Subduction Temperature Errors
2014/04/29
N. Galbraith
Temperature records were removed from the UOP ‘archives’ web area because of
errors, first brought to our attention via email:
I am interested in using the subduction array data, but I have noticed
some peculiarities in the data and I was hoping that you might be able
to help me. At some moorings and at some times (sufficiently often to
show up on a three month average) the water temperature is higher at 10m
and 30m depth than it is at 1m depth. It is occasionally also higher at
50m depth. I have looked in the single publication referenced at the
bottom of the page
http://uop.whoi.edu/uopdata/subduction/subduction.html
that I have access to, but no mention is made of any such feature. Do
you know how or where I might go to access the WHOI technical notes, or
who I might contact to obtain further information about this and other
subduction array data issues?
Many thanks in advance for your help,
John Kennedy
John Kennedy Climate Monitoring and Research Scientist Met Office
FitzRoy Road
Exeter
EX1 3PB
United Kingdom Tel: +44 (0)1392
885105
Fax: +44 (0)1392 885681 E-mail: john.kennedy@metoffice.gov.uk
http://www.metoffice.gov.uk Global climate data sets are available from
http://www.hadobs.org
1) There are times when we might expect a temperature inversion:
- A colder but presumably fresher layer was at time seen to slide in under
the surface mixed layer (see xbt profiles 1-9), so as you went deeper thru
that layer and came out the bottom you hit warmer but presumably saltier
water so that the temp profile has inversions but I'd expect that density did
not.
- Rain is falling. Rain falls at the local wet bulb, usually colder than SST, forms
shallow fresh lenses for a day or so depending on the wind mixing. The third
profile from the left on the xbt profiles 180-189 might be evidence of such a
fresh lense. Other profiles in this plot look to have a slight increase with
depth of surface mixed layer temp.
2) the moorings lacked salinity sensors
Ideally, I would have done a quality control step on the moored temperature
data by computing density and looking to see if the density profile was
stable. This is what we do now on more recent deployments where we
accept as real salinity compensated temperature inversions, or, in high time
resolution data, evidence of overturning.
However, subduction was an early long-term mooring experiment and we
only had temp sensors that would last. So no way to do the stability check,
and against the time desribed in 1) above when there are real inversions, we
found no good way to go after detecting and fixing false temp offsets.
3) a mix of instruments
This experiment drew down the resources at my lab at WHOI and at the folks
we partnered with at Scripps Inst of Oceanography (SIO). So a mooring like
Sub 3 NW had a Vector Averaging Wind Recorder for SST, a WHOI Brancker
temp logger at 1 m, an SIO temp logger at 10 m, then WHOI Brancker temp
loggers at 30, 50, 60, 70.... Given this, I worry about the calibration of the
NW3 10m temp vs the NW3 30 m temp as two different labs, two different
processing schemes were involved.
Then there is the issue of clock drift. These instruments had a sampling rate
of 15 min typically. We checked the crystal clocks in the processing and tried
to do linear interpolations to get to corrected time bases, but I suspect after 8
months we might have a skew of 1 or 2 sample intervals, which in the
presence of internal wave variability might, depending on who lags or leads,
give the type of ± temp difs at high frequency in your plot. If one or both of
the instruments was at or near the base of the mixed layer, I'd expect
something like this as the based moved up and down.
4) the 1 m vs 10 m
I am most concerned about the cooler 1 m temp on your other plot. It looks
to have the same cool bias before and after the diurnal peak. I do not think
this is a geophysical signal.
5) strategy
The issues above led to not implement a correction procedure on the
archived data. From XBTS and CTDs there are at time temp inversions; we
did not have salinity and could not do a stability check.
If I was to work on this data, I would do the following: 1) look for times of
strong wind (so there is good mixing) and take the temps from within the
mixed layer. 2) Assume there is strong enough stirring that the mixed layer
should be isothermal as well as isopycnal. 3) Identify there persistent biases
between temps in the "mixed layer" and correct for them (to what? a bit
arbitrary here to choose which is correct, but I'd add the SST from the met
record in to include that and might correct to that as it was deeper than any
skin temp). 4) check and see if a bit of bias correction helps the fast time
repsonse issue.
5) if the fast time sampling temp data still shows plus and minus difs, one last
effort can be made to verify time base issues. I do a coherence and phase
calculation, looking to see if as you go to high frequencies the phase tails off
to one side of 0 or the other, then I shift one time series with respect to the
other by one sample interval and rerun the coherence/phase; sometimes by
introducing a time shift you bring the phase back to zero.
John, I hope this helps. Basically, there are limits to the data. Our group site
http://uop.whoi.edu shows other moored data sets that we would be happy
to share. In the years since the Subduction experiment, instrumentation has
improved. The moored temp loggers we use now keep their calibrations to
about 0.005 deg C and their clocks are good to a few minutes a year. And we
have moored salinity now too. Let me know if we can help with other data.
Best regards,
Bob Weller
2014/04/29 Update
I did a quick look at the temperature files on the web site, listing the mean value for
each bin and then plotting those where the difference between the means showed
an overall inversion. Some of these were caused just by a shallow instrument
ending early and then the temps warming up, so a deeper bin had a higher mean.
cd /Volumes/Buoy2_HD/Users/Shared/archive/work/Subduction/fromOcelotWeb
infiles=dir('*.epic');
for ii=1:length(infiles)
infile=infiles(ii).name;
clear mday temp depth meta
[mday, temp, depth, meta ]=get_epic_data(infile);
fprintf('\n%s %.1f rate(minutes)\n', infile, median(diff(mday)*24*60));
for dd=1:length(depth)
gv=find(temp(dd,:) < 100);
fprintf('%.1f %.2f %.2f\n', depth(dd),mean(temp(dd,gv)),median(temp(dd,gv)));
end
end
found inversions in
subc2t.epic:
50.0 21.23 21.10
60.0 21.44 21.29
subne3t.epic - this one has a bad bin, shows many problems
10.0 19.87 19.15
30.0 20.11 19.46
subnw2t.epic
130.0 17.90 17.88
150.0 17.96 17.93
subnw3t.epic
1.0 20.29 19.72
10.0 20.31 19.75
Sub 1 NE
Depth (m)
.5
10
30
50
60
70
80
90
100
110
130
150
200
300
400
580
750
1500
Model
VAWR
VMCM
VMCM
VMCM
TPOD
VMCM
TPOD
VMCM
TPOD
VMCM
TPOD
VMCM
VMCM
TPOD
TPOD
TPOD
TPOD
TPOD
SN
704
041
021
039
3274
032
3265
022
3288
030
3269
028
018
3300
3305
3268
3286
3293