Atmospheric response to Indian Ocean Dipole forcing: changes of Southeast China winter precipitation under global warming

Abstract

To investigate the relationship between autumn Indian Ocean Dipole (IOD) events and the subsequent winter precipitation in Southeast China (SEC), observed fields of monthly precipitation, sea surface temperature (SST) and atmospheric circulation are subjected to a running and a maximum correlation analysis. The results show a significant change of the relevance of IOD for the early modulation of SEC winter precipitation in the 1980s. After 1980, positive correlations suggest prolonged atmospheric responses to IOD forcing, which are linked to an abnormal moisture supply initiated in autumn and extended into the subsequent winter. Under global warming two modulating factors are relevant: (1) an increase of the static stability has been observed suppressing vertical heat and momentum transports; (2) a positive (mid-level) cloud-radiation feedback jointly with the associated latent heating (apparent moisture sink Q₂) explains the prolongation of positive as well as negative SST anomalies by conserving the heating (apparent heat source Q₁) in the coupled atmosphere–ocean system. During the positive IOD events in fall (after 1980) the dipole heating anomalies in the middle and lower troposphere over the tropical Indian Ocean are prolonged to winter by a positive mid-level cloud-radiative feedback with latent heat release. Subsequently, thermal adaptation leads to an anticyclonic anomaly over Eastern India overlying the anomalous cooling SST of the tropical Eastern Indian Ocean enhancing the moisture flow from the tropical Indian Ocean through the Bay of Bengal into South China, following the northwestern boundary of the anticyclonic circulation anomaly over east India, thereby favoring abundant precipitation in SEC.

Appendix

Following Yanai et al. (1973), the approximations of atmospheric apparent heat source Q₁ and apparent moisture sink Q₂ are,

$$Q_{1} = C_{p} \left[ {\frac{\partial T}{\partial t} + V \cdot \nabla T + \omega \frac{\partial \theta }{\partial P}\left( {\frac{P}{{P_{0} }}} \right)^{K} } \right]$$

$$Q_{2} = - L\left[ {\frac{\partial q}{\partial t} + V \cdot \nabla q + \omega \frac{\partial q}{\partial P}} \right]$$

where T, V, ω, θ, q represent air temperature, horizontal wind, vertical velocity, potential temperature and specific humidity respectively, with the following constants K = 0.286, C_p = 1004 J/kg K. L is calculated as below,

$$L = 4.2*[597.3 - 0.566*(T - 273.16)]$$

And the vertical integrals of atmospheric apparent heat source 〈Q₁〉 and apparent moisture sink 〈Q₂〉 are calculated as below,

$$\left\langle {Q_{1} } \right\rangle = \frac{1}{g}\int_{100}^{{p_{s} }} {Q_{1} dp}$$

$$\left\langle {Q_{2} } \right\rangle = \frac{1}{g}\int_{300}^{{p_{s} }} {Q_{2} dp}$$

where P _s represents surface pressure, 100 and 300 are pressure levels in hPa; g is the gravitational constant (9.81 m/s²).