To further that effort I have gone over the following talk and typed up another Unauthorized Notes. Trenberth is gracious enough to pack his talk with slides and outlines, so most of these notes are simply his slides transcribed. {curly brackets indicate my own comments}
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Dr. Kevin Trenberth: The Role of the Oceans in Climate
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March 9, 2011
SFUNews - on YouTube
Uploaded by SFUNews on Apr 14, 2011
The Role of the Oceans in Climate
Kevin Trenberth: Senior Scientist and Head of the Climate Analysis Section
National Center for Atmospheric Research, Boulder, Colorado. This lecture is part of SFU's 2011 global warming seminar series "Global Warming: A Science Perspective".
Regardless of one's perspective the effects of global warming are a quantifiable set of environmental results. That is why the SFU Dean of Science Office invited some of the world's leading scientists to present results of their research in this six-part series of talks. The series is designed to speak to a general audience of undergraduate and graduate students, faculty from across the Faculty of Science and the University and interested members of the public.
For more information, visit IRMACS
(Interdisciplinary Research in the Mathematical and Computational Sciences)
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Introduction
03:00 - . . . I’m not an oceanographer, I’m a meteorologist but I have been asked to speak here about the roll of the ocean and climate. . .
03:20 - Will talk mainly about the physical climate system...
03:30 - describing the roll of the different components of the climate system
04:00 - slide #1:
Role of Atmosphere
* The atmosphere is the most volatile component of the climate system
* Winds in jet streams exceed 100 mph or even 200 mph: winds move energy around.
* Thin envelope around planet 90% within 10 miles of surface 1/400th of the radius of Earth: clouds appear to hug the surface from space.
{or as I like to put it, “like the finest silk upon your arm.”}
* The atmosphere does not have much heat capacity.
* “Weather” occurs in troposphere (lowest part)
* Weather systems: cyclones, anticyclones, col and warm fronts tropical storms/hurricanes move heat around: mostly upwards and polewards.
~ ~ ~
5:00 - slide #2:
Role of Oceans
* The oceans cover 70.8% of the Earth’s surface.
* The oceans are wet: water vapor from the surface provides source for rainfall and thus latent heat energy to the atmosphere.
* The heat capacity of the atmosphere is equivalent to that of 3.5 meters of ocean.
The oceans slowly adjust to climate changes and can sequester heat for years.
* The ocean is well mixed to about 20 m depth in summer and over 100 m in winter. An overall average of 90 m would delay climate response by 6 years.
* Total ocean: mean depth 3800 meters.
* Would add delay of 230 if rapidly mixed. In reality, the response depends on rate of ventilation of water through the thermocline (vertical mixing).
* Estimate of delay overall is 10 to 100 years.
* The ocean currents redistribute heat, fresh water, and dissolved chemicals around the globe.
~ ~ ~
06:30 - The ocean is not in equilibrium with the climate. . .
06:50 - schematic of dynamic ocean currents . . .
07:15 - slide #3 -
The Role of Land
* Heat penetration into land with annual cycle is ~2 m.
* Heat capacity of land is much less than water:
- Specific heat of land 4.5 less than sea water
- For moist soil maybe factor of 2
* Land plays lesser role than oceans in storing heat.
Consequently:
* Surface air temperature changes over land are large and occur much faster than over the oceans.
~ ~ ~
08:30 - slide #4:
Role of Ice
Major ice sheet, e.g., Antarctica and Greenland. Penetration of heat occurs primarily through conduction.
-> The mass involved in changes from year to year is small but important on century scales.
Unlike land, ice melts -> changes in sea level on larger time-scales.
Ice volumes: 28,000,000 km2 water is in ice sheets, ice caps and glaciers.
Most is in the Antarctic ice sheet which, if melted, would increase sea level by ~65 meters, vs Greenland ~ 7 meters and other glaciers and ice caps 0.35 meter.
In Arctic: sea ice ~ 3-4 meters thick
Around Antacrctic ~ 1-2 meter thick
~ ~ ~
09:55 - slide #5
Role of Coupling
El Niño - Southern Oscillation ENSO
Some phenomena would not otherwise occur:
ENSO is a natural mode of the coupled ocean-atmosphere system
ENSO: EN (ocean) and SO (atmosphere) together:
Refers to whole cycle of warming and cooling
ENSO events have been going on for centuries
(records in corals, and in glacial ice in S. America)
ENSO arises from air-sea interactions in the tropical Pacific
El Niño: warm phase, La Niña: cold phase
EN events occur about every 3-7 years
~ ~ ~
10:40 - slide #6
Energy on Earth (a)
The main external influence on Planet Earth is from radiation.
Incoming solar shortwave radiation is unevenly distributed owing to the
geometry of Earth-sun system, and the rotation of the Earth.
Outgoing longwave radiation is more uniform.
~ ~ ~
11:45 - slide #7
Energy on Earth (b)
The incoming radiant energy is transformed into various forms (internal hear, potential energy, latent energy, kinetic energy) moved around in various ways primarily by the atmosphere and oceans, stored and sequestered in the ocean, land, and ice components of the climate system, and ultimately radiated back to space as infrared radiation.
~ ~ ~
12:45 - slide #7 continued
Energy on Earth
An equilibrium climate mandates a balance between the incoming and outgoing radiation and that the flows of energy are systematic. These drive the weather systems in the atmosphere, currents in the ocean, and fundamentally determine climate. And they can be perturbed, with climate change.
~ ~ ~
13:10 - schematic of atmosphere/ocean dynamics
{getting into the nitty gritty}
14:30 - slide #8
Top of atmosphere net radiation - global map
time lapse progression
15:25 - slide #9
Net Radiation TOA (top of atmosphere) - global map
16:50 - slide #10
Annual mean net surface flux - global map
(net energy going into the ocean)
17:05 - Trenberth’s presentation is interrupted by a lister question, questioner is inaudible. . .
17:20 - “The variation with latitude is actually pretty close to what you’d expect from the geometry as it turns out. There’s a small effect from changes in clouds, but it is very much secondary. . .
17:40 - {Trenberth returning to his presentation and slide #10}
18:20 - slide #11
Departures from annual mean: Equivalent ocean heat content
(Ignores annual cycle in ocean heat transports)
19:00 - slide #12
Total Upward Surface Energy Flux (net surface flux)
time-lapse map image
19:45 - slide #13
The Annual Cycle of Ocean Energy Tendency
(WOA, Climatology)
21:05 - slide #14
Ocean only
{cross sections}
21:45 - Divergence of the atmospheric energy transports
“Divergence out of the tropics, in both hemispheres tropics and sub-tropics and
Convergence into higher latitudes”
Atmosphere in action
{or as I like saying our global heat engine in action ;-) }
23:10 - slide #15
ERBE - period meridional energy transport
satellite measurements of top of atmosphere transport... radiative imbalance
25:00 - ocean current play key role in climate system
25:10 - The Changing Climate
25:20 - slide #16
The famous “Global Energy Flows” diagram 2009
within ‘this somewhat two dimensional view of this world’
overall energy flows within the climate system.
26:30 - overall energy imbalance positive = absorbing more than radiating
28:10 - slide #17
comparing ocean and land absorption of heat - “CERES period March 2000 to May 2004”
29:30 - slide #18
Global temperature and carbon dioxide anomalies through 2010
overall changes in temperature over time
30:25 - slide #19
Changes in SSTs zonally averaged relative to 1961-90
(ocean sea surface temperature)
32:10 - thermohaline and ocean circulation
33:00 - El Niño is the way the tropical Pacific cools itself.
33:30 - {Indian ocean most susceptible to global warming, comparing the three major ocean basins}
33:45 - slide #20
Global increases in SST are not uniform. Why
* Tropical Indian Ocean has warmed to be competitive as warmest part of global ocean.
* Tropical Pacific gets relief from global warming owing to ENSO?
* Atlantic has MOC/THC
The historical patterns of SST and NOT well simulated by coupled models!
Relates to ocean uptake of heat and ocean heat content.
The result is an imprint on global weather patterns:
~ ~ ~
34:20 - slide #21
Ocean heat content and sea level
Global warming from increasing greenhouse gases creates an imbalance in radiant
at the Top-Of-Atmosphere: now order 0.9 Wm-2.
Where does this heat go?
Main sink is ocean: thermosteric sea level rise associated with increasing ocean heat content.
Some melt sea ice: no change in SL
Some melt land ice.
SL increases much more per unit of energy from land-ice melt: ratio about 30 to 90 to 1.
Sea-ice melt does not change sea level.
~ ~ ~
35:30 - slide #22
Changes in ocean state from 1950-1960s to 1990-2000s (IPCC 2007)
a general view of what has happened in the oceans over recent time
36:50 - slide #23
Energy Content Change (figure 5.4, IPCC AR4)
1961-2003 ~ 1993-2003
38:05 - slide #24
AMOC: Sampling Issues
problems of the fragmentary nature of some of the data
39:30 - side #25
Is ocean warming accelerating?
Annual ocean heat content 0-700m relative to 1961-90 average
(Ishii et al 2006; Willis et al 2004, Levitus WOA)
40:00 - what’s the cause of the early 80s & 2004 temperature declines?
Data artifacts. Since then Argo problems - XBT drop rate problems identified.
40:45 - instrument behaviors and assumptions... also much of this data was taken for purposes other than climate study...
41:30 - this record has been dropped out. . .
41:00 - slide #26
Revised ocean heat content - World Ocean Yearly HC, 0-700m
1957-1990 reference period
results of reprocessing of this data.
42:45 - slide #27
Ocean heat content to 700m
Palmer et al. OceanObs’09
more up to date version including reprocessing results from various groups
• Domingues et al.
• Ishii and Kimoto
• Willis et al.
• Lyman and Johnson
• Palmer et al.
• Levitus et al.
• Gouretski and Reseghetti
{http://www.ncdc.noaa.gov/bams-state-of-the-climate/2009-time-series/ohc}
~ ~ ~
43:00 - what about all the variations between various reprocessed results?
43:30 - slide #28
Lyman et al. 2010 Nature
Applied the seven methods used to a single data set and studied the resulting spread...
... ‘ I think they should have been a bit more critical and said, well actually some of these are not the right way to do these, all these can’t be equally right...
how best to analysis data is still an evolving process...
~ ~ ~
44:30 - slide #29
Ocean heat content 0-2000m -
ARGO data set
Van Schuckmann et al. JGR 2009
45:40 - slide #30
Ocean heat content 0-2000m
SST 2003-2008; 1990-2008, 10m depth ARIVO-WOA05
Tempts; difference for 2003-2008 - WOA05, Levitus et al.
46:55 - slide #31
Trenberth comments on Van Schuckmann
* VS did not provide 0-700m OHC vs 0-2000m
* Some floats are programmed to go only to 1000m
and do not go to 2000m, so that coverage decreases with depth
* How come all the error bars are the same even though coverage is increasing?
* How good is the quality of the sensors over this time? Up to 30% report negative pressures at the surface.
~ ~ ~
47:50 - ARGO instrument described
“how to process that data is still not a solved problem”
48:50 - slide #32
Ocean heat content is increasing
Ocean heat content anomalies
Lyman 2010; Schuckmann 2009; Trenberth 2010 in Nature
49:40 - slide #33
Ocean fresh water ~ Or ~ The ocean salinity budget
“I want to comment on the ocean fresh water...”
The single most important role of the oceans in climate is that they are wet!
49:50 “... these are the kinds of key questions...”
slide #34
Melting ice
IPCC estimated melting ice contribution to SL (sea level) rise was 1.2mm/yr for 1992 to 2003.
* How much is missed?
* Is the Antarcdtic and Greenland melt a transient or not?
* Many glaciers are not monitored
* Ocean warming may change basal melting: poorly known
* Ice sheets, buttressing by ice shelves poorly modeled
* Concern future SL rise underestimated
* Need process studies and improved models
* Changes salinity: fresh water budget
* affects ocean currents (MOC)
50:50 - melting ice effecting salinity
51:00 - slide #35
Snow cover and Arctic sea ice are decreasing
Arctic sea ice area decreased by 2.7% per decade (summer: -7.4%/decade) Up to 2006:
2007: 22% lower than 2005
2008: second lowest
2010: third lowest
~ ~ ~
{Update: courtesy of Deanna Conners Oct 07, 2011
http://earthsky.org/earth/arctic-sea-ice-reached-record-lows-in-2011~ ~ ~
Arctic sea ice reached record lows in 2011
“Arctic sea ice losses during 2011 were the second-greatest in the satellite record dating back to 1979, according to an official NSIDC report.
“On October 4, 2011, the National Snow and Ice Data Center (NSIDC) released an official report detailing historic losses of Arctic sea ice in 2011. Sea ice losses during 2011 were the second-lowest in the satellite record dating back to 1979.”}
51:50 - slide #36
Hydrological Cycle
52:15 - slide #37 Divergences of water fluxes from E-P estimates over the oceans:
Evaporation over Precipitation
54:05 - slide #38
New estimate of fresh water transport in ocean from new values of E-P over ocean plus new river discharge estimates from Dai and Trenberth (2002).
55:00 - slide #39
A) Mean salinity 1951-2000
C) Mean E-P 1980-1993 m3/yr
B) Linear trends pss/50yr (top)
Durack and Wijffels 2010 JC
“Regions of the oceans that were saltier are getting saltier, regions that were fresher are getting fresher. . . so that relates to changes in global warming and the effects on the hydrological cycle”
55:40 - slide #40
Cross section images of changes by basin and structure as a function of depth.
Linear trends pss/50yr
Durack and Wijffels JC
Subduction on isopycnals appears to account for much of the subsurface changes
55:55 - slide #41
Sea level is rising in 20th century
Raters of sea level rise:
* 1.8 ± 0.5mm yr-1, 1961-2003
* 1.7 ± 0.5mm yr-1, 20th century
* 3.1 ± 0.7mm yr-1, 1993-2003
Sea level rise:
* 0.17m ± 0.05mm 20th century
56:35 - slide #42
What about 2003 to 2008?
Global mean surface temperature
“Then we come to this question, what’s happened to the period when the over all heat content uptake by the ocean has been less, after 2004. . . where has global warming gone.
57:20 - slide #43
Can we track energy since 1993 when we have had good sea level measurements?
Global Climate Data
mean surface temperature anomalies
Global Net Energy Budget
“Looking at the temperature at the Top Of Atmosphere, observations only since about 2000 on...
... where’s this missing heat going...”
58:55 - slide #44
Where does energy go? 2004-2008
59:45 - slide #45
Missing energy in CCSM4?
examining some model runs for 21st century
1:01:55 - slide #46
In CCSM4, during periods with no sfc T rise, the energy goes into the deep ocean, somehow.
RCP4.5 - 1: 21st Century Ocean Heat Content
Stasis also in upper OHC: but not for full depth ocean: heat below 700m
Heat is going into deeper ocean
Difficulty has to do with how we can account for what’s happening in the deep ocean.
1:03:50 - slide #47
Where does the heat go?
Questions regarding the mechanisms driving variability in deep ocean heat content remain. Both the CCSM4 and observations suggest that ENSO plays a necessary, if not sufficient, role. Strong recent ENSO events, including the El Niño of 1997/98 and the La Niña of 2007/08 exert a strong influence on trends in global temperature computed across this period.
Similarly, cooling decades from the CCSM4 are bounded by El Niño events at their initiation and La Niña events are their termination. Yet other intervals bounded by El Niño and La Niña are not accompanied by significant cooling. Our current work focuses on understanding this variable association between ENSO and global temperature trends.
1:04:35 - slide #48
Evolution of recent ENSO
Equatorial Pacific SST (°C). 0-300m Heat Content (°C)
Examining ocean heat distribution and warming
1:07:20 - slide #49
The challenge is to better determine the Heat Budget at the surface of the Earth on a continuing basis:
Provides for changes in heat storage of oceans, glacier and ice sheet melt, changes in SSTs and associated changes in atmosphere circulation, some aspect of which should be predicable on decadal time scales.
Several models now can simulate major changes like the Sub-Sahara African drought beginning in the 1960s, the 1930s “Dust Bowl” era in North America, given global SSTs.
Can coupled models predict these evolutions (Not so far).
But there is hope that they will improve.
In any case models should show some skill simply based on the current state, when it becomes well known and properly assimilated into models:
Need better observing system!
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1:09:05 - question:
I was interested that you attributed the fluctuations in temperature in your CCSM4 model simulation to El Niño, southern oscillation within the models. Based on what you’re saying here I wonder if you could comment on the ability of these models now to simulate El Niño. . . ?
1:09:35 - answer:
Yes, the comment relates very much to the ability of the climate models to simulate ENSO events and it hasn’t been very good. I don’t think any of the models that were in the last IPCC report would be what I would call good. The NCAR model now has a very realistic looking ENSO sequence, but it has larger magnitude than is observed in nature.
{Pulls up slide #45: Missing energy in CCSM4?}
(1:10:25) What we’ve done is to look at these kinds of events when the stasis occurred in the surface temperature, and we’ve done some composites of these, and this is what the pattern of sea surface temperature looks like around the world, and where it’s significant. So it turns out it relates very much to La Niña kind of patterns in this particular model. During La Niña the Pacific is relatively cloud free, the sun is beating down, the heat goes into the ocean it builds up in the western Pacific, which is what I showed in the observations, and it gets buried sort of at depth. But it turns out it’s not just equatorial, in our analysis of this ocean heat content the preliminary findings are that most of the heat is getting buried in the sub-tropics, up to about 40° latitude and it’s off at the equatorial which is where we don’t have such good observing system.
It’s mainly in the Pacific, it’s not in the Atlantis and it’s not the Indian Ocean, it’s more in the Pacific and so if the model is anything like right it suggests where we should start looking harder at observations and we’ve got a project to indeed try to do that using the ARGO data, we just wish that oceanographers could get the ARGO data so that it looks the same from one group to another to another. At the moment if you look at six or eight analysis you get six or eight answers. So that’s still a part of the challenge in terms of the observations.
A challenge in terms of observing the system better, but also a challenge in modeling the system better and I think there will be a project within the next IPCC report specifically to look at performance of models with regard to things like El Niño
1:12:25 - question
Is there evidence of a slowing of the thermohaline circulation.
1:12:35 - answer
No that I’m aware of, the noise, the variability is the main thing that’s being emphasized at the moment. There is certainly a suggestion that it may slow as you go into the future. This does relative not only to the thermodynamics component which has been the main thing that’s been talked about for warming at the high latitudes can potentially do that. But, it also relates very much with what happens with regard to E-P {evaporation - precipitation} these changes of salinity that also effect the density of and that varies quite a bit from one model to another as it turns out.
I don’t believe the observational evidence, because the variability is so large, is sufficient to indicate on way or the other.
1:13:30 - question
On this curve the missing energy, there is no missing energy, you have put it all in the deep ocean and that’s based strictly on energy. You don’t have a mechanism for transporting heat to the deep ocean?
1:13:50 - answer
There are sub-tropical over turning circulations in the Pacific that clearly play a substantial role in moving heat around, although the main thought that they are mainly involved in the top 400m of the ocean, or something like that, and so there is some overturning that occurs in both hemispheres that is clearly important. Largely involving the tropics to the sub tropics, these sub-tropic overturning circulations.
But, the other part of it that needs to be explored a lot more and where there is potential for depositing heat at much greater depths is in the western part of the Pacific and in particular in association with the boundary currents, the East Australian current and the Kuroshio Current {http://whatonearth.olehnielsen.dk/currents.asp} and there, there are clearly components that go down below a thousand meters. There’s quite a lot of variability, those are the most dynamic features in the oceans, the currents can be a meter per second, same thing with the Gulf Stream. And so it’s those regions also where I think we need to look harder as well.
1:15:15 - question
Putting the scale on it, the ocean is 6 km deep at that point, so we just don’t know about the convection...
1:15:30 - answer
We don’t know well what is going on. You know the sampling of the deep ocean is quite fragmentary there are a couple papers Craig and Johnson, Purcell and Johnson have done an analysis of data that do exist. Sarah Gillie has also looked at this over the southern oceans, suggesting indeed that there’s evidence for warming at greater depths, generally permeating though. The oceanographers are not quite sure how this happens... I think there is still the general thought is that the abyss, the deep part of the ocean is relatively... there’s not much action there and things change slowly.
Though conduction rather than convection moving stuff around than it should be very slow process. There are things like tides, that are continually pumping the ocean up and down. And in some places at greater depths are known to cause mixing which is probably important. That may not be fully taken into account, and so it may be that there are ways and places where heat is mixed down more readily than others, but it may not be everywhere. And some seasonality can come into play, that’s certainly the case in the North Atlantic for instance. I mean the main convection that occurs in the ocean is when you get in the winter the cold dry outbreaks and this large heat fluxes into the atmosphere that cools the surface of the ocean and then that cool water sinks and so this overturning within the ocean. And so in winter time you can make very large changes to a few kilometers depth in association with those kinds of events and there can be better connectivity.
But in general especially in a warming climate, as your warming the surface it’s more buoyant and tends to sit there and the oceans become more stable and which has some ramifications of course for marine eco-systems as well.
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