Ocean feedbacks on the Afro-Asian monsoon during the Mid-Holocene

Yan ZHAO, Pascale Braconnot, Olivier Marti and PMIP working group on coupled simulations Laboratoire des Sciences du Climat et de l'Environnement (LSCE), Bat. 712, Orme de Merisiers, 91191 Gif-sur-Yvette, France

Email : yan.zhao@cea.fr

• The mid-Holocene (6,000-year BP) is characterized by more vigorous summer monsoon because the seasonal cycle of insolation was increased (decreased) by 5% in the northern hemisphere (southern hemisphere) compared to today (Fig 1).

• In the first phase of Paleoclimate Modeling Intercomparison Project (PMIP, http://www-lsce.cea.fr/pmip), it was established that the atmosphere alone simulations produce changes in qualitative agreement with paleoclimate data. Important mismatches were found, which have been attributed to the changes in ocean circulation or land surface cover that were neglected (Joussaume et al., 1999, Gophys. Res.Lett.,26,859-862).

• Now several coupled simulations of the mid-Holocene are available (Table 1), which offers us the possibility to test how different coupled models reproduce past conditions over the ocean, and how the ocean feedback alter the mean seasonal cycle. Here we foucs on tropical Atlantic and Indian Ocean during boreal summer (JAS).

1. Introduction

Fig 1: insolation change at 6 ka Table 1: Characteristics of the coupled simulations

MODEL

RESOLUTIONFLUX

CORRECTIONCO2

ATMLong x lat (levels)

OCEANLong x lat (levels)

  CTRL6000 yr BP

CSM1.2 T31 (18) 102 x 116 (25) none 280 280

UKMO (HADCM2) 96 x 73 (19) 96 x 73 (20) SST, SSS 323 323

IPSL-CM1 64 x 50 (11) 92 x 76 (31) none 345 345

MRI2 72 x 46 (15) 144 x 111 (23) SST, SSS 345 345

ECHAM3/LSG T21 64 x 32 SST, SSS 345 280

ECBILT T21 (3) 64 x 32 (12) none 345 345

FOAM R15(18) 2.8x1.4(16) none 345 345

3. Late Monsoon retreat over northwestern Indian Ocean

All the coupled model simulations produce a clear delayed warming in autumn during the process of monsoon retreat over the Indian Ocean. This delayed warming is not found in atmosphere alone experiment, indicating it results from air-sea interactions. Variables such as surface salinity (sss), mixed layer depth, precipitation (pr), heat flux also have corresponding response over the region.

Eq. (a)

1/cH0* (Q6 – Q0);

Eq. (b)

1/c*(Q6/H6 -Q0/H0);

Pr.

SST

Thermal Inertia

Mixed layerdepth

dT/dt

Stratification

Role of change in mixed layer depth: • The rate of warming of the mixed layer is thus: dT/dt=Q/(ρcH) C: the heat capacity of water, Q: net surface heat flux, H: the mixed layer depth, T: SST

• The difference of warming rate is: (dT/dt)6 – (dT/dt)0=1/c{Q6/H6 – Q0/H0}, (a) or when H0=H6, (dT/dt)6 – (dT/dt)0=1/cH0{Q6 – Q0}, (b)

CSM: Pr/Ts(color for ts) October

Fig.5 Changes in monthly mean precipitation and surface temperature on October, and monthly mean evolution of ocean surface variables over the interested box (55-75E,5-15N)

Budget = Fsns - Flns - Shflx - Lhflx

Mixed layer depth (6-0k)

Monthe

IPSL CSM

Fig.7

Fig.6 Evolution of Temperature change according to the simplified model over box: 55-75E,5-15N

Local air-sea feedback in autumnChange in mixed layer account

about 50% SST waring

Zonal mean Meridianl Heat transport (top 100m); W/m2

Zonal meam Surface heat flux over Atlantic (ºC)

JAS zonal mean Heat Transport over Atlantic(Gw/m)

Change in surface heat flux over box:

60W-20W,10-20N where wind decrease

Q (Heat Fulx)

= Sw – LE – Lw - Hs

Sw: net solar radiation

LE: laternt heat flux

All the coupled model simulations produce a clear dipole-like structure of SST anomaly over tropical Atlantic, warmer in north of 5~10N and colder in south of 5~10N than present (Fig2). This structure is, to the first order, the response of insolation forcing, but local wind-evaporation-SST feedback and Ekman transport also play a role. The following figures are taken as exapmle from one of the coupled model IPSL-CM1.

Ekman transport From N to S(over 0-10N)

MonsoonWind (W)

Trade Wind (E)over ocean

10-20N

Land-seacontrast

SST

Wind-Evap-SSTFeedback

(over 10-20N)Summer

Insolation

Local feedback

SST anomaly dipole over tropical Atlantic

More Precipitation Over West Africa

SLP decrease

+dipole structure

sharp gradient

6k – 0k: TS (color), SLP (pink contour)

6k – 0k, wind and wind speed (color) at 850hpa

Zonal mean SST over AtlanticFig.2

Fig3:

Fig4

Evolution of mean temperature(T) of the mixed layer (H): ρcHdT/dt=Q+div(uT)+diffusionρ: density of water, c: the heat capacity of water, Q:the net surface heat flux and u the ocean current

Local feedback during summer monsoon season

2. SST Anomaly dipole over tropical Atlantic

2.1 All the models clearly exhibit a dipole-like structure of SST anomalies over tropical Atlantic during boreal summer, which reforces the western African monsoon rainfall;

2.2 The dipole-like structure is, to first order, the response to insolation change; this structure is reinforced by a positive local feedbak of wind-evaporation-SST over 10-20N and by Ekman transporting heat from north to south over 0-10N.

3.1 A delayed Indian monsoon retreat is identified by all the coupled models over Northwestern Indian Ocean in autumn; 3.2 This delayed retreat is attributed to a positive feedback involving the change in mixed layer depth, local warming, precipitation and large scale advection.

4. Conclusions

Wind decreaseEvaporation decrease

SST increaseEkman transport from N to S

Div(uT)<0