Frontogenesis of the Angola–Benguela Frontal Zone

A diagnostic analysis of the climatological annual mean and seasonal cycle of the Angola–Benguela Frontal Zone (ABFZ) is performed by applying an ocean frontogenetic function (OFGF) to the ocean mixing layer (OML). The OFGF reveals that the meridional confluence and vertical tilting terms are the most dominant contributors to the frontogenesis of the ABFZ. The ABFZ shows a well-pronounced semiannual cycle with two maximum (minimum) peaks in April–May and November–December (February–March and July–August). The development of the two maxima of frontogenesis is due to two different physical processes: enhanced tilting from March to April and meridional confluence from September to October. The strong meridional confluence in September to October is closely related to the seasonal southward intrusion of tropical warm water to the ABFZ that seems to be associated with the development of the Angola Dome northwest of the ABFZ. The strong tilting effect from March to April is attributed to the meridional gradient of vertical velocities, whose effect is amplified in this period due to increasing stratification and shallow OML depth. The proposed OFGF can be viewed as a tool to diagnose the performance of coupled general circulation models (CGCMs) that generally fail at realistically simulating the position of the ABFZ, which leading to huge warm biases in the southeastern Atlantic.</p

9. Las diversas facetas de El Niño y sus efectos en la costa del Perú

El fenómeno El Niño es el modo dominante de la variabilidad interanual en el Océano Pacífico, resultando de un proceso de interacción entre el océano y la atmósfera en el Pacífico Tropical, Las últimas Investigaciones demuestran que existen varias facetas de este fenómeno que varían según las modalidades de interacción entre el océano y la atmosfera así como sus ubicaciones. Existen por lo menos dos tipos de El Niño, con expresiones diferentes sobre la Temperatura Superficial del Mar en el Pacifico Tropical y en la costa de Perú: uno que se desarrolla en el Pacifico Central (tiende a estar asociado a condiciones oceánicas más frías que favorecen el estado árido de la costa peruana y condiciones oceánicas hypóxicas), y otro que se desarrolla en el Pacifico Este (que transforma la costa peruana en una “típica” zona tropical, caracterizada por aguas costeras calientes y oxigenadas, y una lluvia intensa). Hoy en día, los esfuerzos de investigación para entender los mecanismos involucrados en los diferentes tipos de El Niño han sido reforzados, dado que, en las últimas décadas, se ha incrementado la frecuencia de ocurrencia de estos eventos en el Pacifico Central, sugiriéndose que podría ser una consecuencia del cambio climático. El perfeccionamiento de los modelos regionales acoplados tanto océano - atmosfera como océano - biogeoquímlco, tiene como objetivo mejorar la comprensión de la vulnerabilidad de la biosfera peruana al cambio climático y proponer un paradigma que represente la bimodalidad de la variabilidad interanual en el Pacifico Tropical.El Niño est le mode dominant de la variabilité interannuelle dans l’océan Pacifique, résultant d’un processus d’interaction entre l’océan et l’atmosphére dans le Pacifique tropical. Les recherches récentes montrent qu’il existe plusieurs facettes de ce phénomène qui varient selon les modalités d’interaction entre l’océan et l’atmosphére et leurs emplacements. Il y a au moins deux types de El Niño, avec des expressions différentes sur la Température de surface dans le Pacifique tropical et le long de la cote du Pérou: un qui se déroule dans le Pacifique central (associé á des conditions océaniques froides qui favorisent l’état aride de la cote péruvienne et des conditions océaniques d’hypoxie), et un autre qui a lieu dans le Pacifique oriental (qui transforme la cote péruvienne en une zone tropicale «typique», caractérisé par des eaux cótiéres chaudes et oxygénées, et de fortes pluies). Aujourd’hui, les efforts de recherche pour comprendre les mécanismes impliqués dans les différents types de El Niño ont été renforcés, en raison de l’accroissement de la fréquence d’occurrence de ces événements dans le Pacifique central au cours des dernières décennies a accru, suggérant qu’ll pourrait s’agir d’une conséquence du changement dimatique. L’optimisation des modeles régionaux couplés océan - atmosphére et océan - blogéochimiques, vise à améliorer la compréhension de la vulnérabilité de la biosphére péruvienne au changement dimatique et de proposer un paradigme qui représente la bimodalité de la variabilité Interannuelle dans le Pacifique tropical.The El Niño phenomenon is the dominant mode of inter-annual variability in the Pacific Ocean, which results from the ¡nteraction between the ocean and atmosphere in the tropical Pacific. Recent research shows that there are several facets of this phenomenon, which vary according to the modalities of ¡nteraction between the ocean and atmosphere, as well as their locations. There are at least two types of El Niño with different expresslons on the sea surface temperature in the tropical Pacific and on the coast of Peru: one that takes place in the Central Pacific (which tends to be associated with colder oceanic conditions who favoring the aridity of the Peruvian coast and the ocean conditions hypoxic), and another that takes place in the Eastern Pacific (which transforms the Peruvian coast in a “typical” tropical zone, with warm and oxygenated Coastal waters, and heavy rain). Nowadays, research efforts to understand the mechanisms involved in the different types of El Niño have been strengthened, since in recent decades has increased the frequency of these events in the Central Pacific, suggesting that ¡t might be a result of climate change. The ¡mprovement of both regional models coupled ocean - atmosphere and ocean - biogeochemical aims to Improve the understanding of the vulnerability of the Peruvian biosphere to climate change, and propose a paradigm that represents the bimodality of the Inter-annual variability in the tropical Pacific

Extratropical cyclones and the projected decline of winter Mediterranean precipitation in the CMIP5 models

The Mediterranean region has been identified as a climate change "hot-spot" due to a projected reduction in precipitation and fresh water availability which has potentially large socio-economic impacts. To increase confidence in these projections, it is important to physically understand how this precipitation reduction occurs. This study quantifies the impact on winter Mediterranean precipitation due to changes in extratropical cyclones in 17 CMIP5 climate models. In each model, the extratropical cyclones are objectively tracked and a simple approach is applied to identify the precipitation associated to each cyclone. This allows us to decompose the Mediterranean precipitation reduction into a contribution due to changes in the number of cyclones and a contribution due to changes in the amount of precipitation generated by each cyclone. The results show that the projected Mediterranean precipitation reduction in winter is strongly related to a decrease in the number of Mediterranean cyclones. However, the contribution from changes in the amount of precipitation generated by each cyclone are also locally important: in the East Mediterranean they amplify the precipitation trend due to the reduction in the number of cyclones, while in the North Mediterranean they compensate for it. Some of the processes that determine the opposing cyclone precipitation intensity responses in the North and East Mediterranean regions are investigated by exploring the CMIP5 inter-model spread

Extremes in temperature and precipitation around the Mediterranean basin in an ensemble of future climate scenario simulations

International audienceA variable-grid atmospheric general circulation model, the LMDZ, with a local zoom over the Mediterranean is used to investigate potential future changes in climate extremes around the Mediterranean basin. Changes in extremes of annual minimum and maximum temperature, winter and summer 24-h maximum precipitation are discussed under the IPCC-A2 emission scenario. Three time slices of 30 years are chosen to represent respectively the end of the 20th century, the middle and the end of the 21st century. The boundary conditions were taken from the outputs of three global coupled climate models: from the Institut Pierre-Simon Laplace (IPSL), Centre National de Recherches Météorologiques (CNRM) and Geophysical Fluid Dynamics Laboratory (GFDL). These three global scenarios were used to estimate uncertainties associated with climate models. Extreme events are expressed in terms of return values, estimated from a Generalized Extreme Value distribution fitted to annual or seasonal extremes. The changes in distribution of extremes are analyzed to elucidate the nature of the changes in extremes. Magnitudes and main spatial patterns of the changes in extremes show a quite good consistency among three global scenarios. Comparison between changes in the middle and at the end of the 21st century does not reveal any remarkable discontinuity in future climate evolution. The maximum of warming occurs over Northeastern Europe for annual minimum temperature and over South Europe for annual maximum temperature. Averaged over the region, increase in cold extremes exceeds that in warm extremes. Changes in temperature extremes are mostly associated with shift of whole distribution (location parameter change) and in addition, for cold extremes, with changes in interannual variability, measured by the scale parameter. Mean precipitation changes are characterized by strong reduction belt over the Mediterranean and South Europe in winter, spring and summer. Precipitation extremes increase in all seasons except summer. These changes are predominantly associated with changes in the scale, but also with changes in the position and shape of the distribution. In general terms, it is suggested that the Mediterranean basin will experience a warmer climate with less total precipitation but more intense precipitation events

ENSO regimes : reinterpreting the canonical and Modoki El Nino

We propose that the first two empirical orthogonal function (EOF) modes of tropical Pacific sea surface temperature (SST) anomalies do not describe different phenomena (i.e., El Nino-Southern Oscillation (ENSO) and "El Nino Modoki") but rather the nonlinear evolution of ENSO. We introduce two new uncorrelated indices (E and C), based on the leading EOFs, that respectively account for extreme warm events in the eastern and cold/moderate warm events in the central equatorial Pacific, corresponding to regimes with different evolution. Recent trends in ENSO can be described as an increase in the central Pacific (C) variability that is associated with stronger cold events, as well as a reduction in the eastern Pacific (E) variability within the cold/moderate warm regime, consistent with model projections. However, little can be said observationally with respect to the extreme warm regime

Weather regime dependence of extreme value statistics for summer temperature and precipitation

International audienceExtreme Value Theory (EVT) is a useful tool to describe the statistical properties of extreme events. Its underlying assumptions include some form of temporal stationarity in the data. Previous studies have been able to treat long-term trends in datasets, to obtain the time dependence of EVT parameters in a parametric form. Since there is also a dependence of surface temperature and precipitation to weather patterns obtained from pressure data, we determine the EVT parameters of those meteorological variables over France conditional to the occurrence of North Atlantic weather patterns in the summer. We use a clustering algorithm on geopotential height data over the North Atlantic to obtain those patterns. This approach refines the straightforward application of EVT on climate data by allowing us to assess the role of atmospheric variability on temperature and precipitation extreme parameters. This study also investigates the statistical robustness of this relation. Our results show how weather regimes can modulate the different behavior of mean climate variables and their extremes. Such a modulation can be very different for the mean and extreme precipitation

Relation between Large-Scale Circulation and European Winter Temperature: Does It Hold under Warmer Climate?

ISI Document Delivery No.: 628GA Times Cited: 3 Cited Reference Count: 32 Cited References: Berner J, 2007, J ATMOS SCI, V64, P117, DOI 10.1175/JAS3822.1 Boe J, 2006, J GEOPHYS RES-ATMOS, V111, DOI [10.1029/2005JD006889, 10.1029/JD006889] Cassou C, 2005, J CLIMATE, V18, P2805, DOI 10.1175/JCLI3506.1 Christiansen B, 2005, J ATMOS SCI, V62, P2528, DOI 10.1175/JAS3490.1 Corti S, 1999, NATURE, V398, P799 D'Andrea F, 1998, CLIM DYNAM, V14, P385, DOI 10.1007/s003820050230 Dufresne JL, 2002, GEOPHYS RES LETT, V29, DOI 10.1029/2001GL013777 EFRON B, 1993, STAT APPL PROBABILIT, V57 Goubanova K, 2007, GLOBAL PLANET CHANGE, V57, P27, DOI 10.1016/j.gloplacha.2006.11.012 Houghton J. T., 2001, CLIMATE CHANGE 2001 Hourdin F, 2006, CLIM DYNAM, V27, P787, DOI 10.1007/s00382-006-0158-0 Hurrell J. W., 2003, GEOPHYS MONOGR SER, V134, DOI [10.1029/GM134., DOI 10.1029/GM134] Huth R, 1999, CLIMATE RES, V13, P91, DOI 10.3354/cr013091 KIMOTO M, 1993, J ATMOS SCI, V50, P2645, DOI 10.1175/1520-0469(1993)0502.0.CO;2 LEGRAS B, 1985, J ATMOS SCI, V42, P433, DOI 10.1175/1520-0469(1985)0422.0.CO;2 Majda AJ, 2006, P NATL ACAD SCI USA, V103, P8309, DOI 10.1073/pnas.0602641103 MICHELANGELI PA, 1995, J ATMOS SCI, V52, P1237, DOI 10.1175/1520-0469(1995)0522.0.CO;2 Najac J, 2009, CLIM DYNAM, V32, P615, DOI 10.1007/s00382-008-0440-4 Palmer TN, 1999, J CLIMATE, V12, P575, DOI 10.1175/1520-0442(1999)0122.0.CO;2 Plaut G, 2001, CLIMATE RES, V17, P285, DOI 10.3354/cr017285 SanchezGomez E, 2005, GEOPHYS RES LETT, V32, DOI 10.1029/2005GL023990 Schubert S, 1998, INT J CLIMATOL, V18, P1419, DOI 10.1002/(SICI)1097-0088(19981115)18:133.3.CO;2-Q Solomon S, 2007, CLIMATE CHANGE 2007: THE PHYSICAL SCIENCE BASIS, P19 Stephenson DB, 2004, Q J ROY METEOR SOC, V130, P583, DOI 10.1256/qj.02.146 Tank AMGK, 2002, INT J CLIMATOL, V22, P1441, DOI 10.1002/joc.773 Tibaldi S., 1990, Tellus, Series A (Dynamic Meteorology and Oceanography), V42A, DOI 10.1034/j.1600-0870.1990.t01-2-00003.x Uppala SM, 2005, Q J ROY METEOR SOC, V131, P2961, DOI 10.1256/qj.04.176 VAUTARD R, 1990, MON WEATHER REV, V118, P2056, DOI 10.1175/1520-0493(1990)1182.0.CO;2 von Storch H., 2001, STAT ANAL CLIMATE RE Wilby R.L., 2004, GUIDELINES USE CLIMA Yiou P, 2008, NONLINEAR PROC GEOPH, V15, P365 Yiou P, 2004, GEOPHYS RES LETT, V31, DOI 10.1029/2003GL019119 Goubanova, K. Li, L. Yiou, P. Codron, F. 3 AMER METEOROLOGICAL SOC BOSTON J CLIMATEThe idea of using large-scale information to predict local climate variability is widely exploited in climate change impact studies as an alternative to computationally expensive high-resolution models. This approach implies the hypothesis that the statistical relationship between large-scale climate states and local variables defined for the present-day climate remains valid in the altered climate. In this paper, the concept of weather regimes is used to deduce a relationship between large-scale circulation and European winter temperature. The change in temperature with increased greenhouse gases is, however, not homogeneous among the individual regimes. As a result, the impact of the weather regimes on local temperature changes varies in the future, limiting its usefulness for refining temperature changes to the small scale

Weather regime dependence of extreme value statistics for summer temperature and precipitation

Extreme Value Theory (EVT) is a useful tool to describe the statistical properties of extreme events. Its underlying assumptions include some form of temporal stationarity in the data. Previous studies have been able to treat long-term trends in datasets, to obtain the time dependence of EVT parameters in a parametric form. Since there is also a dependence of surface temperature and precipitation to weather patterns obtained from pressure data, we determine the EVT parameters of those meteorological variables over France conditional to the occurrence of North Atlantic weather patterns in the summer. We use a clustering algorithm on geopotential height data over the North Atlantic to obtain those patterns. This approach refines the straightforward application of EVT on climate data by allowing us to assess the role of atmospheric variability on temperature and precipitation extreme parameters. This study also investigates the statistical robustness of this relation. Our results show how weather regimes can modulate the different behavior of mean climate variables and their extremes. Such a modulation can be very different for the mean and extreme precipitation

Seasonal variability of the Ekman transport and pumping in the upwelling system off central-northern Chile (∼ 30° S) based on a high-resolution atmospheric regional model (WRF)

Two physical mechanisms can contribute to coastal upwelling in eastern boundary current systems: offshore Ekman transport due to the predominant alongshore wind stress and Ekman pumping due to the cyclonic wind stress curl, mainly caused by the abrupt decrease in wind stress (drop-off) in a cross-shore band of 100 km. This wind drop-off is thought to be an ubiquitous feature in coastal upwelling systems and to regulate the relative contribution of both mechanisms. It has been poorly studied along the central-northern Chile region because of the lack in wind measurements along the shoreline and of the relatively low resolution of the available atmospheric reanalysis. Here, the seasonal variability in Ekman transport, Ekman pumping and their relative contribution to total upwelling along the central-northern Chile region (∼  30° S) is evaluated from a high-resolution atmospheric model simulation. As a first step, the simulation is validated from satellite observations, which indicates a realistic representation of the spatial and temporal variability of the wind along the coast by the model. The model outputs are then used to document the fine-scale structures in the wind stress and wind curl in relation to the topographic features along the coast (headlands and embayments). Both wind stress and wind curl had a clear seasonal variability with annual and semiannual components. Alongshore wind stress maximum peak occurred in spring, second increase was in fall and minimum in winter. When a threshold of −3  ×  10⁻⁵ s⁻¹ for the across-shore gradient of alongshore wind was considered to define the region from which the winds decrease toward the coast, the wind drop-off length scale varied between 8 and 45 km. The relative contribution of the coastal divergence and Ekman pumping to the vertical transport along the coast, considering the estimated wind drop-off length, indicated meridional alternation between both mechanisms, modulated by orography and the intricate coastline. Roughly, coastal divergence predominated in areas with low orography and headlands. Ekman pumping was higher in regions with high orography and the presence of embayments along the coast. In the study region, the vertical transport induced by coastal divergence and Ekman pumping represented 60 and 40 % of the total upwelling transport, respectively. The potential role of Ekman pumping on the spatial structure of sea surface temperature is also discussed