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Three related questions: Why is the ITCZ north of the equator in the SE Pacific?

Three related questions: Why is the ITCZ north of the equator in the SE Pacific? Why is the SE Pacific so cold? Why is the SE Pacific subtropical anticyclone so close to South America?. 1. 2. 3. Without clouds but with upwelling… still to warm. With clouds (prescribed).

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Three related questions: Why is the ITCZ north of the equator in the SE Pacific?

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  1. Three related questions: • Why is the ITCZ north of the equator in the SE Pacific? • Why is the SE Pacific so cold? • Why is the SE Pacific subtropical anticyclone so close to South America? 1 2 3

  2. Without clouds but with upwelling… still to warm With clouds (prescribed)

  3. Y. Wang et al. 2004 Cooling effect of the SCu deck over the SEP further enhance the subsidence and hence the southerly flow

  4. The extensive and persistent deck of SCu over the SSEP plays an important role in the regional and global climate by substancially reducing the ammount of solar radiation that reaches the sea surface 60-80% temp. freq. LTM bi-monthly albedo Albedo ( nubosidad)

  5. Cloud field has significant spatial variability Cu SCu Sc

  6. Austral winter: Mountain only is quite good w(700 hPa) Stream function at 900 hPa Full (heat+mnt) Mnt. only Anticyclone split

  7. Austral summer: Continental heating is essential w(700 hPa) Stream function at 900 hPa Full (heat+mnt) Mnt. only Subsidence in right place but only ¼ of the full value

  8. Continental heatingeffect The Gill model for tropical convection: prescribed at equator V at low levels and w Subsidence to the SW V and p at low levels Upper level high to the SW of convection

  9. Ciclo anual y distribución espacial Long-term-mean OLR (shaded) and 200 hPa wind

  10. t ~ 12-24 hr t < 2 hr 12 km A A Div Calentamiento diabático Este 0 km f = 0 B B Con f < 0 Sur Inicialmente, el calentamiento diabático (calor latente) asociado a la convección profunda genera una alta presión en altura y una baja en superficie (¿?). En respuesta a los gradientes de presión así generados aparece divergencia en altura y convergencia en superficie. En esta primera etapa todo ocurre en un mismo eje de la vertical. Si la convección se mantiene por varias horas, el efecto de Coriolis genera una circulación anticiclonica en altura y ciclonica en superficie (modificada por la fricción superficial). Aun todo esta en fase en la vertical.

  11. Si la circulación en altura es quasi-geostrofica la advección de vorticidad relativa es muy pequeña (¿?). Sin embargo, la circulación en altura produce advección de vorticidad planetaria anticiclonica en el sector oeste de la alta ciclonia en el sector este (¿?). Si la situación se mantiene por varios días/semanas el anticiclón se intensifica al SW de la zona de máximo ascenso y se debilita al NE. Además, se genera una zona de convergencia en altura al W de la zona de ascenso, y en consecuencia la subsidencia se incrementa al W de la zona de convección. Por compensación, también aparece un centro de baja presión al SW de la zona de convección, y en consecuencia vientos del S en su borde este. t ~ días t ~ semanas 12 km B v < 0 A Con Div v > 0 A Este 0 km f = 0 B B f < 0 Sur

  12. Efecto monsonal (Rodwell and Hoskins 2001) • Calentamiento diabático en zona de precipitación • Expansión del calentamiento por ondas de Kelvin/Rossby • Advección fría al Oeste del continente • Incremento de la subsidencia sobre Pacifico SE

  13. Efecto monsonal (Rodwell and Hoskins 2001)

  14. Mountain induced circulation: subsidence west of the Andes Tropical Easterlies Subtropical Westerlies Extratropical Westerlies

  15. Only Andes

  16. July 850 hPa winds January 200 hPa Z CTRL CTRL NSAO NSAO

  17. Zonal flow – Andes interaction Monsonal connection (austral summer only) Svedrup balance ↑ subsidence Equatorward flow MBL Entrainment ↑ Low Trop. Stability Drying of MBL ↑ coastal upwelling ↑ Stratus clouds ↑ Evaporative cooling Oceanic transport ↑ MBL cooling ↓Solar rad. at sfc ↓ SST (near shore and offshore)

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