****************************************************************************************************************

第331回 大気海洋物理学・気候力学セミナー のおしらせ

日　時： 12月19日(木) 9:30 - 12:00
Date ： Thu., 19 Dec. 9:30 − 12:00
場　所： 低温科学研究所 3階 講堂
Place ： Institute of Low Temperature Science, 3F Auditorium

Speaker： Mensah Vigan (Institute of Low Temperature Science/PD)
Title： Comparison of the effects of the long-term trend and the 18.6 years tidal cycle on the intermediate waters of the Okhotsk Sea and the Oyashio

****************************************************************************************************************

 Comparison of the effects of the long-term trend and the 18.6 years tidal cycle on
the intermediate waters of the Okhotsk Sea and the Oyashio
 Mensah Vigan 発表要旨：





　In order to understand the impact of changes of the Okhotsk Sea
Intermediate Water (OSIW) and the Western Subartic Water (WSAW) on the
Oyashio water (OYW) at various time scales, time series and bi-monthly
climatologies of the Okhotsk Sea and North Pacific temperature, salinity,
 dissolved oxygen, and isopycnal layer thickness are produced. The time
scales of interest in this study are the long-term trend, whose signal
is represented by comparing data between 1925-1990 and 1990-2017, and
the 18.6 year tidal cycle, represented by climatologies for the high-
and low-tidal cycle. The OSIW (WSAW) is found to impact the Oyashio
water properties mostly for densities greater (smaller) than 26.8 σθ.
The long-term trend is characterized by an increase of the OYW
temperature (salinity) and decrease in O2 at all seasons and all sigma
layers, with a maximum temperature increase >0.5°C for all layers above
27.0 σθ during winter. This temperature increase is due to the combined
increase of the temperature of both OSIW and WSAW. The thickness of OYW
layers increases moderately (~10 m) for densities greater than 26.8 σθ,
which is due to the combined effect of a moderate decrease of OSIW and a
large increase of the WSAW layer thickness.
The temperature and salinity (O2 and thickness) in the Oyashio decrease
(increase) clearly at all seasons and all layers during the high 18.6
year tidal period, despite moderate variations of the OSIW and WSAW
properties. The OSIW layer thickness is seen to increase by up to 15 m
in the range 26.8 - 27.0 σθ, whereas WSAW layer thickness only changes
slightly. We then presume that the changes of properties in the Oyashio
during the 18.6 year period is primarly due to variations of the outflow
of OSIW from the Sea of Okhotsk to the Pacific Ocean. To confirm this
possibility, the volume transport of OSIW across a section in the
Oyashio is calculated based on T-S properties combined with baroclinic
geostrophic velocities estimated from our dataset and satellite-derived
surface velocities. These preliminary estimations yield a volume
transport of OSIW of 1.6 Sv during the high 18.6 year tidal cycle and 1.
0 Sv during the low tidal cycle. The 18.6 years tidal cycle causes a
temperature decrease in the Oyashio which is equivalent in scale to the
temperature increase due to the long-term trend. This highlights the
importance of this tidal cycle in affecting the water properties in the
North Pacific Ocean, and hints at a non-negligible impact of this cycle
on the climate in the subpolar region of the Pacific.

-----
連絡先