Abstract
We analyse the dependence of the cloud radiative effect (CRE) and cloud amount on mid-tropospheric pressure velocity (ω500) and sea surface temperature (SST) and point out the shortcomings of using these two proxies separately as means to separate cloud regimes. A bivariate approach is proposed to overcome these shortcomings and it is used to systematically investigate marine cloud properties at different spatial and time scales in the present-day (1985–2001) tropical climate. During the 1997–1998 El Nino, the greatest regional change in CRE and cloud cover coincides with the greatest local change in circulation and SST. In addition, we find that the cooling effect of the stratiform low clouds reduces at the rate of approximately 1 W/m2 per percent of cloudiness reduction in the subsident cold pools of the Pacific ocean. During El Nino, the transition between different cloud regimes gives rise to opposing cloud feedbacks. The sign of the total feedback is controlled by the cloud optical thickness. More generally, we find that the largest part of the cloud response to El Nino, when averaged over the tropical Pacific, is not directly associated with ω500 and SST changes, so other factors must play a role as well.
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