What is the reference mean referred to in this paper?
January 27, 2024 11:39 AM Subscribe
World ocean heat content and thermosteric sea level
change (0–2000 m), 1955–2010, which is the source of this data, provides changes in ocean heat content (OHC) since 1955. But what was the ocean heat content in 1955?
They say early on:
It's "the OHC in 1955 + 24x10^22 J", but where can the OHC in 1955 be found?
They say early on:
We use the term “ocean heat content” as opposed to “ocean heat content anomaly” used by some authors because “ocean heat content” is an anomaly by definition. OHC is always computed with a reference mean subtracted out from each temperature observation. Otherwise the OHC computation depends on the temperature scale used.Fair, but what is the "reference mean" in this case? When they say "We have estimated an increase of 24 x 10^22 J" how can I know what the total ocean heat content is?
It's "the OHC in 1955 + 24x10^22 J", but where can the OHC in 1955 be found?
Response by poster: Whoa! Thanks for weighing in.
To make sure I understand correctly, is it right to sat that at iteration number n, the OHC value is:
Tn (the temperature measured at iteration n) minus the mean, which is (T1 + T2 + ... +Tn-1)/(n-1)
?
And if so, at iteration 1 (from 1955), the OHC value is just the temperature measurement because there is no (previous) mean to subtract?
Also, the rough ratio of anomaly to total heat is really helpful for getting a sense of the size of these changes. Thank you!
posted by ignignokt at 7:17 PM on January 27
To make sure I understand correctly, is it right to sat that at iteration number n, the OHC value is:
Tn (the temperature measured at iteration n) minus the mean, which is (T1 + T2 + ... +Tn-1)/(n-1)
?
And if so, at iteration 1 (from 1955), the OHC value is just the temperature measurement because there is no (previous) mean to subtract?
Also, the rough ratio of anomaly to total heat is really helpful for getting a sense of the size of these changes. Thank you!
posted by ignignokt at 7:17 PM on January 27
Best answer: So I think in principle you could calculate the ocean heat anomaly between any two years. What I think your formula actually calculates would be the ocean heat content anomaly between a given year (Tn) and the average of all years before it. In that formula, every year would be compared to a different reference
In practice, you usually choose one reference period to compare a bunch of years to. So here in this paper their reference period is the average of all temperatures from 1955-2006. Then the OHC is calculated for each year by subtracting the average over that reference period.
If you don't mind me asking, do you have any particular goal with this question?
posted by dreyfusfinucane at 8:08 PM on January 27
In practice, you usually choose one reference period to compare a bunch of years to. So here in this paper their reference period is the average of all temperatures from 1955-2006. Then the OHC is calculated for each year by subtracting the average over that reference period.
If you don't mind me asking, do you have any particular goal with this question?
posted by dreyfusfinucane at 8:08 PM on January 27
Response by poster: Oh, interesting. So, it's the anomalies from the average of itself, sort of.
The reason I'm looking into this is because I'm considering using the OHC data in a piece of music I've been working on. It is something of a data sonification, even though conveying data is not the primary goal of the piece. And though (if I end up using it), it will mostly because things end up sounding the way I want, I still want to be able to say what that data is.
On my first look at it, I was having trouble understanding the negative values in the data. Then, upon reading the paper, I saw that they meant that those were measurements that were under some reference mean. And now you've explained what that reference mean is!
So, now I do know that this top layer of the ocean has more heat in it than it did in 1955 (and more than it did in 1956-1975). But does it have more heat than it did in, say, 1930? I'd guess it does, but they didn't measure back that far. I'll look around for some other source that covers that.
Regardless, thank you — I think I now understand this data!
posted by ignignokt at 7:17 PM on January 28
The reason I'm looking into this is because I'm considering using the OHC data in a piece of music I've been working on. It is something of a data sonification, even though conveying data is not the primary goal of the piece. And though (if I end up using it), it will mostly because things end up sounding the way I want, I still want to be able to say what that data is.
On my first look at it, I was having trouble understanding the negative values in the data. Then, upon reading the paper, I saw that they meant that those were measurements that were under some reference mean. And now you've explained what that reference mean is!
So, now I do know that this top layer of the ocean has more heat in it than it did in 1955 (and more than it did in 1956-1975). But does it have more heat than it did in, say, 1930? I'd guess it does, but they didn't measure back that far. I'll look around for some other source that covers that.
Regardless, thank you — I think I now understand this data!
posted by ignignokt at 7:17 PM on January 28
That is really cool! I've always wanted to make different audio plugins from simple ocean models or concepts but have never gotten around to it.
We only began measuring the ocean globally relatively recently (even in 1955 we had extremely little data). Before that we have to rely on models and paleoclimate records which use proxies that indirectly record the ocean temperature like examining the chemical makeup of plankton shells which we know varies with temperature.
I just skimmed it but this paper seems to blend paleoclimate records with a model to estimate OHC in the last millennium.
Best of luck!
posted by dreyfusfinucane at 9:22 AM on January 29
We only began measuring the ocean globally relatively recently (even in 1955 we had extremely little data). Before that we have to rely on models and paleoclimate records which use proxies that indirectly record the ocean temperature like examining the chemical makeup of plankton shells which we know varies with temperature.
I just skimmed it but this paper seems to blend paleoclimate records with a model to estimate OHC in the last millennium.
Best of luck!
posted by dreyfusfinucane at 9:22 AM on January 29
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I'm a physical oceanographer! From my understanding they are measuring the ocean heat content anomaly by subtracting the temperature observed from the mean at that each observation . So they never actually calculate the total heat stored in the ocean.
We can estimate the total heat stored in the top 2000m (what the paper considers) by assuming the average ocean temperature is 4C. If we calculate that out we get the total heat in the top 2000m is roughly on the order of 10^27 so 4 orders of magnitude larger than the change.
This paper is more up to date and includes estimates for the change in ocean heat content in W/m^2.
posted by dreyfusfinucane at 6:15 PM on January 27 [3 favorites]