Subtropical Southeast Pacific Research Articles

Simulation and future projection of the mixed layer depth and subduction process in the subtropical Southeast Pacific

Simulation and future projection of the mixed layer depth and subduction process in the subtropical Southeast Pacific

Distribution and biodiversity patterns of hyperiid amphipods across the coastal-offshore gradient of the sub-tropical Southeast Pacific

The subtropical region of the Southeast Pacific exhibits a strong onshore-offshore gradient in hydrographic conditions and biological production from the eutrophic upwelling zone to the ultra-oligotrophic oceanic area in the central South Pacific gyre (SPG). Across this gradient, zooplankton must cope with either gradual or abrupt changes in environmental conditions. Here, the distribution and diversity of hyperiid amphipods were assessed over this gradient in the upper 1000 m in relation to temperature, salinity, oxygen, chlorophyll-a, sea level anomalies, the bulk of mesozooplankton biomass and the biomass of salps, siphonophores, and other hydrozoans during October/November 2015 from the coastal zone off Chile (27° 00‵ S, 70° 52‵ W) to near Easter Island within the SPG (27° 10‵ S, 109° 20‵ W). The most frequent and abundant species were Hyperioides longipes Chevreux, 1900, Eupronoe minuta Claus, 1879, and Hyperioides sibaginis (Stebbing, 1888). Significant changes in abundance and community structure across the gradient with respect to the evaluated environmental variables and significant correlations of 17 hyperiid species with the gelatinous category other hydrozoans were found. These changes were closely linked to previously defined zonation patterns, which contained distinct species assemblages and a unique dominant species per zone. These zones represented ecoregions based on diversity patterns of hyperiids and other shared species among such ecoregions suggesting a possible ecological connectivity among the zones, promoted by mesoscale eddies travelling westward from the coastal upwelling zone to offshore waters. Environmentally forced zonation and the interactions with mesoscale features are thus suggested as the driving processes maintaining spatial patterns of diversity of the hyperiid community in the Southeast Pacific.

Cutoff lows off the coast of the Atacama Desert under present day conditions and in the Last Glacial Maximum

Abstract Cutoff lows (COLs) over the subtropical Southeast Pacific may have a distinct impact on the local climate in Northern Chile and in the Atacama Desert. They are associated with generally wetter conditions and also influence the thermally driven wind regimes. In this study, mid-tropospheric COLs in the specific region off the coast of the Atacama Desert north of 30°S are analysed for austral winter (April–September). For the recent past climate slightly more than four COLs per winter on average are detected in ERA-Interim reanalysis (1979–2015). From composite analysis we found that a split jet over the Eastern South Pacific is beneficial for the formation of COLs. Depending on the characteristics of the southern jet streak, wave breaking processes are involved, and the majority of the COL events is associated to anti-cyclonic wave breaking equator-wards. More COLs occur in the Last Glacial Maximum (LGM) compared to present day conditions in two models (MPI-ESM-P and CCSM4) of the Paleoclimate Modeling Intercomparison Project (3rd phase; PMIP3). In both models the processes being involved in the COL formation are depicted realistically. The increasing number of winter COLs coincides with a shift of upper-level zonal winds, which facilitates the formation of the split jet structure over the Eastern South Pacific: The non-uniform global cooling during the LGM due to polar amplification induce a decrease of the subtropical jet and a speed up of upper-level zonal winds in the mid-latitudes of the Southern Hemisphere, and these shifts are particularly strong in the South Pacific region west of South America.

Relationships Between Inversion Strength, Lower‐Tropospheric Moistening, and Low‐Cloud Fraction in the Subtropical Southeast Pacific Derived From Stable Isotopologues of Water Vapor

AbstractQuantitative, observational constraints on lower‐tropospheric mixing are crucial for improved models of low‐cloud feedbacks, and analysis of water vapor isotopic composition provides an independent means for generating such constraints. In situ measurements of water vapor isotopic composition from the Chajnantor Plateau, in northern Chile, are merged with sounding data from Antofagasta and satellite measurements of cloud fraction (CF) from the SE Pacific to show an inverse relationship between the estimated inversion strength (EIS) and water vapor export from the marine boundary layer into the free troposphere. When merged with results from the subtropical northern Pacific, the relationship between EIS and water vapor transport is found to be exponential across EIS values ranging from 0 to 15.6 K. The data from Chile are stratified into terciles of EIS with average EIS values of 9, 12.6 and 15.6 K. We show positive relationships between EIS, cloud fraction, and the mixing diagnostic δDv−δDr and negative relationships between EIS and the observed mixing ratios and water vapor δD values at Chajnantor, all of which are consistent with an inverse relationship between inversion strength and water vapor export from the marine boundary layer. Inverse modeling of the isotopic data using a simple process model shows that the average mixing ratios at Chajnantor derived from the marine boundary layer are estimated to be 2.1, 1.15, and 0.84 g/kg, respectively, for the lowest, middle, and highest terciles of EIS. These results can be used to constrain convective parameterizations and models of low‐cloud feedbacks.

Role of horizontal density advection in seasonal deepening of the mixed layer in the subtropical Southeast Pacific

The mechanisms behind the seasonal deepening of the mixed layer (ML) in the subtropical Southeast Pacific were investigated using the monthly Argo data from 2004 to 2012. The region with a deep ML (more than 175 m) was found in the region of (22°–30°S, 105°–90°W), reaching its maximum depth (~200 m) near (27°–28°S, 100°W) in September. The relative importance of horizontal density advection in determining the maximum ML location is discussed qualitatively. Downward Ekman pumping is key to determining the eastern boundary of the deep ML region. In addition, zonal density advection by the subtropical countercurrent (STCC) in the subtropical Southwest Pacific determines its western boundary, by carrying lighter water to strengthen the stratification and form a “shallow tongue” of ML depth to block the westward extension of the deep ML in the STCC region. The temperature advection by the STCC is the main source for large heat loss from the subtropical Southwest Pacific. Finally, the combined effect of net surface heat flux and meridional density advection by the subtropical gyre determines the northern and southern boundaries of the deep ML region: the ocean heat loss at the surface gradually increases from 22?S to 35?S, while the meridional density advection by the subtropical gyre strengthens the stratification south of the maximum ML depth and weakens the stratification to the north. The freshwater flux contribution to deepening the ML during austral winter is limited. The results are useful for understanding the role of ocean dynamics in the ML formation in the subtropical Southeast Pacific.

The PreVOCA experiment: modeling the lower troposphere in the Southeast Pacific

Abstract. The Preliminary VOCALS Model Assessment (PreVOCA) aims to assess contemporary atmospheric modeling of the subtropical South East Pacific, with a particular focus on the clouds and the marine boundary layer (MBL). Models results from fourteen modeling centers were collected including operational forecast models, regional models, and global climate models for the month of October 2006. Forecast models and global climate models produced daily forecasts, while most regional models were run continuously during the study period, initialized and forced at the boundaries with global model analyses. Results are compared in the region from 40° S to the equator and from 110° W to 70° W, corresponding to the Pacific coast of South America. Mean-monthly model surface winds agree well with QuikSCAT observed winds and models agree fairly well on mean weak large-scale subsidence in the region next to the coast. However they have greatly differing geographic patterns of mean cloud fraction with only a few models agreeing well with MODIS observations. Most models also underestimate the MBL depth by several hundred meters in the eastern part of the study region. The diurnal cycle of liquid water path is underestimated by most models at the 85° W 20° S stratus buoy site compared with satellite, consistent with previous modeling studies. The low cloud fraction is also underestimated during all parts of the diurnal cycle compared to surface-based climatologies. Most models qualitatively capture the MBL deepening around 15 October 2006 at the stratus buoy, associated with colder air at 700 hPa.

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

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.

Marine boundary layer over the subtropical southeast Pacific during VOCALS-REx – Part 1: Mean structure and diurnal cycle

Abstract. Atmospheric subsidence over the subtropical southeast Pacific (SEP) leads to a low-level anticyclonic circulation, a cool sea surface and a cloud-topped marine boundary layer (MBL). Observations in this region from a major field campaign during October and November 2008, the VOCALS Regional Experiment, provide ample data to characterize the lower atmospheric features over the SEP. The observations are also useful to test the ability of an area-limited, high-resolution atmospheric model to simulate the SEP conditions. Observations and model-results (where appropriate) improve the characterization of the mean state (Part 1) and variability (Part 2) of the lower troposphere including circulation, MBL characteristics and the upsidence wave. Along 20° S the MBL is generally deeper offshore (1600 m at 85° W) but there is also considerable variability. MBL depth and variability decrease towards the coast and maximum inversion strength is detected between 74–76° W. Weather Research and Forecasting (WRF) simulations underestimate MBL height the most near the coast but improve offshore. Southeasterly trades prevail within the MBL although the wind speed decreases toward the coast. Above the MBL along the coast of Chile, flow is northerly, has a maximum at 3 km, and extends westward to ~74° W, apparently due to the mechanical blocking exerted by the Andes upon the westerly flow aloft. Mean MBL features along northern Chile (18–25° S) are remarkably similar (e.g., MBL depth just below 1 km) in spite of different SST. Observed diurnal cycles of the temperature at the coast and further offshore exhibit a number of conspicuous features that are consistent with the southwestward propagation of an upsidence wave initiated during late evening along the south Peru coast. Furthermore, the passage of the vertical motion results in either constructive or deconstructive interference with the radiatively-forced diurnal cycle of MBL depth. Interference is clearly seen in the soundings at Iquique which are driven by a strong upsidence wave contrary to the radiation-driven cycle, leading to a diurnal cycle opposite of the other sites. Because WRF simulations have a lower MBL height, the speed of the simulated gravity wave is slower than observations and accounts for most of the discrepancy between observed and simulated phase speeds.

Diurnal cycle of surface winds over the subtropical southeast Pacific

The diurnal variability of surface winds over the SE Pacific is characterized with QuikSCAT data for years 2000–2006. The twice daily satellite passes occur around 0600 and 1800 LT (referred to as AM and PM, respectively). The PM‐AM wind difference maximizes in a region between 20°S and 30°S, extending from the Chilean coast up to 75°W. This difference is mostly meridional (along the coast) with seasonal variation characterized by summertime weakening of the morning winds. The observational analysis is supplemented with mesoscale model results for December 2003. They show that the coastal zonal flow increases between 0600 and 1200 local time (LT) but then decreases in the next 6 hours, which explains the absence of an onshore component in the QuikSCAT PM‐AM difference. Additional model diagnostics show that in this region the diurnal cycle hodographs are more simple and larger above the marine boundary layer (MBL) than at the surface. Evaluation of the momentum and temperature budgets reveals the importance of the diurnal cycle of vertical velocity coupled with the thermal structure in controlling the diurnal cycle of the low‐level winds. Around 1300 LT coastal subsidence maximizes in a layer sloping from 1000 m at 36°S to 3000 m at 22°S, inducing the development of a coastal trough at the surface. During the night the passage of a high‐pressure perturbation reverses the surface meridional pressure gradient force, inducing the decrease of the coastal meridional winds. These results stress the important role played by the dynamics of the low troposphere above the MBL (e.g., the heating of the Andes Cordillera to the east) in controlling the wind diurnal cycles near the surface.

The Influence of the Andes on Cutoff Lows: A Modeling Study*

Abstract A cutoff low (COL) pressure system that occurred in March 2005 (late austral summer) over the subtropical southeast Pacific is examined by means of numerical simulations using the Weather and Research Forecasting (WRF) model. The episode exhibited typical features of COLs in this region, including its formation from an elongated northwest–southeast extratropical trough and subsequent intensification off the west coast of South America. During the developing stage, the cyclonic circulation did not extend into the lower troposphere and only upper-level, nonprecipitating clouds were observed at and around the system. When the COL reached the continent it produced moderate but unseasonal rainfall along the semiarid western slope of the Andes cordillera [summit level at ∼5000 m above sea level (ASL)] at the same time that the system experienced a rapid decay. The control simulation used full physics, full topography, and a single domain (54-km grid spacing) laterally forced by atmospheric reanalysis. Model results are in general agreement with upper-air, surface, and satellite observations, and allow a detailed description of the three-dimensional structure of the COL, as well as an evaluation of the vorticity and temperature budgets. A quasi-stationary, amplifying warm ridge over the South Pacific appears as the key precursor feature, in agreement with studies elsewhere. Once the COL formed, it drifted eastward mostly driven by vorticity advection induced by its own circulation, and there was close balance between vertical and horizontal temperature advection near its center. The jet streak along the COL’s periphery migrated from upstream of the COL axis, during the developing stage, to downstream later on. Four sensitivity experiments—reducing/removing topography, suppressing hydrometeors, and using an enlarged domain—were performed to assess the influence of the Andes, the importance of latent heat release, and the effect of the boundary conditions. Comparison among the control and sensitivity runs indicates that the COL formation occurs regardless of the presence of the Andes, and COL dissipation is mainly due to latent heat released in the deep clouds that form over the mountainous terrain. Nevertheless, the Andes cordillera delayed the COL demise by blocking the inflow of warm, moist air from the interior of the continent that otherwise would initiate deep convection in the region of ascending motion downstream of the COL.

Variability of the tropical and subtropical ocean surface latent heat flux during 1989–2000

We assess the consistencies and discrepancies of the interannual variability and decadal trend of the ocean surface latent heat flux (LHF) for the period 1989–2000 among two satellite‐based data sets and two reanalysis products over the tropics and subtropics. The data sets show encouraging agreement on the spatial/temporal variations of the LHF, particularly during 1992–1997, but some discrepancies are noticed. These discrepancies are primarily caused by the differences of the input meteorological state variables used to calculate LHF, particularly for the air specific humidity. Both the satellite‐based data sets and reanalysis products show a statistically significant decadal‐scale positive trend in the LHF, which is independent of the El Niño‐Southern Oscillation, with the strongest trends in the subtropical Southeast Pacific and Southeast Indian Ocean. This trend in the LHF is associated primarily with an increasing trend of the surface wind speed.

Subseasonal Variability of the Southeast Pacific Stratus Cloud Deck*

Abstract Subseasonal variability of the stratus/stratocumulus cloud deck over the subtropical southeast Pacific is studied using satellite and buoy observations as well as the NCEP–NCAR reanalysis. It is found that subseasonal variability in the stratus cloud deck is closely related to variations in surface wind velocity, water vapor, sea level pressure, and 500-hPa geopotential height. An increase in cloud liquid water (CLW) over the subtropical southeast Pacific is found to be associated with the development of an anomalous anticyclonic circulation to the south off the west coast of Chile. The enhanced southerly to southeasterly winds advect cold and dry air into the stratus region against the mean sea surface temperature (SST) gradient. This cold and dry advection, together with increased wind speed, intensifies surface latent and sensible heat fluxes and destabilizes the boundary layer. Anomalous offshore easterlies north of the anomalous anticyclone cause a low-level divergence. The associated subsidence warming, together with the cold advection in the surface layer, strengthens the temperature inversion, conducive to the development of stratus clouds. Buoy observations confirm this subseasonal cloud variability and its relationship with surface meteorological variables. A lead/lag composite analysis indicates that circulation variables such as sea level pressure and surface wind lead cloud liquid water by 1–2 days while SST lags CLW by 1–2 days, suggesting that low-cloud variability is caused by atmospheric circulation changes rather than by the underlying ocean. The dynamic adjustment that leads to cloud fluctuations and possible orographic effects of the Andes are also discussed.

The Diurnal Cycle in Circulation and Cloudiness over the Subtropical Southeast Pacific: A Modeling Study

Abstract The extensive and persistent deck of stratocumulus (Sc) over the subtropical southeast Pacific (SSEP) plays an important role in the regional and global climate. As in other subtropical regions, the Sc form at the top of a marine boundary layer (MBL), capped by the subsidence inversion. A distinctive feature of this subtropical deck is its pronounced dawn-to-afternoon decrease in cloud amount and liquid water path, partially associated with a regular and marked descent of the inversion base and the warming of the lower troposphere. Furthermore, coastal observations in this area reveal a diurnal cycle in air temperature encompassing up to 5 km MSL. In this work, 15-day regional numerical simulations using the fifth-generation PSU–NCAR Mesoscale Model (MM5) in November (austral spring), May (late fall), and January (summer) 2001 were used to document the mean diurnal cycle in circulation and low-level cloudiness over the SSEP. The simulated amplitude, depth, and phase of the diurnal cycle in air te...

CIMAR-5: A Snapshot of the Lower Troposphere over the Subtropical Southeast Pacific

The extensive and persistent deck of stratocumulus (SCu) off the west coast of subtropical South America plays an important role in the regional and global climate, as well as in coastal weather. As in other subtropical regions, the SCu form at the top of a marine boundary layer (MBL) bounded by a relatively cold ocean and a large-scale subsidence inversion. Nevertheless, details of the structure and variability of the lower troposphere over this region are largely unknown. Ship-based meteorological observations taken along a transect at 27°S from the Chilean coast (71°W) to Easter Island (110°W) during the second half of October 1999 provide a preliminary description of the low-level circulation, thermodynamic structure, and cloudiness over the subtropical southeast Pacific. Three types of observations were made: (a) 15-min average of air temperature, relative humidity, solar radiation, pressure, and wind by an automatic weather station on the ship deck; (b) 15-min average of vertical reflectivity and cloud base by a laser ceilometer on the ship deck; and (c) twice-daily rawinsondes. Several cloud and radiation properties were derived from GOES-8 imagery and validated against the ship-based observations. A preliminary assessment of the ability of NCEP–NCAR reanalysis and scatterometer winds in representing the atmosphere over a largely in situ data-void area is also presented. Sea surface temperature and near-surface air temperature increase gradually westward, while near-surface relative humidity remains nearly constant at ~80%. A significant increase in the free-tropospheric moisture indicates an offshore decrease in the large-scale subsidence. Consistently, the MBL evolves from a shallow, well-mixed MBL topped by compact SCu near the coast; to a deeper, decoupled MBL with a cumuli rising into a patchy SCu deck near Easter Island, in a similar fashion to the transition from subtropical-stratus regime to trade-cumulus regime described elsewhere. In addition to these “climatological” features, the ship data also reveal the large sensitivity of the MBL-trade inversion structure to synoptic-scale disturbances over the subtropical Pacific. Cloud droplet effective sizes increase from the coast to open ocean. Furthermore, cloud fraction, cloud-top height, liquid water path, and optical depth all peaked during the morning and reached a minimum by midafternoon.

CIMAR–5: A Snapshot of the Lower Troposphere over the Subtropical Southeast Pacific

CIMAR–5: A Snapshot of the Lower Troposphere over the Subtropical Southeast Pacific