Penhoat, Yves du

Reverdin, G.

Servain, Jacques

Horizon / Pleins textes

Number 47 November 1988 
Sea Surface Temperature: a Comparison between Ship Reports from 
Marine Decks and Ship-of-Opportunity Subsurface Data in the Tropical 
Atlantic Qcean 
Marine reports from merchant ships in 
marine decks are routinely used to analyze 
the large-scale seasonal variability of the sea 
surface temperature (SST). Marine reports, 
together with other in-situ data, are also used 
(Reynolds, 1988) to correct possible biases 
in SST retrieved from-satellite-borne radi- 
ometers (McClain et al., 1985; Strong and 
McClain, 1984). Whether these data are of 
sufficient quality for this purpose is a major 
concern. Indeed, the noise level of the ma- 
nne reports is large (reported to be of the 
order of IOC), and is assessed by Reynolds 
(1988) to be a significant source of uncer- 
tainty in his large-scale monthly SSTanaly- 
sis. 
Here, specifically, it is the systematic bi- 
ases which we discuss for recent years. The 
background question is whether the marine 
reports should be incorporated in  analyses of 
SST for TOGA. Since the early 1950s, the 
marine decks have primarily incorporated 
estimates made in two different ways which 
can differ noticeably: the temperature of 
water collected at the sea surface in an insu- 
lated bucket, or a measurement in the ship- 
intake conduct close to the ship hull. The 
differences between the two measurements 
is expected to be a few tenths of a degree, but 
it is not clear which one is the more accurate 
(Parker, 1988). The comparison by Tabata 
(1978) between the mechanical bathyther- 
mographs of station Papa in the southem 
Gulf of Alaska, and nearby ship reports 
transmitted in real time, suggested that ship- 
intake measurements could have a positive 
bias of 0.2"C. As air-sea temperature differ- 
ences in the marine decks has not evolved 
duringthelast30 years (Follandeial., 1984; 
Barnett, 1986; Wright. 1986). i t  is expected 
-60 -50 -49 -33 -20 -10 0 !O 20 
FIGURE 1 (Reverdin, ei al.) 
Siib.yu@ce ~ciriperaiure dala fo r  December, 1981. The XBT line selecicdfor lhe 
(inulysis is ourlined ot1 a monthly disiribiition of subsurJace profiles in r3ecenll)er. 19SI. 
that, recently, no major change in the biases 
of the SST from marine decks has taken 
place. Many reports in national marine 
decks lack the code for the SST estimation 
technique, and it is often only feasible to 
consider the data as a whole. 
Comparisons between the marine decks 
have been done with other in-situ measure- 
ments, including drifting buoy measure- 
ments (as in the satellitecalibration attempts 
presented in Reynolds [1988]), hydro- 
graphic station data, or moored instruments 
(see, for example, the studies on the clima- 
tological mean field by Reynolds [ 19831 and 
Levitus [19871, or, for recent years, a com- 
parison in Barnett [1984]). These compari- 
sons are often difficult to interpret, as d3u 
are not simultaneous in time and space. 
Recently, in the tropics, data from espcnb- 
able bathythermographs (XBTs) have bwn 
collected for TOGA by ships-of-opportu- 
nity. Usually, these ships run along \veli- 
sampled routes where a large nunthtr  I I :  
marine reports are available, and thers is :i 
larger chance of getting significant resuirs. 
XBTs are not frequently used, and samplinp 
errors will be a significant constraint. Also. 
XBTs are not very accurate instmmenis 
(measurements in a laboratory show that the‘ 
sensor uncertainty is of the order of 0.05"c' 
[Roemmich and Cornuelle,1987]. but thcr:. 
arc also recording errors, and they do n r x  
correctly measure the temperature in th:. 
upper 3 meters). SST is usually otuin;..i 
from a subsurface depth, and this may vary 
with the recording instrument (sometimes, i r  
is also hand digitized). Although laborator!, 
experimcnfi do not show a mean bias (Ro- 
emmich and Cornuelle, 1987), posirive b¡- 
November 19SX 
Fonds Documentaire ORSTOM 
I .  
28 
(o 
2 
2 o 
26 
- sea surface temperature from XBTs 
sea surface temperature from ship reports 
. .  
24 t r I I I 1 I l 
' D J D J D J D I D J D ~ ~ f ~  
1904 1885 I908 1907 1980 1981 1902 1983 
FIGURE 2 (Reverdin, et al.) 
Comparisons between time series of SST. Figure 2a is for a bin centered at 10.5OS and containing 212 XBTprofiles (rms deviation 
with the m a l y i s  is 0.24"C). Figures 2b and 2c are for the bin centered at 6"N. There are 324 XBTprofles (rms deviation with the 
analysis of 0.25"C). 
30 
28 
26 
4 - sea surface temperature from XBTs 
- - - sea surface temperature from ship reports 1 
24 I 1 I I I I 
J D J  
1980 1861 
ases are expected according to earlier in-situ 
studies (Heinmiller, et al. [1983] report a 
0.19"C bias for the T4 type, and a 0.13"C 
bias for the T7 type). 
Here, we investigate one of the best 
sampled lines. Located in the lropical Atlan- 
tic, it runs along the NE-SW axis between 
Cape Verde Islands and Brazil (Fig. 1), and 
has been regularly samplcd since mid-1980 
D I D I D I D I D I D I D  
1 882 1Qß3 lQß4 1985 l S ß 6  1887 
by two ship-of-opportunity programs (man- 
aged respectively by the French agency 
ORSTOR? and by the German agency 
DHD). These ships sample the SST approxi- 
mately every 1 .5" of latitude and, north or 
the Cape Verde Islands, the ship routes arc 
distributedover 5" of longitude; theanalysis 
bins will be 1 .5" in latitude and 5" in longi- 
tude. In 19S3-19S4, we havecomplementcd 
the data-set with subsurface profiles col- 
lected for the French-American cooperative 
program FOCAL-SEQUAL. Except for a 
few interruptions @articularly in late 1982- 
carly 1983), this well samples the low-fre- 
qucncy variability, with a monthly average 
ofthreereportsperbin within6"S-1O0N, and 
two reports farrher poleward. In each bin, 
individual data depart from an analysis of 
the low frequencies with an rms deviation 
TROPICAL OCEAN-ATMOSPIIERE NEWSLETTER November 1985 
- - - - seo surfoce temperatun from ship reports 
24 - I  I I 1 I I l I l 
J D J D J D J D J D J D J D I D  
1980 1981 1882 1883 1884 1805 1906 1887 
smaller than 0.3"C, and sampling uncertain- 
ties on monthly time scales are often no 
more than 0.15"C. 
In the same bins, we gathered marine re- 
ports for 1982-1984 from a marine deck 
compiled at the Deutsches Seewetter Amt 
and the TDF-11 compiled at the American 
National Meteorological Center. The latest 
significant update for these years was car- 
riedoutduring July, 1988. Aftereliminating 
most duplicates and reports with unexpected 
values (a too-large wind or air-sea tempera- 
tarre difference), we are left with a monthly 
average number of reports of the order of 50 
by a 1.5" latitude bin between 1O0Nand4"S, 
and 80 reports along the Brazil coast The 
lower number of reports to the north is the 
unexpected result of the shift to the east of 
the XBT line from the line sampled in the 
marine decks (the French vessels often 
travel closer to Africa than the vessels se- 
lected in the German program). Monthly 
rrrs standard deviations range between 
1 .O"C south of 5"s or north of 10"N to 03°C 
closer to the equator. As these are much 
larger than the ones in the XBT fíes, we can 
expect them to result from random noise, 
and the resulting uncertainty in the monthly 
mean is less than 0.15"C within 10"s-10"N. 
Two examples of bin time-series are pre- 
sented. For the fmt one at 10.5"s (Fig. 2a), 
SST experiences a moderate seasonal cyclc 
which has a similar amplitude every year, 
superimposed to lower frequencies. On the 
second one at 6"N (Fig. 2b), a latitude with 
no sharp spatial gradient in the sea surface 
remperature, SST has a smaller seasonal 
cycle, with a maximum SST in September- 
October when the eastward-flowing North 
Equatorial Countercurrent is well estab- 
lished. At both locations, the XBT analysis 
and the ship reports suggest sin@ar charac- 
teristics of the seasonal variabiiliiy: Therms 
deviation between the two monthly analy- 
ses, which is less than 0.30"C between 6"s 
and 10"N, has.a magnitude comparable to 
the expected rms uncertainties due to sam- 
pling of the two analyses. 
Hcpwever, in the two cases presented, 
temperatures from ship reports are colder 
than the XBT analysis in most months. This 
happens at most latitudes, except between 
10"Nand 14"N,andbetween 3"Nand 1.5"s. 
where there is a larger zonal gradient, with 
colderwaters where the XBT line runs to the 
east of the routes most heavily sampled in 
the marine decks. Elsewhere, the mean bias 
is of the order of -0.20"C. This bias has the 
sign opposite to the one which we expected, 
assuming that it was the positive bias f" 
the ship intake measurements which would 
dominatetheerror. We eliminated thepossi- 
bility of a different sampling of the diurnal 
cycle in the XBT program and the ship 
reports, as in both of them there is the Same 
excess of daily reports over night reports 
(10% more reports during the day time). We 
also separated day and night measurements 
in the analysis of the ship reports, and found 
that the day-night difference averaged 
0.49"C, with little latitudinal or seasonal 
dependence (there was also a daily cycle in 
reported wind stress, assuming a constant 
drag coefficient, with day-time wind 
stresses exceeding night-time wind stresses 
by 12%). The XBT analysis, which is an 
estimateofthe temperaturebelowa 3-meter 
depth, has a much smaller amplitude, and if 
it was the day-time SST in ship reports 
$which was too warm, we would expect the 
SST analysis h m  the marine decks to be 
overestimated. 
We then considered another SST product 
SST-S, derived from marine decks available 
earlier and containing less ship reports than 
in our shipreport analysis (Servain et al., 
1987) for data up to 1984; an update to 1987 
has been compiled later). Random monthly 
differences between SST-S and the XBT- 
based analysis of SST are larger (rms of the 
order of O.4O0C), and random differences 
with the more recent ship report analysis 
have an rms of the order of 0.30"C; SST-S is 
also warmer (Fig. 2c) than the SST from the 
morerecent ship reportanalysis,and in mos[ 
places it is even warmer than the XBT analy- 
sis (the mean difference is O. 1 1°C). We have 
no explanation for these systematic diflcr- 
ences. However, even at 6"N where thc 
interannual variability is small, the seasonal 
variability is portrayed similarly in the dir- 
ferent analyses (Fig. 2b, 2c): there, in  b c  
different records up to 1956, there arc mori5 
year-to-year differences in the minimuni 
temperatures than in themaximum tempcr:i- 
tures. The similarities in the low frequcncic. 
are widespread, and there is hope that intcr- 
annual variability is well portrayed by tlic 
ship reports along well-sampled lines. 
The systematic differences between the 
two SST analyses from ship reports and the 
TROPICAL OCE'NJ-ATMOSPHERE N E W S W E R  November 1958 
4 <  
XBT analyses were found along a ship line 
which is particularly well sampled. We did 
not comprehend what caused the biases, but 
processing of the ship reports is a likely 
source of biases. This suggests that it may be 
difficult to merge ship reports with the vari- 
ety of other SST measurements envisioned 
in TOGA to produce accurate, large-scale 
SST fields. Although the XBTs yield less 
noisy estimates of temperature at a depth of 
3 meters and are easier to control, their 
absolute accuracy should be monitored 
closely as instruments and recording sys- 
tems evolve. 
.i 
References 
Barnett, T.P., 1984: Long-term trends in 
surface temperature over the oceans. 
Mon. Wea. Rev':: 112,303-312. 
Folland, C.K., D.E. Parker, and F.E. Kates, 
1984: Worldwide marine temperature 
fluctuations 1856-1981. Nature, 310, 
670-673. 
Levitus, S., 1987: A comparison of the 
annual cycle of two sea surface tem- 
perature climatologies of the world 
ocean. J .  Phys. Oceanogr., 17, 197- 
214. 
McClain, E.P., W.G. Pichel, and C.C. Wal- 
ton, 1985: Comparative performance of 
AVHRR-based multichannel sea sur- 
face temperature. J .  Geophys. Res., 90, 
11,587-1 1,601. 
Parker, D.E., 1985: A comparison of bucket 
and non-bucket measurements of sea 
surface temperature. Meteor. Off. 
[U.K.], 13 Branch Memorandum 157. 
Reynolds;R.W., 1983: Acomparison of sea 
surface temperature climatologies. J .  
Climatol. Appl. Meteor., 22,447-459. 
Reynolds, R.W., 1988: A real-time global 
sea surface temperature analysis. J .  
Climate, 1,75-86. 
Heinmiller, R.H., C.C. Ebbesmeyer, B.A. 
Taft, D.B. Olson, and O.P. Nikitin, 
1983: Systematic errors in expendable 
bathythermograph (XBT) profiles. 
Deep-sea Res., 30, 1185-1 197. 
Roemmich, D., and B. Comuelle, 1987: 
Digitization and calibration of the ex- 
PendablebathYthermOgraPh. Deep-sea 
Res., 34 (2A): 209-307 
Servain, J., M. Seva, S. Lukas, and G. 
Rougier, 1987: Climatic atlas of the 
tropical Atlantic wind suess and sea 
surface temperature: 1980-1984. 
Ocean-air Interactions, 1, 109-182. 
Strong, A.E., and E.P. McClain, 1984: 
Improved Ocean surface temperatures 
from spacecomparisons with drifting 
buoys. Bull. Amer. Meteor. Soc., 65, 
138- 142. 
Tabata, S., 1978: Comparison of observa- 
tions of sea surface temperatures at 
Ocean Station P and NOAA buoy sta- 
tions and those made by merchant ships 
travelling in their vicinities, in the 
northeast Pacific Ocean. J .  Appl. Me- 
teor., 17,374-385. 
Wright, P.B., 1986: Problems in the use of 
ship observations for the study of inter- 
decadal climate changes. Mon. Wea. 
Rev., 114, 1028-1034. 
Gilles Reverdin 
LODYC, Paris VI,  Tour 14,4 Place 
Jussieu, Paris 05, FRANCE 
Yves du Penhoaf Jacques Servain 
Centre ORSTOM, IFREMER, 29263 
News of the International Tropical Ocean Global Atmosphere Programme 
TOGA, the Tropical Ocean ana Global 
Atmosphere programme, is an international 
research experiment designed to improve 
the understanding of events in the tropical 
oceans and global atmosphere that signifi- 
cantly influence the predictability of sea- 
sonal to interannual variations in the Earth's 
climate. TOGA is acomponent of the World 
Climate Research Programme (WCRP), 
which was established by W O  ancl ICSU, 
and is carried out in association with IOC 
and SCOR. The scientific planning and 
development of TOGA is under the gdd- 
ance of the JSC/CCCO Scientific Steering 
Group for TOGA, assisted by the 1 ~ t a - r "  
tional TOGA Project Office. JSC and to the TOGA program: 
CCCO are the main bodies of WMO-ICSU 
and IOC-SCOR, respectively for formulat- 
ing overall WCRP scientific concepts. 
TOGA'S intergovemmental coordination is 
undertaken jointly by WMO and IOC. The 
following items were prepared by or for thc 
International TOGA Programme office to 
communicate developments of importancc 
NEWS FROM THE INTERNATIONAL TOGA PROJECT OFFICE 
Undoubtedly the most significant recent 
event for the ITPO was the successful site 
survey that was carried out in Gan in Sep- 
tcmbcr by Mr. Pcter Budgen of the U.K. 
Meteorological Office. In a separate article 
Valery Lee explains the details of this proj- 
cct which will lead to upper winds from Gan 
being reported on the Global Telecommuni- 
cation System. 
The growing support for thc proposcd 
TOGA Couplcd Ocean Atmosphere Re- 
sponse Experiment (COARE) in the warm 
pool in the westcm Pacific has bccn moni- 
tored with keen intcrcst. Two future events 
that wil l  be important for the development of 
TOGA COARE are the prescntation by the 
Unitcd States to the second session of the 
Inter-governmental TOGA Board in Dc- 
ccmber 19SS and ~ h c  Intcmationnl Work- 
shop that will be sponsored by France, tlic 
USA and the ITPO in Noumca, 23-30 May 
1989. An announccment about this Work- 
shop appears separatcly in this issue. 
To follow up the recent meeting of the 
TOGA Scientific Steering Group (reportcd 
in TO-AN 46) the TOGA XBT Operations 
and Management Committcc has been insti- 
TROPICAL OCEAN-ATMOSPIIERRE NEII'SLETTER November 1985 


Sea surface temperature : a comparison between ship reports from marine decks and ship-of-opportunity subsurface data in the Tropical Atlantic ocean

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