Sea Level Pressure Patterns Research Articles

Global ocean heat content redistribution during the 1998–2012 Interdecadal Pacific Oscillation negative phase

Previous studies have linked the slowdown in global surface temperature warming during the 1998–2012 period to a negative Interdecadal Pacific Oscillation (IPO) phase. Here, we investigate the changes in ocean heat content (OHC) during this period. We compare two ensembles of coupled model experiments with either zero or observed prescribed tropical Pacific wind stress interannual anomalies. This successfully constrains the global surface temperature, sea level pressure and OHC patterns associated with the IPO phase transition around 1998. The negative IPO phase (1998–2012) is associated with a global ocean heat redistribution. The anomalously cold tropical Pacific Ocean leads to an increased oceanic uptake in this region, and a global OHC increase of 4 × 1022 J. The cold equatorial Pacific also forces mid-latitude wind changes through atmospheric teleconnections, leading to an enhanced wind-driven heat transport convergence at 40°N and 40°S. Enhanced Pacific easterlies also yield an enhanced heat transport to the Indian Ocean via the Indonesian throughflow. As a result, the anomalous Pacific heat uptake is entirely exported towards the North Pacific (~ 50%), Indian (~ 30%) and Southern (~ 20%) Oceans. A significant fraction of this heat is released back to the atmosphere in the North Pacific and Indian basins, and transported across 31°S in the Indian Ocean. Overall, OHC increases most in the Southern Ocean (~ 60% of global changes) and northern Pacific (~ 40%), with negligible changes in the Indian and Atlantic basins. These results point to the major importance of oceanic circulation in re-distributing the Pacific heat uptake globally during negative IPO phases.

An Eastward Shift of the North Pacific Oscillation After the Mid‐1990s and Its Relationship With ENSO

AbstractThe North Pacific Oscillation (NPO), which is characterized by a north‐south dipole‐like pattern of sea level pressure (SLP) in the North Pacific, is an atmospheric circulation that is a key to understanding tropical‐extratropical interactions in the Pacific. We show that the center of the southern lobe SLP in the NPO during boreal winter (December‐January‐February) is shifted to the east after the mid‐1990s compared to before the mid‐1990s. This leads to the change in the relationship between the NPO and El Niño and the Southern Oscillation (ENSO). The NPO is closely associated with the convective forcing in the tropical Pacific during boreal winter before the mid‐1990s. After the mid‐1990s, in contrast, the simultaneous relationship of the NPO and ENSO during boreal winter becomes weak. However, an eastward shift of the NPO's southern lobe SLP during boreal winter causes a close relationship with the ENSO in the winter of the following year through atmosphere‐ocean coupled processes after the mid‐1990s. These results indicate that atmospheric circulation in the North Pacific characterized by the NPO becomes more influential in the tropical Pacific with a lagged time after the mid‐1990s, likely due to the eastward shift in the NPO's structure. We also briefly discuss which processes cause an eastward shift in the NPO's southern lobe SLP.

Decadal Shift of NAO-Linked Interannual Sea Level Variability along the U.S. Northeast Coast

Recent studies have linked interannual sea level variability and extreme events along the U.S. northeast coast (NEC) to the North Atlantic Oscillation (NAO), a natural internal climate mode that prevails in the North Atlantic Ocean. The correlation between the NAO index and coastal sea level north of Cape Hatteras was weak from the 1960s to the mid-1980s, but it has markedly increased since around 1987. The causes for the decadal shift remain unknown. Yet understanding the abrupt change is vital for decadal sea level prediction and is essential for risk management. Here we use a robust method, the Bayesian dynamic linear model (DLM), to explore the nonstationary NAO impact on NEC sea level. The results show that a spatial pattern change of NAO-related winds near the NEC is a major cause of the NAO–sea level relationship shift. A new index using regional sea level pressure is developed that is a significantly better predictor of NEC sea level than is the NAO and is strongly linked to the intensity of westerly winds near the NEC. These results point to the vital importance of monitoring regional changes of wind and sea level pressure patterns, rather than the NAO index alone, to achieve more accurate predictions of sea level change along the NEC.

Atmospheric forcing of upwelling along the south-eastern Baltic coast

The meteorological forcing on the occurrence of upwelling along the south-eastern Baltic Sea coast (Lithuanian-Latvian sector) is analysed in this study. The sea level pressure patterns and the locations of pressure centres inducing and inhibiting upwelling were identified. The research was performed for the years 1982–2017, for the months of May–September, when the sea waters are thermally stratified and the phenomenon is detectable. The frequency of upwelling is the highest in June (approximately 15%), July and August (11–13%) and the lowest in September (7%). The central and northern part of the Lithuanian–Latvian coast is most favourable for upwelling occurrence (frequency up to 20% in summer months). The main features of the sea level pressure patterns that induce upwelling in the research area are positive pressure anomalies spreading over Northern Europe and the Norwegian Sea, while negative anomalies encompass Southern Europe. Airflow around the anticyclonic centres gives a north-eastern component to the wind direction over the Lithuanian-Latvian shore. Two circulation types were recognized as inducing the occurrence of upwelling along the Lithuanian–Latvian coast. Both of them are characterized by the anticyclonic centres located west or northwest of the study area and intensify the northerly or north-easterly airflow over the research area. Different pressure patterns with the negative anomalies of sea level pressure spreading over the North Sea and the positive anomalies underlying Central Europe inhibit upwelling along the Lithuanian–Latvian coast. Such pressure conditions, bring about the western airflow component. More constant western winds restrain the upwelling process and bring about normal thermal stratification of coastal waters. A detailed analysis allowed the recognition of two circulation types inhibiting coastal upwelling in the study area. They reveal dipole patterns of sea level pressure anomalies, but the two inhibiting patterns differ substantially in the intensities and locations of the pressure centres and in wind conditions.

Dynamical response of Mediterranean precipitation to greenhouse gases and aerosols

Abstract. Atmospheric aerosols and greenhouse gases affect cloud properties, radiative balance and, thus, the hydrological cycle. Observations show that precipitation has decreased in the Mediterranean since the beginning of the 20th century, and many studies have investigated possible mechanisms. So far, however, the effects of aerosol forcing on Mediterranean precipitation remain largely unknown. Here we compare the modeled dynamical response of Mediterranean precipitation to individual forcing agents in a set of global climate models (GCMs). Our analyses show that both greenhouse gases and aerosols can cause drying in the Mediterranean and that precipitation is more sensitive to black carbon (BC) forcing than to well-mixed greenhouse gases (WMGHGs) or sulfate aerosol. In addition to local heating, BC appears to reduce precipitation by causing an enhanced positive sea level pressure (SLP) pattern similar to the North Atlantic Oscillation–Arctic Oscillation, characterized by higher SLP at midlatitudes and lower SLP at high latitudes. WMGHGs cause a similar SLP change, and both are associated with a northward diversion of the jet stream and storm tracks, reducing precipitation in the Mediterranean while increasing precipitation in northern Europe. Though the applied forcings were much larger, if forcings are scaled to those of the historical period of 1901–2010, roughly one-third (31±17 %) of the precipitation decrease would be attributable to global BC forcing with the remainder largely attributable to WMGHGs, whereas global scattering sulfate aerosols would have negligible impacts. Aerosol–cloud interactions appear to have minimal impacts on Mediterranean precipitation in these models, at least in part because many simulations did not fully include such processes; these merit further study. The findings from this study suggest that future BC and WMGHG emissions may significantly affect regional water resources, agricultural practices, ecosystems and the economy in the Mediterranean region.

Non-seasonal sea level variations in the Korea Strait and regional atmospheric conditions

Kim, J.-K.; Choi, B.-J., and Lee S.-H., 2018. Non-seasonal sea level variations in the Korea Strait and regional atmospheric conditions. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 581–585. Coconut Creek (Florida), ISSN 0749-0208.To investigate dominant sea level variations in the Korea Strait (KS) and to find their responsible factors, Empirical Orthogonal Function (EOF) analysis was performed on the non-seasonal components of sea level, wind, and sea level pressure data from 1993 to 2014. The non-seasonal sea level variation accounts for 20% of the total sea level variations in the KS. The first EOF mode of the sea level explains 63.8% of the total variance and represents strait-wide simultaneous sea level oscillations. EOF analysis on the surface wind data over the KS and the East China Sea was performed and the first EOF of winds, explaining 49.5% of the total variance, represents the strength of the northeasterly wind. The amplitude time series of the first EOFs from wind and sea level data has correlation coefficients (r) of 0.48. The sea level and wind data have common spectral peaks at periods of 5.1 and 10.2 months. Variance and the first EOF of the steric height in the KS are relatively high around Jeju Island while they are small near the south coast of Korea. Correlation between the amplitude time series of sea level and steric height first modes is low (r = 0.20). The basin-wide simultaneous sea level variations with intra-seasonal and interannual periods in the KS are found to be modulated by variations in the regional surface winds and sea level pressure patterns over the East China Sea and the KS rather than by changes in local steric height.

Influence of the Scandinavian climate pattern on the UK asthma mortality: a time series and geospatial study

ObjectiveIt is well known that climate variability and trends have an impact on human morbidity and mortality, especially during the winter. However, there are only a handful of studies that...

Trends in winter circulation over the British Isles and central Europe in twenty-first century projections by 25 CMIP5 GCMs

Winter midlatitude atmospheric circulation has been extensively studied for its tight link to surface weather, and automated circulation classifications have often been used to this end. Here, eight such classifications are applied to daily sea level pressure patterns simulated by an ensemble of CMIP5 GCMs twenty-first century projections for the British Isles and central Europe in order to robustly estimate future changes in frequency, persistence, and strength of synoptic-scale circulation there. All methods are able to identify present-day biases of models reported before, such as an overestimated occurrence of zonal flow and underestimation of anticyclonic conditions and easterly advection, although the strength of these biases varies among the methods. In future, models show that the zonal flow will become more frequent while the strength of the mean flow is not projected to change. Over the British Isles, the models that better simulate the latitude of zonal flow over the historical period indicate a slight equatorward shift of westerlies in their projections, while the poleward expansion of circulation—expected in future at global scale—is apparent in those models that have large errors. Over central Europe, some classifications indicate an increase in persistence and especially in frequency of anticyclonic types, which is, however, shown to be rather an artifact of some methods than a real feature. On the other hand, the easterly flow is robustly projected to become markedly weaker in central Europe, which we hypothesize might be an important factor contributing to the projected decrease of cold extremes there.

Differences in Meteorological Conditions between Days with Persistent and Non-Persistent Pollution in Beijing, China

We compared the regional synoptic patterns and local meteorological conditions during persistent and non-persistent pollution events in Beijing using US NCEP–Department of Energy reanalysis outputs and observations from meteorological stations. The analysis focused on the impacts of high-frequency (period < 90 days) variations in meteorological conditions on persistent pollution events (those lasting for at least 3 days). Persistent pollution events tended to occur in association with slow-moving weather systems producing stagnant weather conditions, whereas rapidly moving weather systems caused a dramatic change in the local weather conditions so that the pollution event was short-lived. Although Beijing was under the influence of anomalous southerly winds in all four seasons during pollution events, notable differences were identified in the regional patterns of sea-level pressure and local anomalies in relative humidity among persistent pollution events in different seasons. A region of lower pressure was present to the north of Beijing in spring, fall, and winter, whereas regions of lower and higher pressures were observed northwest and southeast of Beijing, respectively, in summer. The relative humidity near Beijing was higher in fall and winter, but lower in spring and summer. These differences may explain the seasonal dependence of the relationship between air pollution and the local meteorological variables. Our analysis showed that the temperature inversion in the lower troposphere played an important part in the occurrence of air pollution under stagnant weather conditions. Some results from this study are based on a limited number of events and thus require validation using more data.

Contribution of atmospheric circulation to recent off-shore sea-level variations in the Baltic Sea and the North Sea

Abstract. The main purpose of this study is to quantify the contribution of atmospheric factors to recent off-shore sea-level variability in the Baltic Sea and the North Sea on interannual timescales. For this purpose, we statistically analysed sea-level records from tide gauges and satellite altimetry and several climatic data sets covering the last century. Previous studies had concluded that the North Atlantic Oscillation (NAO) is the main pattern of atmospheric variability affecting sea level in the Baltic Sea and the North Sea in wintertime. However, we identify a different atmospheric circulation pattern that is more closely connected to sea-level variability than the NAO. This circulation pattern displays a link to sea level that remains stable through the 20th century, in contrast to the much more variable link between sea level and the NAO. We denote this atmospheric variability mode as the Baltic Sea and North Sea Oscillation (BANOS) index. The sea-level pressure (SLP) BANOS pattern displays an SLP dipole with centres of action located over (5° W, 45° N) and (20° E, 70° N) and this is distinct from the standard NAO SLP pattern in wintertime. In summertime, the discrepancy between the SLP BANOS and NAO patterns becomes clearer, with centres of action of the former located over (30° E, 45° N) and (20° E, 60° N). This index has a stronger connection to off-shore sea-level variability in the study area than the NAO in wintertime for the period 1993–2013, explaining locally up to 90 % of the interannual sea-level variance in winter and up to 79 % in summer. The eastern part of the Gulf of Finland is the area where the BANOS index is most sensitive to sea level in wintertime, whereas the Gulf of Riga is the most sensitive region in summertime. In the North Sea region, the maximum sea-level sensitivity to the BANOS pattern is located in the German Bight for both winter and summer seasons. We investigated, and when possible quantified, the contribution of several physical mechanisms which may explain the link between the sea-level variability and the atmospheric pattern described by the BANOS index. These mechanisms include the inverse barometer effect (IBE), freshwater balance, net energy surface flux and wind-induced water transport. We found that the most important mechanism is the IBE in both wintertime and summertime. Assuming a complete equilibration of seasonal sea level to the SLP gradients over this region, the IBE can explain up to 88 % of the sea-level variability attributed to the BANOS index in wintertime and 34 % in summertime. The net energy flux at the surface is found to be an important factor for the variation of sea level, explaining 35 % of sea-level variance in wintertime and a very small amount in summer. The freshwater flux could only explain 27 % of the variability in summertime and a negligible part in winter. In contrast to the NAO, the direct wind forcing associated with the SLP BANOS pattern does not lead to transport of water from the North Sea into the Baltic Sea in wintertime.

Climatological Characteristics of Historical and Future High-Wind Events in Alaska

High winds cause waves, storm surge, erosion and physical damage to infrastructure and ecosystems. However, there have been few evaluations of wind climatologies and future changes, especially change in high-wind events, on a regional basis. This study uses Alaska as a regional case study of climatological wind speed and direction. Eleven first-order stations across different subregions of Alaska provide historical data (1975-2005) for the observational climatology and for the calibration of Coupled Model Inter comparison Project (CMIP5) simulations, which in turn provide projections of changes in winds through 2100. Historically, winds exceeding 25 and 35 knots are most common in the Bering Sea coastal region of Alaska, followed by northern Alaska coastal areas. Autumn and winter are the seasons of most frequent high-wind occurrences in the coastal sites, while there is no distinct seasonal peak at the interior stations where high-wind events are less frequent. An examination of the sea level pressure pattern associated with the highest-wind event at each station reveals the presence of a strong pressure gradient associated with an extratropical cyclone in most cases. Northern coastal regions of Alaska are projected to experience increased frequencies of high-wind events during the cold season, especially late autumn and early winter, when reduced sea ice cover in the late century will leave coastal regions increasingly vulnerable to flooding and erosion.

Linkage between global sea surface temperature and hydroclimatology of a major river basin of India before and after 1980

The frequent occurrence of flood and drought worldwide has drawn attention to assessing whether the hydroclimatology of major river basins has changed. The Mahanadi river basin (MRB) is the major source of fresh water for both Chattisgarh and Odisha states (71 million people approximately) in India. The MRB (141 600 km2 area) is one of the most vulnerable to climate change and variations in temperature and precipitation. In recent years, it has repeatedly faced adverse hydrometeorological conditions. Large-scale ocean–atmospheric phenomena have a substantial influence on river hydroclimatology. Hence global sea surface temperature (SST) linkage with the precipitation and surface temperature of the MRB was analyzed over the period 1950–2012. Significant changes in seasonal correlation patterns were witnessed from 1950–1980 (PR-80) to 1981–2012 (PO-80). The correlation was higher during PR-80 compared to PO-80 between the El Niño region SST versus the maximum temperature (Tmax) in all seasons except the pre-monsoon season and the minimum temperature (Tmin) in all seasons except the monsoon season. However, precipitation correlation changes are not prominent. Like the SST, the correlation patterns of sea level pressure with precipitation, Tmax and Tmin shifted conspicuously from PR-80 to PO-80. These shifts could be related to change in Pacific decadal SST patterns and anthropogenic effects. Fingerprint-based detection and attribution analysis revealed that the observed changes in Tmin (pre-monsoon and monsoon season) during the second half of the 20th century cannot be explained solely by natural variability and can be attributed to an anthropogenic effect.

Impact of atmospheric circulation types on southwest Asian dust and Indian summer monsoon rainfall

This study examines the meteorological feedback on dust aerosols and rainfall over the Arabian Sea and India during the summer monsoon using satellite data, re-analysis and a regional climate model. Based on days with excess aerosol loading over the central Ganges basin during May – September, two distinct atmospheric circulation types (weather clusters) are identified, which are associated with different dust-aerosol and rainfall distributions over south Asia, highlighting the role of meteorology on dust emissions and monsoon rainfall. Each cluster is characterized by different patterns of mean sea level pressure (MSLP), geopotential height at 700hPa (Z700) and wind fields at 1000hPa and at 700hPa, thus modulating changes in dust-aerosol loading over the Arabian Sea. One cluster is associated with deepening of the Indian/Pakistan thermal low leading to (i) increased cyclonicity and thermal convection over northwestern India and Arabian Peninsula, (ii) intensification of the southwest monsoon off the Horn of Africa, iii) increase in dust emissions from Rub-Al-Khali and Somalian deserts, (iv) excess dust accumulation over the Arabian Sea and, (v) strengthening of the convergence of humid air masses and larger precipitation over Indian landmass compared to the other cluster. The RegCM4.4 model simulations for dust-aerosol and precipitation distributions support the meteorological fields and satellite observations, while the precipitation over India is positively correlated with the aerosol loading over the Arabian Sea on daily basis for both weather clusters. This study highlights the key role of meteorology and atmospheric dynamics on dust life cycle and rainfall over the monsoon-influenced south Asia.

Aspects of intraseasonal variability of Antarctic sea ice in austral winter related to ENSO and SAM events

ABSTRACTWe performed an Empirical Orthogonal Function (EOF) analysis to assess the intraseasonal variability of 5–60 day band-pass filtered Antarctic sea-ice concentration in austral winter using a 20-year daily dataset from 1995 to 2014. Zonal wave number 3 dominated in the Antarctic, especially so across the west Antarctic. Results showed the coexistence of stationary and propagating wave components. A spectral analysis of the first two principal components (PCs) showed a similar structure for periods up to 15 days but generally more power in PC1 at longer periods. Regression analysis upon atmospheric fields using the first two PCs of sea-ice concentration showed a coherent wave number 3 pattern. The spatial phase delay between the sea-ice and mean sea-level pressure patterns suggests that meridional flow and associated temperature advection are important for modulating the sea-ice field. EOF analyses carried out separately for El Niño, La Niña and neutral years, and for Southern Annular Mode positive, negative and neutral periods, suggest that the spatial patterns of wave number 3 shift between subsets. The results also indicate that El Niño-Southern Oscillation and Southern Annular Mode affect stationary wave interactions between sea-ice and atmospheric fields on intraseasonal timescales.

The Northern Hemisphere Extratropical Atmospheric Circulation Response to ENSO: How Well Do We Know It and How Do We Evaluate Models Accordingly?

Application of random sampling techniques to composite differences between 18 El Niño and 14 La Niña events observed since 1920 reveals considerable uncertainty in both the pattern and amplitude of the Northern Hemisphere extratropical winter sea level pressure (SLP) response to ENSO. While the SLP responses over the North Pacific and North America are robust to sampling variability, their magnitudes can vary by a factor of 2; other regions, such as the Arctic, North Atlantic, and Europe are less robust in their SLP patterns, amplitudes, and statistical significance. The uncertainties on the observed ENSO composite are shown to arise mainly from atmospheric internal variability as opposed to ENSO diversity. These observational findings pose considerable challenges for the evaluation of ENSO teleconnections in models. An approach is proposed that incorporates both pattern and amplitude uncertainty in the observational target, allowing for discrimination between true model biases in the forced ENSO response and apparent model biases that arise from limited sampling of non-ENSO-related internal variability. Large initial-condition coupled model ensembles with realistic tropical Pacific sea surface temperature anomaly evolution during 1920–2013 show similar levels of uncertainty in their ENSO teleconnections as found in observations. Because the set of ENSO events in each of the model composites is the same (and identical to that in observations), these uncertainties are entirely attributable to sampling fluctuations arising from internal variability, which is shown to originate from atmospheric processes. The initial-condition model ensembles thus inform the interpretation of the single observed ENSO composite and vice versa.

Changed Relationship between Snowfall over the Yeongdong region of the Korean Peninsula and Large-scale Factors

A typical snowfall pattern occurs over the east coastal region of the Korean Peninsula, known as the Yeongdong region. The precipitation over the Yeongdong region is influenced by the cold and dry northeasterly wind which advects over warm and moist sea surface of the East Sea of Korea. This study reveals the influence of large-scale factors, affecting local to remote areas, on the mesoscale snowfall system over the Yeongdong region. The National Centers for Environmental Prediction-Department of Energy reanalysis dataset, Extended Reconstructed sea surface temperature, and observed snowfall data are analyzed to reveal the relationship between February snowfall and large-scale factors from 1981 to 2014. The Yeongdong snowfall is associated with the sea level pressure patterns over the Gaema Plateau and North Pacific near the Bering Sea, which is remotely associated to the sea surface temperature (SST) variability over the North Pacific. It is presented that the relationship between the Yeongdong snowfall and large-scale factors is strengthened after 1999 when the central north Pacific has warm anomalous SST. These enhanced relationships explain the atmospheric patterns of recent strong snowfall years (2010, 2011, and 2014). It is suggested that the newly defined index in this study based on related SST variability can be used for a seasonal predictor of the Yeongdong snowfall with 2-month leading.

Observed modes of sea surface temperature variability in the South Pacific region

The South Pacific (SP) region exerts large control on the climate of the Southern Hemisphere at many times scales. This paper identifies the main modes of interannual sea surface temperature (SST) variability in the SP which consist of a tropical-driven mode related to a horseshoe structure of positive/negative SST anomalies within midlatitudes and highly correlated to ENSO and Interdecadal Pacific Oscillation (IPO) variability, and another mode mostly confined to extratropical latitudes which is characterized by zonal propagation of SST anomalies within the South Pacific Gyre. Both modes are associated with temperature and rainfall anomalies over the continental regions of the Southern Hemisphere. Besides the leading mode which is related to well known warmer/cooler and drier/moister conditions due to its relationship with ENSO and the IPO, an inspection of the extratropical mode indicates that it is associated with distinct patterns of sea level pressure and surface temperature advection. These relationships are used here as plausible and partial explanations to the observed warming trend observed within the Southern Hemisphere during the last decades.

Mistral and Tramontane wind systems in climate simulations from 1950 to 2100

The characteristics of the mesoscale Mistral and Tramontane winds under changing climate conditions are of great interest for risk assessments. In this study, a classification algorithm is applied to identify Mistral and Tramontane-permitting sea-level pressure patterns, thus allowing for estimates of their future characteristics. Five simulations with three regional climate models on a 0.44^circ grid and five global circulation models are assessed for the representative concentration pathways (RCPs) 4.5 and 8.5. Regional climate simulations driven by ERA-Interim are used to test the classification algorithm and to estimate its accuracy. The derived Mistral and Tramontane time series are discussed. The results for the ERA-Interim period show that the classification algorithm and the regional climate models work well in terms of the number of Mistral and Tramontane days per year, but the results overestimate the average length of such events. For both the RCPs, only small changes in Mistral frequency were found in both regional and global climate simulations. Most simulations show a decrease in Tramontane frequencies and average period lengths during the 21st century. Regional climate simulations using RCP8.5 show fewer Tramontane events than those using RCP4.5.

Inter-annual climate variability in Europe during the Oligocene icehouse

New sclerochronological data suggest that a variability comparable to the North Atlantic Oscillation (NAO) was already present during the middle Oligocene, about 20Myr earlier than formerly assumed. Annual increment width data of long-lived marine bivalves of Oligocene (30–25Ma) strata from Central Europe revealed a distinct quasi-decadal climate variability modulated on 2–12 (mainly 3–7) year cycles. As in many other modern bivalves, these periodic changes in shell growth were most likely related to changes in primary productivity, which in turn, were coupled to atmospheric circulation patterns. Stable carbon isotope values of the shells (δ13Cshell) further corroborated the link between shell growth and food availability. Sub-decal oscillations in the 3–7year band in other annually resolved fossil archives were often interpreted as El Niño-Southern Oscillation (ENSO) cycles. This possibility is discussed in the present study. However, combined shell-derived proxy and numerical climate model data lend support to the interpretation of a NAO-like variability. According to numerical climate models, winter sea-level pressure (wSLP) and precipitation rate (wPR) across Central Europe during the Oligocene exhibited a pattern similar to the modern NAO. The simulated NAO index for the Oligocene shows periodicities coherent with those revealed by the proxy data (2.5–6years), yet, on shorter wavelengths than the modern NAO (biennial and 6–10year cycles). Likely, the different paleogeography and elevated atmospheric CO2 concentrations not only influenced the sea-level pressure pattern, but also the temporal variability of the NAO precursor. The present study represents the first attempt to characterize the inter-annual climate variability in Central Europe during the Oligocene and sets the basis for future studies on the early phase of the Cenozoic icehouse climate state.

Seasonal prediction of US summertime ozone using statistical analysis of large scale climate patterns

We develop a statistical model to predict June-July-August (JJA) daily maximum 8-h average (MDA8) ozone concentrations in the eastern United States based on large-scale climate patterns during the previous spring. We find that anomalously high JJA ozone in the East is correlated with these springtime patterns: warm tropical Atlantic and cold northeast Pacific sea surface temperatures (SSTs), as well as positive sea level pressure (SLP) anomalies over Hawaii and negative SLP anomalies over the Atlantic and North America. We then develop a linear regression model to predict JJA MDA8 ozone from 1980 to 2013, using the identified SST and SLP patterns from the previous spring. The model explains ∼45% of the variability in JJA MDA8 ozone concentrations and ∼30% variability in the number of JJA ozone episodes (>70 ppbv) when averaged over the eastern United States. This seasonal predictability results from large-scale ocean-atmosphere interactions. Warm tropical Atlantic SSTs can trigger diabatic heating in the atmosphere and influence the extratropical climate through stationary wave propagation, leading to greater subsidence, less precipitation, and higher temperatures in the East, which increases surface ozone concentrations there. Cooler SSTs in the northeast Pacific are also associated with more summertime heatwaves and high ozone in the East. On average, models participating in the Atmospheric Model Intercomparison Project fail to capture the influence of this ocean-atmosphere interaction on temperatures in the eastern United States, implying that such models would have difficulty simulating the interannual variability of surface ozone in this region.