Winter Mean Sea Level Pressure Research Articles

Changing processes flooding a salt marsh in a microtidal estuary with a drying climate

Estuarine salt marshes globally face numerous threats, not least of which include changing hydrological conditions from human alteration and climate change impact to river flows, sea levels and coastal processes. While changing inundation is evident in many systems, often, the detail of which estuarine processes are changing and to what extent they contribute to flooding and habitat distribution remains unknown. Water levels in the microtidal Swan River Estuary (Derbarl Yerrigan), Western Australia, which has experienced significant climate drying since the 1970's, were disaggregated to assess contributions from tides, mean sea level, barometric effects, river flows and river-tide interactions. These contributions were mapped to the habitat of a salt marsh community. The effect of declining river flows on tides were further assessed by wavelet and harmonic analyses. We found that tides and barometric effects presently dominate flooding events of relevance to the salt marsh community. Declining winter runoff resulted in an increase in the tidal amplitude in the upper estuary. There was also a positive winter mean sea level pressure trend, associated with the winter rainfall decline. Altogether, there was zero net change to flooding of a salt marsh in the estuary from these processes. Therefore, steady sea level rise masked changes in the relative contribution of flooding mechanisms in the estuary which have implications for the stability of the marsh ecosystem. Disaggregating process contributions to salt marsh water levels offers a means to better assess the hydrodynamic processes presently sustaining salt marsh communities and to inform how they might change in the future. These results show that numerous hydrological processes can interact to mask non-stationary changes to estuarine hydrology supporting salt marsh habitat.

A Statistical Downscaling Model Based on Multiway Functional Principal Component Analysis for Southern Australia Winter Rainfall

Abstract We propose a statistical downscaling model based on multiway functional principal component analysis (FPCA) for rainfall prediction. The model mainly explains the relationship between the winter mean sea level pressure (MSLP) and rainfall in southern Australia from the perspective of functional data. In comparison with the traditional approach of feature extraction based on principal component analysis, the multiway FPCA needs fewer principal components not only to capture the most variance in MSLP but also to greatly avoid the loss of spatial information. A functional principal component (FPC) regression is further developed to simulate both current and future rainfall. The main results show that the first five leading FPCs are sufficient to capture the spatial characteristics of winter MSLP, achieving the purpose of efficient dimensionality reduction. Specifically, no more than three FPCs are required to develop the functional downscaling models for the winter rainfall over four studied regions. The functional downscaling model provides a good skill in terms of the correlation higher than 0.7 between the predictions and observations and the ratio of root-mean-square error to the climatology of winter rainfall below 20% over four regions. The developed downscaling models are further used to interpret the MSLP patterns from four CMIP5 climate models [ACCESS1.3, BCC_CSM1.1(m), CESM1(CAM5), and MPI-ESM-MR], which have been used to simulate both present-day and future climate. The resulting downscaled values based on ensemble MSLP provide 1) a closer representation of observed present-day rainfall than the raw climate model values and 2) alternative estimates of future changes in rainfall that arise from changes in MSLP.

How Persistent Are North Atlantic–European Sector Weather Regimes?

Persistent weather regimes in daily North Atlantic–European winter mean sea level pressure (MSLP) fields from the 140-yr Twentieth Century Reanalysis are investigated. The phase space is divided into discrete cells based on quantiles of empirical orthogonal function (EOF) principal components; the cells are thus approximately equally populated. An estimate of persistence is provided in terms of the number of different cells visited for a given trajectory duration. This technique is also applied to the well-known Lorenz63 system, which clearly exhibits two regimes, and the more complex Lorenz96 system where the regime structure is less pronounced. While the analysis identifies the two regimes of both the Lorenz63 and Lorenz96 systems, evidence for comparable regimes in the MSLP data is weaker. Recurrent weather regimes produced by k-means clustering might be expected to be clearly linked to slower-moving regions of phase space, but this is shown not to be the case. Only the region of phase space associated with the negative phase of the North Atlantic Oscillation (NAO) shows any regime-like behavior. Nevertheless, the analysis does reveal some structure to the time evolution of the atmospheric circulation—transitions between neighboring pairs of cells show a preferred direction of evolution in many cases.

Regional climate projections of mean and extreme climate for the southwest of Western Australia (1970–1999 compared to 2030–2059)

Projections of future climate change (1970–1999 compared to 2030–2059) for southwest Western Australia (SWWA) are analysed for a regional climate model (RCM) ensemble using the Weather Research and Forecasting Model with boundary conditions from three CMIP3 general circulation models (GCMs); CCSM3, CSIROmk3.5 and ECHAM5. We show that the RCM adds value to the GCM and we suggest that this is through improved representation of regional scale topography and enhanced land–atmosphere interactions. Our results show that the mean daytime temperature increase is larger than the nighttime increase, attributed to reduced soil moisture and hence increased surface sensible heat flux in the model, and there is statistically significant evidence that the variance of minimum temperatures will increase. Changes in summer rainfall are uncertain, with some models showing rainfall increases and others projecting reductions. All models show very large fluctuations in summer rainfall intensity which has important implications because of the increased risk of flash flooding and erosion of arable land. There is model consensus indicating a decline in winter rainfall and the spatial distribution of this rainfall decline is influenced by regional scale topography in two of the three simulations. Winter rainfall reduction is consistent with the historical trend of declining rainfall in SWWA, which has been attributed in previous research to a reduction in the number of fronts passing over the region. The continuation of this trend is evident in all models by an increase in winter mean sea level pressure in SWWA, and a reduced number of winter front days. Winter rainfall does not show any marked variations in daily intensity.

Downwelling longwave radiation and atmospheric winter states in the western maritime Arctic

Hourly time-series of several atmospheric state variables and downwelling longwave radiation (LWd) collected through the extended winter (mid-December 2007 to March 2008) of the International Polar Year-Circumpolar Flaw Lead (IPY-CFL) ship-board campaign in Amundsen Gulf were analysed. A histogram derived from the hourly LWd time-series revealed a positively (right) skewed frequency distribution. The cold, dry, and mainly clear weather or mean atmospheric states concurrent with the modal and adjacent classes showed that LWd observations within ±30 W m−2 of the modal class mark (160 W m−2) identify the presence of the semi-permanent cold-core high pressure centre which appears on average winter mean sea-level pressure maps for the western maritime Arctic. The warmer, moister, and cloudier weather or mean atmospheric states associated with LWd bins in the right tail of the positively skewed frequency distribution, and the weather coincident with the deepest mean sea-level pressure minima, showed that observations of LWd in the 210–280 W m−2 range identified the passage of baroclinic disturbances. While a right skewed LWd frequency distribution usually occurs, the class mark of the major modal class, and the class marks of the bins in the right tail of the distribution show inter-annual variability of ±10 to 30 W m−2. Owing to its inclusive nature, validation of modelled LWd may provide a comprehensive verification statistic for Arctic climate simulations.

Interannual and interdecadal variations in the North Atlantic Oscillation spatial shift

The spatial shift of the North Atlantic Oscillation (NAO) is analyzed by using the Twentieth Century Reanalysis version 2 dataset and identifying NAO action centers directly on winter mean sea-level pressure (SLP) anomaly maps. The spatial shift of the NAO is characterized by four NAO spatial shift indices: the zonal and meridional shifts of the NAO southern and northern action centers. It is found that the zonal and meridional shift trends of the NAO action centers move along a path of southwest-northwest direction. Spectral analysis shows that the four NAO spatial shift indices have periodicity of 2–6 years and the NAO index has periodicity of 2–3 years in terms of high-frequency variations. On a decadal time scale, the NAO spatial shift indices are closely (positively) related to the NAO index, which is in agreement with previous studies of the relationship between the NAO index and the spatial shift of the NAO pattern. However, there is no relationship between the NAO index and the meridional shift of the northern action center on an interannual time scale. The significant relationship between the NAO index and the interannual variability of NAO spatial shift indices is very likely to be associated with synoptic-scale Rossby wave breaking, which generates surface pressure anomalies and thus affects the phase and pattern of the NAO. The correlations of winter westerly winds over 90°W-0° and the NAO index and the NAO spatial shift indices have a ‘+ − + −’ structure from the Equator to the North Pole. Although there is close correlation between the NAO spatial shift indices and the strength of the zonal winds in the North Atlantic region, the effect of the zonal winds on the NAO spatial shift differs at different latitudes. Hence, the role of the zonal winds is probably a result of the NAO spatial shifts.

Temporal Variability in the Expression of the Arctic Oscillation in the North Pacific

Abstract Although the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) have been identified as important modes of climate variability during the Northern Hemisphere (NH) winter, whether the AO or the NAO is more fundamental to the description of this variability, especially in the North Pacific, is still an open question. An important contributor to this uncertainty is the lack of knowledge of the low-frequency linkages between the North Atlantic and North Pacific Oceans. This paper explores the linkage between the two oceanic basins on interdecadal time scales using the sea level pressure (SLP) field during the twentieth century. In particular, it is shown that the winter mean SLP in the North Pacific was positively correlated with the sign of the NAO during the periods of 1925–50 and 1980–98, which resulted in the classical AO pattern being the dominant mode in the NH. In contrast, during the period of 1951–79, the winter mean SLP in the two basins was decoupled, resulting in a dominant mode that more closely resembled the NAO. Using paleoclimate reconstructions, it is also shown that this interdecadal variability in the North Pacific climate began around 1850, which is nominally considered to be the end of the Little Ice Age.

Arctic dipole anomaly and its contribution to sea ice export from the Arctic Ocean in the 20th century

The winter dipole anomaly (DA) in the Arctic atmosphere and its contribution to sea ice export are investigated by using a high‐resolution coupled global general circulation model. The spatial distributions of the first two leading EOF modes of winter mean sea level pressure (SLP) and geopotential height at 500 hPa north of 70°N obtained by the long‐term simulation (1900–2010) are highly similar to those derived from the National Center for Environmental Prediction and the National Center for the Atmospheric Research (NCEP/NCAR) reanalysis datasets (1948–2004). The first‐leading mode corresponds to the Arctic Oscillation (AO). The DA is defined as the second‐leading mode. The AO and DA account for 59% and 19% of the total variance, respectively. Composite spatial patterns of SLP, sea ice thickness and velocity in the extreme years when both the absolute values of principal component (PC1 and PC2) exceed 1.0 standard deviation indicate that the DA plays a great important role in sea ice export from the Arctic Ocean to the Greenland Sea due to its strong meridionality. Sea ice export is highly promoted (restricted) in the positive (negative) DA phase. The dependence of sea ice export on the DA is comparable to or rather larger than that on the AO.

Decadal and longer changes of the winter sea level pressure fields and related synoptic activity over the North Atlantic

Long-term intensity changes in the winter synoptic activity over the North Atlantic are studied in relation to changes of the winter mean sea level pressure (SLP) fields. Analysis of linear trends has revealed a good agreement between long-term (interdecadal) changes in the intensity of synoptic processes and variations of the winter SLP. On the contrary, no such agreement was found between detrended and low-pass filtered anomalies. There are periods, that are characterised by the enhanced (reduced) synoptic activity attributed to the low (high) index of the North Atlantic Oscillation (NAO). It appears that low-passed anomalies of intramonthly root mean square deviations (RMSD) of SLP are negatively correlated with NAO and East Atlantic (EA) teleconnection patterns over most of the North Atlantic. The low-passed winter SLP anomalies demonstrate both propagating and standing patterns. The latter have a period of about 8 years. While meridional dipole-like structures formed by the winter SLP anomalies are shifted to the west (east) of the North Atlantic, the related anomalies of synoptic activity tend to be located in the eastern (western) part of the region. When decadally averaged, anomalies in the intensity of synoptic activity are strongly linked to the North Atlantic storm track position. The exception is the 1980–1990 decade, characterised by a very high NAO index. During this decade, enhanced synoptic activity is observed to the south of the North Atlantic storm track. Copyright © 1999 Royal Meteorological Society

Time and Space Spectral Analyses of Southern Hemisphere Sea Level Pressure Variability

Abstract This study examines the time-space structure of the standard deviation of daily summer and winter mean sea level pressure over the Southern Hemisphere, as identified in 20 years of analyses generated by the Australian Bureau of Meteorology and two long simulations with a GCM. The unfiltered variability derived from the operational analyses generally display a deal of zonal symmetry, particularly during the summer period, with maxima in the midlatitudes. The percentage of the temporal variance which is explained by the bandpass and low-pass components is calculated; in January and July the percentage of the variance explained by the bandpass data is maximized between 30° and 60°S and assumes values of typically 25%. In general, the low-pass data account for more of the variance and tends to achieve its maxima in low and high latitudes. The greatest contribution to the low-frequency field comes from the planetary-scale waves, particularly at higher latitudes. The synoptic and small-scale waves are ...

Climatic Trends in the Southern Hemisphere

Abstract Observations of monthly mean sea level pressure, surface air temperature, and 500 mb and 300 mb geopotential heights and temperatures are used to study trends in the Southern Hemisphere from 1951–81. The winter mean sea level pressure fell over the Indian/Atlantic half of the hemisphere from the 1950s to the 1960s, and rose over the other half. Generally, these trends reversed from the 1960s to the 1970s. The trends are equivalent barotropic. The trends of temperatures are often regionally dependent. There was a significant warming over Antarctica from the 1960s to 1970s at all upper levels except for a small area on the Indian Ocean side.