Fig. 1

The summer (June—August) monthly climatological average sea surface temperature (SST, ℃) and wind (unit: m·s-1) from 1982 to 2021. The black box A in the figure is the studied nearshore upwelling region, and the black box B is the background region"

Tab. 1

El Niño years and La Niña years classified based on the different PDO phases"

Fig. 2

Variations of averaged sea surface temperature (SST) and upwelling index in the study areas in summer of 1982—2021. (a) averaged SST in offshore background area; (b) averaged SST in nearshore upwelling area; (c) upwelling index is the difference between nearshore sea surface temperature and offshore sea surface temperature. Red dash lines show linear trends of the variation"

Fig. 3

The interannual variation of wind-based upwelling index during summer from 1968 to 2021. The red lines show a linear trend for 1982—2021 and 1968—2021"

Fig. 4

The interannual variation of wind-based upwelling index (WUI) and Ocean Niño Index (ONI), the correlation coefficient of the two is -0.36"

Fig. 5

The variation of wind-based upwelling index during El Niño years (green), La Niña years (red) and natural years (blue). The top orange “+” represents the positive phase of PDO, while the bottom blue “o” represents the negative phase of PDO"

Tab. 2

Wind-based upwelling index of different levels [unit: m3·s-1·(100m)-1]"

Fig. 6

Wind anomalies in the Zhoushan sea in summer in (a) La Niña years and (b) El Niño years (relative to 1968—2021 average), (c) the difference between La Niña year and El Niño year"

Fig. 7

Composites of summer mean wind anomalies in the Zhoushan sea for (a) La Niña years and (b) El Niño years during the PDO positive phases, (c, d) during the negative PDO phases, respectively (relative to 1968—2021, unit: m·s-1)"