Presented on March 2nd, 2022, by

Dr. Takeshi Izumo
Researcher
Research Institute for Development (IRD)
Presently at UMR EIO (Oceanian Island Ecosystems) lab

Abstract:

The El Niño Southern Oscillation (ENSO) is the leading mode of interannual climate variability, with large socioeconomical and environmental impacts. The Recharge Oscillator (RO), one of the two main ENSO conceptual models, considers two independent modes: the fast zonal tilt mode in phase with central-eastern Pacific Temperature (T_E), and the slow recharge mode in phase quadrature. However, usual metrics do notorthogonally isolate the slow recharge mode, being correlated with T_E. Furthermore the optimal Oceanic Heat Content (OHC) region is currently debated.

Here, through an objective approach to optimize RO equations fit to observations, we develop a new recharge index based on T_E-independent OHC_ind (T_E-variability regressed out). The optimal region is the western and southwestern Pacific (5°N-15°S,120°E-155°W) where the long-term OHC recharge occurs, integrating ENSO windstress anomalies: OHC_{ind_w+sw}. Southwestern Pacific OHC anomalies are due to the meridional asymmetry of ENSO-related Ekman pumping, associated with the South Pacific Convergence Zone.

The naturally-emerging question is then: why would a recharge of the southwestern Pacific favor an El Niño onset? Sensitivity experiments with a Linear Continuously Stratified (LCS) model suggest the following answer. The anomalous windstress curl in the Southwestern Pacific forces off-equatorial downwelling (in the La Niña case) Rossby waves, which propagate to the Pacific equatorial band as coastal and then equatorial Kelvin waves. They progressively favor positive OHC anomalies there (rather than zonal current).

To conclude, the OHC_{ind_w+sw} index is more physically and statistically coherent, could reconcile RO conceptual model with observations and climate models, and is more relevant for ENSO operational forecasts diagnostics.

Presented on February 23, 2022, by

Dr. Chuan-Chi Tu
Postdoctoral Fellow
Department of Atmospheric Sciences
National Central University, Taiwan

Abstract: The marine boundary layer jets (MBLJs) over the northern South China Sea during the early summer rainy season over Taiwan are analyzed using 5-yr (2008–12) National Centers for Environmental Prediction Climate Forecast System Reanalysis data with a 6-h interval. The MBLJ is distinctly different from the low-level jets associated with the subsynoptic frontal systems. During this period, the MBLJ events over the northern South China Sea mainly occur during the second half of the monsoon rainy season over Taiwan (after 1 June) and have a wind speed maximum around the 925-hPa level. The MBLJs are mainly related to the subsynoptic-scale pressure gradients related to a relatively deep mei-yu trough over southeastern China and a stronger-than-normal west Pacific subtropical high. Within the MBL, there is a three-way balance among pressure gradients, Coriolis force, and surface friction, with cross-isobar ageostrophic winds pointing toward the mei-yu trough throughout the diurnal cycle. At the jet core, the vertical wind profile resembles an Ekman spiral with supergeostrophic winds >12 m s⁻¹ near the top of the MBL. The MBLJs are strongest at night and close to geostrophic flow in the late afternoon/early evening. This is because the friction velocity and ageostrophic wind decrease during daytime in response to mixing in the lowest levels. The MBLJs play an important role in horizontal moisture transport from the northern South China Sea to the Taiwan area. In the frontal zone, the moisture tongue extends vertically upward. The rainfall production is related to vertical motions in the frontal zone or localized lifting due to orographic effects.

Presented on February 16, 2022, by

Dr. Michael McPhaden
Senior Scientist
NOAA/PMEL
Seattle, Washington

Abstract: In 1929, Dr Friedrich Ritter and his mistress Dore Strauch left their spouses and the turmoil of post-World War I Germany for the remote, rugged and uninhabited volcanic island of Floreana in the Galapagos archipelago. Their dream was to live self-sufficiently in an idyllic tropical setting unspoiled by civilization. Yachts stopping at Floreana after Ritter and Strauch established a homestead reported on their pioneering enterprise to the outside world in the early 1930s. The news created a sensation that subsequently attracted other settlers to the island, one of whom, a mysterious Austrian faux baroness, vexed Ritter and Strauch to the point of open hostility. Not all the participants in this drama survived the experience of colonizing Floreana though. A prolonged drought that gripped the island from 1933 to 1935 led to food shortages and ultimately the death of Dr. Ritter, who unwittingly ate tainted chicken out of desperation. The bizarre intrigues, extraordinary adventures, and struggles to endure on Floreana were chronicled in Strauch’s 1936 memoir “Satan Came to Eden” and a 2013 Hollywood documentary based on it. A story that has not been told is how climate variability, and in particular an extended period of cold La Niña conditions in 1933-35, led to the drought that caused food shortages on the island and the untimely demise of Dr. Ritter. We will use atmospheric reanalyses, contemporaneous marine meteorological observations in the vicinity of islands, and historical accounts from the broader Pacific basin, to describe the evolution of the 1933-35 La Niña and how it affected the human drama as it unfolded on Floreana Island. This protracted La Niña event had impacts felt in other parts of the globe as well and in particular was a major influence on the development of the 1930s Dust Bowl in the southern plains of the United States.

Presented on January 12, 2022, by

Chian-Yi Liu
Professor and Associate Research Fellow
Research Center for Environmental Changes
Academia Sinica, Taiwan

Abstract:

Convective system plays an important role in the global climate including: hydrological cycle; radiative budget; and vertical energy transportation. It is usually associated with severe weather hazards, particularly deep convective clouds. Due to the fast development and short lifecycle, it is difficult to understand the characteristics of convections. This research examines the development of the deep convective cloud over the South China Sea (SCS) and Taiwan during June 2017 using high spatial-temporal geostationary satellite Himawari-8. We track and select convective cases by detecting minimum brightness temperature from 11 µm infrared channel as the cumulus center. Additionally, to investigate the differences in the characteristics of the cloud top from generation to mature, we separate the developing convective lifetime into three stages—Pre-CI, CI, and mature—by using the convective initiation signal.

The cloud top vertical velocities (CTW) are estimated by calculating the change in the cloud top height (CTH), which are found to be clustered 0-4 m s ^-1 and could reach more than 10 m s^-1. The value of CTW tends to increase with height when CTH below 10 km. During the mature stage, regardless of location, frequency is highest when CTW values are large. The cloud optical thickness (COT) is mainly distributed between 0–20 and, with the increase of CTH, the higher occurrence frequency shows a lower value of COT. Over Taiwan, the higher value of COT (> 30) occurs frequently in SCS. Furthermore, the cloud effective radius (Re) is mainly distributed between 20–30 µm. When the CTH is lower than 8 km, the Re in SCS is larger than in Taiwan and on mature stage, the Re decreases with development of CTH. In Taiwan, Re is frequently more than 60 µm. Further analysis shows that the stronger the CTW, the harder it is for convective clouds to develop larger COT and RE.

Presented on November 10, 2021, by

Professor J. David Neelin
Department of Atmospheric and Oceanic Sciences
University of California, Los Angeles (UCLA)

Abstract: Precipitation processes are notoriously complex, so it is not surprising that weather and climate models exhibit deficiencies in simulation of probability distributions of precipitation and their relationship to the water vapor and temperature environment. However, we rely on these models for projections of how the probabilities of extreme precipitation will change in a warming climate, so it is important to seek mechanistic understanding that can increase confidence in processes and associated diagnostics to improve models. Here we argue that it can be helpful to return to fundamental questions about what yields the characteristic shapes of probability distributions. This can be asked for different measures of precipitation including event accumulations, daily-average intensities and the size of spatial clusters. Many of these precipitation statistics can be captured by conceptually simple models, based on economical assumptions. These provide an understanding of the underlying processes and point, for instance, to the important role of the threshold-like transition from dry to precipitating conditions as a function of the thermodynamic environment. An overview will be provided of the dialogue between insights from the simple models and diagnostics of observations and complex model simulations.

Presented on November 3, 2021, by

Professor Yuqing Wang
Department of Atmospheric Sciences & International Pacific Research Center (IPRC)
School of Ocean and Earth Science and Technology
University of Hawai’i at Mānoa

Abstract: In this study, the boundary-layer tangential wind budget equation following the radius of maximum wind, together with an assumed thermodynamical quasi-equilibrium boundary layer is used to derive a new equation for tropical cyclone (TC) intensification rate (IR). A TC is assumed to be axisymmetric in thermal wind balance with eyewall convection becoming in moist slantwise neutrality in the free atmosphere above the boundary layer as the storm intensifies as found recently based on idealized numerical simulations. An ad-hoc parameter is introduced to measure the degree of congruence of the absolute angular momentum and the entropy surfaces. The new IR equation is evaluated using results from idealized ensemble full-physics axisymmetric numerical simulations. Results show that the new IR equation can reproduce the time evolution of the simulated TC intensity. The new IR equation indicates a strong dependence of IR on both TC intensity and the corresponding maximum potential intensity (MPI). A new finding is the dependence of TC IR on the square of the MPI in terms of the near-surface wind speed for any given relative intensity. Results from some numerical integrations of the new IR equation also suggest the finite-amplitude nature of TC genesis. In addition, the new IR theory is also supported by some preliminary results based on best-track TC data over the North Atlantic and eastern and western North Pacific. Compared with the available time-dependent theories of TC intensification, the new IR equation can provide a realistic intensity-dependent IR during weak intensity stage as in observations.

Presented on October 27, 2021, by

Professor Kerry A. Emanuel
Department of Earth, Atmospheric and Planetary Sciences (EAPS)
Massachusetts Institute of Technology (MIT), Cambridge

Abstract: I will present new results from downscaling tropical cyclones from three different 20th century reanalyses, which use only sea surface temperature and sea level pressure observations and show that the resulting time series of various tropical cyclone metrics matches that of historical observations of tropical cyclones. But the same downscaling over the whole globe shows essentially no detectable trends in cyclone activity over the same period. I will speculate about physical drivers of the Atlantic tropical cyclone trends, and also present a risk analysis for Hawaii.

Presented by

Dr. Jiwen Fan
Laboratory Fellow
Pacific Northwest National Laboratory (PNNL)
Richland, Washington

Abstract:

Deep convective clouds play a crucial role in atmospheric circulation, energy, and water cycle of our climate system. The extreme form of such storms produces weather hazards such as large hail, damaging winds and/or tornadoes, and torrential rainfall, causing significant property damages and economic losses. There is a large gap in our fundamental understanding of how human activities modify storm intensity, precipitation, and associated hazards. In this talk, I will present our effort on impacts of anthropogenic aerosols, urbanization, and anthropogenic warming on storm intensity, extreme precipitation, and hailstones.  I will focus on the understandings gained from process-level studies using both advanced observations and high-resolution model simulations. The challenges in observing and modeling such convective storms will be discussed.

Bio:

Dr. Jiwen Fan is a laboratory fellow at Pacific Northwest National Laboratory (PNNL). She received her Ph.D. degree in 2007 from Texas A&M University. Her expertise encompasses atmospheric chemistry, aerosols, clouds, convective systems, and severe storms. Her work contributes to improving physical understanding of the complex aerosol interactions with cloud microphysics and dynamics. Her current work includes (1) physical factors impacting severe convective storms, particularly under the context of urbanization, wildfires, and climate warming., (2) understanding meso-scale convective systems (MCSs) and improving global model capability in simulating MCSs, (3) development of cloud microphysics parameterizations for weather and climate models, and (4) impacts of marine organic aerosols and dust on mixed-phase clouds.

Jiwen has published over 110 peer-reviewed journal articles including the high-impact journals such as Science, Nature Geoscience, Nature Communications, Proceedings of the National Academy of Sciences of the United States of America. She was 2015 AGU ASCENT award for exceptional mid-career scientists. She has led and co-led several large collaborative research projects including the DOE Climate Model Development and Validation (CMDV) on mesoscale convective systems (CMDV-MCS) and the Enabling Aerosol-cloud interactions at GLobal convection-permitting scalES (EAGLES). She is the editor of Journal of Advances in Modeling Earth Systems (JAMES).

Presented by

Dr. Hanqin Tian
Solon & Martha Dixon Professor
Director of International Center for Climate and Global Change Research
School of Forestry and Wildfire Sciences
Auburn University

Abstract:

The terrestrial biosphere can release or absorb the greenhouse gases, carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O), and therefore has an important role in regulating atmospheric composition and climate. Recent assessment (IPCC AR6) indicated that the land biosphere plays a major nature contribution to climate stability by removing around one third of anthropogenic CO₂ emissions from the atmosphere each year. However, anthropogenic perturbation of the land biosphere has altered the carbon and nitrogen cycles, and the resulting increases in the emissions of non-CO₂ greenhouse gases (CH₄ and N₂O) in particular can contribute to climate change. By considering all three major GHGs (CO_2, CH₄ and N₂O ) together, our study shows that the cumulative warming capacity of concurrent biogenic CH₄ and N₂O is a factor of about two larger than the cooling effect resulting from the global land carbon dioxide uptake in the 2000s. Land-use intensification using today’s practices to meet food and energy demands increases anthropogenic GHG emissions, which is not consistent with stabilizing the climate at low temperature scenarios. However, the adoption of climate-smart land use practices to enhance carbon storage as well as reduce non-CO₂ GHG emissions from human-impacted land ecosystems could reverse the biosphere’s current warming role. Therefore, how we manage the global lands needs to become a central part in our strategy to mitigate climate change.

Presented by

Dr. Jong-Seong Kug
Pohang University of Science and Technology (POSTECH)
Pohang, South Korea

Abstract:

With the unprecedented rate of global warming in this century, whether or not human-made climate change is irreversible is the most critical question. Based on idealized CO₂ ramp-up and -down experiments, we show here that the intertropical convergence zone (ITCZ) exhibits irreversible changes. While the ITCZ location does not change much during the CO₂ increasing period, the ITCZ sharply moves south as soon as CO₂ begins to decrease, and its center eventually resides in the Southern Hemisphere. The pattern of the irreversible precipitation changes manifests a permanent extreme El Nino-like pattern, which has distinctive impacts on the global hydrological cycle. It was revealed that the hysteresis behavior of the Atlantic meridional overturning circulation and the delayed energy exchanges between the tropics and extratropics are responsible for the peculiar evolution of the hemispheric temperature contrast, leading to irreversible ITCZ changes.

Key words: ITCZ, Irreversible climate change, AMOC