Recent extreme light sea ice years in the Canadian Arctic Archipelago

16 Pages·2013·1.82 MB·English

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doi:10.5194/tc-7-1753-2013 The Cryosphere A
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Recent extreme light sea ice years in the Canadian Arctic
Archipelago: 2011 and 2012 eclipse 1998 and 2007
S.E.L.Howell1,T.Wohlleben2,A.Komarov3,4,L.Pizzolato5,andC.Derksen1
1ClimateResearchDivision,EnvironmentCanada,Toronto,Canada
2CanadianIceService,EnvironmentCanada,Ottawa,Canada
3DepartmentofElectricalandComputerEngineering,UniversityofManitoba,Winnipeg,Canada
4CentreforEarthObservationScience,UniversityofManitoba,Winnipeg,Canada
5DepartmentofGeographyandEnvironmentalManagement,UniversityofWaterloo,Waterloo,Canada
Correspondenceto:S.E.L.Howell([email protected])
Received:20February2013–PublishedinTheCryosphereDiscuss.:28March2013
Revised:10October2013–Accepted:19October2013–Published:15November2013
Abstract. Remarkably low mean September sea ice area helpedcounteracttheprocessesthatfacilitateextremeheavy
in the Canadian Arctic Archipelago (CAA) was observed iceyears.TherecentextremelightyearswithintheCAAare
in 2011 (146×103km2), a record-breaking level that was associatedwithalongernavigationseasonwithintheNorth-
nearly exceeded in 2012 (150×103km2). These values westPassage.
were lower than previous September records set in 1998
(200×103km2)and2007(220×103km2),andare∼60%
lower than the 1981–2010 mean September climatology. In
this study, the processes contributing to the extreme light 1 Introduction
yearsof2011and2012wereinvestigated,comparedtopre-
vious extreme minima of 1998 and 2007, and contrasted Prior to the Arctic melt season of 2012, perhaps the most
againsthistoricsummerseasonswithaboveaverageSeptem- strikingArcticseaiceeventinthepassivemicrowavesatel-
ber ice area. The 2011 minimum was associated with pos- lite era since 1979 was when the mean September Arctic
itive June through September (JJAS) surface air tempera- sea ice extent and area fell to record lows of 4.3×106km2
ture(SAT)andnetsolarradiation(K∗)anomaliesthatfacil- and 2.78×106km2, respectively (NASATeam; Cavalieri et
itated rapid melt, coupled with atmospheric circulation that al., 1996), in 2007 (Stroeve et al., 2008). This dramatic
restrictedmulti-yearice(MYI)inflowfromtheArcticOcean loss of ice during 2007 was linked to anomalously high
intotheCAA.The2012minimumwasalsoassociatedwith sea level pressure (SLP) over the Beaufort Sea and Cana-
positive JJAS SAT and K∗ anomalies with coincident rapid dian Basin and anomalously low SLP over eastern Siberia
melt, but further ice decline was temporarily mitigated by that transported ice out of the Arctic (Ogi et al., 2008;
atmospheric circulation which drove Arctic Ocean MYI in- Wang et al., 2009), together with high surface air temper-
flow into the CAA. Atmospheric circulation was compara- ature (SAT) over the central Arctic (Comiso et al., 2008)
ble between 2011 and 1998 (impeding Arctic Ocean MYI and the increased bottom melt of sea ice (Perovich et
inflow)and2012and2007(inducingArcticOceanMYIin- al., 2008). Years of gradual thinning of the Arctic sea
flow).However,preconditioningwasmoreapparentleading ice made it susceptible to the 2007 anomalous forcing
up to 2011 and 2012 than 1998 and 2007. The rapid melt events (Maslanik et al., 2007; Lindsay et al., 2009); over
processin2011and2012wasmoreintensethanobservedin the past three decades the Arctic Ocean’s thick multi-year
1998 and 2007 because of the thinner ice cover being more ice (MYI) has been gradually replaced with thinner first-
susceptibletoanomalousthermodynamicforcing.Thethin- year ice (FYI) (Maslanik et al., 2011). In 2012, mean
ner sea ice cover within the CAA in recent years has also September sea ice extent and area fell to 3.61×106km2
and 2.11×106km2, respectively (NASATeam; Cavalieri et
PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion.
1754 S.E.L.Howelletal.:RecentextremelightseaiceyearsintheCanadianArcticArchipelago

Figure 1. The geographical location of the Canadian Arctic Archipelago (CAA) (A) and spatial
Fig.1.ThegeographicallocationoftheCanadianArcticArchipelago(CAA)(A)andspatialdistributionofCAAmeanSeptembertotalice
distribution of CAA mean September total ice concentration (tenths) for the four extreme light
concentration(tenths)forthefourextremelightyears(B).
years (B).

al., 1996), shattering the previous 2007 record. The 2012 conditions in the CAA, but this was partially mitigated by

September mean value represents ∼50% less sea ice area highSLPovertheBeaufortSeawhichfacilitatedtheinflow

withintheArctic comparedtothe1981–2010meanSeptem- of Arctic Ocean MYI into the CAA (Howell et al., 2010).
ber climatology. 2012 sea ice conditions actually fell below Furthermore, unlike 2007, sea ice in 1998 exhibited no ev-
the 2007 record in August, following a dramatic storm that idence of preconditioned thinning leading up the minimum
trackedacrossth eArcticfromSiberiaandintotheCanadian (Howell et al., 2010). In the years that followed the record
Arctic Archipel ago (CAA) (Simmonds and Rudeva, 2012). lighticeyearof1998,theCAAreturnedtoheaviersummer
ParkinsonandC omiso(2013)foundthatdecadesofprecon- ice conditions, but the years that followed 2007 only wit-
ditioned thinning made the sea ice more vulnerable to the nessed a slight increase in end-of-summer ice (apparent in
2012storm.Moreover,Zhangetal.(2013)havearguedthat 2008and2009;Fig.2).Perhapsmorestrikingisthatfiveof
the 2012 record low would still have occurred without the thelightesticeyearsintheCAAsince1968(i.e.2007,2008,
storm.ThelatterresultsareconsistentwiththeviewthatArc- 2010, 2011 and 2012) have all occurred within the last 6yr
ticseaiceisnowthinnerandmorevulnerabletoanomalous (Fig.1c).Consideringtherecentobservedextremelightice
atmosphericforcingcomparedtopreviousdecades(Stroeve yearswithintheCAA,threequestionsneedtobeaddressed:
etal.,2011a). (i)whatweretheprocessesthatcontributedtotherecordex-
WithintheCAA,2011wastherecordlightestseaiceyear, tremelighticeyearsof2011and2012?;(ii)howwerethese
withameanSeptemberareaof146×103km2,but2012was processes similar to or different from 1998 and 2007?; and
a close second at 150×103km2 (Fig. 1a, b). These values (iii)couldtheCAAexperienceaheavysummericecoverin
represent ∼60% less sea ice area within the CAA com- upcomingyears?
paredtothe1981–2010meanSeptemberclimatology.Prior Inordertoanswerthesequestions,thispaperinvestigates
to 2011, the lightest ice year within the CAA occurred in theprocessescontributingtotheextremelightyearsof2011
1998,withameanSeptembericeareaof200×103km2,and and 2012, compares them to those responsible for the pre-
2007 ranked as the second lightest, with a mean Septem- vious extreme minima of 1998 and 2007, and contrasts the
bericeareaof220×103km2 (Fig.1a,b).Interestingly,the processes behind extreme light sea ice years with those be-
27
driving processes behind the 1998 and 2007 minima were hindextremeheavyiceyears.Understandingthedrivingpro-
very different. Atmospheric circulation during the summer cesses behind light and heavy summer sea ice conditions
months of 1998 allowed predominantly warm southerly air within the CAA is important because when global climate
masses to flow over the CAA (Atkinson et al., 2006). The models indicate a seasonally sea ice-free Arctic Ocean (i.e.
resultant anomalously warm SAT and restriction of Arctic less than 1 millionkm2) they still find sea ice to be present
OceanMYIinflowintotheCAAcombinedtograduallyab- within the CAA (e.g. Holland et al., 2006; Sou and Flato,
late the thicker MYI over a longer than normal melt season 2009; Wang and Overland, 2012). Even as climate model
(Howelletal.,2010).In2007,anomalouslywarmJulySAT resolution continues to improve, resolving sea ice thermo-
facilitated an intense and rapid melt which led to light ice dynamicanddynamicprocesseswithinthenarrowchannels
TheCryosphere,7,1753–1768,2013 www.the-cryosphere.net/7/1753/2013/
S.E.L.Howelletal.:RecentextremelightseaiceyearsintheCanadianArcticArchipelago 1755

mationSystemcoveragesandavailableonlinefromtheCIS
(http://ice.ec.gc.ca/).
The CISDA is more accurate than Scanning Multichan-
nelMicrowaveRadiometer(SMMR)andSpecialSensorMi-
crowave/Imager (SSM/I) passive microwave ice concentra-
tion retrievals during the melt season (Agnew and Howell,
2003). The CISDA contains a technological bias (related to
advancesinsensortechnologyandchangesinregionalfocus
due to the emergence of important shipping routes; Tivy et
al.,2011).Themainfocusofthisstudyisonprocessesdur-
inglightversusheavyiceseasons,notthetrendsinicearea
or concentration, therefore the impact of any technological
andmethodologicalchangeswillexertminimalinfluenceon
theresults.CISDAmonthlyaveragesoftotal,MYI,andFYI
area and concentration were calculated for the melt season
months of June to September. The 1981–2010 period was
Figure 2. Time series of mean monthly total ice, multi-year ice (MYI) and first-year ice (FYI)
Fairega. (2km.2T) wimithein stheer Cieasnaodifanm Aercatinc Amrchoinpetlhagloy fotor Jtuanle itco eS,epmtemubletri -(yAe-Da)r friocme 1(9M68-YI) chosentorepresentthehistoricalclimatology,andstandard-
2012. The vertical bars represent the extreme light and heavy years.
a nd first-year ice (FYI) area (km2) within the Canadian Arctic izedanomaliesfortotal,MYI,andFYIwerecalculatedbydi-

Archipelago for June to September (A–D) from 1968–2012. The vidinganomaliesbythe1981–2010climatologicalstandard

v erticalbarsrepresenttheextremelightandheavyyears. deviation.

Theyearsof1972,1979,1997and2004wereselectedas

theextremeheavyiceyearstocompareagainstthefourex-
o ftheCAAwillstillremaindifficult.Knowledgeofthedriv- treme low years of 1998, 2007, 2011 and 2012. 1972 and

in g factors behind heavy and light sea ice conditions in the 1979 rank as the two years with the heaviest September ice

C AAisalsoofparticularimportanceforNorthwestPassage conditionssince1968.1997(5thheaviest;Fig.2)and2004
s hippingactivities(ACIA,2004;AMSA,2009). (6thheaviest;Fig.2)wereselectedinsteadof1978(3rdheav-

Thispaperisorganizedasfollows.Section2describesthe iest;Fig.2)and1986(4thheaviest;Fig.2)becausethesource

dataandmethodsusedintheanalysis.Section3definesthe oftheCAA’sMYIchangedfromFYIagingtoacombination
frameworkofthestudybylookingattheCAA’sextremelight ofFYIagingandArcticOceanMYIinflowin1995(Howell
28
andheavyiceyearswithinahistoricaltimeseries.Section4 etal.,2009).Selectingheavyiceyearsinbothperiodsallows
describes the melt season evolution of sea ice cover in the for a more representative investigation of the processes be-
CAA during extreme light and heavy ice years. Section 5 hindheavyiceyears.Moreover,1978and1986exhibitedan
discussesthethermodynamicanddynamicprocessesduring almostidenticaliceevolutionasthe1stand2ndheaviestice
these two types of seasons. Section 6 discusses the role of years, and discussing their evolutions would have been re-
preconditioned ice thinning on the recent extreme minima. dundant(i.e.theconclusionsofthepaperwouldnotchange).
Section7discussestheimplicationsfornavigatingtheNorth-
west Passage given the recent occurrence of more frequent 2.2 CanadianArcticArchipelago-ArcticOceanice
extremeiceminima.ConclusionsarepresentedinSect.8. exchange
Ice exchange between the CAA and the Arctic Ocean dur-
2 Dataandmethods ing the months of May to September was estimated at
the M’Clure Strait and the Queen Elizabeth Islands (QEI)
2.1 CanadianIceServiceDigitalArchive gates(Fig.1a). Whileiceexchangedoes occurbetweenthe
Amundsen Gulf and the Arctic Ocean, the Amundsen Gulf
Weeklytotal,MYI,andFYIconcentration(tenths)andarea becomes ice-free during the summer months, and ice ex-
(km2) were extracted from the Canadian Ice Service Dig- change provides a negligible source of CAA ice replenish-
ital Archive (CISDA) for 1968 to 2012, from June to Oc- ment during the melt season (Kwok, 2006; Agnew et al.,
tober for the CAA domain shown in Fig. 1a. The CISDA 2008;Howelletal.,2008;CanadianIceService,2011).
is a compilation of Canadian Ice Service regional weekly Ice exchange was derived from sequential RADARSAT-
ice charts that integrate all available real-time sea ice infor- 1orRADARSAT-2syntheticapertureradar(SAR)imagery
mationfromvarioussatellitesensors,aerialreconnaissance, withaspatialresolutionof∼200mpixel−1 usingtheCana-
ship reports, operational model results and the expertise of dian Ice Service’s Automated Ice Tracking System (CIS-
experienced ice forecasters, spanning 1968 to the present ASITS). For extreme light and heavy years (only the 1997
(Canadian Ice Service, 2007). The CISDA ice charts are and 2004 heavy years were within the RADARSAT period)
topologically complete polygon ArcInfo Geographic Infor- the temporal resolution of the RADARSAT imagery was
www.the-cryosphere.net/7/1753/2013/ TheCryosphere,7,1753–1768,2013
1756 S.E.L.Howelletal.:RecentextremelightseaiceyearsintheCanadianArcticArchipelago
between2–5days.Adetaileddescriptionoftheseaicemo- theareafluxuncertaintyis∼12–14km2day−1.Foreachex-
tiontrackingsystemcanbefoundinWohllebenetal.(2013) change gate, sea ice flux estimates from all available image
andKomarovandBarber(2013).Herewebrieflyoutlinekey pairsweresummedovereachmonthforMaytoSeptember.
elementsoftheicetrackingalgorithm.Thesystemoperates
withtwosequentialSARimagesandderivesadetailedmap 2.3 Icethickness
oficedisplacementsforthetimeintervalseparatingtheim-
Ice, Cloud, and land Elevation Satellite (ICESat) and Ice-
ages.Thealgorithmcapturesbothtranslationalandrotational
Bridge ice thickness estimates were obtained prior to the
components of ice motion based on a combination of the
melt season to investigate changes in the thickness of Arc-
phase-andcross-correlationmatchingtechniques.Toreduce
ticOceanMYIthatwouldflowintotheCAAduringthemelt
the computation time, several pyramid levels are generated
season. The ICESat ice thickness estimates are made using
fromtheoriginalSARimages.Ateachresolutionlevelvari-
satellite-basedlaseraltimetryandarethesameusedinKwok
ousicefeatures(e.g.cracks,floes,ridges)suitablefortrack-
etal.(2009)andhaveanuncertaintyof∼0.37m.Afullde-
ing are automatically identified. First, the ice motion is de-
scriptionoftheretrievalmethodologycanbefoundinKwok
rivedatthelowestresolutionlevel.Thenateachconsecutive
etal.(2007)andKwoketal.(2009).IceBridgeicethickness
resolution level the system is guided by the vectors found
estimates are also made by laser altimetry, but from an air-
at the previous resolution level. To eliminate erroneous ice
borneplatformusingtheAirborneTopographicMapperthat
motionvectors,thealgorithmcomparesicedriftvectorsde-
is fully described in Krabill et al. (2002). IceBridge thick-
rivedfromtheforwardpass(trackingfromthefirstimageto
nessdatawereprovidedbytheNationalSnowandIceData
the second one) and the backward pass (tracking from the
Center (Kurtz et al., 2012) using the retrieval methodology
second image to the first one). The remaining vectors are
described by Kurtz et al. (2013). For each year from 2009–
further filtered by thresholding their cross-correlation coef-
2012,weselectedtheclosestIceBridgeflightlinetotheArc-
ficients. Finally, at each resolution level, a high, medium or
ticOceanM’ClureStraitandArcticOceanQEIiceexchange
low level of confidence is assigned to each ice drift vector
gates. The IceBridge flight lines for these gates covered the
according to the cross-correlation coefficient value. The ac-
spatialdomainsshowninblueinFig.1a.ThemeanICESat
curacy of the algorithm was thoroughly examined based on
thicknessforthesameregionwasextractedfrom2004–2008.
thevisualdetectionofsimilaricefeaturesinsequentialSAR
The dates of the IceBridge flight lines and the ICESat time
imagesandgroundtruthedwithice-trackingbeacondatafor
periodisshowninTable1.TheuncertaintyofeachIceBridge
variousiceconditions.InKomarovandBarber(2013)avery
flight line is also shown in Table 1, and Kurtz et al. (2013)
good agreement between the SAR-derived vectors and the
statethattheevaluationoftheaccuracyofthedatasetison-
trajectoriesofdriftingicebeaconslocatedincloseproximity
going.Asaresultweexercisecautionintheinterpretationof
(less than 3km) to the nearest SAR ice motion vectors was
thicknessestimates.
observed.Therootmeansquareerror(RMSE)was0.43km
for36comparisonpoints.
2.4 Ancillarydatasets
Seaiceareafluxwasestimatedusinganapproachsimilar
toKwok(2006)andAgnewetal.(2008).Seaicemotionfor
The date of melt onset and freeze onset from 1979–2012
eachimagepairwasfirstinterpolatedtoeachexchangegate,
withintheCAA(Fig.1a)wasretrievedfromthesatellitepas-
includingabufferregionof∼30kmoneachsideofthegate,
sivemicrowavealgorithmdescribedbyMarkusetal.(2009).
and then sampled at 5km intervals across the gate. Sea ice
SLP and SAT data from 1968–2012 were extracted from
areaflux(F)ateachgatewascalculatedusing:
the National Center for Environmental Prediction-National
X Center for Atmospheric Research (NCEP-NCAR) Reanal-
F = c u 1x, (1)
i i ysis (Kalnay et al., 1996; Kistler et al., 2001). Finally, net
shortwave(K∗)andnetlongwave(L∗)datafrom1979–2012
where1x isthespacingalongthegate(5km),u istheice
i were extracted from North American Regional Reanalysis
motionnormaltothefluxgateattheithlocationandc isthe
i (NARR) (Mesinger et al., 2006). There are large known bi-
sea ice concentration obtained from the CISDA. Assuming
ases in radiative fluxes over the Arctic in re-analyses prod-
the errors of the motion samples are additive, unbiased, un-
ucts (Walsh et al., 2009), therefore despite only starting in
correlated and normally distributed, the uncertainty in area
1979weusedNARRtobettercapturetheicewithinnarrow
fluxacrossthegates(σ )canbeestimatedusing:
f channelsandinletsoftheCAA.
p
σ =σ L/ N , (2)
f e s
3 Extremelightandheavyiceyearswithinthe
where σe∼0.43kmday−1 is the error in SAR-derived ice historicalcontext
velocities deduced from Komarov and Barber (2013), L is
the width of the gate (km) and N is the number of sam- The monthly time series of June to September total ice and
s
ples across the gate. For the M’Clure Strait and QEI gates, MYI area within the CAA is shown in Fig. 2. Total ice has
TheCryosphere,7,1753–1768,2013 www.the-cryosphere.net/7/1753/2013/
S.E.L.Howelletal.:RecentextremelightseaiceyearsintheCanadianArcticArchipelago 1757
Table1.ICESatandIceBridgeicethicknessestimateswithintheCanadianBasinjustnorthoftheArcticOcean–QueenElizabethIslands
(QEI)andArcticOcean–M’ClureStraiticeexchangegates,2004–2012.IceBridgemeanuncertaintyinbrackets.
Year IceThickness(m) IceThickness(m) Sensor Date
QEI M’ClureStrait
2004 5.6 3.5 ICESat Feb–Mar
2005 5.6 3.6 ICESat Feb–Mar
2006 5.1 4.0 ICESat Feb–Mar
2007 5.2 2.8 ICESat Apr–May
2008 3.3 2.3 ICESat Feb–Mar
2009 3.9(0.7) 2.5(0.8) IceBridge 2and21Apr
2010 4.4(0.8) 2.7(0.9) IceBridge 5and21Apr
2011 3.7(0.6) N/A IceBridge 16Mar
2012 3.3(0.8) 1.4(1.1) IceBridge 19and22Mar

been relatively stable in June and July because much of the
sea ice within the CAA remains landfast well into July. In
termsofFYIandMYIforJuneandJuly,therehasbeenade-
creaseinMYIandacorrespondingincreaseinFYI,mostno-
ticeablesince1998.Overtheentireperiod,bothAugustand
September FYI have experienced a slow downward trend,
whereasMYIremainedsteadyandonlybegantodropoffaf-
ter1998.TheSeptembertotalicetimeseriespointsouthow
strong the 1998 anomaly was, as ice conditions were rela-
tively stable until then. While subsequent minima were low
(2007) or lower (2011 and 2012), they are within a context
thatnowfavourslightyears,whichwasnotthecaseforthe
1998minima.
Standardized anomalies are provided in Table 2 for
monthly total, MYI and FYI, for light and heavy ice light
years.Theheavyiceyearstendedtoexperiencemostlyposi-
tivetotalicestandardizedanomaliesforJuneandJuly.Total
iceanomaliesforheavyiceyearsduringAugustandSeptem-
ber were all positive. In addition, total, MYI and FYI for
2011and2012inSeptemberwerelowerthan1998and2007.
The spatial distribution of mean June and September to-
tal ice and MYI for the light ice years is shown in Fig. 3,
and for heavy ice years in Fig. 4. In June, negative total ice
concentration anomalies for light years were located in pe-

ripheralregionsoftheCAA(e.g.AmundsenGulf,M’Clure Figure 3. Spatial distribution of June and September total ice and multi-year ice (MYI)
Strait and Lancaster Sound where melt had already begun). cCoFannicagedn.iatrn3a tA.iornSc taipcn aoAmtricaahlliiepsed l(aitesgnottr h(iAsb)- ufDot)ri. otAhnen olimoghaftl iieJcseu cynaeleacrusla aotnfe dd2 0w1Si2th,e 2pr0ets1ep1em, c2tb0 t0oe7 tr haent d1o 91t8a919l-82 i0wc1ie0th ina nthde
Negative MYI concentration anomalies in June were more clmimuatlotlio-gyye. a rice(MYI)concentrationanomalies(tenths)forthelight
iceyearsof2012,2011,2007and1998withintheCanadianArc-
widespreadthroughouttheCAA(withtheexceptionofpos-
tic Archipelago (A–D). Anomalies calculated with respect to the
itive MYI anomalies across the southern CAA in 1998; see
1981–2010climatology.
Table 2), especially in 2012. June MYI concentrations for
29
the extreme heavy ice years exhibited positive anomalies
throughoutmostoftheCAA.
In September, negative total ice concentration anomalies to anomalous thermodynamic atmospheric forcing events
werelocatedprimarilywithintheQEI,westernParryChan- whichproducelighticeyears.Thelighticeyearscontained
nel and M’Clintock Channel. The negative total ice anoma- virtuallynostrongpositiveMYIconcentrationanomaliesin
lies were particularly strong and widespread in 2011 and September.Forheavyiceyears,only1972experiencednega-
2012. The western Parry Channel and M’Clintock Channel tiveMYIconcentrationanomaliesinthewesternParryChan-
were almost 100% ice-covered during the heavy ice years, nel and M’Clintock Channel. With no coincident total ice
which may suggest that these regions are more sensitive concentration negative anomalies evident during that year,
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1758 S.E.L.Howelletal.:RecentextremelightseaiceyearsintheCanadianArcticArchipelago
Table2.MonthlyJunetoSeptembertotal,multi-yearice(MYI)andfirst-yearice(FYI)standardizedanomaliesforextremelightandheavy
seaiceyearsintheCanadianArcticArchipelago.Thetoprowsarelighticeyearsandthebottomrowsareheavyiceyears.Standardized
anomaliescalculatedwithrespecttothe1981–2010climatology.
Total MYI FYI Total MYI FYI Total MYI FYI Total MYI FYI
2012 2011 2007 1998
Jun 0.99 −2.06 2.23 −0.27 −1.04 0.88 −0.6 −1.03 0.74 −2.12 1.08 −1.67
Jul −1.32 −2.01 1.23 −2.62 −1.23 −0.30 −1.46 −0.98 −0.13 −1.6 1.04 −1.91
Aug −2.66 −1.62 −1.47 −3.27 −1.95 −1.86 −2.03 −1.24 −1.06 −0.94 0.74 −1.87
Sep −2.38 −2.25 −1.21 −2.42 −2.37 −1.52 −1.64 −1.15 −1.19 −1.85 −1.06 −1.20
2004 1997 1979 1972
Jun 0.46 0.34 −0.15 −0.08 0.98 −0.91 1.89 1.98 −1.15 0.28 0.90 −0.97
Jul 0.80 0.30 0.17 −0.04 1.00 −1.01 2.33 1.91 −0.54 1.98 0.08 1.03
Aug 0.97 0.53 0.59 0.56 1.02 −0.43 2.40 2.23 0.38 0.41 1.87 -1.50
Sep 1.55 1.21 0.88 1.58 1.63 0.18 2.29 2.13 1.10 2.58 −0.34 4.25

Over the 45yr record of sea ice conditions within the
CAA, 2011 and 2012 both stand out as extreme light ice
years, even more so than 1998 and 2007, in which the pre-
vious record minima were observed. Regionally, this was
particularly apparent within the western Parry Channel and
M’ClintockChannel(Fig.3).AlthoughFYImeltsmoreeas-
ily because it is thinner than MYI, which in turn is more
likelytofacilitatelighticeconditionsunderanomalousatmo-
sphericforcing,itisimportanttonotethattheearlierbreakup
ofthinnerFYIincreasestheopportunityofMYIinflowfrom
the Arctic Ocean. This process was shown to reduce total
icelossoverthemeltseason(Howelletal.,2008,2009).In
ordertoinvestigatethedrivingprocessesthatcontributedto
the new record low years of 2011 and 2012, and comment
ontheprocessesrequiredfortheCAAtoexperienceaheavy
ice year, we first investigate the melt seasonal sea ice area
progressionduringbothlightandheavyiceyears.
4 MeltseasonprogressionofCanadian
ArcticArchipelagoseaicecover
4.1 Lightyears
Thetimeseriesofweeklytotalice,MYIandFYIfortheex-
Figure 4. Spatial distribution of June and September total ice and multi-year ice (MYI)
cFonicgen.tr4at.ionS apnaomtiaalliesd (itesnttrhisb) ufotri ohenavyo ifce Jyueanrse ofa 2n0d04,S 19e9p7t, e1m979b eanrd 1to97t2a lwiithcien thaen d treme light ice years 1998, 2007, 2011 and 2012 is shown
Canadian Arctic Archipelago (A-D). Anomalies calculated with respect to the 1981-2010
cmlimuatlotlio-gyye. arice(MYI)concentrationanomalies(tenths)fortheheavy in Fig. 5. Total ice for the record low year of 2011 initially

iceyearsof2004,1997,1979and1972withintheCanadianArc- tracked higher than that of the previous record low year of
tic Archipelago (A–D). Anomalies calculated with respect to the 1998 until early July, when the rate of ice loss increased
1981–2010climatology. 30 and eventually reached a record minimum of 97×103km2
on5September.The2011minimumwasconsiderablylower
than observed in 1998 (133×103km2 during the week of
this suggests that FYI survived the melt season in these re-
28September)and2007(175×103km2 duringtheweekof
gions, a conclusion further supported by 1972 having the
10September).Totalicefor2012trackedabovepreviousex-
highestmeanSeptemberFYIareaonrecord(298×103km2;
tremelightyearsandthe1981–2010climatologicalmeanun-
Fig.2d).
tilmid-July,whenitexperiencedaveryrapidrateofdecline
andreachedthesecondlowestminimumintheobservational
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S.E.L.Howelletal.:RecentextremelightseaiceyearsintheCanadianArcticArchipelago 1759

Figure 5. Weekly time series of total ice (A), multi-year (MYI) (B) and first year ice (FYI) (C)
Fig.5.Weeklytimesearireesa o(kfmto2t)a floirc e19(9A8),, 2m0u0l7ti, -2y0e1a1r (aMndY 2I)01(B2 )ina nthdefi Crsatn-aydeiaarni cAerc(FtiYc AI)r(cChi)paerleaago(.k m2)for1998,2007,2011and2012in
theCanadianArcticAr chipelago.

record of 131×103 km2 during the week of 10 September. 4.2 Heavyyears

Howelletal.(2010)characterizedthe2007icelossasrela-

tivelyrapidwhencom paredagainstthegraduallossof1998. The weekly time series of total ice, MYI and FYI for
However,thetotalic etimeseriesinFig.5ashowsthatboth 1972, 1979, 1997 and 2004 are shown in Fig. 6. Both
2011 and 2012 expe rienced an even more rapid rate of de- 1972 and 1979 exhibited a similar slow rate of total ice de-
clinethan2007. cline,reachingminimaof533×103km2 on28Augustand

ThetimeseriesofFYI(Fig.5c)alsohighlightsthegrad- 535×103km2 on6September,respectively.1997and2004

ualdeclinein1998,therapiddeclinein2007andevenmore experienced a greater rate of decline compared to 1972 and

rapid decline during 2011 and 2012. In addition, virtually 1979 and reached minima of 426×103km2 on 28 August
allFYImelteddurin gthelighticeyears.Thetimeseriesof and 431×103km2 on 6 September, respectively. The ex-
MYI (Fig. 5b) indicates that both 1998 and 2011 exhibited tremeheavy iceyearminima, onaverage,represent ∼80%
almost no MYI increases during the melt season, indicat- moreiceareacomparedtotherecordlightminimumof2011.
ing reduced inflow from the Arctic Ocean. 2007 and 2012 The FYI and MYI time series (Fig. 6b, c) for the heavy
did, however, experience MYI increases during August and iceyearshighlightthetwoprocessesas3so1 ciatedwithheavier
September, indicating the inflow of MYI to the CAA from ice-covered summers within the CAA: FYI aging and Arc-
theArcticOcean. tic Ocean MYI import. 1 October is the date each year on
In summary, in all four light ice seasons virtually all the whichanyFYIremainingattheendofthemeltseasonispro-
FYI melted by the end of the summer. The rate of ice loss motedtoMYI(i.e.FYIaging).AlthoughtherateofFYIloss
was relatively slow and persistent in 1998, while the rapid during 1997 and 2004 was greater than in 1972 and 1979,
rateofdeclinein2007wasexceededin2011and2012.MYI itwasstillconsiderablylessthantheFYIlossduringtheex-
increasesduetoimportfromtheArcticOceanmitigatedthe tremelightyears(Figs.5cand6c).Allfourheavyiceseasons
meltseasonicelossin2007and2012,whilethelackofMYI showarapidincreaseinMYIafter1October,rangingfrom
importduringthe1998and2011meltseasonscontributedto ∼75×103km2 for1997to∼250×103km2 for1972.The
therecordlowiceminimasetduringthoseseasons. lattervaluesprovideanapproximateestimateoftheamount
of FYI that has survived the melt season and that was pro-
moted to MYI. This process was almost completely absent
duringtheextremelighticeyearsbecauseofthelackofFYI
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1760 S.E.L.Howelletal.:RecentextremelightseaiceyearsintheCanadianArcticArchipelago

Figure 6. Weekly time series of total ice (A), multi-year (MYI) (B) and first year ice (FYI) (C)
Fig.6.Weeklytimeseraireesa o(fkmto2t)a lfoicr e1(9A72),, m19u7l9ti,- y1e9a9r7( aMndY 2I)0(0B4) ina nthdefi Crsatn-yaedaiarni cAer(cFtiYc IA)r(cCh)ipaerleaagso(. km2)for1972,1979,1997and2004in
theCanadianArcticAr chipelago.

attheendofthemeltseason(Fig.5c).TheroleofMYIim- forcinginseaicemelt.WithrespecttoSAT,thetimeseries
portduringheavyiceyearswasevidentbycomparing1972 ofJJASanomaliesshowspositivedeparturesforallextreme
and1979with1997and2004.AlthoughtheincreaseinMYI lightyears,withthe2012anomalyof2.28◦Cexceedingthe
on 1 October for 1997 and 2004 was small when compared previousrecordanomalyof2.0◦Csetin1998(Fig.8).1998
to1972and1979,therewasalsoanetincreaseinMYIfrom haslongbeenrecognizedasoneofthewarmestyearsinthe
4Juneto24SeptemberthatcanbeattributedtoArcticOcean Canadian Arctic, which exerted a strong influence on CAA
inflow. seaiceconditionsalongwithotherelementsoftheCanadian
In summary, 1972 and 1979 experienced a large end-of- cryosphere (e.g. Jeffers et al., 2001; Alt et al., 2006; Atkin-
season MYI increase from FYI aging, while both 1997 and sonetal.,2006;Howelletal.,2010).TheJJASSATanomaly
2004 experienced an MYI increase from a combination of for the record low ice year of 2011 ranked third at 1.80◦C,
FYI aging and Arctic Ocean MYI inflow. In order to ex- while2007wasthe“coolest”oftheextremelightyears,with
plain these differences in the seasonal evolution of summer ananomalyof0.71◦C(Fig.8).Withrespecttoradiativeforc-
icecoverforbothlightandheavyyears,wenowlookatthe ing,thetimeseriesofJJASanomaliesofK∗,L∗andnetall-
drivingthermodynamicanddynamicprocesses. waveradiation(Q∗)showstrongpositi3v2e K∗ departuresfor
all extreme light years (Fig. 9). Of the 4 lightest ice years,
2007 and 2011 had the strongest positive K∗ anomalies, at
5 Thermodynamicanddynamicprocessesbehind 23Wm−2 and 22Wm−2, respectively (Fig. 9). 2012 ranked
extremeiceyears thirdwithapositiveK∗anomalyof16Wm−2,and1998ex-
periencedthelowestK∗ anomalyoftheextremelightyears,
5.1 Lightyears at9Wm−2(Fig.9).L∗anomaliesfortheextremeyearswere
negative(i.e.1998,2007,and2011)orjustslightlypositive
The dominant JJAS atmospheric circulation pattern during (i.e.2012)(Fig.9).
theextremelightyearsconsistedofhighSLPanomaliesover Thenewrecordextremelighticeyearsof2011and2012
Greenland and the Beaufort Sea (Fig. 7). This atmospheric areassociatedwithpositiveJJASSATandK∗anomaliesthat
circulationpatterntransportswarmairintotheCAA,which inturnhelpeddrivetherapidmelteventsthatbeganinJuly.
influencestheroleofSATinseaicemeltandalsopromotes However,thetimingofmeltonsetisalsoanotherimportant
clearerskies,whichinfluencestheroleofshortwaveradiative factortoconsiderwithrespecttothermodynamicforcingon
TheCryosphere,7,1753–1768,2013 www.the-cryosphere.net/7/1753/2013/
S.E.L.Howelletal.:RecentextremelightseaiceyearsintheCanadianArcticArchipelago 1761

Figure 9. Time series of NARR mean June through September (JJAS) net shortwave, net
longwave Fanigd. n9e.t Talilm-weasveer (iQes*)o fraNdiAatRioRn amnoemanalJieusn (eWthmr-o2)u ginh thSee pCtaenmadbiearn (AJJrActiSc) Archipelago,
1979-2012n.e Atnsohmoratlwieas vceal(cKula∗t)e,dn weitthlo rnesgpwecatv teo (tLhe∗ 1)9a8n1d-2n01et0 aclllim-waatovleog(yQ. ∗)ra-
diation anomalies (Wm−2) in the Canadian Arctic Archipelago,

1979–2012. Anomalies calculated with respect to the 1981–2010

climatology.

Figure 7. Spatial distribution of NCEP/NCAR mean June to September sea level pressure (SLP;
Fimgb.) a7n.omSapliaest ifoarl 20d1i2s, t2r0i1b1u, 2ti0o07n anodf 19N98C (AE-PD)/.N ACnoAmaRliesm caelcaulnateJdu wniteh rteospeScte tpo ttheem -
1981-2010 climatology
be r sea level pressure (SLP; mb) anomalies for 2012, 2011, 2007
an d 1998 (A–D). Anomalies calculated with respect to the 1981– rapid melt. The previous second lightest ice year of 2007

20 10climatology. had a similar melt onset date compared to 2011 and 2012
(Fig.10)andalsoexperiencedarapidmeltevent,butwasless

pronounced (Fig. 5a). K∗ anomalies in 2007 were stronger

thanboth2011and2012,butSAT’swerecooler,whichsug-

geststhatrapidmeltin2007waspotentiallyinfluencedmore

bysolarradiationabsorption.Themeltseasonlengthforthe
33 previousrecordlightesticeyearof1998waslong,withmelt
onset and freeze onset anomalies of −6 days and 14 days,
respectively(Fig.10).Theearlymeltonsetin1998wasim-
portant, as it facilitated more total ice loss early in the melt
season, thus increasing the energy available for melt that in
turncontributedtothegradualmeltoficethroughoutthere-
mainderofthemeltseason(Fig.5a).Comparing1998toboth
2011and2012indicatessimilarpositiveSATanomalies,but 35
K∗anomaliesweremuchlowerin1998,suggestingthatSAT
played more of a role with respect to ice loss. Atkinson et

al.(2006)alsosuggestedthatanomalousSATcontributedto
Figure 8. Time series of NCEP/NCAR mean June through September (JJAS) surface air
Fig.8.TimeseriesofNCEP/NCARmeanJunethroughSeptember theseaicereductionswithintheCAAduring1998.
temperature (SAT) anomalies (C) in the Canadian Arctic Arc◦hipelago, 1968-2012. Anomalies
(JJAS)surfaceairtemperature(SAT)anomalies( C)intheCana- Investigation of the corresponding dynamic processes in-
calculated with respect to the 1981-2010 climatology.
dianArcticArchipelago,1968–2012.Anomaliescalculatedwithre- dicatedthatinadditiontorapidmelt,theextrememinimum
specttothe1981–2010climatology. of2011wasalsodrivenbyJulyandAugustatmosphericcir-

culation restricted inflow of ice from the Arctic Ocean. Ice

areafluxestimatesfor2011showalmostnetzeroexchange

seaiceminima.Perovichetal.(2007)demonstratedthatthe between the Arctic Ocean and QEI gates from May to Au-
melt process is enhanced from an earlier melt onset by in- gustandalargenetinflowof∼21×103km2 inSeptember
creasingtheenergyavailableformeltduringthemeltseason, (Fig.11a).Iceareafluxestimatesindicatealmostzeronetex-
andthisincreasedenergycanexertaninfluenceonthemin- changeacrosstheM’ClureStraitgatefromMaytoSeptem-
imum sea ice area reached over the season. The melt onset berfor2011(Fig.11b).TheweeklyMYItimeseriesfor2011
datesfor2011and2012werenotanomalouslyearly(Fig.10) exhibitedagradualdeclinefromJuneuntilthefirstweekof
and total ice conditions remained near the 1981–2010 cli- September (Fig. 5b), which provides further evidence that
matologyinJune(Fig.5a).However,oncemeltonsetbegan verylittleArcticOceaniceexchangeoccurred.PositiveJune
in 2011 and 2012, the sea ice–albedo feedback intensified toAugustSLPanomaliesoverthecentralArcticOceanand
duetostrongpositiveSATandK∗ anomaliesandfacilitated Greenlandin2011(Fig.7b)contributedtoreducedexchange

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34
1762 S.E.L.Howelletal.:RecentextremelightseaiceyearsintheCanadianArcticArchipelago

Figure 10. Time series of the melt and freeze onset anomalies (days) in the Canadian Arctic
ArchipelagFoi,g 1.91709.-2T0i1m2.e Asneorimesaloiefs tchaelcmuleatletda nwdithfr reeeszpeecot ntos ethtea n1o98m1a-2li0e1s0( cdlaimysa)tology.
intheCanadianArcticArchipelago,1979–2012.Anomaliescalcu-
latedwithrespecttothe1981–2010climatology.

between the Arctic Ocean and CAA, leading up the min-
imum by facilitating stagnant ice motion within the CAA.
Arctic Ocean MYI import via the QEI in September 2011
wascausedbyashiftinatmosphericcirculationthatoccurred
inSeptember(Fig.7b).MYIfor2011alsobegantoincrease
the week after the minimum was reached (Fig. 5b). Similar
to2011,1998alsoexperiencedreducedArcticOceanicein-
flow,althoughJunetoSeptemberSLPanomalieswerediffer-
ent(cf.Fig.7b,d),withatmosphericcirculationdrivingmore

iceoutflowfromtheCAAintotheArcticOcFeiagnuaret b1o1t.h Tthieme series of mean May to September Arctic Ocean-Canadian Arctic Archipelago
M’ClureandQEIgatesinSeptember1998(Fsiega. 1ic1e) .area flux (kmFi2g). a1t1 t.hTei mQeueseernie Esloizfambeeathn IMslaayndtos S(QepEteIm) b(Aer)A arncdti cthOec Mean’C–lure Strait (B) for
Withrespecttoseaicedynamicforcingfor12909182,, 2th0e07m, i2n0-11 anCdan 2a0di1a2n eAstricmticateAdr cfhriopmela RgoADseAa RicSeAaTre. aPoflsuixtiv(ek/mn2e)gaattivteh esign indicates Arctic
imumwasinitiallydrivenbyrapidmelt,butthOicseicaen lionsfslowwa/soutfloQwue. e n ElizabethIslands(QEI)(A)andtheM’ClureStrait(B)for
mitigatedbyArcticOceanMYIinflow.Them ajorityofthis 1998, 2007, 2011 and 2012, estimated from RADARSAT. Posi-
tive/negativesignindicatesArcticOceaninflow/outflow.
inflowoccurredviatheM’ClureStraitinAug ust(Fig.11b),
withMYIduringAugustalsoexhibitingincre ases(Fig.5b).
Theiceinflowislikelyassociatedwiththean omalouslylow ofth3e6 2012meltseason(i.e.theimportedMYIsubsequently
SLPthattrackedacrosstheArcticOceanandintotheCAA
melted;Fig.5b).
(SimmondsandRudeva,2012).FollowingtheAugusticein-
Ice amounts in the CAA for the new record low years of
flowevent,almostzeroneticeareafluxoccurredinSeptem-
2011and2012werenotdrivenbythesamecombinationof
berattheM’ClureStrait,butasmallnetinflowoccurredinto
thermodynamic and dynamic driving processes as 1998 or
the QEI (Fig. 11). These Arctic Ocean ice inflow events re-
2007,butinsteadsharedprocess esrelatedtoboth.Whilepro-
duced the CAA’s rate of total ice decline, preventing total
nouncedmeltevents,eitherrap id(i.e.2007,2011and2012)
iceconditionsfromeclipsingthe2011recordlow.Thisdriv-
or of long duration (i.e. 1998) are a necessary condition for
ing dynamic process observed in 2012 was similar to 2007.
extremelowiceseasonswithin theCAA,dynamicsalsoplay
Although June to September SLP anomalies were different
animportantrole.Specifically,d ynamicscanenhancetheex-
between2012and2007(Fig.7a,c),bothatmosphericcircu-
trememinimabyrestrictingiceinflow(i.e.1998and2011),
lationpatternsfacilitatedArcticOceaninflowintotheCAA.
amplify the extreme minima by facilitating ice outflow (i.e.
Itshouldbenotedhoweverthatthe2012ArcticOceanicein-
1998),orpreventrecord-breakingminimabyfacilitatingice 37
fluxintotheM’ClureStraitwasdrivenby2singleSLPdriv-
inflow(e.g.2007and2012).
ingeventsinAugust(notshown)thatarelostintheAugust
monthlySLPaverage.Thekeydifferencebetween2012and
2007wasthatdespitethenetArcticOceaniceinflowatthe
M’ClureStraitinAugustandatQEIinSeptemberfor2012,
MYI conditions did not remain high through the remainder
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