Variability of polar stratospheric water vapor observed by ILAS

Variability of polar stratospheric water vapor observed by ILAS

L. L. Pan, W. J. Randel, and S. T. MassieNational Center for Atmospheric Research, Boulder, Colorado, USA

H. Kanzawa, Y. Sasano, H. Nakajima, T. Yokota, and T. SugitaNational Institute for Environmental Studies, Tsukuba, Japan

Received 30 July 2001; revised 11 October 2001; accepted 18 October 2001; published 22 October 2002.

[1] We present an analysis of polar stratospheric water vapor from measurements of theImproved Limb Atmospheric Spectrometer (ILAS) onboard the Advanced EarthObserving Satellite (ADEOS). The variability of water vapor is examined usingcontemporaneous potential vorticity fields to separate the vortex from the extravortex air.The overall water vapor distribution from the ILAS measurements agrees qualitativelywith the water vapor climatology in the UARS reference atmosphere (covering 1992–1999), and the space-time variability is consistent with the known stratosphericcirculation. Quantitatively, comparisons between ILAS and Halogen OccultationExperiment (HALOE) equivalent latitude zonal means show agreement to within 5% inthe lower stratosphere (400–800 K potential temperature or 15–30 km). In the upperstratosphere (800–1800 K potential temperature or 30–45 km), however, ILAS data havea 10% to 20% high bias compared to the 8-year (1992–1999) UARS climatology and toHALOE data for the same time period. The ILAS data, covering the time periodNovember 1996 to June 1997, were measured during a winter-spring period when theArctic polar stratosphere was anomalously cold. The ILAS water vapor data suggest anArctic vortex that is more isolated and persistent in the spring than that seen in the UARSclimatology. The spatial and temporal extent of the Arctic vortex this year was very similarto that of the Antarctic vortex. This feature is supported by HALOE data for the samemeasurement period. Using a trajectory model, we show that the NH vortex in spring 1997was significantly more isolated compared to other years in the decade, and this explainsthe unusual confinement of high water vapor within the Arctic vortex observed byILAS. INDEX TERMS: 0341 Atmospheric Composition and Structure: Middle atmosphere—constituent

transport and chemistry (3334); 3349 Meteorology and Atmospheric Dynamics: Polar meteorology; 3309

Meteorology and Atmospheric Dynamics: Climatology (1620); 3319 Meteorology and Atmospheric

Dynamics: General circulation; 3334 Meteorology and Atmospheric Dynamics: Middle atmosphere dynamics

(0341, 0342)

Citation: Pan, L. L., W. J. Randel, S. T. Massie, H. Kanzawa, Y. Sasano, H. Nakajima, T. Yokota, and T. Sugita, Variability of polar

stratospheric water vapor observed by ILAS, J. Geophys. Res., 107(D24), 8214, doi:10.1029/2001JD001164, 2002.

1. Introduction

[2] Water vapor is a long-lived tracer in stratosphere. Itsdistribution and variability has provided a wealth ofinformation on the atmospheric circulation. The mainchemical source of water vapor in the stratosphere ismethane oxidation [Brasseur and Solomon, 1986; LeTexieret al., 1988]. The primary sink in the lower stratosphere isdue to the formation of ice particles, which occurs mainlyin the polar region. An additional sink of water vapor dueto photolysis in the mesosphere [Brasseur and Solomon,1986] also effects the stratospheric water vapor distributionnear the stratopause. The quantity (H2O + 2CH4), oftenreferred to as the total hydrogen index or total water, is

approximately conserved in the lower stratosphere with avalue around 7.5 ppmv [Dessler et al., 1994; Hurst et al.,1999], except when dehydration occurs in the polar regiondue to the formation of ice particles. The climatologicaldistribution of water vapor in the stratosphere [Remsberget al., 1984, 1996; Mote et al., 1996; and Randel et al.,1998] has several well known features: (1) low values nearthe tropical lower stratosphere as a result of air ascendingthrough the cold tropical tropopause region with an annualaverage around 3.8 ppmv [Dessler and Kim, 1999, andreferences therein], (2) increased mixing ratio values withaltitude due to the production of water vapor by methaneoxidation, reaching �7 ppmv near the stratopause, and (3)high values in the polar regions due to the descent of airfrom high altitudes and confinement within the polarvortices, except when dehydration occurs in the polarlower stratosphere. Quantitative information on water

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D24, 8214, doi:10.1029/2001JD001164, 2002

Copyright 2002 by the American Geophysical Union.0148-0227/02/2001JD001164$09.00

ILS 11 - 1

vapor distribution is important for understanding both theradiation budget of the Earth and atmospheric chemistry.Variability of water vapor in the polar region is especiallyrelevant for the latter, since the formation of polar strato-spheric clouds (PSCs) and the subsequent heterogeneouschemistry is chiefly responsible for ozone destruction[Solomon et al., 1986].[3] In this paper, we present the monthly variability of

polar stratospheric water vapor derived from the version5.20 data of the Improved Limb Atmospheric Spectrometer(ILAS) onboard the Advanced Earth Observing Satellite(ADOES). As described by Sasano et al. [1999], ILAS is asolar occultation spectrometer that consists of infrared andnear visible channels. ILAS made profile measurements ofozone, water vapor, nitric acid, as well as aerosol extinctionin the polar stratosphere from November 1996 to June 1997.Descriptions of the instrument performance and retrievalalgorithm can be found in companion papers in this issue[Nakajima et al., 2002; Yokota et al., 2002]. Overalldescription and validation of the version 5.20 water vapordata are given by Kanzawa et al. [2002], where the differ-ences between ILAS water vapor and correlative measure-ments, including both in situ measurements from balloonborne, airborne platforms and satellite measurements, arequantified.[4] The objectives of this paper are (1) to examine the

climatology and variability of the ILAS water vapor data forconsistency with the known stratospheric circulation, (2) tomake comparisons with the water vapor climatologyderived from the UARS Halogen Occultation Experiment(HALOE) and MLS data [Randel et al., 1998], and (3) todemonstrate the dynamic variability of water vapor inrelation to meteorology of the Arctic polar vortex betweenthe ILAS and UARS time periods. Examination of theclimatology or zonal average is a good approach forevaluating data quality and is complementary to detailedcomparisons with correlative measurements. Since the dis-tribution of water vapor is largely controlled by transport inthe stratosphere, the observed variability in the ILAS zonalmean is an important validation for the data set as a whole.[5] The 1996–1997 NH winter-spring period has been

known for its anomalously low temperature in the Arcticpolar stratosphere. The spring Arctic polar vortex wasanomalously strong, similar to the Antarctic polar vortex[Coy et al., 1997]. This has been shown to have significantimpact to the ozone chemistry. Using ILAS observations,Sasano et al. [2000] have estimated that up to 50% of theozone inside the vortex was lost. Arctic PSCs wereobserved by ILAS during January–March 1997 [Hayashidaet al., 2000]. In connection with observed PSCs, ILASmeasurements have also found occurrences of dehydrationinside the Arctic polar vortex during this extremely cold NHwinter [Pan et al., 2002]. In this paper, we examine how themeteorological conditions were manifested in the watervapor zonal mean, which provides a useful perspective ofthe vortex evolution this year.[6] The paper is organized into four sections. This section

serves to define the scope of the paper. The remaining threesections cover the motivation and methodology of equiv-alent latitude mapping for ILAS data, comparisons of theILAS zonal mean with the UARS climatology, and exami-nation of water vapor variability in Arctic polar region in

association with transport across the vortex. A brief sum-mary is given to conclude the paper.

2. Data and Analyses

2.1. ILAS Data and Equivalent Latitude Mapping

[7] The ILAS instrument made measurements at highlatitudes near both the Arctic and Antarctic polar vorticesduring 8 months from November 1996 to June 1997. Thelatitude ranges of sampling, varying month-by-month, werefrom 57 to 73 degrees North and from 64 to 88 degreesSouth [Sasano et al., 1999]. In this section, we show thatalthough the measurements were made within relativelynarrow latitude bands geographically, they covered a widerange dynamically, both because of steep potential vorticitygradients near the vortex edge and the fact that the vortexshapes were often distorted with respect to the pole.[8] As an example, Figure 1 shows the relationship of the

ILAS sampling location to the polar vortex. Given in thefigure are the modified potential vorticity (MPV) [Lait,1994] at the 750 K level for NH for 22 January 1997 andthe locations of ILAS sunrise measurements for 22 and 23January. ILAS measurements were near the 65 N latitudecircle for the two days. The polar vortex was asymmetric(using the 20 PVU contour to represent the edge) andextended from 45�N at the lowest latitude (near 30� east)to 80�N at the highest latitude (near the Date Line). Due tothe asymmetry of the polar vortex, the measurements weremade both inside and outside the vortex. Figure 2 gives thevertical cross-section of the measured water vapor profilesduring the three days of 22–24 January, arranged insequence of the adjacent measurement events, which wereapproximately 25 degree apart in longitude. Overlayingcontours in the figure are the PV field. The horizontalgradient of the PV field and the water vapor distributionare well correlated and mark the edge of the vortex. Highvalues of water vapor mixing ratio mark the interior of thevortex, where water vapor values �6 ppmv or higher reflectthe descent from near the stratopause, and the low valuesindicate the region outside the vortex.[9] Figures 1 and 2 illustrate the wide dynamic range that

the ILAS measurements cover in a given day and theimportance of separating the data by dynamical variableswhen computing statistics or making physical interpreta-tions. To connect the space defined by the dynamic variableof PV to the space defined by the geographical latitude, wemap PV values into their equivalent latitudes and use theequivalent latitude as an alternative way to represent thedata. The equivalent latitudes are determined by comparingthe areas enclosed by the PV contours with those enclosedby the latitudinal circles [e.g., Butchart and Remsberg,1986;Lait et al., 1990; Schoeberl et al., 1992; Randel et al., 1998;Manney et al., 1999]. Figure 3 illustrates the effectivenessof using equivalent latitude as the coordinates to display thedata. Displayed in Figure 3 are ILAS NH middle strato-sphere water vapor data (between 600 and 800 K potentialtemperature range) as a function of latitude, equivalentlatitude and modified PV for the month of January 1997.As shown in the figure, the measurements made within anarrow latitude range near 65�N are well correlated with thedynamic variable of PV. When the data are sorted by PV(expressed as the equivalent latitude), the ILAS sampled

ILS 11 - 2 PAN ET AL.: ILAS ANALYSIS OF POLAR STRATOSPHERIC WATER VAPOR

from approximately 25 degrees to the pole in the equivalentlatitude space, a range representative of both hemispheres.The change of slope around 65�N equivalent latitude in thewater vapor distribution also marks the position of thevortex edge for this month.[10] As illustrated in Figures 1–3, under these condi-

tions, it is advantageous to use the equivalent latitudeinstead of the geographical latitude when computing zonalmean statistics and for comparison with other satelliteclimatologies that are compiled in the same dynamicalcoordinates. We have computed ILAS water vapormonthly zonal mean in equivalent latitude and potentialtemperature (q) coordinates. The data are binned in 4-degree equivalent latitude intervals and q layers varyingroughly according to the standard UARS pressure levels(six per decade of pressure). The PV values used forcomputing equivalent latitudes are derived from UKMO

meteorological analyses [Swinbank and O’Neil, 1994], andwe use the modified PV of Lait [1994] and have applied ascaling factor of (q/420)�9/2.

2.2. UARS Reference Atmosphere

[11] The UARS water vapor climatology used in thispaper is a subset of the UARS reference atmosphere(URL for the project is http://code916.gsfc.nasa.gov/Pub-lic/Analysis/UARS/urap/home.html), which includes a setof climatologies of atmospheric trace gas profiles compiledfrom UARS measurements. The water vapor climatology isderived from a combination of HALOE and MLS data[Randel et al., 1998], using HALOE [Russell et al., 1993;Harries et al., 1996] version 19 (96 months from 1992 to1999) and MLS prototype version 5 (nonlinear retrieval)data (September 1991 to April 1993) [Pumphrey, 1999].The 8-year climatology shown in this paper is derived using

ILAS Measurements Jan 1997 and MPV on 750K

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Figure 1. Arctic polar vortex structure and ILAS samplings position during mid-January 1997. Colorcontours are modified PV (see text) at 750 K level in 5 PVU interval. Red dots mark the locations ofILAS observations during 2 days of 22 and 23 January. Latitudinal circles of 30� and 60�N are indicatedby dotted lines.

PAN ET AL.: ILAS ANALYSIS OF POLAR STRATOSPHERIC WATER VAPOR ILS 11 - 3

a harmonic fit of the seasonal cycle to the 96-month binnedaverages, as described by Randel et al. [1998].

2.3. HALOE Data

[12] To examine and validate the atmospheric variabilityshown in ILAS data, we have computed a HALOE monthlyzonal mean using HALOE version 19 level 2 data for thesame measurement period as ILAS (November 1996 to June1997). The average is computed in identical fashion as for theILAS equivalent zonal mean calculations. The equivalentlatitude mapping of the HALOE data used UKMO PV data.

3. Seasonal Variability of Water Vapor Observedby ILAS

[13] Displayed in Figure 4 is ILAS water vapor monthlyzonal mean for November 1996 to June 1997 period inequivalent latitude-q cross-section. Consistent with the pre-vious section, the ILAS measurements cover equivalentlatitudes from approximately 25� to the pole in each hemi-sphere with no data in the tropics. As a comparison and toprovide a global perspective, water vapor climatology in theUARS reference atmosphere for November, March andJune is shown in Figures 5a–5c. We do not expect theILAS and the UARS climatology to be identical, since thelength of the data records and the sampling intervals forthe data sets went in the climatologies are all very different.Rather, qualitative agreements of the general features are

expected. As an additional comparison in a more quantita-tive level, HALOE data for November 1996 and March,June 1997 are displayed in Figures 5d to 5f.[14] As shown in Figures 4 and 5, the global scale

structure and seasonal variation of water vapor evident inILAS data are consistent with the UARS climatology, andreflect features of stratospheric circulation. The overallpatterns show the general circulation of air rising in thetropics and sinking in the pole. The high value of watervapor mixing ratio in the upper stratosphere (represented byyellow and red in the figures) is due to methane oxidation.The effect of mesosphere photolysis is visible near thewinter polar stratopause (represented by green right belowthe 2000 K level in the figures). Although the ILASmeasurements do not cover the tropics, the general patternof upward bulging contours as a result of the circulation andthe weak latitudinal gradient in the midlatitude as an effectof mixing in the ‘‘surf-zone’’ are readily observed in thezonal mean.[15] The formation and the decay of the polar vortex are

clearly revealed in the ILAS water vapor data. In bothhemispheres, the vortex edge is marked by the steepgradient of water vapor, with high vortex interior valuesdue to the seasonal downwelling. The decay of the vortex inthe middle stratosphere (�800 K or �30 km) in the springhemisphere is shown in NH March and April, with thevortex air persisting longer in the lower stratosphere (�600K or �23 km). This feature is consistent with the seasonal

Figure 2. Color image represents water vapor mixing ratios from ILAS measurements in measurementsequence along the orbit for 3 days, 22–24 January. Overlaying white contours are modified PV in 5PVU intervals.


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Figure 4. ILAS monthly equivalent-latitude zonal mean for the eight months from November 1996 toJune 1997.


variation of vortex breakdown derived from previous obser-vations and model simulations [e.g., Schoeberl et al., 1992;Manney et al., 1994], showing breakdown first in the upperstratosphere and progressing downward in time.[16] The ILAS data also revealed very dry air associated

with dehydration in the Antarctic polar lower stratosphere(around 400 K) during November–December, together witha persistent dry layer in the lower stratosphere over much ofthe globe. These features are consistent with the UARSclimatology, as shown in Figure 5. The climatologicalfeature of dehydration inside the Antarctic vortex is welldocumented from previous observations [e.g., Tuck et al.,1993; Pierce et al., 1994; Rosenlof et al., 1997; Randel et al.,

1998; Pumphrey, 1999; Nedoluha et al., 2000]. AlthoughILAS only observes this feature for November–December,the near global dry layer above the tropopause is mappedmore completely in the UARS data and is formed by rapidisentropic transport following the tropical dehydration [Ran-del et al., 2001].[17] There are also marked differences between the ILAS

data and UARS climatology. Quantitatively, ILAS datashow higher water vapor mixing ratios in the upper strato-sphere and inside the vortex, especially in the SH. Theamount of high bias is consistent with the statistical com-parisons with the correlative measurements, which indicatethat ILAS water vapor is �10% higher in the altitude region

Figure 5. Water vapor climatology from the UARS Reference Atmosphere for November (a), March (b),and June (c), and HALOE equivalent zonal mean for November 1996 (d), March (e), and June 1997 (f ).


of 30–60 km for a majority of the comparisons and within20% for all comparisons with isolated exceptions [Kanzawaet al., 2002], Qualitatively, the descent of air within the NHvortex is much more pronounced in the 1997 ILAS datacompared to the 1992–1999 UARS data. A much moreisolated and persistent vortex in the NH polar region isindicated by the March and April ILAS data as compared tothe UARS climatology.[18] To investigate whether the qualitative difference

between the 1996–1997 ILAS data and the 8-year UARSclimatology is due to physical variability of the atmosphere,we have examined the HALOE data for the same timeperiod. A comparison of HALOE data for November1996to June 1997 (three of the eight months shown in Figure 5)with ILAS data shows many similar features that aredifferent from the 8-year UARS climatology. For example,the Antarctic vortex in the lower stratosphere is morepronounced in November 1996 data from both ILAS(Figure 4) and HALOE (Figure 5d) as compared to theUARS (Figure 5a). More significantly, strong water vaporgradients across the Arctic vortex are found in both ILASand HALOE March 1997 data (Figure 5e), strikingly differ-ent from the UARS climatology (Figure 5b).[19] A more quantitative perspective of these compari-

sons is given in Figure 6, which shows the March NH watervapor equivalent latitude zonal mean at the 655 K q level forall three data sets. The figure shows a reasonable agreementbetween ILAS and HALOE data for the measurement madeduring the same time. Both data sets show a near 2 ppmvincrease of H2O from 55 to 75 degree equivalent latitude.The steep gradients near 60 N equivalent latitude mark thevortex edge in March1997, which do not appear in themultiyear UARS climatology. In the equivalent latituderange 25–50 N, on the other hand, ILAS water vapor is

�0.5 ppmv higher than the HALOE data. Latitudinalsampling difference could be a factor that contributed tothe apparent high bias in ILAS data. ILAS sampled mostlyhigh equivalent latitude air during this month, and theaverages for lower equivalent latitudes are calculated froma small sample and are subject to a higher uncertainty. As aspecific example, the 655 K equivalent latitude zonalaverages are computed from �100 data points per 4-degreeequivalent latitude band for the 50–80 N range, but arefrom �10 data points per equivalent latitude band between25 and 50 N. Indeed, such latitudinal sampling inducedbiases are found in the ozone data comparisons usingequivalent latitude/q coordinates between POAM II dataand HALOE data [Manney et el., 2001].[20] Additional quantitative comparisons for different

altitudes are given in Figure 7, where ILAS, UARS andHALOE March equivalent zonal mean profiles for 68 S andN are shown. Evident in Figure 7, ILAS and HALOE meanprofiles show good agreement in the middle to lowerstratosphere (below 800 K or 30 km), where the differencesare well within 5%. Above 800 K, ILAS data begin to showhigh bias compared to HALOE for both hemispheres, up to�10% in the upper stratosphere. The amount of high biasvaries with latitudes and is different for each month. Thebiases in the NH data are typically less than 10% but arehigher for the SH data at higher equivalent latitudes, up to25% in March, consistent with that discussed by Kanzawaet al. [2002].[21] In the lower stratosphere, both ILAS and HALOE

1997 mean profiles are similar to the UARS climatology inthe SH, but significantly different in the NH. The agreementbetween ILAS and HALOE data provide strong evidencethat the difference between the ILAS and UARS data in theNH polar region is a result of the meteorological conditionsof the 1996–1997 winter-spring seasons, rather than apeculiar bias in the ILAS data. In the following section,we further examine the atmospheric variability manifestedin water vapor observations during this time period.

4. Transport Near the Arctic Polar Vortex for NHSpring 1997

[22] To compare the time evolution of the polar vorticesas inferred from water vapor data, we present, in Figure 8,UARS climatology and the ILAS monthly zonal mean watervapor time-height cross-section at 72�NH and SH equiv-alent latitudes. To facilitate a more direct comparisonbetween the NH and the SH seasonality, we display SHdata from January to December and NH July to June.Although the ILAS measurements only cover two thirdsof the seasonal cycle and have higher mixing ratios in theupper stratosphere, there is a qualitative agreement betweenILAS and UARS in the vortex evolution for the summer toearly winter transition in the SH. In the NH vortex, how-ever, the ILAS data show deeper descent of the wintermaximum, indicating a more isolated and persistent Arcticvortex during 1997. ILAS NH data indicate the descent ofhigh H2O vortex air to about 500 K in q and to April–Mayin time, while the UARS data show a reduced scale in thedescent and a much shorter vortex season at the loweraltitudes. On the other hand, similarities of 1997 Arcticvortex shown in ILAS water vapor to the multiyear UARS

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Figure 6. Comparisons of ILAS (1997), UARS (1992–1999), and HALOE (1997) equivalent latitude zonal meanfor the month of March at 655 K level (�25 km in altitude)for the NH latitudes. The error bars represent the standarddeviation of the fit for the UARS water vapor climatologyand the standard deviation from the mean for the ILAS andHALOE zonal mean.


climatology of the Antarctic vortex is readily observed inFigure 8.[23] To further demonstrate that the anomalous water

vapor distribution observed by ILAS is consistent with themeteorological conditions of the 1996–1997 vortex, wehave performed a tracer transport simulation, using a three-dimensional trajectory model. The calculation uses isen-tropic coordinates with horizontal velocities from theUKMO wind data, and vertical velocities derived from theradiative heating code of Olaguer et al. [1992]. Thesecalculations are very similar to those shown by Manney etal. [1994]. A set of tracers is initiated inside the vortex(choosing 20 PVU contour as the boundary) at the 665 Klevel on 1 March and we examine the change of thedistribution after one month. Among the 7 years we inves-tigated (1993–1999), March 1997 stands out as an extremecase of vortex isolation in terms of the confinement of thetracers.[24] Displayed in Figure 9 are comparisons of tracer

distributions at the beginning and the end of March for1993 (Figure 9a) and 1997 (Figure 9b). We choose 1993 asa representative for other years, both because the 1993results are typical, and because the UARS climatology inthe polar region is largely compiled from the MLS measure-ments (made between September 1991 and April 1993).

Figures 9a and 9b show the number of tracers as a functionof the equivalent latitude on 1 and 31 March for the twoselected years. Figures 9c and 9d show the tracers latitudi-nal-longitudinal position on 1 March for the 2 years at thestarting potential temperature level 665 K. Figures 9e and 9fgive the tracer positions at the end of March and the lowestpotential temperature level the parcels reached thoughdescent. As shown in the figure, during March 1993, about35% of the tracers were transported out of the vortex, butalmost none for 1997. In fact, there is a small polewardchange of tracer distribution from the beginning to the endof March 1997, which reflects the fact that although main-taining its strength, the vortex became smaller by the end ofthe month.[25] To reveal the mechanism of the NH vortex isolation

during spring 1997, we have examined the March PV andwind fields using multiple year NCEP data. The results aregiven in Figure 10, where we show PV and wind speedaveraged along equivalent latitudes for 1 and 31 March,1993–1999, at the 665 K level. The change of PV gradientindicates the vortex edges to be around 60� equivalentlatitude. The maximum of the zonal wind also correspondsto the position of the vortex edge and gives the strength ofthe polar night jet. At the beginning of March, both 1997PV and equivalent latitude zonal wind are similar to other

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Figure 7. Comparisons of ILAS (1997), UARS (1992–1999), and HALOE (1997) equivalent latitudezonal mean profiles for the month of March at 68 S and 68 N. The meanings of the error bars are the sameas in Figure 6.


years. At the end of March, however, the 1997 the windbecame completely anomalous: while the jet was signifi-cantly weakened by the end of the month for all other years,the 1997 jet was as strong as at the beginning of the month.This is also reflected in the higher PV value at the end ofMarch 1997. These results suggest that the more persistentNH polar night jet in early spring 1997 is responsible for theisolation of the Arctic vortex this year, as reflected in theILAS water vapor data. The comparisons in Figures 9 and10 also suggest that the strength of the polar jet is a moredeterminant characteristic for the vortex confinement thanthe PV structure alone.

4.1. Summary

[26] Using equivalent latitude mapping, we have demon-strated that the ILAS measurements at high latitudes cov-

ered a wide dynamic range of the atmosphere. The watervapor monthly equivalent latitude zonal mean compiledfrom the ILAS data contains many key features of strato-spheric circulation from around 25 degrees equivalentlatitude to the poles. Quantitatively, ILAS equivalent zonalmean water vapor agrees well with the HALOE measure-ments during the same time period in the lower stratosphere(within 1 standard deviation of both data sets below 800 K)and shows high bias in the upper stratosphere (around 10%for most months for 800–1800 K, higher in the SHFebruary to March time period). Comparisons with theUARS water vapor climatology show qualitative agree-ments as well as differences. The most remarkable differ-ence is the spatial and temporal extent of the 1996–1997Arctic polar vortex, as inferred from the distribution ofwater vapor. Compared to the UARS climatology, the ILAS

Figure 8. Comparison of UARS versus ILAS water vapor in time-height cross-section 72� SH and NH.Note the time for the NH is shifted 6 months.

Figure 9. (opposite) Tracer distribution from trajectory model simulations. (a) Number of tracers as a function ofequivalent latitude on 1 and 31 March 1993, initially at the 665 K level (�25 km in altitude). As shown in the figure, �35%of the tracers transported to the outside of the vortex by the end of the month. (b) Same as (a) but for 1997. At the end ofMarch, only �0.1% of the tracer got out of the vortex. (c) Contours are MPV in 5 PVU interval and red dots marks theinitial tracer position on 1 March 1993. (d) Same as in (c) but for 1997. (e) PV contours and tracer positions for 31 March1993. All tracers are shown. The 640 K potential temperature level is the lowest isentrope the parcels reached after themonth through descent. (f ) Same as in (e) but for 1997. Similarly, 648 K is the lowest level the parcels reached.


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data show that the extent of the 1996–1997 Arctic vortex issimilar to that of the Antarctic vortex. HALOE water vaporobservations during the same period support that this differ-ence reflects atmospheric variability rather than ILAS databiases. We use a tracer transport simulation to show that thisdifference is due to a more persistent NH polar vortex forthe record cold 1996–1997 NH winter-spring period. Thewind and PV statistics show that the polar jet during spring1997 was much stronger and provided a stronger barrier tothe transport across that vortex edge.[27] Complementary to the correlative data comparisons

[Kanzawa et al., 2002], the comparisons of climatologicalfeatures shown in this paper serve to characterize theoverall quality and the limitations of the ILAS water vapordata. The good agreement between ILAS and HALOEmonthly equivalent zonal means in the Arctic lower strato-sphere and the differences between the ILAS climatologyand the long term UARS climatology in this regionprovide an excellent example of trace gas distribution inresponse to the changes in stratospheric circulation. Theconsistency of the observed water vapor with the atmos-pheric variability validates the scientific value of the ILASV5.20 water vapor product.

[28] Acknowledgments. The authors thank three anonymousreviewers for their helpful comments on the manuscript. This work issupported in part by the National Science Foundation through its support tothe University Corporation for Atmospheric Research, by the NASA UpperAtmosphere Research Satellite guest investigator program, and by theNASA Atmospheric Chemistry Modeling and Analysis Program.

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Figure 10. NCEP March PV and equivalent latitude zonal wind (U) as a function of equivalent latitudefor 1993–1999. 1997 is shown as a solid line.


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��H. Kanzawa, H. Nakajima, Y. Sasano, T. Sugita, and T. Yokota, National

Institute for Environmental Studies, Tsukuba, Japan.S. T. Massie, L. L. Pan, and W. J. Randel, National Center for

Atmospheric Research, 1850 Table Mesa Drive, Boulder, CO 80305, USA.([email protected])


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Variability of polar stratospheric water vapor observed by ILAS