Coupled processes and the tropical climatology : part III : instabilities of the fully coupled climatology

Coupled processes between the equatorial ocean and atmosphere control the spatial structure of the annual mean state in the Pacific region,in particular the warm-pool/cold- tongue structure.At the same time,coupled processes are known to be responsible for the variability about this mean state,in particular the El-Niño/Southern-Oscillation phe- nomenon.In this paper,we consider the connection between both effects of coupling by investigating the linear stability of fully coupled climatologies in an intermediate coupled model.The new element here is that when parameters-such as the coupling strength-are changed,the potential amplification of disturbances can be greatly influenced by a simul- taneous modification of the mean state.This alters the stability properties of the coupled climatology,relative to the flux-corrected cases that have been previously studied.It ap- pears possible to identify a regime in parameter space where ENSO-like unstable modes coincide with a reasonable warm-pool/cold-tongue structure.These unstable modes are mixed SST/ocean-dynamics modes,that is,they arise through an interaction of oscillatory modes originating from ocean dynamics and oscillatory SST-modes.These effects are qual- itatively similar in this fully coupled problem compared to the ?ux-corrected problem,but the sensitivity of the ENSO mode to parameters and external variations is larger due to feedbacks in the climatology

Imperfections of the thermohaline circulation: latitudinal asymmetry and preferred northern sinking

The present Atlantic thermohaline circulation is dominated by deep water formation in the north despite the fact that surface buoyancy forcing has relatively modest latitudinal asymmetry.Many studies have shown that even with buoyancy forcing that is symmetric about the equator,spontaneous symmetry breaking can produce a single overturning cell with intense sinking in the north.This occurs by salt advection at sufficiently large fresh- water forcing.In this symmetry-breaking case,a southern sinking solution and a symmetric solution are also possible.A simple coupled ocean-atmosphere model of the zonally averaged thermohaline circulation is used to examine the effect of latitudinal asymmetries in the boundary conditions.The greater continental area in the northern hemisphere,combined with the slight asymmetry in the observed fresh-water flux,induce a strong preference for the northern sinking solution.Examining the relation to the solution under symmetric conditions,the salt-advection mechanism still acts to enhance the overturning circulation of the northern sinking branch,but multiple equilibria are much less likely to occur within the realistic parameter range.The most plausible shift between equilibria for paleoclimate applications would be between a strong northern sinking branch and a weak northern sinking branch that is an asymmetric version of the thermally driven solution.However,this is possible only in a very limited range of parameters.There is a substantial parameter range where the northern sinking branch is unique.The role of the fractional region of air-sea interaction at each latitude is substantial in producing north-south asymmetry

Rainfall and Dragon-Kings

Previous studies have found broad distributions, resembling power laws for different measures of the size of rainfall events. We investigate the large-event tail of these distributions and find in one measure that tropical cyclones account for a large proportion of the very largest events outside the scaling regime, i.e., beyond the cutoff of the power law. Tropical cyclones are sufficiently rare that they contribute a significant number only in a regime of large event sizes that common rain events almost never reach. The different physical dynamics of tropical cyclones permits a substantial extension of the tail in this large-event regime

Shortwave feedbacks and El Niño-Southern Oscillation: Forced ocean and coupled ocean-atmosphere experiments

International audienc

El Nino/Southern Oscillation

The El Nino/Southern Oscillation (ENSO) phenomenon is the strongest natural climate fluctuation on the short-range climatic time scale. It influences not only regional but also global climate. Within the international TOGA (Tropical Ocean Global Atmosphere) project (1985-1995) observationalists, theoretians and numerical modelers work together to understand ENSO and develop coupled ocean-atmosphere models which can be used for ENSO prediction. This report presents a brief description of the current ENSO theory together with a review on the state of the art in ENSO modeling. ENSO predictability is addressed and an outlook for ENSO research work is given. (WEN)SIGLEAvailable from TIB Hannover: RR 1347(129) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman