Collaborative Research on Sunlight and the Arctic Atmosphere-Ice-Ocean System
• CRREL• Don Perovich (lead PI) and John Weatherly
• UAF• Hajo Eicken, Tom Weingartner, and Jeremy Harbeck
• UW• Bonnie Light, Rebecca Woodgate, Ron Lindsay, Kay Runciman
September 2005 – August 2008
Motivating questions• Evidence of operative ice-albedo feedback processes?
• Are mechanisms that produce polar amplification of global warming in GCMs commensurate with observations?
• Has the AIOS approached a “tipping point” as suggested by model simulations (e.g., Lindsay and Zhang, 2005)?
• What is the role of the atmosphere (cloud cover, precipitation) in modifying the response of the cryosphere to variations in external forcing?
• What is the role of the ocean (through import and storage of solar heat in the mixed layer) in transporting and sequestering solar energy?
Goals• Pan-Arctic maps of shortwave flux variables and parameters
for 1982 – present
• Surface shortwave flux (spatial and temporal variability)
• Snow deposition on sea ice
• Surface albedo
• Quantification of oceanic uptake and transport of solar heat through the Arctic system from oceanographic mooring data over the past 15 years (Woodgate et al., 2005) and satellite-derived sea surface temperature maps
• Synthesis of these data sets, GCM simulations and research findings
VARIABLE SOURCES TIME PERIOD
Radiation & atmospheric variables
Incident solar irradiance NCEP reanalysis, satellite data (ERBE, CERES, ISCCP), GCM simulations 1979-2004
Surface albedo Modeling, dataset integration, AVHRR Pathfinder 1979-2004
Cloud cover Satellite data (ISCCP), GCM simulations 1983-2004
Surface air temperature POLES data set, satellite data, NCEP reanalysis 1970s-2004
State of the ice cover
Ice extent SMMR, SSM/IQuikSCAT
1979-20041999-2005
Ice concentration SMMR, SSM/I 1979-2004
Snow depth NCEP reanalysis, field expts., coastal weather stations, GPCP 1970s-2000s
Ice thickness Field expts., submarine sonar data, satellites 1970s-2000s
Ice melt rates Field expts., models 1970s-2000s
Pond fraction Field expts., pond hydrology model, "national asset” satellites 1970s-2000s
FY / MY fractionsIce concentrationIce types
SSM/I(for freezing season)SMMR, SSM/I (for freezing season)QuikSCAT (for freezing season)
1987-20041978-20031999-2005
Open water fraction RGPS (summer only) 1998-1999
Ice motion RGPSSSMI (Oct-May)AVHRR
1996-20001978-19971978-2003
Ice type - surface roughness Field expts. (incl. Russian monitoring flights), ICESat
1970s-1990s2003-2004
Date of onset of melt & freezeupAccurate timing of melt/freeze
SMMR, SSM/IQuikScat (1999-2005)
1979-20041999-2005
Biogenic/sediment inclusions Field expts., remote sensing 1980s-2000s
Optical properties of snow, ice, water & biogenic/sediment inclusions
Extinction coefficients
Field expts., lab expt., modeling
Scattering coefficients
Lab expts., modeling
Phase functions
Lab expts., modeling
Absorption coefficients
Lab expts.
State of the upper ocean
Influx of heat from Bering Sea Oceanogr. moorings, CTD cruises, satellite SSTs 1990-2004
Influx of heat from Atlantic European VEINS & ASOF Programs 1997-2005
Influx of heat from rivers R-ArcticNet, temperature from field expts. 1970s-2000s
Surface hydrochemistry US-Russian Arctic Hydrochem. atlas, field expts. 1970s-2000s
Thermohaline properties of upper 50 m
Environmental Working Group Atlas, Polar Science Center Hydrographic Climatology (psc.apl.washington.edu/Climatology.html) 1970s-2000s
So far…1. Easy things
• Meetings• Grid (25km x 25km EASE, 1982-2005)• web page for status, updates http://synthesis.apl.washington.edu• acquire data (ERA-40 downwelling Fr, SSMI ice
concentrations)• define a simplified subproblem: assume the ice is opaque
2. Not-so easy things• difficult decisions about difficult data (ice concentration)• GRL manuscript (now in review)
The simplified subproblemIncreasing solar heating of the Arctic Ocean and adjacent seas,
1982-2005: Variability and ice-albedo feedback (GRL, submitted)
- not a budget, just a term - estimate solar heating into the ocean
- Frw = Fr ∙ (1-) ∙ (1-C) - assumption that ice is opaque → “minimum estimate” - problems with C and melt ponds → overestimate - is constant, and appropriate only for open water
- every day, every grid cell - only areas that have ice during the period of observation - instantaneous and cumulative
Map of mean total annual solar input averaged over 1982 – 2005 (units are in MJ m–2)
Maps showing the anomaly of solar heat input into the ocean for 1984, 1995, 1996, and 2005 (units are in MJ m–2).
Maps showing the anomaly of solar heat input into the ocean for 1984, 1995, 1996, and 2005 (units are in MJ m–2).
Map of the linear trend of annual solar heat input to the ocean, with units of percent per year.
Figure 4. Time series of solar heat input for a 50 x 50 km area centered on 75˚ N, 165˚ W.
What’s next?ICE COVER
snow albedo during melt
Advective terms, specifically Bering Sea
Putting all the pieces together . . .
Interface with PP projectPAR? plan: use ERA-40 broadband data and Perovich obs. (in prep)
500 1000 1500 2000 25000.0
0.1
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0.5Complete OvercastSolar Disk Not Visible
4 Sep 5 Sep 7 Sep 10 Sep 11 Sep 14 Sep 20 Sep 21 Sep 22 Sep 23 Sep 25 Sep 26 Sep
Inci
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ar ir
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(W m
-2 n
m-1)
Wavelength (nm)
20 Sept SDNV; C ice = 100; sza = 86.1o
21 Sept SDNV; C ice = 100; sza = 86o
500 1000 1500 2000 25000.0
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7 Aug 9 Aug 10 Aug 11 Aug 12 Aug 18 Aug 22 Aug 23 Aug 27 Aug on ice
Inci
dent
sol
ar ir
radi
ance
(W m
-2 n
m-1)
Wavelength (nm)
Complete OvercastSolar Disk Not Visible
Spatial coverage? Only ice covered areas: leaves out parts of Norweigan, Barents, E. Greenland?Temporal coverage?
-The reviews for the sunlight paper are below. The short story is major revisions. As you will see the reviewers kind of liked it, but wanted more. The good news is that they didn't really challenge the SSMI ice concentrations. The bad news is that they do want more discussion regarding the ERA-40 incidents. Some of the suggestions are fairly straightforward, but there are a few that will be a little complicated. In particular, we need to:
1. Discuss uncertainties in ERA-40 incident.2. Discuss possible issues in transition between ERA-40 and ECMFW.3. Demonstrate temporal behavior of incident and ice concentration. Establish the cause of the increase (i.e. changes in concentration). To do this I suggest dropping the current Figure 2 and replacing it will a figure like the attached (only with more curves) that demonstrates that the incident fluctuates, but hasn't shown a strong trend, while concentration does should a trend.4. Further consideration and discussion of heat input through Bering Strait.5. Do we add 1979-1981? Do we have those years?
-Reviewer #1 (Formal Review):
This is an interesting and well written paper. The connection between the changing Arctic sea ice and solar heat input is an important aspect of the polar climate system.
I must admit, though, that I am a little disappointed with their analysis. The authors say in the abstract "A synthetic approach was taken, combining satellite-derived ice concentrations, incident irradiances....and field observations of albedo over the Arctic Ocean and the adjacent seas". But then a constant value for the albedo was used and, more importantly, they did not distinguish between the variability and trends in incident solar radiation and the variability and trends in ice concentration. If incident solar radiation is not really changing or varying then all trends and patterns are perfect reflections of just changes in ice concentration (something that has been studies before). They say in the discussion that the "increase in heat input was primarily due to reductions in summer ice extent" [p.9, line 180f] but do not quantify this. All the figures show solar heat input, but it would be interesting to see how much of those trends and patterns results from trends inincident solar radiation (mainly from changes in cloud cover I would assume) and how much results from trends in sea ice concentration. There is probably also a correlation between incident solar radiation and ice concentration. Less sea ice may lead to increased cloud cover. The lack of seprating variations in solar radiation and in ice concentration is, in my opinion, a major weakness of this manuscript but it should be something that could easily be done with the data set they collected.
A minor point is that I am wondering whether there is an offset between the ECMWF and ERA-40 solar radiation data. People may have looked at this already so a reference may be sufficient.
Figure 2 shows heat input "from four selected years" [p.7, line 137] ."Selected" always sounds suspicious. If, as they say, heat input was generally below average in the 80s and above average in the 2000s, I suggest to show the actual averaged 80s and 2000s data (if this makes sense).
In summary, I suggest the paper to be accepted with some revisions. As said above, a separation of the effects of incident solar radiation and sea ice concentration should make the manuscript significantly more valuable.
-Reviewer #2 (Formal Review):
General:
Overall I think that this is a potentially useful paper which adds to our understanding of Arctic sea ice loss. However, I think that it needs further work before publication can be recommended, primarily to clarify some of the techniques, assumptions and uncertainties, and to address what I feel are some mis-interpretations.
Specific:
1) Abstract and introduction: The authors are glossing over the issue of sea ice thinning. That sea ice extent is decreasing is clear. However, the direct observational evidence of systematic thinning is not as convincing. Submarine sonar data certainly provide some evidence of thinning, but these data are spotty. The cited paper by Rothrock et al. [2003] was in part modeling based, and as I recall, provided some evidence to at least some thickening after the late 1990s. It certainly makes sense that reductions in extent have been accompanied by thinning, especially due to the observed loss of perennial ice, but the authors need to be very careful of making blanket statements.
2) Introduction, para 1: The ACIA citation should be replaced by, or attended by reference to the study of Zhang and Walsh [2006, J. Climate], who looked at sea ice trends over the period of observations from the IPCC-AR4 models. The ACIA study made use of an earlier generation of models.
3) Introduction, para. 1, sentences 3 and 4: The discussion here is muddled. It is argued that a concurrent reduction in summer sea ice and increased hemispheric warming is the results of increased heating of open water. This makes no sense to me. Is the intended argument that general warming (from greenhouse gas loading) leads to ice loss, and that ice loss is then further enhanced by increased solar heating? The next sentence, arguing that ice loss then "drives further warming at the global scale", doesn't really make much sense unless the authors invoke the "Arctic amplification" of surface air temperature change, which is more of an indirect effect of increased solar heating in that with less ice at summer's end, and more heat in the upper ocean, there will be larger heat losses from the ocean to the lower atmosphere in autumn and winter.
4) Introduction, para. 1, near bottom: Be careful with terminology. The Stroeve et al paper looked at ice extent (the region with at least 15% ice cover), not ice area. Extent and area are different things. Also, be more specific and state that the Stroeve et al. paper looked at the IPCC-AR4 models.
5) Introduction, para. 2: For the non specialist, briefly define "leads" and "polynyas".
6) Introduction, last para. It is stated that use is made of "measurements of the optical properties of water and ice during summer". Ho so? All I see in the Methods section that follows is the use of an assumed open ocean albedo of 0.07.
7) Methods, para. 1: Equation 1 has been corrupted in the .pdf version I have. Double check that it is correct. While mention is made that only solar energy incident on the open ocean is being considered, and that no attempt is made to address penetration of radiation through ice the ice, it would might help the reader to have another sentence to make it crystal clear that nothing is being said here about changes in the albedo of the sea ice itself.
8) Methods, para. 2: There is discussion of the impacts of different sea ice data sets on solar heating. However, unless I've missed something, the ERA-40 downwelling solar radiation data seem to be accepted without any discussion of possible biases. All reanalyses have problems with Arctic cloud cover, which in turn will strongly impact on the surface shortwave flux. I recall that John Walsh (who the authors know) has done some work on this. A climate "jump" might also be introduced when piecing together the ERA-40 records (1982-2001) with fluxes from the ECMWF operation model. This needs to be addressed. Finally, why the odd starting year of 1982? The modern satellite data stream for data assimilation in ERA-40 began in 1979, and is also the start of the SMMR record.
9) Results, para. 2, bottom: Why is the year to year variability in the incident solar flux modest? Presumably this is because cloud cover shows little variability? This goes back to questions raised above regarding the quality of the ERA-40 (and also operational ECMWF) fields.
10) Discussion, para. 4: I'm all for the idea that lateral melting in summer is important in the ice-albedo feedback. On the other hand, I think the authors could be a little more clear in discussing seasonal aspects of the feedback mechanism. The way I might frame this is that even in a greenhouse-warmed world, there is little/no solar radiation over the Arctic ocean in autumn/winter. Hence, much of the heat that is picked up by the ocean in summer is just lost right back to the atmosphere (and to space). The autumn/winter ocean heat loss to the lower atmosphere is actually the climate model "fingerprint" of Arctic amplification of surface air temperature. However, the fact that it represents ocean heat LOSS could itself be viewed as a negative feedback as far as the sea ice cover is concerned. On the other hand, by warming the lower atmosphere, there is more downward longwave radiation to the surface as well, which works the other way. Which process wins? The point is that toperpetuate a strong feedback, you have to hang on to some to the extra heat gained in spring and summer through the long autumn and winter season. Maybe what one needs to do to hang onto some of the ocean heat is to actually grow some ice to shut down the heat loss?
11) Discussion, page 5: The author's need to take a closer look at the evidence for increased heat transport through Bering Strait. My read of the Woodgate et al. paper is that the more obvious signal is high variability, rather than some general trend. As I recall, the heat flux seems to have increased between 2001-2004, but fluxes in 2001 were the lowest of the record. In turn, it was the fairly weak evidence of increased heat flow through Bering Strait that (at least in part) led Shimada et al. to argue instead for a link with redirection of Pacific Surface Water from the shelf slope into the Arctic Ocean. Mention should also be made of the potential role of "pulses" of warm water through into the Arctic Ocean from the Atlantic side [see Polyakov et al., Geophys. Res. Lett, 2005, vol. 32] and the unresolved issue of how one gets this heat up to the surface through the strong halocline There is a recent review paper in "Science" [Serreze et al., 2007, vol. 315] that summarizesthese issues.
