this week (6)— present day climate continued atmosphere-ocean couplings and climate feedbacks...

This Week (6)—Present Day Climate Continued

Atmosphere-Ocean Couplings and Climate Feedbacks

•Atmosphere-Ocean Couplings 1. Water cycle

•Atmosphere-Ocean Couplings 2. Wind Stress-Ocean Surface Circulation

•Thermohaline Circulation: Another Big Pump

•Forcings and Feedbacks in the Climate System

Reading chapter 5 of your textA problem set on atmospheric and oceanic circulationsA reading on natural climate regulation

Today—Coupled Role of Atmosphere and Ocean

•What have we accomplished so far?

•The water cycle: an obvious ocean-atmosphere coupling important for regional and global climate

•Sea Surface Temperature: Looks like solar radiation input, but not quite—atmosphere and ocean exchange heat and momentum

What have we accomplished?

We have been developing the physical basis of present day climate, and an understanding of the physical mechanisms that can cause climate change.

Arrived at a simple, but robust, equation that relates incoming and outgoing radiation to global average T using blackbody radiation physics.

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Changes in solar radiation, Earth’s albedo, and atmospheric absorptivity can alter global average T.

14

1

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Could you have predicted the IPCC recent conclusions?


Present day climate is an average of the weather. Weather is the result of the energy budget not being in balance on a daily and seasonal time scale.

Solar radiation, F=ma, and a rotating spherical planet described general circulation patterns in atmosphere that bring about regional weather and thus climate.

Tropical trade winds, wet and dry seasons in the tropics, deserts at 30N and 30S, mid-latitude storms

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Atmospheric circulations are directly relevant to the the global energy balance and the greenhouse effect.

Vertical motions set the T decrease w/altitude, lead to cloud formation and precipitation. Horizontal motions transport warm wet air northward and cold dry air south.

•Extra-tropical climates depend on heat/moisture transport by atmosphere.

•Clouds affect global climate in a complicated way:•low thick clouds net reflect incoming radiation (coolers) •high-thin clouds net absorb outgoing radiation (warmers)

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Atmosphere-Ocean Couplings

Important Couplings

1. Moisture/Gas Exchange (water cycle)

2. Momentum Exchange (surface wind stress)

3. Heat Exchange

Atmosphere-Ocean Coupling 1: Water Cycle

Key Points

1. Most water evaporates from tropical oceans

2. Most water precipitates over tropical oceans

3. Seasonal variations in precipitation driven by radiation/general atmospheric circulation patterns

4. Water for terrestrial biosphere depends strongly on circulation patterns and (precipitation minus evaporation)

Precipitable Water Distribution

Precipitable water is the total amount of water in a column

Follows seasonal and latitudinal distribution of solar radiation

Precipitable water greatest over oceans, in tropics.

Spatially Resolved Precipitation Rate

Precipitation rate largest over tropics (ITCZ)Note winter NH midlatitude storms over Pacific and Atlantic

Subsiding branches of Hadley Cells

Precipitation Rate Minus Evaporation Rate

green (positive) means surface net gaining wateryellow/brown (negative) means surface net loosing water

Water Cycle and Terrestrial Biosphere

Tundra

Forest

Grassland

Temperature0 oC

Pre

cipit

ati

on

Terrestrial biosphere depends strongly on precipitation and mean temperature which drives evaporation.

Temperature axis can be a “global warming” axis

Water Cycle “Box Model”Burden: amount of material in reservoir

Reservoir: region where material stored; each box

Source/Sink: flow rate into/out of reservoir

Residence time of H2O in atmosphere = 0.013x1015m3/423x1012 m3 yr-1

11 days!; atmospheric water responds quickly to changes in evaporation

Questions

• How long does water reside in the ocean before evaporating?

• Global warming will lead to more evaporation, and thus higher precipitable water, and so more precipitation on average. Does this mean the deserts will get smaller?

Atmosphere-Ocean Coupling 2. Wind-driven Surface Ocean

Circulation

1. Friction of air flow over ocean surface creates drag force which pulls surface water (first ~ 100 m) with it.

2. Circulation of surface water generally follows mean wind direction (but at an angle).

3. Important for the transport of heat and exchange of heat with atmosphere

Key Points

Heat Transport by Ocean and Atmosphere

Tropics MidlatitudesPolar regions

Surface Winds and PressureIn the tropics, 30N – 30S, surface air flows towards the ITCZ, impacted by Coriolis effect and friction.

These are the “trades” (northeasterly in NH, southeasterly in SH)

In mid-latitudes, surface westerly flow





•Deep Ocean Circulation: Another Big Pump


Reading chapter 5 of your textA problem set on atmospheric and oceanic circulationsA reading on natural climate regulation

Upcoming Seminars of InterestThurs. Feb 8 3:30 pm Dr. Ann Fridlind, “Ice formation in mixed-phase Arctic boundary-layer clouds,” JHN 075

Fri. Feb 9 3:30 pm Prof. Steve Warren, “Black Carbon in Arctic Snow and Ice, and Its Effect on Surface Albedo,” JHN 175

Thurs. Feb 15 3:30 pm Dr. Dargan Frierson, “Tropical Circulations in a Hierarchy of Atmospheric Models,” JHN 075

Fri. Feb 16 3:30 pm Dr. Philip Mote, “IPCC Fourth Assessment Report: its significance and the inside story,” JHN 175

Today—Coupled Role of Atmosphere and Ocean

•Surface ocean circulation—upwelling/downwelling

•Sea Surface Temperature: Looks like solar radiation input, but not quite

•Thermo-haline Circulation—how it works and its importance in climate

Atmosphere-Ocean Couplings

Important Couplings

1. Moisture/Gas Exchange (water cycle)

2. Momentum Exchange (surface wind stress)

3. Heat Exchange

Review Surface Ocean CirculationResponds to wind-stress (friction of air flow over

surface drags surface water)

Runs into continents—diverges to complete a “gyre”

Surface Ocean Circulation

Gulf Stream

Gulf Stream is warm surface water heading north on western branch of mid-Atlantic gyre

Carries heat northward, thought to help moderate climate of north Atlantic coastal regions

AVHRR Satellite measurement of SST

Convergence Divergence Downwelling Upwelling

Friction and coriolis force flow ~ 20-40o to right of wind direction in NH (left in SH)—Eckman TransportLeads to areas of convergence (in center of gyres) and divergence (eastern ocean boundaries, near equator)

Equator

Wind

Surface ocean

Divergence and Upwelling At Equator

What vertical motion would convergence induce?

Questions1. As warmer tropical water flows to higher latitudes, it loses heat to the atmosphere. Similarly, cold air blowing from the north removes heat from the ocean.

Where would you expect the colder SSTs, on the western or eastern boundaries of the tropical gyres?

2. Does upwelling or downwelling change this picture at all?

monthly mean SST animation

1. Latitudinal distribution of solar radiation2. Heat exchange with atmosphere3. Circulation patterns (e.g. upwelling)

Sea Surface Temperatures

ftp://ftp.atmos.washington.edu/thornton/ATMS_211/monthsst.ani.gif

Coastal Upwelling/Downwelling

A Better Understanding of Marine Stratus

Flow of water away from coast, draws up cold water from below. Cold water cools air, causes cloud formation.

Surface winds

Ocean surface flow

Deep Ocean CirculationKey Points

1. Unlike atmosphere, ocean is heated from top. Vertical mixing by buoyancy suppressed.

2. Salt and temperature determine water density.

3. Thermo-haline Circulation (THC): cold, salty water sinks at high latitudes forming deep ocean water, upwelling brings deep water up where it warms and heads to higher latitudes to complete circuit.

4. Due to suppressed vertical mixing, circulation times in the ocean are very slow (~ 1000 years to complete circuit).

Water Density

Like the atmosphere, density differences drive vertical transport in the ocean.

Water becomes less dense the warmer it is.

Dissolved salt increases ocean water density.

Mean ocean water density of 1.024 to 1.028 g/cm3

salinity of 34.4 g salt/kg ocean water

Major salt ions: Na+, Ca2+, Mg2+, Cl-, SO42-

(sodium, calcium, magnesium, chloride, sulfate)

Questions1. Does density increase or decrease with depth in

ocean?

2. Does temperature increase or decrease with depth in ocean?

3. Where, latitudinally, are the saltiest regions of the ocean?

Salinity is measured in parts per thousand

Salinity

Thermo-haline Circulation(temperature-salty)

Mixed layer ~ 1 km deep

Middle and deep ocean

Lower latitudes High latitudes

Net sinking: Deep Water formation

Ocean-Atm heat transfer

Sea ice

Cold salty water

•Reduces the influence of the winds

•Insulates the ocean (prevents it from losing heat)

•Rejects salt when it grows / Adds freshwater when it melts

Sea ice influence on the ocean







Reading chapter 5, (2), 3 (p. 51-53) of your textA problem set due Monday Feb. 12A reading on natural climate regulation—see Steve

Today—Forcings and Feedbacks

•Finish Thermo-haline Circulation—its importance in climate

•Radiative Forcings—Definition and example

•Anthropogenic Radiative Forcings•GHG•Aerosols (Direct and Indirect)•Land Use Changes

Thermo-haline Circulation (THC)Deep ocean circulation

A conceptual idea

THC Importance to Climate

1. Deep ocean is an enormous reservoir for heat

2. Overturning brings nutrients up to surface biota

3. Overturning allows greater uptake of gases like CO2 which would otherwise saturate the surface layer

4. Maintains transport of heat to higher latitudes helping to moderate latitudinal temperature gradients

THC Importance to Climate

Temperature proxy record in ice cores show rapid climate change. More negative, colder.

“Younger Dryas” 12,000 – 11,000 years before present a period of glaciation thought to be due to THC slowing or shutdown

T increasing

Question

1. What could cause the THC to slow, or shutdown so quickly?

2. How might global warming affect the THC?

Extra-Credit ActivityTurn in w/Assignment 3

•Think of a location other than Seattle where you have lived.

Characterize the climate of that region.

E.g. Was it rainy in the summer, dry in the winter, or vice versa? Was it humid and tropical with lush vegetation, or was it arid and dry? Always foggy in the summer morning, but clear and sunny during the winter?

Come up with ideas as to why the climate might have been that way.

Climate System Survey

•We have taken a close look at the atmosphere and a large part of the hydrosphere (oceans/sea ice)—two important components of the climate system.

•These two sub-systems and their interaction generally set dominant climate patterns on short time scales (years – decades).

•Longer scale climate change requires consideration of interactions with the biosphere and lithosphere.

Climate Forcings

a perturbation, directly or indirectly, affecting Earth’s energy budget

Temperature Temperature

FINFIN + F

FOUT FOUT

Climate Forcings


FIN + F = FOUT

Initial state: FIN = FOUT

Forcing turned on:

FINnew = FOUT

new

If forcing constant eventually new equilibrium reached:

F = FIN - FOUT

F can be positive or negative

Questions1. The formal definition of a radiative forcing is the

temporary imbalance a perturbation causes in the energy budget: F = FIN – FOUT. If F is less than zero is does it cause warming or cooling?

2. Give an example of a natural (non-human) forcing and an anthropogenic forcing on climate.

3. Suppose Earth’s albedo increased, e.g. 0.28 0.3, is this a positive or negative forcing?

4. I thought we just spent two weeks discussing the consequences of Fin never equaling Fout at any latitude. Why are we now assuming Fin = Fout unless there’s a forcing?

Climate Forcings


Natural Forcings: solar radiation output changes in earth’s orbit volcanic eruptions non-human biota change

Anthropogenic Forcings: greenhouse gasesaerosol particlesland-use changes

Greenhouse Gas Forcing

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Increasing GHG concentration (or mixing ratio) causes to increase

FGHG > 0 Warming, but how big a forcing?

need to know how much increases for a given increase in GHG concentration

Often nonlinear relationship (band saturation!) so not trivial to calculate.

FGHG ~ 2.5 W/m2 (all GHGs since 1750)

Scattering of Radiation by AerosolAerosol particles: tiny suspensions of liquids and solids that range from ~ 0.003 - 10 microns in size

Scatter light most efficiently when wavelength of light similar to size of particle

Typical U.S. Aerosol Size Distributions

Freshurban

Agedurban

rural

remoteWarneck [1999]

Maxima are most common sizes

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Reading chapter 5, (2), 3 (p. 51-53) of your textA problem set due Monday Feb. 12A reading on natural climate regulation—see Steve

Midterm 1 Distribution

Today—Forcings and Feedbacks

•Finish Radiative Forcings

•Feedbacks—Definition and Terminology

•Examples, Examples, Examples•Ice-Albedo Feedback•Water Vapor Feedback•Cloud Feedbacks• Phytoplankton-DMS Feedback

Visibility Reduction by Aerosol (Haze) Scattering

•

Acadia National Park (Northeastern Maine)

clean day moderately polluted day

http://www.hazecam.net/

modis.gsfc.nasa.gov

Smoke particles from biomass burning in Southeast Asia appear as white haze

F ~ - 0.9 W/m2 from direct effect of aerosol

Aerosols Increase Earth’s Albedo

Aerosols scatter a fraction of incoming solar radiation back to space

This is known as the “direct forcing” of aerosols.

Fdirect is negative; but difficult to quantify, why?

Ship Tracks

Off the west coast of the U.S.

“Sulfate Forcing” Mid 20th Century

Aerosol direct effect thought to explain temporary hiatus in T increase

Global Radiative Forcing of Climate Since 1750

IPCC [2001]

To F

Climate Sensitivity

T F

Proportionality constant describes how sensitive climate (global T) is to a forcing or sum of all forcings.

T = F

is climate sensitivity parameter; units: K m2 W-1

determined by feedbacks!

Questions

1. Why do aerosols affect albedo (I.e. incoming radiation?) and greenhouse gases affect outgoing radiation? I.e., why don’t aerosols scatter both?

2. Aerosols are thought to be harmful to human health. They also reduce visibility which affects aviation and national park enjoyment. The EPA is considering reducing the allowed aerosol concentrations any municipality can have in its air.

Will improving air quality enhance, reduce, or have no affect on global warming?

Feedbacks

Cause and effect loops which amplify or dampen initial effect of a forcing.

Typically involve multiple subsystems or components that are coupled by a state variable.

Initial ForcingState Variable Process

or coupling

+/-

+/-

+ increases state variable

- decreases state variable

“feedback loop”

Feedbacks—Key Points

1. Feedbacks (complex cause and effect loops) ultimately control the response of the climate system to a forcing.

2. Feedbacks are the result of the internal sub-systems being coupled.

3. Many feedbacks are poorly understood in terms of the degree of amplification or dampening that will result

4. These uncertainties translates into our models of past and future climate change.

IR Flux--Temperature Feedback

Initial Forcing(e.g. GHG)

Temperature

Example of a negative feedback

What happens if initial forcing causes a decrease in temperature, still negative feedback?

+ Outgoing IR flux increases

+

-

Ice--Albedo Feedback

Initial Forcing(e.g. GHG)

Temperature

Ice melts, dark soils exposed

Example of a positive feedback

What happens if initial forcing causes a decrease in temperature, still positive feedback?

+Albedo

-

+

Feedbacks and Climate Stability

Positive feedbacks are destabilizing

Small initial forcing can snowball into big

effect Negative feedbacks are stabilizing

Large initial forcing can be reigned in

Water Vapor--Temperature Feedback

Initial Forcing(e.g. GHG, solar radiation)

Temperature+

Water Vapor

+

+

Atmosphere holds more water

Increased Greenhouse effect

Atmosphere—Protector of the Oceans?

water trap

If H2O reaches top of atmosphere it is blown apart by UV radiation

H atoms escape to space, never to return

Probable cause for no H2O on Venus

Questions

1. What was the state variable in all of our feedback examples so far? What is a feedback that involves water vapor as the state variable?

2. With temperature as a state variable and cloudiness as a coupling that feeds back on temperature, construct a negative feedback loop.

3. Is there a positive feedback loop involving cloudiness?

Clouds and Cloud Feedback

Initial Forcing(e.g. GHG, solar radiation)

Temperature+Low Clouds+

-


Increased albedo

Temperature+ High

Clouds+

+


Increased greenhouse effect

Uncertain!

Cloud Feedback: Cause of Model Disagreements?

14 models each demonstrate a different climate sensitivity ()

Different climate sensitivities scale with differences in how each model calculates cloud feedback!

Take home: cloud feedback uncertain, but water vapor and ice-albedo feedbacks probably well understood.

Phytoplankton—DMS--Marine Cloud Feedback

Photosynthesis Dimethyl Sulfide**DMS

aerosolscloudines

s

**DMS doesn’t form aerosolsConverted to sulfuric acid which does


Initial Forcing(decreased clouds)

Solar Radiation(Temperature)

+DMS

+

-

Charlson, Lovelock, Andrea, Warren“C.L.A.W.” Hypothesis

Photosynthesis

+

aerosols and cloudiness+


Take home point: can’t ignore biosphere!

Microbes play a large role in sulfur cycle (aerosols), carbon cycle (CO2), stratospheric O3, …

this week (6)— present day climate continued atmosphere-ocean couplings and climate feedbacks...

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