Scientific Background

The processes that influence individual depressions have been extensively studied. The dominant process is baroclinic instability which owes its existence to strong meridional temperature gradients. However, diabatic processes are generally also important. These include both frictional effects (e.g. Valdes and Hoskins, 1988) and latent and sensible heating (e.g. Emanuel et al., 1987). In addition, Farrell (1984) emphasises the importance of existing disturbances as an important initiator of cyclogenesis, particularly for rapid growth. However, the processes that control the oceanic storm-tracks (the seasonal ensemble of mid-latitude storms) have been less widely studied. There has been some exploration of absolute versus convective baroclinic instability as a possible localisation of activity (e.g. Merkine, 1977 and Lin and Pierrehumbert, 1993). Recently Branstator (1995) has shown that five day linear integrations using an ensemble of random initial perturbations to a climatological basic state yeild eddy statistics very similar to those of the observed synoptic time scale eddies, though the variability of the storm-tracks is not addressed. In contrast, Farrell and Ioannou (1995) show that stochastic forcing in the linearised system, including sufficient damping to render it stable, also produces quite realistic results. In either case the growing disturbances will tend to erode the gradient that is producing them, namely the temperature gradient. Thus to understand a storm-track, we need to know about the processes which restore the mean equator-to-pole temperature gradient.

Stephenson and Held (1993) showed that changes in the time mean, regional climate (planetary waves) of the GFDL GCM were, in large part, caused by changes in the vorticity fluxes associated with the storm-tracks. Thus the storms are an integral part of our understanding of regional climate change. Branstator (1995) gives an indication that low frequency anomalies act to determine the high pass transient eddy statistics that act to support them. Hoskins and Valdes (1990) showed that an important part of the forcing of the mean state arose from diabatic heating. This heating field (diagnosed by the residual method from ECMWF data) has a maximum over both N.hemisphere oceanic storm-tracks. It is produced by a combination of strong, transient advection of cold, continental air over a warm ocean, and latent heating in the mature storm (towards the eastern end of the storm track). In part, this diabatic heating can be considered to be directly associated with the storms themselves. This therefore raises the possibility that in some senses the storm-tracks could be self-maintaining features. The North Atlkantic Oscillation (NAO, Bjerknes, 1964 and Rogers, 1984) is a term given to the observed mean sea-level pressure (MSLP) anomaly pattern with oppiste extrema over Iceland and the Azores. In its positive phase it corresponds to enhanced surface westerlies into W.Europe. Rogers (1990) also found it useful to define another dipolar MSLP anomaly pattern based on extrema in S.E.Europe and the N.E.Atlantic (SENA). Rogers (1994) made an initial attempt to relate cyclonic storm anomalies to amplitudes of the NAO and SENA patterns.

The role of the oceans in extra-tropical variability has been widely debated since the 1960s. Three main paradigms are possible : the atmosphere responds passively to the ocean generated SSTs, the ocean responds passively to the atmospheric forcing, or the atmosphere and ocean are both active in producing the variability. The first of these ideas has been widely used in the tropics where it is believed that the boundary forcing of the atmosphere plays a significant role. Because of the stron internal atmospheric instabilities, this idea is less applicable to the mid-latitudes. After examining interannual and interdecadal SST and Sea-Level Pressure (SLP) correlations, Bjerknes (1964) proposed both that the passive ocean paradigm was valid for inter-annual time scales but that basin wide coupled ocean-atmosphere processed were necessary for longer interdecadal time scales. Bjerknes noted that, for interannual time scales, the westerly jet showed local negative correlations with SST anomalies and hence hypothesised that the SST anomalies were the result of the atmospheric fluxes on the oceanic mixed-layer. This has been confirmed for the central and eastern North Atlantic in coupled model experiments (O'Brien and Chassignet, 1995) and in a recent study using fluxes based on COADS observations from 1952-92 to force a simplified oceanic mixed-layer model (Frankignoul et al., 1996). For interdecadal time scales, Bjerknes found basin wide positive correlations and hypothesised that this was due to changes in the poleward heat transport of both the atmosphere and the North Atlantic ocean.

Bjerknes (1964) looked for SST patterns associated with the NAO. Kushnir (1994) found different MSLP/SST anomaly patterns for time-periods longer than or shorter than a decade. The shorter time-scale pattern was similar to that found previously by Deser and Blackmon (1991), with the pressure pattern being similar to the W.Atlantic pattern of Wallace and Gutzler (1981). Warm SSTs went with a surface easterly anomaly and a high to the north. This was like the barotropic response found by Palmer and Sun (1985), and Lau and North (1990). However, it differed from the steady, linear, baroclinic extra-tropical response to low-level heating given by Hoskins and Karoly (1981) and found in a range of prescribed SST AGCM experiments by Kushnir and Held (1994). In contrast the longer time-scale pattern of Kushnir (1994) had an MSLP low downstream of the warm SSTs just as in the idealised models. The differing nature of the thermodynamic and dynamic balances in the atmosphere, the surface fluxes, and the roles of the transient motions in the two cases is presently not understood.