Download Atmosphere of the tropical Pacific region

West Pacific: Atmospheric Circulation
**********************************************************************************
Atmosphere of the tropical Pacific region
This is a summary of the tropical atmospheric circulation by describing in
detail its structure and composition, including the Hadley, Walker and BrewerDobson circulation schemes.
Characterisation of the atmosphere in the Pacific
The Earth’s atmosphere is oxidising and contains a non-equilibrium mixture
of gases. The atmosphere is characterised by different vertical regions based
upon the variation of meteorological and chemical parameters, such as the ozone
concentration or the temperature profile [1]. Figure 1. shows the global mean
vertical profiles of ozone mixing ratio and temperature, the latter demarcating
particular atmospheric layers: troposphere, stratosphere, mesosphere and
thermosphere. Most dynamics and chemistry of crucial importance to the
biosphere occur within the first two atmosphere layers listed above.
Figure 1. Global mean vertical profiles of temperature and ozone mixing ratio [2].
The troposphere ranges from 0 to 10 km in polar regions and up to 20 km
high in the tropics. It is capped by the tropopause, where the temperature gradient
changes from negative to positive at the point of the lowest temperature called the
cold point tropopause (CPT). It is worth mentioning that the radiative transfer
through the atmosphere varies, depending on altitude levels, cloud presence and
atmospheric composition [1]. For the purpose of this CPGS project, the
atmospheric dynamics over the tropical Pacific region will be reviewed in more
detail.
1
Atmospheric characterisation of the Pacific: dynamics
Dynamical processes have a major role in determining the composition of
the atmosphere over the Pacific region. Due to the mixing of air masses of varied
compositions and levels, chemical reactions within the atmosphere occur over
both short and large spatial and temporal scales [3].
Figure 2.General circulation of the troposphere [4].
Non-uniform heating of the Earth produces distinct patterns of rising and
descending air masses, winds, and storms in well-defined cells around the globe
(Figure 2). In the tropics, two major tropospheric circulation cells: Hadley and
Walker cells occur, whereas Brewer Dobson circulation stands as the main
stratospheric circulation scheme.
Figure 3. A schematic illustration of the mean Hadley – Walker circulation in the Pacific
region: a major atmospheric pathway for the transport of mass, momentum and energy
between the tropics and extratropics (easterly trade winds: solid line arrows; deep
convection pathway: vertical arrow; poleward air flow: multiple arrows; high sea level
pressure (SLP) region: H circles; low SLP regions: L circle) [5].
2
Being a meridional component of the tropical tropospheric circulation
(Figure 3), the Hadley cell dominates by moving heat and water vapour
polewards. The boundary layer air, meaning the air from the region where most
emissions occur (0-2 km), is convected over the Equator and then shifted towards
so-called ‘horse latitudes’ (30 oN and 30 oS) [6,7].
Over the equatorial waters where the sea level pressures (SLPs) are low,
the resultant horizontal pressure gradient is weak and winds are light (regions
referred as ‘doldrums’). Here, the warm air rises, often condensing into cummulus
clouds due to the huge amount of liberated latent heat, providing the energy to
drive the Hadley cell. The rising air can reach the tropopause which acts as a
barrier in between troposphere and stratosphere. As air moves poleward from the
tropics it is subjected to the constant cooling by radiation, which forms the belts of
high pressure subtropical highs, which in turn leads to the gradual subsidence of
air over the middle latitudes. Some of the surface air moves back toward the
Equator, along the trade winds. Near the Equator, both northeast and southeast
trades converge along a boundary called the inter-tropical convergence zone
(ITCZ). It is where the air rises and continues its Hadley circulation flow [8].
Being a zonal component of the tropospheric circulation, the Walker
circulation (Figure 3) is driven by the non-uniform heating across the tropics [9].
The zonal overturning involves upwelling at longitudes of heating and downwelling
at longitudes of cooling. The non-uniform distribution of land and sea, along with
the resulting asymmetries in atmospheric heating, leads to Walker circulation
along the equator. The concentration of latent heating over the Maritime continent
forces the Pacific Walker circulation, reinforced by the easterly trade winds across
the equatorial Pacific. Its descending branch maintains the arid climate that
typifies the eastern Pacific [9]. Hosking, et al (2012)[10] reported that the Walker
circulation plays an important role in the seasonality of fast tropical transport from
the lower and middle troposphere to the upper troposphere. At the same time it
impacts the potential supply of surface emissions to the tropospheric tropopause
layer (described later in more detail) and subsequently to the stratosphere.
The convected tropospheric air enters the stratosphere, where it is subject
to Brewer-Dobson circulation (BDC) (Figure 4) [11, 12]. BDC transfers the air
globally in poleward direction and downward from the tropical middle atmosphere.
The stratospheric air is transferred between the Equator and poles on a monthly
timescale, indicative of the strong control by the Coriolis force [11].
3
Figure 4. Brewer-Dobson circulation scheme [12].
Hadley circulation, Walker circulation and BDC are three major circulation
systems which affect the atmospheric transport of air mass from lower parts of the
tropical troposphere, particularly the boundary layer, throughout the free
troposphere, reaching the tropical tropopause layer and then entering
stratosphere. In recent years, extensive studies have been made on the structure
and properties of the interface layer between troposphere and stratosphere,
called the Tropical Tropopause Layer (TTL).
***************************************************************************
4
1. Jacob, D.J. (1999), Introduction to Atmospheric Chemistry, Princeton
University Press, Princeton.
2. U.S. Standard Atmosphere (1976), U.S. Government Printing Office,
Washington, D.C.
3. IPCC, (2001), Climate Change 2001: The Scientific Basis., Cambridge
University Press, Cambridge, United Kingdom.
4. National
Weather
Service,
U.S.,
JetStream,
http://www.srh.noaa.gov/jetstream//global/images/jetstream3.jpg.
5. Schwing F.B., et al., (2002), The Northern Oscillation Index (NOI): a new
climate index for the northeast Pacific, Progress in Oceanography , 53,
115-139.
6. Philander S.G.H, et al., (1987), Simulation of the Seasonal Cycle of The
Tropical Pacific Ocean, Journal of Physical Oceanography, 17, 1986-2002.
7. Quan, X.W., et al., (2004), Change of the Hadley circulation since 1950
The Hadley Circulation: Past, Present, and Future edited by H.F. Diaz and
R.S. Bradley, Kluwer Academic Publishers , 85-120.
8. Ahrens D.C., (1994) Meteorology Today. An introduction to weather,
climate and environment. (5 th edition), West Publishing Company , New
York, U.S.
9. Salby M.L., (2012), Physics of the Atmosphere and Climate, Cambridge
University Press, New York, U.S.
10. Hosking J.S., et al., (2012), Tropical convective transport and the Walker
circulation, Atmos.Chem.Phys. , 12, 9791-9797.
11. Bunzel F., et al., (2013), The Brewer Dobson Circulation in a Changing
Climate: Impact of the Model Configuration, American Meteorological
Society, 1437-1455.
12. Baum
S.,
Brewer
Dobson
http://www.eoearth.org/article/Brewer-Dobson_circulation ,
10/2011.
5
circulation.
updated: