Posted on October 21st, 2013 in Isaac Held's Blog
Zonal (east-west) wind in the lower troposphere (850mb) in two simulations with a 50km resolution atmospheric model with zonally symmetric boundary conditions. Only of longitude within the tropics (30S-30N) is shown. The ITCZ is located at in the upper panel and in the lower panel. Simulations described in Merlis et al 2013. (White, Black) => winds from the (west, east). 6 frames/day for 100 days.
The frequency of formation of hurricanes/typhoons has mostly been studied in the past by trying to develop “genesis indices” – empirical relations between the frequency of storm genesis and the larger scale circulation and thermodynamic structure of the atmosphere. But there is an ongoing transition, picking up steam in a number of atmospheric modeling groups around the world, to using global atmospheric models that simulate hurricanes directly to study how genesis is controlled. One goal of this work is to understand how hurricane frequency responds to the warming resulting from increasing greenhouse gases. Posts #2, 10, and 33 describe some of our recent efforts at GFDL along these lines. That work uses models in a comprehensive setting, with a seasonal cycle and realistic distribution of continents. But Tim Merlis, Ming Zhao, Andrew Ballinger and I have started looking at analogous simulations with global models in more idealized settings.
The animation above is from a model described in Merlis et al 2013. The model has no continents, and no seasonal or diurnal cycles, and the ocean is replaced by a stationary slab of water 20 meters thick, providing some heat capacity and a source of water vapor. The temperature of the slab ocean is predicted by the model. Other than the boundary conditions and lack of seasonal forcing, the model is identical to the one that generates the simulations described in post #2 in which sea surface temperatures are prescribed.
We start with a circulation forced symmetrically between Northern and Southern Hemispheres. Tropical rainfall is then localized in an intertropical convergence zone (ITCZ) centered on the equator. No hurricanes form in this model configuration despite the fact that the model with realistic boundary conditions generates about the right number. Then, just as in Kang et al 2008 (see post #37), we move a given amount of heat within the ocean from high latitudes of one hemisphere to high latitudes of the other hemisphere, causing the tropical rain belt to move some distance off the equator, allowing hurricanes to form. The animations above show two cases, with the ITCZ located roughly at 3N and 8N. The two runs differ only in the prescribed cross-equatorial heat transport. The sensitivity of hurricane number to perturbations in ITCZ latitude int his model is impressive — about a 40% increase per degree latitude poleward displacement of the ITCZ when the ITCZ is at 8N.
[This increase is genesis as the ITCZ is moved off the equator is related to the magnitude of the vorticity in the larger scale environment, a parameter in all empirical genesis indices. Vorticity is the curl of the velocity field. If a fluid is in solid body rotation, with angular velocity , a vector that points in the direction of the axis of rotation, the vorticity is simply . But the atmosphere is a thin shell on the surface of a sphere, so it is primarily the radial (locally vertical) component of the vorticity of the solid body rotation that the storms care about, where is latitude. vanishes at the equator and increases linearly as one moves off the equator.]
You sometimes hear the view expressed that there might be some simple, elegant theory for the average number of tropical cyclones that form per year. I guess this conviction is based on the idea that these cyclones play a fundamental role of some kind in maintaining the climate and that you need a certain number, more or less, to fill that role. (Also, the globally averaged number of tropical storms does not seem to vary much from year to year.) I don’t have much sympathy for this view, as I have never understood what this role might be. I think a more plausible hypothesis is that tropical cyclones are the tail of the dog with weak effects on the general circulation as a whole, at least in a climate at all resembling what we have now. These simulations reinforce my view on this. If boundary conditions are idealized and conditions are modified so that the climate is zonally symmetric and the ITCZ lies along the equator, no hurricanes form in the model (there are a few weak storms that spin off midlatitude fronts penetrating into the subtropics). Has the role that these storms are needed to fill somehow changed with this change in boundary conditions?
Another way of eliminating tropical storms in the model is to reduce the heat capacity of the model “ocean”, the depth of the stationary slab of water. If you take a simulation with a realistic number of hurricanes, this number decreases and eventually approaches zero as the depth of this slab ocean approaches zero. The surface that the atmosphere sees in this limit resembles a water saturated land surface — a swamp. A mature tropical cyclone is a strongly damped vortex that is continually extracting energy from the ocean. If the slab depth is too shallow, then, in response to the energy extraction, the surface cools too much to sustain deep convection. (Tropical cyclone statistics seem to converge for slab depths greater than 20m in our model.) The model’s atmospheric climate as a whole changes in only rather modest ways as this heat capacity is decreased – in the absence of a seasonal cycle.
Once we have moved the ITCZ off the equator, we then increase the model temperature with the total solar irradiance or CO2. The number of hurricanes increases – about 15% per of tropical warming. This is interesting to us because the number of tropical cyclones or hurricanes tends to decrease (or remain roughly constant) with warming in most models — when they are configured with realistic boundary conditions — and this model is no different. In the idealized model the ITCZ moves further poleward with tropical warming, about 0.6 degrees latitude per . If we compensate for this poleward movement by decreasing the cross-equatorial heat flux in the ocean by just the right amount, we find that the number of hurricanes does decrease, by about 10% per tropical warming. With fixed oceanic heat transport, the increase due to displacement of the ITCZ overcompensates for this reduction.
In these particular idealized simulations, the response of hurricane frequency () to warming seems to breaks down into three different problems, each involving very different dynamical mechanisms: the dependence of ITCZ latitude on warming, that is, on an increase in insolation or CO2; the dependence of on warming with fixed ITCZ latitude, and the dependence of on ITCZ latitude at fixed tropical mean temperature.
There are mechanisms relevant for storm development in more realistic climate configurations that are muted or absent in this aqua-planet setup. But even in this idealized aqua-planet model, work underway by Andrew, Tim, and Ming indicates that there are other characteristics of the tropical circulation besides the ITCZ latitude that help control hurricane frequency. So this is still work in progress.
[The views expressed on this blog are in no sense official positions of the Geophysical Fluid Dynamics Laboratory, the National Oceanic and Atmospheric Administration, or the Department of Commerce.]