Seasonal variations of the upper ocean in the western North Pacific observed by an Argo float

1. Seasonal variations of the upper ocean in the western North Pacific observed by an Argo float Naoto Iwasaka(1,2), Fumiaki Kobashi(2),Yosuke Kinoshita(2) and Yuko Ohno(2) (1) Institute of Observational Research for Global Change, JAMSTEC (2)Tokyo University of Marine Science and Technology

2. In 2001-2002, JAMSTEC deployed more than 60 PROVOR type floats in the western North Pacific. • Some of the floats made emergency surfacing just after their deployment and repeated emergency surfacing every 3 days. • One of such floats continued observation for about 9 months in the subtropical region, where there are no strong temperature fronts or strong current system.

3. in the present study ... • We investigated the upper layer variations on seasonal and shorter time scales in the center region of the subtropical gyre from the float observations, where surface and subsurface layers have rarely been observed with high time resolution for long periods of time.

4. Data and background field of the float

5. Float Observation: • WMO ID 5900449 • Float provider: JAMSTEC • Deployment: Dec. 2, 2001. 23.850˚N, 150.068˚E • Last surfacing: Aug.23, 2002. 23.344˚N, 143.080˚E • Profiling cycle: 3 days(*) *repeating emergency surfacing every 3 days • Profiling depth: 2000 dbar • Drifting depth: 2000 dbar • Sensor: SBE-41 • Platform: PROVOR • 88 TS profiles obtained during its life time (about 9 months)

6. Float trajectory Dec. 2, 2001 Aug. 23, 2002 Average drifting speed : 3cm/s SSH: Niiler et al. 2003

7. 3 cm/s 7 cm/s 7 cm/s Time-longitude section of SSH and SST anomalies ℃

8. Other data • Sea Surface Height (SSH): AVISO weekly mean maps • Sea Surface Temperature (SST): TRMM TMI SST (Remote Sensing Systems) • Surface wind: Gridded QuikScat wind data (Remote Sensing Systems) • Surface Heat Flux: NCEP/NCAR reanalysis • Subsurface data: WOA2001 The Horizontal Distribution Maps (HDMap: JAMSTEC)

9. Seasonal and short-term variations in time-depth sections

10. Mixed Layer Depth (MLD) 0.125kgm-3 Depth r MLD r(z) Potential temperature

11. Spring Summer Winter Contour lines: Shadings :

12. Spring Summer Winter

13. Seasonal change • Winter (December to the end of January) Mixed layer: generally deepened, max MLD in the end of January (130m) Surface: cooling • Spring (February to the middle of April) Mixed layer: generally shoaling, Max MLD did not appeared in February or March. MLD largely fluctuating (from 100 to 30m), sometimes unclear Surface: cooling, sometimes warming events • Summer (late April to August) Mixed layer: keeping shallow (20-30m) surface: generally warming

14. Contour lines: about 10 days Shadings : about 10 days

15. Non-seasonal variations • Mixed layre depth (MLD): Period of about 10 days. Large amplitudes seen in spring. • Isopleths in the time-depth section for each property: Periods of about 10 days and a few months. Vertically coherent changes below 150m depth. No seasonality. • Variations with the period of a few months < mesoscale eddies about 10 days < ???

16. Heat budget in the upper 500m column

17. Heat balance in the upper 500m column

18. Horizontal heat advection (HHA) HHA in the mixed layer Geostrophic current < SSH data Horizontal temperature gradient < SST data Ekman advection < surface wind data and SST data (negligible ) correction factor evaluated from WOA2001 and HDMaps HHA in upper 500 m

19. Depth Vertical heat advection (VHA) Assumption: no temperature change in the mixed layer but the MLD changes N th profile N+1 th profile no change in the shape of the temperature profile under the mixed layer

20. Vertical heat advection (VHA) Assumption: (1) no temperature change in the mixed layer (2)no change in the shape of the temperature profile under the mixed layer (3) but with change in the MLD

21. Heat budget in the upper 500m column W/m2 Aug. 2002 Dec. 2001

22. 30-day mean W/m2 VHA HHA SHF TCHC HHA + VHA + SHF TCHC Dec. 2001 Aug. 2002

23. O(TCHC)～O(HHA)～O(VHA) >>O(SHF) • High frequency variations dominate in TCHC and VHA • Dominant HHA variation may be related to the mesoscale eddies passing the float • On the monthly time scale, HHA and VHA are still major components of the heat budget. SHF has smaller but significant contribution to the TCHC.

24. Summary of the present study • The present study shows that the float observation captured a seasonal evolution of surface mixed layer in the western North Pacific continuously for more than 9 months, from December 2001 through August 2002. The result showed that the mixed layer was gradually deepened until it reached its maximum depth in the end of January. From February through the middle of April, the mixed layer varied largely and sometimes disappeared although MLD gradually decreased until the summer mixed layer formed. In summer, shallow surface mixed layer was maintained accompanying sharp thermocline and pycnolcine beneath it. • Heat budget analysis suggested that the vertical temperature advection, or vertical displacement of the thermal structure and the horizontal temperature advection have comparable magnitude and are dominant terms in the heat balance in the upper layer. • The present study demonstrates that the profiling float is a useful tool to observe the surface mixed layer of the ocean.

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