ANTICYCLONES

S J Colucci, Cornell University, Ithaca, NY, USA

Introduction

Anticyclones are regions of relatively high pressure on horizontal surfaces, or high geopotential height on isobaric surfaces, around which air circulates clock-wise in the Northern Hemisphere and counterclock-wise in the Southern Hemisphere. Anticyclones are therefore characterized by negative relative vorticity and low but positive absolute vorticity in the Northern Hemisphere, while in the Southern Hemisphere they are distinguished by positive relative vorticity and low but negative absolute vorticity. On sea-level pressure or geopotential height analyses, they may be subjectively identified by closed isobars or height contours, whereas in vorticity analyses they may objectively be identified by relative vorticity minima in the Northern Hemisphere and maxima in the Southern Hemisphere.

At sea level, anticyclones typically originate as cold, shallow circulations that migrate Equatorward and evolve into warm, subtropical high-pressure systems penetrating well into the troposphere.

Aloft, anticyclones may appear at middle and high latitudes on isobaric surfaces. From hydrostatic con-siderations, these are relatively warm systems. Anti-cyclones aloft are often stationary or westward drifting and thus may block the eastward progress of other weather systems. Anticyclonic circulations at high latitudes may penetrate into the stratosphere where they may be associated with sudden strato-spheric warmings.

Although not as actively researched as cyclones, anticyclones are important because the clear, dry conditions usually associated with them may allow strong nighttime radiative cooling and cold surface

temperatures. The convectively stable air of anticy-clones may allow air pollutants to concentrate near the Earth’s surface. Finally, the blocking action of anti-cyclones aloft may cause persistently anomalous weather conditions at the Earth’s surface.

Structure

Anticyclones may either be cold-core or warm-core systems. An example of each type is presented in Figure 1. Cold-core anticyclones are typically found on the poleward side of the midlatitude baroclinic zone. These are shallow systems with an anticyclonic circulation confined to the lower troposphere.

The geostrophic relative vorticity (curvature in the isobars or geopotential height contours) is anti-cyclonic near the Earth’s surface but becomes anti-cyclonic (or less anticyclonic) by the middle troposphere. The region of high sea-level pressure over south-western Canada inFigure 1is a cold-core anticyclone; note its proximity to a local minimum in the 1000–500 mb thickness which, hydrostatically, is proportional to the vertically averaged temperature in the 1000–500 mb layer. This sea-level anticyclone is located between cyclonic and anticyclonic features at the 500 mb level (Figure 2).

Warm-core anticyclones are found Equatorward of baroclinic zones and are characterized by circulations that remain or may become increasingly anticyclonic from sea level to the middle troposphere. The region of high sea-level pressure over the Arctic inFigure 1is a warm-core anticyclone. Even though this system is at a higher latitude than the cold-core anticyclone over south-western Canada, it is characterized by higher 1000–500 mb thickness (vertically averaged tempera-ture) and it is located near a local maximum in the thickness field. Note inFigure 2that the sea-level 142 ANTICYCLONES

anticyclone is located beneath a well-defined anticy-clone (closed contours around relatively high geopo-tential height) at the 500-mb level.

Both warm-core and cold-core anticyclones are characterized by gently subsiding vertical motion in the troposphere. This subsidence favors clear skies promoting strong nighttime radiative cooling of the Earth’s surface near the centers of these anticyclones.

The adiabatic warming of the sinking air coupled with radiative cooling at the surface often produces an inversion in the vertical temperature profile; this inversion may be eroded or destroyed by daytime radiative heating and vertical mixing in the boundary layer. Regardless, anticyclones are distinguished by strong static stability.

Dynamics

Convergence of mass in the upper troposphere is the primary mechanism responsible for the relatively high sea-level pressure at anticyclone centers. From considerations of gradient wind balance, this mass convergence occurs downwind of anticyclonic circulations, or near regions of anticyclonic vorticity advection. The formation of new, cold-core anticy-clones (or anticyclogenesis) is favored when this

mass convergence occurs over a lower-tropospheric cold-air pool which, hydrostatically, would be associ-ated with relatively high sea-level pressure. Warm anticyclogenesis may occur if mass convergence occurs over relatively high sea-level pressure at lower latitudes.

More commonly, cold anticyclones evolve into warm anticyclones as follows. The circulation around cold anticyclones draws cold air Equatorward, forcing the sea-level pressure to rise locally. The anticyclone relocates towards rising sea-level pressure.

Thus, cold anticyclones usually drift Equatorward with time.

The mass convergence over the cold anticyclone forces air to sink through the troposphere and to adiabatically warm. The anticyclone thus becomes warmer over time and may eventually be located Equatorward of the midlatitude baroclinic zone. Fric-tionally induced mass divergence at the Earth’s surface forces the sea-level pressure to fall at the anticyclone center, which then weakens. The anticyclone may reintensify as a warm system if mass convergence aloft exceeds the lower tropospheric mass divergence near the anticyclone center in magnitude.

Other mechanisms may contribute to anticyclone formation and intensification. While there is usually

Figure 1 Sea-level pressure (solid contours in millibars with leading 9 or 10 omitted, at 4 mb intervals) and 1000–500 mb thickness (dashed contours in dekameters, at 6 dm intervals) at 0000 UTC, 16 December 2000.

ANTICYCLONES 143

very little temperature advection in the lower tropo-sphere over anticyclone centers, the advection of cold air in the upper troposphere over a sea-level anticy-clone center may contribute to its intensification.

Clear conditions near cold anticyclone centers may result in the formation of ice fogs, and radiative heat loss from these fogs may contribute to sea-level pressure rises and anticyclone intensification. How-ever, this effect is believed to be small.

Climatology

Early climatologies of weather systems were construct-ed through manual inspection of sea-level pressure charts and subjective identification of centers of closed isobars. Contemporary investigations employ auto-mated procedures to identify objectively these systems from maxima in sea-level pressure analyses. Such

investigations reveal that, over the Northern Hemi-sphere (Figure 3), sea-level anticyclones are most frequently observed in a band over the midlatitude Pacific Ocean, in a broadband near North America and centered on the Great Lakes, and near Mongolia. Over the Southern Hemisphere, similarly defined sea-level anticyclones are concentrated within a midlatitude (25–451S) band with little longitudinal variation (Figure 4).

In the middle troposphere over the Northern Hem-isphere (Figure 5), anticyclones identified objectively from maxima in geopotential height fields are most frequently observed during the summer at low lati-tudes, especially over continental regions. During the winter, these systems are rare but observed occasion-ally over high latitude oceans. These latter systems may be associated with blocking, as discussed below.

No comparable results exist for the Southern Hemi-sphere to date.

Figure 2 The 500 mb geopotential height (solid contours in dekameters, at 6 dm intervals) and temperature (dashed contours in Celsius, at 51C intervals) at 0000 UTC, 16 December 2000.

144 ANTICYCLONES

Blocking

Warm anticyclones that penetrate deep into the troposphere from the Earth’s surface in midlatitudes may, if stationary and persistent, block the normal

eastward motion of other weather systems. In order for the blocking system to be nearly stationary and persistent, the anticyclonic circulation aloft must be continually maintained on its upstream flank by fluxes of anticyclonic potential vorticity (lower tropospheric warm air and middle tropospheric anticyclonic vorti-city). These fluxes are provided by smaller-scale waves approaching the blocking system. Additionally, the strong static stability of anticyclones helps maintain them against dissolution by convective mixing. Sink-ing motion in the statically stable, anticyclonic environ-ment promotes adiabatic warming and maintenance of a deep, warm anticyclonic system.

Blocking anticyclones may be observed anywhere and at any time, but are favored during the cool season over the oceans, particularly the eastern oceans of the Northern Hemisphere and the eastern Pacific Ocean in the Southern Hemisphere. These locations are also downstream of the principal storm tracks and are also locations of climatologically preferred diffluent flow fields in the middle troposphere. The repeated inter-action of small-scale systems with the diffluent flow enhances the diffluence (by making its poleward branch more anticyclonic) until an anticyclonic circu-lation is established aloft. Quasi-geostrophically, this diffluence is enhanced by the local deposition of anticyclonic potential vorticity. Equatorward of the blocking anticyclone, the normal westerly flow may reverse to easterlies over a considerable longitudinal distance and for periods of a week or more. The blocking anticyclone may even penetrate into the stratosphere, causing a reversal of the flow there from westerly to easterly and sudden stratospheric warm-ings. Thus, blocking anticyclones are of considerable scientific and practical importance.

Impact

Because they are distinguished by clear skies and subsiding air, anticyclones are typically associated with fair weather. A stationary and persistent anticy-clone may produce prolonged fair and dry weather conditions, depleting soil moisture and stressing crops and water supplies. The strong stability of anticy-clones may stagnate air near the Earth’s surface, leading to enhanced concentrations of pollutants.

Clear skies near anticyclone centers favor strong nocturnal cooling near the Earth’s surface; these conditions during the growing season may damage crops. The Equatorward circulation of cold air around anticyclones may cause sudden cold-air outbreaks over midlatitudes. The cold air associated with anti-cyclones may become wedged or dammed against mountain ranges, leading to freezing rain or ice if warm moist air is circulated over the dammed, cold air.

1 1

1 1 1

Figure 3 Sea-level anticyclone track density, in number of centers per month, in the 1980–86 analyses of the European Center for Medium-Range Weather Forecasts. (Reproduced with permission from Sinclair MR and Watterson IG (1999)Journal of Climate 12: 3467–3485. Boston: American Meteorological Society.)

3 1

1 3

Figure 4 As in Figure 3 but for the Southern Hemisphere.

(Reproduced with permission from Sinclair MR and Watterson IG (1999) Journal of Climate 12: 3467–3485. Boston: American Meteorological Society.)

ANTICYCLONES 145

Thus, while perhaps not as dramatic as cyclones, anticyclones have their own unique and interesting features and impacts.