Eddy and Chlorophyll-a Structure in the Kuroshio Extension Detected from Altimeter and SeaWiFS

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1 14th Symposium on Integrated Observing and Assimilation Systems for the Atmosphere, Oceans, and Land Surface (IOAS-AOLS), AMS Atlanta, January 17-21, 21 Eddy and Chlorophyll-a Structure in the Kuroshio Extension Detected from Altimeter and SeaWiFS Peter C. Chu and Yu-heng Kuo Naval Postgraduate School Monterey, CA

2 Mean Surface Dynamic Height 17 cm Contour Kuroshio Extension (KE) (Qiu & Chen JPO)

3 Theories (1) Primary productivity in the oceans is limited by the lack of nutrients in surface water. (2) These nutrients are mostly supplied from nutrient-rich subsurface waters through upwelling and vertical mixing (Barber 1992). (3) In ocean gyres upwelling and vertical mixing are not fully account for the observed productivity (Jenkins & Goldman 1985). (4) Upward pumping of nutrients is through action of meso-scale eddies (McGillicuddy et al. 1999, Segal et al. 1999).

4 Can we identify such a upward pumping mechanism in western Pacific (such as KE) using satellite data?

5 Purpose of the Study To examine the spatial and temporal variability of SeaWiFS chlorophyll-a (Chl-a) in the KE region in relation to eddy structure To identify the upward nutrient pumping mechanism

6 Kuroshio Extension (KE) To the east of Japan the Kuroshio swings eastward to form the Kuroshio Extension. The branching of this current in the region of 16 E results in the movement known as the North Pacific Current. The Kuroshio Extension (KE) current carries warm water at nearly 1 million cubic meters per second (1 Sv) eastward into the North Pacific.

7 Sea Level Anomaly (SLA) SLA is measured by ERS 1/2 and TOPEX/Poseidon satellites at 7-day intervals. SLA has evident annual cycle and mesoscale structure dominated by eddy or Rossby waves.

8 Annual Sea Level amplitude (mm) in the North Pacific Ocean. The Kuroshio Extension route is associate with large annual amplitudes Latitude Annual Sea Level amplitude (mm) in the North Pacific Ocean NOAP NPGS Longitude 65 (mm)

9 Annual Signal of SLA (1) The annual signal is influenced partially by the ocean circulation and partially by the rise and fall of the sea surface that arises from the expansion and contraction of the ocean due to its heating and cooling with seasonal climatic change. (2) Maximum annual elevation change is about ±2 cm

10 Topography of the studied area and Kuroshio Extension axis (marked orange) adopted for the present study. Stations positions 1, 15, and 8 are marked Latitude N Longitude E 2 m 5 m 1 m 2 m 3 m m 5 m Coastline

11 Satellite observed sea level anomaly (cm) in the North Pacific in April 27 Sea level anomalies (cm) for the North Pacific Ocean for April Latitude coast line 6 2m 2m NOAP Longitude

12 Chl-a Concentration Chl-a concentration is computed from the ratio of radiances measured in band-3 (48 5 nm) and band-5 ( nm) according to the following NASA algorithm, [ ] C = exp Ln( L 49 / L 555 a WM WM

13 Satellite observed Chl-a concentration (mg/m3) in the North Pacific in April 27. SeaWiFS Chl a patterns observed in the North Pacific Ocean in April Latitude mg m**-3 chl a coast line 2m 2m NOAP Longitude

14 mg m**-3 chl a C Chl-a concentration G D L atitude A.6 E 2 B F H.4 6. e 3 Cyclonic eddy ÅÆ High Chl-a a Anticyclonic eddy ÅÆ Low Chl-a d c.15 b Longitude (cm) SLA (mm) C Latitude Anticyclonic eddies Æ A, B, C, G D 3 E 2 B A F H 6 7 e a 3 c b Cyclonic eddies Æ 1,2, d Longitude

15 Chl-a (mg m -3 Chl a) seasonal cycle along the route of the Kuroshio Extension. The results for station 1-15 are in grey and for station 16-8 in black Stations Mean Chl (mg m-3 Chl a) Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Months

16 Stations 1-15 SLA (cm) r= r=-.43 (a) SeaWiFS Chl a (mg/m3 Chl a) Stations 16-8 SLA (cm) Correlation between Chl-a and SLA residuals along the route of the Kuroshio Extension (b) SeaWiFS Chl a (mg m-3 Chl a) 1 1.1

17 Conclusions (1) Upward pumping of nutrients by mesoscale eddies in the KE region was identified. (3) SeaWiFS chlorophyll-a concentrations are redistributed at the eddy scale by the eddy surface swirl currents. (4) The zonal scale for chlorophyll perturbations was determined at ~ 46 km