8-1. Research Divisions

Division for Land–Ocean Ecosystem Research Global warming and changes in terrestrial water-material
cycles in the Arctic circumpolar region
 Effects of climate change and anthropogenic forcing on
the terrestrial ecosystem Cloud/rainfall variability in Asian monsoon regions
 Dynamics of phytoplankton in marginal seas and coastal areas
 Climate variability and changing open ocean ecosystem
dynamics and biogeochemical cycle
 Interaction between oceanic waves and climate variations

The Land–Ocean Ecosystem Research Division investigates regional and global energy, water and material cycles,
and physical/biogeochemical processes in the land–ocean ecosystem.
The land research group contributes to advancing our understanding of the mechanisms by which ongoing global
warming and anthropogenic activity influence the terrestrial water cycle and ecosystem. Using field observations, satellite
remote sensing, global meteorological data analysis, laboratory analysis, and model simulation approaches, our group aims
to understand the impact of global warming on hydrological and greenhouse gas cycles in the Arctic region, the dynamics
of the continental scale water cycle, the processes that drive weather and climate over Asia, the interplay between theterrestrial ecosystem and climate, and the detection of early signs of the influence of global warming in Antarctica.
Ocean research was conducted using satellite remote sensing, numerical simulations, and in-situ observations. We
also performed synthesis studies of physical and biogeochemical processes in the ocean and their interactions with theatmosphere and climate. In particular, we are investigating how oceanic heat content, circulation, and surface waves
interact with atmospheric environments and how they are linked to climate and meteorological phenomena such as
tropical cyclones. We are also investigating how variations in ocean circulation, mixing processes, and air–sea fluxes
influence marine ecosystems where phytoplankton are the primary producers. Moreover, we are interested in the possible
impact of the marine ecosystem on physical processes and climate in the ocean and atmosphere.

Main Activities in FY2022
Contribution of summer net precipitation to winter river discharge in permafrost zone of the
Lena River basin
Winter discharge from the Lena River in eastern Siberia has increased over recent decades. However, the impact ofpermafrost thawing and changing hydrological processes induced by climate change on the winter discharge of rivers has
not been well quantified. Herein, using a coupled land surface and a distributed discharge model, we performed trend
evaluations to examine the sensitivity of winter discharge to permafrost thawing and water budget changes in the Lena
River basin from 1979 to 2016. An increasing trend in winter baseflow was found in the upper part of the Lena Riverbasin, where the summer net precipitation demonstrated a statistically significant increase. The increased summer net
precipitation resulted in higher soil moisture in the deepened active layer in late summer and early autumn, which was
linked to the autumn and winter baseflow. These implications were examined from the perspective of the interrelations
among the trends in active layer thickness, soil moisture, and baseflow in the cold season by identifying regions, wherein
all variables exhibited positive trends. The identified source regions were primarily in the upper and lower parts of the
Lena River basin, although winter baseflow was more dominant in the upper regions owing to the freezing effect of theactive layer in the lower region. The thinning of river ice induced by warming temperatures also contributes to an increasein winter river discharge. These results suggest that increased winter discharge was strongly associated with the climate
56 Institute for Space–Earth Environmental Research

8-1. Research Divisionschange-related enhancement of permafrost thawing and an increase in net precipitation that affected soil hydrologicalprocesses, which will be strengthened further in the context of global warming.
(Reference: Hiyama, T., H. Park, K. Kobayashi, L. Lebedeva, D. Gustafsson (2023): Contribution of summer net
precipitation to winter river discharge in permafrost zone of the Lena River basin. Journal of Hydrology, 616, 128797,
doi:10.1016/j.jhydrol.2022.128797)

Nocturnal southerly moist surge parallel to the coastline over the western Bay of Bengal
Nocturnal precipitation is a well-known phenomenon around theHimalayas and the Meghalaya Plateau in South Asia during the summer. Such
precipitation is a major supply source for glaciers in the central and –eastern
Himalayas and the headwaters of major rivers, such as the Ganges andBrahmaputra. In this study, we demonstrate that low-level moist southerlies are
significantly enhanced at night parallel to the coastline over the western Bay
of Bengal (BoB), and then flow onto the Gangetic Plain, enhancing moisturetransport toward land and nocturnal precipitation in South Asia. We refer tothis phenomenon as a nocturnal southerly moist surge. This nocturnal surge
was strongly affected by the diurnal cycle of the thermal and topographic
effects of the Indian subcontinent. At night, a strong low-level westerly jet
appears above the nocturnal stable layer over the Indian subcontinent. Strong
low-level southwesterlies with a low-level jet structure also appear over thewestern BoB, extending from the strait between the southernmost tip of the
Indian subcontinent and Sri Lanka. The low-level westerlies flowing from thesubcontinent and the southwesterlies merge into a single strong southwesterly

Difference in water vapor flux (vectors) and
its divergence (shading) between nighttime(23:00–02:00 LT) and daily mean values

flow, forming the low-level moist surge over the western BoB.
(Reference: Fujinami, H., T. Sato, H. Kanamori, and M. Kato (2022): Nocturnal southerly moist surge parallel to thecoastline

over

the

western

BayofBengal.

GeophysicalResearch

Letters,49,

e2022GL100174,

doi:10.1029/2022GL100174)

Impacts of global warming on summer precipitation trend over northeastern Eurasia during
1990–2010
Summer precipitation in Siberia increased significantly
during the 2000s, particularly in eastern Siberia. However, themechanism controlling this increase in precipitation remains
obscure. This study investigated the impact of global warming
on summer precipitation over northeastern Eurasia usinglarge-ensemble data from historical warming and nonwarming simulations for the period 1990–2010. The positivesummer precipitation trends across Siberia reproduced in theensemble mean of the historical simulation were similar to theobserved data; however, the negative trends observed overnortheast China and Mongolia were not found in the ensemble
mean. An empirical orthogonal function analysis was applied
to the summer precipitation trend over Siberia for each

The composite differences in the linear trends of (a) summer
precipitation and (b) column water vapor flux and low-level precipitablewater between the warming and non-warming experiments contributed
to the strength of the westerly moisture flux over eastern Siberia

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8-1. Research Divisions.
simulation to extract the members with a precipitation trend pattern similar to that of the observations. The first leading
mode in each simulation demonstrated an increase (decrease) in precipitation over eastern Siberia (northeast China),
which is consistent with the spatial features of the observations. In the extracted members, the spatial pattern of thecyclonic (anticyclonic) circulation trend over the northern parts of eastern Siberia (northeast China), associated withdecadal and multidecadal variations, was amplified by global warming, resulting in an increasing trend of the westerly
moisture flux into eastern Siberia from western Siberia (Figure). Additionally, surface heating in northeast China,
enhanced by global warming, may have concurrently intensified the cyclonic circulation over eastern Siberia. Furthermore,
the results suggest that the reduced extent of Arctic sea ice coverage played a role in strengthening the cyclonic circulation
over eastern Siberia and enhancing water vapor transport from the Arctic Ocean, which contributed to the strength of thewesterly moisture flux over eastern Siberia.
(Reference: Kanamori, H., M. Abe, H. Fujinami, and T. Hiyama (2023): Impacts of global warming on summer
precipitation trend over northeastern Eurasia during 1990-2010 using large-ensemble experiments. International Journalof Climatology, 43, 615-631, doi:10.1002/joc.7798)

Algorithm development for satellite-based green Noctiluca red tide and the seasonalvariation in the Upper Gulf of Thailand
Red tides of diverse species occur in coastal
tropical areas, and in recent decades, red tides ofgreen Noctiluca scintillans red tides have been
particularly frequent. In this study, we developed a
red-tide classification algorithm for a satellite
ocean color sensor in the upper Gulf of Thailand(uGoT) to study seasonal variations in red tides.
High green Noctiluca concentrations demonstrated
clear spectral characteristics at blue to green and red
to near-infrared wavelengths with in situ remotesensing reflectance observations. According to the
red tide spectral characteristics, a classification
algorithm using remote sensing reflectance of 488,

Red tide classification algorithm in the upper Gulf of Thailand

531, and 667 nm normalized to the value of 565 nm was developed to distinguish between green Noctiluca red tides at three
concentration levels and other red tides (dinoflagellates, diatoms, blue-green algae, and mixed species), including offshore
and coastal waters in non-red tide conditions (Figure) . Monthly composite data from MODIS from 2003 to 2021 revealed
seasonal variations in the surface distribution and frequency of green Noctiluca and other red tides during the Southwestern(May–September) and Northeastern (October–January) monsoons. Green Noctiluca red tides occurred more offshore fromthe shore and estuaries than other red tides (dinoflagellates and blue-green algae) and occurred more frequently than otherred tides between the mouths of the Tachin and Chao Phraya rivers during non-monsoon periods (February–April). The
frequency and distribution of green Noctiluca and other red tides varied during the monsoon season. By comparing the redtide distribution identified by the satellite with factors caused by the monsoon (ocean wind, precipitation, and river flow),
we were able to elucidate, the spatiotemporal distribution of red tides across the uGoT under Asian monsoon conditions.
This study will help us comprehend the impact of climate change on phytoplankton dynamics.
(References: Luang-on, J., J. Ishizaka, A. Buranapratheprat, J. Phaksopa, J. I. Goes, E. R. Maure, E. Siswanto, Y. Zhu,
Q. Xu, P. Nakornsantiphap, H. Kobayashi, and S. Matsumura (2023): MODIS-derived green Noctiluca blooms in theupper Gulf of Thailand: Algorithm development and seasonal variation mapping. Frontiers in Marine Science, 10,1031901, doi:10.3389/fmars.2023.1031901）
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8-1. Research Divisions

Zonal interaction of interannual oceanic waves in the Tropical PacificThis study examined how oceanic upper-layer wave
motions associated with El Niño and La Niña events andplay a role in the interaction between the western, central,and eastern parts of the ocean basin from the energy
circulation perspective. Recent studies have made it
possible to trace wave energy circulation paths by tracing
group velocity vectors without any borders between theequatorial and mid-latitude dynamics. It is not possible to

Peak values of the energy-flux stream function [Watts] in the easternequatorial Pacific Ocean

trace or quantify the generation, transfer, and dissipation of perturbations in a chain simply by displaying the distribution of theseenergy-flux vectors on a map. To resolve this problem, this study proposes a method to apply Helmholtz decomposition to theenergy flux distribution and diagnose the time series of the peak values of the stream function and potential components. This timeseries implies that the energy flux is integrated across its path, and its time variation can be compared with a well-known index inclimate studies called the Niño 3. In making this comparison, we note that the energy fluxes in this study demonstrate eastward
transfer events during both El Niño and La Niña events and cannot distinguish between downwelling and upwelling Kelvinwaves. However, the Niño 3 index, which represents climatological anomalies in sea surface temperature in the easternequatorial Pacific, can distinguish between downwelling and upwelling Kelvin waves, whereas its quantitative meaning reflects
only the physical component related to potential energy and does not consider the physical component related to kinetic energy.
This study aims to resolve these problems and provide useful validation using the output of hindcast simulations for 51 years.

Rapid increase of surface water pCO2 during 2001–2009 in Sagami Bay
Little Little is known about the rate of increase incoastal seawater pCO2 (pCO2sea) despite its necessity forevaluating future oceanic CO2 uptake capacity. We
examined temporal changes in pCO2sea in central Sagami
Bay during 2001–2009. The weekly pCO2sea wasreconstructed using a time series of the particulateorganic carbon isotope delta (POC-δ13C) of settlingparticles at 150 m from moored sediment trap
deployments. For pCO2sea estimation, an empirical
relationship between suspended POC-δ13C and aqueous

Time series data for (a) pCO2sea (circle) and atmospheric pCO2 (pCO2air,solid line) in the central part of Sagami Bay from 2001 to 2009

CO2 concentrations from repeated ship observations in 2007 and 2008 was applied to the trapped POC-δ13C. The air–sea
CO2 flux was calculated using the air-sea pCO2 difference with the gas transfer velocity. The estimated Bay pCO2sea varied
by 190 μatm (mean 294 μatm) and was mostly below atmospheric pCO2 (pCO2air), resulting in a mean oceanic CO2 uptakeof 30 g m–2 y–1, suggesting that Sagami Bay is an efficient sink for atmospheric CO2. Meanwhile, carbon sequestration to
the mesopelagic layer by particulate carbon export accounted for 60–75% of the CO2 uptake, with the rest likely removed
horizontally via surface water exchange. The pCO2sea demonstrated an increasing trend of +3.9 µatm y–1, approximately
twice that of pCO2air, resulting in the two converging. Concurrently, a decreasing trend in POC export flux and an increasing
trend in the nitrogen isotope delta of the trapped particles were observed. A large summer pCO2sea increasing rate (+4.9 µatmy–1) was observed accompanied by POC concentration decreasing, which resulted in a decrease in CO2 uptake over time.
Long-term summer nutrient depletion and reduced primary production may increase pCO2sea in the bay.
(References: Mino, Y., C. Sukigara, A. Watanabe, A. Morimoto, K. Uchiyama-Matsumoto, M. Wakita, and T. Ishimaru(2023): Rapid increase of surface water pCO2 revealed by settling particulate organic matter carbon isotope time series
during 2001–2009 in Sagami Bay, Japan. Journal of Oceanography, 79, 317–331, doi:10.1007/s10872-023-00688-3）
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