Saturday, May 11, 2013

Dr. Trenberth Lecture: The Role of the Oceans in Climate

The following is a lecture given by Kevin Trenberth explaining the ocean climate connection.  Following the video I've made notes that are,  for the most part, based on the well organized slides that accompanied his talk.  I have also taken the liberty of inserting many links that seemed to me appropriate for those who were curious for further details.

Here is another interesting lesson for those who have a sincere interest in understanding what we are doing to our global heat distribution engine, aka climate.

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Kevin Trenberth: The Role of the Oceans in Climate

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Uploaded 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


Kevin Trenberth: The Role of the Oceans in Climate - lecture notes 

3:00  -    {Dr. Trenberth pointed out that he was a meteorologist and admitted that to him the most important property of the ocean was that it was wet, so he would be talking mainly about the physical climate system and not about the ocean ecology aspects.  

3:30  -   Summarizing the "role of the climate system" 

3:50  -  Role of the atmosphere
*  The atmosphere is the most volatile component of climate system
*  Winds in jet stream exceed 100mph or even 200 mph: winds move energy around
*  Thin envelope around planet 90% within 10 miles of surface 1/400th of the radius of the Earth
*  The atmosphere does not have much heat capacity
*  "Weather" occurs in troposphere (lowest part)
*  Weather systems: cyclones, anticyclones, cold and warm fronts, tropical storms/hurricanes move heat around: mostly upwards and polewards

5:00  -  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 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 years 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.

6:30  -  Ocean is not in equilibrium with the atmosphere

6:55  -  Schematic of the great ocean conveyer of heat, freshwater and salts.

7:15  -  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

* Surface air temperature changes over land are large and occur much faster than over the oceans.
*  Land has enormous variety of features: topography, soils, vegetation, slopes, water capacity
*  Land systems are highly heterogeneous and on small spatial scales
*  Changes in soil moisture affect disposition of heat:
rise in temperature versus evaporation.
*  Changes in land and vegetation affect climate through albedo, roughness and evapotranspiration.

8:20  -  Role of Ice
*  Major ice sheets, 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 time scales.
*  Unlike land, ice melts - changes in sea level on longer time-scales.
*  Ice volumes:  28,000,000 cubic kilometers 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 the other glaciers and ice caps 0.35 meters.

*  In Arctic: sea ice ~ 3-4 meters thick
*  Around Antarctic: ~ 1-2 meters thick

*  Ice is bright: reflects the solar radiation -> ice-albedo feedback
*  The West Antarctic Ice Sheet (WAIS) partly grounded below sea level ->
Warming could alter grounding of ice sheet, making it float, and vulnerable to rapid (i.e. centuries) disintegration. -> rise in sea level of 4-6 meter.
May be irreversible if collapse begins.

Currently biggest changes seen from Greenland.

9:50  -  Role of Coupling - El Niño - Southern Oscillation ENSO
*  ENSO: EN (ocean) and SO (atmosphere) together: Refer 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

10:45  -  Energy on Earth
*  The main external influence on planet Earth is from radiation.
*  Incoming solar shortwave radiation is unevenly distributed owing to the geometry of the Earth-sun system, and the rotation of the Earth.
*  Outgoing longwave radiation is more uniform.

11:50  -  The incoming radiant energy is transformed into various forms (internal heat, potential energy, latent energy, and 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  -  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 the climate.  And they can be perturbed with climate change.

13:05  -  A schematic to indicate the framework Trenberth is going to deal with in explaining heat transfer in the atmosphere.

14:30  -  Top of atmosphere (TOA) net radiation - time lapse data map

15:25  -  Net Radiation TOA 

16:10  -  Net atmospheric heating - diabatic heating

16:50  -  Net surface flux

17:45  -  Annual mean net surface flux

18:20  -  Departures from annual mean: Equivalent ocean heat content

19:05  -  Total Upward Surface Energy Flux - Feb. 1985 - Apr. 1989

20:45  -  Radiation at top of atmosphere compared to actual change in energy heat content in the ocean.

21:10  -  Looking at the ocean . . .  divergence of the atmospheric energy transport . . .  atmosphere in action

26:10  -  Evapo-transpiration's and the hydrological cycles role in moving heat from surface to upper atmosphere.

27:35  -  Climate system is already responding to anthropogenic influences...

28:10  -  Looking at heat transport - oceans vs land - CERES data

30:00  -  Atlantic Ocean

33:00  -  El Niño heat transport in the Pacific Ocean

33:10  -  Indian Ocean - has no corresponding phenomena - monsoons transport heat.

33:30  -  The three major oceans are responding differently causing some atmospheric disruption

33:50  -  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 (meridional overturning/thermohaline circulation)
*  The historical patterns of SST are 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

*  Global warming from increasing greenhouse gases creates an imbalance in radiation at the Top-of-Atmosphere: now order of 0.9 W m-2
*  Where does the heat go?
*  Main sink is ocean: thermosteric sea level rise associated with increasing ocean heat content.
->  Some melts sea ice: no change in sea level - some melts land ice.
*  Sea level 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

35:40  -  Changes in ocean state from 1950-1960s to 1990-2000s (IPCC figure 5.18)

36:50  -  Energy content change - comparing 1961-2003, with 1993-2003
(Topex satellite launched in 1992 to better study sea level rise)

38:10  -  Transection lines measuring "overturning transport" - heat moving towards poles  -  discovered that the variability within the ocean much greater than previously assumed.

39:30  -  Annual ocean heat content 0-700m relative to 1961-90 average

40:00  -  IPCC: Causes of decadal variability not well understood
-  cooling due to volcanism?
-  artefact due to temporally changing observing systems?

Why the apparent drop in ocean heat content during early 2000s?
-  since then:  Argo problems and XBT (expendable bathythermographs) drop rate problems identified and corrected.

41:40  -  All ocean heat content papers pre-2007 are made obsolete by the identification of the various problems with XBT data processing and early ARGO issues.

41:55  -  Revised ocean heat content

42:40  -  Ocean heat content anomaly 0-700m - compilation from many groups - Palmer et al. OceanObs'09

43:00  -  The various studies look roughly the same, but a closer look reveal some striking differences.  Trenberth: "What is going on here?" with the apparent slowing of warming post 2004.

43:25  -  Lyman et al. 2010 Nature - looked at the various ways this data was processed and corrected by the many different teams. Then went and applied them to a single data set.  This is an area of active research.

44:30  -  Von Schuckmann et al. JGR 2009 - "Ocean heat content 0-2000m"
-  used only ARGO data and found ocean warming: OHC of 0.77 W m-2 
which works out to 0.54 W m-2 if averaged globally (still short of 0.9 W m-2)

47:00  -  Trenberth comments on Von Schuckmann study
-  VS did not provide 0-700 m OHC vs 0-2000 m
-  Some floats are programmed to go only to 1000 m and do not go to 2000 m, 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.

49:40  -  Ocean fresh water, or The ocean salinity budget
{FYI - Thermohaline Ocean Circulation - Stefan Rahmstorf (2006)}

49:50  -  Melting ice - IPCC estimated melting ice contribution to SL rise was 1.2 mm/yr for 1992-2003

-  How much is missed?
-  Is the Antarctic and Greenland melt a transient or not?
-  Many glaciers are not monitored
-  Ocean warming may change basal melting: poorly know
-  Ice sheets, buttressing by ice shelves poorly modeled
-  Concern future SL rise underestimated
-  Need process studies and improved models
-  Changes salinity: fresh water budget 
-  Changes in salinity affects ocean currents (MOC)

51:20  -  Calculating how much heat it takes to melt ice
-  To melt 106 km2 ice 1 meter thick (2007) to 10°C = 3.4x1020 J
-  Globally per year since 2004 this is 0.02 W m-2 kjh

52:10  -  Divergence of water fluxes from E-P (evaporation and precipitation) estimates over the oceans - evaporation impacts salinity

53:35  -  {Ocean salinity is also differentially impacted by fresh water river outlets; rain fall; cryosphere melting.}

54:10  -  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  -  Changes in time of mean salinity - impacts on hydrological cycle

55:55  -  Sea level is rising and rate is increasing
*  averaged over 20th century - 1.7mm±0.5mm
*  1961-2003 = 1.8m±0.5mm  
*  1993-2003 = 3.1m±0.7mm

56:35  -  What about the 2003 to 2008 drop in sea level?

57:20  -  Can we track energy since 1993 when we have good sea level measurements?
graph of CO2 level - sea level - mean surface temperature anomalies
Trenberth and Fasulto Science 2011 

57:55  -  Adds Global Net Energy Budget data

58:40  -  stolen emails - Trenberth's "the missing heat travesty" email discussed

59:45  -  missing energy in CCSM4?

1:01:00  -  discussing models and details

1:02:00  -  looking at deep ocean - heat is going into deep ocean - graph:
*  RCP4.5 - 1 21st Century Ocean Heat Content
-  TOA and Surface Anomalies
-  Heat Content Anom by Depth

*  Question 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 variability association between ENSO and global temperature trends.

cross-section look into the Pacific ocean at the equator - ENSO in action

1:06:00  -  then a look into the Indian and Atlantic Oceans

1:07:20  -  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 atmospheric circulation, some aspect of which should be predictable on decadal time scales.

1:09:00  -  Question:  You attributed fluctuation in temperatures CCSM4 model simulation ENSO oscillations... can you comment on the ability of these models now to simulate El Niño... could you comment on the state of the models.

Trenberth gives a frank answer, detailing weaknesses in models, along with touching on how scientists work with models and what they learn and how models evolve.  

1:13:30  -  Question regarding the physical oceanic mechanisms for getting the missing energy into the deep ocean?

Another good frank answer that's better listen to in full.

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By Joe Romm on Mar 25, 2012 at 12:30 pm

By Joe Romm on Nov 21, 2009 at 9:47 am

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