep ocean there is normally
a reverse current resulting in a
closed circulation cell.
7.
Walker Circulation: This is an atmospheric circulation closely
associated with the ocean surface temperature distribution. Due to
winds blowing offshore (easterlies) in the eastern Pacific, with
resultant upwelling, the equatorial waters off the coast of South
America tend to be some 10oC colder than the waters in the western
tropical Pacific. The air is stabilized by the cold water, and is too
heavy to rise and join the Hadley circulation. In the west, the warm
surface waters transfer moisture and heat to the atmosphere, resulting
in the destabilizing of the atmosphere. This rising air in the west,
sinking air in the east, and surface easterlies on the equator result in
the Walker circulation.
8.
El Nino: every 2-7 years, the sea surface temperature in the eastern
equatorial Pacific warms some 4oC, with local and global effects
on the weather patterns. The periodic warming is caused by oceanic
waves (Kelvin and Rossby waves) that take months to years to
propagate back and forth between the western and eastern tropical
Pacific. These waves tend to deepen the mixed layer depth in the
eastern Pacific, causing a reduction in the fish population near the
surface. Since this warming normally reaches its maximum around
Christmas time, the Peruvian and Ecuadorian fishermen named
this phenomenon El Nino, spanish for the “Christ child”.
Normal
El Nino
The warming of the waters in the eastern Pacific weakens and even
reverses the Walker circulation, resulting in dry conditions in Indonesia,
Northern Australia, SE Asia, while causing heavy rainfall in the eastern
and central Pacific. While the Walker circulation (east-west circulation)
decreases in intensity, the intensity of the Hadley circulation (northsouth circulation) increases due to the enhanced equator-pole temperature
gradient, influencing weather at higher latitudes.
9.
Long term sea level rise due to climate change can be a result of:
I) thermal expansion of sea water
II) melting of glaciers and ice caps
Since 1880, global sea level has risen nearly 15 cm.
Changes in Sea Level
Homework:
Barnett, et al., 2005: Penetration of Human-induced warming into
the World’s Oceans, Science, 309, 284-287.