Annual-mean precipitation (colors) and surface winds (arrows). The precipitation data are from TRMM-TMPA for the years 1998– 2012, and the wind information are based on the ECMWF acting reanalysis for the same years. From Schneider et al. (2014).
Almost rain on Earth falls in the tropical rain chugalug known equally the Intertropical Convergence Zone (ITCZ), which on average lies six° northward of the equator. Over the past xv years, information technology has become clear that the ITCZ position can shift drastically in response to remote changes, for example, in Arctic water ice embrace. But current climate models have difficulties simulating the ITCZ accurately, often exhibiting two ITCZs north and south of the equator when in reality there is merely one. What controls the sensitivity of the ITCZ to remote forcings? And how do the model biases in the ITCZ arise?
Paleoclimate studies (e.g., Peterson et al. 2000, Haug et al. 2001) and a serial of modeling studies starting with Vellinga and Forest (2002), Chiang and Bitz (2005) and Broccoli et al. (2006) have revealed one important driver of ITCZ shifts: differential heating or cooling of the hemispheres shifts the ITCZ toward the differentially warming hemisphere. So when the northern hemisphere warms, for example, because northern ice cover and with information technology the polar albedo are reduced, the ITCZ shifts north. This can be rationalized as follows: When the atmosphere receives boosted energy in the northern hemisphere, information technology attempts to rectify this imbalance by transporting energy across the equator from the north to the south. Most atmospheric energy ship near the equator is accomplished by the Hadley circulation, the mean tropical overturning apportionment. The ITCZ lies at the foot of the ascending branch of the Hadley circulation, and the circulation transports free energy in the direction of its upper branch, because energy (or, more precisely, moist static energy) usually increases with meridian in the temper. Southward energy transport beyond the equator so requires an ITCZ north of the equator, so the upper branch of the Hadley circulation tin can cross the equator going from the north to the south.
To sympathize how far away from the equator the ITCZ is located, it helps to consider the steady-state atmospheric free energy residuum
,
where is the vertically integrated energy flux in the atmosphere, is the cyberspace radiative energy input to an atmospheric cavalcade (the deviation between captivated shortwave radiation and emitted longwave radiation), and is the oceanic energy uptake at the surface. The energy rest states that the atmosphere transports energy away from regions of internet energy input (e.g., the torrid zone) toward regions of net energy loss (east.thousand., the extratropics). Broccoli et al. (2006) and Kang et al. (2008) observed that because the ITCZ is located approximately where the meridional atmospheric mass transport in the Hadley circulation vanishes, it is typically also located close to where the atmospheric energy transport vanishes: at the "energy flux equator" (EFE) where . This gives us a handle to obtain a quantitative relation between the EFE or ITCZ and quantities in the atmospheric energy balance. Focusing on the zonal mean (east.g., taken beyond a sufficiently broad longitude sector) and expanding the energy flux effectually the equator (denoted past subscript 0) to get-go order in breadth gives
,
where is Earth'south radius. Equating with the latitude of the EFE or ITCZ implies , and nosotros can solve the to a higher place expansion for :
,
where nosotros have substituted for the equatorial energy flux divergence from the energy balance to a higher place.
The first-order relation for shows that (i) the ITCZ position is farther south the stronger northward the atmospheric energy flux across the equator, and (2) the ITCZ is further from the equator the weaker the cyberspace atmospheric energy input at the equator.
The following sketch illustrates these relations graphically:
Atmospheric meridional energy flux (ruddy) and energy flux equator based on data from the ECMWF interim reanalysis for 1998-2012 (Trenberth and Fasullo 2012). The calorie-free red shading indicates an estimated 0.2 Pow standard error (the actual incertitude is poorly known). The blueish line sketches a hypothetical scenario with strengthened cross-equatorial energy flux. From Schneider et al. (2014).
The figure shows the atmospheric moist static free energy flux in the zonal and almanac mean in the present climate (red line). Given the equatorial values of the free energy flux and of its 'gradient' with latitude , the energy flux equator tin can be determined using the arguments from to a higher place. If the northward cross-equatorial energy flux strengthens (indicated schematically by the blue line), simply the slope remains fixed, the energy flux equator moves due south. Similarly, if increases, the energy flux equator moves toward the equator.
Several previous studies had pointed out that the ITCZ position is proportional to the cross-equatorial energy flux (e.grand., Kang et al. 2008, Frierson and Hwang 2012, and Donohoe et al. 2013). That the net atmospheric energy input modulates the sensitivity of the ITCZ position to the cross-equatorial flux was pointed out in Bischoff and Schneider (2014).
What are some implications of these insights from the free energy balance? The assay draws attention to the importance for the ITCZ of the atmospheric energy residue near the equator. The internet atmospheric energy input near the equator is the modest residual (~xx W m2) of large cancellations between absorbed shortwave radiation (~320 Westward m2), emitted longwave radiations (~250 W k2), and oceanic free energy uptake (~ 50 W m2). Subtle shifts in any of these large terms can lead to relatively large changes in the net atmospheric energy input near the equator and hence big ITCZ shifts. Similarly, the cantankerous-equatorial energy flux (~-0.two Prisoner of war) represents a small residual imbalance between the two hemispheres which each have, for case, shortwave radiative energy gains and longwave radiative energy losses of tens of PW. This makes the ITCZ a sensitive recorder of the atmospheric energy balance, and it probable accounts for the large swings in the ITCZ position inferred from paleoclimatic proxies (see Schneider et al. 2014 for a review).
The results from the free energy rest also point toward a mode of understanding the double-ITCZ bias in climate models. The first-social club expansion above breaks down when the net atmospheric free energy input vanishes. In that case, one needs to go to higher social club in latitude, and multiple solutions for the ITCZ position emerge. We volition discuss this in a future post.
A limitation of the insights from the free energy balance is that they do not provide a closed mechanistic agreement of what controls the ITCZ position. Quantities such as the net atmospheric energy input and the cross-equatorial free energy flux depend on the strength of the Hadley circulation, among other factors, which in plow depends on the ITCZ position. How these are connected mechanistically (for example, through the angular momentum balance) remains a subject of ongoing inquiry.
0 Response to "15. what causes the itcz to move so far north in asia?"
Enregistrer un commentaire