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Lagrangian Observations of the Mode Water Formation in the North Pacific

This workshop discusses the motivation and method behind Lagrangian observations of mode water formation in the North Pacific. Results and their significance in climate research are also presented.

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Lagrangian Observations of the Mode Water Formation in the North Pacific

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  1. 1st Argo Science Workshop Nov. 12-14, 2003，Tokyo Lagrangian Observations of the Mode Water Formation in the North Pacific Toshio Suga (Tohoku University/FORSGC) Hiroko Saito (Tohoku University) Nobuyuki Shikama (FORSGC)

  2. Outline • Motivation: why Lagrangian observations? • Method: Isopycnal APEX • An example of results: formation process of North Pacific Central Mode Water • Discussion • Summary

  3. Mode waters in the North Pacific Subtropical Mode Water (STMW) Central Mode Water (CMW) Eastern STMW (ESTMW) (Hanawa & Talley, 2001) Mode waters are formed in wintertime deep mixed layer, spread widely above or within the permanent pycnocline as pycnostad, and therefore thought to be important for maintenance and temporal variation of the pycnocline structure.

  4. Significance of mode waters in climate research Thickening and cooling ofCMWassociated with mid-1970s regime shift Temperature section along 39ºN 1976/85 winter 76/85-66/75 winter 1966/75 winter Heavy shade: dT/dz < 1.5ºC/100m Light shade: dT/dz < 2.0ºC/100m Yasuda and Hanawa (1997)

  5. Why Lagrangian observations? We know that mode waters are formed in wintertime mixed layer, but… The formation process of mode waters has rarely been observed directly. Our knowledge about the intraseasonal evolution of the formation process is extremely limited: we really don’t know how mode waters acquire their properties. “Quasi-Eulerian” Argo array will definitely provide useful information but might not sufficiently resolve the intraseasonal evolution of the formation process. Even small number of “quasi-Lagrangian” observations would provide good complementary information for revealing the intraseasonal evolution.

  6. Method Instrument: Isopycnal APEX 10 days Sea surface Parking depth: Isopycnal surface Profiling depth: 2000dbar The floats check whether they are at assigned isopycnals +/- 0.1sq every 6 hours and adjust their buoyancy if needed.

  7. Method Deployment of the floats May/Jun 2003: Jul 2002: 26.35sq 26.5sq 26.4sq 26.3sq

  8. １６０ １６５ １７０ １７５E Results ４５N Example of quasi-Lagrangian time series 2002/7/24～2003/3/1 ４０ Jul 24 Mar 1 ３５ 3/1 9/2 12/1 9/2 12/1 3/1 sq MLD 100m Parking depth pressure 350m-thick CMW at 26.02sq !! Denser CMW at 26.1sq presumably formed last winter appears to be entrained into the lighter CMW. Parking depth density 26.5±0.1σθ

  9. Results Temperature 3/1 9/2 12/1 H300: Heat content of upper 300m Undulation below ML ΔH300/ΔT (ΔT = 10days) “Heat loss or gain” gain Heat loss of 400-500Wm-2 required ΔH300/ΔT (Wm-2) Net heat flux (NCEP) q Net heat flux (NCEP) + Ekman heat adv. (NCEP, WOA) loss 3/1 9/2 12/1 Net heat flux of 300-400Wm-2 boosted by Ekman heat advection of 100-150Wm-2 accounts for the large heat loss required to produce the observed thick CMW.

  10. Discussion Mixed layer depth: Feb/Mar climatology (Suga et al., 2003) Net heat flux: Jan climatology (NCEP/NCAR reanalysis) 240~280Wm-2 150~200m Mixed layer ( >350m) much deeper than the climatology and associated large net heat flux (300-400Wm-2) were observed in winter of 2003. These results merely imply large interannual variability, but other interpretations are possible.

  11. Interpretation: extremely thick mixed layer Mixed layer depth during Feb & Mar, 2001 detected by Argo floats (Uehara et al, 2003: see poster) MLD shows mesoscale variability; thicker MLD is associated with an anticyclonic eddy. (see also Oka and Suga, in press: talk this afternoon) The float was possibly located in an anticyclonic eddy throughout the winter of 2003.

  12. Trajectory of the float Mar 1, 2003 Dec 11, 2002 anticyclonic eddy?

  13. Interpretation: extremely large heat flux Positive SST-wind correlation (Nonaka and Xie, 2003) The float was possibly located in a patch of relatively high SST likely associated with an anticyclonic eddy. The water column possibly lost larger heat.

  14. Summary • Intraseasonal evolution of winter mixed layer, ending up with the formation of 350m-thick CMW, was observed by the profiling float in the quasi-Lagrangian manner. • Net heat flux, which is much larger than the climatology, boosted by Ekman heat advection accounts for the large heat loss required. • Extremely larger MLD and net heat flux compared withthe climatology may imply… • large interannual variability of the CMW formation process, or • formation of thick CMW occurring selectively within an anticyclonic eddy which is associated with higher SST and larger heat loss compared with its surroundings.

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