Synoptic-scale analysis of light-rainfall episodes at San Felix Island

(80°W, 27°S)Jan-Jun 2003

José Rutllant José Rutllant && René Garreaud René Garreaud

CIMAR ISLASFONDECYT 1920833FONDECYT 1920833

PRODACPRODACCLIVAR/VAMOS/VOCALSCLIVAR/VAMOS/VOCALS

THE STRATOCUMULUS (THE STRATOCUMULUS (ScSc) CLOUD ) CLOUD COVERCOVER

Role of subtropical Sc in global climate: Role of subtropical Sc in global climate: radiative forcing dominated by the short-radiative forcing dominated by the short-wave component: wave component: “climate refrigerator”. “climate refrigerator”.

A A 4 %4 % reductionreduction in subtropical Sc cloud- in subtropical Sc cloud-cover/albedo is equivalent to a cover/albedo is equivalent to a 2 a 3ºC2 a 3ºC increaseincrease in the mean in the mean global global surface surface temperaturetemperature..

SUBTROPICAL STRATOCUMULUS CLOUD-COVER IN THE SUBTROPICAL STRATOCUMULUS CLOUD-COVER IN THE SUBTROPICAL SOUTH-EASTERN PACIFIC UNDER THE SOUTH-SUBTROPICAL SOUTH-EASTERN PACIFIC UNDER THE SOUTH-

EAST TRADE-WIND REGIME.EAST TRADE-WIND REGIME.

SE Pacific: extensive cold-tongue SE Pacific: extensive cold-tongue spanning over eastern spanning over eastern equatorial Pacific (ITCZ north of equatorial Pacific (ITCZ north of the equator)the equator)

– SE trades vs. Coastline shapeSE trades vs. Coastline shape– Subtropical anticyclone Subtropical anticyclone

large/stronglarge/strong– Orography // coastline. AltiplanoOrography // coastline. Altiplano

* Coastal and equatorial * Coastal and equatorial upwelling, evaporation, cold-upwelling, evaporation, cold-water advection.water advection.

* Complex air-sea interaction * Complex air-sea interaction processes. processes.

* Lack of observational data * Lack of observational data over the ocean over the ocean

>>>>>>>>

FAILURE OF COUPLED MODELS TO FAILURE OF COUPLED MODELS TO SIMULATE THE SEASONAL CYCLE SIMULATE THE SEASONAL CYCLE (Mechoso et al., 1995)(Mechoso et al., 1995)

September-October ClimatologySeptember-October Climatology: :

* surface winds (arrows* surface winds (arrows),),* precipitation over 200 mm/month (orange-yellow)* precipitation over 200 mm/month (orange-yellow)

* stratocumulus cloud cover above 30 % (grey).* stratocumulus cloud cover above 30 % (grey).* Cross-equatorial flow (westerly components downstream) east of * Cross-equatorial flow (westerly components downstream) east of

the curved line. the curved line. Adapted from EPIC (1998).Adapted from EPIC (1998).

Variability in the subsidence over the Variability in the subsidence over the stratocumulus cloud cover stratocumulus cloud cover

HIGH FREQUENCYHIGH FREQUENCY

((coastal stripcoastal strip): diurnal cycle of mass divergence associated with flow over the west ): diurnal cycle of mass divergence associated with flow over the west slope of the Andes (Rutllant et al., 1998, 2003slope of the Andes (Rutllant et al., 1998, 2003))

((subtropical SE Pacificsubtropical SE Pacific): diurnal cycle in convection over the central Andes ): diurnal cycle in convection over the central Andes

highlands (altiplano) (e.g. Rodwell and Hoskins, 2001 )highlands (altiplano) (e.g. Rodwell and Hoskins, 2001 )

((subtropical SE Pacificsubtropical SE Pacific): diurnal wave resulting from ): diurnal wave resulting from thermal/mechanical effects off the coastline/orography bent at 18 ºS (Garreaud and thermal/mechanical effects off the coastline/orography bent at 18 ºS (Garreaud and

Muñoz, 2004)Muñoz, 2004)

((SE PacificSE Pacific). synoptic-scale disturbances including coastal ). synoptic-scale disturbances including coastal lows (Garreaud et al. 2001; Garreaud and Rutllant, 2002)lows (Garreaud et al. 2001; Garreaud and Rutllant, 2002)

LOW FREQUENCY (CLIVAR)LOW FREQUENCY (CLIVAR)

**intraseasonalintraseasonal cycle (MJO’s) cycle (MJO’s)**seasonal seasonal cycle from the Caribbean warm pool (e.g. Enfield, CP)cycle from the Caribbean warm pool (e.g. Enfield, CP)

**interanuuainteranuual cyclel: ENSOl cyclel: ENSO**decadal/interdecadaldecadal/interdecadal cycles (e.g. ENSO-like) cycles (e.g. ENSO-like)

Non-linear behaviour in Non-linear behaviour in HIGH-FREQUENCYHIGH-FREQUENCY disturbances determine disturbances determine LOW FREQUENCYLOW FREQUENCY (CLIVAR)(CLIVAR) variabilityvariability

))

Cross-shore section at 23Cross-shore section at 23ººS (Antofagasta) in austral summer S (Antofagasta) in austral summer afternoonafternoon

Rutllant et al., (1998, 2003)Rutllant et al., (1998, 2003)

*regional atmospheric circulation (thin arrows)*regional atmospheric circulation (thin arrows)* ocean circulation induced by the Ekman drift (thick arrows) * ocean circulation induced by the Ekman drift (thick arrows)

* Height-integrated zonal mass transport over the subsidence inversion (twin arrows)* Height-integrated zonal mass transport over the subsidence inversion (twin arrows)

Coastalsubssidence

Inland distance in km

Mean divergence and diurnal cycle anomalies Mean divergence and diurnal cycle anomalies (Quikscat)(Quikscat)

((adapted from Wood et al., EPIC 2003)adapted from Wood et al., EPIC 2003)

Mean divergence Diurnal cycle (6L-18L)

•Positive mean divergence anomalies (relative to other subtropical west coasts) except at the coastal bend (18ºS).

• Enhanced subsidence in the afternoon (18L) off the coast (especially off subtropical Chile) except at the coastal bend (18ºS).

““Upsidence” wave simulated Upsidence” wave simulated with the MM5 model (Garreaud with the MM5 model (Garreaud

& Muñoz 2004)& Muñoz 2004)

• Vertical (large-scale) winds at 800 hPa (from 15- day regional MM5 simulation, October 2001)

- Subsidence prevails over much of the

SE Pacific during morning and afternoon (10-18 UTC).

- A narrow band of strong ascending motion originates along the coast after

local noon (18 UTC) and propagates southwestward over the following 12

hours, reaching as far west as the IMET

buoy (85W, 20S) by local midnight.

Diurnal vs. Synoptic variability Diurnal vs. Synoptic variability (MM5)(MM5)

(adapted from Garreaud & Muñoz, (adapted from Garreaud & Muñoz, 2004)2004)

Diurnal Diurnal amplitude in amplitude in

800 hPa 800 hPa potencial potencial

temperature temperature equals or equals or exceeds exceeds

synoptic-scale synoptic-scale variability.variability.

High near-shore variability

Closed cells

Robert WoodRobert Wood (UW) (UW)

Pockets of open-cells associated with light precipitation

Closedcells

Pocket of open cells

Stevens et al. (2004)

Stevens et al. (2004)

Stevens et al. (2004)

CIMAR 5CIMAR 5Caldera – Easter IslandCaldera – Easter Island

October 1999 cruiseOctober 1999 cruiseCeilometer Vaisala CT12KCeilometer Vaisala CT12K

(RSMAS)

CIMAR 5Easter Island - Robinson Crusoe Island

November 1999AGOR Vidal Gormaz

A

A

17 November 1999. R/V Vidal Gormaz steaming to A. Selkirk Is.Rain-showers to the NW . Ship under Sc with NW/6 knots winds.

On- board ceilometer Vaisala CT12K (weak precip. Reported between 6 and 8 UTC) 17 November 1999

300 hPa300 hPa SFC SFCShip position Ship position BB

B B

SAN FELIX ISLAND

Precipitation Events

May 2003

DAYHOUR OBS. PP[mm] CL oktas CL type

W present W past Wind

UTCgrad/knots

10 6 0.2 6 3 80 8 140/18

10 12 1.5 6 4 2 8 130/16

12 0 0.3 7 3 80 2 130/08

12 12 0.7 5 5 1 2 120/13

15 6 0.1 5 3 80 2 130/15

15 12 0.2 7 8 25 2 120/09

16 12 1.4 7 8 25 2 150/14

20 12 trace 3 5 1 8 310/13

20 24 1.1 0 0 1 8 270/14

23 12 0.6 8 8 80 8 130/13

27 6 trace 8 5 2 8 130/13

27 12 0.4 8 5 2 2 100/11

29 12 1.2 8 7 80 6 calm

BK (22 Apr 2003)Geopotential height anomalies at 300 hPa

SF

BK (22 Apr 2003)Geopotential height anomalies at 1000 hPa

SF

BK (5 Jan 2003)Geopotential height anomalies at 300 hPa

SF

BK (5 Jan 2003)Geopotential height anomalies at 1000 hPa

SF

BH (26 Jan 2003)Geopotential heights at 300 hPa

BH (26 Jan 2003)Geopotential height anomalies at 300 hPa

SF

BH (26 Jan 2003)Geopotential heights at 1000 hPa

SFSF

BH (26 Jan 2003)Geopotential height anomalies at 1000 hPa

SF

BH (4 March 2003)Geopotential heights at 300hPa

BH (4 March 2003)Geopotential height anomalies at 300hPa

SF

BH (4 March 2003)Geopotential heights at 1000hPa

BH (4 March 2003)Geopotential height anomalies at 1000hPa

SF

WH-T (2 June 2003)Geopotential heights at 300 hPa

SF

WH-T (2 June 2003)Geopotential height anomalies at 300 hPa

WH-T (2 June 2003)Geopotential heights anomalies at 1000 hPa

SF

SH-R (23 May 2003)Geopotential heights at 300 hPa

SF

SH-R (23 May 2003)Geopotential height anomalies at 300 hPa

SH-R (23 May 2003)Geopotential height anomalies at 1000 hPa

SF

SH-R (23 May 2003)Geopotential heights at 1000 hPa

Hovmoeller DiagramGeopotential heights at 500 hPa

May 2003

SF

pp > 1 mm

Preliminary ConclusionsPreliminary Conclusions The majority of the recorded precipitation events during the The majority of the recorded precipitation events during the

study period (study period (43 43 %) at %) at SFI SFI correspond with low-zonal index correspond with low-zonal index conditions, with quasi-barotropic anticyclonic/cyclonic conditions, with quasi-barotropic anticyclonic/cyclonic anomalies at high/subtropical latitudes (anomalies at high/subtropical latitudes (BKBK) off the chilean ) off the chilean coast. coast.

During tha austral summer, when the subtropical upper During tha austral summer, when the subtropical upper tropospheric circulation is mostly controlled by the South tropospheric circulation is mostly controlled by the South American monsoon (American monsoon (BHBH), precipitation events (), precipitation events (2626 %), seem to %), seem to respond to weak disturbances in the trade- wind regime, respond to weak disturbances in the trade- wind regime, some of them with a clear mid-latitude origin. The latter some of them with a clear mid-latitude origin. The latter become (become (WH-TWH-T) in austral autumn-winter) in austral autumn-winter ( (2323 %) %)

Cases with a strong subtropical anticyclone and a ridge aloft Cases with a strong subtropical anticyclone and a ridge aloft ((SH-RSH-R) probably respond to mesoscale features (POC’s ?)) probably respond to mesoscale features (POC’s ?)((99 %)%). .

Measurable precipitation is observed mainly before noon. Measurable precipitation is observed mainly before noon. Most of it appearMost of it appearss with a mixture of Cu and Sc, with with a mixture of Cu and Sc, withinin a partial a partial cloud cover. cloud cover.

BKBK

BK