Marine Chemistry, 35 (1991) 77-84Else vier Sc ience Publishers B.V., A msterd am

Autumn air-sea disequilibrium of CO2 in theSouth Pacific Ocean

77

P.P. Murphy", R.A. Feely8, R.H. Gammon", K.C. Kellyaand L.S. Waterman"a Pacific Marine Environmental Laboratory (PMEL), National Oceanic andAtmospheric

Administration (NOAA). 7600Sand Point Way NE, Seattle. WA 98115-0070. USA"Universlty of Washington. Seattle. WA 981 95. USA

<Climate Monitoring and Diagnostics Laboratory. National OceanicandAtmosphericAdministration.Boulder, CO 80303, USA

(Received 24 September 1990; accepted 22 March 1991 )

ABST RACT

Murphy, P.P ., feel y, R.A., Gammon, R.H. , Kelly, x.c. and Waterman, L.S., 1991. Autumn air-seadis equilibrium orco2 in the South Pacific Ocean. Mar. Chem., 35: 77-8 4.

Accurate assessment of the role of th e South Pacific in the global carbon cycle has been hindered bya paucity of da ta . In particular, the South Pacific has been suggested as a major sink for CO 2 on thebas is of measurements made pr incipally in the western basin. Th e PMEL/NOAA Marine CO 2 Pro­gram has made bas in-wid e CO2 mea su rements over 6 years wh ich now permit estimation of surfaceCO 2 fluxes during the austral autumn. These results suggest that the South Pacific is not a significantsink for atmospheric CO2 during the austral autumn.

INTRODUCTION

Major gaps in the measurement ofoceanic CO2 concentrations in the SouthPacific have hindered an assessment of this ocean's role in global carbon cy­cling. Tans et al. (1990) have suggested that because the net CO2 flux in thisregion is so poorly known, it may be the principal uncertainty in evaluatingthe oceanic role in CO 2 removal from the atmosphere. The first global LlpCOzmap of Keeling (1968) showed almost no data in the South Pacific. Taka­hashi et al. (1986) subsequently provided data for the southwest Pacific andextrapolated from west to east , presenting a zonal map of annual LlpC02 datawith the Pacific equatorial zone oversaturated, and the subtropical and polarSouth Pacific zones unsaturated. The LlpC02 maps of Tans et al. (1990)showed seasonal data, but large gaps for the South Pacific/Southern Oceanstill exist. The new seasonal basin-wide CO2 measurements presented herehelp to fill in some of the data gaps and suggest a new pattern of LlpC02 dis­tribution for the South Pacifi c.

78 P.P. MURPHY ET AL.

Air-sea flux of CO2 can be calculated from L1pCOz data using the followingequation:

F=E(moICm-2yeac l,uatm- 1)XL1pC02 (1)

where F is the flux of COz,E is the gas exchange coefficient, and L1pCOz is theatmosphere-seawater disequilibrium. There is currently no consensus on theform of the wind dependence of E. The formulation of the gas exchange coef­ficient (E) dependence on wind speed used in this work is that of Tans et al.(1990) (an alternative form was given by Liss and Merlivat (1986)):

E=:0.016(W-3) for W23 (2)

E=Ofor W<3 (3)

where W is the wind speed (m s- 1 ).

There is also no consensus on which type of wind data is optimal for cal­culation of the gas exchange coefficient and flux. For the South Pacific, theproblem is compounded by the sparsity of wind data in this region.

The objective of this paper is to present a L1pCOz map for the South Pacificconstructed from basin-wide, seasonal data collected by NOAA over 6 years,compare for one cruise track the wind speeds obtained from four differentdata sets, and construct a preliminary COz flux map using one of these winddata sets.

METHODS

Carbon dioxide was measured in the atmosphere (pC0 2 ) and in surfacewaters (PCO z ) during four separate expeditions (March-April 1984, May1987, May 1988, and February-March 1989). Geographical coverage is fromthe Equator to approximately 600S and from 1600Eto 105°W (Fig. 1).

Aliquots of air pumped continuously from upwind were dried and auto­matically injected into a gas chromatograph, where COz was catalytically con­verted to methane and quantified by a flame ionization detector. All mea­surements are relative to CO2-in-air standards prepared and calibrated by theNOAA Climate Monitoring and Diagnostics Laboratory (CMDL), formerlyGeophysical Monitoring for Climatic Change (GMCC) (Thoning et al.,1987). Data are reported on the World Meteorological Organization X85 scale.

Surface water was pumped from 5 m depth and continuously supplied at arate of approximately 20 l/min- I to a 'shower-head' type seawater-air equi­librator. The equilibrator was designed by R. Weiss and has been describedby Butler et al. (1988). An aliquot of seawater-equilibrated gas was sampledfrom the headspace, dried, and automatically injected to the gas chromato­graph, as for atmospheric samples. Results were corrected into account forthe warming which occurs on pumping seawater to the equilibrator. SeawaterPCO z values were corrected by 4.3% °C- 1

•

CO 2 DISEQUILIBRIUM IN THE SOUTH PACIFIC OCEAN 79

o·0 00 •0 • 0 •0 ~ •10· 8 0 0 0 •.

0 0 •00 ••0 •

20 · 8' 0

0 ··0 0 •

w 0 0 0c

§ 30 · S 00 0 •0 · · 0

0 0 •

It0 0 ••

~o· 80 0 •I 0 • •, 0 •

0 •0 •

BO· 80 0 • •0 o 0 "- 0·

0 0 • •0 • •

0 6. •eo· 8

I~O· E IBO· E 110· W 110· W 140· W 120· W 100· W 10· W 10· W

LONGITUDE

Fig. I. Hydrographic station locations. 0, 1987SAGA II expedition; *, 1988 NOAARITS/C02

expedition; 0, 1984 NOAA RITS/C02 expedition; 6, 1989 NOAA RITS/C02 expedition.

Atmospheric-seawater disequilibrium values (,uatm) were converted frommole fraction concentrations in parts per million of CO2 in dry air as follows:

(4)

where P; is atmospheric pressure (1.0 atm is used for all calculations); Pw isthe equilibrium water vapor pressure at sea surface temperature and salinityof 33%0; and (XC0 2 )w and (XcoJa are the mole fraction concentrations inparts per million of CO2 in the dried gas sample equilibrated with seawaterand in air, respectively.

The data presented here are pC02 data logged at the hydrographic stationsshown as symbols in Fig. 1, and are a subset of the more extensive quasi­continuous data set. The analytical precision of these data is :t 0.5 ppm forair and seawater-equilibrated vapor. The principal uncertainty in these mea­surements is in assessing the warming correction. In all cases, the warming isless than 0.7°C, which translates to an uncertainty in surface seawater PC02

of less than 1.7%.Four different wind data sets were initially examined for one cruise track.

The wind data sets are as follows:(1) Wind speeds from ship deck weather logs were averaged over the 9 h

before and 3 h after the time of the cast at depth.(2) Vector component winds given every 6 h from the Navy Fleet Numer­

ical Oceanographic Center for the actual year of the cruise were converted to

80 P.P. MURPHY ET AL.

wind speeds and smoothed spatially to a 2.5° (north-south) X7.5° (east­west) grid.

(3) Monthly mean wind speeds from the National Climate Center (TDF­11) 30-year climatology evaluated by Harrison (1989) were temporally in­terpolated and smoothed spatially as for case (2), above.

(4) Monthly winds speeds from the ISO-year climatology evaluated by Es­bensen and Kushnir ( 1981 ) were temporally interpolated and smoothed spa­tially as for case (2), above.

RESULTS AND DISCUSSION

From our quasi-continuous measurements of atmospheric and surface waterCO2 concentrations, we constructed a disequilibrium map of LlpC02 (water­air) for the South Pacific (Fig. 2). Positive values indicate surface water ov­ersaturation with respect to the atmosphere, and a potential for gas evasion .Negative values, conversely, indicate a potential for gas invasion. The equa­torial region (0-10 0S) is everywhere oversaturated by as much as 100 patm.The subtropical latitudes (10-50 0S) show an east-west gradient of70 patm,with the west undersaturated ( - 30 patm) and the east oversaturated (+ 40zzatm ). The polar region (50-60 °S) is very slightly undersaturated(LlpC0 2~ 0-1 0 patm) .

This map for LlpC02 in the South Pacific agrees with other such maps inthe major features of equatorial oversaturation and polar undersaturation,but differs from that of Broecker et al. (1986), who used annual rather than

o·

10· S

20· S

wQ

~30· S

40· S

&0· S

•.......

eo· s ~--.:---~----,.---,,----'~r-----...------4~E ~E ~w ~w ~w ~w ~w ~w ~w

LONGITUDE

Fig. 2. L1pC02 (zzatrn ) in the South Pacific du ring the austral autumn.

CO, DISEQUILIBRIUM IN THE SOUTH PACIFIC OCEAN 81

SEA SURFACE TEMPER ATURE (Oe)

O·-f-----L--........-..::::--'- - ........---'- - -'----,.-,

10'5

20"5

LUCl

~ 30'S

~40'S

SO' S

60' Wl00 'W140"W

LONGIT UDE

l BO'60'S .p::::=---r~-.,......='--r....c...-"'?'---,."""--~---,.---t­

140"E

Fig. 3. Sea surface temperature in the South Pacific averaged over February-May taken from amonthly averaged climatology processed from surface marine reports and used by Reynolds(1988).

30

o 8

25

o

201510

o 00

. . . • • . . . ~ . . . • .AtmosphericpC02

1989'. • -e' • • • •• • • • • • • • • • . • • 'Atmospherlc pCO 1984"

o •• 200

00 0

------.,;...------':)

5

-- 110W (10-60'S)-0-- HOW (10-54'S)

280 L-_---I__---I__---l__---l_ _ ---J._ _ --J

o

300

400 r;::::==::::c====:c:==::;-r--,-----,-----,

340

360

320

El 380

Temperature ('C)

Fig. 4. Temperature-Ft'O, relationships along two meridians in the South Pacific Ocean. Theregression line for data taken along 1l00W from 10 to 600S in 1989 has the equation)1= l.72x+334, ,.2=0.79, n=20. The regression line for data taken along 1700 w from 10 to'54°Sin 1984 is described by y=0.388x+ 312, ,.2=0.20, n= 20.

82 P.P. MURPHY ETAL.

9

VI'<,

7E

0ww 5c..VI

0Z 3

~

00 5° 100 15° 2Qo 25° 30°

LATITUDE SOUTH

Fig. 5. Wind speeds (m S-I) during SAGA II (1987) along 165°E from four different data sets.0, Time-space interpolation of monthly speeds from the I 50-year climatology evaluated byEsbensen and Kushnir (1981); f:::" time-space interpolation of monthly speeds from the 30­year climatology evaluated by Harrison (1989); \J, the time-space interpolation of 6-h speedsfrom the Fleet Numerical Oceanography Center; 0, mean of 12 hourly (instantaneous) ship­board measurements.

10 ' W10' W

. ..... . · 1 · ·..I,

.~

•.....•.... .

-,/ ~ .: ~ ~ , - s · · _-,::::,:~ .

. ~~: :~ :..... \~ :.:.~~~~.~~~~:..;;- '. : \ \ .

·.1··. . . ".v : . : . \:. ..~

o '-. .

• 1 \.

;···:······2 ." " "

10' 8 +---~---c--""""C-------.:---':::-':""""--c---"'T""---!I~O' E leO' E 110' W liD ' W

o'

10' 8

20' 8

WC

§ 30' S

~o' 8 ,80' 8

LONGITUDE

Fig. 6. CO2 flux (mol C m- l year-I) in the South Pacific during the austral autumn.

CO, DISEQUILIBRIUM IN THE SOUTH PACIFIC OCEAN 83

seasonal data and extrapolated from western basin measurements into theeastern South Pacific. The Fig. 2 map differs from the January-April map ofTans et al. (1990) in that it incorporates data from the southeastern Pacific.The pattern of our data is generally consistent with that reported by Miyakeet al. (1974), with some differences in the polar regions; however, directcomparison of our data with those of Miyake et al. cannot be made as theseasons of their data collection were not specified.

Although seawater PC02 might be expected to vary with sea surface tem­perature, the nearly horizontal isotherms seen in Fig. 3 indicate that temper­ature does not principally determine CO2 concentrations throughout the en­tire basin. In specific regions, a significant ternperature-Pt.'O, correlation wasfound, as Fig. 4, for the southeastern Pacific between 10 and 60oS, shows(y= 1.72x+334, r2=O.79). In the southwestern Pacific, however, the tem­perature-Pf'O, relationship is weaker (y=0.388x+312, r2 = 0.20 ). Clearly,factors besides temperature, e.g. biological activity, also playa major role indetermining CO2 concentrations in the South Pacific.

CO 2 flux at the air-sea boundary is calculated from LipC0 2 as well as fromthe wind-dependent exchange coefficient (eqns, (1 )-(3)).

To obtain some idea of potential flux uncertainties that result from using aparticular wind data set, wind speeds from four different data sets were eval­uated for one cruise track (Fig. 5). Depending on the wind data set used,averaged wind speeds at a given location can vary by as much as a factor oftwo, limiting the certainty with which E, and therefore CO2 flux, can be de­termined. Further work will attempt to quantify the levelof uncertainty whichmay result from using different wind data sets to calculate CO2 flux in theSouth Pacific.

A preliminary map of CO2 flux in the South Pacific was constructed usinga subset of the quasi-continuous LipC0 2 data and the I50-year climatologicalwind data set analyzed by Esbensen and Kushnir ( 1981 ). This map (Fig. 6)shows a band of water near equilibrium with respect to the atmosphere thatbisects the basin diagonally from the Equator west of 1700W to approxi­mately 55°S in the east. South and west of this band is a major sink region forCO2 • North and east of the diagonal is a major source region. It is not yet clearwhether biological or physical factors are dominant in producing this pattern.Areal averaging of the final results can provide a quantitative measure of thenet source or sink for CO2 in the South Pacific during the austral autumn.Our preliminary results, however, suggest that the South Pacific is not a ma­jor net sink for CO2, at least in the austral autumn.

ACKNOWLEDGMENTS

This work was supported by grants from the Air Resources Laboratory, theEquatorial Pacific Ocean Climate Studies Program and the Climate and

84 P.P. MURPHY ET AL.

Global Change program, all of NOAA. We thank the captain and crew of theNOAA ships "Discoverer" and "Oceanographer" and the Soviet ship "Aka­demik Korolev". We also thank Steve Hankin for assistance with the windsanalysis, Cathy Cosca for programming support, and Ryan Whitney for wordprocessing. This paper is Contribution 1231 from NOAA/Pacific Marine En­vironmental Laboratory.

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