Mean Seasonal Cycle Research Articles

Development and validation of a regional coupled atmosphere lake model for the Caspian Sea Basin

We present a validation analysis of a regional climate model coupled to a distributed one dimensional (1D) lake model for the Caspian Sea Basin. Two model grid spacings are tested, 50 and 20 km, the simulation period is 1989–2008 and the lateral boundary conditions are from the ERA-Interim reanalysis of observations. The model is validated against atmospheric as well as lake variables. The model performance in reproducing precipitation and temperature mean seasonal climatology, seasonal cycles and interannual variability is generally good, with the model results being mostly within the observational uncertainty range. The model appears to overestimate cloudiness and underestimate surface radiation, although a large observational uncertainty is found in these variables. The 1D distributed lake model (run at each grid point of the lake area) reproduces the observed lake-average sea surface temperature (SST), although differences compared to observations are found in the spatial structure of the SST, most likely as a result of the absence of 3 dimensional lake water circulations. The evolution of lake ice cover and near surface wind over the lake area is also reproduced by the model reasonably well. Improvements resulting from the increase of resolution from 50 to 20 km are most significant in the lake model. Overall the performance of the coupled regional climate—1D lake model system appears to be of sufficient quality for application to climate change scenario simulations over the Caspian Sea Basin.

Seasonal sea level variations in the gulf of Cadiz continental shelf from in-situ measurements and satellite altimetry

Long-term time series (1997–2008) of gridded multi-mission near shore altimeter data and in-situ tide gauge records have been analyzed to investigate the seasonal variability of sea level along the Gulf of Cadiz continental shelf and the Strait of Gibraltar. The contribution of different forcing mechanisms including atmospheric pressure, wind, steric anomalies and river runoff was also addressed. Comparison between altimeter and ground-truth data showed that the former can be a valuable tool to study the sea level seasonal cycle near the coast as well as to detect anomalous data from neighboring tide gauges. Overall, both sets of time series presented a similar mean seasonal cycle along the coast, dominated by the annual component and, to a lesser extent, by the semiannual one. Low-frequency atmospheric pressure and steric anomalies were the main forcing factors affecting the sea level seasonal cycle, their effect diminishing toward the Strait of Gibraltar. The combined effect of both forcing factors accounted for 55–61%/62–63% of the observed sea level variance at the tide gauge stations/ altimeter points located outside the Strait of Gibraltar and for only 27%/48% at those placed within the Strait. The influence of local wind was small in all stations except for the westernmost tide gauges. The effect of large river discharges was notable at the tide gauge located within the Guadalquivir river mouth, with 18% of the pressure-adjusted sea level variance explained, and negligible (< 0.1%) at the one placed within the Tinto–Odiel rivers mouth, indicating that such effects only need to be taken into account when dealing with tide gauges located within major river estuaries. Moreover, the effect of river runoff seems to be confined within a small area near the river mouth, thus not affecting the altimeter measurements. A small but significant influence of the wintertime North Atlantic Oscillation on sea level was observed, its effect being mainly reflected through changes in the atmospheric pressure, wind and river discharge.

CMIP5 simulated climate conditions of the Greater Horn of Africa (GHA). Part 1: contemporary climate

The present study is a preliminary interrogation of the ability of ten Earth System Models (ESMs) from the fifth phase of coupled model intercomparison project to characterize seasonal and annual mean precipitation cycle over the Greater Horn of Africa region. Each ESM had at least 2 ensemble members. In spite of distributional anomalies of observations, ESM ensemble means were examined on the basis of gridded precipitation data. Majority of the ten ESMs analyzed correctly reproduce the mean seasonal and annual cycle of precipitation for the period 1979-2008 as compared to gridded satellite-derived observations. At the same time our analysis shows signif- icant biases in individual models depending on region and season. Specifically, a modest number of models were able to capture correctly the peaks of bimodal (MAM and OND) and JJAS rainfall while a few either dragged the onset to subsequent months or displaced the locations of seasonal rainfall further north. Nearly all models were in agreement with their representation of the zonal orientation of spatial pattern of the leading EOF rainfall modes; more so, enhanced precipitation over the Indian Ocean and a dipole mode of precipitation pattern are captured in the first and second mode respectively. Further, the corresponding EOF time series of the ESMs rainfall modes were all in phase with observations. However, all models output were posi- tively biased against observations, with large medians and varied range of anomalies. Therefore, caution needs to be taken when choosing models for applications over the region, especially when ensemble means have to be considered.

Phenological changes of oceanic phytoplankton in the 1980s and 2000s as revealed by remotely sensed ocean‐color observations

We investigated the phenology of oceanic phytoplankton at large scales over two 5‐year time periods: 1979–1983 and 1998–2002. Two ocean‐color satellite data archives (Coastal Zone Color Scanner (CZCS) and Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS)) were used to investigate changes in seasonal patterns of concentration‐normalized chlorophyll. The geographic coverage was constrained by the CZCS data distribution. It was best for the Northern Hemisphere and also encompassed large areas of the Indian, South Pacific, and Equatorial Atlantic regions. For each 2° pixel, monthly climatologies were developed for satellite‐derived chlorophyll, and the resulting seasonal cycles were statistically grouped using cluster analysis. Five distinct groups of mean seasonal cycles were identified for each half‐decade period. Four types were common to both time periods and correspond to previously identified phytoplankton regimes: Bloom, Tropical, Subtropical North, and Subtropical South. Two other mean seasonal cycles, one in each of the two compared 5‐year periods, were related to transitional or intermediate states (Transitional Tropical and Transitional Bloom). Five mean seasonal cycles (Bloom, Tropical, Subtropical North, and Subtropical South, Transitional Bloom) were further confirmed when the whole SeaWiFS data set (1998–2010) was analyzed. For ∼35% of the pixels analyzed, characteristic seasonal cycles of the 1979–1983 years differed little from those of the 1998–2002 period. For ∼65% of the pixels, however, phytoplankton seasonality patterns changed markedly, especially in the Northern Hemisphere. Subtropical regions of the North Pacific and Atlantic experienced a widespread expansion of the Transitional Bloom regime, which appeared further enhanced in the climatology based on the full SeaWiFS record (1998–2010), and, as showed by a more detailed analysis, is associated to La Niña years. This spatial pattern of Transitional Bloom regime reflects a general smoothing of seasonality at macroscale, coming into an apparent greater temporal synchrony of the Northern Hemisphere. The Transitional Bloom regime is also the result of a higher variability, both in space and time. The observed change in phytoplankton dynamics may be related not only to biological interactions but also to large‐scale changes in the coupled atmosphere–ocean system. Some connections are indeed found with climate indices. Changes were observed among years belonging to opposite phases of ENSO, though discernible from the change among the two periods and within the SeaWiFS era (1998–2010). These linkages are considered preliminary at present and are worthy of further investigation.

Spatial linkages between coral proxies of terrestrial runoff across a large embayment in Madagascar

Abstract. Coral cores provide vital climate reconstructions for site-specific temporal variability in river flow and sediment load. Yet, their ability to record spatial differences across multiple catchments is relatively unknown. Here, we investigate spatial linkages between four coral proxies of terrestrial runoff and their relationships between sites. Coral cores were drilled in and around Antongil Bay, the largest bay in Madagascar, and individually analysed for fifteen years of continuous luminescence (G / B), Ba / Ca, δ18Osw and δ13C data. Each coral core was drilled close to individual river mouths (≥ 7 km), and proxy data were compared to modelled river discharge and sediment runoff data for the three corresponding catchments. A reasonable agreement between terrestrial runoff proxies with modelled river discharge and sediment yield was observed. Some inconsistencies between proxy and modelled data are likely linked to proxy behaviour, watershed size and local environmental physiochemical parameters. In general, the further a coral resided from its river source, the weaker the proxy relationship was with modelled data and other corals, due to mixing gradients and currents. Nevertheless, we demonstrate that two coral Ba / Ca and luminescence (G / B) records influenced by the same watershed are reproducible. Furthermore, a strong Ba / Ca relationship was observed between two cores from distant watersheds, with baseline averages in agreement with modelled sediment runoff data. As humic acids behave conservatively in the water column, luminescence (G / B) data gave the highest regional correlations between cores, and showed the most consistent relationship with site specific modelled discharge. No statistical relationship was observed between cores in terms of interannual δ18Osw and δ13C, meaning corals were recording a localised signal at their respective sites, confounded by vital effects. Comparing proxy baseline averages and mean seasonal cycles provided a good overview of the runoff dynamics of the bay system.

Where can we find a seasonal cycle of the Atlantic water temperature within the Arctic Basin?

Recent mooring observations in the Arctic Basin suggest the existence of a seasonality of Atlantic Water (AW) temperature. Here the DRAKKAR global ocean/sea‐ice model is used to examine the seasonal cycle amplitude of AW temperature within the Arctic Ocean and to investigate the possible mechanisms governing this seasonality. The simulation as well as available mooring data reveals that the amplitude of the AW temperature seasonal cycle is significant only in the Nansen Basin along the continental slope, where AW is primarily advected. In the model, the seasonal cycle of the AW temperature is advected from Fram Strait up to St. Anna Trough and then re‐energized by the Barents Sea Branch. This suggests that the seasonal AW temperature signal survives over a finite distance (∼1000 km). Interannual changes in the seasonal cycle amplitude can be as large as the mean seasonal cycle amplitude; thus seasonality is difficult to characterize from observations spanning only a short period. The seasonal bias of in‐situ observations taken during spring and summer does not induce a large error when considering the interannual‐to‐decadal variations of AW temperature, because the seasonal cycle accounts for a small or negligible part of AW temperature variability, even near the inflow region.

Statistical indices of the northward rainfall progression over eastern Africa

AbstractTwo statistical indices are developed to represent the northward progression of rainfall between late January and late July over eastern Africa. The first, a Seasonal Location Index, extracts the seasonal phase associated with a daily rainfall map using a principal component analysis. The second, a Rainfall Cluster Index, describes the seasonal cycle as a progression through a series of rainfall patterns that have been defined using a cluster analysis. The indices are used to identify large seasonal shifts in the latitude of rainfall, to examine when these shifts occur, and to investigate circulation features associated with perturbations ahead of and behind the mean seasonal cycle. These indices are potentially useful for exploring the predictability of rainfall transitions at both short‐range and seasonal timescales. Copyright © 2012 Royal Meteorological Society

Probability Distribution Characteristics for Surface Air–Sea Turbulent Heat Fluxes over the Global Ocean

Abstract To analyze the probability density distributions of surface turbulent heat fluxes, the authors apply the two-parametric modified Fisher–Tippett (MFT) distribution to the sensible and latent turbulent heat fluxes recomputed from 6-hourly NCEP–NCAR reanalysis state variables for the period from 1948 to 2008. They derived the mean climatology and seasonal cycle of the location and scale parameters of the MFT distribution. Analysis of the parameters of probability distributions identified the areas where similar surface turbulent fluxes are determined by the very different shape of probability density functions. Estimated extreme turbulent heat fluxes amount to 1500–2000 W m−2 (for the 99th percentile) and can exceed 2000 W m−2 for higher percentiles in the subpolar latitudes and western boundary current regions. Analysis of linear trends and interannual variability in the mean and extreme fluxes shows that the strongest trends in extreme fluxes (more than 15 W m−2 decade−1) in the western boundary current regions are associated with the changes in the shape of distribution. In many regions changes in extreme fluxes may be different from those for the mean fluxes at interannual and decadal time scales. The correlation between interannual variability of the mean and extreme fluxes is relatively low in the tropics, the Southern Ocean, and the Kuroshio Extension region. Analysis of probability distributions in turbulent fluxes has also been used in assessing the impact of sampling errors in the Voluntary Observing Ship (VOS)-based surface flux climatologies, allowed for the estimation of the impact of sampling in extreme fluxes. Although sampling does not have a visible systematic effect on mean fluxes, sampling uncertainties result in the underestimation of extreme flux values exceeding 100 W m−2 in poorly sampled regions.

Seasonal and interannual variability in the Gulf of Cadiz: Validation of gridded altimeter products

Nine years of gridded multi-mission altimeter products have been validated in the Gulf of Cadiz. The comparison was performed using two tide gauges deployed in the mouths of two estuaries: the Tinto–Odiel system and Guadalquivir River. The averaged seasonal cycle of the sea-level – corrected for atmospheric pressure effect – obtained from the two tide gauges and their closest altimeter points showed good agreement, correlation coefficients being about 0.95 (at both stations). The variability about the mean seasonal cycle observed in the altimeter data is about half that obtained from the tide gauges. The year-to-year analysis of the seasonal cycle confirmed the level of agreement between the two datasets, and indicated lower amplitude errors in the estimate of the annual harmonic (only significant at 95% confidence level in most of the years) in the altimeter data. The estimation of the annual component is affected by sporadic heavy river discharges only observed in the tide-gauge records. In those cases the river flow explained more than 50% of the variance associated with the daily mean sea-level.

Sea Level Variations along the U.S. Pacific Northwest Coast: Tectonic and Climate Controls

Abstract Analyses of the progressive multidecadal trends and climate-controlled annual variations in mean sea levels are presented for nine tide-gauge stations along the coast of the U.S. Pacific Northwest: Washington, Oregon, and Northern California. The trends in relative sea levels are strongly affected by the tectonics of this region, characterized by significant alongcoast variations in changing land elevations measured by benchmarks and global positioning system data. These combined data sets document the existence of both submergent and emergent stretches of shore. The Pacific Northwest sea levels are also affected by variations in the monthly mean seasonal cycles, with its extreme water levels occurring in the winter during strong El Ninos. To quantify this climate control and to derive improved multidecadal sea-level trends, separate evaluations of the winter and summer-averaged measured water levels have been undertaken. The resulting pair of linear regressions for each tide gauge shows a consis...

Modelling Long-Term Variability in Daily Air Temperature Time Series for Southern Hemisphere Stations

Customarily, climate studies of long-range temperature variability have been carried out using annual or monthly averages. The approach mixes the details of short- and long-range variability that are different for air temperature series. This work shows that a useful method for eliminating short-range variability on long-range variability is to apply a sufficiently long (about 2 months) time step to the daily series. An autoregressive integrated moving average model is fitted to daily maximum and minimum temperature anomalies from the mean seasonal cycle, using data from a number of Australian and New Zealand weather stations. The fitted model can be considered as a sum of random walk plus white noise. This enables us to obtain a quantitative long-term description of the temperature variability.

TropFlux: air-sea fluxes for the global tropical oceans—description and evaluation

In this paper, we evaluate several timely, daily air-sea heat flux products (NCEP, NCEP2, ERA-Interim and OAFlux/ISCCP) against observations and present the newly developed TropFlux product. This new product uses bias-corrected ERA-interim and ISCCP data as input parameters to compute air-sea fluxes from the COARE v3.0 algorithm. Wind speed is corrected for mesoscale gustiness. Surface net shortwave radiation is based on corrected ISCCP data. We extend the shortwave radiation time series by using ''near real-time'' SWR estimated from outgoing longwave radiation. All products reproduce con- sistent intraseasonal surface net heat flux variations asso- ciated with the Madden-Julian Oscillation in the Indian Ocean, but display more disparate interannual heat flux variations associated with El Nino in the eastern Pacific. They also exhibit marked differences in mean values and seasonal cycle. Comparison with global tropical moored buoy array data, I-COADS and fully independent mooring data sets shows that the two NCEP products display lowest correlation to mooring turbulent fluxes and significant bia- ses. ERA-interim data captures well temporal variability, but with significant biases. OAFlux and TropFlux perform best. All products have issues in reproducing observed longwave radiation. Shortwave flux is much better captured by ISCCP data than by any of the re-analyses. Our ''near real-time'' shortwave radiation performs better than most re-analyses, but tends to underestimate variability over the cold tongues of the Atlantic and Pacific. Compared to independent mooring data, NCEP and NCEP2 net heat fluxes display *0.78 correlation and (65 W m -2 rms-difference, ERA-I performs better (*0.86 correlation and *48 W m -2 ) while OAFlux and TropFlux perform best (*0.9 correlation and *43 W m -2 ). TropFlux hence provides a useful option for

Investigation of Arctic sea ice and ocean primary production for the period 1992–2007 using a 3-D global ice–ocean ecosystem model

In the Arctic Ocean, both phytoplankton and sea ice algae are important contributors to primary production and the arctic food web. An ice algal ecosystem model was added to fully couple with the global physical model POP-CICE (Parallel Ocean Program-Los Alamos Sea Ice Model) and the open-ocean pelagic ecosystem model. The physical model captured the seasonal and interannual variations of northern hemispheric sea ice extent and area measured by satellite remote sensing for the model period of 1992–2007. The model results showed a reasonable mean seasonal cycle of ice algal production from March to May and subsequent ocean production from May to September in the Arctic. The ice algal production, although smaller than that of the ocean, is of ecological importance as a food source for higher trophic levels during the long arctic winter before ice melt. The simulated mean open-ocean upper 100m primary production within the Arctic Circle was 413±88TgCyr−1 in the years 1998–2006, close to the remote sensing derived estimate of 419±33TgCyr−1 but with higher interannual variations. The mean sea ice algal production in the Northern Hemisphere from 1998 to 2007 was 21.3TgCyr−1, which is in the range of multi-observational estimations of 9–73TgCyr−1 based on in situ measurements. Model-data comparisons were conducted with various regional observations and the observed trend of temporal and spatial variation of the primary production. The model results compared well with the following observations and observed trends: (1) an increase of ocean primary production from 2003 to 2007 in the arctic open water areas as derived from remote sensing data; (2) regional annual ice and ocean primary production measured in the Bering and Chukchi seas and Canadian Basin; (3) primary production rate with phytoplankton size composition and Chl-a concentration along an arctic cruise track in the Chukchi Sea and Canadian Basin from August 2 to September 7, 2008; and (4) observed decadal changes of ocean primary production from the 1990s to 2007 due to rising temperature and increasing open-ocean area in the western Arctic. The changes are shown as a trend of a northward shift of production, with a decrease in the Bering Sea and an increase in the Arctic shelf.

Numerical Investigations of Seasonal and Interannual Variability of North Pacific Subtropical Mode Water and Its Implications for Pacific Climate Variability

Abstract North Pacific Subtropical Mode Water (NPSTMW) is an essential feature of the North Pacific subtropical gyre imparting significant influence on regional SST evolution on seasonal and longer time scales and, as such, is an important component of basin-scale North Pacific climate variability. This study examines the seasonal-to-interannual variability of NPSTMW, the physical processes responsible for this variability, and the connections between NPSTMW and basin-scale climate signals using an eddy-permitting 1979–2006 ocean simulation made available by the Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2). The monthly mean seasonal cycle of NPSTMW in the simulation exhibits three distinct phases: (i) formation during November–March, (ii) isolation during March–June, and (iii) dissipation during June–November—each corresponding to significant changes in upper-ocean structure. An interannual signal is also evident in NPSTMW volume and other characteristic properties with volume minima occurring in 1979, 1988, and 1999. This volume variability is correlated with the Pacific decadal oscillation (PDO) with zero time lag. Further analyses demonstrate the connection of NPSTMW to the basin-scale ocean circulation. With this, modulations of upper-ocean structure driven by the varying strength and position of the westerlies as well as the regional air–sea heat flux pattern are seen to contribute to the variability of NPSTMW volume on interannual time scales.

Can sea surface height be used to estimate oceanic transport variability?

The relation between the sea surface height and the meridional transport across a zonal section at 26.5°N in the North Atlantic is studied by using an eddy resolving ocean state estimate simulated with the Massachusetts Institute of Technology general circulation model. It is shown that the correlation between the zonal sea surface height difference and transport can be substantially increased if the steric height contribution from the seasonal thermocline is removed. The latter explains a substantial part of sea surface height variability, but its effect on transport is weak. It is also found that the zonal steric height difference correlates well with the transport after the contribution of the seasonal thermocline has been removed. There is a similar agreement (with correlation coefficient of 0.63 for the full signal and 0.89 for the mean seasonal cycle) between the meridional transport and steric height based on observations from the Rapid Climate Change (RAPID) project.

Greenhouse gas observations from Cabauw Tall Tower (1992–2010)

Abstract. Since 1992 semi-continuous in-situ observations of greenhouse gas concentrations have been performed at the tall tower of Cabauw (4.927° E, 51.971° N, −0.7 m a.s.l.). Through 1992 up to now, the measurement system has been gradually extended and improved in precision, starting with CO2 and CH4 concentrations from 200 m a.g.l. in 1992 to vertical gradients at 4 levels of the gases CO2, CH4, SF6, N2O, H2, CO and gradients at 2 levels for 222Rn. In this paper the measurement systems and measurement results are described for the main greenhouse gases and CO, for the whole period. The automatic measurement system now provides half-hourly concentration gradients with a precision better than or close to the WMO recommendations. The observations at Cabauw show a complex pattern caused by the influence of sources and sinks from a large area around the tower with significant contributions of sources and sinks at distances up to 500–700 km. The concentration footprint area of Cabauw is one the most intensive and complex source areas of greenhouse gases in the world. Despite this, annual mean trends for the most important greenhouse gases, compatible with the values derived using the global network, can be reproduced from the measured concentrations at Cabauw over the entire measurement period, with a measured increase in the period 2000–2009 for CO2 of 1.90 ± 0.1 ppm yr−1, for CH4 of 4.4 ± 0.6 ppb yr−1, for N2O of 0.86 ± 0.04 ppb yr−1, and for SF6 of 0.27 ± 0.01 ppt yr−1; for CO no significant trend could be detected. The influences of strong local sources and sinks are reflected in the amplitude of the mean seasonal cycles observed at Cabauw, that are larger than the mean Northern Hemisphere average; Cabauw mean seasonal amplitude for CO2 is 25–30 ppm (higher value for lower sampling levels). The observed CH4 seasonal amplitude is 50–110 ppb. All gases except N2O show highest concentrations in winter and lower concentrations in summer, N2O observations show two additional concentration maxima in early summer and in autumn. Seasonal cycles of the day-time mean concentrations show that surface concentrations or high elevation concentrations alone do not give a representative value for the boundary layer concentrations, especially in winter time, but that the vertical profile data along the mast can be used to construct a useful boundary layer mean value. The variability at Cabauw in the atmospheric concentrations of CO2 on time scales of minutes to hours is several ppm and is much larger than the precision of the measurements (0.1 ppm). The diurnal and synoptical variability of the concentrations at Cabauw carry information on the sources and sinks in the footprint area of the mast, that will be useful in combination with inverse atmospheric transport model to verify emission estimates and improve ecosystem models. For this purpose a network of tall tower stations like Cabauw forms a very useful addition to the existing global observing network for greenhouse gases.

New diagnostic estimates of variations in terrestrial water storage based on ERA‐Interim data

AbstractTerrestrial water storage is an important component of the climate system as it determines the partitioning of the water and energy fluxes at the land surface. Since in situ measurements are scarce, there is a strong need for alternative validation data sources.Here we present a new dataset of monthly basin‐scale terrestrial water storage changes (TWSC) derived with the combined atmospheric‐terrestrial water‐balance approach, also referred to as basin‐scale water‐balance (BSWB) approach. It is diagnosed from observation‐constrained reanalysis data for the atmospheric fields and observed streamflow. A previous BSWB dataset derived from the European Center for Medium‐Range Weather Forecasts (ECMWF) reanalysis ERA‐40 provided reasonable estimates of TWSC for mid‐latitude basins. The here presented BSWB dataset is derived from the latest ECMWF reanalysis ERA‐Interim, which is available in real time. The potential real‐time availability of TWSC derived from ERA‐Interim is of great relevance for several agricultural, hydrological, and climate applications.An uncertainty analysis of the derived TWSC highlights the importance of the atmospheric moisture flux convergence fields from the reanalysis for the resulting product uncertainty. We find that the BSWB estimates have a relative uncertainty of about 4–6% in basins larger than 1·5 × 106 km2 compared with the mean seasonal cycle. The ERA‐Interim based dataset compares better with in situ soil moisture, snow depth, and groundwater measurements in the Ob basin and Illinois than the previous ERA‐40 based dataset. We further compare the BSWB datasets to retrievals from the Gravity Recovery And Climate Experiment (GRACE), scatterometer data from the European Remote Sensing (ERS) satellites, and land surface model output from the second phase of the Global Soil Wetness Project (GSWP). We find a better correlation between ERS and the BSWB data than between GRACE and the BSWB data, which might be related to the relatively coarse resolution of GRACE. GSWP model output and the BSWB datasets compare well in most of the studied river basins. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Impact of Earth’s orbit and freshwater fluxes on Holocene climate mean seasonal cycle and ENSO characteristics

We use a state-of-the-art 3-dimensional coupled model to investigate the relative impact of long term variations in the Holocene insolation forcing and of a freshwater release in the North Atlantic. We show that insolation has a greater effect on seasonality and La Nina events and is the major driver of sea surface temperature changes. In contrast, the variations in precipitation reflect changes in El Nino events. The impact of ice-sheet melting may have offset the impact of insolation on El Nino Southern Oscillation variability at the beginning of the Holocene. These simulations provide a coherent framework to refine the interpretation of proxy data and show that changes in seasonality may bias the projection of relationships established between proxy indicators and climate variations in the east Pacific from present day records.

Mean Circulation, Seasonal Cycle, and Eddy Interactions in the Eastern Brazilian Margin, a Nested ROMS Model

A nested configuration of the Regional Ocean Modeling System (ROMS) is used to study the seasonal circulation patterns and mesoscale activity of the Eastern Brazilian Margin (EBM). The EBM encompass an oligotrophic and bathymetrically complex zone in the NW South Atlantic from 8°S to 20°S. Sea-level anomaly data are used to validate the model. Analysis of the mean circulation reveals that the EBM is dominated by seasonal and spatial dynamics of the southward Brazil Current (BC) and the northward North Brazil Undercurrent (NBUC), as well as their connection to the South Equatorial Current (SEC) dynamics. The EBM can be divided in three dynamic provinces, which are seasonally connected either by the permanent main flow or by mesoscale process. In the northern province, from 8°S to 13°S, the NBUC is the major permanent feature and the BC is just a thin flow, confined to the top few meters. As it moves southward, the BC gets deeper and stronger. In the middle province, from 13°S to 16°S, the dominance of the top 0–100-m circulation is seasonally alternated between the southward BC flow and the northward NBUC flow. In the southern province, from 16°S to 20°S, the BC appears as a dominating surface feature. While on the top (0–100 m) the main current presents a pronounced seasonal and spatial variability, on subsurface waters (100–500 m) the NBUC connects the EBM continuously. Finally, analysis of the regional simulation reveals well-defined cyclonic and anticyclonic eddies. They detach from the main flow and translate along the domain throughout the year. The translation patterns are associated with the seasonal variability of the main EBM flow, with anticyclonic mesoscale features translating southward and mesoscale cyclonic features translating northward. On their pathway, these features may come very close to the margin, interacting with the near-shelf flow.

Some features of seasonal variations in the methane content in the atmosphere over northern Eurasia

The climatic trends and basic features of seasonal variations in and anomalies of the concentration of methane in the atmospheric surface layer are considered on the basis of the current notion of the processes that form the global field of methane in the Earth’s atmosphere. Measurement data on the surface concentration of methane, which were obtained in Moscow and at a number of observation stations in Europe and Siberia in the fall-winter period of the first decade of the 21st century, have been analyzed. It is shown that, in the anomalously warm winter months of 2006/2007, the concentration of methane in the atmosphere over Moscow was higher than in the previous and following years. The excess concentration of methane amounted to 10% in March 2007, which is higher than the mean range of seasonal variations in the monthly mean concentration of surface methane. A comparison between the data obtained in Moscow and the data obtained at three stations of the NOAA global monitoring network and at three Russian Hydrometeorological Research Center stations shows the high spatial variability of the methane concentration in the atmosphere over northern Eurasia. The complex and multifactor processes that determine the content of methane in the atmospheric surface layer result in noticeable spatial and interannual deviations from the mean seasonal cycle of its concentration, which can manifest themselves on both regional and global scales. It is possible that the resumed increase in the content of methane in the Earth’s atmosphere recorded in 2007 (after its relative stabilization in the early 2000s) at the global monitoring network was also caused, to some extent, by the anomalously warm winter of 2006–2007 in northern Europe and western Siberia.