Meteorología del Pacifico Sur Oriental

René D. GarreaudDepartamento de Geofísica – Universidad de Chile

www.dgf.uchile.cl/rene

SENAMHI – Perú

Curso de Climatología Sinóptica en la Costa Oeste de América del Sur16-20 Abril 2007; Lima - Perú

Presentación No. 2: Meteorología del Pacifico Sur Oriental

En esta exposición se describen los rasgos climáticos mas relevantes en la región sur-oriental del Océano Pacifico, con especial énfasis en la zona tropical-subtropical y franja costera. Tópicos incluidos en esta exposición incluyen el origen y mantención del anticiclón del Pacifico, los vientos costeros y la surgenciaoceánica.

• El campo de presión y vientos de gran escala• El campo de nubosidad• Mantención del anticiclón subtropical del Pacifico• El campo de viento de meso-escala y surgencia oceánica• Ciclo diario lejos y fuera de la costa

SubtropicalHigh Coastal

jet

SCu deck

SAMITCZ

Andes Mts.

Key atmospheric features over the SEP

1

2

Jan

Apr

Jul

Oct

Annual cycle of SST

Jan

Apr

Jul

Oct

Annual cycle of SLP

The extensive and persistent deckof SCu over the SSEP plays animportant role in the regional andglobal climate by substanciallyreducing the ammount of solar radiation that reaches the sea surface

LTM bi-monthly albedo

60-80%temp. freq.

Albedo ( nubosidad)

Cloud field has significant spatial variability

SCuSc

Cu

Cloud pattern related to MBL depth

Cloud pattern related to MBL depth

Full: Continental monsoon + Hadley cell + MountainsTopo: Hadley cell + Mountains

ω(674 hPa) ψ(887 hPa)

Full

Topo

“The similarities between upper and lower panels imply an important role for the interaction between the zonal mean flow and the topography in sustaining the subtropical anticyclones. The differences emphasize the importance of the zonally asymmetric heating”. (Rodwell and Hoskins 2001)

(Rodwell and Hoskins 2001)

ω v

Ideal monsoon heating@ 25°S-90°E

+ Continental drag

+ Newtonian cooling

+ Ideal. mountain

(Rodwell and Hoskins 2001)

ω(674 hPa) ψ(887 hPa)

Mt.+SAM

Mt.+SAM+SPac

(Rodwell and Hoskins 2001)

Y. Wang et al. 2004

Cooling effect of the SCu deck over the SEP further enhance the subsidence and hence the southerly flow

Formation and maintenanceof the SCu deck

(moist air confined to the MBL)

Enhanced equatorward flowβv ≈ f∂w/ ∂z

Enhanced LS subsidence tokeep ∂T/ ∂t ≈ 0

Net cooling at the top of the SCu(Radiative effect

– condensational heating)

Large Scale Circulation set the stage:

Subsidence (adiab. Warming)+ Equatorward flow (low SST)

= Temperature Inversion

From K. Takahashi (2006)

SCu: Low level, shallow cloud layer at the top of the MBL

Net Cloud Radiative Forcing

ΔR = ΔLW + ΔSH, where Δ• = •clear - • average

For SCu clouds:ΔLW ~ 0 as Tc ~ SSTΔSH < 0 (albedo effect)ΔR < 0 : Cooling effect

SEP SCu: -50 W/m2

Annual cycle of SCu an inter-regional differences largely explained by low level stability: θ700- θsfc (Klein and Hartmann 1993)

LTM annual mean SLP and surface wind speed

(NCEP-NCAR Reanalysis)

Bi-monthly means of the surface wind speed (near the coast, the flow is largely along-shore). An "atmospheric" jet is a closed maxima of wind speed (i.e., you need closed contours of ws). Thus, there are only two jets along the coast: one off central Chile (30°-35°S) and a small one off Sechura (5°S aprox).Nevertheless, there is a conspicuous maximum of wind speed off Pisco (15°S). The wind speed there has a very marked seasonality (from 3 m/s in JF up to +7 m/s in JA). When the maximum is present during austral winter, its along-shore scale is rather short (i.e., a very windy region of about 300 km bounded by "calm" regions to the north and south of it). Of course, its narrowness and seasonality make this maximum particularly susceptible to "climate change" either in the past or future.

Surgencia: Afloramiento de aguas profundas (frías, ricas en O2 y nutrientes) por efecto del viento

Surgenciacostera

Surgenciaecuatorial

V

U

θ

w

V

U

θ

w

Simulated (MM5) structure of the coastal jet

V > 18 m/sGarreaud and Muñoz 2005

−6 −4 −2 0 2 4 60

1000

2000

3000

4000U Budget

a

(m/s/hour)

(m)

Upg

Uc

Uha

Uf

−6 −4 −2 0 2 4 60

1000

2000

3000

4000V Budget

b

(m/s/hour)

(m)

Vpg

Vc

Vha

Vf

−1 −0.5 0 0.5 10

250

500

750

1000T Budget

c

(K/hour)

(m)

Tza

Tma

Tva

Tf

−60 −40 −20 0 20 40 600

250

500

750

1000TKE Budget

d

(m2/s2/hour)(m

)

SBPD

21 vHC

yp d=∂∂

−ρ

vfxp ρ−=∂∂

vuHdC

fvxp

yuv

xuu

tu r−+

∂∂

=∂∂

+∂∂

+∂∂

ρ1

vvHCfu

yp

yvv

xvu

tv d r−+

∂∂

−=∂∂

+∂∂

+∂∂

ρ1

Steady-stateDynamics

•V > 8 m/s off central Chile almost alwayd associatedwith a southerly jet (darkshaded)

•Jet events typically a week long (3-15 days)

•More frequent, strongerand longer in summer.

SQ3. Coastal Jet under Clear Skies…Why?(+300 W/m2 reaching the surface)

Renault et al. 2006

SQ4. Impacts of Jet Events on SST

Renault et al. 2006

Impacts of Jet Events on SST

Wind, SST and SST anomalies

A distinctive feature of this Sc deck is its particularly pronounced diurnal cycle in cloud amount (Minnis and Harrison 1984; Rozendaal et al.

1995) and LWP (Bretherton et al. 2003; Wood et al. 2002), that is highly relevant to the quantification of the true impact of Sc on climate

(Bergman and Salby 1997).

Bretherton et al. 1995

θ-θ q - q

Mean diurnal cycle at Antofagasta (Rutllant et al. 2003)

MM5 results

• Significant diurnal cycle in θ up to 5 km ASL• Subsidence interrupted by period of upward motion• Cooling largely produced by vertical advection

W C W C W C

MM5 results

Diurnal cycle of vertical velocity at 800 hPa MM5 results

Coastal Diurnal Cycle