Growing Pacific Linkage with the Interannual Variability of North Atlantic Explosive Cyclogenesis

Explosive cyclones (ECs), defined as developing extratropical cyclones that experience pressure drops of at least 24 hPa in 24 hours, are impactful weather events which occur along highly populated coastal regions in the eastern United States. These storms occur due to a combination of atmospheric and surface processes, such as jet stream intensification and latent heat release at the ocean surface. Even though previous literature has elucidated the role of these processes in EC formation, the sources of interannual variability that impact seasonal EC frequency are not well known. To analyze the sources of interannual variability, we track cases of ECs and dissect them into two spatial groups: those that formed near the east coast of North America (coastal) and those in the North Central Atlantic (high latitude). The frequency of high-latitude ECs is strongly correlated with the North Atlantic Oscillation, a well-known feature, whereas coastal EC frequency exhibits a growing relationship with an atmospheric wave-train emanating from the North Pacific in the last 30 years. This wave-train pattern of alternating high-and-low pressure resulted in resulted in heightened upper-level divergence and baroclinic instability along the east coast of North America. Using a coupled model experiment, we show that the tropical Pacific Ocean is the main driver of this atmospheric wave train and the subsequent enhancement seasonal baroclinic instability in the North Atlantic.

Increased Interannual Variability in the Dipole Mode of Extreme High-temperature Events over East China during Summer after the Early 1990s and Associated Mechanisms

The dominant mode of the interannual variability in the frequency of extreme high-temperature events (FEHE) during summer over eastern China showed a dipole mode with reversed anomalies of FEHE over northeastern and southern China. This study found that the interannual variability of this dipole mode underwent an interdecadal increase after the early 1990s. The anomalous atmospheric circulation responsible for the FEHE dipole mode was associated with the air-sea interaction over the western tropical Pacific and North Atlantic. Due to the weakened correlation between the SST in the tropical Pacific and in the Indian Ocean after the early 1990s, a meridional atmospheric wave train induced by the anomalous SST around the Maritime continent (MCSST) was intensified during 1994–2013, which was also contributed by the increased interannual variability of MCSST. However, under the influence of the anomalous SST in the Indian Ocean concurrent with the anomalous MCSST, the meridional wave train was weakened and contributed less to the dipole mode during 1972–1993. In addition, the dipole mode was associated with the atmospheric wave trains at middle-high latitude, which were different during the two periods and related to different air-sea interaction in the North Atlantic. The interannual variability of the dipole mode induced by the associated SST anomalies in the North Atlantic during 1994–2013 was significantly larger than that during 1972–1993. Therefore, the interannual variability of the dipole mode was increased after the early 1990s.

Drivers behind the summer 2010 wave train leading to Russian heatwave and Pakistan flooding

Summer 2010 saw two simultaneous extremes linked by an atmospheric wave train: a record-breaking heatwave in Russia and severe floods in Pakistan. Here, we study this wave event using a large ensemble climate model experiment. First, we show that the circulation in 2010 reflected a recurrent wave train connecting the heatwave and flooding events. Second, we show that the occurrence of the wave train is favored by three drivers: (1) 2010 sea surface temperature anomalies increase the probability of this wave train by a factor 2-to-4 relative to the model’s climatology, (2) early-summer soil moisture deficit in Russia not only increases the probability of local heatwaves, but also enhances rainfall extremes over Pakistan by forcing an atmospheric wave response, and (3) high-latitude land warming favors wave-train occurrence and therefore rainfall and heat extremes. These findings highlight the complexity and synergistic interactions between different drivers, reconciling some seemingly contradictory results from previous studies.

Sea ice and large-scale atmospheric variability around East Antarctica

<p><b>​​Antarctica’s sea ice cover is an important component in the global climate system. The variability and recent trends of sea ice concentration are, however, not accurately reproduced by models. Evaluating model performance is hampered because the processes that determine sea ice distribution are not yet well understood, particularly in the East Antarctic region. Here I explore the relationships between recent climate variability and sea ice around East Antarctica, the spatial variability in these relationships, and the impacts that these may have on other aspects of the climate and cryosphere. To achieve this, I analysed satellite-derived HadlSST sea ice concentration (SIC) alongside ERA5 atmospheric reanalysis data for the period between 1979-2018.</b></p> <p>I found that variability in sea ice coverage around East Antarctica was affected by El Niño Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), the Southern Annular Mode (SAM), and Zonal Wave 3 (ZW3). Additionally, I found that the influence of each of these modes varied spatially and temporally, and that sea ice variability affected how regional scale climate responded to changes in large-scale circulation. Summer and autumn SIC around Dronning Maud Land between 10°E and 70°E exhibited a statistically significant negative correlation with the Niño 3.4 index. Analysis of ERA5 data suggests that a southward‐propagating atmospheric wave train triggered by SST anomalies in the tropical Pacific extends into Dronning Maud Land and alters sea ice concentration by encouraging meridional airflow. Shifts in meridional flow in Dronning Maud Land affected sea ice thermodynamically, by altering local heat transport and in turn altering sea ice formation and melt. </p> <p>Sea ice around the Western Pacific sector (WPS) of East Antarctica showed a significant association with variability in the IOD and the SAM. The IOD was correlated with SIC in all seasons but summer. The IOD-SIC relationship is likely driven by an IOD-associated atmospheric wave-train which propagates polewards from the tropical Indian Ocean to Wilkes Land, altering regional circulation and in turn affecting SIC through changes to local climate and sea ice transport. The correlation between WPS SIC and the SAM shifts from positive in summer and autumn to negative in winter and spring, and is likely due to the influence of the SAM on katabatic winds and coastal polynyas, which in turn affect SIC. </p> <p>A significant correlation was observed between SIC variability around East Antarctica and precipitation variability across the continent and the near-coastal Southern Ocean. Further analysis showed that SIC affected how continental precipitation responded to large-scale atmospheric circulation, including modes such as ZW3 and the SAM. Specifically, increased southward moisture flux was only associated with increased precipitation in the inland coastal regions of the continent when SIC was anomalously low. These findings suggest that any future decrease in sea ice may result in greater coupling of climate variability with continental precipitation.</p>

Sea ice and large-scale atmospheric variability around East Antarctica

<p><b>​​Antarctica’s sea ice cover is an important component in the global climate system. The variability and recent trends of sea ice concentration are, however, not accurately reproduced by models. Evaluating model performance is hampered because the processes that determine sea ice distribution are not yet well understood, particularly in the East Antarctic region. Here I explore the relationships between recent climate variability and sea ice around East Antarctica, the spatial variability in these relationships, and the impacts that these may have on other aspects of the climate and cryosphere. To achieve this, I analysed satellite-derived HadlSST sea ice concentration (SIC) alongside ERA5 atmospheric reanalysis data for the period between 1979-2018.</b></p> <p>I found that variability in sea ice coverage around East Antarctica was affected by El Niño Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), the Southern Annular Mode (SAM), and Zonal Wave 3 (ZW3). Additionally, I found that the influence of each of these modes varied spatially and temporally, and that sea ice variability affected how regional scale climate responded to changes in large-scale circulation. Summer and autumn SIC around Dronning Maud Land between 10°E and 70°E exhibited a statistically significant negative correlation with the Niño 3.4 index. Analysis of ERA5 data suggests that a southward‐propagating atmospheric wave train triggered by SST anomalies in the tropical Pacific extends into Dronning Maud Land and alters sea ice concentration by encouraging meridional airflow. Shifts in meridional flow in Dronning Maud Land affected sea ice thermodynamically, by altering local heat transport and in turn altering sea ice formation and melt. </p> <p>Sea ice around the Western Pacific sector (WPS) of East Antarctica showed a significant association with variability in the IOD and the SAM. The IOD was correlated with SIC in all seasons but summer. The IOD-SIC relationship is likely driven by an IOD-associated atmospheric wave-train which propagates polewards from the tropical Indian Ocean to Wilkes Land, altering regional circulation and in turn affecting SIC through changes to local climate and sea ice transport. The correlation between WPS SIC and the SAM shifts from positive in summer and autumn to negative in winter and spring, and is likely due to the influence of the SAM on katabatic winds and coastal polynyas, which in turn affect SIC. </p> <p>A significant correlation was observed between SIC variability around East Antarctica and precipitation variability across the continent and the near-coastal Southern Ocean. Further analysis showed that SIC affected how continental precipitation responded to large-scale atmospheric circulation, including modes such as ZW3 and the SAM. Specifically, increased southward moisture flux was only associated with increased precipitation in the inland coastal regions of the continent when SIC was anomalously low. These findings suggest that any future decrease in sea ice may result in greater coupling of climate variability with continental precipitation.</p>

Atmospheric wavenumber-4 driven South Pacific marine heat waves and marine cool spells

Marine heat waves (MHW) and cool spells (MCS) can both positively and negatively impact marine ecosystems with potentially large societal and economic impacts. Here, I examine the global teleconnections of MHW/MCS in the southern hemisphere and Tasman Sea. When MHW/MCS are defined with respect to a linear warming trend, there is little evidence that MHW in the Tasman Sea are changing in either frequency or intensity but may be lasting longer. MCS may be becoming weaker and less frequent. I show that MHW/MCS in the Tasman Sea co-occur with corresponding events in the Atlantic, Indian, and eastern-Pacific Oceans, and these southern hemisphere events are likely driven by stalling of a global wavenumber-4 (W4) atmospheric wave, leading to anomalously weak north-easterly winds during MHW or strong south-westerly winds during MCS. Thus, the key to predicting MHW/MCS is in understanding what causes the atmospheric W4 wave to stall.

Southern Hemisphere Atmosphere Wavenumber 4 driven Marine Heat Waves and Marine Cool Spells

When SST anomalies are defined with respect to a changing baseline and normalised by their 90th percentile, the Tasman Sea is one of the southern hemisphere hotspots of marine heat waves (MHW) and marine cool spells (MCS). There is little evidence that MHW or MCS are increasing in either frequency or intensity, although the duration of MHW has increased from 8 d four decades ago to 26 d now. On average, Tasman Sea MHW/MCS co-occur with MHW/MCS in the Atlantic, Indian, and eastern-Pacific Oceans, in a wavenumber 4 (W4) pattern. Canonical MHW and MCS show they are likely driven by a stalling of the eastward propagation of a W4 atmospheric wave. During MHW, this slow down leads to near-stationary anomalously high and low air pressure areas driving anomalous north-easterly winds over the Tasman Sea. During MCS, similar a slow-down occurs, but shifted by one-half wavelength zonally.

Large-scale dune aurora event investigation combining Citizen Scientists' photographs and spacecraft observations

&lt;p&gt;Recently, citizen scientist photographs led to the discovery of a new auroral form called &quot;the dune aurora&quot; which exhibits parallel stripes of brighter emission in the green diffuse aurora at about 100 km altitude. This discovery raised several questions, such as (i) whether the dunes are associated with particle precipitation, (ii) whether their structure arises from spatial inhomogeneities in the precipitating fluxes or in the underlying neutral atmosphere, and (iii) whether they are the auroral manifestation of an atmospheric wave called a mesospheric bore. This study investigates a large-scale dune aurora event on 20 January 2016 above Northern Europe. The dunes were observed from Finland to Scotland, spanning over 1500 km for at least four hours. Spacecraft observations confirm that the dunes are associated with electron precipitation and reveal the presence of a temperature inversion layer below the mesopause during the event, creating suitable conditions for mesospheric bore formation. The analysis of a time lapse of pictures by a citizen scientist from Scotland leads to the estimate that, during this event, the dunes propagate toward the west-southwest direction at about 200 m/s, presumably indicating strong horizontal winds near the mesopause. These results show that citizen science and dune aurora studies can fill observational gaps and be powerful tools to investigate the least-known region of near-Earth space at altitudes near 100 km.&lt;/p&gt;