UpperUpper--Ocean Processes Ocean Processes and Airand Air--Sea Interaction in Sea Interaction in the Indonesian Seasthe Indonesian Seas

Janet Sprintall, Janet Sprintall, Scripps Institution of Oceanography, USAScripps Institution of Oceanography, USA

Arnold L. Gordon, Asmi M. Napitu, Arnold L. Gordon, Asmi M. Napitu, LDEO, USALDEO, USA

Ariane KochAriane Koch--LarrouyLarrouy, LEGOS, France, LEGOS, France

Susan E. WijffelsSusan E. Wijffels, CSIRO, Australia, CSIRO, Australia

the Indonesian Seas and the Indonesian Seas and ThroughflowThroughflow (ITF)(ITF)

• the only tropical inter-ocean exchange site (~15 Sv)• transports heat and freshwater from Pacific into Indian Ocean• pressure gradient between Pacific (high) and Indian Ocean (low) (Wyrtki, 1987)• ascending branch of Walker Circulation• closely coupled to the Australasian Monsoon system, MJO, ENSO and IOD• extends across Indo-Pacific warm pool • many many islands, deep basins, wide and shallow marginal seas• mixing from strong tides and enhanced air-sea heat and freshwater fluxes

• the only tropical inter-ocean exchange site (~15 Sv)• transports heat and freshwater from Pacific into Indian Ocean• pressure gradient between Pacific (high) and Indian Ocean (low) (Wyrtki, 1987)• ascending branch of Walker Circulation• closely coupled to the Australasian Monsoon system, MJO, ENSO and IOD• extends across Indo-Pacific warm pool • many many islands, deep basins, wide and shallow marginal seas• mixing from strong tides and enhanced air-sea heat and freshwater fluxes

Strong Mixing

Koch-Larrouy et al. (2007)Sprintall et al., (2014)

Signatures of theSmax in the thermocline and Smin in the Intermediate layer disappear quickly in the Indonesian seas through vigorous mixing from tides, air-sea interaction and complexbathymetry to form cool and fresh Indonesian Seas Water masses

Regional T-S Plots (color coded by depth)

Processes that Drive SST Variability

Kida and Richards, JGR, 2009Kida and Wijffels, JGR, 2012

February

August

SE Monsoon: Wind driven upwelling and Arafura shelf-break cools SST

NW Monsoon: Also large P -> high R/O warms SST by limiting latent heat release & mixing. Freshwater caps trap heat in near surface

Annual SST: Monsoon driven upwelling not the whole story!

Seasonal SST: Role of the ITF in Summer

Kida and Wijffels, JGR, 2012

Standard Deviation Note weak seasonality in Flores Sea. What other processes are important to SST here?

Flores Sea: Control NO-ITF

Heat balance shows in austral summer, lateral advection warms SST while vertical mixing and Q cools SST, i.e the ITF impacts SST by advection of warm water

Resolving Mixing is Important

Models that include tides can reproduce the rainfall (Δ20%), SST (Δ2oC) and heat flux (Δ20 Wm-2) patterns observed in the Indonesian Seas than those without

Koch-Larrouy et al (2010)Kida and Wijffels (2012) Jochem and Potemra (2008)Sprintall et al. (2014)

Annual SST and precipitation differences in coupled simulations with/without tides

OLR (Rainfall)

Intraseasonal SST

Largest intraseasonal SST variance (55-60%) found in Banda Sea, Timor Sea, and in the Sulawesi Sea.

Napitu, Gordon and Pujiana, submitted, 2014

Contribution of intraseasonal variability to total SST variance

Intraseasonal SSTCoherence of SST and OLR at intraseasonal timescales.

Highest amplitude in Banda Sea and Timor Sea

OLR leads SST by 1-2 weeks

Napitu, Gordon and Pujiana, submitted, 2014

Mixing Impact on Intraseasonal OLR

Koch-Larrouy et al (in prep)

Std dev OLR bandpass filtered (30-1d)

OBS

CTL

TIDES

Coupled model with tidal parameterization reduced intraseasonal variability in good agreement with observations

Oceanic Processes Influence SST Too!

Napitu, Gordon and Pujiana, submitted, 2014

Indo-Pacific MJO Surface Forcing

Drushka, Sprintall, Gille and Wijffels, J. Climate, 2012

OLR U-WND STRESS Qo MJO Composites (Nov-Apr) based on Wheeler and Hendon (2004) Index

Active MJO phase convective cells (OLR<0) lead strong westerly wind anomalies in Indian Ocean (esp. IAB in phases 5-7) but are more aligned in Pacific, and so have different impacts on the SST and mixed layer.

Variations in Q (QLH) more closely follow wind stress magnitude

MLD Response to MJO Forcing

Drushka, Sprintall, Gille and Wijffels, J. Climate, 2012

MLD MLD-T • Argo data MLD amplitudes ~ 10 m• Largest signal in IAB ~0.6C• No profiles available in Indonesian

seas

• Spatial patterns resemble t and Q• Active MJO: diabatic cooling &

wind stirring cause MLD deepening and cooling

• Suppressed MJO: Surface warming and light winds lead to MLD shoaling and warming

• MLD-T lags Q by one phase, i.e., consistent with model that Q drives MLD-T

Barrier Layer (BL) Influence on MJO

Drushka, Sprintall and Gille, JGR, 2014

THIN BL THICKHeat Flux

Precip

Wind Stress

Zonal V

Temperature

•Thin BL:- 15% stronger heat flux and wind stress; 10% stronger P; higher MLD-T; deeper isothermal layer

•Thick BL:- entrainment cooling during MJO reduced so MJO drives weaker SST anomaly

•Modulation of SST by BL thickness can have significant consequences for response of ocean to MJO and in turn, the feedbacks of ocean to atmosphere on MJO time scales

Summary

• SST and upper ocean characteristics in the Indonesian Seas are the product of several competing processes (both atmospheric and oceanic driven) over many time scales

• Response may be to local (winds, tides, inertial waves, air-sea interaction) and remote (Kelvin waves) forcing

• Mixing is important! The efficiency of mixing processes depends on stratification, bathymetry and the background oceanic and atmospheric large-scale conditions that vary across the Indonesian seas.

Few Direct Observations of Mixing in Indonesian Seas

shear

strain

dissipation rate

diapycnal diffusivity

• Near-inertial phase lines evident in shear and strain, leading to pulses of mixing every 4.4 days, the wave period

• Turbulence mixing occurs when low Ri=N2/S2 (base of MLD)

• At time of observations, local winds were light -> down-ward propagating internal wave likely generated 3 weeks prior and ~200 km away.

Alford and Gregg, JGR, 2001

Direct measure of vertical mixing : INDOMIX

CTDIndirectOverturnThorpe method

AboveStraits

Banda SeaFar field

Nearfield

VMPdirectDissipationMeasurement

Koch-Larrouy, Atmadipoera, van Beek, Madec, Aucan, Lyard, Grelet, and Souhart, submitted DSR, 2015

Discussion Issues• Mixing Processes:

– Where does mixing occur? At what depths? Seasonal preference?• Impact of the ITF advection stream on SST and air-sea interaction

– if ITF absent or significantly reduced, may enhance the zonal SST gradient• Shallow versus Deep Basins?• Precipitation and presence of barrier layer?

– Large P -> high R/O warms SST by limiting latent heat release & mixing. Also freshwater caps can trap heat in near surface

– Presence of a barrier layer may significantly affect SST anomalies. • SST gradients in response to MJO forcing

– zonal SST gradient in response to seasonal migration across Indonesia and its atmospheric convective activity (convective limit at SST ~27.5C)

• Diurnal variability in SST and surface layer?• Strong vs. Weak Wind scenarios?• Regional variability within Indonesian Seas? EEZ Issues? Use of ROV?• What is the impact on biology? Ecosystem/fisheries interest and also might

feedback via solar absorption to SST• Need in situ SST and MLD observations within Indonesian Seas to validate

remotely sensed data and model output

MJO Driven Deep Vertical Motions

Drushka, Sprintall and Gille, JGR, 2014

MJO composite dT/dz at thermocline depth shows eastward propagation of 0.5C anomaly along the equator at speed of ~2 m/s consistent with Kelvin wave speed (and slower than 5 m/s propagation speed of MJO)

Nov-Apr Mean T along equator

T Anomaly at Thermocline Depth

ITF: Influence on CirculationITF: Influence on Circulation

Song, Gordon and Visbeck, JGR, 2004

• ITF cools and freshensthrough mixing in the Indonesian Seas and also the Indian Ocean thermocline [on isopycnals] relative to the ambient thermocline. cool

Low salinity

ITF

Temperature on sigma-0 = 25.0, upper thermocline

Salinity on sigma-0 = 25.0, upper thermocline

freshwater input

IndonesianMix-Master

Seasonal SST: The role of Mixing

Kida and Wijffels, JGR, 2012

SST(CTRL-NO_TIDE)

Annual

February

SST(NOWind-NOWindTide)

February

• Along shelf-break- enhanced bottom mixing lifts the thermocline and enables intrusion of thermocline water onshore.• Shallow seas -water column well mixed so vertical mixing does not result in enhanced cooling.

North Nusa Tenggara: well mixed thermocline water from Makassar intrudes below and upwells to cool SST in NW monsoon. NE Ekman flow advects this to Banda Sea.

MJO Driven MLD Heat Budgets

Drushka, Sprintall, Gille and Wijffels, J. Climate, 2012

dT/dt Q Horadv Vertent• Mixed layer temperature variations

driven by anomalous Q• Central I.O. rapid deepening of

MLD during onset of active MJO induces entrainment cooling

• No entrainment cooling in WPAC despite MLD deepening is related to barrier layer (water below MLD is same T and so no net cooling).

• Using a fixed MLD overestimates heat budget by 40% during active MJO (i.e. using a slab MLD will overestimate the T changes forced by MJO -> important consideration for models simulating MJO temperature and air-sea interaction associated with MJO

• Large error bars!• Event to event variation!

Barrier Layers Influence SST

Drushka, Sprintall and Gille, JGR, 2014

THIN BL

THICK BL

Difference

• MJO composite surface temperature anomalies are stronger when BL is thin

• Modulation of SST by BLT can have significant consequences for response of ocean to MJO and feedbacks of ocean to atmosphere on MJO time scales