Mean Seasonal Changes Research Articles

Eastern Boundary Circulation and Hydrography Off Angola: Building Angolan Oceanographic Capacities

AbstractThe eastern boundary region off Angola encompasses a highly productive ecosystem important for the food security of the coastal population. The fish-stock distribution, however, undergoes large variability on intraseasonal, interannual, and longer time scales. These fluctuations are partly associated with large-scale warm anomalies that are often forced remotely from the equatorial Atlantic and propagate southward, reaching the Benguela upwelling off Namibia. Such warm events, named Benguela Niños, occurred in 1995 and in 2011. Here we present results from an underexplored extensive in situ dataset that was analyzed in the framework of a capacity-strengthening effort. The dataset was acquired within the Nansen Programme executed by the Food and Agriculture Organization of the United Nations and funded by the Norwegian government. It consists of hydrographic and velocity data from the Angolan continental margin acquired biannually during the main downwelling and upwelling seasons over more than 20 years. The mean seasonal changes of the Angola Current from 6° to 17°S are presented. During austral summer the southward Angola Current is concentrated in the upper 150 m. It strengthens from north to south, reaching a velocity maximum just north of the Angola Benguela Front. During austral winter the Angola Current is weaker, but deeper reaching. While the southward strengthening of the Angola Current can be related to the wind forcing, its seasonal variability is most likely explained by coastally trapped waves. On interannual time scales, the hydrographic data reveal remarkable variability in subsurface upper-ocean heat content. In particular, the 2011 Benguela Niño was preceded by a strong subsurface warming of about 2 years’ duration.

Consistency of climate change projections from multiple global and regional model intercomparison projects

We present an unprecedented ensemble of 196 future climate projections arising from different global and regional model intercomparison projects (MIPs): CMIP3, CMIP5, ENSEMBLES, ESCENA, EURO- and Med-CORDEX. This multi-MIP ensemble includes all regional climate model (RCM) projections publicly available to date, along with their driving global climate models (GCMs). We illustrate consistent and conflicting messages using continental Spain and the Balearic Islands as target region. The study considers near future (2021–2050) changes and their dependence on several uncertainty sources sampled in the multi-MIP ensemble: GCM, future scenario, internal variability, RCM, and spatial resolution. This initial work focuses on mean seasonal precipitation and temperature changes. The results show that the potential GCM–RCM combinations have been explored very unevenly, with favoured GCMs and large ensembles of a few RCMs that do not respond to any ensemble design. Therefore, the grand-ensemble is weighted towards a few models. The selection of a balanced, credible sub-ensemble is challenged in this study by illustrating several conflicting responses between the RCM and its driving GCM and among different RCMs. Sub-ensembles from different initiatives are dominated by different uncertainty sources, being the driving GCM the main contributor to uncertainty in the grand-ensemble. For this analysis of the near future changes, the emission scenario does not lead to a strong uncertainty. Despite the extra computational effort, for mean seasonal changes, the increase in resolution does not lead to important changes.

An analysis of Southeastern US prescribed burn weather windows: seasonal variability and El Niño associations

Fire plays an important role in wildland ecosystems, critical to sustaining biodiversity, wildlife habitat and ecosystem health. By area, 70% of US prescribed burns take place in the Southeast, where treatment objectives range widely and accomplishing them depends on finding specific weather conditions for the effective and controlled application of fire. The climatological variation of the preferred weather window is examined here using two weather model reanalyses, with focus on conditions critical to smoke dispersion and erratic fire behaviour. Large spatial gradients were evident in some months (e.g. 3× change across the Appalachian Mountains in winter). Over most of the Southeast, availability of preferred conditions in summer was several (up to 8) times less than in autumn or winter. We offer explanation for this variability in terms of the mean seasonal changes of key weather conditions (especially mixing height and transport wind). We also examine the interannual variability of the preferred weather window for linkage to the tropical Pacific (1979–2010). Associations with the subset of El Niño events identified by outgoing-longwave-radiation suggest skilful seasonal fire weather forecasts are feasible. Together, these findings offer a predictive tool to prioritise allocation of scarce prescribed fire resources and maximise annual area treated across this landscape.

Seasonal mean temperature changes control future heat waves

AbstractIncreased temperature will result in longer, more frequent, and more intense heat waves. Changes in temperature variability have been deemed necessary to account for future heat wave characteristics. However, this has been quantified only in Europe and North America, while the rest of the globe remains unexplored. Using late century global climate projections, we show that annual mean temperature increases is the key factor defining heat wave changes in most regions. We find that commonly studied areas are an exception rather than the standard and the mean climate change signal generally outweighs any influence from variability changes. More importantly, differences in warming across seasons are responsible for most of the heat wave changes and their consideration relegates the contribution of variability to a marginal role. This reveals that accurately capturing mean seasonal changes is crucial to estimate future heat waves and reframes our interpretation of future temperature extremes.

Creating regional climate change scenarios over southern South America for the 2020’s and 2050’s using the pattern scaling technique: validity and limitations

Dynamical downscaling of global climate simulations is the most adequate tool to generate regional projections of climate change. This technique involves at least a present climate simulation and a simulation of a future scenario, usually at the end of the twenty first century. However, regional projections for a variety of scenarios and periods, the 2020s or the 2050s, are often required by the impact community. The pattern scaling technique is used to estimate information on climate change for periods and scenarios not simulated by the regional model. We based our study on regional simulations performed over southern South America for present climate conditions and two emission scenarios at the end of the twenty first century. We used the pattern scaling technique to estimate mean seasonal changes of temperature and precipitation for the 2020s and the 2050s. The validity of the scalability assumptions underlying the pattern scaling technique for estimating near future regional climate change scenarios over southern South America is assessed. The results show that the pattern scaling works well for estimating mean temperature changes for which the regional changes are linearly related to the global mean temperature changes. For precipitation changes, the validity of the scalability assumption is weaker. The errors of estimating precipitation changes are comparable to those inherent to the regional model and to the projected changes themselves.

Seasonal changes in photosynthesis, respiration and NEE of a mixed temperate forest

We analysed 6 years (1997–2002) of continuous eddy covariance CO 2 fluxes over a mixed temperate forest in the Belgian Campine region and present the long-term mean seasonal changes in net ecosystem exchange (NEE), gross primary production (GPP) and total ecosystem respiration (RE). Seasonal changes in NEE, GPP and RE were tightly correlated, with extremes in July and December/January. Nonetheless, maximum rates of RE slightly lagged behind the maximum rates of GPP. We observed a substantial increase in NEE at optimum light conditions (reduced uptake) above 24 °C. Because this increase is unlikely to be related only to the increase in RE, we hypothesize that it also indicates a significant decrease in GPP under dry and warm meteorological conditions. Because this increase in NEE was better correlated with atmospheric vapour pressure deficit than with temperature, we further hypothesize that the observed change in NEE was predominantly related to the effect of high atmospheric evaporative demand on GPP via stomatal closure than to a negative-temperature response. Because dry conditions were rare in the maritime climate of this study site, its impact on the annual GPP or even on NEE was small (<1% and 10%, respectively).

Mean Seasonal Changes and Recent Trends of Dissolved Inorganic Nutrients in Okkirai Bay, Japan.

Mean seasonal changes in water temperature, salinity, silicate, phosphate, dissolved inorganic nitrogen, and dissolved oxygen (DO) in Okkirai Bay, Japan, were presented based on the monthly averaged data measured from 1979 to 1995. Recent trends of these properties were presented using 12-month running means. As a result, isopleths of mean seasonal changes showed a high nutrient concentration in winter and spring, and the vertical profile tended to be uniform. Recent trends showed great effects due to the Oyashio cold water that is rich in nutrients and DO, which came close to coastal waters in 1984, and that the water temperature has gradually increased since then, in line with decreasing nitrate, ammonium nitrogen, and DO.

Mean seasonal changes of dissolved inorganic nutrients in the Ofunato estuary.

Mean seasonal changes of dissolved inorganic nutrients (silicates, phosphates, and nitrogen compounds) in the Ofunato estuary were represented on the basis of monthly measurements from April 1979 to March 1989, and were compared with those in coastal waters. Estuarine nutrients increased in all layers in winter and spring, and decreased in the middle layer in summer and autumn, during which, however, high concentrations were observed in the surface and the bottom layer. A rapid depletion of nutrients occurred from February to March in the estuary in relation to phyto-plankton blooms, and from April to May in coastal waters. Estuarine nutrients were greatly influenced by fluxes of nutrients from the river into the surface and from bottom sediments into the bottom, as well as fluxes from coastal waters in which the seasonal changes were mainly attributed to those in the oceanographic structure.

Cyclic seasonal variation in plasma lipid and lipoprotein levels: The lipid research clinics coronary primary prevention trial placebo group

Seasonal plasma lipid and lipoprotein cycles were studied in 1446 hypercholesterolemic 35–59 year-old men followed for 7 years as the placebo group of the Lipid Research Clinics (LRC) Coronary Primary Prevention Trial (CPPT). Separate periodic time series were calculated for each study participant; mean parameter estimates were obtained by vector algebra. Highly significant ( p < 0.001) synchronous sinusoidal seasonal cycles, peaking in the first month of winter, were demonstrated for plasma levels of total (TOT-C), low-density lipoprotein (LDL-C), and high-density lipoprotein (HDL-C) cholesterol. Their mean seasonal changes (nadir to zenith) were 7.4, 6.4, and 0.8 mg/dl, respectively. An irregular but statistically significant seasonal pattern was also observed for plasma triglyceride (TG) levels, with peak levels in the autumn. The variation of these seasonal effects among subgroups and geographic locales and their correlation with seasonal weight and dietary patterns yielded few clues as to their underlying etiologic mechanisms.

Empirical Relations Between Seasonal Changes in Meridional Temperature Gradients and Meridional Fluxes of Heat

Abstract The correlation of the zonal mean meridional flux of heat in the atmosphere-ocean system and of its various components with the zonal mean 1000 mb meridional temperature gradient are calculated from analyses of the mean seasonal changes in these quantities. The correlations for the total atmospheric flux and its dominant components are typically 90% or more. The correlation of the eddy flux with the temperature gradient is particularly high (typically 97%) but only if the transient and stationary components are added together. There appears to be no correlation between the ocean flux and the temperature gradient in low latitudes. In cases where the correlation is high, the seasonal changes are used to derive empirical relations between the fluxes and the gradient. Seasonal changes in the total flux in the atmosphere-ocean system are twice as strong as would be implied by a linear diffusion law. The empirical relations for the seasonal changes in the eddy flux of sensible heat indicate that it is ...

The influence of biological activity and physical stability in determining the chemical distributions of inorganic phosphate, silicate and nitrate

Temperature, salinity and nutrient structure at a station, E1, in the English Channel 22 miles from Plymouth have been described for many individual years (see Armstrong &amp; Butler, 1968, for example). By averaging the results gathered over many years (1903–61) the mean seasonal changes in the vertical distributions have been obtained (Pingree &amp; Pennycuick, 1975). In this paper these distributions have been extended to incorporate data to 1975 and as well as presenting up-to-date mean distributions for temperature, salinity and inorganic phosphate, the mean distributions for silicate and nitrate are included. The purpose of the present work is to interpret the observed mean seasonal changes in the inorganic nutrient distributions in the light of recent measurements giving continuous variations of chemical properties (Armstrong &amp; Lafond, 1966), and in particular, from the knowledge gained from continuous vertical chlorophyll ‘a’ determinations (Lorenzen, 1966; Holligan &amp; Harbour, 1977; Pingree, Holligan &amp; Head, 1977).