Rossby wave and eddy

1
Rossby wave and eddy in the North Pacific
Subtropical Countercurrent (STCC)
Qinyu Liu, Lijuan Li and Wei Liu Physical
Oceanography Lab. Ocean-Atmosphere Interaction
and Climate Lab, Ocean University of China,
Qingdao, 266003, China)
2
Outline

1. Motivation
2. Amplification features of the Rossby wave in STCC
   
3. Eddies in the STCC and Their Possible Effects on
   the South China Sea
4. The Vortex Street to the west of the Big
   Island
5. Summary

3
Motivation
Chelton and Schlax (1996) speculated that the
topography may be the source of the long Rossby
wave amplification.
21N
The location of Rossby wave amplification is
variation with time
4
Zonal current
Northwest STCC
STCC to west of Hawaii Islands
Vertical Shear
Two branches ( Kobashi and Kawamura 2003)
5
Problems

• How about the amplification features of the
  Rossby wave in STCC area?
• Whether those Rossby wave or some eddy in the
  STCC can enter the Luzon strait?
• Is there ocean vortex Street or not, when North
  Equator Current overpass the Hawaii Islands ?

6
Data

• Sea Surface Height Anomaly The Maps of Sea Level
  Anomaly (MSLA), altimeter products by the CLS
  Space Oceanography Division
• Drifter data Global Drifter Center (GDC) and
  the Marine Environmental Data Service (MEDS)

7
Amplification features of the Rossby wave
(80-210 day) when westward propagation
Low PV
8
Stability analysis
The necessary condition for longer Rossby wave to
be unstable
,

,
This condition is easy to be fulfilled when there
is Mode water in Northwest STCC
9
Conclusion(1)Amplification features of the
Rossby wave

• In both STCC branches, the SSHA exhibits
  remarkable oscillations with period of 80-210
  days, corresponding to westward propagation of
  free Rossby waves.
• The amplitude of those Rossby waves appears
  larger in the west end of the Northwest STCC than
  that in the east end, seemingly due to the
  stronger baroclinic instability in the North
  STCC, because mode water exists in subsurface
  layer.

10
Eddy according single drifter path and SSHA
Eddy radii vs. longitude in the area between
5?N-25?N,
Eddy radii vs. longitude in the area between
5?N-25?N,
Estimate radius 98 km298km westward eddy
speed approximately 0.098 m/s in STCC
11
Approximately 200 drifters in the area around the
Luzon Strait (18?N-22?N and 121?E-125?E), only 4
of the total (2) actually entered the SCS
through the Luzon Strait, and all the four
drifters entered the SCS in the winter.
(1979-1998)
12
DecFeb.
Sep.-Nov.
15/323gt5 winter(19792004)
13
Conclusion(2)

• Eddies in the STCC, corresponding to the Rossby
  wave, move westward at an average speed of
  approximately 0.098 m/s and their average radius
  is about 200 km (wavelengh800km).
• These westward eddies from the North Pacific
  interior are unlikely to enter the South China
  Sea through the Luzon Strait.

14
Vortex Street to the west of the Big
Island (AugDec 1995).
Drifter
SSHA
15
Drifter 1989 to 1998
EOF first mode (21.2) SSHA 19952001
two symmetrical arrays of cyclonic and
anticyclonic vortices similar to the pattern of
the vortex Street and often apperars
16
Conclusion (3)

• there is the vortex pair with the orbital period
  of 10-11days and the radius of 58-68 km to the
  west of Hawaii Islands.
• The obvious feature of the SSHA field is that
  there are two symmetrical arrays of cyclonic and
  anticyclonic vortices similar to the pattern of
  the vortex Street .

17
Thank you!