Contribution of ocean overturning circulation to tropical rainfall peak in the Northern Hemisphere

Abstract

Rainfall in the tropics is largely focused in a narrow zonal band near the Equator, known as the intertropical convergence zone. On average, substantially more rain falls just north of the Equator 1 . This hemispheric asymmetry in tropical rainfall has been attributed to hemispheric asymmetries in ocean temperature induced by tropical landmasses. However, the ocean meridional overturning circulation also redistributes energy, by carrying heat northwards across the Equator. Here, we use satellite observations of the Earth's energy budget 2 , atmospheric reanalyses 3 and global climate model simulations to study tropical rainfall using a global energetic framework. We show that the meridional overturning circulation contributes significantly to the hemispheric asymmetry in tropical rainfall by transporting heat from the Southern Hemisphere to the Northern Hemisphere, and thereby pushing the tropical rain band north. This northward shift in tropical precipitation is seen in global climate model simulations when ocean heat transport is included, regardless of whether continents are present or not. If the strength of the meridional overturning circulation is reduced in the future as a result of global warming, as has been suggested 4 , precipitation patterns in the tropics could change, with potential societal consequences.

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

“…This mechanistic link has been demonstrated in aqua-planet simulations [Kang et al, 2009;Frierson et al, 2013;Voigt et al, 2014] and in models with realistic geography , but these studies did not make comparisons against synergistic observations to assess model performance. Our simulations show that the model response in each of the three idealized methods (STRAT, CLOUD, and OCEAN) are broadly similar indicating that the results are unlikely to be a side effect of our methodology.…”

“…Idealized aqua-planet simulations clearly demonstrate that low-level moisture transport in the atmospheric Hadley cell opposes the atmospheric energy transport [Voigt et al, 2013[Voigt et al, , 2014Frierson et al, 2013]. Obviously, aqua-planet configurations cannot be used to quantify the impact of equilibrating hemispheric albedos on model performance (e.g., in continental precipitation and the African monsoon jump), as they do not include continents of any kind.…”

North Atlantic forcing of tropical Indian Ocean climate

“…10), which contributes to anomalous energy transport from the southern hemisphere to the anomalously cold northern hemisphere (c.f. 24,84 ). The increased energy demand of the northern hemisphere during Heinrich Stadials is particularly large in the boreal winter season when the northern hemisphere cooling is strongest (Supplementary Information Fig.…”

“…The tropical mean circulation responds to the Northern Hemisphere cooling (and Southern Hemisphere warming) by generating anomalous energy transport from the southern hemisphere to the Northern Hemisphere. This energy transport is accomplished by a reorganization of the mean Hadley circulation involving an anomalous northward cross-equatorial flow in the upper branch accompanied by an anomalous southward flow in the lower branch 24 . Over the southern Indian Ocean, this results in a weakening of the annual mean Hadley circulation involving a southward shift of its rising branch, reflected by a north-south dipole structure in the mid-tropospheric vertical velocity anomaly (Fig.…”

On the evolution of Atlantic Meridional Overturning Circulation Fingerprint and implications for decadal predictability in the North Atlantic

“…Some other mechanisms, rather than direct changes in the ocean heat transport convergence/divergence, must be important for linking the AMOC anomaly at northern high latitudes with the SST anomaly in the subtropical/tropical North Atlantic south of 34°N. In fact, the subtropical/tropical SST anomaly is more likely to be affected by coupled ocean-atmosphere processes, such as the AMOC-induced intertropical convergence zone (ITCZ) shifts, changes in the Hadley circulation and trade wind [Zhang and Delworth, 2005;Kang et al, 2008;Frierson et al, 2013;Robson et al, 2014], the wind-evaporation-SST (WES) feedback [Xie and Philander, 1994;Mahajan et al, 2011a], and changes in the NAO-like large-scale atmospheric response to AMOC variability [Hodson et al, 2014;Omrani et al, 2014;Robson et al, 2014]. In addition, the AMOC-induced SST anomaly at lower latitudes can also be amplified by the cloud feedback and the coupled feedback between SST, African dust, and Sahel rainfall [Wang et al, 2012].…”