Abstract
We used ocean color data of chlorophyll-a (CHL) from the period 1998 to 2017 to investigate phytoplankton phenology during winter–spring in association with the Kuroshio Extension (KE) instability. In the areas south of the KE, CHLs tended to be higher in winter during periods of unstable KEs (compared to stable KEs) which were attributed to the increases in nutrient and light availability. Nutrients were supplied from the deep layer due to physical processes indicated by negative sea surface height anomalies (SSHAs) and shallow mixed layer depths (MLDs). The increase in light availability could be attributed to greater exposure of phytoplankton to light in the shallower MLD. The same physical processes also explained higher CHLs in spring during unstable KEs. We also found that CHLs could possibly be lower during unstable KEs in spring which might be related to warmer SSTs in winter–spring. On average, the onset of the phytoplankton spring bloom south of the KE tended to be 1–3 weeks earlier during the period of unstable KEs than during the period of stable KEs. Whether this difference of 1–3 weeks impacts high-trophic-level organisms should be investigated in future studies.
Highlights
The Kuroshio Extension (KE) jet defined by 170-cm sea surface height in the westernNorth Pacific Ocean can be either weak or strong during certain periods of time
During the period of unstable KE was more characterized by negative anomalies (Figure 2c) consistent with the result of the study by Lin et al [10]. These negative sea surface height anomalies (SSHAs) south of the KE during the period of unstable KEs were mainly attributed to two physical factors
First is the negative SSHAs that are propagated westward from the central Pacific Ocean and remotely induced by large-scale wind stress curl anomalies associated with the positive phase of Pacific DecadalOscillation (PDO) or negative phase of North Pacific Gyre Oscillation (NPGO) [1–4]
Summary
The Kuroshio Extension (KE) jet defined by 170-cm sea surface height in the westernNorth Pacific Ocean can be either weak or strong during certain periods of time. The weak (strong) KE jet is associated with westward propagation of negative (positive) sea surface height anomaly (SSHA). As mentioned by Oka et al [7], in the areas north of the KE during the period of unstable KEs, the westward propagating negative SSHAs turn into positive SSHAs in the upstream portion of KE east of Japan due to more anticyclonic eddies detached to the north from the KE. This explains the negative (in the south) and positive (in the north) SSHA seesaw observed during the period of unstable KEs [5,10]
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