Wednesday, May 3, 2017

ERSST and Sea Ice

I use the NOAA ERSST V4 SST (Sea surface temperature) dataset as part of TempLS. It has the virtue of coming out promptly at the start of the month, and of course is the product of a lot of scientific work. But it has two nuisance aspects. One that I described last month, is that its 2x2° cells don't align very well with the coastal boundaries, and some repair action is needed. The other is the treatment of sea ice. ERSST returns values (if it can) for all non-land regions, and where there is sea ice, returns -1.8°C, which is the melting point of ice in sea, and so is indeed presumably the temperature of the water. But it isn't much use as a climate proxy there. Polar air over ice is often very much colder.

My aim is to mark these regions as no result, so that they will be interpolated, mostly from land. But that is complicated because, while -1.8 is clear enough, there are often temperatures close to that, which presumably mean mostly ice, or maybe ice for part of the month. So I have used a cut-off of -1°C.

I have been working recently with land masks to improve the accuracy of TempLS near coasts. My preferred version uses a triangular mesh with nodes at measurement points, so triangles will often be part land, part sea. It would be desirable to ensure that the implied interpolation uses land values for land locations. I'll post soon on how this can be done. But it sharpens the problem of sea ice, because the land mask doesn't recognise it. So I need to use some data, and ERSST is to hand, to mark this as land rather than sea.

So I have been reviewing the criterion for making that determination. I actually still think that -1°C is reasonable. To see that, I mapped the ERSST grid for Jan-Mar 2017 to show where the in-between regions are. I used WebGL.

It might seem that WebGL is overkill, since the polar regions can be easily projected onto 2D. But the WebGL facility makes it the easiest way. I just set all positive temperatures to zero, use the GRID type so I don't have to work out triangles, and then the color mapping automatically devotes the color range to the region of interest (and makes a color key).

So here is the plot (drag to see poles); in those months (radio buttons) it is Arctic that is of most interest. You can see that most of the region expected to be sea ice is in fact at -1.8C, and the fringe regions are intermediate. But there are also regions around the Canadian islands, for example, which show up as higher than -1.8, but would be expected to be frozen. A level of -1 seems to capture all that, without unduly modifying the front to clear ocean.



You'll notice the small white circle at the pole. That comes because ERSST goes from -88° to 88°. The grid triangles actually connect cell centers, which is the origin of the shading. It also means that if just one of the nodes is land, the triangle will be marked black. That is why the coast outlines aren't just rectangular.

17 comments:

  1. Even where data exists in recent years, Arctic SST anomalies using ERSSTv4 (or HadSST3) are problematic because there is often no real baseline climatology reference for comparison - only -1.8ºC. I think the GISS approach makes most sense, where they seem to discard all data in grid cells without valid baseline climatology figures (i.e. if climatology = -1.8).

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    1. Yes, that is certainly a problem. Not so painful in TempLS, where there is no fixed reference period. But there is still a required minimum period of admissible (> -1C) readings.

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    2. Could be wrong but I'm not sure lack of fixed reference period in TempLS makes much difference since the input ERSSTv4 data has already been processed using a fixed reference period.

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    3. Paul,
      I don't think the prior processing matters, since the LS fitting overrides that. Any previously calculated offset would just shift the LS offset, but leave the same anomaly. LS helps because you don't need to exclude points because of a gap between the reference period and the present data; it just needs enough recent history to create an offset.

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  2. The white spot in Arctic is larger (4x?) than the corresponding spot in Antarctica. It seems unlikely that ERSST cover -89° to 88°?

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    1. Thanks, Erik,
      It was actually 3° radius; I was covering -89 to 87. I had been thinking that because I had 89 cell centre data points, I must have 89 cells. But no - when plotting the centres become vertices. I've fixed it (just shifts the grid 1° north).

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  3. You could create a mask for each month, each one the union of the standard land mask and the maximum sea ice extent for that month in those grid cells.

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    1. CCE,
      Yes, that is what I am planning to do.

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    2. OK. I got the impression that there would be some kind of dynamic mixing of SST and SAT in the same grid cells through time (not sure how that would work).

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    3. CCE,
      I hope to post in about 8 hours on that. Yes, there will be a kind of mixing, in the triangles for each time point. But the underlying mask will be as you say - month-by-month setting the sea mask to NA where ERSST records ice (that's why I want to get the criterion right).

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  4. Nick

    I understand if you consider both atmosphere and oceans there is never a pause in global warming. The forcing from extra GHG's is constant. On the other hand, if you're only talking about the atmosphere....?
    Here is an an analogy that explains my confusion:

    "It took me 6 hours to hike 12 miles. I'm a fast walker and had time to stop for a long, leisurely lunch (hiatus).

    Scientist: "I can produce evidence you never actually stopped for lunch"

    Cliff: "What are you talking about?"

    Scientist: "Look at this trend chart. I've determined your overall rate of travel was 2 mph"

    Cliff (looking confused): ".....that's the silliest argument I've ever heard.""

    What am I missing?
    Sent from my iPhone

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    1. Cliff
      "What am I missing?"
      1. This post isn't about trend in any way
      2. No proper scientist would say that a trend disproved the lunch stop
      but
      3. You got there, despite the hiatus. That is what the trend is saying. You actually did cover twelve miles in six hours. And if we keep pumping out GHGs, we'll get there too. We may or may not like it.

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    2. Nick

      I goofed - intended to comment on your most recent post.

      I understand there is no pause in the "trend" of warming. This doesn't mean there aren't hiatus's, as illustrated by the lunch stop. Hiatus from "warming" and hiatus from "trend of warming" are not the same thing. If this distinction isn't made clear, it can be really confusing.

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    3. As skeptics often point out, from 1998 to 2014 there was a hiatus in warming. Scientists, in turn, point out there was no hiatus in the trend. Both statements are accurate. It seems like each party is talking past each other.

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    4. Cliff,

      Scientist: "I can produce evidence you never actually stopped for lunch"

      What you're missing is that no-one is arguing this. In your analogy what some scientists are saying is that the lunch stop did not alter the vector of the journey - the hiker is still ultimately going to the same destination. Furthermore, lunch stops on long hikes are completely normal events (as is short-term deviation from long-term trend in global average temperature, to connect the analogy back) so why is so much attention being paid to this particular lunch stop?

      I also don't think you're accurately characterizing scientists and "skeptics" here. It is scientists who are taking both positions on the hiatus, depending on their focus of study, and are ultimately both correct according to different definitions. "Skeptics" on the other hand are generally arguing for assigning a much greater level of importance to the lunch stop than the rest of the journey, and indeed tend to suggest that the existence of the lunch stop brings into question whether the hiker will ever actually reach the destination.

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  5. Nick

    What I'm trying to say is just because there wasn't a pause in the overall trend, doesn't mean there wasn't a pause. Not everyone understands this.

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    1. I think people understand that. If you want to drive across town to get somewhere in an hour, you'll need to maintain an average speed. There will be hiatuses, but that doesn't change the issue of driving ten miles in an hour.

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