The slowdown tends to be greater for stronger tropical cyclones

Understanding the impact of climate change on tropical cyclones (TCs) has become a hot topic. The slowdown of TC translation speed contributes greatly to the locally accumulated TC damage. While the recent observational evidence shows that TC translation speed has decreased globally by 10% since the mid-twentieth century, the robustness of the trend is questioned by other studies as effects of changes in observational capability can strongly affect the global trend. Moreover, none of the published studies considered dependence of TC slowdown on TC intensity. This is the caveat of these analyses as the effect of TC slowdown is closely related to TC intensity. Here, we investigate the relationship between TC translation speed trend and TC intensity, and reveal possible reasons for the trend. We show that the global slowing trend without weak TC moments (≤ 17 m s-1) is about double of that with weak TC moments in a recent study. This is because the slowing trend is dominated by strong TCs’ trend. Stronger (weaker) TCs tend to be controlled more by upper-level (lower-level) steering flow, and the calculated trend of upper-level steering flow is much larger than that of lower-level steering flow. This may be an important reason for the large difference between the slowing trend without weak TC moments and that with weak TC moments. Furthermore, the changes of TC tracks (including inter-basin trend and latitudinal shift), which are partly attributed to data inhomogeneity, make a much larger contribution to the slowing trend, compared with the weakening of tropical circulation, which is related to anthropogenic warming.

A Diagnostic Model for The Large-Scale Tropical Circulation Based on Moist Static Energy Balance

In this study we present a diagnostic model for the large-scale tropical circulation (vertical motion) based on the moist static energy equation for first baroclinic mode anomalies (MSEB model). The aim of this model is to provide a basis for conceptual understanding of the drivers of the large-scale tropical circulation changes or variations as they are observed or simulated in Coupled Model Inter-comparison Project Phase (CMIP) models. The MSEB model is based on previous studies relating vertical motion in the tropics to the driving forces of the tropospheric column heating rate, advection of moisture and heat, and the moist stability of the air columns scaled by the first baroclinic mode. We apply and evaluate the skill of this model on the basis of observations (reanalysis) and CMIP model simulations of the large-scale tropical vertical motion. The model is capable of diagnosing the large-scale pattern of vertical motion of the mean state, annual cycle, interannual variability, model-to-model variations and in warmer climates of climate change scenarios with correlations of 0.6-0.8 and nearly unbiased amplitudes for the whole tropics (30°S-30°N). The skills are generally better over oceans at large scales and worse over land regions. The model also tends to have an upward motion bias at higher latitudes, but still has good correlations in variations even at the higher latitudes. It is further illustrated how the MSEB model can be used to diagnose the sensitivity of the tropical vertical motion to the forcing terms of the models for the mean state, seasonal cycle and interannual variability such as El Nino. The model clearly illustrates how the seasonal cycle in the circulation is driven by the incoming solar radiation and how the El Nino shift in the Walker circulation results mainly from the sea-surface temperature changes. Overall, the model provides a very good diagnostic tool to understand tropical circulation change on larger and longer (>month) time scales.

Understanding changes in tropical circulations in a future warmer climate using a cloud-resolving model and a conceptual model

<p>The interaction between large-scale tropical circulations and moist convection has been the focus of a number of studies. However, projections of how the large-scale tropical circulation may change under global warming remain uncertain because our understanding of this interaction is still limited.</p><p>Here, we use a cloud-resolving model (CRM) coupled with a supra-domain scale (SDS) parameterisation of the large-scale circulation to investigate how tropical circulations driven by sea-surface temperature (SST) gradients change in a future warmer climate. Two popular SDS parameterisation schemes are compared; the weak temperature gradient approximation and the damped-gravity-wave approximation. In both cases, the large-scale vertical velocity is related to the deviation of the simulated density profile from a reference profile taken from the same model run to radiative-convective equilibrium.</p><p>We examine how the large-scale vertical velocity profile varies with surface temperature for fixed background profile (relative SST) as well as how it varies with the surface temperature of the reference profile (background SST). The domain mean vertical velocity appears to be very top-heavy with the maximum vertical velocity becoming stronger at warmer surface temperatures. The results are understood using a simple model for the thermodynamic structure of a convecting atmosphere based on an entraining plume. The model uses a fixed entrainment rate and the relative humidity from the cloud-resolving model to predict a temperature profile. The vertical velocities calculated from these predicted temperature profiles is similar to the vertical velocity structures and their behaviour in a warmer climate that we see in the CRM simulations. The results provide insight into large scale vertical velocity structures simulated by SDS parameterisation schemes, providing a stepping stone to understanding the factors driving changes to the large-scale tropical circulation in a future warmer climate.</p>

An MSE budget view on seasonal and CO2-induced ITCZ shifts in the TRAC-MIP model ensemble

<p>One of the grand challenges of climate is predicting and modeling tropical rainfall. Here, we address a specific problem of this grand challenge, namely how does the vertical structure of the atmosphere affect the tropical circulation and the position of the ITCZ during the seasonal cycle and in response to increased CO<sub>2</sub>. The tropical circulation can be described by the column-integrated budget of moist static energy (MSE). We use this framework in the TRAC-MIP model ensemble to investigate the role of the vertical structure of the tropical atmosphere in setting the anti-correlation between the ITCZ location and the atmospheric energy transport.</p><p>TRACMIP "The Tropical Rain belts with an Annual cycle and Continent - Model Intercomparison Project" is a set of idealized simulations that are designed to study the tropical rain belt response to past and future forcings. TRACMIP includes 13 comprehensive CMIP5-class atmosphere models and one simplified atmospheric model. Importantly, TRACMIP includes a slab ocean with prescribed ocean heat transport. This leads to a closed surface energy balance and forces the annual-mean ITCZ to be north of the equator, consistent with today’s climate.</p><p>We use the MSE budget framework to diagnose the seasonal evolution of vertical velocity from the energetic terms in the MSE budget equation. We obtain a diagnostic expression for the vertical velocity. By means of the MSE budget framework we estimate the efficiency of exporting energy from the atmospheric column, which is defined as the gross moist stability (GMS). The GMS characterizes the stability of the tropical troposphere related to moist convective processes in the tropospheric column. We use the MSE and GMS analysis to disentangle the impact of deep and shallow circulations on energy transport, vertical velocity and hence precipitation in an objective manner.</p><p>Through this work we aim to elucidate to what extent model uncertainty in simulations of future ITCZ changes are caused by model differences in the vertical structure of the atmosphere. We also hope to use the results to advance our understanding of the tropical climate and to assess the plausibility of simulated changes in tropical rainfall.</p>

Cloud-Radiative Impacts On Tropical Circulation Change in GFDL AM4.1

<pre>We use the Geophysical Fluid Dynamics Laboratory (GFDL) state-of-the-art AM4.1 atmospheric model to assess the impact of clouds on the change in tropical circulation. Slab-ocean experiments where cloud microphysical properties are locked to either the pre-industrial or 4xCO<sub>2</sub> conditions allow us to cleanly separate the circulation changes into a part caused by the cloud radiative effects (CREs), and to a part caused by the CO<sub>2</sub> changes. The CO<sub>2</sub>-induced SST changes are shown to dominate the response in the boundary layer, but are rivaled by the impacts of CREs in the mid to upper troposphere. The reduction in the east-to-west sea level pressure difference over the Pacific is solely caused by the increasing CO<sub>2</sub> and SST, but they only account for about half of the change in the mid-tropospheric Walker circulation. The weakening of the free-tropospheric circulation is shown to be mostly caused by the near-equal contributions the CO<sub>2</sub> and CREs make to the changes in dry-static and gross moist stability. Also, concerning the <span>meridional</span> circulation, we show that the response in the strength of the southern branch of the Hadley cell is largely due to CREs, while they have a much smaller impact in the north.</pre>

The tropical atmospheric conveyor belt: a Lagrangian perspective of the large-scale tropical circulation

<div> <div>The Hadley circulation (HC) is a key element of the climate system. It is traditionally defined as the zonally averaged meridional circulation in the tropics, therefore treated as a zonally symmetric phenomenon. However, differences in temperature between land and sea cause zonal asymmetries on Earth, dramatically affecting the circulation. The longitudinal dependence of the HC evokes questions about where and when the actual large scale tropical circulation occurs. In this study, we look into the connection between the longitudinally dependent HC and the actual large scale movement of air in the tropics using a coupled Eulerian and Lagrangian approach. Decomposing the velocity field, we identify the components affecting the actual circulation. In addition, we calculate trajectories of air parcels to analyze the actual movement. We propose an alternative definition for the circulation, that describes the actual path of air parcels in the tropics, as a tropical conveyor belt. The Indo-Pacific warm pool is the driver of the circulation, where air converges and ascends, then moves westward and poleward before entering the jet stream, moving eastward with it, eventually beginning its descent near the Americas. Furthermore, using an idealized moist GCM, we explore how tropical asymmetries affect the circulation and discuss the possible mechanisms controlling the tropical conveyor belt.</div> </div>