A Climatology of Rossby Wave Breaking along the ...matt/Hitchman_AMS_Papers/PostelHitchman_1999.pdf(e.g., Juckes and McIntyre 1987; Polvani and Plumb 1992; Waugh and Plumb 1994; Norton

1 FEBRUARY 1999 359P O S T E L A N D H I T C H M A N

q 1999 American Meteorological Society

A Climatology of Rossby Wave Breaking along the Subtropical Tropopause

GREGORY A. POSTEL AND MATTHEW H. HITCHMAN

Department of Atmospheric and Oceanic Sciences, University of Wisconsin—Madison, Madison, Wisconsin

(Manuscript received 20 October 1997, in final form 8 April 1998)

ABSTRACT

Ten years (1986–95) of global analyses from the European Centre for Medium-Range Weather Forecasts areused to investigate the temporal and spatial distributions of Rossby wave breaking (RWB) at 350 K along thetropopause, herein defined by the 61.5 potential vorticity (PV) unit (1026 K m2 s21 kg21) contours. Thoughmany studies acknowledge RWB as an important contributor to the complex of mixing processes in the at-mosphere, there exists no prior climatological study of its distribution near the tropopause.

As in previous studies, RWB is identified in the global analyses by southward directed PV gradients. At 350K, RWB along the tropopause occurs preferentially during summer over the midoceans, in relative proximityto the planetary-scale high pressure systems in the subtropics. Isentropic trajectories at 350 K show that outflowfrom the tops of these subtropical highs directly participates in RWB over the adjacent, downstream oceanicregions.

Two regions are highlighted in this study: the North Pacific during boreal summer and the South Atlanticduring austral summer. Synoptic maps of breaking Rossby waves in these regions are provided to reveal theacute tropopause folding in the meridional plane, which characteristically accompanies RWB. The rich interactionbetween the tropical flow and the extratropical westerly current exhibited by these cases suggests that thesubtropical highs serve as important agents in the coupling between the tropical troposphere and the extratropicalstratosphere. As expected from theoretical considerations, the locations where RWB occurs most frequently,known as ‘‘surf zones,’’ are shown to coexist with regionally weak time-mean wind speeds and horizontalgradients of PV at 350 K.

1. Introduction

Using daily Northern Hemisphere (NH) meteorolog-ical analyses and satellite data to construct midstratos-phere synoptic charts of potential vorticity (PV), Mc-Intyre and Palmer (1983) provided the first detailed ob-servational evidence of breaking planetary waves in thestratosphere. Over the course of several days, continen-tal-scale tongues of relatively high PV were peeled awayfrom the polar vortex’s edge and advected (primarilyisentropically) into tropical latitudes. They called thisquasi-horizontal, severe distortion of PV contours‘‘planetary wave breaking.’’ Planetary (or Rossby) wavebreaking is characterized by the rapid (on the order ofa day) and irreversible deformation of material contours.Further elucidation of the Rossby wave breaking (RWB)concept is given in McIntyre and Palmer (1984, 1985).

Analytical and numerical studies have composed con-sistent analogs of the observed structure of breakingRossby waves. The theory of nonlinear critical layers,

Corresponding author address: Gregory A. Postel, Department ofAtmospheric and Oceanic Sciences, University of Wisconsin—Mad-ison, 1225 W. Dayton St., Madison, WI 53706.E-mail: [email protected]

for example, yields an analytical solution for the dis-tribution of material contours in a critical layer, whichevokes the familiar Kelvin cat’s eye pattern and resem-bles RWB (e.g., Stewartson 1978; Warn and Warn 1978;Haynes and McIntyre 1987). Additionally, numericalmodeling experiments have simulated RWB at spatialscales finer than present observational capabilities allow(e.g., Juckes and McIntyre 1987; Polvani and Plumb1992; Waugh and Plumb 1994; Norton 1994). Fromthese studies, it is apparent that the velocity field thatattends RWB drives a cascade of motion to increasinglyfine scales, wherein intensification of constituent gra-dients and the enhancement of transport and mixingprocesses occur simultaneously.

Detection of RWB’s large-scale structure or formfrom observations remains a daunting problem sincethere is no unique signature or shape by which one canalways identify it (McIntyre and Palmer 1985). None-theless, the empirical approach to studying its mor-phology, though unable to reveal detail due to data res-olution constraints, can contribute significantly to ourunderstanding of atmospheric chemistry issues and themaintenance of certain features of the general circula-tion.

Leovy et al. (1985) showed that the ozone distributionin the middle stratosphere, and thus the global distri-

360 VOLUME 56J O U R N A L O F T H E A T M O S P H E R I C S C I E N C E S

FIG. 1. Schematic of Rossby wave breaking events over (a) the North Pacific and (b) the SouthAtlantic, on the 350 K isentropic surface. The thick contours represent the tropopause. The hatchedregions denote surf zones, where the meridional gradient of PV is regionally reversed.

bution of the total column amount of ozone, is a sen-sitive function of the advective processes associatedwith RWB. Trepte et al. (1993) implicated a role formidlatitude RWB motions in the poleward detrainmentof Mount Pinatubo aerosol out of the Tropics.O’Sullivan and Hitchman (1992) used a mechanisticmodel of the middle atmosphere to study the interactionbetween Rossby waves forced at the extratropical tro-popause and inertial instability in the equatorial lowermesosphere. Their results suggested that inertial insta-bility is intimately involved in RWB in the subtropicalwinter mesosphere. Nakamura (1994) illustrated howdeformation of the PV field associated with RWB in theupper troposphere over Europe during winter assists thedevelopment of blocked regimes. Chen (1995) sug-gested that the large-scale breaking of material contours,which routinely attends occluding tropospheric depres-sions, is primarily responsible for the vigorous cross-tropopause mass exchange observed in all seasons onand below 330 K.

This paper describes the first climatological study ofRWB along the tropopause at 350 K. Since large-scalemotions near the tropopause are approximately adiabaticand frictionless on synoptic timescales, an isoline ofpotential vorticity on an isentropic surface closely sim-

ulates a material contour. For this reason, this studyemployed a ‘‘PV-u’’ (Hoskins 1991) view of RWB,whereby its detection was based solely on the evolutionof PV on the 350 K surface.

The lack of small-scale precision in global datasetsand the nonsingular appearance of breaking Rossbywaves make establishing a climatology of RWB diffi-cult. To do so, an objective definition of the process isrequired. Following Baldwin and Holton (1988) in theirclimatology of the stratospheric polar vortex and plan-etary wave breaking, we regard the signature of RWBas a southward-directed (‘‘reversed’’) PV gradient of aselected strength. Further details of how its signaturewas extracted from the data are provided in section 2.

An example representative of the reversed PV gra-dient configurations at the tropopause, or tropopausefolds, that accompany RWB is schematically depictedin Fig. 1 for each of the two focal regions in this study:the North Pacific during boreal summer and the SouthAtlantic during austral summer. Analogous to the oftenobserved tropopause folding in the vertical dimensionassociated with occluding tropospheric cyclones, thetropopause folds detected in this study lie in the quasi-horizontal plane defined by the 350 K isentropic surface.

In this study, the tropopause is designated as the 1.5


FIG. 2. Meridional cross section of the zonal-mean and time-averaged (1986–95) potentialtemperature (K), the 1.5 PV unit contour (thick dashed line), and the 1.2 3 1022 K m21 contourof the vertical gradient of potential temperature (thick solid line). The shaded region approximatelydelineates the ‘‘Middleworld.’’

PV unit (1026 K m2 s21 kg21) contour in the NH andthe 21.5 PV unit contour in the Southern Hemisphere(SH), following Hoskins et al. (1985). Though accept-able for interpreting large-scale dynamical processes inthe subtropics, alternative representations of the tro-popause may be more appropriate very near the equatorand in midlatitudes (e.g., Hoerling et al. 1991). Figure2 illustrates a meridional slice of the potential temper-ature (K) field. The 61.5 PV unit contours (thick dashedlines) and the 1.2 3 1022 K m21 contour of the verticalgradient of potential temperature (thick solid line) aresuperposed. In this view, the 350 K level intersects 61.5 PV unit contours in the subtropics, wherein theyclearly establish an appropriate boundary between thetroposphere and the highly stratified regime above. Inthe deep Tropics, the potential temperature gradientmore accurately outlines the distinction between the tro-posphere and the stratosphere.

The tropopause folding detected in this study ex-emplifies interaction between the troposphere andstratosphere (e.g., Andrews et al. 1987). As shown byFig. 2, the 350 K level lies near the center of the ‘‘Mid-dleworld’’ (Hoskins 1991), which is the horizontal slabof atmosphere encompassing only those isentropes thatcross the tropopause but do not intersect the earth’ssurface. The selection of 350 K as the isentrope uponwhich RWB was detected provided a robust perspectiveof the RWB-induced interaction between the tropicaltroposphere and the extratropical stratosphere. Quanti-tative assessment of the net transport between the tro-posphere and stratosphere induced by RWB along thetropopause, which warrants diagnosis of its vertical de-pendence, will be reported in a later manuscript. Thepresent purposes of isolating the frequency distributionof RWB in space and time along the tropopause at the350 K level highlight preferred times and places in

which the tropical troposphere and the extratropicalstratosphere interact.

It will be shown in section 3 that RWB at 350 Koccurs most frequently during summer, over the mid-oceans. Isentropic trajectories provided in section 4show that particularly active ducts through which RWBpromotes interaction between the tropical flow and tran-sient disturbances in the extratropical westerly currentare found on the poleward and eastward edges of theupper-level, semipermanent subtropical highs. Synopticcharts of PV valid during individual RWB events, pro-vided in section 4, reveal that the RWB distributiondepends on the ‘‘fragmentation’’ of the subtropicalhighs, herein defined as a process in which chunks oftropospheric PV are stripped from the high pressuresystems and advected downstream into the midlatitudestratosphere. Further insight into the RWB distributionsis obtained in section 5 by examining composite summerflows near the tropopause in light of current theory. Asummary and discussion are given in section 6. Forreference, in the remainder of this paper, the NH summerrefers to June, July, and August (JJA), and NH winterrefers to December, January, and February (DJF). TheSH summer (winter) refers to DJF (JJA).

2. Data and analysis methods

This study employs ten years (1986–95) of meteo-rological data produced by the European Centre for Me-dium-Range Weather Forecasts (ECMWF). Generatedfrom operational assimilation procedures, the data con-sist of twice daily uninitialized globally analyzed fieldsof geopotential height, temperature, and horizontal windon a 2.58 3 2.58 grid at all standard levels in the tro-posphere and stratosphere. Though slight differences ex-ist between uninitialized and initialized ECMWF ana-


lyses at the smallest horizontal scales, especially in theirrotational wind component, they nonetheless exhibitgood agreement in the fields of geopotential height,wind, and temperature (e.g., Trenberth 1992; Dunkerton1995). Trenberth (1992) provides details of the ECMWFproducts.

Inspection of the data showed that P,

[u cos(f)]u nfz lP 5 f 2 1 , (1)5 6r a cos(f) a cos(f)closely approximates Ertel’s potential vorticity near thetropopause [cf. Andrews et al. 1987, Eq. (3.1.4)]. Tocompute the three-dimensional distribution of P in iso-baric coordinates, second-order accurate centered dif-ference derivatives of the wind field were used for therelative vorticity and cubic splines were used for thestatic stability. The isobaric distribution of P was lin-early interpolated to isentropic surfaces to recover fieldsof PV in isentropic coordinates. Absolute values of thehorizontal gradients of PV on isentropic surfaces, asshown in Fig. 10, were computed using centered dif-ferences in the zonal and meridional dimensions. Linearinterpolation was also used to transform isobaric dis-tributions of the Montgomery streamfunction and windfields onto isentropic levels.

To limit the inclusion in the RWB climatology oftropopause folds induced by scales of motion decidedlysmaller than Rossby waves, the data used by the al-gorithms, which tagged the tropopause folds in spaceand time were spectrally truncated to include only thezonal mean and first 18 zonal wavenumbers at every2.58 lat. These data have an equivalent horizontal gridinterval of 2.58 lat 3 108 long.

Three objective steps were employed to create theRWB climatology. The first was an areal search overall latitudes on the 350 K surface, at each analysis timein the 10-yr record, for a northward decrease from 21.0to 22.0 PV units in the SH, or from 12.0 to 11.0 unitsin the NH, within 108 lat. If met, these criteria ensuredthat the meridional gradient of PV was reversed at thetropopause and associated with a strong poleward pro-trusion of relatively low PV from the Tropics into mid-latitudes. The locations and times that satisfied theselatitudinally independent tropopause fold conditionswere temporarily stored.

The second step was an areal search at each time forconsecutive longitudes at which the first criterion wasmet. If such a series was detected, all but the western-most location was removed from storage to ensure thatthe entire series was counted only once. This removedmultiple tallies for a single tropopause fold whose at-tendant PV field deformation produced reversed PV gra-dients at consecutive longitudes.

The third step aimed to prevent the inclusion of re-versed PV gradients associated with secluded masses oftropospheric PV in the stratosphere. In this procedure,if stratospheric PV existed at any equatorward latitude

for at least five consecutive grid points (508 of long) ona westward search from a location selected by the firsttwo criteria, the location retained by steps one and twowas discarded (see Fig. 1 for reference). This helped toremove tropopause folds associated with nearly andcompletely detached streams of tropospheric PV fromthe RWB climatology. These strips, or pieces, of PVoften denoted residue of the RWB process, followingthe poleward and eastward surge of tropospheric air intohigher latitudes.

To unambiguously link RWB along the tropopause at350 K in the summer hemisphere to the subtropicalhighs, isentropic back-trajectories were derived to revealair parcel paths during RWB. To show that air associatedwith RWB often comes from these high pressure sys-tems, extraction of individual breaking waves from thetropopause fold distribution that defined the climatologywas required. Owing to its transient nature, a singlebreaking wave may induce RWB (folds in the tropo-pause) at irregularly distributed locations in space andtime. Since the three automated steps outlined abovewere unable to sequester individual breaking waves, thisstudy employed an additional subjective visual screen-ing of the PV field evolution associated with each tro-popause fold in the climatology to retain one fold perbreaking Rossby wave event. The location and time ofthe fold that denoted the inception of a single eventwere preserved.

For each breaking wave detected, a set of five tra-jectories was derived from the 350 K wind field, as-suming adiabatic flow, using the twice daily ECMWFdata on the 2.58 3 2.58 grid. A ‘‘central’’ parcel wasinitialized at the grid point and time where the associatedtropopause fold was tagged, and four additional parcelswere initialized at the same time at locations 58 lat and58 long away from the central grid point (see Fig. 7 forillustrations of these configurations). Each set was in-tegrated backward in time for 7 days in 1-h incrementsusing a fourth-order Runge–Kutta scheme, which em-ployed a linear interpolation routine to retrieve the ve-locity field between analysis times.

The 350 K Montgomery streamfunction and hori-zontal winds, valid at the last time in the backwardintegration, were employed to elucidate the trajectories’histories. For the North Pacific sector, parcels that, atthe last time step, were located equatorward of the high-est open Montgomery streamfunction contour and be-tween 408 and 1208E were defined to be associated withthe high pressure system over southern Asia. Similarly,in the South Atlantic sector, parcels that, at the last timestep, were located equatorward of the highest openMontgomery streamfunction contour between 1008 and208W were defined to be associated with the high overSouth America. These open streamfunction contoursserved as references for the instantaneous high pres-sure systems on the 350 K surface. Closed contoursoften did not entirely encompass the anticyclonic shearassociated with these systems, especially on their equa-


FIG. 3. Histogram of the total number of tropopause folds detectedduring the 1986–95 period at 350 K, as a function of month, for (a)the NH and (b) the SH.

torward flanks. In addition, closed contours did not al-ways exist at the final time step in the trajectory al-gorithm, despite clear evidence of a high pressure sys-tem in other fields.

3. RWB climatology along the subtropicaltropopause

Figure 3 reveals the total number of tropopause foldsdetected per month during the 1986–95 period. Thesehistograms clearly illustrate the summertime predomi-nance for RWB in both hemispheres. Figure 3a showsthat in the NH, RWB-induced tropopause folds occurredmost often from June–September. A month-to-monthcomparison reveals that RWB in the NH was detectednearly eight times more often in July than in January.The RWB frequency in the SH exhibits a similar tem-poral asymmetry. Figure 3b shows that RWB in the SHwas most often observed during December, January, andFebruary.

The spatial distribution of the summertime tropopausefold count accumulated during the 1986–95 period isprovided in Figs. 4 (NH, JJA) and 5 (SH, DJF). Though

tropopause folds at 350 K were observed at nearly alllongitudes in the northern summer, Fig. 4a strongly sug-gests that RWB was particularly common near the date-line and near 508W. This is especially evident in Fig.4b, which shows the tropopause fold count (indicatedby the thick contours) and the summer-mean 150-hPageopotential height and wind fields (superposed for laterreference). According to Fig. 4b, the peaks at 1808 and508W evident in Fig. 4a, compose two distinct regionsthat preferentially favor RWB: near the middle of thesubtropical Pacific and Atlantic Oceans, respectively.

Figure 5a shows the austral summer tropopause foldcount. The SH flow clearly favored RWB in the lon-gitudes near 108W, 708E, 1208W, and to a lesser extentnear 1608E. Figure 5b reveals that these peaks composelarge-scale regional maxima in the RWB frequency overthe South Atlantic, southern Indian, the eastern SouthPacific, and to a lesser extent, extreme western portionsof the South Pacific oceanic basins, respectively. As inFig. 4b, the summer-mean 150-hPa geopotential heightand wind fields are superposed on the tropopause foldcount for later reference.

It has been shown thus far, that in the 1986–95 period,RWB at 350 K was detected most often during summerover the midoceans. Since the locales where tropopausefolding is most common define where Rossby wavesmost often break, they may be thought of as regional‘‘surf zones.’’ The hatched regions in Fig. 1, wherebreaking Rossby waves have reversed the PV gradientlocally, schematically depict surf zones over the NorthPacific and South Atlantic Oceans.

In the next section, direct pathways from the sub-tropical highs to the surf zones immediately polewardand eastward from them are uncovered. Analysis of is-entropic trajectories reveals that air is rapidly trans-ported from the high pressure systems into the surf zonesduring individual RWB events. This active participationin tropopause folding over the midoceans suggests thatthe subtropical highs’ circulations provide a favorableenvironment for RWB.

4. A link between the subtropical highs and RWB

According to the geopotential height and wind fieldsshown in Figs. 4b and 5b, the centers of the upper-levelsubtropical highs reside over southern Asia (908E) andMexico (1058W) during the northern summer and oversouthern Africa (258E), northern Australia (1408E), theSouth Pacific (1808), and South America (608W) duringthe southern summer. These large-scale anticyclonicvortices may be regarded as the upper flow associatedwith the summer monsoon circulations (e.g., Ramage1971; Hastenrath 1991; Douglas et al. 1993). Thoughthese systems exist over regions where observations arerelatively sparse (Daley 1991), most notably in the SH,upper-air studies have long since depicted their season-ally and geographically dependent structures (e.g., Gut-man and Schwerdtfeger 1965; Neyama 1968; Krish-


FIG

.4.

Nor

ther

nH

emis

pher

etr

opop

ause

fold

coun

tsu

mm

edov

erea

chJJ

Adu

ring

1986

–95

at35

0K

.(a

)H

isto

gram

ofit

slo

ngit

udin

aldi

stri

buti

on.

(b)

Mer

cato

rpr

ojec

tion

ofit

sho

rizo

ntal

dist

ribu

tion

show

nby

the

unla

bele

dth

ick

cont

ours

,co

ntou

red

from

5by

4;th

e15

0-hP

age

opot

enti

alhe

ight

sin

met

ers

(thi

nla

bele

dco

ntou

rs)

and

win

dve

ctor

s(m

s21)

aver

aged

over

each

JJA

duri

ng19

86–9

5.

FIG

.5.

Sam

eas

inF

ig.

4bu

tfo

rD

JFin

the

SH

.Tro

popa

use

fold

coun

tin

(b)

isco

ntou

red

from

4by

1.


FIG. 6. Histogram of the percent of trajectories initialized withinbreaking Rossby waves over (a) the North Pacific and (b) the SouthAtlantic, which were coupled to the southern Asian and South Amer-ican anticyclone systems, respectively, as a function of the longitudeat which the tropopause fold was detected.

namurti 1970; Newell et al. 1972; Van Loon and Jenne1972; Chu and Hastenrath 1982; Dunkerton 1995).

The circulations associated with the subtropical highsexhibit marked daily variations. Consequently, season-al-mean views of their attendant geopotential height andwind fields present unrealistically smoothed versions oftheir real-time flow structures. For example, over south-ern Asia, daily observations (not shown) reveal thatseveral anticyclonic circulation centers often coexist ina broad longitudinal band from roughly 408 to 1408E(e.g., Neyama 1968). Similar longitudinal asymmetries,though not as extensive, are also observed on a dailybasis over South America during the austral summer.Thus, for many purposes, it is more appropriate to viewthe subtropical highs as a system of high pressure cellsrather than a single anticyclone.

Figures 4b and 5b indicate that RWB occurred inrelative proximity to the planetary-scale subtropicalhighs. Relatively short distances exist between the peaksin the tropopause fold count and the highest summer-mean geopotential heights associated with the subtrop-ical highs immediately upstream. In the NH, for ex-ample, though 8200 km separates the time-mean centerof the Asian high from the center of the surf zone overthe North Pacific, the 14 200-m closed contour of theanticyclone system is approximately 11 500 km wide inthe zonal dimension. Even less of a gap is readily ap-parent between the upper-level high over Mexico andthe North Atlantic surf zone, given that RWB occursrelatively frequently just several grid points east andnorth of the highest time-mean geopotential heights. Inthe SH, the high pressure systems are smaller, yet thedistances to the centers of the surf zones to their southand east are still several thousands of kilometers. None-theless, inspection of the PV field’s evolution shows thatthese pathways are rapidly traversed by the southeast-ward advection of PV contours during RWB, on time-scales of the order of 1 day.

Trajectories were computed for each breaking Rossbywave event detected over the two focal regions: theNorth Pacific (between 1408E and 1308W) and the SouthAtlantic (between 408W and 208E). During the 10 borealsummers studied, 156 RWB events were documentedover the North Pacific sector. During the 10 austral sum-mers analyzed, 67 RWB events were documented overthe South Atlantic sector.

Of the 780 trajectories initialized within the NorthPacific surf zone, 25.5% were linked to the Asian an-ticyclone system. Of the 335 trajectories initializedwithin the South Atlantic surf zone, 20.9% were linkedto the South American high. Figures 6a and 6b illustratethe percentage of trajectories that were coupled to theanticyclone systems as a function of the longitude ofthe breaking Rossby wave within which they were ini-tialized. For example, Fig. 6a reveals that 36% of thetrajectories initialized within RWB events at 1408E weretraced back to the Asian high pressure system. Figure6b shows that 40% of the trajectories initialized within

RWB events at 408W were traced to the South Americanhigh. Both figures suggest that outflow from the sub-tropical highs is less likely to attend RWB over thedownstream oceans the further east the events occur.

Trajectory paths for two selected RWB events areprovided in Figs. 7a and 7b. These events are proto-typical examples of the link between the subtropicalhighs and RWB. Figure 7a shows an event that occurredover the North Pacific during June 1988, and Fig. 7billustrates another that was detected in the South At-lantic sector in December 1993. The 350 K PV fieldcorresponding to the break time is superposed in bothcases, with absolute values greater than 3 PV units shad-ed for clarity. The locations at which the five parcelswere initialized are identified by the large dots.

In both of the cases presented in Fig. 7, only one ofthe five trajectories linked the surf zone to the upstreamhigh pressure system. Maps of the streamfunction andwind fields at each analysis time (not shown) reveal thatthe trajectory initialized at 358N and 1758E in the NorthPacific RWB event of 1200 UTC 18 June 1988, asshown in Fig. 7a, departed the Asian high pressure sys-tem approximately 72 h earlier. The path between theSouth American high pressure regime and the SouthAtlantic surf zone was more rapidly traversed by the airparcel tagged at 358S and 158W (shown in Fig. 7b)


FIG. 7. Isentropic back-trajectories, PV unit contours, and wind vectors (m s21) at 350 K. Trajectory paths wereinitialized at the large solid dots: at (358N, 1758E), (358N, 1758W), (308N, 1808), (258N, 1758E), and (258N, 1758W)at 1200 UTC 18 June 1988 in (a); and at (258S, 158W), (258S, 58W), (308S, 108W), (358S, 158W), and (358S, 58W) at0000 UTC 18 December 1993 in (b). Winds and PV are valid at 1200 UTC 18 June 1988 in (a), and at 0000 UTC 18December 1993 in (b). Absolute values of PV greater than 3 (9) units are lightly (heavily) shaded.

during the RWB event of 0000 UTC 18 December 1993.After 36 h, isentropic outflow from the top of the SouthAmerican high attended the RWB event several thou-sand kilometers to its south and east.

The direct coupling that the Asian and South Amer-ican anticyclonic systems exhibited with the surf zonesover the North Pacific and South Atlantic Oceans, re-spectively, reveals that these subtropical highs partici-pate in RWB. Figures 8 and 9 illustrate the same twoRWB events shown in Fig. 7. Figures 8a–c show theNorth Pacific RWB event, and Figs. 9a–c illustrate theSouth Atlantic case.

The North Pacific sequence shows the eastwardstretching of PV contours on the east flank of the an-ticyclonic shear associated with the well-developedAsian high pressure system, which was centered near258N and 1008E. On 1200 UTC 17 June, an intensewesterly flow (.40 m s21) existed along the north sideof the Asian high with relatively vigorous northeasterlywinds near 208N and 1508E. In an anticyclonic manner,these flows rapidly advected tropospheric PV eastward,and stratospheric PV toward the southwest. By 0000UTC 20 June 1988, fragments of tropospheric air (e.g.,near 1658W and 328N), ripped from the eastern edge of


FIG. 8. PV unit contours and wind vectors (m s21) at 350 K during a RWB event over the North Pacific at (a) 1200UTC 17 June 1988, (b) 1200 UTC 18 June 1988, and (c) 0000 UTC 20 June 1988. Absolute values of PV greater than1.5 (9) units are lightly (heavily) shaded.


FIG. 9. Same as in Fig. 8 but over the South Atlantic at (a) 0000 UTC 15 December 1993, (b) 0000 UTC 18December 1993, and (c) 0000 UTC 19 December 1993.


the Asian anticyclonic circulation system, appeared inthe Western Hemisphere, just south of a westerly currentexceeding 40 m s21. Also at that time, fragments ofstratospheric PV appeared in the tropical easterly jet(e.g., Hastenrath 1991) at 158S over southeast Asia.

Similarly, during the RWB event over the South At-lantic, exhibited by Figs. 9a–c, the rapid poleward andeastward extension of tropospheric air from the highpressure system centered over the South American in-terior coincided with the northwestward injection ofstratospheric PV deep into the Tropics. By 0000 UTC19 December, a synoptic-scale fragment of troposphericair extended into the Eastern Hemisphere, equatorwardof a speedy westerly wind regime and collocated withan anticyclonic gyre at 308S and 58E.

Both cases reveal that, in a matter of days, PV con-tours are pulled (and/or pushed) poleward and eastwardfrom the subtropical highs, and folded over anticyclon-ically into a pattern analogous to that depicted in Fig.1. Eventually, strips of PV are dislodged from the an-ticyclone systems, concluding the ‘‘fragmentation’’ pro-cess. By the last days shown in each sequence, air withrelatively high PV (shaded), characteristic of strato-spheric origin, lies adjacent to air with PV values char-acteristic of the tropical troposphere.

Details of the relative influences that the subtropicalhigh pressure systems and the extratropical currentshave on the instigation and sustenance of RWB eventsare currently under investigation by the authors. None-theless, it has been established herein that air streamspoleward and eastward from the geographically tiedhigh pressure systems and attends RWB in the surfzones over the downstream oceans. The air parcel tra-jectories shown outline corridors (schematically de-picted by the curved arrows in Fig. 1) through whichthe tropical troposphere and extratropical stratospheredirectly and rapidly interact.

5. Time-mean flows associated with the RWBdistribution

The concentration of RWB at 350 K over the mid-oceans, east of the subtropical highs, from June throughSeptember in the NH and from December throughMarch in the SH is testimony to the fact that, by certainmeasures, the near-tropopause flow at 350 K is moredisturbed in summer than in winter. We now show thatweak PV gradient and wind fields are characteristic ofthe summertime current over the midoceans, in relativeproximity to the anticyclone systems. Theoretical con-siderations suggest that these flow attributes are nec-essary precursors to and responses of the temporal andspatial asymmetries in the RWB distributions.

a. Observed time-mean PV gradients and windspeeds

Horizontal gradients of PV on the 350 K level at thetropopause during summer are generally weaker than

those during the other seasons. For example, during the10-yr period studied, 72% of all longitudes in the NHexhibited weaker JJA-mean gradients than those aver-aged over DJF. Similarly, at nearly all (.97%) longi-tudes in the SH, summer-mean gradients were weakerthan the seasonal averages for autumn, winter, andspring.

A view of the spatial distribution of the PV gradientfield is provided in Fig. 10. Magnitudes of the horizontalgradients of PV (thin contours) on the 350 K surfaceare superposed on the RWB count (unlabeled thick con-tours). In the NH (as shown in Fig. 10a), two notableweaknesses in the PV gradient field protrude north-eastward from the Tropics: near 1808 and near 508W.The RWB frequency maxima are collocated with theaxes of relatively weak PV stratification off the eastcoasts of Asia and North America. In the SH (Fig. 10b)regional minima in the PV gradient field extend south-eastward from the subtropical highs, shown in Fig. 5bto reside over southern Africa (258E), the South Pacific(1808), and South America (608W), toward the maximain the RWB frequency over the southern Indian, south-eastern Pacific, and South Atlantic Oceans. As in theNH, slices of relatively strong gradients bulge equator-ward and westward on the equatorward flanks of therelatively weak gradients over the midoceans.

In addition, relatively weak winds were detectedwhere the RWB frequencies peaked. Figure 11 illus-trates summer-mean wind speeds at 350 K, averagedover the 10 yr of interest. In the northern summer (asshown in Fig. 11a), maxima in the RWB tally coexistwith the poleward and eastward tilted light wind regimesover the midoceans, east and south of the time-mean jetmaxima (.20 m s21) observed over the east coasts ofthe Eurasian and North American continents. Thoughthe summertime midlatitude winds are generally stron-ger in the SH than in the NH (as shown in Fig. 11b),axes of southeastward tilted corridors of relatively lightwinds extend over the proximate longitude bands, whichhosted the RWB maxima.

b. Physical interpretations

The fact that RWB most often occurred in close prox-imity to chronically weak PV gradient and wind regimesfollows well-explored lines of theoretical reasoning. Inaccordance with ‘‘PV thinking’’ (Hoskins et al. 1985),weakly stratified PV fields allow relatively large parcelexcursions along PV gradients. The summertime flow’spreference for RWB may, in part, be attributed to therelative ease with which the subtropical highs’ outflowdeforms the characteristically weak PV gradients near-by. Relatively strong PV gradients and the absence ofthe large-scale divergent flow associated with the sum-mertime monsoon circulations may contribute to thedearth of RWB at 350 K during winter. In addition,observations suggest that gradients of potential vorticityare weak in weak wind regimes. Thus, as Rossby wave


FIG. 10. Mercator projections of the magnitudes of the summer-mean horizontal gradients ofPV (10212 K m s21 kg21), drawn by thin and labeled contours, on the 350 K surface. Valuesgreater than 2.5 (5) 3 10212 K m s21 kg21 are lightly (heavily) shaded. NH values, temporallyaveraged over each JJA in the 1986–95 period, are shown in (a). SH values, temporally averagedover each DJF in the 1986–95 period, are shown in (b). Contours (thick) of the correspondingtropopause fold counts, as shown in Figs. 4b and 5b, are superposed.

energy encounters the weak time-mean currents overthe midoceans, the background flow offers relativelylittle resistance to wave action attempts to displace, de-form, and fold over PV contours (e.g., McIntyre andPalmer 1984; Hoskins et al. 1985). Observations haveshown this to be the case. For example, Nakamura(1994) showed that RWB-like deformation of the PVfield occurs downstream of diffluent zones, where weakwesterly flow prevails, when Rossby waves propagateinto these regions from stronger westerlies upstream.

From an alternative point of view, it is almost certainthat as eastward propagating Rossby waves enter theweaker ambient winds downwind of diffluent zones,differences between the waves’ phase speeds and theambient flow speeds decrease. According to critical lay-er theory, these locally reduced differences could in partaccount for the structure of the RWB distributionsshown here. In fact, numerical simulations of Rossbywaves propagating toward their critical latitude (e.g.,

Held and Phillips 1990), which lies where the phase andmean flow speeds match, show RWB-like PV field evo-lution.

On a final note, the juxtaposition of relatively strongand weak PV gradients (shown in Fig. 10) is consistentwith the previously referenced numerical work, whichhas shown that fluid motions within RWB breaking re-gimes simultaneously enhance and weaken constituentgradients in nearby regions. It is likely, then, that theweakly stratified PV fields over the midoceans (asshown in Fig. 10) are at least in part a result of theregionally enhanced mixing associated with RWB (e.g.,McIntyre and Palmer 1983). This idea naturally extendsfrom the condition that was herein defined to be a sig-nature of RWB, namely a reversed PV gradient.

Thus, to the extent that PV thinking, principles ofcritical layer theory, and results from previous RWBexperiments apply to the observations presented in thispaper, the time-mean flow regimes on the east sides of


FIG. 11. Same as in Fig. 10 except wind speeds (m s21) instead of PV gradients. Wind speedcontours are labeled. Values greater than 15 (30) m s21 are lightly (heavily) shaded.

the subtropical highs and over the midoceans provideevidence of and for the preferential recurrence of RWB.

6. Summary and discussion

Though many studies acknowledge RWB as an im-portant contributor to the complex of mixing processesin the atmosphere, there exists no prior survey of itsspatial and temporal distributions near the tropopause.Results of this study have established that RWB inducedtropopause folding at 350 K occurs preferentially overthe summertime oceans, downstream of the subtropicalhigh pressure systems. The juxtaposition of the tropicaltroposphere and extratropical stratosphere in these re-gions fosters mixing between these two very differentflow regimes.

Outflow from the subtropical highs is directly in-volved with RWB, and participates in the troposphere–stratosphere interaction induced by the attendant tro-popause folding. To this extent, troposphere–strato-sphere interaction is geographically rooted to regionsadjacent and downstream of the anticyclone systems, in

accordance with the tropopause fold distributions shownin Figs. 4 and 5. Implications that the east sides of thesubtropical highs support such interaction via RWB arefirmly supported by the aforementioned results fromChen (1995) and Dunkerton (1995). As Chen (1995)pointed out, the upper-level quasi-stationary subtropicalhighs are easy to break, based on their close proximityto their critical line, and should thus enhance cross-tropopause mass exchange above 340 K in the summerhemisphere.

The communication between the tropical troposphereand extratropical stratosphere focused on in this paperbehaves quite differently from that described by classicteleconnection theory in which linear, quasi-rotationalwaves propagate away from low latitudes into the ex-tratropics (e.g., Hoskins and Karoly 1981; Sardeshmukhand Hoskins 1988; Hoskins and Ambrizzi 1993).Though the trajectories shown in section 4 suggest thatthe RWB distributions are, in part, manifestations of thecoupling between the divergent outflow atop the sub-tropical highs and the traveling eddies in the midlati-tudes, the air parcel paths reveal connections that are


direct and rapid interactions, more akin to local Hadleycell overturnings (e.g., Charney 1963; Paegle et al.1983).

The climatological results presented in section 3 weremodestly sensitive to changes in the tropopause defi-nition (TD). The monthly dependence of the RWB fre-quency at 350 K (as depicted in Fig. 3) varied little withincreases in magnitude of the TD from 61.5 to 64.0PV units. In each experiment, the maximum and min-imum in the tropopause fold tally occurred in July (Feb-ruary) and March (August) in the NH (SH), respectively.However, the number of folds detected per month inboth the NH and the SH decreased rapidly with increasesin the TD from the 61.5 PV unit threshold: by a factorof 2 at the 62.5 PV unit TD and by a factor of 8 at the64.0 PV unit TD. In addition, the locations and shapesof the surf zones (as denoted by the thick contours inFigs. 4b and 5b) were nearly unaffected by TD changes;only a slight poleward and eastward migration accom-panied increases in the TD. It seems likely that the RWBfrequency’s limited sensitivity to TD variations (onlythe total tropopause fold count was notably affected) isa direct consequence of the fact that the higher TDvalues represent stratospheric PV unit contours, onwhich the wave activity amplitude has been dampenedconsiderably during its upward propagation from thetroposphere.

Alterations to the RWB climatology induced by in-creases in the magnitude of the minimum PV gradient(from 1.0 PV units per 108 lat to 3.0 PV units per 108lat) required to accompany RWB mimicked those pro-duced by changes in the TD. In particular, in both hemi-spheres the spatial and temporal dependencies of theRWB frequency remained nearly steady with each PVgradient change. However, the total number of tropo-pause folds per location (and per month) decreased withincreasing gradient strength. This suggests that thoseRWB events associated with relatively strong reversedPV gradients occur less often, but in the usual placesand at the usual times.

Though the NH and SH RWB distributions exhibitremarkable similarities, there are some noteworthy dif-ferences. Approximately twice as many tropopausefolds were detected in the NH (than in the SH), despitethe relative absence of RWB in multiple longitude bands(Fig. 4a). It is likely that the comparatively strong mid-latitude wind speeds characteristic of the SH midlatitudeflow between roughly 208W and 1008E, evident in Fig.11b, more broadly distributed the austral summer RWBdistribution in the Eastern Hemisphere by advecting theattendant tropopause folds eastward across the SouthAtlantic and southern Indian Oceans. Inspection of nu-merous RWB events in this region (not shown) supportsthis notion.

It is interesting to note that the preferred regions forRWB in both the northern and southern summers appearover the middle of the oceans, near where the residenceof semipermanent troughs in the upper troposphere is

well documented (e.g., Krishnamurti 1971). As shownby Figs. 7–9, RWB along the subtropical tropopause isattended by flows that pull high (in the absolute value)PV from the stratospheric reservoir equatorward intosubtropical latitudes. Following PV thinking concepts,regionally high PV values in the upper troposphere, suchas those that are drawn into the subtropics by the afore-mentioned processes, coincide with depressions in theupper-tropospheric mass field. It therefore seems pos-sible that the fragmentation of the subtropical highs as-sociated with RWB influences the intensity and locationof the climatological upper troughs in the summer hemi-sphere over the oceans.

The authors are currently investigating the source(s)of the Rossby wave energy, which folds the 350 K tro-popause in the regions outlined above. Perhaps Rossbywaves propagating upward and equatorward from themidlatitude troposphere into the subtropics reach theircritical latitude and assist the wave breaking describedhere.

Acknowledgments. This work was supported by NSFGrant 9120110 and NASA Grants NAG5-2722, NAS1-96030, and NAG5-2806. The ECMWF data were ac-quired from the National Center for Atmospheric Re-search, which is sponsored by the National ScienceFoundation.

REFERENCES

Andrews, D. G., J. R. Holton, and C. B. Leovy, 1987: Middle At-mosphere Dynamics. Academic Press, 489 pp.

Baldwin, M. P., and J. R. Holton, 1988: Climatology of the strato-spheric polar vortex and planetary wave breaking. J. Atmos. Sci.,45, 1123–1142.

Charney, J. G., 1963: A note on large-scale motions in the Tropics.J. Atmos. Sci., 20, 607–609.

Chen, P., 1995: Isentropic cross-tropopause mass exchange in theextratropics. J. Geophys. Res., 100, 16 661–16 673.

Chu, P.-S., and S. Hastenrath, 1982: Atlas of upper-air circulationover tropical South America. UW-Madison MET Publication No.82.05.Cl, 237 pp. [Available from University of Wisconsin—Madison, Department of Atmospheric and Oceanic Sciences,1225 W. Dayton St., Madison, WI 53706.]

Daley, R., 1991: Atmospheric Data Analysis. Cambridge UniversityPress, 457 pp.

Douglas, M. W., R. A. Maddox, and K. Howard, 1993: The MexicanMonsoon. J. Climate, 6, 1665–1677.

Dunkerton, T. J., 1995: Evidence of meridional motion in the summerlower stratosphere adjacent to monsoon regions. J. Geophys.Res., 100, 16 675–16 688.

Gutman, G., and W. S. Schwerdtfeger, 1965: The role of latent andsensible heat for the development of a high pressure system overthe subtropical Andes in the summer. Meteor. Rundsch., 18, 1–7.

Hastenrath, S., 1991: Climate Dynamics of the Tropics. Kluwer Ac-ademic, 488 pp.

Haynes, P. H., and M. E. McIntyre, 1987: On the representation ofRossby wave critical layers and wave breaking in zonally trun-cated models. J. Atmos. Sci., 44, 2359–2382.

Held, I. M., and P. J. Phillips, 1990: A barotropic model of the in-teraction between the Hadley cell and a Rossby wave. J. Atmos.Sci., 47, 856–869.

Hoerling, M. P., T. K. Schaack, and A. J. Lenzen, 1991: Global ob-jective tropopause analysis. Mon. Wea. Rev., 119, 1816–1831.


Hoskins, B. J., 1991: Towards a PV-u view of the general circulation.Tellus, 43, 27–35., and D. J. Karoly, 1981: The steady linear response of a sphericalatmosphere to thermal and orographic forcing. J. Atmos. Sci.,38, 1179–1196., and T. Ambrizzi, 1993: Rossby wave propagation on a realisticlongitudinally varying flow. J. Atmos. Sci., 50, 1661–1671., M. E. McIntyre, and A. W. Robertson, 1985: On the use andsignificance of isentropic potential vorticity maps. Quart. J. Roy.Meteor. Soc., 111, 877–946.

Juckes, M. N., and M. E. McIntyre, 1987: A high-resolution one-layer model of breaking planetary waves in the stratosphere.Nature, 328, 590–596.

Krishnamurti, T. N., 1970: Observational study of tropical upper tro-pospheric motion field during Northern Hemisphere summer.Tech. Rep. 70-4, The Florida State University, Tallahassee, FL,51 pp. [Available from Florida State University, Department ofMeteorology, 404 Love Building, Tallahassee, FL 32306-4250.], 1971: Tropical east–west circulations during the northern sum-mer. J. Atmos. Sci., 28, 1342–1347.

Leovy, C. B., C.-R. Sun, M. H. Hitchman, E. E. Remsberg, J. M.Russell III, L. L. Gordley, J. C. Gille, and L. V. Lyjak, 1985:Transport of ozone in the middle stratosphere: Evidence for plan-etary wave breaking. J. Atmos. Sci., 42, 230–244.

McIntyre, M. E., and T. N. Palmer, 1983: Breaking planetary wavesin the stratosphere. Nature, 305, 593–600., and , 1984: The ‘‘surf zone’’ in the stratosphere. J. Atmos.Terrest. Phys., 46, 825–849., and , 1985: A note on the general concept of wave breakingfor Rossby and gravity waves. Pure Appl. Geophys., 123, 964–975.

Nakamura, H., 1994: Rotational evolution of potential vorticity as-sociated with a strong blocking flow configuration over Europe.Geophys. Res. Lett., 21, 2003–2006.

Newell, R. E., J. W. Kidson, D. G. Vincent, and G. J. Boer, 1972:The General Circulation of the Tropical Atmosphere and Inter-

actions with Extratropical Latitudes. Vol. 1. The MIT Press, 258pp.

Neyama, Y., 1968: The morphology of the subtropical anticyclone.J. Meteor. Soc. Japan, 46, 431–441.

Norton, W. A., 1994: Breaking Rossby waves in a model stratospherediagnosed by a vortex-following coordinate system and a tech-nique for advecting material contours. J. Atmos. Sci., 51, 654–673.

O’Sullivan, D. J., and M. H. Hitchman, 1992: Inertial instability andRossby wave breaking in a numerical model. J. Atmos. Sci., 49,991–1002.

Paegle, J., J. N. Paegle, and F. P. Lewis, 1983: Large-scale motionsof the Tropics in observations and theory. Pure Appl. Geophys.,121, 947–982.

Polvani, L. M., and R. A. Plumb, 1992: Rossby wave breaking, mi-crobreaking, filamentation, and secondary vortex formation: Thedynamics of a perturbed vortex. J. Atmos. Sci., 49, 462–476.

Ramage, C. S., 1971: Monsoon Meteorology. Academic Press, 296pp.

Sardeshmukh, P. D., and B. J. Hoskins, 1988: The generation of globalrotational flow by steady idealized tropical divergence. J. Atmos.Sci., 45, 1228–1251.

Stewartson, K., 1978: The evolution of the critical layer of a Rossbywave. Geophys. Astrophys. Fluid Dyn., 9, 185–200.

Trenberth, K. E., 1992: Global analyses from ECMWF and atlas of1000 to 10 mb circulation statistics. NCAR Tech. Note NCAR/TN-3731STR., NCAR, 191 pp. [Available from UCAR Publi-cations, P.O. Box 3000, Boulder, CO 80307.]

Trepte, C. R., R. E. Veiga, and M. P. McCormick, 1993: The polewarddispersal of Mount Pinatubo volcanic aerosol. J Geophys. Res.,98, 18 563–18 575.

Van Loon, H., and R. L. Jenne, 1972: The zonal harmonic standingwaves in the Southern Hemisphere. J Geophys. Res., 77, 992–1003.

Warn, T., and H. Warn, 1978: The evolution of a nonlinear criticallevel. Stud. Appl. Math., 59, 37–71.

Waugh, D. W., and R. A. Plumb, 1994: Contour advection with sur-gery: A technique for investigating finescale structure in tracertransport. J. Atmos. Sci., 51, 530–540.

Documents

A Climatology of Rossby Wave Breaking along the ...matt/Hitchman_AMS_Papers/PostelHitchman_1999.pdf(e.g., Juckes and McIntyre 1987; Polvani and Plumb 1992; Waugh and Plumb 1994; Norton