Pendant le fort El Nino 1982-1983, deux réseaux d'observations océanographiques ont fourni des données continues dans le Pacifique tropical : le réseau de mesure du niveau de la mer créé et maintenu par l'Université d'Hawaii et le réseau XTB - navires marchands du groupe SURTROPAC, Centre ORSTOM de Nouméa - La topographie dynamique mensuelle 0-400 db est calculée le long de la route Nouméa-Japon (20°S-20°N, 150°E-165°E), en utilisant des relations T-S réelles, pour faire ressortir les anomalies interannuelles. Cette topographie est comparée aux valeurs mensuelles du niveau de la mer. Ce très bon accord obtenu entre les variations spatio-temporelles déduites des deux réseaux montre qu'ils se complètent utilement dans la surveillance des anomalies climatiques. (Résumé d'auteur

Donguy, Jean-René

Eldin, Gérard

Wyrtki, K.

Horizon / Pleins textes

I .  
I 
October I 9 8 G  ' Number31 
Sea Level and Dynamic Topography in the Western Pacific 
Dwring 1982-83 El Niño 
During the strong 1982.83 EI Niño, two 
oceanographic networks gathered continu. 
ous data in the troplcal Pacific: the sealevel 
network established and opented by the Uni- 
versity of Hawaii, and the XBT network using 
ships ofopportunity established and operated 
by the Groupe SURTROPAC of the Centre 
ORSIDM de Noum& The latter is a joint 
Fmce4J.S. prognm and covers three main 
routes: Noum&Japan, Noum&a-California. 
and Noum6a-Tahfti-F'anamS. 
Sii?j¿¿ce cimrlation deriwdfrom sen Iewl In 
the mtmi Pacific 
Monthly maps of sea-level anomaly in the 
Pacific for 1982.83 (Wyvy.rtki, 1985a) show 
abundant data in the western Pacific (Figure 
1). particularly in the vicinity of the NouniCa- 
Japan shipping route. Sea-level deviations 
from the long-term mean at each station 
have been plotted along this route in the form 
of a space-time diagnm for the years 1979 to 
1983 (Figure 2). From 1979 to May 1982, 
deviations of sea level are small and exceed 
10 cm only on a few occasions, indicating 
that the mean topography of the se3 surface 
was relatively undisturbed during this period. 
In June 1982 a strong negative anomaly de 
vcloped near 1O"Nat thelocationoftheCoun- 
tercurrent trough. The anomaly rcached a 
peak value of -30 cm in January 1983 and 
lasted until June 1983. South of the equator, 
sea level also dropped in late 1982, but the 
lowest deviation was reached near 10% in 
July 1983. Sea level returned to normal In 
June In the Northern Flemlspherc and not 
before the end of 1983 in the Southern Hemis- 
phere. 
The rapid drop of sea level near 10% in 
1982 indicates an intensification of the North 
Equatorial Countercurrent. In 1983, in the 
Southern Hemisphere, the depression of sea 
level also indicates an anomalous circulation. 
North of the minimum, between the equator 
and loos, surface castward flow moved warm 
waters to the eastern Paciflc. Sea-level 
anomalies during the 1982-83 warm event 
showed a southward displacement of the sub- 
tropical gyre in the southwestern Pacifc and 
consequently a strong slope in sea level bet- 
ween 10' and 203, inducing a strong geos- 
trophic flow from east towest (Wyrtkì. 1984). 
l h e  westwad flow south of the minimum 
would be identified as the South Equatorial 
Current. 
1 2 0 ' ~  
- I  - 
using actual T-S relationships. to emphasize 
in temual  anomalies. 
Avenge T.S relationships have been com- 
puted for the Pacific Ocmn (Emery and 
Dewar, 1982). These T-S relationships must 
be Improved - particularly in the southwwt- 
ern Pacific for several rearons: Ihey are limited 
to 27% although surface temperature can 
p;rsS 30°C aid they do not extend south of 
IO'S. hloreover, thrue TS relationships are 
long-term averages and they do not represent 
In temual  fluctuations. which are important 
in the western tropical Pacific as shown by 
surface salinity data (Donguy and Dessier. 
1983). Consequently, the T-S curves have 
been modied  by the following methtd. 
In the western trupical Pacifc, all salinity 
proflles exhibit a similar shape - an increase 
from Oie surface to a more or less well-dcfìned 
salinity mivxiinuni in the thermocline and a 
decrease downaard. Below the salinity 
maximum, T4 curves of cruises carried out 
during the period of El Niiio show only weak 
devlations of salinity from the nican and reg- 
ular T-S relationdiips can be used (Morlil\rc 
and RCbcrt, 1985). From die snlitiity 
maxlmum up to the surface, sca-surface salin- 
h and temperature mthcrcd bv shins of OD- 
Variations of the sea surface can also be 
Suface cimclniionfruni XßT in ifJe U w m t  
monitored by means of dynamic topography 
computed with data from the XßT netw-ork 
Monthly dynamic topography relative to 400 
decibars has been computed along the . 
Noum&.Japan route (20% - 20%, 150"E - 
Pncific 
o. 
165"E) between 1980 and 1984 (Firmre 3 l  ' . '  
20' 
s 
30' 
linear relationship is taken between the 
monthly averaged surface temperature-salin. 
ity and the salinity maximum on the T-S curve 
(Figure 4). Although the actual relation is not 
always linear, the exact choice of the relation 
dects the results only slightly, as long as it 
describes a continuous increase in salinity. 
Monthly T.S cuwes are obt:iined for the 
lations. Results show up to eight dynamic ccn- 
tinieten difference in dynamirheight relative 
to 400 decibmconipared to cJlculatitinsper. 
bmied wlth a constant T.S relationship. 
From 1980 to 1981, the usual pattern oc. 
curs (Figure 3) as earlier described by Wynkl 
(1974): a maximum of dynmic height at 5"s 
(South Euuarorlal Ridae) and a minimum at 
period 1980.84 and uscd for d y ~ ~ m ~ i c  C ~ I C U -  
portunity i;l the SU~~IROPAC ' p r o b  &e , B"N (North EqunGrial ~ountercurrent 
used to complete the curves. In order to Trough). There was noappeamceofse;lsond 
spectfy W t y  within the surface layer, a variations during this time. From August 1981 
October 1986 7l?OPICAL 0CEAN.ATMOSPHEhF NEWWTIIX 
'. ! 
3'1 $ANY. 1989 
ORSTOM  FON^ Documentaire 
Cote 5 
No I: i l 
20' :.R 
0' 
S 
20' 
1979 
FIGURE 3 (Dongiy, Eldin, und Wj571kkl) 
Dyttainic topogmpby in dpunric centfineten, 1979-1W4, relarira to 400 db, dong Uze ship- 
ping n i t e  from New Caledonia to Jupean. 
to February 1982, the South Equatorial Ridge 
that this area was occupied by lowdensity 
water due to a great heat content. At that 
time easterly winds prevailed and surface sa- 
linity was high (Donguy and Dessier, 1983). 
These features could be interpreted as the 
existence of a bulld-up In the western Pacific 
associated with a westward current from at 
the calculation made by K ' y ~  (1985b) who 
found a positive heat anomaly in 1981 by 
integration of sea level between 15% and 
15%. 
Starting in June 1982, d)namic heights de- 
crcascdbetwecn 15Wand 15%, butespcrlally 
, expanded From the equator to 15%. Implying lest 5"N to 5%. They are du) consistent with 
a 
,! 
1ROPIc4I. OcEiw-AliUOPHERE NEWSLEGWZ 
in the vicinity of SoN. The decreare Seems to 
propagate southward, reaching 10% in March 
1983. This feature is very similar to the sea. 
level anomaly observed at the same time (Fi. 
gure 2) and can be Interpreted in die same 
way. Between Septeniber 1982 and April 
1983. during the mature phase of the wann 
event both the North Equatorial Countercur- 
rent and the " l i  Equatorial Current were 
strengthened. In the vicinity of the equator, 
flat dynamic topography indicates the pre- 
sence of a weak csnvard current. 
In the Southern'Hemispliere a dynamic 
trough develops during 1983, instead of the 
usual South Equatorial Ridge, and induces a 
strong westward South Equatorial Current be- 
tween 10% and 15% (Wyrtkl. 1985a). This 
feature seems characteristic of El Niño condi. 
tlons as noticed by Donguy et nL (1985). The 
end of the warm event is apparent in the 
beginning of 1984 by a return to conditionr 
observed prior to 1981. 
Colrclrlsio?ls 
The excellent agreement ofthe time-space 
pattems of variations of both sea level and 
dynamic height indicates that the two 
parameters can be used jointly to monitor 
the variability of ocean circulation. Reben et 
nL (19ß5) have already shown that sea level 
can be used to monitor both dynamic height 
and heat content a t  Individual stations. The 
next important step will bc to combine the 
coverage of D T  lines h d  ofsealevel stations 
for the preparation of maps ofdynamic height 
and heat content for the equatorial Pacific. 
The data From the two networks are com 
plenicnlnry in other aspects as well. They 
combine the spatial coverage of XBT lines 
sampled at a particular time with the continu- 
ous record of sea level at discrete stations. 
These two networks will probably constitute 
the basic large-scale monitoring elfort of the 
1DGA proyam. 
October I98G 
, 
I 
Refereces 
Donguy,J.R, and A. Dessier, 1983: EI Niño-like 
events observed in the tropical Pacific. 
Moir Wea Reu., 111. 2137-2139. 
Donguy, J.R., G. Eldin, A. MorliCre, and J.P. 
Ili.lxrt, 1985: Vulabillty of dyn:imlc top)g  
raphy and equatorlal currents In relation 
to hydrtxlimatlc conditions of the Western 
Pacific. Occ'nirogr 7top., 19, 155- 160. 
Emery, WJ., andJ.S. Dewar, 1982: Mean temp 
erature-salinity, salinity-depth, and temper. 
ature-depth curves for the North Atlantic 
and Nonh Pacific. Prop. Ocenttogr, If, 
MorliCre, A., and J.P. RCbert, 1985: Mean hyd. 
nrgnphic conditions in South WLyt Pacific. 
Rapport ScientiRque et Technique, 
O R S i M  NoumCa, no. 33,35 pp. 
219.305. 
RCben, J.P., J.R Donguy. G. Eldm and K 
Wynki. 1985: Relations behveen sea level, 
themlocline dcpth, hiat content, and 
dynamic height in the tmpiral Pacific 
0cean.J GwpQr Res, m, 1 I ,  719-1 I, 725. 
Wyrtkl, K. 1974: 71n- d'~imiilc tojmgtripl~y 
Ilawail Institute of Gcmpliysics, Repon 
HIG-74.5. Univenity of Iiamaii, 19 pp. 
Wynki, K. l98.i: A southvcud displacenient 
of the subtropical gyre in the South h c x c  
during the 1982-83 EI Niiio. l bp .  Ocem- 
Abfios Newsbrr., No. 23, p. 14. 
I I I  t/n- pnci/ic dtirftlg ti= EI m o  of 1982 
and 1983. Time Seria of O c m  hleasure- 
nicnts, vol. 11, 1.O.C. Tech. Sedm No. 30, 
Unesco, Paris, pp. 43-54. 
Of IhC rt1dj-k 0 C e f l l l  f l l ld  / fS / r t iC~l t i l !~ fJt lS  
Wynki, K, 1 9 8 5 ~  h f O l l t / J ~  IIUljX Of Sen I C t d  
3 
Wyrtki, K. 1985b Water displacements in the 
Pacific and die genesis of  El Niiio cycles. 
I. Gt'OjL!I'S Res, 90, 7129-7132. 
A Global Sea-Level Network: How Many Gauges are Enough? 
For the past two centudes, tide gauges have 
provided the only m r m  of measuring mean 
sea level (MSL) and have usually been instal- 
led for specific local purposes (e.g., harbor 
navigation, storm surge, or tsunanii warning) 
unconnected with global circulation in- 
terests. In recent times, however, hlSL has 
come to be regarded as an essential oceanog- 
raphic parameter for use in the tropics as a 
proxy for heat content and as a measure 
worldwide of geostrophic surface currents. 
The first attempt to weld strategically placed 
gauges into a global network is the proposal, 
now called "GLOSS" (Global %-Level Ob- 
serving System), designed for the 1DGA prog- 
nm as well as for the WOCE period and 
beyond. This plan (IOC, 1986), which can bc 
taken as a first model for a continuing global 
network, envisages a total of approximately 
250 gauges (150 of which already exist), the 
main selection criterla for which are (1) the 
allocation of a gauge to each island or each 
island-group at intervals not closer than 500 
km; and (2) the allocation of gauges along 
continental coasts at intervals not less than 
IO00 km. The authors of the proposal 
(primarily Prof. K Wynkl and Dr. D.T. Fu&) 
stress that these lnitial criteria are somewhat 
arbitrary, make no allowance for different sea- 
level variability in different regions, and will 
require modification in the light of experi- 
ence. In fact, the results of the investigation 
described below show that these are quite 
reasonable initial criteria, although for full 
global coverage the MSL data-set will need 
expansion by means of precise satellite al- 
timetry. 
TROPIW OCEAN-ATMOSPHERE NEWSLEITER 
It is possible to evaluate Oie suitability of 
the suggested spacinp of criteria ( I )  and (2) 
f" the existing worldwide tide-gauge re- 
cords stored by the Permanent Service for 
Mean S a  Level (f?WSL). \Illile preference 
in the choice of gaogr~ for the purposrs of a 
globd network must be given to those gauges 
with mximum exposure to the open ocean, 
all records msII conrain a contribution from 
local forcing (Le., local air-pressure fluctua- 
tions, wind setup. and runoff) which has to 
be understood and removed from the sea- 
level time series from each station. Usually 
the local effccts are larger along coastlines 
with a wider contlncntal shelf. If lofal cUccts 
dominate, then the gaug~5become unsuitable 
for global monitoring as a much denser reg 
ional network required to provide intercon- 
nection of Mormation across the network 
Figure I(a-n) show the average (zerolag) 
correlation between pain of stations dong 
the coastlines and in island groups for which 
suffìcient data exist. Annual \dulues of hlSL 
have been used to provide an estimate of the 
average correlation over time scales from two 
ycars to decad~s. In addition. each station re- 
cord has been de-trended so the correlations 
should not include a contribution from the 
secular changes in sea level due to land move- 
ments or low-frequency climatic change. The 
requirement for the optimum spacing for a 
global nehwrk In this frequency range can 
be expresscd as an average correlation of 0.5 
between statlons, La, at a distance apart at 
which half the variance of the sea-lwel varia- 
bdty at two stations is correlated. With such 
a requirement, thcn the variability in common 
behvecn dilferent elements of the nehvork, 
which will be representative of the regional 
variability, should certainly be extractable by 
statistical techniques such as Principal Com 
ponents Analysis. 
111 general, Figure I(3.n) suppons the 
hitherto arbitrary dcdgn spacing (motivated 
primarily by cost and other practicalities) be- 
tween sea-level measuring stations as 
specified in criteria ( I) and (2) of IOC ( 1986). 
Ille American Pacific data (Fiyre 1 [b.nl), 
which contain strong contributions from EI 
Nino events, sugest that a spacing wider than 
1000 km could be silficient in that area, al- 
thougli aiere are in hct very few opentioinl 
gaug~s along the southern parts of the 
coastline (Figure I[d]). Enclosed sc& (Fiyre 
111 and m]) also show a high degreeofcoher- 
ence behveen gauges and may, for global 
studies purposes, be represented by relatively 
few tide gauges. lhe weakest average corre- 
lation between stations for distances up to 
1000 km is shown in the data from the Euro- 
pean Atlantic coastline (Dehark to Spdn, 
including the British Isles (Figure 1[k]). How- 
ever. this figure includes gauges around the 
North and Irisliseas which will not necessarily 
be representative of ocean conditions and 
which contain a large amount of "local" var- 
iability as discussed above; two gauges, New- 
lyn and La CoruiW, 780 km apan and closer 
tllan most to the open ocm show an 
adequate correlation of0.73. For criterion ( I), 
Figure 1 (ef) shows data from the central 
PaciEc and Philippines, respectively, and s u p  
ports the use of a 5 0 0 -  to IOOO-km spacing 
in thesc areas. More detailed discussion of 
October 198G 


Sea level and dynamic topography in the Western Pacific during 1982-83 El Nino

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