Vertical diffusivity and tropical
SST
Ryo Furue
in collaboration with J. McCreary and Z. Yu
March 18, 2008
1/10
Eastern tropical SST anomalies
Obs. and IPCC
AR4 coupled
models (plot
provided by Z. Yu).
2/10
Eastern tropical SST anomalies
Obs. and IPCC
AR4 coupled
models (plot
provided by Z. Yu).
◮
◮
◮
2/10
Not enough low-level clouds?
Easterly too strong?
....
Eastern tropical SST anomalies
Obs. and IPCC
AR4 coupled
models (plot
provided by Z. Yu).
◮
◮
◮
◮
2/10
Not enough low-level clouds?
Easterly too strong?
....
Incorrect ocean upwelling?
Model
The model is COCO 3.4 (Hasumi 2000, 2006) with
◮ H = 4000 m;
◮ [0, 100◦ ] × [40◦ S, 40◦ N];
◮ 2◦ × 1◦ res., 36 levs;
◮ constant salinity;
3/10
Model
The model is COCO 3.4 (Hasumi 2000, 2006) with
◮ H = 4000 m;
◮ [0, 100◦ ] × [40◦ S, 40◦ N];
◮ 2◦ × 1◦ res., 36 levs;
◮ constant salinity;
◮ PP mixing with κb = 0 (standard).
◮ Other explicit diffusion = 0, too.
3/10
Forcings
The model is forced by
◮ M : Indonesian Throughflow;
◮ T ∗ (y): restoring at the surface;
◮ τ x : basin-wide trades;
◮ τ y : southerly along South America
⇒ coastal upwelling;
◮ τ e : inducing upwelling
⇒ Costa Rica Dome.
4/10
Tropical subthermocline currents
Std.: κb = 0
Velocity
integrated for
11◦ –14◦ C.
The thermostad water flows eastward and upwells at the eastern boundary and CRD.
5/10
Tropical subthermocline currents
The thermostad water flows eastward and upwells at the eastern boundary and CRD.
5/10
Impact of κb
SST diff.:
(κb = 0.1) − (κb = 0)
6/10
Impact of κb
κb = 0
κb = 0.1
6/10
Impact of κb on upwelling
w&T
κb = 0
κb = 0.1
7/10
Mechanism?
Hypotheses:
1. Because κb changes vertical structure in
the upwelling regions;
8/10
Mechanism?
Hypotheses:
1. Because κb changes vertical structure in
the upwelling regions;
2. Because the thermostad water is eroded by
κb on its way to the upwelling regions.
8/10
Mechanism?
Hypotheses:
1. Because κb changes vertical structure in
the upwelling regions;
2. Because the thermostad water is eroded by
κb on its way to the upwelling regions.
Two additional experiments:
◮ κb = 0.1 only near the eastern
boundary (4◦ ) in the southern hemisphere;
◮ κb = 0.1 only in the equatorial band (±6◦ ).
8/10
Equatorial κb is the reason
Diff: (κb = 0.1) − (κb = 0)
9/10
Equatorial κb is the reason
Diff: (κb = 0.1) − (κb = 0)
Diff: (eq. κb = 0.1) − (κb = 0)
The thermostad water is eroded on its way to
the upwelling regions.
9/10
Summary and discussion
Effect of vertical diffusion
Thermostad water upwells along the equator.
=⇒ Lower SST along the equator; higher SST
in the upwelling regions.
10/10
Summary and discussion
SST differences when κb = 0.01 → 0.5 cm2 /s in a
0.5◦ × 0.5◦ model, iROAM (Richards & Miyama):
March
Contour interval 0.5oC
October
10/10
Summary and discussion
Effect of vertical diffusion
Thermostad water upwells along the equator.
=⇒ Lower SST along the equator; higher SST
in the upwelling regions.
10/10
Summary and discussion
Effect of vertical diffusion
Thermostad water upwells along the equator.
=⇒ Lower SST along the equator; higher SST
in the upwelling regions.
Implications
In coupled models,
◮ Correct upwelling necessary;
◮ Decadal adjustment timescales;
◮ Correct source and pathway of thermostad
water.
10/10