Modulation of Kuroshio transport by mesoscale eddies at the Luzon Strait entrance

Abstract

Measurements of Kuroshio Current velocity at the entrance to Luzon Strait along 18.75 N were made with an array of six moorings during June 2012 to June 2013. Strong positive relative vorticity of the order of the planetary vorticity f was observed on the western flank of the Kuroshio in the upper 150 m. On the eastern flank, the negative vorticity observed was about an order of magnitude smaller than f. Kuroshio transport near its origin is computed from direct measurements for the first time. Kuroshio transport has an annual mean of 15 Sv with a standard deviation of 3 Sv. It is modulated strongly by impinging westward propagating eddies, which are identified by an improved eddy detection method and tracked back to the interior ocean. Eight Kuroshio transport anomalies >5 Sv are identified; seven are explained by the westward propagating eddies. Cyclonic (anticyclonic) eddies decrease (increase) the zonal sea level anomaly (SLA) slope and reduce (enhance) Kuroshio transport. Large transport anomalies of >10 Sv within O(10 days) are associated with the pairs of cyclonic and anticyclonic eddies. The observed Kuroshio transport was strongly correlated with the SLA slope (correlation 5 0.9). Analysis of SLA slope data at the entrance to Luzon Strait over the period 1992-2013 reveals a seasonal cycle with a positive anomaly (i.e., an enhanced Kuroshio transport) in winter and spring and a negative anomaly in summer and fall. Eddy induced vorticity near the Kuroshio has a similar seasonal cycle, suggesting that seasonal variation of the Kuroshio transport near its origin is modulated by the seasonal variation of the impinging mesoscale eddies.

Large variability of the Kuroshio at 23.75°N east of Taiwan

“…Rather, we concentrate on the relationship between the Kuroshio transport and the SSH gradient. Lien et al [2014] found that the Kuroshio transport anomaly is well correlated with the zonal gradient of SSHa at 18.75 N along a transect northeast of Luzon Island. Their observation confirms that the satellite SSHa well reflects the baroclinic structure of Kuroshio [Guinehut et al, 2009] and can be used to provide a measure of the Kuroshio transport northeast of Luzon Island.…”

“…The Kuroshio transport or its anomaly is often related with the zonal gradient of sea surface height (SSH) or SSHa [e.g., Lien et al, 2014]. Figure 9 shows time series of the SSH, SSHa, and zonal gradient of SSH along 23.75 N from August 2012 to September 2014.…”

“…Furthermore, the existence of SCSIW east of Taiwan as well as the dynamic link between SCSIW in the Okinawa Trough and that upstream off southeastern Taiwan has yet to be clarified. To address these uncertainties, an ongoing Observations of the Kuroshio Transports and their Variability (OKTV) project, as a counterpart of the Origins of the Kuroshio and Mindanao Current (OKMC) project Lien et al, 2014], has conducted nine comprehensive ship surveys along the KTV1 transect (Figure 1) between September 2012 and September 2014. Results from the new data set are compared with the climatology of the Kuroshio.…”

Combining Observations from Multiple Platforms across the Kuroshio Northeast of Luzon: A Highlight on PIES Data

“…The former measures both the geostrophic and ageostrophic components of the current; whereas only the absolute geostrophic current is derived from PIES. Some of the periods of large discrepancies between the two volume transport time series may be associated with the mesoscale eddy-Kuroshio interactions reported in Lien et al (2014), as in June, July, and December 2012, while some others seem unrelated to such influence (August-September 2012). The detailed analysis of the differences between ADCPand PIES-derived transports is the subject of an ongoing study.…”

“…8b). In this case, transports are for the layer between the surface and 600 dbar, following Lien et al (2014), who extrapolate their ADCP measurements to these levels in order to capture a larger part of the Kuroshio transport. In both comparisons, the zonal integration range is set to 122.108-122.878E, which corresponds to the zonal extent of the ADCP array.…”