Chap. 3
Regional climates in tropics
3.1 Regional climates
3.2 Ocean circulations
3.3 Structure of the InterTropical
Convergence Zone (ITCZ)
3.4 Monsoon circulations and
associated jets
sommaire
3.2 Ocean circulations
Surface current
Reminder : wind stress is generated by the effect of the wind on
the surface ocean
Which is the origin of the South Equatorial Current (SEC)
and the North Equatorial Current (NEC) ?
⇨ these surface current are driven by the wind stress
Courants océaniques de surface (moyenne annuelle).
Source : d’après White et al., 1992
3.2 Ocean circulations
Subsurface current
Which is the origin of equatorial undercurrent btw 100/200 m ?
40 cm
SEC
100 m
z
Equatorial
undercurrent
200 m
West
East
Circulations along the Equatorial Pacific
Green arrows above sea-level ocean show the wind stress
Arrows in the ocean show oceanic circulations
Source : Météo-France (F.Beucher)
• In regions next to the equator (< 150 km), trades winds pile up
water on the western Pacific, and thus give rise to eastwards
pressure force (next to equator, no influence of the Coriolis Force),
which in turns drives a subsurface current between 100 and 200 m
deep, called ‘Cromwell current’ or Equatorial undercurrent.
3.2 Ocean circulations
Subsurface current
Which is the origin of equatorial undercurrent btw 100/200 m ?
40 cm
SEC
100 m
z
Equatorial
undercurrent
200 m
West
East
Circulations along the Equatorial Pacific
Green arrows above sea-level ocean show the wind stress
Arrows in the ocean show oceanic circulations
Source : Météo-France (F.Beucher)
• The surface and subsurface currents explain why downwelling
occur on the western flank of Pacific (high SST) and upwelling on
the eastern flank (low SST).
• For Atlantic Ocean : same circulation.
3.2 Ocean circulation
Ekman divergence and equatorial upwelling
Which is the origin of the Equatorial upwelling ?
E
E
Source : Météo-France (F.Beucher)
• The oceanic Ekman mass transport, E, is directed at right
angles to the right (left) of τ in the northern (southern)
hemisphere. The magnitude of E is proportional to the
strenght of τ.
• Following this rule, at the equator, E is directed away
from the equator producing divergence and upwelling
along the equator.
3.2 Ocean circulations
Upwellings and SST
Monthly mean of Sea surface température
Source : RéAnalyse NCEP 1981-2002
• The equatorial upwelling
the coastal upwelling
and
are pronounced
in the sectors of Eastern Pacific and Eastern Atlantic,
which explains that cold tongues of SST occur
in these area.
3.2 Ocean circulations
Ocean-atmosphere coupling : upwelling areas
Annual Précipitations (m.).
Sources : Dorman et Bourke (79,81), Dorman (82), Baumgartnet et Reichel (75
• As atmosphere-ocean coupling plays an important
role in tropics (latent heat and sensible fluxes are linked
with the SST) shallow convection (St/Sc or shallow Cu)
and rare rain (
) occur in upwelling areas :
along the equator + E. Pacific + E. Atlantic
3.2 Ocean circulations
Surface current
Which is the origin of the north Equatorial Counter
Current (ECC) located at the surface and between a
latitud band of 4°N-10°N ?
Courants océaniques de surface (moyenne annuelle).
Source : d’après White et al., 1992
3.2 Ocean circulations
Surface current
Location of the North ECC :
15°N
z
2°N
Source : Pond et Pickard (83)
• The minimum of trades winds occur between 2°N and
15°N in phase with the location of the North ECC.
• The piling-up of water on the western side of ocean is
backing through the North ECC in surface and through
the Equatorial undercurrent in subsurface
3.2 Ocean circulations
Surface current : ECC
Sea Surface Température in August.
Source : RéAnalyse NCEP 1981-2002
ECC
• Since the north Equatorial Counter Current (ECC) is directed
eastwards, warm waters are also advected from the Western Pacific
towards the Eastern Pacific
⇨ we observe within the latitud band of ECC (2°N-15°N) a zone of
TSM maximale (>= 28°C).
• By coupling ocean-atmosphere, deep convection (
) is
enhanced within this latitud band = mean location of the
InterTropical Convergence Zone (ITCZ)
sommaire chap.3
3.2 Ocean circulations
Surface current : ECC
Sea Surface Température in August.
Source : RéAnalyse NCEP 1981-2002
ECC
• We can also explain the fast increasing os SST (+2 à 3 °C)
just northwards the equator by the process of downwelling
situated at 4-5°N.
sommaire chap.3
3.2 Ocean circulations
Ekman convergence and downwelling
Source : Météo-France
(F.Beucher)
• We remind that the Ekman transport E is proportional to the
intensity of the wind stress τ.
• Since the southeasterlies decrease while they approach the ITCZ,
the Ekman transport decrease too :
⇨ we observe a strong convergence of Ekman towards 4°N
⇨ producing downwelling and fast increasing of SST
sommaire chap.3
3.2 Circulations océaniques
Ocean-atmosphere coupling : zone de TSM maxi
Annual Précipitations (m.).
10°N
Sources : Dorman et Bourke (79,81), Dorman (82), Baumgartnet et Reichel (75
• As the ocean-atmosphere coupling plays an important role
under tropics (flux of latent heat and sensible heat are linked
to SST), we observe heavy rains over areas of SST maximum
(>28°C)
• Under annual mean, the ITCZ ( ) is located between 5°N10°N over Central Pacific – Eastern Pacific - Atlantic
sommaire chap.3
chap 3.3 : ZCIT
References
- Baumgartner, A., Reichel, E., 1975 : The World water balance.
Elsevier, Amsterdam, Oxford, New York, 179 pp.
- Dorman, C. E. , 1982 :4Indian Ocean Rainfall’. Tropical OceanAtmosphere Newsletter,10,4.
- Dorman, C., E., Bourke, R.,R., H., 1979 :’Precipitation over the Pacific
Ocean’, 30°N to 30°S. Mon. Wea. Rev., 107, 896-910
- Dorman, C., E., Bourke, R.,R., H., 1981 :’Precipitation over the Atlantic Ocean’,
30°N to 30°S. Mon. Wea. Rev., 109, 554-563
-Pond, S., PIckard, G. L., 1983 : Introduction dynamic oceanography.
Second Edition, Pergamon Press, Oxford, New York, Toronto, Sidney,
Paris, Frankfurt, 329 pp.
-White et Warren B., 1992 :’Reflection of interannual Rossby waves at
the maritime western boundary of the tropical Pacific’; Journal of
Geophysical Research, Washington, DC, vol.97, n°C9, pp. 14305-14322.