an introduction to the inter-tropical convergence zone (itcz) chia-chi wang dept. atmospheric...
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An introduction to the Inter-tropical Convergence Zone
(ITCZ)
Chia-chi Wang
Dept. Atmospheric Sciences
Chinese Culture University
Acknowledgment:
Prof. Gudrun Magnusdottir
Earth system science
University of California, Irvine
Visible Channel (~0.65 um)
ITCZ
Scale of different weather systems
Idealized simulations of Hadley cells
Northern hemisphere Summer Spring or fall
Strong cell in the winter hemisphere
Contour: streamfunction
pres
sure
Climatological flow
Zonal and time average of the wind field from the high resolution NCEP analysis from Aug—Oct, 2000—2002.
u
v
Climatological ITCZ
Waliser and Gautier 1993. Averaged from 17 years of monthly Highly reflective cloud (HRC) data. The number of days per month the given grid point was covered by a large-scale deep convective system, subjectively determined.
The shallow meridional circulation
A return flow atop the atmospheric boundary layer (ABL), around 2-4 km, in the winter hemisphere (Zhang et al. 2004)
Meridional wind & RH (Tropical Atmosphere-Ocean (TAO) ship soundings) (Zhang et al. 2004)
Alt
itud
e (k
m)
Alt
itud
e (k
m)
Latitude
1-11 Nov. 2000, 95W
Aug.-Dec., 1995-2002, 95W-110W
Satellite images of the shallow ITCZ (20 Sept 2000)
VS
IR
Some interesting topics• ITCZ tends to stay in the NH.• Double ITCZ in the eastern Pacific• South Pacific Convergence Zone (SPCZ)• East Pacific ITCZ and cold tongue complex• ITCZ breakdown• Equatorial waves (Rossby waves and Kelvin w
aves) v.s. ENSO• West-ward propagating disturbances (ex. East
erly waves) • Madden-Julian Oscillation (MJO) • Mesoscale processes related to cloud clusters
ITCZ breakdown
1988, IR Another case of ITCZ breakdown
Why is it important?
• To understand the basic dynamics of the ITCZ on synoptic timescale.
• An efficient way to pool vorticity in regions in the tropics which represents the early stages of tropical cyclonegenesis.
• Might contribute meridional moisture transport (need more data and study).
• It is an interesting topic.
Vortex roll-up simulation
Vertical heating profile
Horizontal heating profile
Wang and Magnusdottir (2005)
Relative vorticity ζ
+-
Potential vorticity (PV)
NPSP
Planetary vorticity, f
EQ
PV = ( f + ζ) / h
PV is conserved when there is no friction and no diabatic heating.
~850 hPa Potential Vorticity
PVU
Barotropic instability
(strong horizontal wind shear)
Idealized simulation of ITCZ breakdown
Wang and Magnusdottir (2005)
Characteristics of the ITCZ on synoptic timescale:
• The breakdown can be self-induced
• The new ITCZ re-forms within a few days
A strong mode of synoptic timescale lifecycle
North Eastern Pacific
u = 0 m/s
u = 2.6 m/s
easterly
Dispersion curve for equatorial Rossby
waves (n = 1)
Longwave approximation:Non-dispersive waves
Phase speed = 8 m/s
Magnusdottir and Wang 2008
ITCZ composite
(lag-regression between the raw vorticity data and the ITCZ index)
Magnusdottir and Wang 2008
Summary• The ITCZ goes through stages of
formation, undulation, breakdown, and dissipation on synoptic timescale.
• The ITCZ is a constructive feature. It is not a group of thermal-driven convective cells lined up in a row.
• A non-dispersive wave packet propagates westward in a constant phase speed.
Future work
• Its vertical structure needs more study.
• Wet convection in model simulation
• The interaction with other phenomena (on other timescales) such as MJO and ENSO.
• Mechanisms to form the ITCZ in details?
25-day running variance of the filtered vorticity
El Nino
El Nino
El Nino
La Nina
La NinaEl Nino
El Nino
El Nino
La Nina La Nina
Magnusdottir and Wang 2008
An elongated convergent area induced by a weak disturbance
Wang et al. 2010
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