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

1. Local feedback SLP decrease SST Wind-Evap-SST Feedback (over 10-20N) Trade Wind (E) over ocean 10-20N Summer Insolation Land-sea contrast Monsoon Wind (W) Ekman transport From N to S (over 0-10N) More Precipitation Over West Africa SST anomaly dipole over tropical Atlantic 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 1. Introduction 2. SST Anomaly dipole over tropical Atlantic 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. • 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). Zonal mean SST over Atlantic 6k – 0k: TS (color), SLP (pink contour) Fig.2 dipole structure + sharp gradient 6k – 0k, wind and wind speed (color) at 850hpa Wind decrease Evaporation decrease SST increase Ekman transport from N to S Fig 1: insolation change at 6 ka Table 1: Characteristics of the coupled simulations Evolution of mean temperature(T) of the mixed layer (H): ρcHdT/dt=Q+div(uT)+diffusion ρ: density of water, c: the heat capacity of water, Q:the net surface heat flux and u the ocean current Change in surface heat flux over box: 60W-20W,10-20N where wind decrease Zonal meam Surface heat flux over Atlantic (ºC) Fig3: Q (Heat Fulx) = Sw – LE – Lw -Hs Sw: net solar radiation LE: laternt heat flux Div(uT)<0 3. Late Monsoon retreat over northwestern Indian Ocean Zonal mean Meridianl Heat transport (top 100m); W/m2 JAS zonal mean Heat Transport over Atlantic(Gw/m) 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. Local feedback during summer monsoon season Fig4 Mixed layer depth (6-0k) 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) 4. Conclusions Budget = Fsns - Flns - Shflx - Lhflx Monthe 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. • 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) Eq. (b) 1/c*(Q6/H6 -Q0/H0); Fig.6 Evolution of Temperature change according to the simplified model over box: 55-75E,5-15N IPSL CSM Eq. (a) 1/cH0*(Q6 – Q0); Change in mixed layer account about 50% SST waring Local air-sea feedback in autumn Stratification Pr. Mixed layer depth SST dT/dt Thermal Inertia Fig.7