Ocean Circulation and Water Mass Characteristics around the Galápagos Archipelago Simulated ...

17 Pages·2015·6.33 MB·English

Hindawi Publishing Corporation
International Journal of Oceanography
Volume 2014, Article ID 198686, 16 pages
http://dx.doi.org/10.1155/2014/198686
Research Article
Ocean Circulation and Water Mass Characteristics around
the Galápagos Archipelago Simulated by a Multiscale Nested
Ocean Circulation Model
YanyunLiu,1,2,3LianXie,1JohnM.Morrison,4DanKamykowski,1andWilliamV.Sweet1,5
1DepartmentofMarine,Earth,andAtmosphericSciences,NorthCarolinaStateUniversity,Raleigh,NC27695-8208,USA
2CooperativeInstituteforMarineandAtmosphericStudies,UniversityofMiami,Miami,FL33129-1098,USA
3AtlanticOceanographicandMeteorologicalLaboratory,NOAA,4301RickenbackerCauseway,Miami,FL33129-1026,USA
4DepartmentofPhysicsandPhysicalOceanography,UniversityofNorthCarolinaatWilmington,Wilmington,
NC28403-5606,USA
5NationalOceanService,NOAA,CenterforOperationalOceanographicProductsandServices,SilverSpring,
MD20910-3281,USA
CorrespondenceshouldbeaddressedtoYanyunLiu;[email protected]
Received29September2013;Accepted12December2013;Published11February2014
AcademicEditor:LakshmiKantha
Copyright©2014YanyunLiuetal.ThisisanopenaccessarticledistributedundertheCreativeCommonsAttributionLicense,
whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkisproperlycited.
OceancirculationandwatermasscharacteristicsaroundtheGala´pagosArchipelagoarestudiedusingafour-levelnested-domain
oceansystem(HYCOM).Themodelsensitivitytoatmosphericforcingfrequencyandspatialresolutionisexamined.Resultsshow,
thatwithprescribedatmosphericforcing,HYCOMcangenerallysimulatethemajorElNin˜oeventsespeciallythestrong1997-
1998events.Waterssurroundingthearchipelagoshowalargerangeoftemperatureandsalinityinassociationwithfourdifferent
currentsystems.WestzonesofIsabellaandFernandinaIslandsarethelargestupwellingzones,resultingfromthecollisionofthe
EquatorialUndercurrent(EUC)withtheislands,bringingrelativelycolder,saltywaterstothesurfaceandmarkingthelocation
ofthehighestbiologicalproduction.Modelresults,whichagreewellwithobservations,showaseasonalcycleinthetransport
oftheEUC,reachingamaximumduringthelatespring/earlysummerandminimuminthelatefall.Thefarnorthernregionis
characterizedbywarmer,fresherwaterwiththegreatestmixedlayerdepthasaresultofPanamaCurrentwatersenteringfrom
thenortheast.WatermassesovertheremainderoftheregionresultfrommixingofcoolPeruCurrentwatersandupwelledCold
Tonguewatersenteringfromtheeast.
1.Introduction upwellingzones.Becauseofthisstrongtemporalandspatial
variability, it would be extremely difficult to monitor or
The Gala´pagos Archipelago (Figure1) lies in the equatorial explain the sources of the ecological variability using an
easternPacificOcean,about1000kmwestfromthecoastof observing system alone. The work described here results
Ecuador,SouthAmerica.Itcanreasonablyclaimtopossess fromacombinedfieldremotelysensedandmodelingeffort
the most distinctive marine flora and fauna and unique to study the connectivity of the upwelling dynamics over
speciesforanyareaofitssizeworldwide[1].ThusinMarch theGMR.During2006-2007,aseriesof5archipelago-wide
1998,EcuadorcreatedtheGala´pagosMarineReserve(GMR), shipbornesurveys,supportedbymooringsin5ofthemajor
2
whichisanextensivearea,over140,000km anditconsists upwellingregionstogetherwithremotelysensedobservation
ofanextremelyvariableecologicalsysteminspaceandtime anddevelopmentofahigh-resolutionhydrodynamicalmodel
[2]. The main reason why the GMR features such great of the GMR, were carried out by a joint program of the
diversityliesinitspositioninginacomplextransitionzone University of North Carolina Wilmington, North Carolina
between tropical and subtropical waters and intense local State University, Mote Marine Laboratory, Charles Darwin
2 InternationalJournalofOceanography
×103
2 0
1 −0.5
Wolf −1
0
−1.5
1.5
de −2
atitu 0.5 Santiago −2.5
L
−0.5 Fernandina
Santa Cruz −3
−1 Isabella San Cristobal
−3.5
Floreana
−1.5 −4
−2
−92.5 −92 −91.5 −91 −90.5 −90 −89.5 −89 −88.5
Longitude
Figure1:BathymetrymapandsamplingstationsoftheGala´pagosArchipelago.
Research Station, and the Gala´pagos Nation Park Service tothenorthoftheequator,thesoutheasttradewindsbecome
whichwassupportedbyNASA’sBiodiversityandEcological dominant, and the major source waters for the SEC are
MonitoringandBiologicalOceanographyPrograms.Thefirst the cold Peruvian Ocean current and the Peruvian Coastal
results of the model development and testing are described Current (the Humboldt Current), enhancing the contribu-
in this paper, with the observed ocean spatial and time tions to the SEC with the colder (18∘C–20∘C), saltier (>35)
seriesdatausedtovalidatethemodelingeffort.Asmightbe productivewaters.ThedifferentsourcewatersfortheSECas
expected, the biological response to physical forcing within itflowsthoughtheGMR,frombothnorthernandsouthern
the Gala´pagos Marine Reserve is the ultimate goal of this hemispheres, as well as topographically induced upwelling
activity. whichoccurswhentheeastwardflowingEUCrunsintothe
TheGala´pagosArchipelagoecosystemismainlyaffected Gala´pagosplatformarethecauseofthetremendousdiversity
by waters from three different surface and subsurface cur- aroundthearchipelago.
rents. The major currents affecting the Gala´pagos are the A few studies have quantified the primary production
westwardSouthEquatorialCurrent(SEC)andtheeastward around the archipelago, especially higher concentrations in
Equatorial Undercurrent (EUC) [3–5]. The SEC drives sur- westofIsabella[9,10]fromtheupwellingEUC.Houvenaghel
facewatersovertheentireregionaroundtheGala´pagosand [8] discussed the oceanographic setting of the Gala´pagos
isaffectedbywarmernorthernhemispherewatersandbythe and concluded that the EUC was the major oceanographic
Panama Current and cool Humboldt Current generated in phenomenaaffectingtheGala´pagos. Hayes [11] pointedout
thePeruupwellingzone,aswellascoolsouthernhemisphere thatthesealeveldifferencesacrosstheArchipelagoincrease
waters.ThewarmPanamaCurrentinfluencesprimarilythe astheEUCstrengthens,measuredbyshallowmooredpres-
northern islands, while the cool Humboldt (Peru) Current sure gauges. Steger et al. [7] found that the near-surface
influencesprimarythesouthernandcentralportionsofthe waters to the west of the Archipelago during November
archipelago.TobalancethewestwardSECasitflowstothe 1993 were predominately upwelled EUC waters, and as a
westalongtheequator,theeastwardEUC(alsoknownasthe consequence were cooler, saltier and higher in nutrients
CromwellCurrent)[6]developsasasubsurface(subthermo- and iron than waters found over the eastern portion of the
cline)compensationagainstthewestwardSEC.Thecollision Archipelago. In addition, the extent of the plume shown
of the subsurface EUC with the Gala´pagos platform results by Feldman [10] could only be maintained by upwelling of
intopographicallyinducedupwellinginwesternarchipelago EUCwatersthatarecoolerandhigherinnutrientsthanthe
(particularly along Isabella Island). This upwelled water is waterssurroundingtheplume.Palacios[12]alsofoundthat
relativelyrichiniron,themacronutrientneededforgrowth the temporal variability of ocean color and SST around the
bymostorganisms,andtheproductionishighinmanyareas Gala´pagos was dominated by the seasonal migration of the
oftheGala´pagosandsurroundingwaters[7]. ITCZandassociatedstrengtheningoftheSECandEUC.
The Gala´pagos are greatly affected by seasonal atmo- Another distinctive feature associated with Gala´pagos
spheric and oceanographic variability [8]. During the hot coastal waters that the greatest sea temperature anomalies
wet season (December to May), the Intertropical Conver- occur during El Nin˜o and La Nin˜a events [13]. Recently,
genceZone(ITCZ)migratessouthwardtowardstheequator, the frequency and severity of El Nin˜o events appear to
the northeast trades become more prevalent, and warmer have increased and this is a concern for the conservation
(>25∘C), fresher (<34), and less-productive waters of the ofendangeredspecies.ElNin˜oeventsnowoccur2–7times
PanamaCurrentflowsouthwardintothearchipelago.During morefrequentlythantheydid7000–15,000yearsago[14,15].
thedryGaru´aseason(MaytoNovember),theITCZmigrates The 1982-83 and 1997-98 El Nin˜o events, in particular,
InternationalJournalofOceanography 3
appear to have been the most extreme for many centuries, 30N Domain2(0.48×0.24)
and have had a devastating effect on marine life in the
Gala´pagos region. During the El Nin˜o event period, the 20N Interdomain (0.04×0.02)
EquatorialUndercurrentweakens,theupwellingweakensor 10N 1N
e
d
sretodpuscaeldt,otgheethperri,mthaerysuprrfoadcuecwtiaotenrdweacrrmeass,emsaancdrofinsuhtrnieunmtsbaerres utitaL 100S 10S
diminish[10,16].SotherelationshipbetweenextremeENSO Domain3(0.12×0.06)
eventsandlossofmarinebiodiversityinGala´pagosneedsto 20S W W W W
2 1 0 9
9 9 9 8
be investigated as a matter of local and global importance. 30S
Such a study has direct implications to predictions of loss
120E 140E 160E 180 160W140W120W100W 80W 60W
of global biodiversity associated with ocean warming and
Longitude
climatechange.
PreviousmodelingstudiesbyBrentnall[17]andEdenand Figure 2: Model domain size: the outer domain is global ocean
Timmerman [18] firstly described the response of currents (not shown here, with the resolution of 1.44∘ × 0.72∘), and the
and sea surface temperature (SST) to the presence of the intermediatedomains(coveringtheeasternPacificOcean,withthe
Gala´pagos. Eden and Timmerman [18] found changes in resolutionof0.48∘× 0.24∘and0.12∘× 0.06∘),andtheinterdomain
theregionalpatternofSSTofwestoftheGala´pagosdueto withthehighestresolutionof0.04∘×0.02∘.
island-induced changes in TIW activity to be as important
as those resulting from direct interaction with the mean
flow (e.g., partially deflecting the EUC). Cravatte et al. [19]
shallowwateryieldofmorereliableoceancirculationmakes
studied the seasonal, interannual, and decadal variability
∘ it ideal to apply to the GMR. The islands lie in the Pacific
in eastern Pacific Ocean using 2 ocean general circulation
Ocean about 1,000km from the south American coast and
model (OGCM) and described that adding the Galapagos
arestraddlingtheequator.Thereare13largeislands,6smaller
Archipelagotothemodelbathymetryonlyinducesverylocal
ones,and107isletsandrocks,withatotallandareaofabout
changes in the equatorial Pacific. The OGCM studies by
8,000squarekilometers,togetherwithuncountedseamounts
Karnauskas et al. [5] described the effect of the Gala´pagos
∘ (Stuart Banks, personnel communication). The islands are
IslandsontheequatorialPacificColdTongueusingthe0.25
volcanic in origin and several volcanoes in the west of the
ocean model. As suggested by Karnauskas et al. [20], the
archipelagoarestillveryactive.Thesevolcanoesrisefroma
EUCintheeasternPacificOceancanbeunderestimateddue
depthof>3500mwiththemainplatformat∼500mdepth.As
to insufficient ocean resolution. This paper focuses on the
shownbyChassignetetal.[22],HYCOMwasabletogenerate
oceanographicvariabilityatdifferentspatialscalesassociated
amorerealisticEUCthanseveralotherpopularOGCMs[22].
with the biological productivity and biogeography of the
Since understanding the dynamics of the EUC is crucial to
variousmarineecosystemsacrosstheGMRassimulatedbya
this study, HYCOM is an appropriate choice for simulating
high-resolutionoceangeneralcirculationmodel,specifically,
theoceancirculationaroundtheGMR[24].
the Hybrid Coordinate Ocean Model (HYCOM) [21–23].
The details of the HYCOM equations and numerical
For the first time, the ocean model is downscaled from
algorithms, along with a description and validation of the
the global ocean to the GMR region using the multiscale
hybridcoordinategenerator,canbefoundinBleck[21].
ocean circulation model system. The rest of the paper is
organizedasfollows:inSection2,wedescribetheHYCOM
modelconfigurationanddosomesensitivityexperiments.In 2.2. Model Configuration and Numerical Experiments. A
Section3,thesimulatedtropicaloceancirculationaroundthe suite of HYCOM simulations was carried out using a nest-
GMR using QuikSCAT and NCEP forcing is demonstrated ing scheme of 4 ever increasing resolution domains (see
andcomparedwiththeobservationcruiseresults.Resultsare Figure2).Thelargestdomaincoverstheentireglobalocean
discussedinSection4andsummarizedinSection5. with a grid size of 1.44∘ × 0.72∘ in the zonal and merid-
ionaldirections,respectively.Theseconddomainembedded
within the global domain uses a grid size of 0.48∘ × 0.24∘
2.HYCOMModelandData coveringtheequatorialPacificOcean.Thethirddomainhas
a grid size of 0.12∘ × 0.06∘. Finally, the highest resolution
2.1. Model Description. HYCOM is a primitive equation domain has a grid size of 0.04∘ × 0.02∘ (about 4.45km ×
∘ ∘
ocean general circulation model using density, pressure, 2.21km) that is centered on the GMR (92.16 W–88.96 W,
∘ ∘
and sigma coordinates in the vertical coordinate. It has 1.68 S–1.68 N).Alldomainshaveaverticalresolutionof26
evolvedfromtheMiamiIsopycnicCoordinateOceanModel layers that stretch or shrink vertically as a function of total
(MICOM) andisisopycnalintheopenstratifiedoceanbut depth according to the hybrid coordinate frame discussed
reverts to a terrain-following coordinate in shallow coastal above.
regions, and to 𝑧-level coordinates near the surface in the Themodelwasinitializedwithtemperatureandsalinity
mixed layer. This generalized vertical coordinate approach from the Levitus monthly climatology [25, 26] and run
is dynamic in space and time via the layered continuity for 12 years. It was driven by daily surface wind stress,
equationandpermitstheexistenceofzerothicknesslayers. surfaceairtemperature,surfaceatmosphericspecifichumid-
HYCOM’s ability to seamless transition between deep and ity, net shortwave radiation, net long-wave radiation, and
4 InternationalJournalofOceanography
0.5
1N 0.7 1N 0.5 0.5 0.7
0.4
0.5N 0.6 0.6 0.5 0.6 0.5N 00..6 0.6 0.35 0.6
0.50S 0.550.6 0.6 0.5 0.5 0.50S 0.07.765 0.6 0.60.5 0.5
0.7
1S 0.6 0.4 1S 0.7 0.4
1.5S 0.3 1.5S 0.65 0.3
92W 91W 90W 89W 92W 91W 90W 89W
(a) Reso=0.04 (b) Reso=0.12
0.7
0.51NN 0.6 0.06.55 0.5 0.5 00..67 0.15NN 0.8 0.06.8 0.50.40.3 00..67
0 0.7 0 0.7 0.9 0.9 0.7
0.65 0.6 0.9 0.8
0.5 0.5
0.5S 0.5S
0.65 0.8
0.9
1S 0. 0.4 1S 0.4
65 0.6 0.65 0.6
0.7
1.5S 0.65 1.5S
0.3 0.3
92W 91W 90W 89W 92W 91W 90W 89W
(c) Reso=0.48 (d) Reso=1.44
Figure3:Simulationswithdifferentwindsforcing,fromthe6-hourlywind(a),dailymeanwind(b),weeklymeanwind(c),tothemonthly
windforcing(d).
0.5 0.6 0.55
1N 0.5 1N
0.65 0.6 0.65
0.5N 0.6 0.6 0.6 0.5N 0.55
0 0.65 0.6 0.5 0.4 0.6 0 0.55 0.6 0.6
0.5S 0.7 0.6 0.55 0.5S 0.65 0.55
0.7 0.5 0.6 0.75 0.7 0.5
1S 0.7 0.6 0.45 1S 0.6 0.65 0.45
1.5S 0.6 1.5S 0.6
0.4 0.4
92W 91W 90W 89W 92W 91W 90W 89W
(a) Reso=0.04 (b) Reso=0.12
0.7 0.6 0.6 0.7
1N 0.580.6 0.62 0.65 1N 0.65
0.5N 0.62 00..6646 0.6 0.5N0 0.65 0.65 0.6
0
0.55 0.55
0.5S
−−10−..155SSS 0.6 0.62 0.64 0.6 6 0.68 00..455 1.521SSS 000..56.55 0.65 00..455
0.4 0.4
92W 91W 90W 89W 92W 91W 90W 89W
(c) Reso=0.48 (d) Reso=1.44
∘ ∘ ∘ ∘
Figure4:Simulationswithdifferenthorizontalresolutions(0.04,0.12,0.48,and1.44).
InternationalJournalofOceanography 5
Zonal velocity at EQ(reso=0.04) Zonal velocity at EQ(reso=0.12)
0 0
− 0.4 −0.3 −0.4 −0.6 −0.4
0.30. −0.3 −0.1 0.2 −0.2 0.2
1
0 0 0
50 0 0.1 0.1 50 0.1
0 0 0
epth 100 0. 1 0 0.10.2 −0.1 epth 100 0.20 0.2 −0.1
D D
−0.2 −0.2
150 0.1 −0.1 −0.3 150 0 0 −0.3
0 −0.32.0− −0.4 −0.4 −0.2 −0.4
200 −0.5 200 −0.5
92W 91W 90W 89W 92W 91W 90W 89W
Longitude Longitude
(a) (b)
Zonal velocity at EQ(reso=0.48) Zonal velocity at EQ(reso=1.44)
0 0
−0.6 −0.4 0.2 −0.25 −0.15 0.2
−0.2 0
0
50 0.1 50 0.15 0.1
0.2
0 0
0.2 0.15
epth 100 0.4 −0.1 epth 100 00.0.15 −0.1
D D 0
6 −0.2 −0.2
0. −0.05
150 −0.3 150 −0.3
0.4
−0.4 −0.4
0.1
200 −0.5 200 −0.5
92W 91W 90W 89W 92W 91W 90W 89W
Longitude Longitude
(c) (d)
Figure5:Verticalsectionsofzonalcurrent(m/s)alongtheequatorfortheupper200maroundtheGMRwithdifferenthorizontalresolutions
∘ ∘ ∘ ∘
(a)0.04,(b)0.12,(c)0.48,and(d)1.44.
precipitationfieldsobtainedfromNCEPreanalysis[27].The University(http://www.pmel.noaa.gov/vents/staff/chadwick/
latentandsensibleheatfluxeswerecalculatedduringmodel galapagos.html). The wind field used to drive the ocean
runs using the model sea surface temperature and the bulk circulation in this subregion was derived from the 25km
formulation[28,29].Boundaryconditionswereprovidedby resolutionQuikSCATwindfieldinsteadoftheNCEP/NCAR
bufferzonesthataretengridpointswidewithinwhichtem- reanalysisfromyear2000toyear2006.Themodelwasrun
perature,salinity,andinterfacedeptharerelaxedtoLevitus for twelve years from 1995 to 2006, and fields from the
climatological values that have been vertically remapped to last two years of the simulation (years 2005 and 2006) are
hybridverticalcoordinates.TheKPPverticalmixingmodel analyzedhere.
of Large et al. [30] was used, which has been shown to
perform well in open ocean settings. For the experiments
withtheresolutionof1.44∘,0.48∘,and0.12∘,thebathymetry 2.3. Sensitivity Experiments. Two types of sensitivity exper-
was derived from ETOPO2, which is a digital data base of iments were conducted to test the ocean variability around
seafloorandlandelevationsona2-minutelatitude/longitude theGMR.Oneexperimentwastotesthowmarinemesoscale
grid and is interpolated onto the various model grids. The variabilitydependsontemporalandspatialscalesofthewind
∘
bathymetry for the simulation of the resolution of 0.04 forcing (Expt.1); the other is to test how marine variability
(near the Gala´pagos Islands) domain is derived from the dependsondifferentmodelhorizontalresolutions(Expt.2).
∘
0.01 bathymetry of William Chadwick at Oregon State TheENSOvariabilitywasalsosimulatedusingthemodelto
6 InternationalJournalofOceanography
quantifytheabilityofthemodeltogiverealisticsimulations 30
ofENSOeventsaroundtheislands.
29
28
2.3.1.WindForcingExperiments. Inthewindforcingexperi-
27
ments(Expt.1),theHYCOMmodelwasrunwithfourdiffer-
entwindsforcing:monthlymeanforcing,weeklymeanforc- 26
ing,dailymeanforcing,and6-hourlywindforcing.Thehigh )C
∘( 25
frequencydailymeanforcingand6-hourlywindforcingwere T
S
downloaded from http://www.cdc.noaa.gov/cdc/reanalysis/, S 24
while the monthly mean forcing and weekly mean forcing 23
werecomputedbyusingthedailymeanforcingdata.Figure3
22
shows the standard deviation of sea surface temperature
(SST)withthefourdifferentwindforcing.Fromthisfigure, 21
thesimulationwiththemonthlywindforcinghasthelargest
20
∘
standard deviation (0.7 C) over most of the GMR, while 2005 2006 2007
the standard deviations from the simulations with the high Year
frequency wind forcing are similar but much smaller than Reso=0.04 Reso=1.44
thosewiththelowfrequencyforcing.Theresultshowsthat Reso=0.12 Obs
monthly wind forcing is not adequate to resolve the ocean Reso=0.48
variabilityaroundthearchipelago;therefore,especiallyinour
∘ ∘
regionofinterest,itisessentialtousehighfrequencywind Figure 6: The region-averaged around GMR (92.16W–88.96W,
∘ ∘
1.68S–1.68N) simulated SST under four different horizontal res-
forcing.
∘ ∘ ∘ ∘
olutions (0.04, 0.12, 0.48, and 1.44) and the region-averaged
observedSST.
2.3.2. Model Resolution Experiments. In the horizontal res-
olution experiments (Expt.2), the model was run centered
∘ ∘ ∘ ∘ Table1:Correlationcoefficientsbetweentheobservedandsimu-
on the GMR (92.16 W–88.96 W, 1.68 S–1.68 N) with four ∘ ∘ ∘
different horizontal resolutions—0.04∘ (0.04∘ × 0.02∘), 0.12∘ latedSSTusingthedifferentresolutionof0.12,0.48,and1.44 in
(0.12∘×0.06∘),0.48∘(0.48∘×0.24∘),and1.44∘(1.44∘×0.72∘). thefourdifferentregions(showninFigure7).
Figure4displaystheSSTstandarddeviationatthesedifferent Resolution Region1 Region2 Region3 Region4
resolutions.AsshownbyFigure4,thesimulationsaroundthe 0.12∘ 0.69 0.59 0.55 0.89
∘
GMRshowthata0.04 resolutionismoresuitabletostudy ∘
0.48 0.67 0.53 0.48 0.83
thevariabilityinandaroundtheGala´pagos. ∘
1.44 0.56 0.46 0.36 0.72
In the equatorial Pacific Ocean, the zonal transport
is mainly dominated by the eastward EUC and the
westward SEC. The changes to the EUC are also found ∘ ∘ ∘ ∘ ∘ ∘
2.16 S–2.16 N;region2:90 W–85.68 W,2.16 S–2.16 N;region
with different resolutions (Figure5). Figure5 presented the ∘ ∘ ∘ ∘ ∘ ∘
3: 90 W–85.68 W, 6.5 S–2.16 S; region 4: 90 W–85.68 W,
vertical sections of zonal current along the equator for the ∘ ∘
2.16 N–6.5 N)simulatedSSTusingthreedifferenthorizontal
upper 200m around the GMR with different horizontal ∘ ∘ ∘
resolutions. The core of the EUC is located at 50−100m. resolutions (0.12 , 0.48 , and 1.44 ) and the corresponding
region-averagedobservedSST.Table1showsthecorrelation
With increasing model resolution, the EUC speed is
coefficients between the observed and simulated SST. The
decreased and elevated to the surface with the inclusion
∘ resultshowsthatthesimulationusinghigherresolutionhas
of the Gala´pagos Islands under the fine 0.04 resolution.
thehighercorrelationwithobserveddata.Allaboveresults
Figure6showstheregion-averaged(withinthewholeGMR
show that the simulated SST using higher resolution (i.e.,
domain) SST simulated using four different horizontal ∘
∘ ∘ ∘ ∘ 0.04 resolution) is more suitable to study the variability in
resolutions (0.04 , 0.12 , 0.48 , and 1.44 ) and the region-
andaroundtheGala´pagos.
averaged observed SST. The observational datasets used for
comparisonwithmodeloutputcamefromthe0.25∘ ×0.25∘
microwave (MW) + infrared (IR) optimally interpolated 2.3.3.ENSOSimulations. Asafinaltestoftheconfigurations
(OI) microwave SST product from 2002 to present tobeusedinthisstudy,ElNin˜o/SouthernOscillation(ENSO)
(http://www.remss.com/sst/microwave oi sst browse.html). variability was simulated using HYCOM driven by NCEP
∘
These figures show that the simulated SST with a res- windforcing.TheSSTanomaliesintheNino3.4region(5 S–
∘ ∘ ∘ ∘
olution of 0.04 is the closest to the observed SST. The 5 N,150 W–90 W)areoftenusedtoindexENSOvariability
correlation coefficient between the two reaches 0.72 (𝑃 < [31].Figure8showstheobserved(blue)andHYCOMsimu-
0.05), while the correlation coefficients between the simu- latedSSTanomalies(red)inNino3.4region.Thecorrelation
∘
latedandobservedSSTusingthedifferentresolutionsof0.12 , coefficient between the simulate SSTA and observed SSTA
∘ ∘
0.48 ,and1.44 are0.67,0.61,and0.32,respectively.Figure7 is 0.883. This result shows that, with prescribe atmospheric
depicts the region-averaged (see Figure7(e): four differ- surfaceforcing,HYCOMcangenerallysimulatethemajorEl
∘ ∘
ent regions in eastern Pacific—region 1: 97.2 W–92.88 W, Nin˜oeventsespeciallythestrong1997-1998events.Therefore,
InternationalJournalofOceanography 7
30 31
29 30
28 29
27 28
)C 26 )C 27
∘( 25 ∘( 26
T T
S 24 S 25
S S
23 24
22 23
21 22
20 21
2005 2006 2007 2005 2006 2007
Year Year
Reso=0.12 Reso=1.44 Reso=0.12 Reso=1.44
Reso=0.48 Obs Reso=0.48 Obs
(a) SSTinregion1 (b) SSTinregion2
30
31
30 28
29
28 26
∘()TC 2267 ∘()TC 24
SS 25 SS 22
24
20
23
22 18
2005 2006 2007 2005 2006 2007
Year Year
Reso=0.12 Reso=1.44 Reso=0.12 Reso=1.44
Reso=0.48 Obs Reso=0.48 Obs
(c) SSTinregion3 (d) SSTinregion4
15N
10N
e 5N Region 3
d
Latitu 50S Region 1 RReeggiioonn 24
10S
15S
100W 95W 90W 85W 80W 75W
Longitude
(e) Regionmap
∘ ∘ ∘ ∘
Figure7:Theregion-averaged(fourdifferentregionsineasternPacific,seeFigure7(e),region1:97.2W–92.88W,2.16S–2.16N;region2:
∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘
90W–85.68W,2.16S–2.16N;region3:90W–85.68W,6.5S–2.16S;region4:90W–85.68W,2.16N–6.5N)simulatedSSTunderthree
∘ ∘ ∘
differenthorizontalresolutions(0.12,0.48,and1.44)andtheregion-averagedobservedSST.
we feel confident that the HYCOM should give realistic observational cruise results, focusing on the surface water
simulationsofthelarge-scalecirculationaroundtheislands masscharacteristicsandflowpatterns.Itshouldbenotedthat
duringENSOeventsaswellasnormalyears. it is not expected that the modeled fields (which represent
greater than 10,000 grid points over the water regions of
the GMR) accurately portray the observed fields (which
3.ModelSimulationsComparedto representgriddeddatafromapproximately60–70datapoints
Observations depending on the cruise) but should portray the major
featuresofthewatermassesandcurrents(fromourvalidation
3.1.SimulationofT-SandOceanCurrentsaroundGMR. Here experimentsusingSST,themodeldatamayactuallyportray
we compare the simulated circulation around the GMR to thefeaturesbetterthantheobserveddata).
8 InternationalJournalofOceanography
Theobservationdatausedinthestudywerecollecteddur- 4
ingfoursurveys(seestationpositionsinFigure1)conducted 3
from17to28March,2005(Mar2005),fromNovember22to )C 2
December3,2005,(Nov/Dec2005),fromJune26toJuly4, ∘(A 1
2006(Jun/Jul2006),andfrom14to23November,2006(Nov ST 0
S
−1
2006), onboard the Gala´pagos National Park Service’s M/N
−2
SierraNegra.Eachcruisewas8to12dayslongandoccupied
1996 1998 2000 2002 2004 2006
∼70hydrographicstationsaroundtheGMR[32].Surfacedata
Time (year)
included underway measurements of temperature, salinity,
dissolved oxygen, and fluorescence data using a Seabird Figure8:Theobserved(blue,dashed)andHYCOMsimulatedSST
Electronics SBE 19+ system (CTD) that was averaged into anomalies (red, solid) in Nino3.4 region. The grey shaded areas
30mintimeintervals. indicatethefourcruiseperiods.
(a) March 2005. The March 2005 cruise occurred during
the hot, wet season, a period characterized by warm SST
depth (MLD) to compare with the observed chlorophyll
associatedwiththesouthwardadvectionofwarmer,northern
(Figure14(a)) during November/December 2005. During
waters from the Panama Current and a weakening of the ∘
this period, the modeled coldest (20 C, see Figure9(b))
southeast trade winds. Figures 9(a) and 10(a) display the
and saltiest (34.7, see Figure11(b)) waters were found in
simulated and observed SST. Figures 11(a) and 12(a) dis-
the west of Isabella Island and north of Fernandina Island
play the simulated and observed sea surface salinity (SSS).
andcorrespondedwiththelargestupwellingzonesfoundin
Figure13(a)displaythesimulatedmixedlayerdepth(MLD)
late spring/early summer as the EUC strengthened. These
to compares with the observed chlorophyll (Figure14(a))
features were consistent with the in situ T-S characteris-
duringMarch2005.FromFigures9(a)and11(a),thewarmest
tics observed during this cruise period (see Figures 10(b)
and the last salty waters are located within the north and
and 12(b)). Figure13(b) shows that the MLD was slightly
south regions of the GMR and the coolest and the saltiest
shallower in the west near the cooler, upwelling region.
waters are found in the west of Isabella and Fernandina, ∘
The warmest (25.0 C) and the freshest (33.7) waters were
indicativeoftopographicallyinducedupwellingoftheEUC
found in north of the Archipelago. The far northernregion
[10].Theseresultsareconsistentwiththeobservationcruise
was characterized with warmer, fresher water and had the
T-S characteristics (see Figures 10(a) and 12(a)). Also the
deepest MLD, and the other regions fell between the range
MLD(Figure13(a))wastheshallowest(∼10m)inthewestof
ofupwellingregionandfarnorthregion.BoththeSSTand
IsabellaandFernandinaduringtheMarch2005cruise.The
SSSdistributionsshowedaneast-to-westgradientassociated
coldestandthesaltiestwaterhadthehighestchlorophyllcon-
with higher values of chlorophyll concentrations indicative
centration(seeFigure14(a)).Thehighestsurfacechlorophyll
of higher phytoplankton biomass. The maximum values of
𝑎concentrationsobservedof3.25mgm−3 werefoundinthe chlorophyll𝑎werefoundinwestofIsabellaIsland,andthe
west of Isabella Island and the north of Fernandina Island. minimumchlorophyll𝑎concentrationswereobservedalong
The minimum chlorophyll 𝑎 concentration observed was theequatorintheGMR(Figure14(b)).Also,theshallowest
−3
0.05mgm locateddirectlyinsoutheastofIsabellaIsland. MLD was simulated over cooler waters at depth (Figures
The simulated surface layer flow (Figure15(a)) showed 13(b)and9(b)),anotherindicationofstrongupwelling.The
thatthewestwardflowoftheSECcollidedwiththeeastward modeled surface layer flow (Figure15(b)) was completely
flowoftheEUCinbothnorthandsouthofIsabellaIsland. dominated by the westward flow of the SEC throughout
NeartheArchipelago,theSECsplintersintonorthwardand the GMR during this period. The EUC (Figure16(b)) had
southwardlobesthatcontinuetothewestwithafewdegrees weakenedconsiderably.
off the equator [33]. The mixed layer flow (Figure16(a)) is
mainlydominatedbytheeastwardflowofEUCaroundthe (c) June 2006. The June/July 2006 cruise occurred during
Gala´pagos. The EUC is quite strong (150cm/s), has its core a transition period, leaving the wet season and entering
below the thermocline, and drives an eastward flow near theGaru´aseason,withthestrengtheningofsoutheasttrade
surface(∼20m).Themixedlayerdepthshoalsto15minthe winds and SST significantly (+1.2∘C) above climatology
westofIsabellaandFernandinaIslands,whichindicatesthe (Figure8). Figures 9(c) and 10(c) display the simulated and
upperreachofaweakened,centrallylocatedEUC. observed SST. Figures 11(c) and 12(c) display the simulated
andobservedseasurfacesalinity(SSS).Figure13(c)displays
(b) November/December 2005. The November/December the simulated mixed layer depth (MLD) to compare with
2005 cruise occurred during a transition from the Garu´a the observed chlorophyll (Figure14(c)) during June 2006.
to the wet season with lessening of the southeast trades. The model mean temperature was 24.4 ± 1.7∘C throughout
∘
The simulated mean SST was cooler than climatology by the GMR (Figure9(c)) with the warmest waters, 26.0 C,
approximately −0.5∘C during the transition, as seen from found in north of the archipelago and the coldest waters,
∘
the ENSO anomaly (Figure8, grey area). Figures 9(b) and 24.2 C,foundinthewestofIsabellaandFernandinaIslands
∘
10(b) display the simulated and observed SST. Figures 11(b) (Figure9(c)). The simulated SST was the warmest (26.0 C)
and 12(b) display the simulated and observed sea surface and SSS was the freshest in northeast of the Archipelago,
salinity(SSS).Figure13(b)displaysthesimulatedmixedlayer forminganincreasinggradienttothewestwiththehighest
InternationalJournalofOceanography 9
1.15 25.225 2 5 2 5 222667.5 1.15 24 242.45 222667.5
0.5 24.8 25 24.8 2255.5 0.5 23 23.5 2255.5
23
0 24.5 0 24.5
22
−0.5 25.2 2243.5 −0.5 22 2243.5
.5
−1−.51 25.4 25 25 25.2 2232.5 −1−.15 23 2 3 2232.5
22 22
−92 −91.5 −91 −90.5 −90 −89.5 −89 −92 −91.5 −91 −90.5 −90 −89.5 −89
(a) Mar2005 (b) Nov/Dec2005
1.5 27 1.5 27
26 24 26.5 27 27 26.5
1 26 1 26 26
0.5 255 25.5 2255.5 0.5 25.5 26.5 2255.5
−0.05 24.3.5 24.5 4 2244.5 −0.50 24.5 2 5.5 26 2244.5
242 2 23.5 25 23.5
−1 23 −1 5 23
22.5 5. 22.5
−1.5 24 −1.5 2
22 22
−92 −91.5 −91 −90.5 −90 −89.5 −89 −92 −91.5 −91 −90.5 −90 −89.5 −89
(c) Jun/Jul2006 (d) Nov2006
Figure 9: HYCOM simulated sea surface temperature for (a) March 2005, (b) November/December 2005, (c) June/July 2006, and (d)
November2006.
27 27
1.5 1.5 5
1 26 25 26 1 24. 24 26
0.5 2 4 5 0.5 23 24
2 24 25 22 25
0 0
−0.5 2 3 25 24 −0.5 2210 23 24
2 4
−1−.51 25 25 25 23 −1−.51 2021 22 22.5 22.5 22 2222.5 23
22 22
−92 −91.5 −91 −90.5 −90 −89.5 −89 −92 −91.5 −91 −90.5 −90 −89.5 −89
(a) Mar2005 (b) Nov/Dec2005
27 27
1.15 2323.5 242.55 25.5 26 1.51 26 26 26
0.5 2212 24 25 0.5 2525.5 25
0 0 24
−0.5 21.5 24 24.5 24 24 −0.5 23 25.5 24
25
−1−.51 22.5 23.5 23.5 23 −1−.51 2 4 24.5 24.5 23
22 22
−92 −91.5 −91 −90.5 −90 −89.5 −89 −92 −91.5 −91 −90.5 −90 −89.5 −89
(c) Jun/Jul2006 (d) Nov2006
Figure10:Theobservedseasurfacetemperaturefor(a)March2005,(b)November/December2005,(c)June/July2006,and(d)November
2006cruises.
10 InternationalJournalofOceanography
34.6 34.6
1.5 1.5
33.6 33.8 34.4 33.8 34.4
1 1 34.1 33.9
34.2 34 34.2
0.5 3 0.5
0 3.433 34 0 3 4.4 34.1 34
.8 33.8 33.8
−0.5 33.8 33.6 −0.5 34.5 3 4.6 33.6
3
−1−.51 33.6 4 34 33.8 33.8 3 4 3333..42 −1−.51 34.3 34.2 3333..42
−92 −91.5 −91 −90.5 −90 −89.5 −89 −92 −91.5 −91 −90.5 −90 −89.5 −89
(a) Mar2005 (b) Nov/Dec2005
34.6 34.6
1.5 33.8 34.2 34.4 1.5 33.8 33.6 34.4
1 34 1 3 4
−00..505 34.6434 34.2 34.4 333443..28 −00..505 3344.3.244.5 34 333.38.7 333443..28
34.4.8 33.6 33.6
−1 3 −1
−1.5 34.6 34.4 34.6 3333..42 −1.5 34.2 34.1 34 3333..42
−92 −91.5 −91 −90.5 −90 −89.5 −89 −92 −91.5 −91 −90.5 −90 −89.5 −89
(c) Jun/Jul2006 (d) Nov2006
Figure11:HYCOMsimulatedseasurfacesalinityfor(a)March2005,(b)November/December2005,(c)June/July2006,and(d)November
2006.
1.15 33 33.5 34 34.5 3 5 34.5 1.15 33.8 33.6 3 3.4 33.2 33 32.8 32.6 34.5
0.5 34 34 0.5 34.2 34
0 34.5 34 33.5 0 34.6 33.5
−0.5 −0.5
−1−.15 3 4 33.5 34.5 35 33 −1−.51 34.4 34 33.433.2 33
−92 −91.5 −91 −90.5 −90 −89.5 −89 −92 −91.5 −91 −90.5 −90 −89.5 −89
(a) Mar2005 (b) Nov/Dec2005
1.5 34.5 34.5 34 34.5 1.5 33.6 34.5
0.51 35 3 3.5 34 0.51 34.2 33.8 33.6 34
0 35 5 0
−0.5 34. 33.5 −0.5 34.6 33.5
−1 3 −1 34.6 33.8
−1.5 5 34.5 33 −1.5 34.2 33.8 34 34 33
−92 −91.5 −91 −90.5 −90 −89.5 −89 −92 −91.5 −91 −90.5 −90 −89.5 −89
(c) Jun/Jul2006 (d) Nov2006
Figure12:Theobservedseasurfacesalinityfor(a)March2005,(b)November/December2005,(c)June/July2006,and(d)November2006
cruises.