Relationship between topography, tropospheric wind, and frequency of mountain waves in the upper mesosphere over the Kanto area of Japan

Imaging observations of OH airglow were performed at Meiji University, Japan (35.6° N, 139.5° E), from May 2018 to December 2019. Mountainous areas are located to the west of the imager, and westerly winds are dominant in the lower atmosphere throughout the year. Mountain waves (MWs) are generated and occasionally propagate to the upper atmosphere. However, only four likely MW events were identified, which are considerably fewer than expected. There are two possible reasons for the low incidence: (1) MWs do not propagate easily to the upper mesosphere due to background wind conditions, and/or (2) the frequency of MW excitation was low around the observation site. Former possibility is found not to be a main reason to explain the frequency by assuming typical wind profiles in troposphere and upper mesosphere over Japan. Thus, frequency and spatial distribution of orographic wavy clouds were investigated by analyzing images taken by the Himawari-8 geostationary meteorological satellite in 2018. The number of days when wavy clouds were detected in the troposphere around the observation site (Kanto area) was about a quarter of that around the Tohoku area. This result indicates that frequency of over-mountain flow which is thought to be a source of excitation of MWs is low in Kanto area. We also found that the angle between the horizontal wind direction in troposphere and the orientation of the mountain ridge is a good proxy for the occurrence of orographic wavy clouds, i.e., excitation of MWs. We applied this proxy to the topography around the world to investigate regions where MWs are likely to be excited frequently throughout the year to discuss the likelihood of "MW hotspots" at various spatial scale. Graphical Abstract

Precipitation Changes in Semi-arid Regions in East Asia Under Global Warming

The semi-arid regions of East Asia are located in the transition area between regions dominated by the monsoon system and by westerly winds; their interaction is the key to understand precipitation changes, especially in the summer. Our results show that the enhancement of both the monsoon and westerly winds occurs in wet years, leading to stronger convergence and more rainfall. Weakening of both the monsoon and westerly winds occurs in dry years and results in less rainfall. Such interaction between the monsoon and westerlies is not constant; the boundary of their effects is changing all the time. As the monsoon strengthens, it shifts to the west in wet years and covers most of the semi-arid regions, and the negative effect of the El Niño-Southern Oscillation (ENSO) system on precipitation in the semi-arid regions becomes obvious. However, westward expansion has not been evident over the past 70 years in historic data. In the future, the monsoon will obviously expand westward, and the precipitation over the Loess Plateau will gradually increase as the monsoon boundary expand westward until the end of the 21st century. This change indicates that more rainfall will occur in the semi-arid regions of East Asia, which could dramatically change the ecological environment, especially over the Loess Plateau.

Impact of the El Niño type and PDO on the Winter Sub-seasonal North American Zonal Temperature Dipole via the Variability of Positive PNA Events

In recent years, the winter (from December to February, DJF) North American surface air temperature (SAT) anomaly in midlatitudes shows a “warm west/cold east” (WWCE) dipole pattern. To some extent, the winter WWCE dipole can be considered as being a result of the winter mean of sub-seasonal WWCE events. In this paper, the Pacific SST condition linked to the WWCE SAT dipole is investigated. It is found that while the sub-seasonal WWCE dipole is related to the positive Pacific North American (PNA+) pattern, the impact of the PNA+ on the WWCE dipole depends on the El Niño SST type and the phase of Pacific decadal Oscillation (PDO). For a central-Pacific (CP) type El Niño, the positive (negative) height anomaly center of PNA+ is located in the western (eastern) North America to result in an intensified WWCE dipole, though the positive PDO favors the WWCE dipole. In contrast, the WWCE dipole is suppressed under an Eastern-Pacific (EP) type El Niño because the PNA+ anticyclonic anomaly dominates the whole North America. Moreover, the physical cause of why the El Niño type influences PNA+ is further examined. It is found that the type of El Niño can significantly influence the location of PNA+ through changing North Pacific midlatitude westerly winds associated with the Pacific Hadley cell change. For the CP-type El Niño, the eastward migration of PNA+ is suppressed to favor its anticyclonic (cyclonic) anomaly appearing in the west (east) region of North American owing to reduced midlatitude westerly winds. But for the EP-type El Niño, midlatitude westerly wind is intensified to cause the appearance of PNA+ anticyclonic anomaly over the whole North America due to enhanced Hadley cell.

Evolution Of The Westerly Winds Belt In The Middle Latitudes Of The Southern Hemisphere Since The Last Glacial Maximum

<p><b>The Southern Westerly Winds (SWW) are a symmetric component of the global climate system that govern the modern climate of all Southern Hemisphere landmasses south of ~30°S. Changes in the strength and latitudinal position of the SWW influence the precipitation patterns in the southern mid-latitudes, and have been postulated as fundamental drivers of ocean-atmospheric CO2 exchange since the Last Glacial Maximum (LGM: ~34.0-18.0 ka). Despite their role in modern and past climatic dynamics, the evolution of the SWW at locations within their zone of influence is still uncertain; this is largely because of the paucity of paleoclimate records with well constrained chronology, adequate sampling resolution and an appropriate depositional setting. Resolving these issues will help understand the behaviour of the SWW in the past at different spatial (regional and hemisphere) and temporal (centennial to multi-millennial) scales. Here I present new paleoclimate data based on the examination of detailed chronologies of fossil pollen, charcoal and chironomids preserved in lake sediments from western Patagonia: Lago Emerenciana (43°S) and Lago Pintito (52°S) and New Zealand’s southwestern South Island: Lake Von (45°S). These data, spanning a broad range of the SWW zone of influence, provide insights into the role of shifting SWW in environmental and climate dynamics of the middle latitudes of the Southern Hemisphere spanning the last ~24,000 years.</b></p> <p>In the first study site, I performed detailed fossil pollen and charcoal analyses from sediment cores collected from Lago Emerenciana, a relatively small closed-basin lake located in northwestern Patagonian (43°S), to examine past vegetation, fire regime and climate change during the last ~24,000 years. I detect very low temperature and increased precipitation between ~24.0 and ~17.0 ka, followed by a warming trend and reduced precipitation between ~17.0 and ~14.3 ka. A cold reversal and increased precipitation regime occurred between ~14.3 and ~12.4 ka, followed by a return to warming and a slight decline in precipitation between ~12.4 and ~11.0 ka. I identify warmer temperatures and a major decline in precipitation at the beginning of the Holocene between ~11.0 and ~9.0 ka, conditions that persisted until ~6.2 ka. Centennial to millennial precipitation variability occurred during the last ~6200 years. </p> <p>In the second study site, I developed high resolution fossil pollen and charcoal records, along with an exploratory chironomid record from sediment cores obtained from Lake Von, a small closed-basin lake located in the southwestern sector of the South Island of New Zealand (45°S), to examine vegetation, fire and climate trends spanning the last ~18,000 years. I observe a trend toward warming and relatively dry conditions between ~18.0 and ~14.8 ka with relatively wet conditions between ~18.0 and ~16.7 ka, increased precipitation between ~16.7 and ~14.8 ka, and cooling conditions and enhanced precipitation between ~14.8 and ~12.8 ka, followed by a marked drop in precipitation between ~12.6 and ~11.2 ka. I detect warmer and diminished precipitation between ~10.8 and ~7.2 ka, followed by lower temperature and enhanced precipitation between ~7.2 and ~3.7 ka. The mid-late Holocene is also characterised by alternating dry and wet oscillations of millennial- and centennial-scale phases with low precipitation between ~6.0 and ~5.2, ~4.4 and ~4.1, ~3.7 and ~2.9, and ~1.9 and ~0.56 ka, and increased precipitation in the intervening intervals. In the third study site, I produced high resolution fossil pollen and charcoal records from sediment cores I collected from Lago Pintito, a small and shallow closed-basin lake located in southwestern Patagonia (52°S). This record allows the detection of past vegetation, fire and hydroclimatic shifts at millennial and centennial scales over the last ~17,000 years. From these data, I identify cold and dry conditions between ~17.0 and ~16.4 ka, increased precipitation between ~16.4 and ~14.2 ka and ~12.5 and ~11.4 ka, and intense precipitation but lower in magnitude than the neighbouring intervals between ~14.2 and~12.5 ka. I detect a major decline in precipitation at the beginning of the Holocene between ~11.4 and ~6.8 ka, followed by centennial-scale changes in precipitation until the present. </p> <p>The comparison between precipitation variability reconstructed from the records from western Patagonia (Lago Emerenciana and Lago Pintito) and New Zealand’s southwestern South Island (Lake Von) allows the inference of SWW changes at a hemispheric scale during and since the LGM, based on the premise that there is a strong and positive correlation between zonal wind speeds and local precipitation in these regions. The results of this thesis suggest: i) strong SWW influence at 43°S between ~24.0 and ~17.5 ka, ii) a southward shift of the SWW between ~17.5 and ~16.5 ka and reduced SWW influence north of 52°S, iii) strengthening and/or a northward shift of the SWW between ~16.5 and ~ 14.5 ka, with strong SWW influence between 52°S and 43°S, iv) a northward shift of the SWW between ~14.5 and ~12.6 ka which resulted in stronger SWW influence between 43°S and 46° S and weaker SWW influence at 52°S, v) a southward shift of the SWW between ~12.6 and ~11.2 ka leading to weaker SWW influence between 43°S and 46°S and stronger SWW influence at 52°S, vi) a generalized multi-millennial decline in the strength of the SWW between ~11.2 and ~7.2 ka, and vii) high variability in the SWW in Western Patagonian and New Zealand’s southwestern South Island during the last ~7200 years. Based on these findings, I postulate that hemisphere-wide changes in the position and/or strength of the SWW have modulated the atmospheric CO2 concentration through wind-driven upwelling of CO2-rich deep waters in the high southern latitudes during and since the LGM.</p>

Evolution Of The Westerly Winds Belt In The Middle Latitudes Of The Southern Hemisphere Since The Last Glacial Maximum

<p><b>The Southern Westerly Winds (SWW) are a symmetric component of the global climate system that govern the modern climate of all Southern Hemisphere landmasses south of ~30°S. Changes in the strength and latitudinal position of the SWW influence the precipitation patterns in the southern mid-latitudes, and have been postulated as fundamental drivers of ocean-atmospheric CO2 exchange since the Last Glacial Maximum (LGM: ~34.0-18.0 ka). Despite their role in modern and past climatic dynamics, the evolution of the SWW at locations within their zone of influence is still uncertain; this is largely because of the paucity of paleoclimate records with well constrained chronology, adequate sampling resolution and an appropriate depositional setting. Resolving these issues will help understand the behaviour of the SWW in the past at different spatial (regional and hemisphere) and temporal (centennial to multi-millennial) scales. Here I present new paleoclimate data based on the examination of detailed chronologies of fossil pollen, charcoal and chironomids preserved in lake sediments from western Patagonia: Lago Emerenciana (43°S) and Lago Pintito (52°S) and New Zealand’s southwestern South Island: Lake Von (45°S). These data, spanning a broad range of the SWW zone of influence, provide insights into the role of shifting SWW in environmental and climate dynamics of the middle latitudes of the Southern Hemisphere spanning the last ~24,000 years.</b></p> <p>In the first study site, I performed detailed fossil pollen and charcoal analyses from sediment cores collected from Lago Emerenciana, a relatively small closed-basin lake located in northwestern Patagonian (43°S), to examine past vegetation, fire regime and climate change during the last ~24,000 years. I detect very low temperature and increased precipitation between ~24.0 and ~17.0 ka, followed by a warming trend and reduced precipitation between ~17.0 and ~14.3 ka. A cold reversal and increased precipitation regime occurred between ~14.3 and ~12.4 ka, followed by a return to warming and a slight decline in precipitation between ~12.4 and ~11.0 ka. I identify warmer temperatures and a major decline in precipitation at the beginning of the Holocene between ~11.0 and ~9.0 ka, conditions that persisted until ~6.2 ka. Centennial to millennial precipitation variability occurred during the last ~6200 years. </p> <p>In the second study site, I developed high resolution fossil pollen and charcoal records, along with an exploratory chironomid record from sediment cores obtained from Lake Von, a small closed-basin lake located in the southwestern sector of the South Island of New Zealand (45°S), to examine vegetation, fire and climate trends spanning the last ~18,000 years. I observe a trend toward warming and relatively dry conditions between ~18.0 and ~14.8 ka with relatively wet conditions between ~18.0 and ~16.7 ka, increased precipitation between ~16.7 and ~14.8 ka, and cooling conditions and enhanced precipitation between ~14.8 and ~12.8 ka, followed by a marked drop in precipitation between ~12.6 and ~11.2 ka. I detect warmer and diminished precipitation between ~10.8 and ~7.2 ka, followed by lower temperature and enhanced precipitation between ~7.2 and ~3.7 ka. The mid-late Holocene is also characterised by alternating dry and wet oscillations of millennial- and centennial-scale phases with low precipitation between ~6.0 and ~5.2, ~4.4 and ~4.1, ~3.7 and ~2.9, and ~1.9 and ~0.56 ka, and increased precipitation in the intervening intervals. In the third study site, I produced high resolution fossil pollen and charcoal records from sediment cores I collected from Lago Pintito, a small and shallow closed-basin lake located in southwestern Patagonia (52°S). This record allows the detection of past vegetation, fire and hydroclimatic shifts at millennial and centennial scales over the last ~17,000 years. From these data, I identify cold and dry conditions between ~17.0 and ~16.4 ka, increased precipitation between ~16.4 and ~14.2 ka and ~12.5 and ~11.4 ka, and intense precipitation but lower in magnitude than the neighbouring intervals between ~14.2 and~12.5 ka. I detect a major decline in precipitation at the beginning of the Holocene between ~11.4 and ~6.8 ka, followed by centennial-scale changes in precipitation until the present. </p> <p>The comparison between precipitation variability reconstructed from the records from western Patagonia (Lago Emerenciana and Lago Pintito) and New Zealand’s southwestern South Island (Lake Von) allows the inference of SWW changes at a hemispheric scale during and since the LGM, based on the premise that there is a strong and positive correlation between zonal wind speeds and local precipitation in these regions. The results of this thesis suggest: i) strong SWW influence at 43°S between ~24.0 and ~17.5 ka, ii) a southward shift of the SWW between ~17.5 and ~16.5 ka and reduced SWW influence north of 52°S, iii) strengthening and/or a northward shift of the SWW between ~16.5 and ~ 14.5 ka, with strong SWW influence between 52°S and 43°S, iv) a northward shift of the SWW between ~14.5 and ~12.6 ka which resulted in stronger SWW influence between 43°S and 46° S and weaker SWW influence at 52°S, v) a southward shift of the SWW between ~12.6 and ~11.2 ka leading to weaker SWW influence between 43°S and 46°S and stronger SWW influence at 52°S, vi) a generalized multi-millennial decline in the strength of the SWW between ~11.2 and ~7.2 ka, and vii) high variability in the SWW in Western Patagonian and New Zealand’s southwestern South Island during the last ~7200 years. Based on these findings, I postulate that hemisphere-wide changes in the position and/or strength of the SWW have modulated the atmospheric CO2 concentration through wind-driven upwelling of CO2-rich deep waters in the high southern latitudes during and since the LGM.</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.

Understanding convective storms in a tropical, high-altitude location with in-situ meteorological observations and GPS-derived water vapor

We investigate convective storms over the Sabana de Bogotá, a high-altitude and densely populated area in the Colombian tropical Andes. Convective events are identified using infrared satellite images and in-situ precipitation data. As expected, convection shows a strong early-afternoon peak during the two rainy seasons. Previous studies hypothesize that early-afternoon westerly winds and their moisture advection from the warmer Magdalena Valley are the main explanatory mechanism for intense storms. We find that early-afternoon westerlies are present in 78% of the rainy season days, but convective events develop in only 26% of the days. Thus, although westerlies seem necessary for convection due to the convergence they generate, they only occasionally generate storms, and are therefore not a good predictor. Furthermore, reanalysis data indicate that precipitable water vapor (PWV) at the Magdalena Valley is anomalously low during convective days, suggesting that moisture converges locally instead of being advected from the west. Based on composites of surface wind speed, air temperature, pressure and GPS-derived PWV, we identify the most prominent signals associated to deep convection: a weaker than average wind speed throughout the morning, higher than normal values of surface air temperature towards noon, followed by an anomalous steep increase of PWV and wind speed. These features indicate that convection results from a strong diurnal forcing facilitated by convergence of westerly winds, combined with sufficient water vapor convergence, with a timescale of about 3 hours. This highlights the relevance of high temporal resolution monitoring of PWV offered by Global Navigational Satellite System stations.

Twenty-first century Southern Hemisphere impacts of ozone recovery and climate change from the stratosphere to the ocean

Changes in stratospheric ozone concentrations and increasing concentrations of greenhouse gases (GHGs) alter the temperature structure of the atmosphere and drive changes in the atmospheric and oceanic circulation. We systematically investigate the impacts of ozone recovery and increasing GHGs on the atmospheric and oceanic circulation in the Southern Hemisphere during the twenty-first century using a unique coupled ocean-atmosphere climate model with interactive ozone chemistry and enhanced oceanic resolution. We use the high emission scenario SSP5-8.5 for GHGs under which the springtime Antarctic total column ozone returns to 1980s levels by 2048 in our model, warming the lower stratosphere and strengthening the stratospheric westerly winds. Novel results of this study include the springtime stratospheric circulation response to GHGs, which is characterized by changes of opposing sign over the Eastern and Western Hemispheres, the opposing responses of the Agulhas leakage to ozone recovery and increasing GHGs, and large uncertainties in the prediction of atmospheric and oceanic circulation changes related to whether the ozone field is prescribed or calculated interactively. By performing a thorough spatial analysis of the predicted changes in the stratospheric dynamics, we find that the GHG effect during spring exhibits a strong dipole pattern, which contrasts the GHG effect during the rest of the year and which was previously not reported, as it cannot be detected when zonal means are considered. Over the Western Hemisphere, GHGs drive a warming of the lower stratosphere and a weakening of the westerlies, while over the Eastern Hemisphere they drive a cooling and a strengthening of the westerlies. Associated with these changes, planetary waves break higher up in the stratosphere over the Eastern Hemisphere, strengthening the polar downwelling and inducing dynamical warming in the upper stratosphere, while weakening the downwelling and inducing dynamical cooling in the lower stratosphere. The opposite occurs over the Western Hemisphere. Because the changes in the Western Hemisphere dominate during November in our model, we find that during this month GHGs lead to a weakening of the lower branch of the Brewer-Dobson Circulation, reinforcing the weakening caused by ozone recovery. At the surface, the westerly winds weaken and shift equatorward due to ozone recovery, driving a weak decrease in the transport of the Antarctic Circumpolar Current and in the Agulhas leakage, which transports warm and saline waters from the Indian into the Atlantic Ocean. The increasing GHGs drive changes in the opposite direction that overwhelm the ozone effect. The total changes at the surface and in the oceanic circulation are nevertheless weaker in the presence of ozone recovery than those induced by GHGs alone, highlighting the importance of the Montreal Protocol in mitigating some of the impacts of climate change. We additionally compare the combined effect of interactively calculated ozone recovery and increasing GHGs with their combined effect in an ensemble in which we prescribe the CMIP6 ozone field. This second ensemble simulates a weaker ozone effect in all the examined fields. The magnitude of the difference between the simulated changes at the surface and in the oceanic circulation in the two ensembles is as large as the ozone effect itself. This shows that the choice between prescribing or calculating the ozone field interactively can affect the prediction of changes not only in the atmospheric, but also in the oceanic circulation.

Dynamical evolution of a minor sudden stratospheric warming in the Southern Hemisphere in 2019

This study analyzes the Japanese 55-year Reanalysis (JRA-55) dataset from 2002 to 2019 to examine the sudden stratospheric warming event that occurred in the Southern Hemisphere (SH) in 2019 (hereafter referred to as SSW2019). Strong warming at the polar cap and decelerated westerly winds were observed, but since there was no reversal of westerly winds to easterly winds at 60° S in the middle to lower stratosphere, the SSW2019 is classified as a minor warming event. The results show that quasi-stationary planetary waves of zonal wavenumber 1 developed during the SSW2019. The strong vertical component of the Eliassen–Palm flux with zonal wavenumber 1 is indicative of pronounced propagation of planetary waves to the stratosphere. The wave driving in September 2019 shows that the values are larger than those of the major SSW event in 2002 (hereafter referred to as SSW2002). Since there was no pronounced preconditioning (as in SSW2002) and the polar vortex was already strong before the SSW2019 occurred, a major disturbance of the polar vortex was unlikely to have taken place. The strong wave driving in SSW2019 occurred in high latitudes. Waveguides (i.e., positive values of the refractive index) are found at high latitudes in the upper stratosphere during the warming period, which provided favorable conditions for quasi-stationary planetary waves to propagate upward and poleward.

Is our dynamical understanding of the circulation changes associated with the Antarctic ozone hole sensitive to the choice of reanalysis dataset?

This study quantifies differences among four widely used atmospheric reanalysis datasets (ERA5, JRA-55, MERRA-2, and CFSR) in their representation of the dynamical changes induced by springtime polar stratospheric ozone depletion in the Southern Hemisphere from 1980 to 2001. The intercomparison is undertaken as part of the SPARC (Stratosphere–troposphere Processes and their Role in Climate) Reanalysis Intercomparison Project (S-RIP). The reanalyses are generally in good agreement in their representation of the strengthening of the lower stratospheric polar vortex during the austral spring–summer season, associated with reduced radiative heating due to ozone loss, as well as the descent of anomalously strong westerly winds into the troposphere during summer and the subsequent poleward displacement and intensification of the polar front jet. Differences in the trends in zonal wind between the reanalyses are generally small compared to the mean trends. The exception is CFSR, which exhibits greater disagreement compared to the other three reanalysis datasets, with stronger westerly winds in the lower stratosphere in spring and a larger poleward displacement of the tropospheric westerly jet in summer. The dynamical changes associated with the ozone hole are examined by investigating the momentum budget and then the eddy heat and momentum fluxes in terms of planetary- and synoptic-scale Rossby wave contributions. The dynamical changes are consistently represented across the reanalyses and support our dynamical understanding of the response of the coupled stratosphere–troposphere system to the ozone hole. Although our results suggest a high degree of consistency across the four reanalysis datasets in the representation of these dynamical changes, there are larger differences in the wave forcing, residual circulation, and eddy propagation changes compared to the zonal wind trends. In particular, there is a noticeable disparity in these trends in CFSR compared to the other three reanalyses, while the best agreement is found between ERA5 and JRA-55. Greater uncertainty in the components of the momentum budget, as opposed to mean circulation, suggests that the zonal wind is better constrained by the assimilation of observations compared to the wave forcing, residual circulation, and eddy momentum and heat fluxes, which are more dependent on the model-based forecasts that can differ between reanalyses. Looking forward, however, these findings give us confidence that reanalysis datasets can be used to assess changes associated with the ongoing recovery of stratospheric ozone.