Variation of upper tropospheric clouds and water vapor over the Indian ocean

Abstract. The upper tropospheric (UT) ice water content (IWC) and water vapor (H₂O) observed by the Microwave Limb Sounder (MLS) show dominant dipole mode variability over the Indian Ocean. This is characterized by the oscillating differences between the Western and Eastern Indian Ocean (WIO and EIO) with greater amplitude in JJA and SON than in other seasons. We denote δ X = X_WIO–X_EIO, with X being H₂O and IWC at three UT levels (215 hPa, 147 hPa and 100 hPa) as well as sea surface temperature (SST), following the definition for previously identified Indian Ocean Dipole (IOD) variability. We found a strong positive correlation of δIWC at three UT levels with δSST, and a relatively weak positive correlation of δIWC with Nino 3.4 SST, suggesting that the UT clouds over the Indian Ocean are largely controlled by local thermally-driven circulation while teleconnection to ENSO plays a secondary role. The change per degree of δSST for δIWC in SON is 5.5 mg m⁻³ C⁻¹ at 215 hPa, 1.6 mg m⁻³ C⁻¹ at 147 hPa and 0.13 mg m⁻³ C⁻¹ at 100 hPa (the 7-yr mean δIWC is −4.7 mg m⁻³, −1.6 mg m⁻³ and −0.13 mg m⁻³ at 215 hPa, 147 hPa and 100 hPa respectively). For δH₂O, the per degree δSST change of 41.2 ppmv C⁻¹ corresponds to a strong increase at 215 hPa and a decrease of −0.23 ppmv C⁻¹ (−0.18 ppmv C⁻¹) at 100 hPa (147 hPa), respectively. The Nino 3.4 SST has a relatively weak positive (negative) correlation with δ H₂O at 215 hPa (100 hPa). The increase of δH₂O at 215 hPa with increasing δSST is associated with the sharper contrast in convective intensity while the decrease of δH₂O at 100 hPa with increasing δSST is a signature of the "convective cold top" and temperature control of 100 hPa H₂O. For H₂O, the 147 hPa marks a transition from the convection-controlled 215 hPa to the temperature-controlled 100 hPa.