An ongoing cooling in the eastern Pacific linked to eastward migrations of the Southeast Pacific Subtropical Anticyclone

Abstract A regional model is used to study the radiative effect of boundary layer clouds over the southeast Pacific on large-scale atmosphere circulation during August–October 1999. With the standard settings, the model simulates reasonably well the large-scale circulation over the eastern Pacific, precipitation in the intertropical convergence zone (ITCZ) north of the equator, and marine boundary layer stratocumulus clouds to the south. In a sensitivity experiment with the radiative effect of liquid clouds south of the equator over the eastern Pacific artificially removed, boundary layer clouds south of the equator almost disappear and precipitation in the ITCZ is reduced by 15%–20%, indicating that the stratocumulus clouds over the southeast Pacific have both local and cross-equatorial effects. Examination of the differences between the control and sensitivity experiments indicates that clouds exert a net diabatic cooling in the inversion layer. In response to this cloud-induced cooling, an in situ anomalous high pressure system develops in the boundary layer and an anomalous shallow meridional circulation develops in the lower troposphere over the equatorial eastern Pacific. At the lower branch of this shallow circulation, anomalous boundary layer southerlies blow from the boundary layer high toward the northern ITCZ where the air ascends. An anomalous returning flow (northerly) just above the cloud layer closes the shallow circulation. This low-level anomalous shallow circulation enhances the subsidence over the southeast Pacific above the cloud layer, helping to maintain boundary layer clouds and temperature inversion there. Meanwhile, the strengthened cross-equatorial flow near the surface enhances moisture convergence and convection in the ITCZ north of the equator. This in turn strengthens the local, deep Hadley circulation and hence the large-scale subsidence and boundary layer clouds over the southeast Pacific. This positive feedback therefore enhances the interhemispheric climate asymmetry over the tropical eastern Pacific.

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Marine boundary layer over the subtropical southeast Pacific during VOCALS-REx – Part 2: Synoptic variability

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-9-26063-2009 ◽

2009 ◽

Vol 9 (6) ◽

pp. 26063-26094 ◽

Cited By ~ 3

Author(s):

D. A. Rahn ◽

R. D. Garreaud

Keyword(s):

Boundary Layer ◽

Vertical Velocity ◽

Large Scale ◽

Marine Boundary Layer ◽

Subtropical Anticyclone ◽

Easterly Wind ◽

Entrainment Velocity ◽

Synoptic Variability ◽

Synoptic Forcing ◽

Southeast Pacific

Abstract. In the second part of this work we study the day-to-day variability of the marine atmospheric boundary layer (MBL) over the subtropical southeast Pacific using primarily results from a numerical simulation that covered the whole VOCALS-REx period (October–November 2008). In situ and satellite-derived observations of the MBL height in the offshore region indicate rapid, significant variations (from 500 m to 1700 m a.s.l. over a few days) during October. These MBL changes are connected with the passage of midlatitude troughs that altered the large-scale environment over the VOCALS-REx region. In contrast, the synoptic forcing and MBL changes were less prominent during November. Modelled and observed MBL depth at Point Omega (20° S, 85° W) compare quite well during October (but the simulation is on average 200 m lower) while in November the simulation does not perform as well. Each term in the prognostic local MBL height equation (horizontal MBL height advection, large scale vertical velocity at MBL top, and entrainment velocity) is calculated explicitly from the simulation except the entrainment velocity which is calculated as the residual of the other terms in the equation. While the vertical velocity and residual terms are opposing and generally have the largest magnitude on average, it is the variability in the advection that explains most of the large changes in the MBL depth. Examination of several cases during VOCALS-REx suggests that the advective term is in turn largely controlled by changes in wind direction, driven by midlatitude activity, acting on a MBL that generally slopes down toward the coast. In one phase, the subtropical anticyclone is reinforced and extends toward the Chilean coast, leading to easterly wind that advects low MBL heights from the coast as far as Point Omega. The opposite phase occurs after the passage of an extratropical cyclone over southern Chile, leading to southwesterly wind that advects a deeper MBL towards subtropical latitudes.

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Microphysical variability in southeast Pacific Stratocumulus clouds: synoptic conditions and radiative response

Atmospheric Chemistry and Physics ◽

10.5194/acp-10-6255-2010 ◽

2010 ◽

Vol 10 (13) ◽

pp. 6255-6269 ◽

Cited By ~ 43

Author(s):

D. Painemal ◽

P. Zuidema

Keyword(s):

Boundary Layer ◽

Cloud Droplet ◽

Static Stability ◽

Radiative Properties ◽

Free Troposphere ◽

Subtropical Anticyclone ◽

Lower Cloud ◽

Synoptic Conditions ◽

Southeast Pacific ◽

Stratocumulus Clouds

Abstract. Synoptic and satellite-derived cloud property variations for the southeast Pacific stratocumulus region associated with changes in coastal satellite-derived cloud droplet number concentrations (Nd) are explored. MAX and MIN Nd composites are defined by the top and bottom terciles of daily area-mean Nd values over the Arica Bight, the region with the largest mean oceanic Nd, for the five October months of 2001, 2005, 2006, 2007 and 2008. The ability of the satellite retrievals to capture composite differences is assessed with ship-based data. Nd and ship-based accumulation mode aerosol concentrations (Na) correlate well (r = 0.65), with a best-fit aerosol activation value dln Nddln Na of 0.56 for pixels with Nd>50 cm−3. The adiabatically-derived MODIS cloud depths also correlate well with the ship-based cloud depths (r=0.7), though are consistently higher (mean bias of almost 60 m). The MAX-Nd composite is characterized by a weaker subtropical anticyclone and weaker winds both at the surface and the lower free troposphere than the MIN-Nd composite. The MAX-Nd composite clouds over the Arica Bight are thinner than the MIN-Nd composite clouds, have lower cloud tops, lower near-coastal cloud albedos, and occur below warmer and drier free tropospheres (as deduced from radiosondes and NCEP Reanalysis). CloudSat radar reflectivities indicate little near-coastal precipitation. The co-occurrence of more boundary-layer aerosol/higher Nd within a more stable atmosphere suggests a boundary layer source for the aerosol, rather than the free troposphere. The MAX-Nd composite cloud thinning extends offshore to 80° W, with lower cloud top heights out to 95° W. At 85° W, the top-of-atmosphere shortwave fluxes are significantly higher (~50%) for the MAX-Nd composite, with thicker, lower clouds and higher cloud fractions than for the MIN-Nd composite. The change in Nd at this location is small (though positive), suggesting that the MAX-MIN Nd composite differences in radiative properties primarily reflects synoptic changes. Circulation anomalies and a one-point spatial correlation map reveal a weakening of the 850 hPa southerly winds decreases the free tropospheric cold temperature advection. The resulting increase in the static stability along 85° W is highly correlated to the increased cloud fraction, despite accompanying weaker free tropospheric subsidence.

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Variability of the Southeast Pacific Subtropical Anticyclone and its impact on sea surface temperature off north-central Chile

Ciencias Marinas ◽

10.7773/cm.v41i1.2338 ◽

2015 ◽

Vol 41 (1) ◽

pp. 1-20 ◽

Cited By ~ 25

Author(s):

Santiago Ancapichun

Keyword(s):

Surface Temperature ◽

Sea Surface Temperature ◽

Sea Surface ◽

Subtropical Anticyclone ◽

North Central ◽

Central Chile ◽

Southeast Pacific

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Marine boundary layer over the subtropical southeast Pacific during VOCALS-REx – Part 2: Synoptic variability

Atmospheric Chemistry and Physics ◽

10.5194/acp-10-4507-2010 ◽

2010 ◽

Vol 10 (10) ◽

pp. 4507-4519 ◽

Cited By ~ 44

Author(s):

D. A. Rahn ◽

R. Garreaud

Keyword(s):

Boundary Layer ◽

Vertical Velocity ◽

Large Scale ◽

Marine Boundary Layer ◽

Subtropical Anticyclone ◽

Easterly Wind ◽

Entrainment Velocity ◽

Synoptic Variability ◽

Southeast Pacific ◽

Height Change

Abstract. In the second part of this work we study the day-to-day variability of the marine atmospheric boundary layer (MBL) over the subtropical southeast Pacific using primarily results from a numerical simulation that covered the whole VOCALS-REx period (October–November 2008). In situ and satellite-derived observations of the MBL height in the offshore region indicate rapid, significant variations (from 500 m to 1700 m a.s.l. over a few days) during October. These MBL changes are connected with the passage of midlatitude troughs that altered the large-scale environment over the VOCALS-REx region. In contrast, the synoptic forcing and MBL changes were less prominent during November. Modelled and observed MBL depth at Point Omega (20° S, 85° W) compare quite well during October (but the simulation is on average 200 m lower) while in November the simulation does not perform as well. In the prognostic local MBL height equation the height change, the horizontal MBL height advection, and the large scale vertical velocity at MBL top are calculated explicitly from the simulation. The entrainment velocity is calculated as the residual of the other terms in the equation. While the vertical velocity and residual terms are opposing and generally have the largest magnitude on average, it is the variability in the advection that explains most of the large changes in the MBL depth. Examination of several cases during VOCALS-REx suggests that the advective term is in turn largely controlled by changes in wind direction, driven by midlatitude activity, acting on a MBL that generally slopes down toward the coast. In one phase, the subtropical anticyclone is reinforced and extends toward the Chilean coast, leading to easterly wind that advects low MBL heights from the coast as far as Point Omega. The opposite phase occurs after the passage of an extratropical cyclone over southern Chile, leading to southwesterly wind that advects a deeper MBL towards subtropical latitudes.

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The Low-Level Jet off the West Coast of Subtropical South America: Structure and Variability

Monthly Weather Review ◽

10.1175/mwr2972.1 ◽

2005 ◽

Vol 133 (8) ◽

pp. 2246-2261 ◽

Cited By ~ 131

Author(s):

RenéD. Garreaud ◽

Ricardo C. Muñoz

Keyword(s):

Wind Speed ◽

South America ◽

West Coast ◽

Inversion Layer ◽

Surface Winds ◽

The West ◽

Subtropical Anticyclone ◽

Central Chile ◽

Low Level ◽

Southeast Pacific

Abstract The subtropical anticyclone over the southeast Pacific drives low-level southerly flow along the west coast of South America. In turn, the alongshore flow induces coastal upwelling that supports a wealth of fishery resources. Within this region, satellite data, marine reports, and coastal observations indicate the existence of a southerly coastal jet (i.e., a maximum of wind speed) off central Chile (26°–36°S). The mean features and variability of this southerly jet is documented in this work using 4 yr of satellite-derived sea surface winds, complemented by satellite-derived cloud amount fields and atmospheric reanalysis. Furthermore, analysis of in situ data and model results of a well-defined jet event during October 2000 allows a preliminary description of the jet’s three-dimensional structure and a comparison with the northerly jet off the coast of California. Southerly jet events off central Chile occur year-round, but they are more frequent during spring–summer (over 60% of the time). The jet is characterized by an elongated maximum of surface wind speed (∼10 m s−1) with its axis at about 150 km off the coast and a cross-shore scale of about 500 km. The two Quick Scatterometer (QuikSCAT) fields per day (a.m. and p.m. passes) allow a rough estimate of the amplitude of the diurnal cycle of the surface winds, which appears to be remarkably small in the region of the jet. The jet events are associated with the passage of a midlatitude ridge over the southeast Pacific strengthening the subtropical anticyclone. Upstream and over the jet region the coastal deck of stratocumulus clouds tends to dissipate in contrast to an increase in cloudiness downstream of the jet. In the case study the jet core resides at the top of the marine boundary layer (MBL)/inversion layer. Weak offshore flow prevails above the jet axis, and even weaker onshore flow prevails in the MBL. Consistent with its subtropical location the jet is embedded in a region of large-scale subsidence; nevertheless a mesoscale area of mean upward motion is simulated just downstream of the jet core.

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Occurrence and biogeographic significance of Heliaster (Echinodermata: Asteroidea) from the Pliocene of southwest Florida

Journal of Paleontology ◽

10.1017/s0022336000058960 ◽

1988 ◽

Vol 62 (01) ◽

pp. 126-132 ◽

Cited By ~ 8

Author(s):

Douglas S. Jones ◽

Roger W. Portell

Keyword(s):

Central American ◽

Eastern Pacific ◽

Whole Body ◽

Marine Fauna ◽

Western Atlantic ◽

Southwest Florida ◽

Extant Species ◽

Geologic Record ◽

Central American Isthmus

Whole body asteroid fossils are rare in the geologic record and previously unreported from the Cenozoic of Florida. However, specimens of the extant species,Heliaster microbrachiusXantus, were recently discovered in upper Pliocene deposits. This marks the first reported fossil occurrence of the monogeneric Heliasteridae, a group today confined to the eastern Pacific. This discovery provides further non-molluscan evidence of the close similarities between the Neogene marine fauna of Florida and the modern fauna of the eastern Pacific. The extinction of the heliasters in the western Atlantic is consistent with the pattern of many other marine groups in the region which suffered impoverishment following uplift of the Central American isthmus.

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