A seasonal study of the atmospheric dynamics over the Iberian Peninsula based on circulation types

A seasonal analysis of the atmospheric circulation over the Iberian Peninsula (IP) based on circulation types (CTs) obtained from sea level pressure and 500-hPa geopotential height is presented. The study covers the period of 1958–2008, when a high variability and important changes in winter and spring precipitation and temperature have been reported. Frequency, persistence, and the most probable transitions of the circulation types are analyzed. Among the clustering methods available in the literature, two of the most reliable classification methods have been tested, K-means and simulated annealing and diversified randomization. A comparison of both methods over the IP is presented for winter (DJF). The quality of the circulation types obtained through both methods as well as the better stability achieved by K-means suggest this method as more appropriated for our target area. Twelve CTs were obtained for each season and were analyzed. The patterns obtained were regrouped in five general situations: anticyclonic, cyclonic, zonal, summertime, and hybrid-mixed. The analysis of frequencies of these situations offers a similar characterization of the atmospheric circulation that others previously obtained by subjective methods. The analysis of the trends in frequency and persistence for each CT shows few significant trends, mainly in winter and spring with a general decrease of the cyclonic patterns and an increase of the anticyclonic situations. This can be related to the negative precipitation trends reported by other authors. Regarding the persistence, an interesting result is that there is a high interannual variability of the persistence in autumn and spring, when patterns can persist longer than in other seasons. An analysis of the most probable transitions between the CTs has been performed, revealing the existence of cyclic sequences in all seasons. These sequences are related to the high frequency of certain patterns such as the anticyclonic situations in winter. Finally, a clear seasonal dependence of the transitions between cyclonic situations associated with extratropical disturbances was found. This dependence suggests that the transitions of low-pressure systems towards the south of the IP are more likely in spring and autumn than in winter.     

Estimating impacts of warming temperatures on California's electricity system

Despite a clear need, little research has been carried out at the regional-level to quantify potential climate-related impacts to electricity production and delivery systems. This paper introduces a bottom-up study of climate change impacts on California's energy infrastructure, including high temperature effects on power plant capacity, transmission lines, substation capacity, and peak electricity demand. End-of-century impacts were projected using the A2 and B1 Intergovernmental Panel on Climate Change emission scenarios. The study quantifies the effect of high ambient temperatures on electricity generation, the capacity of substations and transmission lines, and the demand for peak power for a set of climate scenarios. Based on these scenarios, atmospheric warming and associated peak demand increases would necessitate up to 38% of additional peak generation capacity and up to 31% additional transmission capacity, assuming current infrastructure. These findings, although based on a limited number of scenarios, suggest that additional funding could be put to good use by supporting R&D into next generation cooling equipment technologies, diversifying the power generation mix without compromising the system's operational flexibility, and designing effective demand side management programs.     

The behavior of vertically integrated boundary-layer winds

This research provides a study of the behavior of vertically integrated boundary-layer winds (IBLWs). This information should be helpful for both theoretical and practical applications, e.g., boundary-layer parameterization in general circulation models, air pollution models, and low-level parachuting operations. The study concerned itself with winds integrated up to a height of 300 m in the United States. The only data suitable for studying the behavior of IBLWs for such heights were measurements taken on tall towers. An extensive search indicated that data were readily available from only four towers instrumented to or above 300 m. These were located in rural locations in Texas, Oklahoma, Nevada, and South Carolina. At each location, means and coefficients of variation of IBLWs were computed, plotted as a function of time since sunrise/sunset, and compared. The research shows that IBLWs may differ appreciably in magnitude at different locations; may have considerable universality in behavior, and may also be ‘regionally’ dependent; and that IBLWs have a pronounced diurnal variation linked to sunrise/sunset time.     

Denudation and retreat of the Serra do Mar escarpment in southern Brazil derived from in situ‐produced 10Be concentration in river sediment

The Serra do Mar escarpment, located along the southeastern coast of Brazil, is a high‐elevation passive margin escarpment. This escarpment evolved from the denudation of granites, migmatites and gneisses. The granites outcrop in the form of a ridge along the escarpment crest, due to its differential erosion (‘sugarloaf’ hills) from the surrounding lithologies. Several studies suggest that the passive margin escarpments are actively retreating toward the interior of the continent. However, no prior study has calculated the long‐term denudation rates of Serra do Mar to test this hypothesis. In this study, we measured the in situ‐produced ¹⁰Be concentration in fluvial sediments to quantify the catchment‐wide long‐term denudation rates of the Serra do Mar escarpment in southern Brazil. We sampled the fluvial sediments from ten watersheds that drain both sides of the escarpment. The average long‐term denudation rate of the oceanic side is between 2.1‐ and 2.6‐fold higher than the rate of the continental side: 26.04 ± 1.88 mm ka^‐1 (integrating over between 15.8 ka^‐1 and 46.6 ka^‐1) and 11.10 ± 0.37 mm ka^‐1 (integrating over between 52.9 ka^‐1 and 85.4 ka^‐1), respectively. These rates indicate that the coastal base level is controlling the escarpment retreat toward the continental high lands, which is consistent with observations made at other high‐elevation passive margins around the globe. The results also demonstrate the differential erosion along the Serra do Mar escarpment in southern Brazil during the Quaternary, where drainages over granites had lower average denudation rates in comparison with those over migmatites and gneisses. Moreover, the results demonstrate that the ocean‐facing catchments have been eroded more intensely than those facing the continent. The results also reveal that drainage over the granites decreases the average denudation rates of the ocean‐facing catchments and the ‘sugarloaf’ hills therefore are natural barriers that slowly retreat once they are exhumed. Copyright © 2013 John Wiley & Sons, Ltd.     

Summer interactions between weather regimes and surface ocean in the North-Atlantic region

This study aims at understanding the summer ocean-atmosphere interactions in the North Atlantic European region on intraseasonal timescales. The CNRMOM1d ocean model is forced with ERA40 (ECMWF Re-Analysis) surface fluxes with a 1-h frequency in solar heat flux (6 h for the other forcing fields) over the 1959–2001 period. The model has 124 vertical levels with a vertical resolution of 1 m near the surface and 500 m at the bottom. This ocean forced experiment is used to assess the impact of the North Atlantic weather regimes on the surface ocean. Composites of sea surface temperature (SST) anomalies associated with each weather regime are computed and the mechanisms explaining these anomalies are investigated. Then, the SST anomalies related to each weather regime in the ocean-forced experiment are prescribed to the ARPEGE Atmosphere General Circulation Model. We show that the interaction with the surface ocean induces a positive feedback on the persistence of the Blocking regime, a negative feedback on the persistence of the NAO-regime and favours the transition from the Atlantic Ridge regime to the NAO-regime and from the Atlantic Low regime toward the Blocking regime.     

Evolution de la structure de la couche limite planetaire sur des sites oceaniques et continentaux

We have studied the evolution of the planetary boundary layer using both oceanic and continental stations. This has been possible through the analysis of twice-daily temperature radiosondes at three different oceanic stations (Point A; Point K; and Azores) and also at three continental stations (Berlin, Trappes and Madrid) all situated in the Northern Hemisphere. We have studied the annual evolution of mixing layers and of temperature inversion levels; an annual evolution presenting a minimum in winter and a maximum in summer has been observed in the continental stations; the oceanic stations present a lesser variation and in the opposite sense. As for the elevated inversion layers, their maximum frequency can be observed in summer at the oceanic stations and in winter at the continental stations. We have shown that considering the studied stations, both the level and the frequency of the elevated inversion layers are similar at 00 h and at 12 h; such a result is important because these layers regulate the intensity of the exchanges between the ground and the free atmosphere. Using the equivalent coefficient, we have determined the annual variation of the vertical exchanges between the surface and different altitudes. At the continental stations, the vertical exchanges are more important in summer than they are in winter; the opposite behaviour occurs at the oceanic stations.     

An upgraded version of the Eta model

Upgrades implemented over a number of years in an open source version of the Eta model, posted at the CPTEC web site http://etamodel.cptec.inpe.br/, are summarized and examples of benefits are shown. The version originates from the NCEP’s Workstation Eta code posted on the NCEP web site http://www.emc.ncep.noaa.gov/mmb/wrkstn_eta, which differs from the NCEP’s latest operational Eta by having the WRF-NMM nonhydrostatic option included. Most of the upgrades made resulted from attention paid to less than satisfactory performance noted in several Eta results, and identification of the reasons for the problem. Others came from simple expectation that including a feature that is physically justified but is missing in the code should help. The most notable of the upgrades are the introduction of the so-called sloping steps, or discretized shaved cells topography; piecewise-linear finite-volume vertical advection of dynamic variables; vapor and hydrometeor loading in the hydrostatic equation, and changes aimed at refining the convection schemes available in the Eta. Several other modifications have to do with the calculation of exchange coefficients, conservation in the vertical diffusion, and diagnostic calculation of 10-m winds. Several examples showing improved performance resulting from the dynamics changes are given. One includes a case of unrealistically low temperatures in several mountain basins generated by a centered vertical advection difference scheme’s unphysical advection from below ground, removed by its replacement with a finite-volume scheme. Another is that of increased katabatic winds in the Terra Nova Bay Antarctica region. Successful forecast of the severe downslope zonda wind case in the lee of the highest peaks of the Andes is also shown, and some of the recent successful verification results of the use of the upgraded model are pointed out. The code is used at numerous places, and along with setup information it is available for outside users at the CPTEC Eta web site given above.     

ENSO anomalies over the Western United States: present and future patterns in regional climate simulations

Surface temperature, precipitation, specific humidity and wind anomalies associated with the warm and cold phases of ENSO simulated by WRF and HadRM are examined for the present and future decades. WRF is driven by ECHAM5 and CCSM3, respectively, and HadRM is driven by HadCM3. For the current decades, all simulations show some capability in resolving the observed warm-dry and cool-wet teleconnection patterns over the PNW and the Southwest U.S. for warm and cold ENSO. Differences in the regional simulations originate primarily from the respective driving fields. For the future decades, the warm-dry and cool-wet teleconnection patterns in association with ENSO are still represented in ECHAM5-WRF and HadRM. However, there are indications of changes in the ENSO teleconnection patterns for CCSM3-WRF in the future, with wet anomalies dominating in the PNW and the Southwest U.S. for both warm and cold ENSO, in contrast to the canonical patterns of precipitation anomalies. Interaction of anomalous wind flow with local terrain plays a critical role in the generation of anomalous precipitation over the western U.S. Anomalous dry conditions are always associated with anomalous airflow that runs parallel to local mountains and wet conditions with airflow that runs perpendicular to local mountains. Future changes in temperature and precipitation associated with the ENSO events in the regional simulations indicate varying responses depending on the variables examined as well as depending on the phase of ENSO.     

Climate change and the incidence of a forest pest in Mediterranean ecosystems: can the North Atlantic Oscillation be used as a predictor?

Many forest pest species strongly depend on temperature in their population dynamics, so that rising temperatures worldwide as a consequence of climatic change are leading to increased frequencies and intensities of insect-pest outbreaks. In the Mediterranean area, the climatic conditions are strongly linked to the effects of the North Atlantic Oscillation (NAO). The aim of this work is to analyze the dynamics of the pine processionary moth (Thaumetopoea pityocampa), a severe pest of Pinus species in the Circunmediterranean, throughout a region of southern Spain, in relation to NAO indices. We related the percentage of forest plots with high defoliation by pine processionary moth each year with NAO values for the present and the three previous winters, using generalized linear models with a binomial error distribution. The time series is 16-year long, and we performed analyses for the whole database and for the five main pine species separately. We found a consistent relationship between the response variable and the NAO index. The relationship is stronger with pine species living at medium-high altitudes, such as Aleppo (P. halepensis), black (P. nigra), and Scots (Pinus sylvestris) pine, which show the higher defoliation intensities up to 3?years after a negative NAO phase. The results highlight, for the first time, the usefulness of using global drivers in order to understand the dynamics of pest outbreaks at a regional scale, and they open the window to the development of NAO-based predictive models as an early-warning signal of severe pest outbreaks.     

Fingerprinting of the Atlantic meridional overturning circulation in climate models to aid in the design of proxy investigations

It is desirable to design proxy investigations that target regions where properties reconstructed from calibrated parameters potentially carry high-fidelity information concerning changes in large-scale climate systems. Numerical climate models can play an important role in this task, producing simulations that can be analyzed to produce spatial “fingerprints” of the expected response of various properties under a variety of different scenarios. We will introduce a new method of fingerprinting the Atlantic meridional overturning circulation (AMOC) that not only provides information concerning the sensitivity of the response at a given location to changes in the large-scale system, but also quantifies the linearity, monotonicity and symmetry of the response. In this way, locations that show high sensitivities to changes in the AMOC, but that exhibit, for example, strongly nonlinear behavior can be avoided during proxy investigations. To demonstrate the proposed approach we will use the example of the response of seawater temperatures to changes in the strength of the AMOC. We present results from an earth-system climate model which has been perturbed with an idealized freshwater forcing scenario in order to reduce the strength of the AMOC in a systematic manner. The seawater temperature anomalies that result from the freshwater forcing are quantified in terms of their sensitivity to the AMOC strength in addition to the linearity and monotonicity of their response. A first-order reversal curve (FORC) approach is employed to investigate and quantify the irreversibility of the temperature response to a slowing and recovering AMOC. Thus, FORCs allow the identification of areas that are unsuitable for proxy reconstructions because their temperature versus AMOC relationship lacks symmetry.