Link between the double-Intertropical Convergence Zone problem and cloud biases over the Southern Ocean

Abstract

The double-Intertropical Convergence Zone (ITCZ) problem, in which excessive precipitation is produced in the Southern Hemisphere tropics, which resembles a Southern Hemisphere counterpart to the strong Northern Hemisphere ITCZ, is perhaps the most significant and most persistent bias of global climate models. In this study, we look to the extratropics for possible causes of the double-ITCZ problem by performing a global energetic analysis with historical simulations from a suite of global climate models and comparing with satellite observations of the Earth's energy budget. Our results show that models with more energy flux into the Southern Hemisphere atmosphere (at the top of the atmosphere and at the surface) tend to have a stronger double-ITCZ bias, consistent with recent theoretical studies that suggest that the ITCZ is drawn toward heating even outside the tropics. In particular, we find that cloud biases over the Southern Ocean explain most of the modelto-model differences in the amount of excessive precipitation in Southern Hemisphere tropics, and are suggested to be responsible for this aspect of the double-ITCZ problem in most global climate models. tropical precipitation | model biases | cloud radiative forcing | atmospheric energy transport | general circulation P recipitation is essential to life, with its variation tightly linked to water and food security. Providing the best estimate of future trends in precipitation has always been a primary goal of global climate models. For this reason, global climate models are closely scrutinized not only on their ability to simulate large-scale dynamics but also on their skill in simulating precipitation distributions at regional scales. One naturally only trusts model forecasts of precipitation if there is substantial fidelity in simulating the current precipitation climatology.Because precipitation features are related with processes occurring at a tremendous range of time and spatial scales, their simulation remains challenging. The main precipitation feature that most global climate models have difficulty capturing is the Intertropical Convergence Zone (ITCZ) in the deep tropics at around 6°N, a narrow latitude band with some of the most intense rainfall on Earth. Despite decades of work by modeling centers around the world, the double-ITCZ problem, in which excessive precipitation is produced in the Southern Hemisphere tropics resembling the stronger Northern Hemisphere ITCZ, remains the largest precipitation bias of most state-of-the-art global climate models. There has been little progress in reducing this bias over recent years (1-3) (Figs. 1 A and B and 2A).The double-ITCZ bias is most apparent in the strip 5-15°S over the central and east Pacific, and a similar feature is visible in the Indian and Atlantic Oceans in most models. Most of the proposed reasons for tropical precipitation biases involve local mechanisms within or close to the tropics, for example, warm sea surface temperature errors in the coastal upwelling region off Peru (4, 5), ofte...

Southern Ocean albedo, inter-hemispheric energy transports and the double ITCZ: global impacts of biases in a coupled model

“…Donohoe and Battisti (2012) have previously noted that the spread in maximum meridional heat transport in the CMIP3 models is primarily a function of cloud properties. Hwang and Frierson (2013) have shown that the spread in Southern Ocean cloud biases in the CMIP5 models explains much of the difference in southern hemisphere tropical precipitation biases in those models. The impact of these biases may not easily be constrained by blunt instruments which target the TOA energy budgets.…”

“…A southward cross-equatorial energy transport anomaly induces a northward moisture flux anomaly which pushes the ITCZ further north, and vice versa. For these purposes, the energy and moisture fluxes at the equator can be considered proxies for the upper and lower branch of the southern branch of the Hadley circulation, thereby modulating the ITCZ location (see Hwang and Frierson 2013, Figure 4, for a schematic of this mechanism). The use of slab ocean configurations has been justified on the basis it provides a closed surface energy budget.…”

The impact of equilibrating hemispheric albedos on tropical performance in the HadGEM2‐ES coupled climate model

“…A particularly relevant study is that of Hwang and Frierson [2013] (HF13), who showed a striking relationship between the "precipitation asymmetry index" (defined as model precipitation over the latitudinal range 0-20°N minus that for 0-20°S normalized by the mean tropical precipitation over 20°N-20°S) and cross-equatorial energy transport in CMIP5 models. An analysis of HIST, STRAT, CLOUD, and OCEAN (supporting information, Figure S9) indicates that the striking linear relationship shown in the multimodel ensemble of HF13 is also evident in our simulations with a gradient of À1.5 PW per unit HF13 precipitation asymmetry index which is very similar to that found in HF13.…”

Meridional atmospheric heat transport constrained by energetics and mediated by large-scale diffusion

“…The energetic perspective also provides a framework for understanding the latitudinal position of the Inter-tropical Convergence Zone (ITCZ): annual-mean ascent north of the equator permits net MSE to be transported southward across the equator in the upper branch of the Hadley Cell, as required to balance stronger heating of the northern hemisphere atmosphere (Kang et al 2008;Frierson and Hwang 2012;Hwang and Frierson 2013;Donohoe et al 2013Donohoe et al , 2014). An implication is that the peak in zonalmean rainfall resides north of the equator in the annual mean due to hemispheric asymmetry of high-latitude surface heat fluxes which, in turn, reflects northward OHT across the equator due to meridional overturning in the Atlantic Ocean (Figs.…”