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The Role of Large-Scale Dynamic and

Diabatic Processes in the Generation of

Cut-off Lows over Northwest Africa

PETER KNIPPERTZ and JONATHAN E. MARTIN

Department of Atmospheric & Oceanic Sciences, University of Wisconsin–Madison,

Madison, Wisconsin, USA

Submitted to

Meteorology and Atmospheric Physics

With 13 figures

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Summary

The present observational study addresses the role of large-scale dynamic and diabatic

processes leading to the generation of the deep upper-level troughs/cut-offs, characteristically

involved in cases of extreme precipitation over West Africa during the cool season. The

elongated potential vorticity (PV) streamers associated with the observed troughs form as a

result of an equatorward transport of high-PV air downstream of a large ridge over the central

North Atlantic. Negative PV advection in the ridge leads to a thinning of the PV streamer. If

another PV ridge is present downstream of the streamer, this process produces a long-lived,

stationary PV cut-off near West Africa that eventually gets re-absorbed by the midlatitude PV

reservoir. In situations with a prior anticyclonic wave-breaking event over Europe, high PV

near the Iberian Peninsula impedes a complete cut-off of the streamer.

The rapid amplification of the involved PV ridges is achieved through a combination of

negative horizontal PV advection and diabatic reduction of upper-level PV through latent

heating, often in connection with explosive baroclinic developments. A strong upper-level jet

and the feeding of warm, moist air into the cyclo- and frontogenetic region by a low-level jet

ahead of the surface cold front (often called a warm conveyor belt; WCB) promote the rapid

deepening and the intense latent heating. Diabatic PV tendencies are highest where the WCB

rises over the surface warm front to the northeast or east of the cyclone centre. The

acceleration of the participating extratropical jets is fuelled by momentum fluxes and

convective outflow from the (sub-) Tropics. Implications of the described mechanism for the

general circulation over the North Atlantic are discussed.

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1. Introduction

In a recent paper Knippertz and Martin (2005; KM05 hereafter) analyzed extreme

precipitation events affecting West Africa during the cool season. During their case I (09–11

January 2002) extraordinary dry season precipitation fell in the northern West African

Tropics (Senegal, Mauritania, Mali). During case II (30 March–01 April 2002) heavy rainfall

was observed at the arid southern side of the Atlas chain in northwestern Africa. Case II has

also been investigated with respect to its hydrological impacts (Fink and Knippertz, 2003)

and the dynamics of the tropical plume (TP) and the strong subtropical jet (STJ) streak

accompanying this event (Knippertz, 2005). Both cases are characterized by the succession of

two upper-level cyclonic disturbances penetrating to low latitudes to the west of the

precipitation zones within 3–5 days (KM05). As we will show below with the help of maps

of potential vorticity (PV) on isentropic surfaces (Hoskins et al., 1985), the respective first

disturbances take the form of distinct and long-lived cut-offs from the midlatitudes. They do

not produce significant precipitation, but initiate a poleward transport of tropical moisture at

low- to midlevels, thereby creating a potentially unstable air mass (KM05). The respective

second disturbances then generate the ascent required to release this instability.

In the present study we investigate the large-scale dynamics and diabatic factors that lead to

the equatorward penetration and cutting-off of these extratropical upper-level disturbances.

We will show that this process involves rapid baroclinic developments over the western

North Atlantic several days before the formation of the deep troughs near Africa. Explosively

deepening surface cyclones (termed “bombs”) occur frequently in this region during

wintertime and usually form downstream of a mobile trough and within or poleward of the

maximum westerlies (Sanders and Gyakum, 1980; Roebber, 1984). The deepening is usually

accompanied by strong latent heat release, usually producing a divergent “outflow jet” to the

northeast of the storm centre and an amplification of the downstream ridge (e.g. Stoelinga,

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1996). From a PV perspective this amplification can be viewed as a combination of advection

of low-PV air through the jet flow ahead of the trough and diabatic reduction of PV at the

tropopause through latent heating (Hoskins et al., 1985, chapter 7; Stoelinga, 1996;

Massacand et al., 2001; Posselt and Martin, 2004).

Massacand et al. (2001) have shown that the latent heating within a pronounced frontogenetic

band can affect the equatorward transport of stratospheric PV to the east of the amplified

downstream ridge and is therefore crucial for the formation of a PV streamer in this region.

The modification of upper-level structures by low PV originating in the outflow of tropical

cyclones during their transition to extratropical systems is a similar effect (Jones et al., 2003).

Thorncroft and Jones (2000) suggest that the low-PV outflow might promote downstream

anticyclonic wave breaking (AWB), which is often associated with the equatorward

penetration and cutting-off of a positive PV anomaly (Thorncroft et al., 1993). Often the

AWB also produces a cut-off negative PV anomaly and an associated blocking event at

midlatitudes (Pelly and Hoskins, 2003).

The aim of the present paper is to demonstrate the importance of upstream explosive

cyclogenesis, diabatic PV reduction and ridge amplification for the generation of the low-

latitude troughs/cut-offs involved in the heavy rainfall events over West Africa. The main

results section of this paper contains separate analyses for each case (sections 3 and 4). It is

preceded by an overview over the employed data (section 2) and it is followed by a summary

(section 5) and a short discussion of the results (section 6).

2. Data

For the representation of all large-scale atmospheric variables, twice daily (00 and 12 UTC)

European Centre for Medium-Range Weather Forecasts (ECMWF) Tropical Ocean and

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Global Analysis (TOGA) data were used with 2.5˚×2.5º grid spacing on standard pressure

levels (Trenberth, 1992). The General Meteorological Analysis Package (GEMPAK) was

employed for the interpolation to isentropic levels and the computation/display of various

meteorological parameters (e.g. PV). Water vapour (WV) and infrared (IR) images from the

geostationary GOES and Meteosat satellites have been obtained through the Space Science

and Engineering Center of the University of Wisconsin. Surface weather information was

taken from daily weather maps issued by the National Weather Service of the USA and by

the German Weather Service.

3. Case I (January 2002)

(a) Isentropic PV maps

With the help of PV maps on the 325-K isentropic surface Fig. 1 shows the formation of a

cut-off low near the West African coast during 04–06 January 2002. The thick lines on this

figure display PV advection at 325 K. The shading depicts PV tendencies due to latent

heating in the 250–200 hPa layer, which includes or is located just above the 325-K level in

the extratropics. The calculation of this parameter follows the method described in Cammas

et al. (1994), and Posselt and Martin (2004) and is based on the parameterization of latent

heating rates employed by Emanuel et al. (1987).

At 00 UTC 04 January the situation over the western North Atlantic is characterized by a

distinct, positively tilted PV trough over the southeastern USA and a broad PV ridge over the

downstream Atlantic Ocean. A fairly large region of negative PV advection is found along

the eastern flank of this ridge (Fig. 1a). Twelve hours later, the PV trough has moved

eastward and a surface cyclone with a core pressure of 995 hPa has formed underneath the

region of positive PV advection ahead of the trough (Fig. 1b). A second cyclone forms in the

region of strong PV gradients near Newfoundland. Moderate diabatic PV reduction is

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associated with this cyclogenesis and causes a small region of negative PV advection farther

downstream. The ongoing advection of low-PV air has extended the ridge northeastward and

has contributed to a narrowing of the downstream trough near 25ºW. By 00 UTC 05 January

the PV trough and cyclone over the western North Atlantic have changed rather little, but the

northern cyclone has quickly moved northeastward and deepened by 23 hPa (Fig. 1c).

Considerable latent heating accompanies this explosive intensification and leads to a

substantial reduction of the upper-level PV in this region. Large negative PV advection

downstream of the cyclone supports the continuing northeastward extension of the PV ridge

and the creation of a narrow, north–south oriented PV streamer over the eastern North

Atlantic.

Over the next 12 hours the northern low deepens further to 944 hPa and moves away from the

region of strong PV gradients towards the southern tip of Greenland (Fig. 1d). With a total

deepening of 45 hPa in one day this system fulfils the criteria for the bomb classification by

Sanders and Gyakum (1980) and ranks above the 99th percentile of the climatology by

Roebber (1984). To the east of this system diabatic PV erosion has become extreme, reaching

values of –3 PVU day–1 (1 PVU = 10–6 m2 s–1 K kg–1). The associated widespread and strong

advection of low-PV air pushes the leading edge of the PV ridge quickly northeastward. With

another region of low PV over western Europe this process results in a cut-off of the PV

streamer close to northwest Africa. This almost circular PV cut-off has a fairly high core

value of more than 6 PVU.

At the same time the surface low over the central North Atlantic is still located ahead of the

upper PV trough and has slightly intensified (Fig. 1d). During 06 January this cyclone moves

northeastward along the strong PV gradient and deepens by 15 hPa (Figs. 1e and f). The

associated diabatic PV reduction to the north of this system and the resulting downstream

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negative PV advection help to create a huge PV ridge centred over Scotland with PV values

below 2 PVU as far north as 70ºN (Fig. 1f). Meanwhile the system close to the southern tip

of Greenland fills slowly. The PV cut-off close to the northwest African coast is almost

stationary during 06 January and displays a strong cyclonic circulation around its base

(Figs. 1e und f).

Figure 2 shows the analogous analysis for the period 07–09 January during which the first

cut-off system gets re-absorbed by the midlatitude PV reservoir and is replaced by a second

cyclonic disturbance. At 00 UTC 07 January a region of high PV is found near the Iberian

Peninsula, to the south of the extremely amplified PV ridge over northwestern Europe

(Fig. 2a). The inverted meridional PV gradient in this region is a clear sign of an AWB (see

Postel and Hitchman, 1999). At 12 UTC on this day the cut-off over northwest Africa begins

to merge with this high PV air (Fig. 2b). Prior to this, localized diabatic heating to the

southeast of the cut-off centre has already weakened the PV anomaly (Figs. 1f, 2a and b). The

fact that no precipitation was observed over northwest Africa during this time (KM05) is

most likely related to evaporation in the deep and dry Saharan planetary boundary layer.

The synoptic situation over the western North Atlantic during this period shows great

similarities to Figs. 1a and b. At 00 UTC 07 January a deep, positively tilted PV trough is

located over the eastern half of the USA, upstream of a broad ridge with negative PV

advection along its eastern side (Fig. 2a). A cyclone forms ahead of this trough and moves to

the New England coast over the next 12 hours, associated with massive diabatic PV

destruction (Fig. 2b). Again the advection of low-PV air leads to a northeastward extension

of the PV ridge and a narrowing of the PV trough downstream. At 00 UTC 08 January a

second surface low forms to the northeast of Newfoundland, also accompanied by

widespread and strong diabatic PV tendencies to the northeast of its core (Fig. 2c). The PV

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ridge to its east has further amplified and a distinct PV streamer has formed over the eastern

North Atlantic, much like the situation three days earlier (Fig. 1c). By this time the cut-off

over northwestern Africa has been completely re-absorbed by the tongue of higher PV values

over the Mediterranean (Fig. 2c).

Between 00 UTC 08 and 00 UTC 09 January the southern surface low intensifies very little

and remains near the zone of strong PV gradients ahead of the PV trough (Figs. 2c–e). The

system farther to the north moves to the eastern side of Greenland and deepens by 29 hPa to

958 hPa, which classifies it as a bomb. This period is again characterized by widespread

diabatic PV reduction and a northeastward extension of the PV ridge. In contrast to the first

disturbance, the PV streamer does not cut off from the midlatitude PV reservoir, but connects

with the high PV air around southwestern Europe at 12 UTC 08 January (Fig. 2d). The two

PV anomalies together establish a broad cyclonic circulation over West Africa and the

adjacent eastern Atlantic (vectors in Figs. 2d–f), which impedes the seclusion of a single

positive PV anomaly in contrast to the situation three days earlier. The large southward

extension of this circulation suggests the existence of a positive PV anomaly at a more

elevated isentropic surface that influences the flow at 325 K. The connection between the two

PV features also results in the seclusion of a large region of very low-PV air over northern

Europe that slowly drifts eastward (Figs. 2d–f) and is connected with a substantial blocking

high at the surface (not shown). Finally, by 12 UTC 09 January the surface cyclone near

Greenland has started to fill, but diabatic PV destruction, negative PV advection and the

extension of the ridge continue (Fig. 2f). The low-pressure system to the east of the PV

trough has deepened to 975 hPa. Around this time the first precipitation was observed over

Senegal (KM05).

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(b) Formation of the cut-off and stratospheric intrusion

In the previous subsection we have shown that the formation of the PV cut-off and trough,

respectively, is related to the southward advection of high PV air towards the Atlantic Ocean

near West Africa. Air with a PV of more than 2 PVU on the 325-K isentropic surface is

usually of stratospheric origin and thus very dry. In a situation of a large southward

displacement, the stratospheric air subsides on the equatorward sloping isentropic surfaces to

midlevels, creating a substantial dry anomaly. In the absence of clouds the WV channel of

geostationary weather satellites can detect this midlevel dryness. Figure 3 shows Meteosat

WV images for 0430 UTC 05 and 0030 UTC 09 January. The respective times correspond to

the cut-off of the first disturbance (Figs. 1c and d) and the largest intensity of the second

disturbance (Fig. 2e).

In Fig. 3a the stratospheric intrusion is evident as a conspicuous dry line that follows the

western edge of the upper PV trough and wraps around its base. If we assume the

conservation of PV, the sharp cyclonic turning at the base of the trough is most likely related

to the increase in relative vorticity needed to compensate the strong decrease in the Coriolis

parameter f occurring in the northerly flow and the decrease in vertical stability that results

when stratospheric air enters the less stratified upper-troposphere. This process is likely to

support the formation of a secluded PV maximum with a closed cyclonic circulation. Similar,

but somewhat weaker signs of a stratospheric intrusion are evident on the WV image for

0030 UTC 09 January (Fig. 3b). This appears to be consistent with the somewhat less

dramatic synoptic development over the North Atlantic (compared to the first disturbance,

see section 3a), but could also be related to obscuration by upper-tropospheric moisture and

clouds. The initial formation of the TP that accompanied the heavy precipitation over West

Africa (see KM05) is evident in the southeastern corner of Fig. 3b.

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Both Figs. 3a and b display conspicuous dry regions over the subtropical Atlantic to the

southwest of the cyclonic circulation centres (around 22ºN, 30ºW and around 18ºN, 40ºW,

respectively). According to KM05 and Knippertz (2005) the drying is related to the

subsidence caused by the combination of the ageostrophic motions associated with the large-

scale extratropical wave and with outflow from tropical convection that can penetrate far into

the subtropics due to the weak PV gradients in the eastern portion of the ridge. Lower-level

divergence underneath the subsidence enhances the trade winds over the eastern North

Atlantic, which in turn triggers more convection over the equatorial Atlantic. The enhanced

outflow from this convection accelerates the downstream STJ and further increases

convergence, subsidence and drying in the subtropics as revealed by a more intense dry

region in Fig. 3b as compared to Fig. 3a. The consequence is a locally enhanced Hadley

overturning and the formation of a TP over West Africa (KM05). Pronounced dry regions to

the west of TPs have been found in a number of studies (e.g. McGuirk and Ulsh, 1990;

Knippertz, 2005).

(c) Upstream baroclinic developments

In this subsection we will investigate factors that support the explosive baroclinic

development over the western North Atlantic and the associated PV ridge amplification,

which have been demonstrated to be instrumental in producing the low-latitude disturbances

near West Africa. According to the literature, factors that accompany rapid deepening include

explosive jet acceleration, widespread divergence at upper-levels, the superposition of the

secondary circulations in the entrance and exit regions of two upper-level jet streaks and the

formation of a strong poleward directed low-level jet (LLJ; Uccellini et al., 1984; Uccellini

and Kocin, 1987; Uccellini, 1999). Further crucial processes are strong sensible and latent

heat fluxes and a decrease in vertical stability (Cammas et al., 1994; Stoelinga, 1996;

Uccellini, 1999). Most of these factors will be discussed in the following analysis.

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Figure 4 shows GOES IR images with superimposed streamlines and isotachs at the 335-K

isentropic level for the period of explosive cyclogenesis and rapid ridge amplification (see

section 3a, Figs. 1a–c). The 335-K level is located near the tropopause at midlatitudes (250–

200 hPa) and reaches down to below 400 hPa in the Tropics. Given the tendency of the large-

scale atmospheric flow at upper-levels to conserve its potential temperature Θ, it is more

adequate to consider jet evolutions on isentropic than on pressure surfaces (see Uccellini et

al., 1984).

At 00 UTC 04 January an elongated, southwest–northeast oriented upper-level jet of up to

79 m s–1 dominates the large-scale flow over the extratropical western North Atlantic

(Fig. 4a). It corresponds with the region of large PV gradients in Fig. 1a. On the anticyclonic

flank of the jet is a cloud band that extends into the tropical East Pacific. Divergent outflow

from this band might support the jet acceleration. In the jet entrance region over the Gulf of

Mexico, where ageostrophic winds reach 32 m s–1 (not shown), the streamlines display strong

confluence between the southwesterly flow ahead of a broad trough in the tropical East

Pacific and the westerly to northwesterly flow upstream of the positively tilted extratropical

trough over the eastern USA (c.f. Fig. 1a). The former airflow is most likely associated with

large poleward momentum fluxes into the extratropical jet.

Between 00 and 12 UTC 04 January wind speeds in the jet core explosively increase to

92 m s–1 and then reach a rarely observed maximum of 95 m s–1 to the southeast of

Newfoundland by 00 UTC 5 January (Fig. 4b). The anticyclone to the south of the jet has

rapidly stretched northeastward and a trough begins to penetrate equatorward to the west of

West Africa in agreement with the PV map shown in Fig. 1c. The extratropical portion of the

cloud band has thickened substantially (Fig. 4b) and strong divergence is found along the jet

axis (not shown).

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Figure 5a depicts wind vectors, Θ and frontogenesis at 925 hPa and reveals that the cloud

band coincides with a region of very pronounced frontogenesis. The western portion of the

upper-jet follows a distinct low-level baroclinic zone with a Θ contrast of more than 35 K

between maritime Canada and the subtropical Atlantic (Figs. 4a and 5a). Farther to the north,

the baroclinic and frontogenetic zone wraps around the explosively deepening surface

cyclone. The strong frontogenesis is related to the eastward advection of cold (and dry) air

off the North American continent underneath the upper-jet core and a southwesterly to

southerly LLJ of relatively warm (and moist) air over the North Atlantic. The LLJ shows

characteristics of the so-called warm conveyor belt (WCB; Browning, 1990). Underneath the

entrance region of the upper-level jet strong, but not very frontogenetic, cold advection is

found in the vicinity of Bermuda. Figures 1c and 5a reveal that the strongest diabatic PV

destruction occurs at the northeastern end of the frontogenetic zone, where the WCB rises

over the (in this case ill-defined) low-level warm front. Here the latent heating within the

cloud band affects a region of relatively high PV at 325 K (i.e. a relatively low tropopause).

The analysis of Uccellini et al. (1984) provides a possible interpretation for the development

described above. When the upper-trough over North America approaches the strong jet from

the west (see Fig. 4a), the resulting decrease in wavelength of the trough-ridge system over

the western North Atlantic makes it increasingly difficult to balance the flow around the ridge

crest, leading to a phase of inertial instability at upper-levels. In fact very unusual

ageostrophic winds of up to 53 m s–1 (69% of the total wind) at 330 K are observed in the jet

exit region to the north of the Azores (not shown). The strong ageostrophic motions are

associated with widespread divergence at upper-levels and a frontogenetic circulation at

lower levels. The LLJ together with strong heat fluxes from the ocean surface sustain strong

latent heat release within the frontogenetic cloud band. The resulting massive divergent

outflow further accelerates the upper-jet and thereby helps to sustain the inertial instability. In

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addition, the latent heat release causes diabatic PV destruction and supports the rapid

amplification of the ridge over the North Atlantic. No direct precipitation observations are

available over this region, but grid point values of up to 100 mm simulated by the model

employed by KM05 and satellite estimates of the same magnitude corroborate the existence

of a massive latent heat release. A detailed analysis of the described mechanism is beyond the

scope of this paper.

Figures 5b and 6 show the analogues analysis for the second disturbance. Again a southwest–

northeast oriented jet of 74 m s–1 and a positively tilted upstream trough are observed near the

east coast of North America at 12 UTC 7 January (Fig. 6a). A clear connection to the tropical

Pacific as on 04 January is not observed. The comma cloud near maritime Canada is

associated with the surface cyclogenesis and strong diabatic PV tendencies shown in Fig. 2b.

By 12 UTC 8 January the jet accelerates to a maximum of 83 m s–1 (Fig. 6b). At that time

some inflow from the Tropics into the jet is indicated by the streamline analysis. As before,

the anticyclone over the South American north coast has extended northeastward and a

trough starts digging southward to the west of Africa. A weak cloud band stretches

northeastward along the anticyclonic shear side of the jet, from Central America into the

North Atlantic, where the clouds thicken substantially and show a strong cyclonic curvature.

This region is characterized by strong frontogenesis between a northwesterly flow of cold air

from the Labrador Strait and a southerly LLJ of warm air farther to the east (Fig. 5b). The

strongest upper-level PV destruction occurs, where the WCBs of the two cyclones rise over

their respective surface warm fronts (Figs. 2d and 5b). Over all, the second development

shows great similarities to the evolution three days earlier, but most of the features involved

(the upper-jet, surface cyclone, frontogenesis etc.) are less extreme, consistent with the

generation of a somewhat less intense disturbance near West Africa.

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4. Case II (March/April 2002)

(a) Isentropic PV maps

As for Figs. 1 and 2, Figs. 7 and 8 show PV maps at 325 K for case II. However, since the

evolution occurs somewhat less explosively than during case I, we decided to show the PV

distribution every 24 rather than every 12 hours. At 00 UTC 21 March 2002 a distinct PV

trough is found over the central North Atlantic (Fig. 7a). A surface cyclone has formed on its

eastern side with a core pressure of 984 hPa. Large diabatic PV tendencies are analyzed to the

northeast of this system, followed by a region of negative PV advection further downstream.

One day later, the cyclone has deepened by 20 hPa to 964 hPa in association with strong

positive PV advection (Fig. 7b) and thus fulfils the definition of a “bomb”. According to

Sanders and Gyakum (1980) and Roebber (1984) both the time of the year and the location

are unusual for such a development. Martin and Otkin (2004) describe a similarly unusual

case over the North Pacific. With both diabatic PV reduction and negative PV advection

continuing, a large PV ridge forms and begins to wrap cyclonically around the rapidly

deepening low (Fig. 7b). In addition, two weak surface cyclones are found on the eastern side

of a broad PV trough over eastern North America.

By 00 UTC 23 March the cyclone over eastern Canada has intensified with a “bomb”-like

deepening rate of 21 hPa (Fig. 7c). The system to the southeast has also deepened to 989 hPa.

The associated diabatic PV tendencies are only moderate, probably due to the dry

environment over the continent. Nevertheless there is a considerable amplification of the

downstream ridge with widespread negative PV advection on its eastern side. The deep

cyclone over the central North Atlantic has moved northward and started to fill. A region of

diabatic PV reduction and downstream negative PV advection is still found at the northern

end of the now largely amplified PV ridge over the eastern North Atlantic/western Europe.

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At 00 UTC 24 March the cyclone over the central North Atlantic has crossed 60ºN and

weakened to 988 hPa, while the PV ridge has slowly moved to western Europe (Fig. 7d). The

cyclone over Canada has further deepened to 966 hPa. The downstream PV ridge has

stretched northeastward and the negative PV advection at its northeastern edge thins the

north–south oriented PV streamer along 20ºW. As in Fig. 3, dark filaments mark the streamer

in WV imagery (not shown).

By 00 UTC 25 March a compact, circular PV cut-off with a core value of 6 PVU and a closed

circulation around the centre has formed near the Canary Islands (Fig. 7e). This cut-off is

unusually far away from the midlatitude stratospheric reservoir of high PV. A huge region of

high 325-K PV and low surface pressure stretches from Newfoundland to the Norwegian Sea.

Figure 7e shows only one of the embedded cyclone centres. Along the southern flank of this

region the flow is westerly to southwesterly across the entire North Atlantic. The large-scale

situation is almost stationary over the following 24 hours (Fig. 7f).

Figure 8 shows the PV evolution between 27 March and 01 April, the time when the first cut-

off gets re-absorbed by the midlatitude PV reservoir and a new disturbance forms over the

North Atlantic. At 00 UTC 27 March the PV cut-off is still stationary over the Canary Islands

(Fig. 8a). Over the North Atlantic a surface low with a core pressure of 984 hPa forms. Over

the following 12 hours the low quickly moves northeastward with the broad anticyclonic flow

and weakens (Fig. 8b). This process is associated with an amplification of the downstream

PV ridge and a subsequent AWB over Europe. The resulting region of high PV over

southwestern Europe connects with the nearby PV cut-off that eventually gets re-absorbed on

29 March (Fig. 8c). This process is quite similar to that shown in Figs. 2a–c. In the meantime

a broad PV trough with a shallow surface cyclone ahead of it slowly moves eastward over

eastern North America (Figs. 8a–c). The core pressure of the low stays above 1000 hPa

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during this period, but substantial diabatic PV reduction and negative PV advection far

downstream of this system contribute to an amplification and broadening of the large PV

ridge over the North Atlantic. At 00 UTC 29 March the northern end of the PV streamer to

the east of the ridge begins to merge with the high-PV air over western Europe (Fig. 8c).

Thereby the low-PV air in the breaking PV ridge over northern Europe gets secluded (cf.

Fig. 2d). As in Fig. 3b, WV imagery displays a dark filament along the western edge of the

PV streamer (not shown).

On 30 and 31 March a shallow eastward moving PV trough causes large positive PV

advection over the western North Atlantic, resulting in a moderate deepening and a quick

eastward displacement of the surface cyclone in this region (Figs. 8d and e). The AWB

farther downstream thins the connection of the PV streamer to the high PV over Europe.

Around the time displayed in Fig. 8e, rainfall began under the southwesterly flow to the east

of the now strongly positively tilted PV trough (Fink and Knippertz 2003; KM05). Finally,

by 00 UTC 01 April, the surface low over the North Atlantic has rapidly intensified to

978 hPa and has moved northeastward to about 60ºN, 16ºW. The accompanying

amplification of the PV ridge over the British Isles leads to a cutting-off of the PV streamer

over southwestern Europe. This development differs markedly from case I (see Figs. 2e–f).

(b) Upstream baroclinic developments

The 335-K streamlines and isotachs at 00 UTC 21 March show an elongated jet streak near

the east coast of North America with a southwesterly inflow from the tropical Pacific

(Fig. 9a). With a wind speed maximum of just above 70 m s–1 this jet is weaker than the jets

that characterized case I. A cloud band stretches along the anticyclonic shear side of the jet

and then curves cyclonically into maritime Canada, where substantial PV reduction was

analyzed (Fig. 7a). Divergent outflow from the high clouds over the southeastern USA is

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likely to feed into the jet maximum just downstream. The conspicuous comma cloud of the

cyclone over the central North Atlantic is evident around 35ºW. This region is also

characterized by large PV tendencies (Fig. 7a).

By 00 UTC 23 March the jet has moved eastward without great changes in wind speed

(Fig. 9b), but the intensification of the large upper-level cyclonic circulation over eastern

Canada has triggered a rapidly deepening surface cyclone (see Figs. 7b and c). While the

clouds along the equatorward side of the jet weaken, a thick cloud band wraps around this

cyclone. Figure 10a shows that this band is characterized by strong cold frontogenesis

between a westerly flow of cold air off the North American continent and a southerly flow of

warm air over the Atlantic. As before, the largest diabatic PV reduction occurs, where the

WCB rises over the intensifying low-level warm front (see Fig. 7c). The streamlines in

Fig. 9b underline that the ridge formation over the North Atlantic is considerably less

dramatic than during case I (cf. Figs. 4 and 6), consistent with the weaker jets, cyclogenesis

and PV tendencies. The cyclone over the central Atlantic generates localized frontogenesis

over the British Isles, but strongest diabatic PV tendencies are located at the northern end of

the southerly flow of warm air to the northeast of the cyclone centre (Figs. 7c and 10a).

Figure 11 shows the circulation at 335 K for 00 UTC 28 March, the time of the second

significant PV ridge amplification over the North Atlantic (see Fig. 8). This situation differs

fundamentally from the ones shown in Figs. 4, 6 and 9. The jet along the east coast of North

America is relatively weak and no connection to the Tropics is found (Fig. 11). The upper-

trough over the eastern USA is also not very pronounced, and the associated surface cyclone

is fairly shallow (see Fig. 8b). The most striking feature in Fig. 11 is a thick anticyclonically

curved cloud band over the western North Atlantic and eastern Canada. Figure 10b shows

that this band is coincident with strong low-level frontogenesis in connection with the distinct

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southerly flow on the western side of a huge anticyclone over the North Atlantic. As in all the

cases described above, the diabatic PV reduction maximizes near the point where this low-

level flow of warm air crosses the surface warm front to the northeast of the cyclone centre

(Figs. 8b and 10a). One factor that might support this unusual development is the

superposition of the right entrance region of the extratropical jet along the North American

east coast and the left exit region of a STJ streak over the southern USA, resulting in highly

diffluent flow in this region (c.f. the wintertime cases described by Uccellini and Kocin,

1987). The STJ appears to be related to northward momentum fluxes and convective outflow

ahead of a positively tilted trough over the North Pacific (Fig. 11).

5. Summary

This study has examined the large-scale evolutions that led to the equatorward penetration

and cutting-off of extratropical upper-level PV troughs in two cases of heavy rainfall over

West Africa described by KM05. Both cases involved the serial passage of two positive-PV

disturbances through this region. The first initiated a moisture transport from the Tropics, and

the second was directly associated with the precipitation event (see KM05). All four

disturbances were included in the present investigation, which was based on ECMWF

analyses and satellite images. The evolution of the respective first and second PV

disturbances are schematically summarized in Figs. 12 and 13. The most important findings

are:

• The upper-level disturbances examined took the form of elongated PV streamers that

formed through an equatorward transport of high-PV air downstream of a high-amplitude

ridge (Figs. 12b and 13a). Dry lines in WV imagery to the west of the troughs indicate

stratospheric intrusions.

19

• The respective first disturbances were accompanied by deep negative PV anomalies to the

east, so that the continuous thinning of the PV streamer through negative PV advection in the

upstream ridge finally produced a cut-off close to the West African coast that remained

stationary for several days (Fig. 12). Subsequent AWB over Europe led to an advection of

high-PV air to the Iberian Peninsula and the cut-off eventually got re-absorbed (see Fig. 13a).

• At their early stages, the respective second disturbances evolved similarly to the

description above (compare Figs. 12b and 13a). At later stages, however, the intensifying PV

streamers merged with high-PV air close to southwestern Europe resulting from the

respective previous development, impeding a cutting-off from the midlatitude PV reservoir

(Fig. 13b). This process also generated a seclusion of low-PV air over northern Europe

associated with a blocking situation.

• The rapid amplification of the PV ridges over the North Atlantic was achieved through

negative PV advection and diabatic reduction of upper-level PV due to latent heating

(Figs. 12b and 13a). In most cases these processes were related to explosive cyclogenesis and

strong frontogenesis. A southerly LLJ ahead of the intensifying elongated surface cold front

fed warm, moist air into the cyclo- and frontogenetic region, probably associated with strong

sensible and latent heat fluxes (Uccellini et al., 1984). This feature has been termed a WCB

(e.g. Browning, 1990; see Figs. 12b and 13a). In most cases the diabatic PV reduction

maximizes in the cloud-head portion of the cyclone, where the WCB rises over the (at times

ill-defined) surface warm front at the crest of the low-level thermal ridge. This is a typical

location for heavy rainfall, and the relatively low tropopause there allows strong impacts on

the upper-level PV distribution.

• Factors contributing to the rapid baroclinic developments were (1) pronounced low-level

baroclinicity related to the strong land–sea contrast along the North American east coast,

(2) a strong upper-level jet that appears to be supported by momentum fluxes and divergent

20

outflow from low latitudes (c.f. Hurrell and Vincent, 1990) and (3) an upper-level

trough/positive PV anomaly approaching from the west (c.f. Uccellini, 1999; see Figs. 12a–b

and 13a). During case I signs of low dynamic stability (explosive jet acceleration, strong

ageostrophic motions, widespread upper-level divergence) were observed (Uccellini et al.,

1984). During case II strong frontogenesis was related to the superposition of the left exit

region of the STJ and the right entrance region of the polar jet (c.f. Uccellini and Kocin,

1987).

• In general, the evolution is faster and more dramatic during the January case, when low-

level baroclinicity, upper-level jet speeds and cyclone deepening rates are greater.

6. Discussion and outlook

The analysis presented in this paper indicates that the described mechanism has a preferred

geographic location connected to the cool season quasi-permanent baroclinic zone along the

North American east coast that promotes rapid cyclone deepening (Sanders and Gyakum,

1980; Roebber, 1984). It is also consistent with the finding by Eckhardt et al. (2004) that

intense WCBs are most frequent in the western ocean basins of the NH midlatitudes. As

shown above, these WCBs can be associated with substantial diabatic PV reduction. On the

other hand, PV streamers and cut-offs are most frequent over the Atlantic Ocean to the west

of West Africa and the Iberian Peninsula (H. Wernli, University of Mainz, 2005, pers.

comm.), where upper-troughs play an important role for the cool season precipitation

variability (Knippertz, 2004). If we assume that the presented cases are extreme examples of

a fairly common process, these occurrence maxima might be physically related.

In contrast to large parts of the Tropics, the low latitude eastern Atlantic Ocean is

characterized by mean upper-level westerlies during the cool season and, together with the

21

eastern Pacific Ocean, has been referred to as a “westerly duct” (Webster and Holton, 1982).

A number of statistical studies claim that the westerly basic current allows a penetration of

extratropical Rossby waves to low latitudes, resulting in an enhancement of tropical

convection (Hartmann and Liebmann, 1984; Kiladis and Weickmann, 1992, 1997;

Iskenderian, 1995; Kiladis, 1998). The present study corroborates a connection between

enhanced low-latitude cloudiness (and precipitation) and a large-scale extratropical Rossby

wave, but the high amplitude of the waves, together with the diabatic and highly nonlinear

interaction processes described here and in Knippertz (2005), suggest a far more complicated

picture than that of a linear wave dispersion. In fact, the statistical results of Kiladis and

Weickmann (1997) also show a highly amplified ridge over the North Atlantic upstream of

enhanced cloudiness near West Africa (their Figs. 5c and 6c). It therefore seems plausible

that the low latitude westerlies are, at least partly, generated by persistent cut-offs of the kind

presented in this paper rather than a result of purely tropical processes.

In idealized numerical experiments Thorncroft et al. (1993) demonstrated that AWBs and PV

cut-offs occur as a result of baroclinic wave developments without the involvement of moist

processes (their LC1 life cycle). Being exclusively based on observational data the present

study cannot quantify the importance of latent heating for the wave amplification, the AWB

and the cut-off formation. Consequently an important next step will be a test of the net effect

of diabatic processes on the large-scale PV wave by suppressing it in a model simulation.

Comparable studies by Stoelinga (1996) and Massacand et al. (2001) suggest an important

role for PV modification through condensational heating in the formation of the downstream

ridge and PV streamer.

In addition, future work should include a more statistical evaluation of cases of deep

equatorward penetrations/cut-offs of extratropical troughs over the Atlantic and Pacific

22

Oceans in order to better understand and to compare the processes affecting the two main

westerly duct regions. The cases analyzed here suggest that meridional momentum fluxes

from the tropical eastern Pacific affect the evolution over the North Atlantic. Since these are

sensitive to the El Niño/Southern Oscillation (ENSO; e.g. Shapiro et al., 2001), this relation

could be further examined to better understand the observed linkages of ENSO to West

African precipitation (e.g. Nicholson and Kim, 1997; Knippertz et al., 2003).

Acknowledgements

The scientific results presented in this paper were accomplished during a two-year

postdoctoral research scholarship from the German Science Foundation (DFG; Grant KN

581/1–1) at the Department of Atmospheric & Oceanic Sciences of the University of

Wisconsin–Madison under the supervision of Prof. Jonathan E. Martin and Prof. emer. Stefan

L. Hastenrath. We would like to thank Pete Pokrandt for his assistance in data and software

issues.

23

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Figure captions

Fig. 1. Isentropic PV maps at 325 K for (a) 00 UTC 04, (b) 12 UTC 04, (c) 00 UTC 05,

(d) 12 UTC 05, (e) 00 UTC 06 and (f) 12 UTC 06 January 2002. Thin isopleths show PV

every 1 PVU starting with 2 PVU, and thick isopleths depict PV advection every 10–4 PVU s–

1 with negative values dashed and the zero-contour omitted; vectors show total winds greater

than 15 m s–1 (scale in bottom right corner); shading indicates negative diabatic PV

tendencies in the 200–250 hPa layer (legend below figure; in PVU day–1). A 9-point

smoothing algorithm has been applied to the PV advection field in order to suppress noise.

Circles and numbers indicate the positions and core pressures (in hPa) of selected surface

low-pressure systems.

Fig. 2. As Fig. 1 but for 7–9 January 2002.

Fig. 3. Meteosat WV satellite image for (a) 0430 UTC 05 and (b) 0030 UTC 09 January

2002.

Fig. 4. GOES IR satellite images with superimposed 335-K streamlines and isotachs (dashed

lines every 10 m s–1 starting with 50 m s–1) for 00 UTC on (a) 04 and (b) 05 January 2002.

Fig. 5. Frontogenesis at 925 hPa for (a) 00 UTC 05 and (b) 12 UTC 08 January 2002. Thin

isopleths show potential temperature every 5 K; vectors display total winds greater than

5 m s–1 (scale in bottom right corner); shading depicts frontogenesis (legend below figure; in

K (100 km)–1 day–1). The low-pressure systems from Figs. 1 and 2 are marked.

Fig. 6. As Fig. 4 but for 12 UTC on (a) 07 and (b) 08 January 2002.

Fig. 7. As Fig. 1 but for 00 UTC on (a) 21, (b) 22, (c) 23, (d) 24, (e) 25 and (f) 26 March

2002.

28

Fig. 8. As Fig. 7 but for 27 March–01 April 2002.

Fig. 9. As Fig. 4 but for 00 UTC on (a) 21 and (b) 23 March 2002.

Fig. 10. As Fig. 5 but for 00 UTC on (a) 23 and (b) 28 March 2002.

Fig. 11. As Fig. 4 but for 00 UTC 28 March 2002.

Fig. 12. Schematic depiction of the cut-off formation from the respective first PV

disturbances. Thick black lines show an arbitrary PV contour with positive (negative)

anomalies indicated by a plus (minus). Thick black arrows mark the upper-level jet, thin grey

arrows the low-level WCB. Grey-shaded regions are characterized by large diabatic PV

erosion, and the dashed line indicates large negative PV advection. The Ts mark the position

and the intensity of a surface cyclone. (a) Initial PV wave, (b) explosive cyclogenesis and

wave amplification, and (c) cut-off formation.

Fig. 13. As Fig. 12 but for the respective second PV disturbances. Here the cut-off formation

is impeded by the merging of the PV streamer with the large reservoir of positive PV

resulting from a prior AWB to the east of the amplifying wave.

29

Fig. 1. Isentropic PV maps at 325 K for (a) 00 UTC 04, (b) 12 UTC 04, (c) 00 UTC 05, (d) 12 UTC 05, (e) 00 UTC 06 and (f) 12 UTC 06 January 2002. Thin isopleths show PV every 1 PVU starting with 2 PVU, and thick isopleths depict PV advection every 10–4 PVU s–

1 with negative values dashed and the zero-contour omitted; vectors show total winds greater than 15 m s–1 (scale in bottom right corner); shading indicates negative diabatic PV tendencies in the 200–250 hPa layer (legend below figure; in PVU day–1). A 9-point smoothing algorithm has been applied to the PV advection field in order to suppress noise. Circles and numbers indicate the positions and core pressures (in hPa) of selected surface low-pressure systems.

30

Fig. 2. As Fig. 1 but for 7–9 January 2002.

31

Fig. 3. Meteosat WV satellite image for (a) 0430 UTC 05 and (b) 0030 UTC 09 January 2002.

32

Fig. 4. GOES IR satellite images with superimposed 335-K streamlines and isotachs (dashed lines every 10 m s–1 starting with 50 m s–1) for 00 UTC on (a) 04 and (b) 05 January 2002.

33

Fig. 5. Frontogenesis at 925 hPa for (a) 00 UTC 05 and (b) 12 UTC 08 January 2002. Thin isopleths show potential temperature every 5 K; vectors display total winds greater than 5 m s–1 (scale in bottom right corner); shading depicts frontogenesis (legend below figure; in K (100 km)–1 day–1). The low-pressure systems from Figs. 1 and 2 are marked.

34

Fig. 6. As Fig. 4 but for 12 UTC on (a) 07 and (b) 08 January 2002.

35

Fig. 7. As Fig. 1 but for 00 UTC on (a) 21, (b) 22, (c) 23, (d) 24, (e) 25 and (f) 26 March 2002.

36

Fig. 8. As Fig. 7 but for 27 March–01 April 2002.

37

Fig. 9 As Fig. 4 but for 00 UTC on (a) 21 and (b) 23 March 2002.

38

Fig. 10. As Fig. 5 but for 00 UTC on (a) 23 and (b) 28 March 2002.

39

Fig. 11. As Fig. 4 but for 00 UTC 28 March 2002.

40

Fig. 12. Schematic depiction of the cut-off formation from the respective first PV disturbances. Thick black lines show an arbitrary PV contour with positive (negative) anomalies indicated by a plus (minus). Thick black arrows mark the upper-level jet, thin grey arrows the low-level WCB. Grey-shaded regions are characterized by large diabatic PV erosion, and the dashed line indicates large negative PV advection. The Ts mark the position and the intensity of a surface cyclone. (a) Initial PV wave, (b) explosive cyclogenesis and wave amplification, and (c) cut-off formation.

41

Fig. 13. As Fig. 12 but for the respective second PV disturbances. Here the cut-off formation is impeded by the merging of the PV streamer with the large reservoir of positive PV resulting from a prior AWB to the east of the amplifying wave.

42

ADDRESS FOR GALLEY PROOFS Dr. Peter Knippertz Institut für Physik der Atmosphäre Johannes Gutenberg-Universität Mainz Becherweg 21 D-55099 Mainz GERMANY