A Preliminary Study of Mesoscale Eddy effects on the Formation of the NPSTMW

1. ARGO 12-14 November 2003 Tokyo, Japan A Preliminary Study of Mesoscale Eddy effects on the Formation of the NPSTMW Aijun Pan & Qinyu Liu Physical Oceanography Lab. Ocean University of China，Qingdao, P.R. China

2. Introduction 1 • Pycnostador thermostad • Formation mechanism • Spreading pathway • Interannual and long-term change (Masuzawa, 1969; Stommel, 1979; Hanawa and Hoshino, 1988; Suga and Hanawa, 1990; Bingham et al., 1992; Suga and Hanawa, 1995a, b; Suga and Hanawa, 1995; Yasuda and Hanawa, 1999; Taneda et al., 2000, Yasuda and Kitamura., 2003)

3. Introduction 2 Recently, several analysis suggest that formation and distribution of STMW are in more “turbulent” fashion and affected by mesoscale eddies. • Actually, the direct evidence was first given by Suga (1995), who • pointed out that anticyclonic eddy retained fairly thick STMW • during its westward movement. • Qiu (1999) further verified that eddy activity is remarkably high in • the KR region using T/P altimetry data. • Ebuchi and Hanawa [2000, 2001] estimated there are typically 2 to 3 • anticyclonic and cyclonic eddies each per year generated in the • vicinity of the KE with T/P data.

4. deep thermocline deep mixed layer STMW formation Anticyclonic eddies STMW spreading retain thick STMW continue • Latest work done by Uehara et al [2003] (With ARGO and T/P data) Conceptually Uehara et al [2003] (a) MLD in winter (b) STMW thickness in summer and autumn (Shown by color of plots)

5. Questions First, former works seem to exaggerate the role of anticyclonic eddies during the STMW formation process, and not much attention is given to cyclonic eddy • Also, stronger evidences are still in need to support former conclusions

6. Goal Our Goal is to take a snapshot of the mesoscale eddy effects on the STMW formation with the use of instantaneous ARGO profiling floats, and further, to make a quantitative estimation of anticyclonic and cyclonic eddy effects on STMW formation.

7. Data • SST from Tropical Rain Measuring Mission (TRMM) • satellite’s microwave imager (TMI), available • since December 1998 at 0.25º resolutions • [Wentz et al. 2000]. Besides ARGO, two other datasets are used: • SLA from merged T/P-ERS. It is on a MERCATOR • 1/3 grid and a temporal resolution of 7 days • [Le Traon et al., 1998;Ducet et al., 2000]

8. South North Cross-isopycnal flow Lateral induction :MLD Vertical pumping According to Williams (1989; 1991) Low PV water mass subducted cross the mixed layer front in North-South direction

9. Argo floats being chosen in February • Framed region by Suga and Hanawa [1990].

10. Argo floats being chosen in February

11. Isotherms (CI=2ºC) • isopycnals (CI=0.3 kgm-3) • shaded (PV<1.5×10-10m-1s-1 • dashed (MLD,SSD+ 0.125 kgm-3) (a) SST from TRMM (shaded) (b) SLA from T/P-ERS (shaded) Feb • First section (points) • (2001\02\25-26) • Second section (pentagram) • (2001\02\26-27)

12. March • (a) SST from TRMM • (b) SLA from T/P-ERS • Points(Argo floats) • Isotherms (CI=2ºC) • isopycnals (CI=0.3 kgm-3) • shaded (PV<1.5×10-10m-1s-1 • dashed (MLD,SSD+ 0.125 kgm-3)

13. Formation process of STMW using climatology XBT (at 144ºE). • T.: CI~2.0ºC • dotted (MLD) • shaded PV less than 1×10-10 m-1s-1

14. Generally, whether there is a mesoscale eddy or not determines greatly the local wintertime vertical mixing process, the distribution of the mixed layer, and hence the formation of the NPSTMW. The existence of anticyclonic (warm) eddy enhances the local vertical mixing process and helps to the formation of NPSTMW, while cyclonic (cold) eddy is unfavorable for the deepening of the mixed layer and hinders the formation of mode water.

15. Based on above conclusions, it can be inferred that interannual variations of eddy activities in the NPSTMW formation would inevitably impact the local vertical deep mixing and thus, affecting the formation of NPSTMW.

16. SSHA: Jan. 06, 1993 SSHA: Feb. 03, 1993 SSHA: Mar. 10, 1993 SSHAs in J/F/M of 1993 (T/P-ERS) 1993

17. SSHA: Jan. 01, 1997 SSHA: Feb. 19, 1997 SSHA: Mar. 19, 1997 SSHAs in J/F/M of 1997 (T/P-ERS) 1997

18. Total number of grid points in winter (J/F/M) that the SSHAs less than -30 cm in the zone of [30º-34ºN, 140º-170ºE] are calculated. The intensity of cyclonic eddy is defined as the J/F/M number deviations from the mean J/F/M number value of 1993 to 1998. Area and intensity of eddy

19. 13 x 10 3 2 ) 1 3 0 Formate Rate (m -1 -2 -3 -4 92 93 94 95 96 97 98 99 Time (years) By calculating the volume of the low potential vorticity water mass in winter (J/F/M) circumvented by . The intensity of vertical mixing is defined as J/F/M volume each winter deviations from the mean J/F/M volume value.

20. Summary • Compared to climatology data, ARGO provides us a great chance to capture the mesoscale features during the formation process of NPSTMW • Anticyclonic (warm) eddy helps to the vertical deep convection and the formation of NPSTMW, while cyclonic (cold) eddy plays a negative role • Cyclonic eddies prevail in the classical STMW formation region in late 1990s and hence greatly weakens the wintertime vertical mixing and the formation of NPSTMW

21. The End

22. Mesoscale eddies Characteristics of mesoscale eddies • Timescale: weeks to months • Horizontal scales: tens of km to the low hundreds of km. • Translatory velocity: several kilometers per day (rather slow) • However, velocities of water motion in eddies are much greater • than background current. • Effects: greatly affects the “oceanic weather”, the instantaneous • distributions of current velocities, temperature, salinity, • and so on. Eddy is a circulation system in which the water follows closed Circular or elliptic paths; can be cyclonic or anti-cyclonic (Tomczak and Godfrey, 2001) V.M.Kamenkovich, M.N.Koshlyakov and A.S.Monin (1986)