Atmosphere Ocean

(1)

Effects of freshwater flux (FWF) forcing on interannual climate variability

in the tropical Pacific

Rong-Hua Zhang

(2)

2

Effects of freshwater flux (FWF) forcing on interannual climate variability

in the tropical Pacific

Rong-Hua Zhang

(3)

mixed layer

Processes involved in ENSO: Forcings & feedbacks

Thermocline

SST wind

Heat Flux

(4)

4

mixed layer

Thermocline

Processes involved in ENSO: Forcings & feedbacks

Thermocline

SST wind

Pre

Eve

Heat Flux Freshwater Flux

(5)

El Nino La Nina

El Nino and La Nina Precipitation Anomaly Patterns Warm Pacific Cold Pacific

Red: positive precipitation anomalies Blue: negative precipitation anomalies (normalized [by mean] anomalies, i.e., σ/μ)

(6)

6

Roles of freshwater flux (FWF) forcing &

related salinity effect

in the Tropical Pacific Ocean

• Climate

9 One atmos forcing component;

9 Large FWF anomalies induced by ENSO;

9 Some unique FWF/salinity related phenomena;

9 Significant modulating effects;

9 Positive feedback

….

• Water/hydrological cycles ….

• Data assimilation ….

• Global warming ….

(7)

Challenge in freshwater forcing & ocean salinity issues

¾ Studies mostly on wind/heat flux, less on FWF;

¾ Studies mostly on ocean modeling, less on coupled ocean-atmos modeling;

¾ FWF forcing not adequately represented in models;

¾ Uncertainty in observations & data products;

¾ Intermodel differences

Intermediate ocean models level OGCMs

layer OGCMs

¾ Systematic biases & errors in SSS simulations:

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8

(9)

(10)

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Some approaches to improving SSS & SST simulations & predictions

1. Continue to improve parameterizations^:

Great focus on We & Kv, (but Te & Se equally important !!)

2. Different ocean & coupled models:

Intermediate ocean model:

Layer models: isopycnal coordinate, …

Level models: z-coordinate (e.g., GFDL MOM) (Te & Se depiction in different models !!)

3. Flux/bias corrections in ocean & coupled models

(better mean climatology!!)

4. MOS (model output statistics) corrections

(get SSTAs first and then try to correct them regardless of reasons!!)

5. Ocean data assimilation

(11)

Taking into account

freshwater flux (FWF) forcing

¾ Identify bias sources for SSS simulation;

¾ Understanding basic processes involved;

¾ Improving model simulations of salinity;

¾ Improving ENSO simulations & prediction

;

¾ Support for

satellite mission to measure SSS.

This work

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12

Outline

• Introduction

• A hybrid coupled model (HCM_OGCM )

Ocean: The Cane-Gent OGCM Atmosphere: Wind stress: SVD-based;

Heat flux: Seager et al.;

FWF: SVD-based:

FWF = (P-E)_clim+ α_FWF • (P-E)_inter

• The standard HCM_OGCM simulation

• Sensitivity experiments:

• A FWF-induced positive feedback

• Summary

α_FWF=1.0

α_FWF=0.0 α_FWF=2.0

(13)

Pre Pre

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14

A SVD-based Empirical Model for (P-E)

_inter

FWF = (P-E)_clim+ α_FWF • S_vd ( SST_inter )

• P, E : ECHAM 4.5 AMIP run

• SST: Reynolds &

Smith

(15)

S

_vd

( SST

_inter

) = (P-E)

_inter

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16

Hybrid Coupled Model at ESSIC/UMD

• The Gent-Cane ocean model

A sigma-layer, reduced-gravity OGCM with

(1) A hybrid mixing scheme

Chen, Rothstein & Busalacchi (1995)

(2) Coupling to an advective atmos mixed layer model (Murtugudde, Seager & Busalacchi 1995)

(3) Model specifications:

Tropical Pacific domain: 25N-25S; 31-layers;

Resolution: 1 deg in longitude and 0.5 deg in latitude

• An empirical atmospheric wind stress anomaly model (SVD-based)

σσσ

(17)

OGCM

SST_inter=SST-SST_clim

τ=τ

_clim

+

^τinter

A hybrid coupled ocean-atmosphere model

(P-E)_clim+ (P-E)_inter

Heat Flux (HF)

SSS Buoyancy Flux (Q_B)

Freshwater Flux (FWF)

Atmosphere Ocean

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18

OGCM

τ=τ

_clim

+

^τinter

A hybrid coupled ocean-atmosphere model

(P-E)_clim and/or relaxation

Heat Flux (HF)

Atmosphere Ocean

(P-E)_clim and/or relaxation

(19)

Data & Model experiments

0 . 0 1 . 0 2 . 0

⎧⎪

⎨⎪

⎩

τ=τ_clim+ ^τinter

(P-E)_clim+ (P-E)_inter

Heat Flux (HF)

α_FWF(P-E)_inter

α_FWF=

=> -1.0

=> 5.0

• Wind stress, P, E : ECHAM 4.5 AMIP run 1950-1997

• SST: Reynolds & Smith

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20

Effects of freshwater flux (in general):

FWF = (P-E)_clim+ (P-E)_inter

Sea surface sailinty (SSS):

=> density => stratification & stability => mixing Buoyancy flux (Q

_B

):

Q_B = α•HF/(ρC_p)+β•S₀• FWF = Q_T+ Q_S

Gaining (+) => more bouyant (lighter) =>

stable => shallow mixed layer => less entrainment

(21)

Effects of anomalous FWF during ENSO:

+ +

-

+

-

⁺

La Nina

- -

- +

+ -

El Nino

MLD SST Q_B

SSS Q_S

Q_T SST

SSS & Q

_B^{=α•HF/(ρC}_p^)+β•S₀^{• FWF =Q}_T^+Q_S

(as represented at Nino4 site)

(22)

22

Effects of anomalous FWF during ENSO:

+ +

-

+

-

⁺

La Nina

?

- -

- +

+ -

El Nino

MLD SST Q_B

SSS Q_S

Q_T SST

?

SSS & Q

(23)

Effects of anomalous FWF during ENSO:

+ +

-

+

-

⁺

La Nina

- -

- +

+ -

El Nino

MLD SST Q_B

SSS Q_S

Q_T SST

Less entrainment More

stable

Less Less negative

More warming More

positive

Freshening

more Salty

shallow

deepening

SSS & Q

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24

α_FWF=1

(25)

α_FWF=0

(26)

26

α_FWF=2

(27)

(28)

28

(29)

(30)

30

mixed layer

thermocline

Processes involved

Depth

Precip

Evap SST

P-E

(31)

mixed layer

thermocline

Processes involved

Depth

Precip

Evap SST

SSS P-E

Density/Mixing /Entrainment

(32)

32

mixed layer

thermocline

Processes involved

Depth

Precip

Evap SST

MLD Q_B P-E

(33)

mixed layer

thermocline

Processes involved

Depth

Precip

Evap SST

SSS MLD

Q_B P-E

SST

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34

(35)

Buoyancy flux (Q

_B

) and its relation with Q

_T

& Q

_S

FWF = (P-E)_clim+ α_FWF • (P-E)_inter

α•HF/(ρC_p)+β•S₀• FWF =Q_T+Q_S=Q_B _Less

negative

Less positive

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α_FWF=2 Q_T+ Q_S = Q_B

(37)

SST P-E SSS_Diff SST_Diff

MLD_Diff

A FWF-induced positive feedback

El Nino La Nina

(38)

38

SST P-E SSS_Diff SST_Diff

MLD_Diff

Reinforce ^FWF

-induced Origin

al anomaly

A FWF-induced positive feedback

El Nino La Nina

(39)

( a ) SSS

( b ) SST

α_FWF=0.0 α_FWF=1.0

α_FWF=2.0

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40

The std of selected anomaly fields

for α_FWF =0.0, α_FWF =1.0 and α_FWF =2.0.

α

_FWF= 2.0

(Enhanced run)

α

_FWF= 1.0

(Standard run)

α

_FWF= 0.0

(Clim run) Niño4 region

0.92 0.76

0.67

Niño3 SST

0.64 0.57

0.53

Niño12 SST

1.08 1.24

1.49 Q_B

1.58 0.65

0.0 Q_S

1.95 1.72

1.49 Q_T

0.23 0.19

0.16 τ_x

9.0 6.7

5.6 MLD

0.97 0.85

0.76 SST

0.28 0.16

0.11 SSS

(41)

Summary

• Demonstrate

a positive feedback

induced by FWF

;

• FWF: compensating effect on Q

_T

for Q

_B

;

• Different role of FWF vs. heat flux;

• Significant effects on interannual variability

> 10% differences in SST

> 20% differences in SSS

• FWF is a clear source for model biases

• Taking into account this

^atmos ^forcing

component for better ENSO simulation & prediction

(42)

42

(43)

El Nino La Nina

El Nino and La Nina Precipitation Anomaly Patterns Warm Pacific Cold Pacific

Red: positive precipitation anomalies Blue: negative precipitation anomalies (normalized [by mean] anomalies, i.e., σ/μ)

(44)

44

Atmosphere Ocean

Effects of freshwater flux (FWF) forcing on interannual climate variability

in the tropical Pacific

Effects of freshwater flux (FWF) forcing on interannual climate variability

in the tropical Pacific

El Nino and La Nina Precipitation Anomaly Patterns Warm Pacific Cold Pacific

Roles of freshwater flux (FWF) forcing &

related salinity effect

• Climate

Challenge in freshwater forcing & ocean salinity issues

Some approaches to improving SSS & SST simulations & predictions

Taking into account

freshwater flux (FWF) forcing

¾ Identify bias sources for SSS simulation;

¾ Understanding basic processes involved;

¾ Improving model simulations of salinity;

¾ Improving ENSO simulations & prediction

¾ Support for

Outline

A SVD-based Empirical Model for (P-E)

S

( SST

) = (P-E)

Hybrid Coupled Model at ESSIC/UMD

• The Gent-Cane ocean model

(3) Model specifications:

• An empirical atmospheric wind stress anomaly model (SVD-based)

OGCM

τ=τ

+

A hybrid coupled ocean-atmosphere model

OGCM

τ=τ

+

A hybrid coupled ocean-atmosphere model

Data & Model experiments

Effects of freshwater flux (in general):

Sea surface sailinty (SSS):

=> density => stratification & stability => mixing Buoyancy flux (Q

):

Gaining (+) => more bouyant (lighter) =>

stable => shallow mixed layer => less entrainment

Effects of anomalous FWF during ENSO:

-

-

SSS & Q

Effects of anomalous FWF during ENSO:

-

-

?

?

SSS & Q

Effects of anomalous FWF during ENSO:

-

-

SSS & Q

Processes involved

Processes involved

Processes involved

Processes involved

Buoyancy flux (Q

) and its relation with Q

& Q

α

α

α

Summary

a positive feedback

;

• FWF: compensating effect on Q

for Q

;

• Different role of FWF vs. heat flux;

• Significant effects on interannual variability

> 10% differences in SST

> 20% differences in SSS

• FWF is a clear source for model biases

• Taking into account this

El Nino and La Nina Precipitation Anomaly Patterns Warm Pacific Cold Pacific

Thank you !!!