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Objectives understand differences between ocean and atmosphere that cause the ocean to be the “memory” of the climate system. Understand how atmosphere and ocean are forced and how they interact. Investigate some large scale climate patterns. Outline:
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Relative thicknesses of earth, ocean and troposphere (lower atmosphere).
Consequence: Ocean and atmosphere motions are mostly 2-dimensional.
- density and energy content
Energy = mass x heat capacity x temperature (broadly)
Heat capacity of air ~ 1000 J/kg deg K
Density of air ~ 1.2 kg/m3 at sea level
Heat capacity of water ~ 4000 J/kg deg K
Density of water is 1000 kg/m3
So, 1 meter of water has roughly as much stored energy as the whole lower atmosphere (troposphere)
-ocean slower, more massive, has longer timescales (more “memory”).
Blue dashed line is outgoing radiation, red line is incoming radiation. So why aren’t tropics getting steadily hotter and poles steadily colder?
Consequence: Ocean and atmosphere motions are driven by thermal gradient (difference between Tequator and Tpole).
Consequence: Ocean is heated from above, Atmosphere is heated from below. Ocean is more stable than the atmosphere.
Heat from: sun (shortwave), evaporation and precipitation (latent), conduction (sensible), emission (longwave)
Freshwater: precipitation and evaporation
The only external input here is solar radiation. Otherwise, the interaction between ocean and atmosphere occurs as a system. What is lost by one component is gained by the other (or by the land surface or cryosphere).
Conservation of mass
In a perfect stable world, oceans would only carry heat within the gyres, not between them.
Consequences: What we observe rarely looks like the “mean” picture described. Ocean and atmosphere are turbulent and hard to predict.
(Thermal and freshwater forcing)
Ideal gas law PV=nRT…
And density = mass/volume
if you keep all constant then, T -> V but
P -> T
Also, as S ->
So warm water rises, cold water sinks
And fresh water rises, salty water sinks,
Density (or temperature) from South to North - pycnocline or thermocline = maximum upper ocean density or temperature gradient.
Consequences: Cold water made at poles encircles the globe, gradually upwelling to the surface. This also occurs at mid-latitudes. (Storing surface conditions for ~ 500 years)
Consequence: Relatively short changes in wind (several days), drive large ocean changes that affect ecosystems down to physics. Feedbacks!
October-March air temperature anomalies
Above average winter and spring
Temperatures in Northwestern North
America, below average temperatures
In the southeastern US.
Above average winter and spring
Rainfall in the southern US and
Northern Mexico, below average
Precipitation in the interior Pacific
Northwest and Great lake regions.
DJF Precipitation Anomalies
Table 1: summary of North American climate anomalies associated with extreme phases of the PDO.
Warm Phase PDO
Cool Phase PDO
Ocean surface temperatures in the northeastern and tropical Pacific
October-March northwestern North American air temperatures
October-March Southeastern US air temperatures
October-March southern US/Northern Mexico precipitation
October-March Northwestern North America and Great Lakes precipitation
Northwestern North American spring time snow pack
Winter and spring time flood risk in the Pacific Northwest
TOPEX/Poseidon satellite measures the sea surface height. The image shows
a horsehoe of higher than (warm) water in the western Pacific (red and white)
and lower than average (cool) blue and purple water in the eastern Pacific.
Warm PDO phase: enhanced coastal ocean productivity in Alaska
but inhibited productivity in Washington and Oregon.
Cold PDO phase: reversed fortune.
Quotes from News:
August/September 1972 (Pacific Fisherman)
“Bristol Bay (Alaska) salmon run a disaster”
“Gillnetters in the lower Columbia received an unexpected bonus when the largest run
of spring chinook since counting began in 1938 entered the river.”
1995 Yearbook (Pacific Fishing)
“Alaska set a new record for its salmon harvest in 1994, breaking the record set the year
“ Columbia spring chinook fishery shut down; west coast troll coho fishing around.
* The Positive NAO index phase shows a stronger than usual subtropical high pressure center and a deeper than normal Icelandic low.
* The increased pressure difference results in more and stronger winter storms crossing the Atlantic Ocean on a more northerly track.
* This results in warm and wet winters in Europe and in cold and dry winters in northern Canada and Greenland
* The eastern US experiences mild and wet winter conditions
* The negative NAO index phase shows a weak subtropical high and a weak Icelandic low.
* The reduced pressure gradient results in fewer and weaker winter storms crossing on a more west-east pathway.
* They bring moist air into the Mediterranean and cold air to northern Europe
* The US east coast experiences more cold air outbreaks and hence snowy weather conditions.
* Greenland, however, will have milder winter temperatures
source http://www.ldeo.columbia.edu/NAO by Martin Visbeck
The NAO index is defined as the anomalous difference between the polar low and the subtropical high during the winter season (December through March)
Instrumented records are limited in their ability to examine decadal to centennial scale climate variability. Paleo proxies, including tree rings, ice cores and corals, are now being used in an effort to determine the climate dynamics and forces involved.
Fig. 3. Mean spatial distribution of C. finmarchicus and C. helgolandicus abundances in the Northeast Atlantic and the North Sea, during years of low and high NAO. Colourscales are proportional to log-abundances of the species. Log-abundances of each species over that area have been estimated for each month from 1962 to 1992 by means of kriging interpolation. This procedure resulted in a series of regular maps from which values have been averaged over the periods of high and low NAO (respectively 60 and 72 months).
-Density! Leads to differences in energy
content, memory, types of forcing.
-Sun, but interactions between ocean and
atmosphere occur through momentum
(wind) forcing,and through buoyancy
(heating and freshwater).
-Local processes (short time scale),
ENSO 2-3 year timescale,
NAO, PDO decadal timescales.