Linkages between Japanese sardines and gyre spin-up

Shoshiro Minobe

(Hokkaido University)

Data & Method• Data

– Log Residual Recruitment for Japanese sardine, Pacific group (e.g., Yatsu et al. 2005 FO) calculated by Dr. Honda• RPS (ReProductive Success) with correction due to density

effect– NCEP-NCAR reanalysis data– Our own subsurface temperature data from WOD 2009

• 0-400 m averaged temp (similar to sea-level height)– Satellite altimeter data

• Method– correlation– Linear Rossby wave model

• At each latitude, sea-level heights due to Rossby waves propagating westward are calculated from wind-stress curl

Feeding ground (summer)

Nursery ground (spring)

Spawning ground (winter-early spring)

Schematics of areas for Sardine, Pacific group, low-stock level period.

Simultaneous correlation bewteen LNRR and SST

DJF 　 SST と同年の LNRR との相関

R < -0.4 over

DJF 　 KESA SST & LNRR

Kuroshio extension southern region (KESA)

SST explains well sharp decrease of LNRR in the end of 1980s.

Noto & Yasuda (1999) suggested KESA SST controls sardine mortality (close to RPS) for the period 1979-1994. Warm SST shallower mixed layer bad nutrient supply larger mortality.

0-400 m averaged temp & LNRR

Recent increase may also be explained (insufficient data in recent years).

Subsurface temp explains 1960s LNRR increase.

Nishikawa & Yasuda (1999) suggested current speed is important for mixed layer based on OFES (ocean model) for the period 1988-1994. Faster current short resident time of water in Kuroshio Extension shallower mixed layer bad nutrient supply larger mortality.

LNRR ： SST & 0-400m temp

Corr. DJF 　 SST & LNRRDJF 　 KESA -SST & LNRR

Rossby waves?

Corr. 0-400 temp & LNRR

Subsurface temp explains LNRR better than KESA SST.

KESA 0-400 m -temp & LNRR (r=0.63)

Ekman transport

Wind stress

Div.

Conv.

N

Rossby waves

Current

Eastward velocity anomalies to the west of eastward wind stress anomalies with a time-lag of Rossby wave propagation.

Lag correlation: Eastward wind-stress & LNRR

Westward wind stress anomalies leading to high LNRR is consistent with Rossby wave hypothesis.

Rossby waves?

Correlation in sea-level height between satellite altimeter SLH & Rossby model (Qiu 2003)Kuroshio

extension west of 160E cannot

explained

ロスビー波モデル

Not only KESA temperature south of Japan is also related as a gyre spin-up & down?

Correlations onto LNRR with Sea-level height and zonal velocity

But this is in the region where Rossby wave model fails.

Sea surface height EOF1 (Satellite)

Prominent decadal variability.

Jet-trapped Rossby waves (thin-jet theory) might play a role.

Lag correlation between PC1 and Sea surface height.

Conclusions

• Reproductive success expressed by LNRR for the Japanese sardine Pacific group is highly correlated with subsurface temperature in the Kuroshio extension southern area and in western subtropical gyre.

• It is interesting study in future whether gyre spin-up/down in addition to local speed up/down of the Kuroshio extension play an important (e.g., by transporting low-nutrient waters from the south).