INTERNATIONAL JOURNAL OF CLIMATOLOGY
Int. J. Climatol. 26: 283–301 (2006)
Published online 3 February 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/joc.1255
AN EVENT-BASED JET-STREAM CLIMATOLOGY AND TYPOLOGY
PATRICK KOCH,* HEINI WERNLI and HUW C. DAVIES
Institute for Atmospheric and Climate Science, ETH Zürich, Switzerland
Received 23 December 2004
Revised 30 June 2005
Accepted 13 July 2005
ABSTRACT
A novel climatology is developed for upper-tropospheric jet streams, which is complementary to and an alternative for
the traditional depictions of the time-mean jets. It entails identifying the occurrence of a jet event at a given location and
then compiling the spatial frequency distribution of such events. The resulting climatology, derived using the ERA-15
reanalysis data set of the ECMWF for the period 1979–1993 indicates that (1) in both hemispheres the annual cycle of jet
events takes the form of comparatively smooth transition from a quasi-annular structure in summer to a more spiral-like
structure in winter with a temporally asymmetric return to the summer pattern; (2) the hemispheres differ primarily in
the amplitude of the frequencies and the longitudinal overlap of the spiral portion of the pattern.
In addition, the jet events are subdivided using a two-class typology comprising shallow and deep jets whose vertical
shear (sic. baroclinicity) are/are not confined principally to the upper troposphere. This provides a conceptually simple
and dynamically meaningful classification since deep jets are more likely to spawn tropospheric-spanning cyclones. The
accompanying climatology displays important longitudinal variations and significant inter-hemispheric differences.
A comparison is drawn between these new and conventional climatologies and typologies. Also, comments are proffered
on the relationship between, on the one hand, the patterns of jet frequency including the differing distributions of the
shallow and deep types and, on the other hand, the location of the time-mean jets and the downstream storm tracks.
Copyright  2006 Royal Meteorological Society.
KEY WORDS:
jet stream; jet classification; global climatology; hemispheric contrasts
1. INTRODUCTION
A jet stream in the form of a comparatively narrow, fast-flowing current of air located at the tropopause level
and almost circumnavigating the globe with large-amplitude meanders is one of the atmosphere’s most striking
and ubiquitous flow features. The attribution of the jet’s first detection is a subject of debate (see Kington,
1999; Kraus, 1999; Phillips, 1999), but early observation of the movement of cirrus clouds in England (Ley,
1879) and subsequent pilot balloon measurements of wind speed in Japan (see Lewis, 2003) were certainly
indicative of such airstreams. Jets were often experienced in situ by pilots during World War II, and the
term ‘jet stream’ is probably derived from this period (Seilkopf, 1939). Thereafter, jet streams have come to
form a key feature in routine synoptic analyses and a central ingredient in dynamical theories of large-scale
atmospheric flow.
Their existence is notable on two grounds. From a practical and physical standpoint, their presence influences
air transport operations not only because of the attendant high winds but also because the jet often connotes
a region of clear-air turbulence (Shapiro, 1980). Concomitantly, the high wind speed can transport in situ
pollutants over large distances in short time periods, and the strong lateral and vertical wind shears astride
the jet-maximum, ensuring strong dispersion of an originally localized distribution of a passive scalar. In
addition, a strong jet stream is invariably co-located with a break in the tropopause and also very often with
* Correspondence to: Patrick Koch, Institute for Atmospheric and Climate Science, ETH Hönggerberg, CH-8093 Zürich, Switzerland;
e-mail: [email protected]
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a tropopause fold (see e.g. Shapiro and Keyser, 1990), and these sub-structures are favourable regions for
troposphere–stratosphere exchange of mass and chemical constituents including pollutants.
From a dynamical and theoretical standpoint, the jet stream is of particular significance. First, the jet signifies
the existence of an underlying band of enhanced baroclinicity and, hence, of available potential energy and the
possible seat of cyclonic development. Second, in the extratropics, the along-flow variation of a jet’s strength
and direction, in particular, at jet entrance and exit regions, have been dynamically linked to, and are a seminal
precursor sign for, surface cyclogenesis and anticyclogenesis. In effect, the along-stream variations relate to
upper-level divergence (e.g. Sutcliffe, 1939; 1947), regions of confluence and diffluence (Namias and Clapp,
1949), and attendant transverse ageostrophic circulations across the jet (see e.g. Uccellini, 1990; Keyser, 1999
and references therein). Third, the jet is dynamically linked to, and is an observational surrogate for, the coaligned band of a strong potential vorticity (PV) gradient on the in situ isentropes. There are indications that
a time-mean structure of the band influences the quasi-horizontal propagation of both stationary large-scale
Rossby waves (see e.g. Hoskins and Ambrizzi, 1993, Massacand and Davies, 2001) and its more transient
features exert a profound effect upon and reflects the presence of synoptic-scale waves (Nakamura and Sampe,
2002 Schwierz et al., 2004).
In the extratropics, the origin of the band and the jet is associated with the scale contraction of the ambient
PV gradient during deep baroclinic development (Davies and Rossa, 1998). In the subtropics, it corresponds
to the poleward margin of the upper branch of the Hadley circulation.
40
0
400
38
360
0
30
30
2
8
20
20
40
340
340
320
6
0 30
2
300
32
340
2
300
4
20
20
Pressure [hPa]
360
J1
200
10
380
360
500
3
700
850
30
0
320
280
30
0
(a)
2
0
30
60
90
Latitude [°]
m/s
ID
m/s
60
60
50
50
40
ID
NP
GM
(b)
40
NP
30
GM
30
20
20
10
10
(c)
Figure 1. Mean features for the wintertime (DJF) in the NH from the ERA15 period: (a) latitude-height cross section of zonally averaged
potential vorticity (shaded, in pvu, the 2-pvu line being indicated by a solid bold line), zonal wind flow (dash-dotted lines, contours are
20, 30, 40, 50 and 60 ms−1 ), and potential temperature (solid lines, contours every 4 K). J1 indicates a jet core. Mean horizontal wind
speed in ms−1 on the (b) 200-hPa and (c) 300-hPa surfaces
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JET-STREAM CLIMATOLOGY AND TYPOLOGY
The foregoing considerations serve to underline the case for establishing a climatology of jet streams.
Traditionally, climatological information has been derived from either depictions of the time and longitude
mean of the zonal-velocity displayed in a latitude-height cross section or from charts of the time-mean wind
speed on selected near-tropopause pressure surfaces (see e.g. Blackmon et al., 1977; Lau et al., 1981; Hoskins
et al., 1989). Examples of such depictions, derived here for the mean wintertime (DJF) conditions of the ERA15 period, are shown in Figure 1. The Northern Hemisphere latitude-height section (Figure 1(a)) displays a
single jet of some 40 ms−1 located at approximately 200 hPa on the time-mean tropopause at 30 ° N. The
latitude–longitude sections (Figures 1(b,c)) capture a spiral-like structure that extends from off the coast of
North Africa over central Asia to Japan, extending over the Pacific and central North America, and ending
with a tongue off the eastern seaboard that terminates between Iceland and the United Kingdom. There are
notable amplitude variations along the spiral with maxima over Arabia and the eastern seaboards of both Asia
and North America, and there are significant quantitative differences between the patterns on the 200- and
300-hPa surfaces.
The spatial distributions portrayed in the foregoing time-mean climatologies provide only limited information on the major day-to-day space-time variations in the direction, strength, latitudinal location, and elevation
of jet streams. In effect, the displayed climatologies are by construct biased toward sampling the more quasisteady features and, hence, by default toward the more regular subtropical flow as opposed to transient and
meandering extratropical flow.
Likewise, the typology of jet streams based upon the traditional latitudinally based subdivision of jets into
subtropical and polar-front classes is not unambiguously applicable to the observed spiral pattern.
Here, we propose a novel form of jet-stream climatology that is based explicitly upon detecting the presence
of jet streams, and the resulting climatology lends itself to a simple, objective, two-class typology of the
detected jets. The basis for, and an outline of, the procedures are described in Section 2. The resulting
climatology is computed for both hemispheres, and the seasonal evolution of the patterns is displayed,
described, and discussed in Section 3. The corresponding patterns for the typologies are set out in Section 4.
Thereafter, (Section 5) a comparison of the derived climatology and typology is made with that for other
time-mean and transient-eddy variables and inferences concerning dynamical implications are drawn.
2. INGREDIENTS OF THE ALTERNATIVE CLIMATOLOGY
2.1. The rationale
Figure 2 illustrates, in the same format as Figure 1, the instantaneous distribution of jet streams. Coinspection of the two figures demonstrates a striking contrast between the time mean and the instantaneous
distributions.
The time-mean latitude-height section (Figure 1(a)) captures only a single jet (the so-called subtropical
jet). In contrast, the instantaneous section (Figure 2(a)) reveals a complex of jets (cf Serebreny et al., 1962;
Shapiro et al., 1999) that takes the form of a triple-jet structure comprising a subtropical jet (J1), and two
jets (J2 and J3) located in close proximity in the extratropics. At this particular time instant and longitude,
all three jets were aligned almost zonally, but nevertheless they carried different signatures in terms of their
vertical location, distortion of the tropopause, and the depth of the underlying zone of enhanced baroclinicity.
Likewise, the time-mean charts (Figure 1(b), (c)) bear a qualitative resemblance in the subtropics to their
instantaneous counterparts (Figures 2(b,c)) albeit with the expected smoother distribution, but the displays
carry no vestige of strong transient and finer-scale jet-structures in the extratropics.
Our objective is to derive a jet climatology and typology that reflects the actual day-to-day occurrences
of jet streams and the depth of their baroclinicity while circumventing the drawbacks associated with the
time-mean climatological displays. This requires a procedure that objectively detects and then records the
presence of jets at all latitudes and different tropopause elevations, and contemporaneously distinguishes two
jet classes on the basis of the vertical structure.
Copyright  2006 Royal Meteorological Society
Int. J. Climatol. 26: 283–301 (2006)
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P. KOCH, H. WERNLI AND H. C. DAVIES
400
20
8
30
20
2
20
20
0
34
320
6
J3
300
4
30
20
20
20
20
20
0
30
500
2
30
40
30
3
4020
2
32
0
40
30
2
20
50
J2
2
J1
30
Pressure [hPa]
340
40
0
36
40
20
300
30
40
2
200
340
10
360
40
30
30
0
36
380
380
3
320
700
850
300
28
0
2
30
0
20
0
30
60
90
Latitude [°]
(a)
m/s
ID
m/s
60
60
50
50
40
ID
NP
GM
(b)
40
NP
30
GM
30
20
20
10
10
(c)
Figure 2. Instantaneous features in the NH at 12 UTC, 27 January 1989: (a) latitude-height cross section at 5 ° W (same layout as
Figure 1(a), with J1, J2 and J3 indicating jet cores). Horizontal wind speed (in ms−1 ) on the (b) 200-hPa and (c) 300-hPa surfaces
(solid black line indicates the 30-ms−1 isotach)
2.2. Data set and analysis procedure
2.2.1. Data set. The new climatology is derived using the so-called ERA-15 data set (see e.g. Gibson
et al., 1997) of the European Centre for Medium-Range Weather Forecasts (ECMWF). The data is available
globally for the entire 15-year period between 1979 and 1993 with a temporal resolution of 6 h and a vertical
resolution at tropopause heights of ca 30 hPa. For the purpose of the present study, the pertinent wind data
is interpolated onto a regular 1° × 1° latitude–longitude grid. The data set’s temporal span of 15 years is
adequate for indicating the overall jet-frequency distribution, whereas for examining trends in that frequency
it would be desirable to use a data set spanning a longer time period, like the more recently available ERA-40
set.
2.2.2. Jet-event climatology. This data set is first used to derive a jet climatology based upon the detection
of a jet event’s occurrence. This is achieved in a three-step procedure, and the events are recorded and
accumulated to derive the climatology.
Firstly, the horizontal wind speed is computed at every time instance and every grid point between two
pre-specified pressure levels (p1 and p2 ), and is then averaged vertically between these two levels. This yields
Copyright  2006 Royal Meteorological Society
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JET-STREAM CLIMATOLOGY AND TYPOLOGY
m/s
60
50
ID
ID
40
NP
NP
GM
30
GM
20
10
(a)
(b)
Figure 3. An illustration of the method used to identify jet events. Panel (a) shows the αvel field in ms−1 at 12 UTC, 27 January 1989
(solid black line indicates the 30-ms−1 isotach) and panel (b) the distribution of the jet events after application of a wind speed criterion
of 30 ms−1 . See text for further details
at every instant a horizontal distribution of a scalar variable, αvel, defined by,
p2
1
αvel =
(u2 + v 2 )1/2 dp,
p2 − p1 p1
(1)
where u and v are respectively the zonal and meridional wind components. The two pressure levels are selected
to be 100 and 400 hPa, respectively, so that they span the latitudinal height variations of the tropopause and
the accompanying sub- and extratropical jets (see e.g. Figure 2(a), and also Palmén and Newton, 1969,
Figure 4.7). A similar vertical averaging procedure has been introduced (e.g. Massacand et al., 1998) to
simultaneously sample the PV variations associated with sub- and extratropical jets. An example of the
resulting αvel field is shown in Figure 3(a). A comparison with the distribution of the analysed wind speed
at the 200- and 300-hPa fields for the same time instant (Figures 2(b,c)) indicates that the αvel field has
the same overall pattern and amplitude as the single-level fields while retaining the finer-scale instantaneous
features in the extratropics.
Secondly, a minimum threshold criterion is set for the value of αvel, and each grid point meeting this
criterion is then identified as being in the vicinity of a jet-stream ‘event’. Thus every 6 h, an ‘event’ pattern is
available in the form of a disjoint of meandering structures. The selection of the threshold value is based upon
the requirement that the derived pattern adequately captures the spatial structure of the jet streams evident
in the instantaneous 200- and 300-hPa fields while concomitantly excluding localized large-flow values not
associated specifically with a spatially coherent jet stream. Systematic studies (Koch, 2004) conducted to
examine the sensitivity to the choice of αvel (see e.g. Figure 3(a)) indicate that a threshold criterion of
30 ms−1 is appropriate, and this value is adopted hereafter. An example of the resulting pattern is shown in
Figure 3(b), and a comparison of Figures 2 and 3 lends credence to the choice.
Finally, the temporal averaging of the jet-event patterns over a specified time period establishes the spatial
distribution of the mean frequency for that period. Such a climatology is computed for each month and season
of the ERA-15 time period.
2.2.3. Jet typology. The jet-event data set derived with the fore-mentioned procedure is then used for an
additional climatology based upon a two-class typology. Jet events are subdivided into two categories on the
basis on a characteristic of the jet’s vertical shear as measured by the following index
vrel =
(v200 − v500 )
,
v200
Copyright  2006 Royal Meteorological Society
(2)
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P. KOCH, H. WERNLI AND H. C. DAVIES
where v200 and v500 are the horizontal wind speeds at the 200- and 500-hPa levels respectively. In effect, the
index is a measure of the upper-tropospheric wind shear (sic. baroclinicity) normalized by the wind speed at
200 hPa.
The adoption of this index is based on the following rationale. A jet at the tropopause level is indicative of
an underlying zone of enhanced baroclinicity, and theoretical considerations suggest that this baroclinicity can
more readily spawn a tropospheric-spanning cyclone, provided it is significant in both the upper and lower
troposphere. (Conversely, a strong jet without an accompanying low-level zone of baroclinicity is not expected
to parturate a cyclone.) In effect, the proposed index provides an inverse measure of the jet’s ‘cyclogenetic
potential’.
To calibrate the index, note that a jet
• with predominantly upper-tropospheric baroclinicity would render vrel → 1;
• accompanied by predominantly low-level baroclinicity would render vrel → 0;
• of uniform shear in pressure and of the form,
v = −A(p − 1000)/1000,
(3)
where p is the pressure in hectopascal, would set vrel ≈ 0.4.
In the light of the foregoing and the pragmatic factors set out in the next subsection, a two-class jet typology
is established by stipulating a subdivision criterion of
vrel = 0.4.
(4)
The original jet-event data set is then subdivided into comparatively shallow (vrel > 0.4) jets confined
to the upper troposphere and deeper (vrel < 0.4) jets. Hereafter, we refer to these two shallow and deep
classes as respectively SL and DL jet types.
2.3. Comments and Critique
The procedure outlined above is an avowedly synoptic-statistical approach to the climatology of a jet
stream. It delivers the latitude–longitude frequency distribution of both its occurrence and that of two of its
sub-components (shallow and deep jet types). Here we comment on the overall concept and the procedure’s
limitations and possible shortcomings.
In concept, it differs intrinsically from global circulation climatologies that document the latitude–longitude
structure of a flow variable’s time-mean and its standard deviation. For example, the time-mean flow structure
of the jet signifies the average state through which quasi-steady planetary-scale Rossby waves propagate and
within which baroclinic eddies grow and decay, and that the variance is a measure of the strength and location
of the attendant storm tracks. In contrast, the present jet-event climatology does not entail a split into mean
and transient features, but is rather a statistical record of the transient existence of the meandering jet streams
themselves. It follows that the event-based nature of the present climatology carries implications for the
interpretation of the results.
Two possible shortcomings in the appropriateness of recording a jet’s occurrence relate respectively to the
lateral extent and along-stream coherency of a jet stream. With regard to the lateral extent, a broad jet extending
over several hundred kilometres could be recorded contemporaneously at several grid points located astride
the jet. In effect, the record signifies the occurrence of an event in the vicinity. With regard to the along-stream
coherency, a highly localized high wind speed associated with, e.g. a jet streak or large-amplitude orographic
wave could be recorded as a jet event even in the absence of a coherent and elongated jet stream. In mitigation,
we note that a jet streak usually denotes the presence of a sub-synoptic PV anomaly in the vicinity. If the
anomaly is adjacent to the jet, it increases the jet’s maximum wind and decreases its lateral extent, whereas if
the anomaly is located away from the jet, its lateral scale and depth imply that it will usually not register an
αvel = 30 ms−1 . Note that in harmony with the above remarks, the jet-stream pattern displayed in Figure 3
Copyright  2006 Royal Meteorological Society
Int. J. Climatol. 26: 283–301 (2006)
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JET-STREAM CLIMATOLOGY AND TYPOLOGY
suggests that the empirically adopted threshold criterion of αvel = 30 ms−1 tends to largely circumvent these
two possible shortcomings. In particular, the figure shows that the jet event corresponds to comparatively
elongated and narrow jet streams, with no evidence of highly localized jet streaks.
The other major empirically based element in the analysis procedure is the selection of the vrel = 0.4
criterion to distinguish between the two jet types. The pragmatic justification for the choice is based on the
following considerations. Comparison of the winter climatologies obtained with vrel values ranging from 0.1
to 0.9 (not shown here, but available in Koch, 2004 and at http://iacweb.ethz.ch/staff/pkoch) shows that (1) the
population of the two types is highly skewed for vrel < 0.3 and vrel > 0.7, but (2) the derived SL and DL
frequency patterns of both hemispheres retain the same spatial structure although differing quantitatively for
vrel in the range [0.3–0.5].
3. JET-EVENT CLIMATOLOGY
3.1. Seasonal distributions
Figures 4 and 5 display respectively the seasonal mean frequency distributions of jet events for the Northern
and Southern Hemispheres. Note that for the seasonal time period, the frequency percentage at a given location
equates approximately to the seasonal average of the number of days a jet is present in the vicinity.
DJF
180°
%
MAM
180°
%
90
120°W
20°N
120°E
75
40°N
60°N
90
120°W
20°N
75
40°N
60°N
60
ID
120°E
NP
60
ID
NP
45
GM
45
GM
30
60°W
30
60°W
60°E
60°E
15
15
0°
0°
JJA
SON
%
180°
%
180°
90
120°W
20°N
120°E
75
40°N
60°N
90
120°W
75
60°N
NP
45
GM
30
60°E
45
GM
30
60°W
60°E
15
0°
60
ID
NP
60°W
120°E
40°N
60
ID
20°N
15
0°
Figure 4. Seasonal mean distribution of the jet events in percentage in the NH for the whole ERA15 period. Seasons are defined as
winter (DJF), spring (MAM), summer (JJA), and autumn (SON)
Copyright  2006 Royal Meteorological Society
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P. KOCH, H. WERNLI AND H. C. DAVIES
DJF
MAM
%
0°
%
0°
GM
GM
90
60°W
20°S
60°E
75
40°S
60°S
90
60°W
20°S
60°E
75
40°S
60°S
60
60
SP
SP
45
45
30
120°W
30
120°W
120°E
120°E
15
15
ID
ID
180°
JJA
180°
SON
%
0°
%
0°
GM
GM
90
90
60°W
20°S
60°E
40°S
60°S
75
60°W
20°S
60°E
40°S
60°S
60
SP
60
SP
45
45
30
120°W
75
120°E
30
120°W
120°E
15
15
ID
ID
180°
180°
Figure 5. Seasonal mean distribution of the jet events in percentage in the SH for the whole ERA15 period. Seasons are defined as
summer (DJF), autumn (MAM), winter (JJA), and spring (SON)
3.1.1. Northern Hemisphere. In the Northern Hemisphere (NH), the overall pattern has a single band
spiral-like structure that is strongest in winter and collapses into a weak almost-annular ring in summer.
In the winter season (DJF), the band starts south of the Canary Islands and ends one circumnavigation
later over Brittany. Along the band, there are zones of distinctively higher frequency (>75%) extending from
the central Sahara to the mid-Pacific and from the eastern United States to the western Atlantic. Peak values
(>90%) occur in a zone from eastern Asia to the mid-Pacific.
In spring (MAM), the spiral is more fragmented with zonally contracted zones of high frequency maxima
over North Africa (>60%), Japan (>75%) and the southern United States (>45%) separated by pronounced
breaks over central Asia and the western seaboard of North America. In effect, the pattern is almost separated
into two spiral arms. In summer (JJA), a more annular structure with a break over Europe is located distinctly
poleward of the distribution in other seasons, and the frequency values are substantially weaker (<45%).
In autumn (SON), the annular pattern is evident, but now with an arm extending southwestward from the
Middle East to North Africa. In comparison with spring, the band is reduced in amplitude over North Africa,
but there is a zone of high frequencies (>60%) extending from eastern Asia to the mid-Pacific and another
centred over Nova Scotia.
3.1.2. Southern Hemisphere. In the Southern Hemisphere (SH), the overall pattern has a more concentric
form that persists all year with an embedded, seasonally varying, finer-scale interior structure. The latter can
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Int. J. Climatol. 26: 283–301 (2006)
JET-STREAM CLIMATOLOGY AND TYPOLOGY
291
be viewed either as an annular ring with reduced frequency, centred at the longitude of New Zealand or
as two segments – a zonally aligned subtropical band west of South America that transits downstream to a
tighter extratropical spiral.
In winter (JJA), the zonal band extends from Madagascar over Australia and out to the central South Pacific
(with frequency values attaining values of over 90%), and the spiral segment extends from South America
poleward of South Africa and New Zealand.
In spring (SON), the pattern retains a similar structure but with significantly lower frequency values. Also,
the second segment now extends to circumscribe the globe. It thereby serves to emphasize the reduced values
in the New Zealand sector or equivalently, the double-banded structure extending from over the Tasman Sea
to the central Pacific. In summer (DJF), the subtropical band is absent, and the second segment has migrated
poleward to form an annular structure centred at ∼40 ° S with a band of high frequency (60%) extending from
the mid-Atlantic to southwest of Tasmania. In autumn (MAM), both the subtropical band and the spiral are
present albeit with reduced frequency compared with the spring values.
3.1.3. Further remarks. Particularly noteworthy features of the seasonal climatology patterns include (1) the
comparatively limited longitudinal overlap of the spiral-like frequency distribution in the boreal winter
compared to the more extended overlap of its austral counterpart, (2) the comparatively low values of the
frequency of the annular structure in the boreal summer, compared to much higher values for its austral
counterpart, (3) the presence of numerous comparatively narrow elongated bands of very high frequency in
both hemispheres during the non-summer seasons (e.g. the regions over Asia and out to the Pacific in the NH
and over Australia and out to the mid-Pacific in the SH), (4) the co-existence in the Australia-to-mid-Pacific
sector of two zonally aligned bands of frequency maxima bands indicative of the possible co-occurrence of
bands of enhanced baroclinicity, and (5) the hint of a temporal asymmetry of the evolution in both hemispheres
as captured in the differing patterns for the equinoctial seasons, with a comparatively higher frequency and
a more pronounced signature of the equatorward portion of the spiral in spring.
3.2. Annual cycle
It was noted above that the temporal evolution is not merely a symmetric meridional migration of the
patterns plus an annual amplitude cycle. Further insight on the cycle can be gleaned from the single-month
climatologies (not shown here but available in Koch, 2004 and at http://iacweb.ethz.ch/staff/pkoch).
The month-to-month evolution in both hemispheres is comparatively smooth, rendering the fore-displayed
seasonal distributions reasonably representative. Here, we provide a cursory description of the key features
of the cycle in the two hemispheres.
In the NH, the pattern is narrowest in width and weakest in frequency during July when it forms an annular
ring with a break over Europe. During August, it retains its shape but with a slight increase in frequency,
and in September there is a distinctly larger increase with maxima centred over the eastern seaboards of
Asia and North America. In the October to December period, the spiral-like structure emerges with the
westward extension of the Asian band over North Africa and out into the Atlantic and the contemporaneous
poleward displacement of the eastern ends of the pre-existing bands of high frequency over the Pacific and
Atlantic. In January and February, the spiral-like structure is at its widest, with large frequency maxima over
the eastern US seaboard, Japan and the Red Sea. Thereafter, from March to April the frequency decreases
relatively uniformly with an accompanying contraction of the spiral band and a subsequent recovery of the
mid-summer annular pattern.
In the SH, the simple annular pattern of summer with the higher frequency values in the eastern hemisphere
is most evident during January and February. From March to May, it accretes an arm that extends westward
with time from the mid-Pacific to north New Zealand (in March) to the central Indian Ocean (in May). By
June, a spiral-like structure emerges as the annular region is reduced in width and frequency in the midPacific, and an accompanying shift in the point of accretion appears over South America. From July through
to September, the pattern retains its spiral-like structure with a distinct frequency minimum from the midPacific to Australia. Thereafter, from October to December the annular structure is gradually recovered via an
in situ reduction in the frequency values of the arm in November and its virtual disappearance in December.
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P. KOCH, H. WERNLI AND H. C. DAVIES
Previous studies have often stressed the difference in the jet characteristics of the two hemispheres. It has
been repeatedly noted that, in SH winter, two jets are present at the time, and longitudinally averaged cross
sections are allied to a double-jet structure over much of the Pacific and Australia in time-mean charts. It has
been adduced that these differences relate to the marked topographic and orographic differences between the
two hemispheres.
In contrast, we note here that there is a marked similarity in the temporal evolution of the annual cycle in
the two hemispheres. In particular, the annular structure in the summer frequency pattern is slowly disrupted
in autumn by the accretion of an arm that results in a more spiral-like pattern by mid-winter, and in turn the
annular pattern is recovered in late spring with the gradual disappearance of that arm. From this standpoint,
the notable inter-hemispheric difference is the longitudinal extent of the arm.
4. JET-EVENT TYPOLOGY
Figures 6 and 7 display respectively the seasonal mean frequency distributions of shallow (SL) and deep
(DL) jet events for the Northern Hemisphere. Again the frequency percentage at a given location equates
approximately to the seasonal average of the number of days that such a jet type is present in the vicinity.
Figures 8 and 9 provide the corresponding distributions for the Southern Hemisphere.
DJF
180°
MAM
%
180°
%
90
120°W
20°N
120°E
75
40°N
60°N
90
120°W
20°N
75
40°N
60°N
60
ID
120°E
60
ID
NP
NP
45
GM
45
GM
30
60°W
30
60°W
60°E
60°E
15
15
0°
0°
JJA
SON
%
180°
%
180°
90
90
120°W
20°N
120°E
75
40°N
60°N
120°W
120°E
75
40°N
60°N
60
ID
20°N
NP
45
GM
60°E
60°W
60°E
30
15
15
0°
45
GM
30
60°W
60
ID
NP
0°
Figure 6. Seasonal mean distribution of the jet events of the SL category (in percentage) for the NH. Seasons are defined as in Figure 4
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JET-STREAM CLIMATOLOGY AND TYPOLOGY
DJF
MAM
%
180°
180°
%
90
120°W
20°N
120°E
40°N
60°N
75
90
120°W
120°E
40°N
60°N
60
ID
20°N
NP
75
60
ID
NP
45
GM
45
GM
30
60°E
60°W
30
60°E
60°W
15
15
0°
JJA
0°
SON
%
180°
%
180°
90
120°W
20°N
120°E
90
120°W
75
40°N
60°N
120°E
75
40°N
60°N
60
ID
20°N
60
ID
NP
NP
45
GM
45
GM
30
60°E
60°W
30
60°E
60°W
15
0°
15
0°
Figure 7. Seasonal mean distribution of the jet events of the DL category (in percentage) for the NH. Seasons are defined as in Figure 4
4.1. Northern Hemisphere
A comparison of Figures 6 and 7 indicates that SL events are significantly more frequent than their DL
counterparts and generally occupy a more equatorward location. The SL events are most frequent over, and
are predominantly confined to, the continental land masses and their eastward margin, whereas DL events
predominate over the oceans downstream of the SL events.
The continental bands of the SL patterns are strongest and broadest in winter, with maxima in excess of
75, 90 and 45% located respectively over the Red Sea, eastern Asia, and the southeastern United States. They
are weakest and thinnest in JJA with the maxima reduced to 30, 45, and 30% respectively. In winter and
spring, the axes of the bands are located near 30 ° N, in autumn they are nearer to 35 ° N, and in summer they
migrate to around 40 ° N over Asia and 45 ° N over North America.
Likewise, the oceanic DL bands are strongest and broadest in winter with maxima in excess of 45%
located respectively over the western Pacific and the western Atlantic. They are weakest in summer, but
the maxima still amount to over 30%. In comparison with the SL bands, the axes of the DL bands show
comparatively little seasonal variation. The axes are located between 40° and 60 ° N in all seasons, except for
a double band structure in winter on the western Pacific seaboard whose equatorward component is centred
at around 25 ° N.
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P. KOCH, H. WERNLI AND H. C. DAVIES
DJF
0°
%
%
0°
MAM
GM
GM
90
60°W
20°S
60°E
75
40°S
60°S
90
60°W
20°S
60°E
75
40°S
60°S
60
SP
60
SP
45
45
30
120°W
30
120°W
120°E
120°E
15
15
ID
ID
180°
180°
JJA
SON
%
0°
%
0°
GM
GM
90
60°W
20°S
60°E
75
40°S
60°S
90
60°W
20°S
60°E
75
40°S
60°S
60
SP
60
SP
45
45
30
120°W
30
120°W
120°E
120°E
15
15
ID
ID
180°
180°
Figure 8. Seasonal mean distribution of the jet events of the SL category (in percentage) for the SH. Seasons are defined as in Figure 5
4.2. Southern Hemisphere
In the SH, the frequency of SL (Figure 8) and DL (Figure 9) events possesses a quasi-annular structure,
and both annulae exhibit comparatively little latitudinal variation with season except for the DL events over
the South Pacific. Both SL and DL events favour the sector between 60° and 190 ° E that extends from the
central western Pacific to the central eastern Pacific. It is also noteworthy that the maximum in the DL pattern
is located upstream (westward) of the maximum in the SL pattern in the non-summer seasons. In the austral
winter (JJA), the frequency peaks of SL and DL events are comparable (60–75%), whereas in spring and
autumn the two austral maxima of SL events (<60% and 45%) are less than the DL values (∼60%).
The quasi-annular SL frequency distribution is located between 20° and 35 ° S. It is strongest in winter (JJA)
with a longitudinally extended maximum in excess of 60%. It is weakest in summer (DJF) when it breaks
up into two, with gaps centred in the southern and eastern Pacific Ocean, each portion with a frequency of
less than 30%. The DL annulus is located between 40° and 60 ° S, but in winter it acquires an equatorward
arm that extends outward from the central eastern Pacific over central Australia and on into the mid western
Pacific. It has comparable peak frequencies (75%) in all four seasons.
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JET-STREAM CLIMATOLOGY AND TYPOLOGY
DJF
MAM
%
0°
0°
GM
%
GM
90
60°W
20°S
60°E
90
60°W
75
40°S
20°S
60°E
40°S
60°S
75
60°S
60
60
SP
SP
45
45
30
120°E
120°W
30
120°W
120°E
15
JJA
15
ID
ID
180°
180°
SON
%
0°
%
0°
GM
GM
90
90
60°W
20°S
60°E
60°W
75
40°S
60°S
20°S
60°E
75
40°S
60°S
60
60
SP
SP
45
45
30
30
120°W
120°W
120°E
120°E
15
15
ID
ID
180°
180°
Figure 9. Seasonal mean distribution of the jet events of the DL category (in percentage) for the SH. Seasons are defined as in Figure 5
4.3. Inter-hemispheric differences
The characteristics described above identify notable spatial and temporal differences between the SL and
DL climatologies of the two hemispheres, and these include (1) both the SL and DL types of the Southern
Hemisphere possess a more annular appearance than their NH counterparts; (2) the SL and DL types occupy
an overlapping longitudinal band in the SH with the latter type being displaced somewhat upstream of
the former, whereas there is less of a longitudinal overlap in the Northern Hemisphere with the DL type
being displaced downstream of the SL type; (3) in winter, the SL and DL types possess similar (different)
amplitudes in the austral (boreal) hemispheres, and these values are respectively less than and greater than
their NH counterparts; (4) in spring and autumn the two austral maxima of SL events (<60 and 45%) are
less than the DL values (∼60%), and this is the reverse of their northern counterparts; and (5) the DL pattern
has comparable peak frequencies (75%) in all four seasons in the SH in comparison with the 50% reduction
between winter and summer of the counterpart NH peaks.
These are significant inter-hemispheric dynamical differences, and their origin and relationship to the
different topographic distributions of the two hemispheres merits attention.
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P. KOCH, H. WERNLI AND H. C. DAVIES
5. RELATIONSHIP TO OTHER CLIMATOLOGIES
Standard compilations of the free atmosphere’s mean and transient wind field climatologies tend to focus
primarily on the more baroclinically active winter season. For the most part, we also emphasize this in this
section.
5.1. Time-mean wind and PV fields
First we compare the seasonal time-mean distributions of the wind speed at 200 hPa for both hemispheres,
derived for the ERA 15 data set (see Figures 11 and 12) with the analogue figures for the jet-stream frequency
(see Figures 4 and 5). These two climatological parameters are radically different in construct – one a timemean field that can more readily pinpoint the presence and location of quasi-steady jets, and the other a purely
event-based field that does not discriminate between steady and transient features. Nevertheless, there is a
similarity in the overall patterns of the two climatologies.
This resemblance is favoured under conditions where (1) the jet is quasi-steady and undergoes only weak
intra-seasonal oscillations in its latitudinal location; (2) the jet is highly transient but undergoes only small
amplitude wave-like meanders about a mean path; and (3) the jet is highly transient and undergoes largeamplitude meanders (including possibly reversal in direction), but there is a proportional relationship between
the jet’s frequency of occurrence at a location and its time-mean wind strength at that same location. The
quasi-steady criterion (setting 1) is often appropriate in the subtropics, whereas the weak meander assumption
(setting 2) does not apply to breaking waves in the upper troposphere. The proportionality relationship (setting
3) is a plausible, but only a posteriori verifiable assertion. It is plausible because on the one hand the timeaverage wind speed value is related to the number of jet events and, on the other hand, the frequency value
itself is strength-related via the application of the threshold criterion.
For the winter season, the patterns are strikingly similar. In the subtropics, it presumably reflects the quasisteady character of the flow. In the extratropics, it suggests linkage between the jet frequency and the mean
wind speed. Indeed, the poleward portion of the NH contours of the frequency values () and the wind speed
(V ) show a reasonable, if not remarkable, geographical correspondence that matches the linear proportionality
relationship
≈ 1.5 · V
(5)
for a wide range of values of both parameters. For example, south of Iceland, the mean wind is ∼20 ms−1 ,
and the frequency indicates that a jet is present in the vicinity on average about once every 3 days.
It was noted in the ‘Introduction’ that the jet stream can serve as an observational surrogate for the location
of a strong PV gradient on tropopause-intersecting isentropes. Indeed, the January time-mean tropopause-level
isentropic PV gradient field in the NH (Massacand and Davies, 2001) features two zonally elongated but
longitudinally separated bands centred respectively over Japan and Nova Scotia, and both extend downstream
into mid-ocean. Thus, these bands correspond to the regions of highest time-mean wind speed, baroclinicity
and available potential energy, and moreover bear a close correspondence to the jet-event climatology.
5.2. Subtropical and polar-front jets
It has been customary to classify atmospheric jets into two broad categories – the subtropical jet (STJ) and
mid-latitude polar-front jet (PFJ) (see e.g. Krishnamurti, 1961; Palmén and Newton, 1969; Bluestein, 1993;
Nakamura and Shimpo, 2004). A third appellation – the Arctic polar front – has also been recorded (Shapiro
et al., 1987) to identify the less frequent occurrence of jets on the lower-level Arctic tropopause, but these
systems tend to be less coherent.
The characteristics attributed to the STJ and PFJ are listed in Bluestein (1993). The STJ is regarded as a
quasi-steady and spatially continuous wintertime phenomenon located at ∼200 hPa at the poleward extremity
of the Hadley cell (i.e. in the ca 25–35° latitudinal belt). In contrast, the PFJ is assumed to be a highly transient
all year–round phenomenon at ∼250–300 hPa and located above the extratropical frontal zone of enhanced
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baroclinicity. A sketch of the relative locations of the STJ and PFJ in the Northern Hemisphere winter was
given by Riehl (1962). It shows a distinct geographical separation between two spatially continuous wave
number–three jet patterns of differing phase such that they are in closer proximity over Japan, the eastern
seaboard of the United States, and eastern Europe.
With reference to the two-class typology introduced in this study, the baroclinicity of the STJ is by default
expected to be confined principally to the upper troposphere, whereas the PFJ is expected to overlay a deep
baroclinic zone.
A cursory inspection of Figures 8–11 indicates that the STJ and PFJ categorization bears comparison with
the present SL and DL typology. However, the present typology does not a priori invoke a latitudinal and
temporal subdivision, and thereby permits the categorization of jets that do not readily match the STJ/PFJ
split. An example of the latter is the Northern Hemisphere wintertime spiral that crosses from the sub- to the
extratropics.
An illustration of the relative spatial distribution of the SL and DL events is provided in Figure 12, and this
constitutes an analogue of Riehl’s earlier figure. The geographical distribution of the 20% frequency domains
of the SL and DL types shows a significant spatial overlap, with the DL jets located partly within and partly
poleward of the SL band in both hemispheres.
180°
DJF
m/s
180°
MAM
m/s
60
120°W
20°N
120°E
60
120°W
50
40°N
60°N
20°N
50
40°N
60°N
40
ID
120°E
40
ID
NP
NP
30
GM
30
GM
20
60°E
60°W
20
60°E
60°W
10
10
0°
0°
180°
JJA
m/s
180°
SON
m/s
60
60
120°W
20°N
120°E
120°W
50
40°N
60°N
120°E
50
40°N
60°N
40
ID
20°N
40
ID
NP
NP
30
GM
30
GM
20
60°E
60°W
20
60°E
60°W
10
0°
10
0°
Figure 10. Mean seasonal horizontal wind speed (in ms−1 ) at 200 hPa for the ERA15 period in the NH
Copyright  2006 Royal Meteorological Society
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P. KOCH, H. WERNLI AND H. C. DAVIES
DJF
m/s
0°
MAM
0°
GM
m/s
GM
60
20°S
60°E
60°W
50
40°S
60
60°W
20°S
60°E
50
40°S
60°S
60°S
40
40
SP
SP
30
30
20
120°W
20
120°W
120°E
120°E
10
JJA
10
ID
ID
180°
180°
m/s
0°
SON
0°
GM
m/s
GM
60
60°W
20°S
60°E
50
40°S
60°S
60
60°W
20°S
60°E
50
40°S
60°S
40
40
SP
SP
30
30
20
120°W
20
120°W
120°E
120°E
10
10
ID
ID
180°
180°
Figure 11. Mean seasonal horizontal wind speed (in ms−1 ) at 200 hPa for the ERA15 period in the SH
5.3. Storm tracks
Standard storm track climatologies for the NH winter season, whether of the band-pass meridional wind or
a geopotential variance in mid-troposphere (Blackmon et al., 1977; Lau et al., 1981; Hoskins et al., 1989) or
the PV variance at tropopause elevations (Massacand and Davies, 2001), show two zonally separated tracks.
One begins downstream from the peak in the mean wind speed/PV gradient off Japan and extends up to North
America. The other is located downstream from the second peak in wind speed/PV gradient over Nova Scotia
and extends toward Iceland and northern Europe. It has been argued that the available energy associated with
the time-mean structure is released downstream in the form of baroclinic eddies. This assertion’s generality
is limited by the lack of such eddies downstream of the pronounced mean wind-speed maxima over North
Africa in the NH and also south of Australia in the SH. However, it is consistent with the SL nature of the
bands in the latter two regions.
Inter-comparison of the NH climatologies of the time mean winds and storm tracks with the corresponding
climatology of jet events shows that the latter possess high frequency values upstream of and over the westward
portion of both major storm tracks with a marked reduction toward the downstream end. Again consistent
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Int. J. Climatol. 26: 283–301 (2006)
JET-STREAM CLIMATOLOGY AND TYPOLOGY
299
GM
ID
SP
NP
GM
(a)
(b)
ID
Figure 12. Geographical winter distribution of the 20% frequency patterns of jet events of the SL (grey shaded) and DL (hatched)
categories for (a) the Northern and (b) the Southern Hemispheres
with the SL/DL typology, these domains of reduced frequency are regions characterized predominantly by
DL events.
The foregoing comparison raises two issues: (1) the nature of the distinction between high frequency bands
that are/are not associated with a storm track and (2) the cause for the reduction of the jet-frequency values
toward the end of the storm tracks. We consider these issues in turn.
In the NH, the almost ubiquitous bands centred over the eastern seaboard of Asia and North America are
located above regions of strong surface temperature gradients and downstream of respectively the Himalayas
and the Rockies. In contrast, the North African band, which does not spawn a storm track, does not have a
strong underlying thermal contrast (i.e. an SL domain). Also, it is present only in the non-summer months
and is located at the poleward extremity of the local manifestation of a Hadley circulation. In the Southern
Hemisphere, there is again a storm-track-spawning band located downstream of the Andes.
An inference arising from the existence of these two disparate jet types relates to the appropriateness of
standard theoretical and numerical modelling studies of baroclinic instability. In such studies, the basic state
is often taken to be the time-mean zonal flow field (cf Figure 1(a)) without accounting for the foregoing
distinction.
The reduction of the jet occurrences toward the end of the storm tracks could in principle be attributable to
either less frequent or spatially more dispersed jet events at a given longitude, or to a narrowing/weakening
of the jets within the storm track. The latter is more consistent with synoptic assessment and theoretical
considerations. A narrowing is consistent with PV-frontogenesis within the baroclinic wave development, and
weakening would relate to the dissolution or mixing of the enhanced zone of baroclinicity beneath the jet in
the mature phase of cyclonic development.
6. FINAL REMARKS
In this study, a novel climatology has been presented for upper-tropospheric jet streams, which is complementary to, and an alternative for, the traditional depictions of the time-mean jets. It involves identifying
and objectively determining the occurrence of a jet event and compiling the spatial frequency distribution of
such events. The resulting climatology provides direct information on the spatial distribution, annual cycle,
and the inter-hemispheric differences of jet occurrences. It is shown that the geographical distribution resembles that of the time-mean wind speed, that the annual cycle produces dissimilar equinoctial distributions
in both hemispheres, but that the hemispheres differ in the spatial structure and amplitude of the frequency
distribution.
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P. KOCH, H. WERNLI AND H. C. DAVIES
In addition, the data set for jet events is subdivided into a two-class typology that constitutes an alternative to
the customary split into SFJ and PFJ. The associated climatology possesses major inter-hemispheric differences
in the spatial distribution, frequency, and seasonal cycle of the two jet types.
Also the new climatologies, considered in tandem with the standard climatologies for the mean and variance
of the upper-tropospheric fields, shed light on the relationship between the maxima in patterns of jet frequency,
the location time-mean jets, and the downstream storm tracks.
The database that forms the kernel of this method – a register of the space-time occurrence of jets – can
be utilized directly or refined in various ways to derive further climatologies. For example, it can be used
to identify and categorize the co-occurrence of double jets at a given longitude and, thereby, help in the
evaluation of the temporal development and breakdown of such structures.
ACKNOWLEDGMENT
It is a pleasure to thank MeteoSwiss for granting access to the ERA15 data set.
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