Simulated Sea Surface Salinity Variability in the Tropical Pacific

Xiaochun Wang Yi Chao

JPL/CaltechTerrain-Following Ocean Models User Workshop

Seattle , Aug. 4-6Acknowledgement: Discussions with J. McWilliams, P. Marchesiello, A. Shchepetkin , T. Song, Y. Golubev about the use of ROMS model. Computations were performed on computers of JPL and NASA Ames Research Center.

Scientific Issues

• What is the Sea Surface Salinity (SSS) structure associated with the ENSO phenomenon?

• What are the detailed features of SSS seasonal cycle?

• Are there any salinity signatures associated with the Tropical Instability Waves (TIWs)?

Motivation

• The effect of salinity on dynamics is gradually realized, e.g. barrier layer, influence on SSH, … However, salinity has very sparse observation.

• Aquarius mission, measure monthly SSS with accuracy of 0.2 psu with horizontal resolution of 100km

Pacific ROMS (Regional Ocean Modeling System)• Horizontal resolution, 50km, 45S-65N, 99E-70W• 20 S-coordinate levels • KPP scheme• Open boundary condition • 1990-2001 NCEP-NCAR Daily Forcing (Wind

stress, heatflux, precipitation and evaporation)• 5-day (pentad) average

Validation of Model Simulation

• TAO Data• 5-day • 99% Sig.

SSS Change on InterannualTimescale

• 1-yr lowpassed• Freshening

during ENSO

Spatial Pattern Associated with ENSO

• EOF Analysis• Consistent with Ob.• Variance (24.9%)

Seasonal Cycle of SSS

• EOF Analysis• Variance (66.0%)• ITCZ

Annual Mean

SSS Features Associated with TIWsSSS SST and Current

50day Highpassed

• Westward Propagation

• Seasonal Change

• Intreannual Change

• 50day Highpassed

SSS Features of TIWs

Spatial Features of TIWs

.Fresh

.Warm

.Upward

.Southward

.Eastward

TIWs SSS Variability is along EQ

Why TIWs SSS Variability is along EQ?

Meridional Advection !

SSS Variability on Different Timescales

Total

Interannual

Seasonal

Intraseasonal(<50day)

Summary• The western tropical Pacific is a region of large

variability on different timescales, interannual, seasonal, and intraseasonal.

• Though ENSO events show up as a freshening of the whole equatorial Pacific, the eastern equatorial Pacific is not a center of SSS variability on interannual timescale.

• On timescale shorter than 50days, besides the variability center in the western tropical Pacific, there is a variability center along the equator in the eastern equatorial Pacific associated with the TIWs.