The impact of meridional variations in cloud albedo on tropical climatology, and biases, in Earth system models

A salient feature of the tropical Pacific is the pronounced east-west gradient in sea surface temperature along the equator. This temperature gradient is coupled to the atmospheric zonal circulation and oceanic thermocline tilt. Together they define the Warm pool, Cold tongue, Walker circulation Complex (WCWC). Asymmetry in the SST and precipitation fields about the equator sets the position of the Intertropical Convergence Zone (ITCZ). This is a proposal to study the effect of meridional variation in cloud albedo across the entire Pacific basin on this tropical climatological state in Earth system models.

Coupled model simulations of the tropical Pacific, while improved over the past decades, still show significant biases. A cold bias within the cold tongue region, a cold tongue that extends far west into the warm pool, and the double-ITCZ problem remain persistent issues in climate models. The mean east-west temperature gradient along the equator in the CMIP5 models varies between 3 and 8 degrees C, compared to nearly 6 degrees C in the observations. Developing a comprehensive understanding of what determines this temperature gradient and related characteristics of the fully coupled ocean-atmosphere system is critical for understanding tropical climatology and potential biases in climate models. Often, this question is treated as a local problem in the equatorial band – improvements are sought by tuning local parameters affecting the properties of deep convection in the warm pool or the amount of shortwave radiation reaching the eastern Pacific. However, the strength of the cold tongue is ultimately controlled by the temperature of waters subducted in the extra-tropics and transported to the equator by the ocean subtropical cells (STC). Consequently to understand tropical biases one needs to treat this coupled problem in a broader geographical context and consider latitudinal variations in the main dynamical factors.

Our preliminary analysis indicates that the meridional gradient in cloud albedo is one of these key factors. A close relationship exists across the pre-industrial CMIP5 simulations and our preliminary numerical experiments that connects the mean east-west gradient in upper-ocean temperatures and the contrast in cloud albedo between the extra-tropical and tropical Pacific. For example, when extra-tropical cloud albedo is higher than observed, or tropical cloud albedo is too low, the east-west temperature gradient is stronger than observed. Thus, we propose that in coupled climate models the zonal SST gradient and the related characteristics of the tropical ocean-atmosphere system (e.g. zonal winds and the thermocline tilt) are largely controlled by the meridional gradient in cloud albedo between the equator and the extra-tropics. Further, we propose that it is the inter-hemispheric albedo contrast that controls the position of the ITCZ. To investigate these problems we will conduct (i) sensitivity experiments with CESM, in which we systematically modify cloud properties affecting cloud albedo, (ii) a theoretical analysis of the coupled system with cloud feedbacks included, and (iii) an analysis of CMIP5 models focused on the effects of latitudinal variations in cloud albedo on the tropical climate.

Relevance to long-term NOAA goals and current solicitation: The overarching goal of this study is to understand the fundamental physical mechanisms that control tropical climatology and model biases. This objective is directly relevant to the current solicitation and longer-term NOAA goals, since simulating tropical climate correctly is critical for climate prediction on a variety of timescales from seasonal to interannual, to decadal and longer. Funding from this grant will support cross-disciplinary training of a postdoctoral associate at Yale, Dr. Natalie Burls.