Pressure Anomalies Research Articles

Synchronous Variation Patterns of Monthly Sea Ice Anomalies at the Arctic and Antarctic

Abstract Sea ice variability in the opposite polar regions is examined holistically by applying the self-organizing map (SOM) method to global monthly sea ice concentration data over two periods. The results show that the variability modes of sea ice decrease in the Arctic correspond to an overall sea ice increase in the Antarctic, and vice versa. In particular, the monthly sea ice anomaly patterns are dominated by in-phase variability across the Arctic that is stronger in the marginal seas particularly the Barents Sea than the central Arctic Ocean. The corresponding Antarctic sea ice variability is characterized by a zonal wavenumber-3 structure or a dipole pattern of out-of-phase variability between the Bellingshausen/Amundsen Seas and the rest of the Southern Ocean. The frequency of occurrence of these dominant patterns exhibits pronounced seasonal as well as decadal variability and the latter is closely related to the Pacific decadal oscillation and Atlantic multidecadal oscillation. Other less frequent patterns seem to be associated with the central Pacific El Niño and spatially heterogeneous interannual variability of sea surface temperature (SST) in the Indian and the Atlantic Oceans. The dominant modes explain 57% of the four-decade domain-averaged trends in the annual polar sea ice concentration, with more explained in the eastern than western Arctic Ocean and in the Weddell Sea and the Amundsen Sea in the Antarctic. The spatial patterns of the leading modes can be largely explained by the dynamic (sea ice drift) and thermodynamic (sea ice melt) effects of the anomalous atmospheric circulations associated with SST and sea level pressure anomalies. Significance Statement The purpose of this study is to extract the main modes of monthly global sea ice concentration variability in the past four decades, explain the mechanisms behind the occurrences of these modes, and examine the contributions of these modes to the trend in annual global sea ice concentration. Sea ice extent in the past four decades has shown a significant declining trend in the Arctic and a slight, but significant increasing trend in the Antarctic. By jointly analyzing the sea ice variability and trends in the two polar regions, the results here provide a reference for what might have contributed to the opposite sea ice trends in Arctic and Antarctic and highlight the important influence of large-scale sea surface temperature anomalies on the trends in the two polar regions.

Impact of SST on Present and Future Extreme Precipitation in Hokkaido Investigated Considering Weather Patterns

AbstractThis study investigated the impact of sea surface temperature (SST) on extreme precipitation events in North Japan and its relation to synoptic weather patterns using large‐ensemble climate simulations. Eight weather patterns were identified by applying cluster analysis to sea level pressure anomalies for selected days with extreme precipitation derived from a 3000‐year historical climate experiment. Interannual variability of extreme precipitation days associated with two specific weather patterns, characterized by a weak low‐pressure system and an atmospheric river (AR), significantly correlated with that of SST over the Sea of Japan, with a correlation coefficient of 0.37 and 0.53, respectively. In higher SST years, the increased atmospheric moisture can increase the extreme precipitation in the inland area of Hokkaido for the weather pattern associated with a weak low‐pressure system, whereas it appears to enhance orographic precipitation along the western slopes of mountains for the pattern resembling AR. These results indicate that the effect of local SST on extreme precipitation strongly depends on the weather patterns. In the future projection, the two weather patterns that are sensitive to SST over the Sea of Japan show a sharp increase of more than 4 times under the 4 K warming climate. Moreover, the magnitude of extreme precipitation was also found to increase with SST, broadly following the Clausius–Clapeyron relation in both historical and warmed climates. These results suggest increased risk of heavy precipitation associated with such weather patterns over North Japan in the future.

Seasonal predictions of energy-relevant climate variables through Euro-Atlantic Teleconnections

The goal of this analysis is the better understanding of how the large-scale atmospheric patterns affect the renewable resources over Europe and to investigate to what extent the dynamical predictions of the large-scale variability might be used to formulate empirical prediction of local climate conditions (relevant for the energy sector). The increasing integration of renewable energy into the power mix is making the electricity supply more vulnerable to climate variability, therefore increasing the need for skillful weather and climate predictions. Forecasting seasonal variations of energy relevant climate variables can help the transition to renewable energy and the entire energy industry to make better informed decision-making. At seasonal timescale climate variability can be described by recurring and persistent, large-scale patterns of atmospheric pressure and circulation anomalies that interest vast geographical areas. The main patterns of the North Atlantic region (Euro Atlantic Teleconnections, EATCs) drive variations in the surface climate over Europe. We analyze reanalysis dataset ERA5 and the multi-system seasonal forecast service provided by Copernicus Climate Change Service (C3S). We found that the observed EATC indices are strongly correlated with surface variables. However, the observed relationship between EATC patterns and surface impacts is not accurately reproduced by seasonal prediction systems. This opens the door to employ hybrid dynamical-statistical methods. The idea consists in combining the dynamical seasonal predictions of EATC indices with the observed relationship between EATCs and surface variables. We reconstructed the surface anomalies for multiple seasonal prediction systems and benchmarked these hybrid forecasts with the direct variable forecasts from the systems and also with the climatology. The analysis suggests that hybrid methodology can bring several improvements to the predictions of energy relevant Essential Climate Variables.

Coupled mode of cloud, atmospheric circulation, and sea ice controlled by wave-3 pattern in Antarctic winter

This study examines coupled relationships among clouds, atmospheric circulation, and sea ice in Antarctic winter. We find that the wave-3 pattern dominates the leading covariability mode among cloud, atmospheric circulation, and sea ice. Both horizontal transport and vertical motion contribute to cloud formation, resulting in maximum cloud anomalies spatially between maximum meridional wind and pressure anomalies in the coupled system. The radiative effect of the clouds related to the wave-3 pattern can generate sea ice anomalies up to 12 cm thick in one month in the Amundsen Sea. It also strengthens the sea ice anomalies that are directly induced by low-level atmospheric circulation anomalies. In addition, the radiative forcing of the leading cloud mode in the lower troposphere is suppressed by the dynamic and thermodynamic effects of the circulation anomalies. These discoveries provide a better understanding of Antarctica’s interactive processes, and also offer physical evidence for climate model validations.

Climatology of Lake-Effect Snow Days Along the Southern Shore of Lake Michigan: What Is the Sensitivity to Environmental Factors and Snowband Morphology?

The Laurentian Great Lakes have substantial influences on regional climatology, particularly with impactful lake-effect snow events. This study examines the snowfall, cloud-inferred snow band morphology, and environment of lake-effect snow days along the southern shore of Lake Michigan for the 1997–2017 period. Suitable days for study were identified based on the presence of lake-effect clouds assessed in a previous study and extended through 2017, combined with an independent classification of likely lake-effect snow days based on independent snowfall data and weather map assessments. The primary goals are to identify lake-effect snow days and evaluate the snowfall distribution and modes of variability, the sensitivity to thermodynamic and flow characteristics within the upstream sounding at Green Bay, WI, and the influences of snowband morphology. Over 300 lake-effect days are identified during the study period, with peak mean snowfall within the lake belt extending from southwest Michigan to northern Indiana. Although multiple lake-effect morphological types are often observed on the same day, the most common snow band morphology is wind parallel bands. Relative to days with wind parallel bands, the shoreline band morphology is more common with a reduced lower-tropospheric zonal wind component within the upstream sounding at Green Bay, WI, as well as higher sea-level pressure and 500-hPa geopotential height anomalies to the north of the Great Lakes. Snowfall is sensitive to band morphology, with higher snowfall for shoreline band structures than for wind parallel bands, especially due south of Lake Michigan. Snowfall is also sensitive to thermodynamic and flow properties, with a greater sensitivity to temperature in southwest Michigan and to flow properties in northwest Indiana.

Classification of Synoptic Patterns With Mesoscale Mechanisms for Downslope Windstorms in Korea Using a Self‐Organizing Map

AbstractDownslope windstorms (DWs) are responsible for wildfires, wind gusts, and turbulence on the lee side of the Taebaek Mountains in the Yeongdong (YD) region of Korea. We classified the synoptic conditions of the windstorms in the YD region using a self‐organizing map (SOM). For windstorm events from 1979 to 2019, sea level pressure anomalies were used to train the SOM. It was found that the synoptic patterns could be classified into three representative types: (1) the south‐high and north‐low pattern in the spring, (2) the west‐high and east‐low pattern in the winter, and (3) the strong low‐pressure system passing through the northern part of Korea. At 850 hPa, prevailing southwesterly and northwesterly flows with warm and cold advections were dominant in Types 1 and 2, respectively. And, Type 3 presented a well‐developed baroclinic system of cyclone. Adiabatic warming by downslope windstorm was the strongest in Type 1, which was likely to have a huge impact on the spread of wildfires. Three mesoscale generation mechanisms were examined under different synoptic patterns. The hydraulic‐jump theory was dominant for the windstorms in Type 2 due to upstream flows with moderate Froude numbers and inversion layers. The partial reflection of mountain waves was found in all types but was more frequent in Type 1 than others. DWs with wave breakings at critical levels mostly occurred in Type 1. This objective classification of weather patterns responsible for DWs in the YD region may improve predictions and future projections of this event, particularly in light of ongoing climate changes.

Impact of the Scandinavian Pattern on Long-Lived Cold Surges over the South China Sea

Abstract This study investigates the influence of the Scandinavian (SCA) pattern on long-lived cold surges over the South China Sea (SCS). The results show that, different from the short-lived ones, the majority of long-lived cold surges over the SCS are preceded by a negative phase of the quasi-stationary SCA pattern in the extratropics, which is characterized as a primary cyclonic center over the Scandinavian Peninsula and two anticyclonic ones over the North Atlantic and central Siberia. This connection is mainly conducted through a continuous amplification of the high pressure anomalies over East Asia. On the other hand, the SCA-related anomalies also reveal identical responses as an increase in sea level pressure over East Asia and northerly flows over the SCS. Besides, the SCA pattern may influence the long-lived cold surges over the SCS by facilitating blocking occurrences through the extensive and quasi-stationary anticyclone over central Siberia. The present results have an implication for the extended weather forecast: long-lasting circulation anomalies, such as the SCA pattern, can affect long-lasting weather phenomena in the regions that are located remotely in both the zonal and meridional directions, such as long-lived cold surges over the SCS.

A recent weakening of winter temperature association between Arctic and Asia

Arctic warming and its association with the mid-latitudes has been a hot topic over the past two decades. Although many studies have explored these issues, it is not clear how their linkage has changed over time. The results show that winter low tropospheric temperatures in Asia experienced two phases over the past two decades. Phase I (2007/2008–2012/2013) was characterized by a warm Arctic and cold Eurasia, and phase II by a warm Arctic and warm Eurasia (2013/2014–2018/2019). A strengthened association in winter temperature between the Arctic and Asia occurred during phase I, followed by a weakened linkage during phase II. Simulation experiments forced by observed Arctic sea ice variability largely reproduce observed patterns, suggesting that Arctic sea ice loss contributes to phasic (or low-frequency) variations in winter atmosphere and makes the Arctic–Asia temperature association fluctuate over time. The weakening of the Arctic–Asia linkage post-2012/2013 was associated with amplified and expanded Arctic warming. The corresponding anomalies in sea level pressure resembled a positive-phase North Atlantic Oscillation during phase II. This study implies that the phasic warm Arctic—cold Eurasia and warm Arctic—warm Eurasia patterns would alternately happen in the context of Arctic sea ice loss, which increases the difficulty in correctly predicting Asian winter temperature.

Mechanism of interannual variability of ocean bottom pressure in the South Pacific

The study of ocean bottom pressure (OBP) is useful for understanding the variability that contributes to sea level change. Previous studies have reported the strong OBP anomalies in the Southern Ocean on different timescales. In this study, the characteristic and mechanisms of the energetic interannual OBP variability in the southeastern Pacific are examined using 14 years of Gravity Recovery and Climate Experiment (GRACE) data. It is found that the OBP anomalies are positive (negative) related to the convergence (divergence) of Ekman transport forced by local winds variability. Such local winds are attributed to atmospheric teleconnections, particularly the second Pacific-South American (PSA2). The sea level pressure (SLP) anomalies associated with the positive phase of PSA2 shows a wavenumber-3 structure in the high latitude of South Pacific, which benefits a strong and persistent anticyclone over the southeastern Pacific, leading to the positive OBP anomalies there. Moreover, El Niño–Southern Oscillation (ENSO) plays an important role in the concurrent OBP variability during austral spring (August–November) and leads the austral autumn (March–June) OBP variability by 1 season. These results highlight the influence of atmospheric teleconnections on interannual OBP variability and are validated by a mass conservation (non-Boussinesq) ocean model, which is expected to not only better understanding of OBP mechanisms in a longer time, but also predict OBP variation in the global scale.

Залежність виникнення поривів вітру від аномалій тиску та температури повітря на території Північно-Західного Причорнорномор’я

In order to reduce the devastating consequences of extreme wind gusts, a significant amount of research is being carried out in the world to study the conditions for the occurrence of this phenomenon, the dynamics of its spatio-temporal distribution, and a possible potential change in the wind regime against the background of modern climate change. Nowadays, it appears that not much research has been carried out to determine the possible impact of climate change on future wind gusts on the territory of Ukraine and, in particular, in the Northern Black Sea region. Global warming scenarios are expected to change the severity and frequency of future wind gusts by the end of this century. Considering this, a study was made of the spatio-temporal features of the formation of wind gusts in the North-Western Black Sea region for the period 2005-2020, and the dependence of their occurrence on air temperature and atmospheric pressure anomalies, which can be used to analyze possible changes of future wind regimes as a result of climate change. To analyze the occurrence of wind gusts in the North-Western Black Sea region, data from three stations located in different parts of the region were selected. The data of METAR telegrams on wind gusts at the Odessa-AMSC, Mykolaiv, Kherson stations and surface weather observations data for the period from January 2005 to December 2020 were used as source material in the work. For statistical analysis of daily air temperature and pressure anomalies, NCEP/NCAR Reanalysis archive data for the same period were used. The obtained results show that in the territory of the North-Western Black Sea region over the past 15 years, wind gusts in the gradation of 13-15 (m.s-1) prevailed, and the speed of wind gusts in most cases rised with an increase of the daily temperature anomaly (from 0.0 to 5.0 °С) and a decrease of the daily pressure anomaly (from 0 to -7.0 hPa).

Link of ENSO and Southern Annular Mode as elements of Global Atmospheric Oscillation

Based on the monthly average data of NCEP/NCAR Reanalysis, using a specially developed technique, the interannual variability of the sea level pressure (SLP) anomalies associated with the Global Atmospheric Oscillation (GAO) for the period 1950-2021 was studied. The field of mean deviations of the SLP anomalies during the El Niño – Southern Oscillation (ENSO) from the interannual GAO variability has been calculated. It is shown that the variability of the SLP anomalies during the GAO contains, in addition to the mode associated with the ENSO, also the mode of interannual climatic variability associated with the Antarctic Oscillation (AAO) – the Southern Annular Mode (SAM). It was found that the GAO does not include a part of the SLP anomaly variability associated with El Niño and La Niña events of the Central Pacific type. At all other grid nodes, the field of mean deviations of the SLP anomalies in the ENSO from the GAO has SLP differences slightly different from zero and formally statistically insignificant. This means that in the interannual climatic variability of the SLP associated with the GAO, there are only two modes: ENSO and SAM. With the help of cross-wavelet analysis, the relationships between the ENSO and SAM indices were studied. Negative relationships were found between these indices on fluctuation periods of about 11 years. An analysis of the time series of these indices and the time series of total solar irradiance (TSI) made it possible to put forward a hypothesis about the influence of synchronization and desynchronization of quasi-11-year changes in solar activity and ENSO on the weakening and strengthening of negative relationships between ENSO and SAM at periods of oscillations of about 11 years.

The Recent Droughts of 2019/20 in Southern Africa and Its Teleconnection with ENSO Events

This study is motivated to highlight the variability of recent drought hotspots in the region of southern Africa in terms of the seasonal and annual rainfall regimes and their possible spatial linkage with the 1950-2020 seasonal El Niño/Southern Oscillation (ENSO). Some evidence is found on possible links between the occurrence of drought hotspots in the region in terms of seasonal and mean annual runoff and warm ENSO events. This was revealed by the existence of a strong and nearly-strong positive linear correlation between Seasonal and annual rainfall depths and the warm seasonal ENSO indices explained by the southern oscillation index represented by the sea level pressure (SLP) anomalies data obtained from the National Oceanographic and Aeronautics Administration (NOAA). Considering the entire southern African region, 41% of the surface areas exhibit moderate (r > 0.25) and strong (r > 0.5) correlation coefficients in terms of the December to February quarter rainfall and ENSO indices. Above 50% confidence interval in the correlation between seasonal rainfall and ENSO during DJF quarters is found in 74% of the surface area of the region of southern Africa. The high confidence interval of the positive correlation coefficients is an indication that substantial variance of precipitation during ENSO years is accounted for by the warm ENSO events. The areas with pronounced lower rainfalls and droughts associated with ENSO activity in the region include larger and some pockets of various countries in southern Africa, including but not limited to Angola, Botswana, Lesotho, Namibia, Zambia, Zimbabwe, South Africa and Mozambique. The recent drought events of 2019/2020, and previously in 2015/16 in this region with wider regional impacts can be explained by the ENSO phenomena.

Seasonally Dependent Future Changes in the U.S. Midwest Hydroclimate and Extremes

AbstractThis study investigates the responses of the hydroclimate and extremes in the U.S. Midwest to global warming, based on ensemble projections of phase 6 of the Coupled Model Intercomparison Project and the multimodel initial-condition large-ensemble simulations. The precipitation response features a seasonally dependent change with increased precipitation in April–May but reduced precipitation in July–August. The late-spring wetting is attributed to the enhanced low-level moisture-transporting southerlies, which are induced by regional sea level pressure anomalies linked to the poleward shift of the North American westerly jet (NAWJ). The late-summer drying is attributed to the weakened storm track, which is also linked to the poleward NAWJ shift. The seasonally dependent future changes of the Midwest precipitation are analogous to its climatological seasonal progression, which increases over late spring as the NAWJ approaches the Midwest and decreases over late summer as the NAWJ migrates away. In response to the mean precipitation changes, extremely wet late springs (April–May precipitation above the 99th percentile of the historical period) and extremely dry late summers (below the 1st percentile) will occur much more frequently, implying increased late-spring floods and late-summer droughts. Future warming in the Midwest is amplified in late summer due to the reduced precipitation. With amplified background warming and increased occurrence, future late-summer droughts will be more devastating. Our results highlight that, under a time-invariant poleward jet shift, opposite precipitation changes arise before and after the peak rainy month, leading to substantial increases in the subseasonal extremes. The severity of such climate impacts is obscured in projections of the rainy-season mean.

Strengthening Southern Hemisphere Westerlies and Amundsen Sea Low Deepening Over the 20th Century Revealed by Proxy‐Data Assimilation

AbstractWinds and pressure over the Southern Ocean are critical to many aspects of the climate system, but the brevity of climate data in this region makes it challenging to interpret recent changes. Here, we reconstruct 20th century sea level pressure and zonal surface wind anomalies over the Southern Ocean, using data assimilation with a global paleoclimate proxy database and four climate‐model priors. The reconstructions agree well with instrumental reanalysis products, especially in the circumpolar westerly and Pacific regions. We observe significant strengthening in the midlatitude Pacific westerlies, associated with a deepening Amundsen Sea Low, throughout the 20th century in all four reconstructions. When the prior includes anthropogenic forcing, we observe poleward‐shifting circumpolar westerlies throughout the 20th century. Our results highlight the combined roles of natural variability and anthropogenic forcing, and the zonally asymmetric character of atmospheric circulation changes at high southern latitudes, with implications for Antarctic ice sheet change.

QUASI-STATIONARY PLANETARY WAVES IN MID-LATITUDE STRATOSPHERE–MESOSPHERE IN WINTER 2011-2020

The 10-year climatology (2011–2020) of quasi-stationary planetary waves in the mid-latitude stratosphere and mesosphere (40–50N, up to 90 km) has been analyzed. Longitude–altitude sections of geopotential height and ozone have been obtained using the Aura MLS satellite data. It is found that stationary wave 1 propagates into the mesosphere from the North American High and Icelandic Low, which are adjacent surface pressure anomalies in the structure of stationary wave 2. Unexpectedly, the strongest pressure anomaly in the Aleutian Low region does not contribute to the stationary wave 1 formation in the mesosphere. The vertical phase transformations of stationary waves in geopotential height and ozone show inconsistencies that should be studied separately.

Relationship between sea surface temperature anomalies in the Southwestern Atlantic Continental Shelf and atmospheric variability on intraseasonal timescales

The intraseasonal (IS) variability of the sea surface temperature (SST) in the Southwestern Atlantic Continental Shelf (SWACS, 45–33° S—70–50° W), and its relationship with that in the atmosphere, was studied for the austral warm season. SST satellite data (11-km resolution NOAA CoastWatch Program) and data of different atmospheric variables (Reanalysis1 NCEP/NCAR and ERA-Interim) were used. Data were filtered using a 10–90 day filter to isolate the IS variability. A Principal Component analysis was applied then to the filtered SST anomalies (SSTA) and the activity of the leading modes was described through the corresponding temporal series. The first three modes are significant. EOF1 (25.7% of variance) exhibits SSTA of opposite sign to the north/south of 42° S. EOF2 (9.0%) and EOF3 (5.1%) are related with centers of SSTA of opposite sign located off the Uruguayan coast and in the middle shelf. Composites of SSTA and of key atmospheric variables were made considering the days in which the main modes were active. They show that the SSTA described by the three modes are associated with distinctive regional sea level pressure anomalies that, in turn, seem to be related to atmospheric Rossby wave trains extending from the Australia area towards South America. The corresponding atmospheric wave sources vary depending on the mode. These results show, therefore, that the SSTA in the SWACS exhibit significant IS variability that is, in part, locally and remotely influenced by atmospheric anomalies oscillating on similar timescales. These ocean–atmosphere teleconnections could help to improve ocean predictability at those timescales in the future.

Association between El Niño and extreme temperatures in southern South America in CMIP5 models. Part 2: future climate projections

This study provides an overview of projected future changes in the probability density function (PDF) and variability of the equatorial Pacific sea surface temperature in the El Niño 3.4 region (SST3.4). We analyzed how these changes affect the relationship with extreme temperature events in southern South America under global warming. We used 2 different emission scenarios for climate change simulations in the Coupled Model Intercomparison Project Phase 5 (CMIP5): RCP4.5 and 8.5, and for 2 future periods: 2031-2060 and 2071-2100. The Kolmogorov-Smirnov test was applied to compare the PDFs of SST3.4 between historical and future periods. We found that the PDFs of SST3.4 for both future periods differed significantly from the PDFs of the historical period for both scenarios. Changes in SST3.4 under greenhouse warming will alter the intensity and position of the teleconnection patterns. In particular, we observed that the mean difference in sea level pressure anomalies between El Niño and La Niña will weaken in the future. This result could explain why, in general, no significant correlations or quantile regressions were projected between SST3.4 and extreme temperature indices in southern South America for both the near and far future. The projections of the best-performing models in the historical period did not provide much clarity because they differ from each other but showed us that the individual models simulate changes in the relationships between SST3.4 and extremes temperature according to the period and scenario.

A weather regime characterisation of winter biomass aerosol transport from southern Africa

Abstract. During austral winter, a compact low cloud deck over the South Atlantic contrasts with clear sky over southern Africa, where forest fires triggered by dry conditions emit large amounts of biomass burning aerosols (BBAs) in the free troposphere. Most of the BBA burden crosses the South Atlantic embedded in the tropical easterly flow. However, midlatitude synoptic disturbances can deflect part of the aerosol from the main transport path towards southern extratropics. In this study, the first objective classification of the synoptic variability controlling the spatial distribution of BBA in southern Africa and the South Atlantic during austral winter (August to October) is presented. By analysing atmospheric circulation data from reanalysis products, a six-class weather regime (WR) classification of the region is constructed. The classification reveals that the synoptic variability is composed of four WRs, representing disturbances travelling at midlatitudes, and two WRs accounting for pressure anomalies in the South Atlantic. The WR classification is then successfully used to characterise the aerosol spatial distribution in the region in the period 2003–2017, in both reanalysis products and station data. Results show that the BBA transport towards southern extratropics is controlled by weather regimes associated with midlatitude synoptic disturbances. In particular, depending on the relative position of the pressure anomalies along the midlatitude westerly flow, the BBA transport is deflected from the main tropical route towards southern Africa or the South Atlantic. Moreover, the WRs accounting for midlatitude disturbances show organised transition sequences, which allow one to illustrate the evolution of the BBA northerly transport across the region in the context of a wave pattern. The skill in characterising the BBA transport shown by the WR classification indicates the potential for using it as a diagnostic/predictive tool for the aerosol dynamics, which is a key component for the full understanding and modelling of the complex radiation–aerosol–cloud interactions controlling the atmospheric radiative budget in the region.

Strengthening of the relationship between West China Autumn Rain and arctic oscillation in the mid-1980s

“West China Autumn Rain” (WCAR), as the last rainy season in China, is inclined to flooding and secondary disasters which often cause severe damages to the development of socio-economy and safety of people's lives. It is thus crucial to explore influential factors and associated physical processes for the anomalies in WCAR. In this study, based on the gridded precipitation data and reanalysis data during 1961–2013, we presented that the WCAR-AO relationship has strengthened in the mid-1980s, which varies from an insignificant correlation to a significant positive one. During the latter period (the significant period), the positive AO phase corresponds to a strong anticyclonic circulation anomaly over the region from northeast Asia extending to the North Pacific in the middle and lower troposphere, concurrent with a northward displacement of the East Asian jet (EAJ) in the upper troposphere. These atmospheric circulation situations favor the increase of WCAR, through their influences on the moisture and dynamical conditions. During the former period (the insignificant period), however, the association of the AO with above atmospheric circulations is weakened, thus diluting the WCAR-AO linkage. The unstable relationship between the AO and aforementioned atmospheric circulations may be due to the change in the AO structure. In the latter period, associated with the positive AO phase is a typical seesaw pattern featured by negative sea level pressure (SLP) anomalies in the Arctic and positive SLP anomalies in the North Atlantic and the North Pacific, whereas the anomalous center in the North Pacific becomes less pronounced in the former period. This discrepancy may be attributed to stronger wave activity arising from the SST warming from the former period to the latter period over the North Pacific.

Self-attraction and loading feedback on ocean dynamics in both shallow water equations and primitive equations

We introduce the effects of self-attraction and loading (SAL) to the momentum equations of a barotropic global ocean tide model as well as a baroclinic general circulation model. We show for the M2 tidal dynamics, that an explicit treatment of SAL is favourable compared to a scalar approximation or even disregarding the effects all together. For the general circulation the influence on the ocean bottom pressure is shown to be about 1 hPa for short time periods along the coasts and about 0.5 hPa for longer periods in resonant basins in the southern ocean. In contrast to previous studies, we additionally examine the effects of surface pressure anomalies over the continents in the computation of SAL which is shown to have an influence of about 0.5 hPa along the coasts. With those results, we demonstrate that the explicit treatment of SAL is important for satellite applications.