Negative Pressure Anomalies Research Articles

Evolution of Antarctic Sea Ice Ahead of the Record Low Annual Maximum Extent in September 2023

AbstractThe 2023 Antarctic sea ice extent (SIE) maximum on 7 September was the lowest annual maximum in the satellite era (16.98 × 106 km2), with the largest contributions to the anomaly coming from the Ross (37.7%, −0.57 × 106 km2) and Weddell (32.9%, −0.49 × 106 km2) Seas. The SIE was low due to anomalously warm (>0.3°C) upper‐ocean temperatures combined with anomalously strong northerly winds impeding the ice advance during the fall and winter. Northerly winds of >12 ms−1 in the Weddell Sea occurred because of negative pressure anomalies over the Antarctic Peninsula, while those in the Ross Sea were associated with extreme blocking episodes off the Ross Ice Shelf. The Ross Sea experienced an unprecedented SIE decrease of −1.08 × 103 km2 d−1 from 1 June till the annual maximum. The passage of quasi‐stationary and explosive polar cyclones contributed to periods of southward ice‐edge shift in both sectors.

Anomalies Of Isobaric Fields In The Southwestern Indian Ocean During El Nino Climatic Events

The el Nino climate event has produced upheavals in the global climate. Among many others is the prolonged drought in the Sahel. In fact, these upheavals are the direct consequences of changes in the centers of action. Therefore, we are interested in knowing their configuration in the South-West Indian Ocean basin, SWOI, both on the ground and at altitude during the two seasons, summer and winter in the southern hemisphere. It has been observed that there are negative pressure anomalies in the western part of the SWOI and positive ones in its eastern part; moreover the geopotential levels 700hPa and 500hPa include positive anomalies in the north and negative anomalies in the south. These mid-altitude results continue at high altitude, with the tropopause experiencing low anomalies on the equator side and high anomalies on the south pole side. As a result on the other hand, the southern winter experiences more moisture than normal. The significance of the images showing isobaric field anomalies was tested by the Student t-test.

Seasonal Antarctic pressure variability during the twentieth century from spatially complete reconstructions and CAM5 simulations

As most permanent observations in Antarctica started in the 1950s, understanding Antarctic climate variations throughout the twentieth century remains a challenge. To address this issue, the non-summer multi-decadal variability in pressure reconstructions poleward of 60°S is evaluated and assessed in conjunction with climate model simulations throughout the twentieth and early twenty-first centuries to understand historical atmospheric circulation variability over Antarctica. Austral autumn and winter seasons show broadly similar patterns, with negative anomalies in the early twentieth century (1905–1934), positive pressure anomalies in the middle twentieth century (1950–1980), and negative pressure anomalies in the most recent period (1984–2013), consistent with concurrent trends in the SAM index. In autumn, the anomalies are significant in the context of estimates of interannual variability and reconstruction uncertainty across most of the Antarctic continent, and the reconstructed patterns agree best with model-generated patterns when the simulation includes the forced response to tropical sea surface temperatures and external radiative forcing. In winter and spring, the reconstructed anomalies are less significant and are consistent with internal atmospheric variability alone. The specific role of tropical SST variability on pressure trends in these seasons is difficult to assess due to low reconstruction skill in the region of strongest tropical teleconnections, the large internal atmospheric variability, and uncertainty in the SST patterns themselves. Indirect estimates of pressure variability, whether through sea ice reconstructions, proxy records, or improved models and data assimilation schemes, will help to further constrain the magnitude of internal variability relative to the forced responses expected from SST trends and external radiative forcing.

The influence of the Pacific Decadal Oscillation on North Central China precipitation during boreal autumn

ABSTRACTDecadal variations in autumn (September–October–November, SON) precipitation over North Central China since the 1940s are investigated using observations and reanalysis data. They are found to be significantly correlated negatively with the Pacific Decadal Oscillation (PDO). Further analyses indicate that the inter‐decadal changes in SON precipitation over North Central China result from anomaly patterns in the large‐scale atmospheric circulation in the mid‐latitude Northern Hemisphere associated with the PDO. Specifically, during the negative phase of the PDO (1945–1976 and 2003–2014), an anomalous low pressure and cyclonic circulation are seen west of Lake Baikal, while positive pressure anomalies are located over the East Asia‐Japan‐North Pacific region, which weakens the East Asian trough (EAT). The weakened EAT favours southerly winds over North Central China. The negative pressure anomalies around Mongolia and Lake Baikal induce large‐scale ascent motion to the west of the EAT, which also produces southerly advection of warm and moist air into North Central China, leading to increased precipitation there. These results reinforce the notion that PDO has a large impact on SON rainfall over North Central China on decadal timescales and highlight the need to further examine how the PDO induces the Eurasia atmospheric teleconnection pattern (e.g. East Atlantic/Western Russia pattern).

Stratospheric polar vortex splits and displacements in the high‐top CMIP5 climate models

AbstractSudden stratospheric warming (SSW) events can occur as either a split or a displacement of the stratospheric polar vortex. Recent observational studies have come to different conclusions about the relative impacts of these two types of SSW upon surface climate. A clearer understanding of their tropospheric impact would be beneficial for medium‐range weather forecasts and could improve understanding of the physical mechanism for stratosphere‐troposphere coupling. Here we perform the first multimodel comparison of stratospheric polar vortex splits and displacements, analyzing 13 stratosphere‐resolving models from the fifth Coupled Model Intercomparison Project (CMIP5) ensemble. We find a wide range of biases among models in both the mean state of the vortex and the frequency of vortex splits and displacements, although these biases are closely related. Consistent with observational results, almost all models show vortex splits to occur barotropically throughout the depth of the stratosphere, while vortex displacements are more baroclinic. Vortex splits show a slightly stronger North Atlantic surface signal in the month following onset. However, the most significant difference in the surface response is that vortex displacements show stronger negative pressure anomalies over Siberia. This region is shown to be colocated with differences in tropopause height, suggestive of a localized response to lower stratospheric potential vorticity anomalies.

Statistical Seasonal Rainfall Forecast in the Neuquén River Basin (Comahue Region, Argentina)

A detailed statistical analysis was performed at the Neuquén river basin using precipitation data for 1980–2007. The hydrological year begins in March with a maximum in June associated with rainfall and another relative maximum in October derived from snow-break. General features of the rainy season and the excess or deficits thereof are analyzed using standardized precipitation index (SPI) for a six-month period in the basin. The SPI has a significant cycle of 14.3 years; the most severe excess (SPI greater than 2) has a return period of 25 years, while the most severe droughts (SPI less than −2) have a return period of 10 years. The SPI corresponding to the rainy season (April–September) (SPI9) has no significant trend and is used to classify wet/dry years. In order to establish the previous circulation patterns associated with interannual SPI9 variability, the composite fields of wet and dry years are compared. There is a tendency for wet (dry) periods to take place during El Niño (La Niña) years, when there are positive anomalies of precipitable water over the basin, when the zonal flow over the Pacific Ocean is weakened (intensified) and/or when there are negative pressure anomalies in the southern part of the country and Antarctic sea. Some prediction schemes using multiple linear regressions were performed. One of the models derived using the forward stepwise method explained 42% of the SPI9 variance and retained two predictors related to circulation over the Pacific Ocean: one of them shows the relevance of the intensity of zonal flow in mid-latitudes, and the other is because of the influence of low pressure near the Neuquén River basin. The cross-validation used to prove model efficiency showed a correlation of 0.41 between observed and estimated SPI9; there was a probability of detection of wet (dry) years of 80% (65%) and a false alarm relation of 25% in both cases.

Radionuclide Transport Behavior in a Generic Geological Radioactive Waste Repository

We performed numerical simulations of groundwater flow and radionuclide transport to study the influence of several factors, including the ambient hydraulic gradient, groundwater pressure anomalies, and the properties of the excavation damaged zone (EDZ), on the prevailing transport mechanism (i.e., advection or molecular diffusion) in a generic nuclear waste repository within a clay-rich geological formation. By comparing simulation results, we show that the EDZ plays a major role as a preferential flowpath for radionuclide transport. When the EDZ is not taken into account, transport is dominated by molecular diffusion in almost the totality of the simulated domain, and transport velocity is about 40% slower. Modeling results also show that a reduction in hydraulic gradient leads to a greater predominance of diffusive transport, slowing down radionuclide transport by about 30% with respect to a scenario assuming a unit gradient. In addition, inward flow caused by negative pressure anomalies in the clay-rich formation further reduces transport velocity, enhancing the ability of the geological barrier to contain the radioactive waste. On the other hand, local high gradients associated with positive pressure anomalies can speed up radionuclide transport with respect to steady-state flow systems having the same regional hydraulic gradients. Transport behavior was also found to be sensitive to both geometrical and hydrogeological parameters of the EDZ. Results from this work can provide useful knowledge toward correctly assessing the post-closure safety of a geological disposal system.

Indian Ocean Dipole and southern high latitude precipitation: possible links

Southern high latitude precipitation during austral spring in relation to the Indian Ocean Dipole (IOD) and ENSO is investigated in the present study. Both the IOD and ENSO generate Rossby waves trains that create positive and negative pressure anomalies. These anomalous pressure centres generate meridional moisture fluxes that impact the precipitation. Influence of the IOD is detected mainly in the Ross sea region, where the southward moisture transport induced by the low pressure cell enhances precipitation. During strong IOD years, east Antarctica near 100°E, is also characterised by enhanced precipitation induced by the southward moisture transport by a high pressure cell located south of Australia. In the Dronning Maud Land, precipitation is linked to the moisture advection through the Atlantic during ENSO years and not during the IOD years.

Palaeoglacial and palaeoclimatic conditions in the NW Pacific, as revealed by a morphometric analysis of cirques upon the Kamchatka Peninsula

The distribution of glacial cirques upon the Kamchatka peninsula, Far Eastern Russia, is systematically mapped from satellite images and digital elevation model data. A total of 3758 cirques are identified, 238 of which are occupied by active glaciers. The morphometry of the remaining 3520 cirques is analysed. These cirques are found to show a very strong N bias in their azimuth (orientation), likely resulting from aspect-related variations in insolation. The strength of this N bias is considered to indicate that former glaciation upon the peninsula was often ‘marginal’, and mainly of cirque-type, with peaks extending little above regional equilibrium-line altitudes. This is supported by the fact that S and SE-facing cirques are the highest in the dataset, suggesting that glacier-cover was rarely sufficient to allow S and SE-facing glaciers to develop at low altitudes. The strength of these azimuth-related variations in cirque altitude is thought to reflect comparatively cloud-free conditions during former periods of glaciation. It is suggested that these characteristics, of marginal glaciation and comparatively cloud-free conditions, reflect the region's former aridity, which was likely intensified at the global Last Glacial Maximum, and during earlier periods of ice advance, as a result of the development of negative pressure anomalies over the North Pacific (driven by the growth of the Laurentide Ice Sheet), combined with other factors, including an increase in the extent and duration of sea ice, a reduction in global sea levels, cooler sea surface temperatures, and the localised growth of mountain glaciers. There is published evidence to suggest extensive glaciation of the Kamchatka Peninsula at times during the Late Quaternary, yet the data presented here appear to suggest that such phases were comparatively short-lived, and that smaller cirque-type glaciers were generally more characteristic of the period.

Synoptic conditions of heavy snowfalls in europe

Synoptic conditions of heavy snowfalls at four stations in urope (slo, remen, molensk and udapest) were analyzed using daily data of snow cover depth from 50 winter seasons (1960/1961–2009/2010). Composite maps of the sea level pressure and 500 geopotential height means and anomalies were presented for the days with an increase in snow depth by ≥10 cm, for each station separately. In the same way, maps were presented for the 850-hPa temperature and the precipitable water content. Additionally, 48 h back trajectories of air masses for the chosen days with the highest amount of snowfall were constructed, using the model. Negative anomalies of sea level pressure and 500-hPa heights, which correspond to low‐pressure systems spreading over urope, are prerequisites for the occurrence of heavy snowfalls at each analyzed location. In udapest, heavy snowfalls appear as a result of cyclonic cold fronts from the editerranean ea. Another typical location of snow‐bringing low‐pressure systems is the altic ea region. Negative pressure anomalies over the orth tlantic with a low‐pressure centre over the orwegian ea cause snowfalls in slo. There are two main sources of air masses bringing abundant snowfalls in urope: the tlantic region and editerranean. According to back trajectories, the humid air masses are transported over long distances and on the way they are shifted from the low troposphere in the source regions to the upper layers.

Cross‐shelf transport of warm and saline water in response to sea ice drift on the Laptev Sea shelf

Oceanographic moorings and conductivity‐temperature‐depth (CTD) surveys from September 2009 to September 2010 are used to describe recent changes in the Laptev Sea hydrography and to highlight wind‐ and ice‐driven surface Ekman transport as the mechanism to translate these changes from the outer‐ to the inner‐shelf bottom waters. In February 2010, moored oceanographic instruments recorded a sudden increase in temperature (+0.8°C) and salinity (+ >3) near the bottom of the inner Laptev Sea shelf. Such warm and saline waters had not been previously observed on the inner shelf in winter. They likely originated from the basin and were first observed during a summer 2009 CTD survey in the northwestern shelf break region, subsequently spreading east and shoreward across the Laptev Sea shelf. The changes were introduced to the mooring site by the first of a series of bottom‐intensified flow events with velocities reaching 20 cm s−1, topographically guided along a relic submarine river valley. Each of the flow events coincided with negative pressure anomalies at the mooring site and offshore‐directed (upwelling‐favorable) winds and ice drift. We suggest that the observations to first order resemble a simplified two‐dimensional two‐layered ocean, where offshore surface Ekman transport is compensated for by a barotropic shoreward response flow near the bottom. In this paper, we use one of the first comprehensive long‐term Laptev Sea datasets to highlight ice‐ocean‐atmosphere interactions in early and late winter and discuss the role of freshwater, stratification, and ice mobility on under‐ice circulation on the Laptev Sea shelf.

Analysing and combining atmospheric general circulation model simulations forced by prescribed SST: northern extratropical response

The ECHAM 3.2 (T21), ECHAM 4 (T30) and LMD (version 6, grid-point resolution with 96 longitudes × 72 latitudes) atmospheric general circulation models were integrated through the period 1961 to 1993 forced with the same observed Sea Surface Temperatures (SSTs) as compiled at the Hadley Centre. Three runs were made for each model starting from different initial conditions. The mid-latitude circulation pattern which maximises the covariance between the simulation and the observations, i.e. the most skilful mode, and the one which maximises the covariance amongst the runs, i.e. the most reproducible mode, is calculated as the leading mode of a Singular Value Decomposition (SVD) analysis of observed and simulated Sea Level Pressure (SLP) and geopotential height at 500 hPa (Z500) seasonal anomalies. A common response amongst the different models, having different resolution and parametrization should be considered as a more robust atmospheric response to SST than the same response obtained with only one model. A robust skilful mode is found mainly in December-February (DJF), and in June-August (JJA). In DJF, this mode is close to the SST-forced pattern found by Straus and Shukla (2000) over the North Pacific and North America with a wavy out-of-phase between the NE Pacific and the SE US on the one hand and the NE North America on the other. This pattern evolves in a NAO-like pattern over the North Atlantic and Europe (SLP) and in a more N-S tripole on the Atlantic and European sector with an out-of-phase between the middle Europe on the one hand and the northern and southern parts on the other (Z500). There are almost no spatial shifts between either field around North America (just a slight eastward shift of the highest absolute heterogeneous correlations for SLP relative to the Z500 ones). The time evolution of the SST-forced mode is moderatly to strongly related to the ENSO/LNSO events but the spread amongst the ensemble of runs is not systematically related at all to the intensity of Nino3.4 SST anomalies. The leading reproducible mode in JJA is clearer and more skilful for SLP than for Z500 and also seems related to the SST time evolution of tropical Pacific. It is characterised by an out-of-phase between the whole North Pacific and a horseshoe shaped area from Eastern Siberia and Gulf of Mexico. The leading OM mode found in MAM and SON, are quite close to the DJF one (at least for the modelled anomalies), but they are less skilful than in DJF. The most skilful mode (i.e. SLP-Z500 mode in DJF and SLP mode in JJA) is almost similar to the most reproducible one during these particular seasons. In MAM and SON, the SST-forced pattern is very close to the wintertime one. The warm episodes in the central and eastern tropical Pacific are then associated with negative pressure anomalies at the sea level and also at 500 hPa over the whole North Pacifkc and from SE US Coast to Western Europe (from SE US Coast to Scandinavia for Z500) and positive pressure anomalies on Central Canada, north of 55°-60°N across the North Atlantic and also over Northern Siberia (in MAM). The variance forced by SST are lower in MAM and SON than in DJF and, as suggested above, the skill of this SST-forced mode is weak in MAM and almost close to zero in SON.

On the late twentieth century decrease in Australian east coast rainfall extremes

AbstractA change from negative to positive circulation anomalies over New South Wales (NSW) during the 1970s resulted in a decrease in east coastal low‐pressure systems and accompanying rain. Negative pressure anomalies over eastern Australia, anomalous onshore low‐level winds and larger NSW rainfall totals since 1950, occur in the cool phase of the IPO rather than the warm phase (post‐1976). Larger NSW extreme annual rainfall totals occurred prior to 1977 and were characterized by negative mean sea level pressure (MSLP) anomalies over eastern Australia. In contrast, extreme annual rainfall totals from 1977 were smaller and characterized by positive MSLP anomalies. Copyright © 2008 Royal Meteorological Society

The effects of El Niño–La Niña on reproductive parameters of elephant seals feeding in the Bellingshausen Sea

AbstractAim To assess the impacts of El Niño–La Niña events on the pup weaning mass and diet of female southern elephant seals (Mirounga leonina) feeding in the Bellingshausen Sea, Antarctica, and to understand the ecological processes that drive these impacts.Location Atlantic southern elephant seal weaning mass and diet were measured at King George Island (62º14′ S, 58º30′ W). Feeding areas for pregnant female seals from King George Island are located west of Alexander Island in the Bellingshausen Sea.Methods Data on weaning mass were collected between 1985 and 1994 during the breeding season (September–November). Moulting females were anaesthetized and cephalopod beaks were isolated and identified from stomach contents obtained from stomach lavages. Sea‐surface temperature anomaly (SSTA) data for the ‘El Niño 3.4’ geographical region (5º N–5º S, 120º W–170º W) were used to define El Niño–Southern Oscillation (ENSO) event years (grouped as El Niño, La Niña and Neutral) as well as the strength of each ENSO event year. Using data from the US National Center for Environmental Prediction, temperature, sea ice concentration and atmospheric pressure anomalies in the Bellingshausen Sea were calculated from March to August, corresponding to the feeding period of pregnant female seals.Results Positive temperature anomalies and negative pressure anomalies in the Bellingshausen Sea were observed during La Niña years and negative temperature anomalies and positive pressure anomalies during El Niño years. These data correlate with sea ice concentration anomalies, which are highly negative during La Niña years and highly positive during El Niño years. Warm temperature conditions in the Bellingshausen Sea during La Niña years are strongly related to both higher weaning mass in elephant seals and to an increase in squid beaks in the stomach contents of females.Main conclusions It is possible that higher elephant seal weaning masses in La Niña years correlate with warmer waters in the Bellingshausen Sea leading to the rapid growth of squid and their more frequent descents to depths frequented by elephant seals. This results in increased predation by pregnant females, leading to a greater mass among weaned pups. This hypothesis may guide future research about interactions between climate and the marine biosphere.

Evidence for recent changes in a surface‐air warming singularity in late winter over central North America

Evidence for an earlier onset of spring in recent years is supported by daily temperature data from central North America. From 1976–1999, mean temperatures during February 15th through 24th were appreciably higher (5°C or more over large areas) than during 1952–1975. This is associated with a strengthened Aleutian Low for those calendar days during 1976–1999, with negative pressure anomalies extending well into North America. This apparently reflects a decrease in strong, cold, high‐pressure systems moving from western Canada into the north central United States during late February. These changes appear to be related to conditions in and over the North Pacific Ocean, possibly in conjunction with global warming, although the latter signal is not clearly obvious. Comparison of time series of daily minimum temperatures for the earlier period (1952–1975) and the recent period (1976–1999) indicates other times of the year when an appreciable warming has occurred; such warming is largely confined to the first half of the calendar year.

Vorticity and pressure anomalies in the western Mediterranean during El NiñO/Southern Oscillation extremes

By means of a statistical study of percentiles, applied to bi-monthly seasons of composite 2-year periods centred in the year of occurrence of a warm or cold ENSO extreme, we show the climatic influence of these events on the western Mediterranean area. The study is developed over a 30-year period (1961–1990). The variables used are the number of cyclonic days and the number of days having negative pressure anomalies in the area. Daily sea-level atmospheric pressure data have been used to estimate both daily mean wind and vorticity in the study zone. The main influence found is that after a warm extreme year, during the March–April bi-month, there is a decrease in the number of days with both cyclonic circulation and negative pressure anomaly. © 1997 by the Royal Meteorological Society. Int. J. Climatol. 17: 475–482, 1997.

Large-Scale Atmospheric Circulation Features of Warm and Cold Episodes in the Tropical Pacific

Ship observations of sea surface temperature (SST), sea level pressure and surface wind, and satellite measurements of outgoing longwave radiation (OLR) (an indicator of deep tropical convection) are used to describe the large-scale atmospheric circulation over the tropical Pacific during composite warm and cold episodes. Results are based on linear regression analysis between the circulation parameters and an index of SST in the tropical Pacific during the period 1946–85 (1974–89 for OLR). Warm episodes along the Peru coast (i.e., El Niño events) and basin-wide warmings associated with the Southern Oscillation are examined separately. Charts of the total as well as anomalous fields of SST, sea level pressure, surface wind and OLR for both warm and cold episodes are presented. SST and surface wind anomalies associated with warm episodes are consistent with the results of Rasmusson and Carpenter (1982). El Niño events are characterized by strong positive SST anomalies along the coasts of Ecuador and Peru and along the equator eastward of 130°W, and by an equatorward expansion and intensification of the Inter Tropical Convergence Zone (ITCZ) over the eastern Pacific. Basin-wide warm episodes exhibit positive SST anomalies along the equator eastward of 170°E, a southward expansion and intensification of the ITCZ, and an eastward shift and strengthening of the Indonesian convective zone. The movements of the precipitation zones are in good agreement with anomalous large scale surface wind convergence, Meridional wind anomalies dominate the anomalous surface convergence throughout the tropical Pacific. Surface winds are consistent with the sea level pressure distribution, with down-gradient flow near the equator, and with Ekman balance in the subtropics. A center of below normal sea level pressure over the equatorial eastern Pacific, distinct from the negative pressure anomalies over the subtropical southeast Pacific, is observed during basin-wide warm episodes. This equatorial feature is highly correlated with local SST and appears to be a boundary layer phenomenon. There is a net increase in deep convection over the tropical Pacific during warm episodes. Enhanced convection in the ITCZ during warm years is not accompanied by a net increase in surface wind convergence. A comparison between precipitation and surface wind convergence suggests that moisture convergence extends through a deeper layer in the equatorial western Pacific than in the ITCZ over the eastern Pacific. The contrasting distributions of surface relative humidity, total cloudiness and air-sea temperature difference over the eastern tropical Pacific during basin-wide warm and cold episodes are described in the context of boundary layer processes.

On Atmospheric Pressure Oscillations Along the Pacific Coast of North America, 1873–1963

Abstract An analysis is made of 90-year sea level atmospheric pressure records along the Pacific coast of North America and in adjacent regions. An investigation of the power spectra of monthly mean and extreme pressure fluctuations in the frequency range between 0 and 6 cycles per year showed that significant periodicities were associated only with the annual and semiannual pressure oscillations, and that there have been no secular changes. The amplitude of the annual pressure oscillation varies between 1 and 7 mb, that of the semiannual one between 1 and 3 mb. Large amplitude changes over short distances are observed when proceeding from a maritime to a desert environment. The extreme pressure range at individual stations varies from 33 mb in Hawaii to 94 mb in Alaska. The mean duration of positive and negative pressure anomalies (from long term monthly means) is of the order of 2 or 3 months; the extreme duration of these anomalies varies between 10 and 20 months. The probability functions for the extr...