Large-scale Sea Level Pressure Field Research Articles

Influence of Long-Term Changes in the Large-Scale Sea Level Pressure Field on the Wind Regime and the Wind Stress Curl in the Black Sea

Purpose. The paper is aimed at studying the relationship between the wind regime and the wind stress curl in the Black Sea and the long-term changes in the large-scale sea level pressure field in winter months. Methods and Results. The data on wind speed and sea level pressure in January – February from the NCEP/NСAR reanalysis for 1948–2018 are used. Based on the 6-hour data, the synoptic conditions accompanied by high and low values of the wind stress curl in the sea were determined. The synoptic situations in which a vast anticyclone is located north and northeast of the sea, and the area of low pressure – to the southwest of the sea in the Mediterranean region, are accompanied by the northeast and east winds, and by the cyclonic curl predominance. On the contrary, passing of the cyclones to the north of the sea and increase of pressure to the southwest are followed by the westerly and southwesterly winds, and by the anticyclonic curl predominance. Extremely high monthly mean values of the cyclonic curl were observed in those years, when the area occupied by the Siberian anticyclone increased and expanded westward, so that the Black Sea was on the southwestern periphery of its spur. Extremely low values of the anticyclonic curl were noted when the Azores anticyclone area expanded to the Mediterranean region. The wind stress curl changes on the multidecadal scales have shown its relation to the global changes in the field of the sea level pressure and the sign of the pressure anomalies at the low latitudes. Conclusions. The opposite sign of the surface pressure anomalies to the northeast and southwest of the sea is accompanied by the highest values of the wind stress curl.

A multivariate extreme wave and storm surge climate emulator based on weather patterns

Coastal floods often coincide with large waves, storm surge and tides. Thus, joint probability methods are needed to properly characterize extreme sea levels. This work introduces a statistical downscaling framework for multivariate extremes that relates the non-stationary behavior of coastal flooding events to the occurrence probability of daily weather patterns. The proposed method is based on recently-developed weather-type methods to predict extreme events (e.g., significant wave height, mean wave period, surge level) from large-scale sea-level pressure fields. For each weather type, variables of interest are modeled using Generalized Extreme Value (GEV) distributions and a Gaussian copula for modelling the interdependence between variables. The statistical dependence between consecutive days is addressed by defining a climate-based extremal index for each weather type. This work allows attribution of extreme events to specific weather conditions, enhancing the knowledge of climate-driven coastal flooding.

Mistral and Tramontane wind speed and wind direction patterns in regional climate simulations

The Mistral and Tramontane are important wind phenomena that occur over southern France and the northwestern Mediterranean Sea. Both winds travel through constricting valleys before flowing out towards the Mediterranean Sea. The Mistral and Tramontane are thus interesting phenomena, and represent an opportunity to study channeling effects, as well as the interactions between the atmosphere and land/ocean surfaces. This study investigates Mistral and Tramontane simulations using five regional climate models with grid spacing of about 50 km and smaller. All simulations are driven by ERA-Interim reanalysis data. Spatial patterns of surface wind, as well as wind development and error propagation along the wind tracks from inland France to offshore during Mistral and Tramontane events, are presented and discussed. To disentangle the results from large-scale error sources in Mistral and Tramontane simulations, only days with well simulated large-scale sea level pressure field patterns are evaluated. Comparisons with the observations show that the large-scale pressure patterns are well simulated by the considered models, but the orographic modifications to the wind systems are not well simulated by the coarse-grid simulations (with a grid spacing of about 50 km), and are reproduced slightly better by the higher resolution simulations. On days with Mistral and/or Tramontane events, most simulations underestimate (by 13 % on average) the wind speed over the Mediterranean Sea. This effect is strongest at the lateral borders of the main flow—the flow width is underestimated. All simulations of this study show a clockwise wind direction bias over the sea during Mistral and Tramontane events. Simulations with smaller grid spacing show smaller biases than their coarse-grid counterparts.

Pressure effects on past regional sea level trends and variability in the German Bight

The impact on a large-scale sea level pressure field to the regional mean sea level changes of the German Bight is analysed. A multiple linear regression together with an empirical orthogonal function analysis is used to describe the relationship between the sea level pressure and the regional mean sea level considering the time period 1924–2001. Both, the part of the variability and of the long-term trend that can be associated with changes in the sea level pressure, are investigated. Considering the whole time period, this regression explains 58 % of the variance and 33 % of the long-term trend of the regional mean sea level. The index of agreement between the regression result and the observed time series is 0.82. As a proxy for large-scale mean sea level changes, the mean sea level of the North East Atlantic is subsequently introduced as an additional predictor. This further improves the results. For that case, the regression explains 74 % of the variance and 87 % of the linear trend. The index of agreement rises to 0.92. These results suggest that the sea level pressure mainly accounts for the inter-annual variability and parts of the long-term trend of regional mean sea level in the German Bight while large-scale sea level changes in the North East Atlantic account for another considerable fraction of the observed long-term trend. Sea level pressure effects and the mean sea level of the North East Atlantic provide thus significant contributions to regional sea level rise and variability. When future developments are considered, scenarios for their future long-term trends thus need to be comprised in order to provide reliable estimates of potential future long-term changes of mean sea level in the German Bight.

Springtime connections between the large-scale sea-level pressure field and gust wind speed over Iberia and the Balearics

Abstract. This paper investigates, by means of Singular Value Decomposition analysis, the springtime relationships between the mean sea-level pressure field over the North Atlantic and the regional wind gusts over the Iberian Peninsula, identifying the main atmospheric circulation patterns linked to gust wind speed anomaly configurations. The statistical significance of the obtained modes is investigated by means of Monte Carlo approach. The analysis highlighted that the covariability is dominated by two main large-scale features of the atmospheric circulation over the North Atlantic. The first mode relates to Iberian gust wind speeds to the Scandinavian pattern (SCAND), linking the large-scale pattern to above-normal wind gusts. The second covariability mode, associated with the North Atlantic Oscillation (NAO) pattern, correlates with maximum wind speeds over Iberia. An enhanced spring NAO pattern is related to positive (negative) wind gust correlations over northern (southern) Iberia. To find true relationships between large-scale atmospheric field and the gust wind speeds, composite maps were built up to give an average atmospheric circulation associated with coherent wind gust variability over Iberia.

Sea level variability at Adriatic coast and its relationship to atmospheric forcing

Abstract. Sea level (SLH) variability at the Adriatic coast was investigated for the period 1872–2001 using monthly average values of observations at 13 tide gauge stations. Linear trends and seasonal cycles were investigated first and removed afterwards from the data. Empirical Orthogonal Functions (EOF) analysis was used further on remaining anomalies (SLA) to extract the regional intermonthly variability of SLH. It was found that the leading EOF and its principal component (PC) explain a major part of SLA variability (92%). The correlation between the reconstructed SLA, based on leading EOF and its PC, and overlapping observed SLA values for selected tide gauge stations is between 0.93 and 0.99. Actual SLH values at tide gauge stations can be reconstructed and some gaps in the data can be filled in on the basis of estimated SLA values if reasonable estimates of long-term trends and seasonal cycles are also available. A strong, seasonally dependent relationship between SLA at the Adriatic coast and atmospheric forcing, represented by sea level pressure (SLP) fields, was also found. Comparing the time series of leading PC and gridded SLP data for the period 1948–2001, the highest correlation coefficients (r) of –0.92 in winter, –0.84 in spring, –0.66 in summer, and –0.91 in autumn were estimated for a SLP grid point located in northern Italy. The SLP variability on this grid point contains information about the isostatic response of the sea level at the Adriatic coast, but can also be treated as a sort of teleconnection index representing the large-scale SLP variability across central and southern Europe. To some extent the large-scale SLP variability that affects the SLA at the Adriatic coast can be related to the North Atlantic Oscillation (NAO), because significant correlations were found between the NAO index and the first PC of SLA (rwinter=–0.56, rspring=–0.45, rsummer=–0.48, and rautumn=–0.43) for the period 1872–2001. The use of partial least-squares (PLS) regression between large-scale SLP fields and SLA only slightly improved the description of the SLA dependence on SLP forcing in comparison to the single grid point approach. A strong relationship between atmospheric pressure and the sea level could represent an additional possibility for filling in the gaps in the tide gauge data. Keywords. Oceanography: general (Climate and interannual variability) – Oceanography: physical (Air-sea interactions; sea level variations)

Cod and climate variability in the Barents Sea

Interannual variability of temperature in the Kola section (Barents Sea) and the abun- dance as 0-group (age 5 mo) and recruits (age 3 yr), spawning stock biomass, and survival of Arcto- Norwegian cod in the Barents Sea were related to climate variability using a multivariate regression model. The results show that in the Barents Sea the temperature anomalies are significantly and highly correlated to climate variables such as large-scale sea-level pressure fields and the North Atlantic Oscillation index. A significant and high correlation was detected between the temperature in the Barents Sea and both the 0-group index and recruitment of Arcto-Norwegian cod. A phase lag of 2 yr appears in recruitment. The high model skill and excellent correlation indicate that it is possi- ble to predict the future development of Arcto-Norwegian cod stocks from climate-change scenarios.

Analysis of the sea-level variability along the Chinese coast and estimation of the impact of a CO2-perturbed atmospheric circulation

The relationship between monthly sea-level data measured at stations located along the Chinesecoast and concurrent large-scale atmospheric forcing in the period 1960–1990 is examined. Itis found that sea-level varies quite coherently along the whole coast, despite the geographicalextension of the station set. A canonical correlation analysis between sea-level and sea-levelpressure (SLP) indicates that a great part of the sea-level variability can be explained by theaction of the wind stress on the ocean surface. The relationship between sea-level and sea-levelpressure is analyzed separately for the summer and winter half-years. In winter, one factoraffecting sea-level variability at all stations is the SLP contrast between the continent and thePacific Ocean, hence the intensity of the winter Monsoon circulation. Another factor that affectscoherently all stations is the intensity of the zonal circulation at mid-latitudes. In the summerhalf year, on the other hand, the influence of SLP on sea-level is spatially less coherent: thestations in the Yellow Sea are affected by a more localized circulation anomaly pattern, whereasthe rest of the stations is more directly connected to the intensity of the zonal circulation. Basedon this analysis, statistical models (different for summer and winter) to hindcast coastal sealevelanomalies from the large-scale SLP field are formulated. These models have been testedby fitting their internal parameters in a test period and reproducing reasonably the sea-levelevolution in an independent period. These statistical models are also used to estimate thecontribution of the changes of the atmospheric circulation on sea-level along the Chinese coastin an altered climate. For this purpose the ouput of 150 year-long experiment with the coupled ocean-atmosphere model ECHAM1-LSG has been analyzed, in which the atmospheric concentrationof greenhouse gases was continuously increased from 1940 until 2090, according to theScenario A projection of the Intergovermental Panel on Climate Change. In this experimentthe meridional (zonal) circulation relevant for sea-level tends to become weaker (stronger) inthe winter half year and stronger (weaker) in summer. The estimated contribution of thisatmospheric circulation changes to coastal sea-level is of the order of a few centimeters at theend of the integration, being in winter negative in the Yellow Sea and positive in the China Seawith opposite signs in the summer half-year.

Seasonal Sea-level Variations in San Francisco Bay in Response to Atmospheric Forcing, 1980

The seasonal response of sea level in San Francisco Bay (SFB) to atmospheric forcing during 1980 is investigated. The relations between sea-level data from the Northern Reach, Central Bay and South Bay, and forcing by local wind stresses, sea-level pressure (SLP), runoff and the large-scale sea-level pressure field are examined in detail. The analyses show that the sea-level elevations and slopes respond to the along-shore wind stressTyat most times of the year, and to the cross-shore wind stressTxduring two transition periods in spring and autumn. River runoff raises the sea-level elevation during winter. It is shown that winter precipitation in the SFB area is mainly attributed to the atmospheric circulation associated with the Aleutian Low, which transports the warm, moist air into the Bay area. A multiple linear regression model is employed to estimate the independent contributions of barometric pressure and wind stress to adjusted sea level. These calculations have a simple dynamical interpretation which confirms the importance of along-shore wind to both sea level and north-south slope within the Bay.