Changhyun Yoo

Impacts of the north and tropical Atlantic Ocean on the Antarctic Peninsula and sea ice

In recent decades, Antarctica has experienced pronounced climate changes. The Antarctic Peninsula exhibited the strongest warming of any region on the planet, causing rapid changes in land ice. Additionally, in contrast to the sea-ice decline over the Arctic, Antarctic sea ice has not declined, but has instead undergone a perplexing redistribution. Antarctic climate is influenced by, among other factors, changes in radiative forcing and remote Pacific climate variability, but none explains the observed Antarctic Peninsula warming or the sea-ice redistribution in austral winter. However, in the north and tropical Atlantic Ocean, the Atlantic Multidecadal Oscillation (a leading mode of sea surface temperature variability) has been overlooked in this context. Here we show that sea surface warming related to the Atlantic Multidecadal Oscillation reduces the surface pressure in the Amundsen Sea and contributes to the observed dipole-like sea-ice redistribution between the Ross and Amundsen–Bellingshausen–Weddell seas and to the Antarctic Peninsula warming. Support for these findings comes from analysis of observational and reanalysis data, and independently from both comprehensive and idealized atmospheric model simulations. We suggest that the north and tropical Atlantic is important for projections of future climate change in Antarctica, and has the potential to affect the global thermohaline circulation and sea-level change.

Mechanisms of Arctic Surface Air Temperature Change in Response to the Madden-Julian Oscillation

AbstractUsing lagged composites and projections with the thermodynamic energy equation, in this study the mechanisms that drive the boreal winter Arctic surface air temperature (SAT) change associated with the Madden–Julian oscillation (MJO) are investigated. The Wheeler and Hendon MJO index, which divides the MJO into 8 phases, where phase 1 (phase 5) corresponds to reduced (enhanced) convection over the Maritime Continent and western Pacific Ocean, is used. It is shown that the more zonally localized (uniform) tropical convective heating associated with MJO phase 5 (phase 1) leads to enhanced (reduced) excitation of poleward-propagating Rossby waves, which contribute to Arctic warming (cooling). Adiabatic warming/cooling, eddy heat flux, and the subsequent change in downward infrared radiation (IR) flux are found to be important for the Arctic SAT change. The adiabatic warming/cooling initiates the Arctic SAT change, however, subsequent eddy heat flux makes a greater contribution. The resulting SAT chan...

Modulation of the boreal wintertime Madden‐Julian oscillation by the stratospheric quasi‐biennial oscillation

Madden-Julian oscillation (MJO), the dominant mode of intraseasonal variability in the tropical troposphere, has a significant impact on global weather and climate. Here we present that the year-to-year variation of the MJO activity shows significant changes with the quasi-biennial oscillation (QBO) in the tropical stratosphere. Specifically, the boreal winter MJO amplitude, evaluated by various metrics, is typically stronger than normal during the QBO easterly phase at 50 hPa and weaker than normal during the QBO westerly phase at 50 hPa. This relationship, which is possibly mediated by the QBO-related static stability and/or vertical wind shear changes in the tropical upper troposphere and lower stratosphere, is robust whether or not the activeness of the MJO or QBO is taken into account. This result suggests a new potential route from the stratosphere that regulates the organized tropical convection, helping to improve the prediction skill of the boreal winter MJO.

Stratospheric Control of the Madden–Julian Oscillation

AbstractInterannual variation of seasonal-mean tropical convection over the Indo-Pacific region is primarily controlled by El Nino–Southern Oscillation (ENSO). For example, during El Nino winters, seasonal-mean convection around the Maritime Continent becomes weaker than normal, while that over the central to eastern Pacific is strengthened. Similarly, subseasonal convective activity, which is associated with the Madden–Julian oscillation (MJO), is influenced by ENSO. The MJO activity tends to extend farther eastward to the date line during El Nino winters and contract toward the western Pacific during La Nina winters. However, the overall level of MJO activity across the Maritime Continent does not change much in response to the ENSO. It is shown that the boreal winter MJO amplitude is closely linked with the stratospheric quasi-biennial oscillation (QBO) rather than with ENSO. The MJO activity around the Maritime Continent becomes stronger and more organized during the easterly QBO winters. The QBO-rela...

A Rossby Wave Bridge from the Tropical Atlantic to West Antarctica

AbstractTropical Atlantic sea surface temperature changes have recently been linked to circulation anomalies around Antarctica during austral winter. Warming in the tropical Atlantic associated with the Atlantic multidecadal oscillation forces a positive response in the southern annular mode, strengthening the Amundsen–Bellingshausen Sea low in particular. In this study, observational and reanalysis datasets and a hierarchy of atmospheric models are used to assess the seasonality and dynamical mechanism of this teleconnection. Both the reanalyses and models reveal a robust link between tropical Atlantic SSTs and the Amundsen–Bellingshausen Sea low in all seasons except austral summer. A Rossby wave mechanism is then shown to both explain the teleconnection and its seasonality. The mechanism involves both changes in the excitation of Rossby wave activity with season and the formation of a Rossby waveguide across the Pacific, which depends critically on the strength and extension of the subtropical jet over...

Rossby Waves Mediate Impacts of Tropical Oceans on West Antarctic Atmospheric Circulation in Austral Winter

Recent studies link climate changearound Antarctica to the sea surface temperature of tropical oceans, with teleconnections from the Pacific, Atlantic, and Indian Oceans making different contributions to Antarctic climate. In this study, the impacts of each ocean basin on the wintertime Southern Hemisphere circulation are identified by comparing simulation results using a comprehensive atmospheric model, an idealized dynamical core model, and a theoretical Rossby wave model. The results herein show that tropical Atlantic Ocean warming, Indian Ocean warming, and eastern Pacific cooling are all able to deepen the Amundsen Sea low located adjacent to West Antarctica, while western Pacific warming increases the pressure to the west of the international date line, encompassing the Ross Sea and regions south of the Tasman Sea. In austral winter, these tropical ocean basins work together linearly to modulate the atmospheric circulation around West Antarctica. Further analyses indicate that these teleconnections critically depend on stationary Rossby wave dynamics and are thus sensitive to the background flow, particularly the subtropical/midlatitude jet.Nearthesejets, wind shearis amplified,which strengthens the generation of Rossby waves. On the other hand, near the edges of the jets the meridional gradient of the absolute vorticity is also enhanced. As a consequence of the Rossby wave dispersion relationship, the jet edge may reflect stationary Rossby wave trains, serving as a waveguide. The simulation results not only identify the relative roles of each of the tropical ocean basins in the tropical‐Antarctica teleconnection, but also suggest that a deeper understanding of teleconnections requires a better estimation of the atmospheric jet structures.

On the Causal Relationship between Poleward Heat Flux and the Equator-to-Pole Temperature Gradient: A Cautionary Tale

AbstractAn increase in the poleward heat or energy transport is often ascribed to a strengthening of the equator-to-pole gradient in temperature or in the top-of-the-atmosphere (TOA) net radiation. While this attribution conforms to the well-established flux–gradient relationship, a counterexample is shown here, demonstrating that a forced atmospheric circulation, triggered by enhanced convection over the western tropical Pacific warm pool and suppressed convection over the eastern tropical Pacific and Indian Oceans, can cause the equator-to-pole gradient in the TOA net radiation to increase.

Persistent Multiple Jets and PV Staircase

Abstract The persistence of multiple jets is investigated with a quasigeostrophic, two-layer, β-plane channel model. Linearly unstable normal modes are found to be capable of qualitatively describing the eddy fluxes of the nonlinear model. For a persistent double jet (PDJ) state, the most unstable normal mode has its largest amplitude located between the two jets, with a downshear tilt that acts to keep the jets separated. The opposite tilt occurs for a double jet state that is intermittent. An analysis of these normal modes, which utilized the concept of counterpropagating Rossby waves (CRWs), suggests that the downshear tilt in the interjet region hinges on the presence of critical latitudes only in the lower layer. This conclusion in turn suggests that the initial generation of the persistent jets requires L/Cgy < r−1, where L is the distance between the wave source (jet) and sink (interjet), Cgy is the meridional group velocity, and r is the linear damping rate. Similar CRW analysis for a conventional...

Boreal Winter MJO Teleconnection in the Community Atmosphere Model Version 5 with the Unified Convection Parameterization

AbstractThe Madden–Julian oscillation (MJO), the dominant mode of tropical intraseasonal variability, influences weather and climate in the extratropics through atmospheric teleconnection. In this study, two simulations using the Community Atmosphere Model version 5 (CAM5)—one with the default shallow and deep convection schemes and the other with the unified convection scheme (UNICON)—are employed to examine the impacts of cumulus parameterizations on the simulation of the boreal wintertime MJO teleconnection in the Northern Hemisphere. It is demonstrated that the UNICON substantially improves the MJO teleconnection. When the UNICON is employed, the simulated circulation anomalies associated with the MJO better resemble the observed counterpart, compared to the simulation with the default convection schemes. Quantitatively, the pattern correlation for the 300-hPa geopotential height anomalies between the simulations and observation increases from 0.07 for the default schemes to 0.54 for the UNICON. These...

SPARC Local Workshop on “WCRP Grand Challenges and Regional Climate Change”

SPARC (Stratosphere–Troposphere Processes and their Role in Climate) is one of the core projects of the World Climate Research Program (WCRP), coordinating international efforts to address relevant issues in climate and climate prediction via better understanding of the stratosphere– troposphere system. SPARC is a broad umbrella body that supports many scientific activities solving environmental issues, such as atmospheric dynamics and predictability, chemistry and climate, and long-term records for understanding climate. Much of the scientific activity of SPARC is directly linked to the WCRP’s “grand challenges”, particularly the issues of “Clouds, Circulation and Climate Sensitivity”, “Weather and Climate Extremes”, “Near-term Climate Prediction”, “Carbon Feedbacks in the Climate System”, and “Melting Ice and Global Consequences”. A SPARC local workshop was held at the Korea Polar Research Institute, 18–20 October 2017, in Incheon, South Korea, focusing on “WCRP Grand Challenges and Regional Climate Change” to promote international collaboration on these issues. This SPARC-sponsored workshop aimed at not only international collaborations with Asian scientists, but also capacity development for SPARC-related activities in Asia. Active discussions among 79 scientists and students from 10 different countries (Fig. 1) filled the workshop and sessions of various SPARC and SPARC-related research topics on stratosphere–troposphere coupling, atmospheric composition, Arctic climate, and climate change and variability. We briefly describe them below.