I specialize in meteorology and climate dynamics, with a particular focus on the activity of mobile cyclones and anticyclones (so-called storm tracks), which significantly influence daily weather and climate variability, and their interactions with underlying jet streams. I also investigate atmosphere-ocean interactions in midlatitudes.

My research aims to deepen our understanding of the interactions between phenomena of different scales within the climate system. By "connecting" weather-scale fluctuations with climate change and variability, I strive to obtain insights that contribute to improving their forecasts, predictions, and projections.

The ultimate goal of my research is to apply this knowledge to build a more resilient society, ensuring the safety of people's lives and livelihoods and creating a more secure, sustainable future for all. 

Transient eddies (cyclones & anticlones) and storm track activity

In the midlatitude regions where we live, day-to-day weather variability is primarily driven by migratory high and low-pressure systems. For instance, it's well-known that weather often deteriorates as a migratory low-pressure system (extratropical cyclone) approaches from the west.

In the Northern Hemisphere, two areas with particularly active migratory high- and low-pressure system activity are near Japan to the western North Pacific, and from the eastern coast of North America to the North Atlantic (see figure below). Theoretically, the development of migratory high and low-pressure systems is known to be proportional to the north-south temperature difference (baroclinicity). Indeed, regions with active migratory high and low-pressure system activity, known as storm tracks, correspond well with the jet streams that are present in areas of large north-south temperature differences. These storm tracks are also located near oceanic frontal zones, which play a crucial role in maintaining the atmosphere's significant baroclinicity.

Assessing Storm Track Activity Based on Local Time Filtering (Eulerian Method)

Objective Identification of Migratory Low and High-Pressure Centers (Lagrangian Method)

Separating the Contribution of Cyclonic and Anticyclonic Eddies in Eulerian Statistics

Based on the multi-lateral approach mentioned above, I have applied our storm track activity evaluation data (such as tracking results and Eulerian statistics) to various datasets. This includes not only multiple reanalysis datasets but also model data like CMIP6 and large-ensemble climate model simulations (d4PDF), from which we've generated monthly mean data. If you are interested in collaborative research, please feel free to contact Satoru Okajima.

"Midwinter Minimum" of the North Pacific Storm Track Activity

When we look at the seasonality of migratory high and low-pressure activity, the North Atlantic (right side of each panel in the figure below) shows the most vigorous activity in midwinter (January), when baroclinicity (which is roughly equivalent to jet stream wind speed aloft) is at its maximum. However, the North Pacific (left side of each panel in the figure below) exhibits a peculiar seasonality: migratory high and low-pressure activity weakens in midwinter, when baroclinicity is at its peak, compared to early winter or early spring.

This phenomenon, termed the "midwinter minimum" of North Pacific storm track activity, contradicts theoretical expectations. As a result, it has been studied for over 30 years since its discovery as a test case for mid-latitude meteorology and climate dynamics, with various mechanisms proposed. Nevertheless, many aspects of its mechanism remain unclear, making it one of the unsolved problems in meteorology and climate dynamics.

I have pursued research from multiple angles to clarify the mechanism behind this "midwinter minimum" in North Pacific storm track activity: 

• Seasonality of cyclone and anticyclone frequency: Okajima, S., H. Nakamura, Y. Kaspi (2023). Distinct roles of cyclones and anticyclones in setting the midwinter minimum of the North Pacific eddy activity: a Lagrangian perspective, J. Climate. Journal Site

• Quantitative eddy energetics: Okajima, S., H. Nakamura, Y. Kaspi (2022). Energetics of transient eddies related to the midwinter minimum of the North Pacific storm-track activity, J. Climate. Journal Site

• Relative importance of cyclonic and anticyclonic contributions ("anticyclonic suppression"): Okajima, S., Nakamura, H., Kaspi, Y. (2024a). Anticyclonic suppression of the North Pacific transient eddy activity in midwinter. Geophys. Res. Lett., 51, e2023GL106932. Journal Site

• Future Changes: Okajima et al., Autumn Meeting of the Meteorological Society of Japan 2024

Seasonality and Long-Term Modulation of Migratory Cyclone Activity in East Asia

Long-Term Modulation of Bomb Cyclone Frequency

• Kuwano-Yoshida, A., S. Okajima, H. Nakamura (2022). Rapid increase of explosive cyclone activity over the midwinter North Pacific in the late 1980s, J. Climate. Journal Site

The results of this research are introduced on the following page.

京都大学

Midlatitude air-sea interactions

The Earth's atmosphere and ocean are intimately connected, and their interactions are indispensable for understanding the formation and variability of the climate system. In midlatitude regions, including Japan, daily weather changes and seasonal to interannual and decadal climate variations are closely linked to these atmosphere-ocean interactions.

I specifically focus on the influence of the midlatitude ocean on the atmosphere, investigating its impact on large-scale atmospheric circulation and migratory high and low-pressure activity.

I have investigated how anomalies in sea surface temperature (SST) in the North Pacific affect large-scale atmospheric circulation through numerical experiments using an atmospheric general circulation model (AGCM).

I contributed to "Oceanic and Hemispheric Scale Vertically Coupled Variability in the Atmosphere-Ocean-Cryosphere System" in the Meteorological Research Book No. 244, titled "Hotspots in the Climate System: New Developments in Mid-Latitude Atmosphere-Ocean Interaction Research."

Impact of the Prominent North Pacific Warm SST Anomaly in the 2011 Warm Season on the Atmosphere

I focused on the prominent warm water anomaly in the North Pacific during the summer of 2011 and pointed out that its atmospheric response exhibited significant seasonality from summer to autumn.

• Okajima, S., H. Nakamura, K. Nishii, T. Miyasaka, A. Kuwano-Yoshida: Assessing the importance of prominent warm SST anomalies over the midlatitude North Pacific in forcing large-scale atmospheric anomalies during 2011 summer and autumn, J. Climate, 27, 3889-3903, 2014. Journal Site

Atmospheric Response to Decadal Variability of the Winter North Pacific Subarctic Frontal Zone

I investigated the atmospheric response to SST variations associated with the decadal variability of the winter North Pacific Subarctic Frontal Zone and its maintenance mechanisms. This research revealed that the response possesses "dynamical mode" characteristics, maintained by energy conversion from the background field.

• Okajima, S., H. Nakamura, K. Nishii, T. Miyasaka, A. Kuwano-Yoshida, B. Taguchi, M. Mori, Y. Kosaka: Mechanisms for the Maintenance of Wintertime Basin-Scale Atmospheric Response to Decadal SST Variability in the North Pacific Subarctic Frontal Zone, J. Climate, 31, 297-315, 2018. Journal Site

Influence of Mid-Latitude Oceanic Frontal Zones on Water Cycling Between Migratory Cyclones and Anticyclones

By quantitatively evaluating the water budget within cyclonic and anticyclonic regions, we clarified that the water cycling between migratory high and low-pressure systems is enhanced by the oceanic frontal zones in the North Pacific and North Atlantic.

• Okajima, S., Nakamura, H., Spengler, T. (2024b). Midlatitude oceanic fronts strengthen the hydrological cycle between cyclones and anticyclones. Geophysical Research Letters, 51, e2023GL106187.Journal Site

Impact of Fine Structures in Oceanic Frontal Zone Sea Surface Temperature on Atmospheric Reanalysis

Using JRA-55CHS, a sub-product of JRA-55 that provides high-resolution horizontal SST as a lower boundary condition, we evaluated the impact of the fine structures of the oceanic frontal zone's SST field on atmospheric reanalysis.

• Masunaga, R., H. Nakamura, H. Kamahori, K. Onogi, S. Okajima: The JRA-55CHS: An atmospheric reanalysis constructed with high-resolution sea surface temperature, SOLA, 14, 2018, 6-13. Journal Site

(Compound) Extreme events

Compound Events of Marine Heatwaves and Hot-Wet Extremes in East Asia During Summer 2023

Joint research is carried out with several national and international organizations. (The following are in chronological order and affiliation at the time the joint research was carried out)

• Domestic: JAMSTEC, JMA, MRI, Kyoto University, Toyama University, Port and Airport Research Institute, The University of Tokyo, Hokkaido University

• International: Weizmann Research Institute (Israel), University of Bergen (Norway), Florida State University (USA), University of Barcelona (Spain), NOAA (USA), Hanyang University (South Korea)

• Grant-in-Aid for Transformative Research Areas (A), Planned Research (Co-Investigator):
  "Frequent Atmospheric and Oceanic Heatwaves and Persistent Cold Spells" (FY2024–FY2028)
  → Part of the research area "Habitable Japan: Sustainability of the Atmospheric and Oceanic Environment as the Foundation for the Survival of the Island Nation Japan", featured on the project website.
  → Serving as one of the coordinators of the Early-Career Researchers' Network "ECHOES" within "Habitable Japan".

• Grant-in-Aid for Early-Career Scientists (Principal Investigator):
  "Distinct Seasonal Characteristics and Variability Mechanisms of South-Coast Cyclones" (FY2022–FY2025)

• Grant-in-Aid for Scientific Research (B) (Co-Investigator):
  "A Novel Atmospheric Circulation Dynamics Approach Using an Innovative Diagnostic Method to Evaluate the Behavior of Atmospheric Vortices by Polarity" (FY2022–FY2025)

• JSPS Research Fellowship for Young Scientists (Principal Investigator):
  "Impacts of Midlatitude Ocean Temperature Variability on Large-Scale Atmospheric Circulation" (FY2015–FY2016)

Environmental Research and Technology Development Fund (Co-Investigator):

• "Quantification and Understanding of Compound Extreme Weather and Climate Events Related to Future Changes in Japan and the Asia-Pacific Region" (FY2024–FY2026)

• "Improved Communication of Extreme Weather Projections with Reduced Uncertainty Based on Physical Understanding of Future Changes" (FY2022–FY2023)

• Grant-in-Aid for Transformative Research Areas (A) "The past, present, and future of "misbehaving climate" and humans" (Collaborator) (FY2024–FY2028)

• MEXT-Program for the Advanced Studies of Climate Change Projection (SENTAN) (Collaborator): (FY2022–FY2026)

• Arctic Challenge for Sustainability III (ArCS III) (Collaborator): (FY2025–FY2029)

• Arctic Research Acceleration Project (Collaborator): (FY2020–FY2024)

• Program on Open Innovation Platforms for Industry-academia Co-creation (COI-NEXT) "ClimCORE" (Collaborator) (FY2020–FY2030)

• Grant-in-Aid for Scientific Research on Innovative Areas (Collaborator) "Mid-latitude ocean-atmosphere interaction hotspots under the changing climate" (FY2019–FY2023)