C. Mechoso

University of California, Los Angeles, Atmospheric & Oceanic Sciences, Faculty Member

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Papers by C. Mechoso

Climate Dynamics �2001) 18: 255±276 Ó Springer-Verlag 2001

An ensemble of twenty four coupled oceanatmosphere models has been compared with respect to their... more An ensemble of twenty four coupled oceanatmosphere models has been compared with respect to their performance in the tropical Paci®c. The coupled models span a large portion of the parameter space and di€er in many respects. The intercomparison includes TOGA �Tropical Ocean Global Atmosphere)-type models consisting of high-resolution tropical ocean models and coarse-resolution global atmosphere models, coarse-resolution global coupled models, and a few global coupled models with high resolution in the equatorial region in their ocean components. The performance of the annual mean state, the seasonal cycle and the interannual variability are investigated. The primary quantity analysed is sea surface temperature �SST). Additionally, the evolution of interannual heat

Additional contributions to CMIP5 regional sea level projections resulting from Greenland and Antarctic ice mass loss

Environmental Research Letters, 2015

The impact of Greenland and Antarctic ice sheet mass loss on regional sea level is evaluated here... more The impact of Greenland and Antarctic ice sheet mass loss on regional sea level is evaluated here under RCP 4.5 and RCP 8.5 scenarios for the period 2081-2099. To this end, estimates of associated fresh water sources are added to the Max Planck Institute for Meteorology s Earth System Model (MPIESM) ocean component and the dynamical impact is quantified in terms of the difference in sea level relative to previous CMIP5 runs. Overall, the addition of these freshwater sources have only a small impact on regional sea level variations relative to the global mean (<2cm in magnitude). However, in some regions, notably in the North Atlantic and Arctic Ocean, an additional increase in regional steric sea level by 4-8 cm can be obtained, which is ∼20% more than the previous climate model response. Climate feedbacks can have additional sea level impacts regionally, e.g., through changes in the wind forcing or surface freshwater fluxes. Overall, the dynamical regional sea level response to the polar ice mass loss is of the same order as the simulated decadal sea level variability.

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On the Early Response of the Climate System to a Meltwater Input from Greenland

Journal of Climate, 2014

The early response of the atmosphere–ocean system to meltwater runoff originating from the Greenl... more The early response of the atmosphere–ocean system to meltwater runoff originating from the Greenland ice sheet is studied using a coupled atmosphere–ocean general circulation model (AOGCM). For this purpose, AOGCM ensemble simulations without and with associated ocean freshening around Greenland are compared. For freshwater perturbations initiated in northern winter, the mean response for the first three months shows the emergence of negative sea surface temperature (SST) anomalies in the Denmark Strait, in association with enhanced oceanic advection by the East Greenland Current. The response also shows negative SST anomalies in the North Atlantic associated with enhanced westerlies at the ocean surface. Additionally, the baroclinic atmospheric cyclonic circulation east of Greenland intensifies, and anticyclonic circulations with equivalent barotropic structures develop over western Europe and the North Pacific Ocean. Simulations by the atmospheric component of the AOGCM indicate t...

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Variability and Predictability of West African Droughts: A Review on the Role of Sea Surface Temperature Anomalies

Journal of Climate, 2015

The Sahel experienced a severe drought during the 1970s and 1980s after wet periods in the 1950s ... more The Sahel experienced a severe drought during the 1970s and 1980s after wet periods in the 1950s and 1960s. Although rainfall partially recovered since the 1990s, the drought had devastating impacts on society. Most studies agree that this dry period resulted primarily from remote effects of sea surface temperature (SST) anomalies amplified by local land surface–atmosphere interactions. This paper reviews advances made during the last decade to better understand the impact of global SST variability on West African rainfall at interannual to decadal time scales. At interannual time scales, a warming of the equatorial Atlantic and Pacific/Indian Oceans results in rainfall reduction over the Sahel, and positive SST anomalies over the Mediterranean Sea tend to be associated with increased rainfall. At decadal time scales, warming over the tropics leads to drought over the Sahel, whereas warming over the North Atlantic promotes increased rainfall. Prediction systems have evolved from sea...

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Routes of Transport across the Antarctic Polar Vortex in the Southern Spring

Journal of the Atmospheric Sciences, 2012

Transport in the lower stratosphere over Antarctica has been studied in the past by means of seve... more Transport in the lower stratosphere over Antarctica has been studied in the past by means of several approaches, such as contour dynamics or Lyapunov exponents. This paper examines the problem by means of a new Lagrangian descriptor, which is referred to as the function M. The focus is on the southern spring of 2005, which allows for a comparison with previous analyses based on Lyapunov exponents. With the methodology based on the function M, a much sharper depiction of key Lagrangian features is achieved and routes of large-scale horizontal transport across the vortex edge are captured. These results highlight the importance of lobe dynamics as a transport mechanism across the Antarctic polar vortex.

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Isentropic Transport within the Antarctic Polar-Night Vortex: Rossby Wave Breaking Evidence and Lagrangian Structures

Journal of the Atmospheric Sciences, 2013

The trajectories in the lower stratosphere of isopycnic balloons released from Antarctica by Vorc... more The trajectories in the lower stratosphere of isopycnic balloons released from Antarctica by Vorcore and Concordiasi field campaigns during the southern springs of 2005 and 2010 showed events of latitudinal transport inside the stratospheric polar vortex, both away from and toward the poleward flank of the polar-night jet. The present paper applies trajectory-based diagnostic techniques to examine mechanisms at work during such events. Reverse domain-filling calculations of potential vorticity (PV) fields from the ECMWF Interim Re-Analysis (ERA-Interim) dataset during the events show irreversible filamentation of the PV fields in the inner side of the polar-night jet, which is a signature of planetary (Rossby) wave breaking. Balloon motions during the events are fairly consistent with the PV filaments. Events of both large (~15° of arc length) and small (~5° of arc length) balloon displacements from the vortex edge are associated, respectively, with deep and shallow penetration into...

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Relationships between Precipitation Anomalies in Uruguay and Southern Brazil and Sea Surface Temperature in the Pacific and Atlantic Oceans

Journal of Climate, 1998

This study focuses on precipitation in Uruguay and the Brazilian state of Rio Grande do Sul, whic... more This study focuses on precipitation in Uruguay and the Brazilian state of Rio Grande do Sul, which extend along the Atlantic coast of southern South America. The present paper has two principal goals: 1) to describe the annual cycle of precipitation and 2) to investigate the relationships between its anomalies and those in sea surface temperature (SST) in the Pacific and Atlantic oceans. The dataset is provided by 40 rainfall stations almost evenly distributed in space and covers the period 1917-80. The tools used in support of this research include principal component and canonical correlation analyses. It is found that total precipitation tends to be evenly distributed during the year. The largest spatial variability in the monthly deviations from the annual mean appears as a west-east (inland-coastal) dipole with the largest positive values in the west during early fall and midspring, and in the east along the Atlantic coast during winter. The second mode of rainfall variability appears as a north-south dipole with the largest positive values in the south during late summer and late fall, and in the north during early spring and early summer. The third mode appears primarily as a north-south dipole along the western boundary with the largest positive values in the southwest during fall and in the northwest during early spring. These modes explain 60%, 19%, and 8% of the total variance. Five subregions are identified according to similarities between the characteristics of the annual cycles in their rainfall stations. It is shown that there are significant relationships between anomalies in rainfall and in SST in the Pacific and Atlantic oceans. Some of these relationships confirm the results of previous studies, such as the links between the El Niño-Southern Oscillation phenomenon in the equatorial Pacific Ocean and rainfall anomalies in Uruguay during late spring-early summer and late fall-early winter. Other relationships have not been reported before, such as the links between SST anomalies in the southwestern Atlantic Ocean and rainfall anomalies in the entire region during October-December and April-July. It is also found that when SST anomalies are considered in both oceans simultaneously, their links with rainfall anomalies are in some cases enhanced and in others weakened.

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Role of Land Surface Processes in South American Monsoon Development

Journal of Climate, 2006

This study explores the role of vegetation biophysical processes (VBPs) in the structure and evol... more This study explores the role of vegetation biophysical processes (VBPs) in the structure and evolution of the South American monsoon system (SAMS) with an emphasis on the precipitation field. The approach is based on comparing ensemble simulations by the National Centers for Environmental Prediction general circulation model (GCM) in which the land surface parameterization in one ensemble includes an explicit representation of vegetation processes in the calculation of surface fluxes while the other does not [GCM/Simplified Simple Biosphere Model (SSiB) and GCM/Soil, respectively], but with similar monthly mean surface albedo and initial soil moisture. The ensembles consist of five pairs of 1-yr integrations differing in the initial conditions for the atmosphere. The results show that, during the austral summer, consideration of explicit vegetation processes does not alter the monthly mean precipitation at the planetary scale. However, at continental scales, GCM/SSiB produces a more...

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The GCM–Reality Intercomparison Project for SPARC (GRIPS): Scientific Issues and Initial Results

Bulletin of the American Meteorological Society, 2000

To investigate the effects of the middle atmosphere on climate, the World Climate Research Progra... more To investigate the effects of the middle atmosphere on climate, the World Climate Research Programme is supporting the project "Stratospheric Processes and their Role in Climate" (SPARC). A central theme of SPARC, to examine model simulations of the coupled troposphere-middle atmosphere system, is being performed through the initiative called GRIPS (GCM-Reality Intercomparison Project for SPARC). In this paper, an overview of the objectives of GRIPS is given. Initial activities include an assessment of the performance of middle atmosphere climate models, and preliminary results from this evaluation are presented here. It is shown that although all 13 models evaluated represent most major features of the mean atmospheric state, there are deficiencies in the magnitude and location of the features, which cannot easily be traced to the formulation (resolution or the parameterizations included) of the models. Most models show a cold bias in all locations, apart from the tropical tropopause region where they can be either too warm or too cold. The strengths and locations of the major jets are often misrepresented in the models. Looking at three-dimensional fields reveals, for some models, more severe deficiencies in the magnitude and positioning of the dominant structures (such as the Aleutian high in the stratosphere), although undersampling might explain some of these differences from observations. All the models have shortcomings in their simulations of the present-day climate, which might limit the accuracy of predictions of the climate response to ozone change and other anomalous forcing.

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STOIC: a study of coupled model climatology and variability in tropical ocean regions

Climate Dynamics, 2002

We describe the behaviour of 23 dynamical ocean-atmosphere models, in the context of comparison w... more We describe the behaviour of 23 dynamical ocean-atmosphere models, in the context of comparison with observations in a common framework. Fields of tropical sea surface temperature (SST), surface wind stress and upper ocean vertically averaged temperature (VAT) are assessed with regard to annual mean, seasonal cycle, and interannual variability characteristics. Of the participating models, 21 are coupled GCMs, of which 13 use no form of¯ux adjustment in the tropics. The models vary widely in design, components and purpose: nevertheless several common features are apparent. In most models without¯ux adjustment, the annual mean equatorial SST in the central Paci®c is too cool and the Atlantic zonal SST gradient has the wrong sign. Annual mean wind stress is often too weak in the central Paci®c and in the Atlantic, but too strong in the west Paci®c. Few models have an upper ocean VAT seasonal cycle like that observed in the equatorial Paci®c. Interannual variability is commonly too weak in the models: in particular, wind stress variability is low in the equatorial Paci®c. Most models have diculty in reproducing the observed Paci®c horseshoe' pattern of negative SST correlations with interannual NinÄ o3 SST anomalies, or the observed Indian-Paci®c lag correlations. The results for the ®elds examined

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The Atmospheric Circulation Around Antarctica: Linear Stability and Finite-Amplitude Interactions with Migrating Cyclones

Journal of the Atmospheric Sciences, 1980

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A multivariate assessment of climate change projections over South America using the fifth phase of the Coupled Model Intercomparison Project

International Journal of Climatology

Weaker AMOC in coupled models inhibits north tropical Atlantic impact on ENSO

&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;... more &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;p align=&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;justify&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;span&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;Observational studies have reported that interannual variability of sea surface temperature in two tropical Atlantic regions can act as ENSO predictors in different seasons and periods: boreal summer Atlantic Nino (AN) in negative phases of the Atlantic Multidecadal Variabil- &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#732; ity (AMV); and boreal spring tropical north Atlantic (TNA) in positive AMV. The robustness of the AMV role in the interbasin connection remains an open question due to the short observational record. Using observations and pre-industrial climate model simulations, we demonstrate for the first time that latitudinal displacements of the Atlantic ITCZ act as a switch for the type of inter-basin teleconnection. During periods in which the Atlantic ITCZ is further equatorward (northward) AN (TNA) impacts ENSO. This ITCZ location can be 1 affected by several factors, including the inter-hemispheric SST gradients associated with AMV.Coupled models success in capturing the AN-ENSO connection. Nevertheless, they have difficulties in reproducing the TNA-ENSO connection because they overestimate rainfall in the southern tropical Atlantic. The TNA-ENSO connection occurs sporadically during periods when the ITCZ is shifted further northward in association with strong heat transports by the AMOC. Weaker AMOC periods in coupled models don&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;t present the TNA-ENSO connection. State-of-the-art models still need to improve for correctly representing tropical Atlantic impact on ENSO.&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;/span&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;/p&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;

Observed warming over northern South America has an anthropogenic origin

Climate Dynamics

We investigate whether the recently observed trends in daily maximum and minimum near-surface air... more We investigate whether the recently observed trends in daily maximum and minimum near-surface air temperature (Tmax and Tmin, respectively) over South America (SA) are consistent with the simulated response of Tmin and Tmax to anthropogenic forcing. Results indicate that the recently observed warming in the dry seasons is well beyond the range of natural (internal) variability. In the wet season the natural modes of variability explain a substantial portion of Tmin and Tmax variability. We demonstrate that the large-scale component of greenhouse gas (GHG) forcing is detectable in dry-seasonal warming. However, none of the global and regional climate change projections reproduce the observed warming of up to 0.6 K/Decade in Tmax in 1983-2012 over northern SA during the austral spring (SON). Thus, besides the global manifestation of GHG forcing, other external drivers have an imprint. Using aerosols-only forcing simulations, our results provide evidence that anthropogenic aerosols also have a detectable influence in SON and that the indirect effect of aerosols on cloud’s lifetime is more compatible with the observed record. In addition, there is an increasing trend in the observed incoming solar radiation over northern SA in SON, which is larger than expected from natural (internal) variability alone. We further show that in the dry seasons the spread of projected trends based on the RCP4.5 scenario derived from 30 CMIP5 models encompasses the observed area-averaged trends in Tmin and Tmax. This may imply that the observed excessive warming in the dry seasons serve as an illustration of plausible future expected change in the region.

Baroclinic-to-Barotropic Pathway in El Niño–Southern Oscillation Teleconnections from the Viewpoint of a Barotropic Rossby Wave Source

Journal of the Atmospheric Sciences

The baroclinic-to-barotropic pathway in ENSO teleconnections is examined from the viewpoint of a ... more The baroclinic-to-barotropic pathway in ENSO teleconnections is examined from the viewpoint of a barotropic Rossby wave source that results from decomposition into barotropic and baroclinic components. Diagnoses using the NCEP–NCAR reanalysis are supplemented by analysis of the response of a tropical atmospheric model of intermediate complexity to the NCEP–NCAR barotropic Rossby wave source. Among the three barotropic Rossby wave source contributions (shear advection, vertical advection, and surface drag), the leading contribution is from shear advection and, more specifically, the mean baroclinic zonal wind advecting the anomalous baroclinic zonal wind. Vertical advection is the smallest term, while surface drag tends to cancel and reinforce the shear advection in different regions through damping on the baroclinic mode, which spins up a barotropic response. There are also nontrivial impacts of transients in the barotropic wind response to ENSO. Both tropical and subtropical barocl...

El Niño–Southern Oscillation Sea Level Pressure Anomalies in the Western Pacific: Why Are They There?

Journal of Climate, 2015

Although sea level pressure (SLP) anomalies in the western Pacific have long been recognized as a... more Although sea level pressure (SLP) anomalies in the western Pacific have long been recognized as an integral part of the classic Southern Oscillation pattern associated with El Niño–Southern Oscillation (ENSO), there is an unresolved question regarding the dynamics that maintain these anomalies. Traditional studies of the ENSO response in the tropics assume a single deep baroclinic mode associated with the tropospheric temperature anomalies. However, the SLP anomalies in the western Pacific are spatially separated from the baroclinic signal in the NCEP–NCAR reanalysis, CMIP5 models, and an intermediate complexity model [a quasi-equilibrium tropical circulation model (QTCM)]. Separation of ENSO SLP anomalies in the tropical Pacific into baroclinic and barotropic components indicates that the barotropic component contributes throughout the tropics and constitutes the primary contribution in the western Pacific. To demonstrate the roles of baroclinic and barotropic modes in ENSO telecon...

Climatology and Hydrology of the Plata Basin

Response of a Coupled Ocean–Atmosphere Model to Greenland Ice Melting

Surveys in Geophysics, 2011

We investigate the transient response of the global coupled ocean-atmosphere system to enhanced f... more We investigate the transient response of the global coupled ocean-atmosphere system to enhanced freshwater forcing representative of melting of the Greenland ice sheets. A 50-year long simulation by a coupled atmosphere-ocean general circulation model (CGCM) is compared with another of the same length in which Greenland melting is prescribed. To highlight the importance of coupled atmosphere-ocean processes, the CGCM results are compared with those of two other experiments carried out with the oceanic general circulation model (OGCM). In one of these OGCM experiments, the prescribed surface fluxes of heat, momentum and freshwater correspond to the unperturbed simulation by the CGCM; in the other experiment, Greenland melting is added to the freshwater flux. The responses by the CGCM and OGCM to the Greenland melting have similar patterns in the Atlantic, albeit the former having five times larger amplitudes in sea surface height anomalies. The CGCM shows likewise stronger variability in all state variables in all ocean basins because the impact of Greenland melting is quickly communicated to all ocean basins via atmospheric bridges. We conclude that the response of the global climate to Greenland ice melting is highly dependent on coupled atmosphereocean processes. These lead to reduced latent heat flux into the atmosphere and an associated increase in net freshwater flux into the ocean, especially in the subpolar North Atlantic. The combined result is a stronger response of the coupled system to Greenland ice sheet melting.

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Achieving superlinear speedup on a heterogeneous, distributed system

IEEE Parallel & Distributed Technology: Systems & Applications, 1994

Choosiug the appropriate computer arch itectu res and masking communication with computation can ... more Choosiug the appropriate computer arch itectu res and masking communication with computation can produce superlinear speedup. he CASA Gigabit Network Testbed, part of NSF and ARPA's Gigabit Project, is investigating whether a "metacomputer" consisting of widely distributed, heterogeneous supercomputers connected by a high-speed (Gbit per sec

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High-performance computing and networking for climate research

Lecture Notes in Computer Science, 1995

ABSTRACT The numerical simulation of Earth&#39;s climate stresses advances in computing and n... more ABSTRACT The numerical simulation of Earth&#39;s climate stresses advances in computing and networking. This paper discusses the parallelization and distribution of the UCLA general circulation model of the atmosphere-ocean system both in a single multi-processor computer or a distributed computer environment. It is argued that a ‘metacomputer’ consisting of a CRAY C90 connected to a massively parallel processor by a gigabit per second network can result in a substantial speedup in the execution of the code, which can eventually be superlinear. Such a speedup is realized in spite of overheads due to latency and communication delays.

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