The interaction of oceanic jets with the midlatitude storm tracks

• Main Author: O'Reilly, Christopher Horkesley
• Format: Text
• Language: unknown
• Published: Imperial College London 2013
• Subjects: Pacific; North Atlantic; North Atlantic oscillation
• Online Access: https://dx.doi.org/10.25560/19231; http://spiral.imperial.ac.uk/handle/10044/1/19231

Two mechanisms through which oceanic jets and the atmospheric storm tracks interact in midlatitudes are considered. Firstly, the response of a two-layer ocean model to large- scale stochastic forcing, a simplified model of forcing by the North Atlantic Oscillation, is investigated. Long Rossby waves are excited at the eastern boundary of the square model basin and the waves are baroclinically unstable. A novel aspect is that the instability leads to the generation of zonal jets throughout the domain. Unlike other theories of jet generation, the jets are actually wave-like in nature, and result directly from the instability. The “jets” appear when averaging the zonal velocity field over fixed periods of time. The longer the averaging period, the weaker the jets as the latter are actually time-varying. The jets occur for a wide range of stochastic forcing strength and the presence or not of a time mean circulation. The mechanism described here thereby provides an explanation for the recent observations of alternating zonal jets. The response of the Pacific storm track to the variability of the Kuroshio Extension jet is then studied. An index of the Kuroshio Extension front strength is produced using sea surface temperature and sea surface height observations. The index reflects the strengthening and weakening of the SST gradient associated with the bimodal states of the Kuroshio, and composites of the atmospheric state are presented during its positive and negative phases. The anomalous response of the transient eddy heat transport resembles a zonal dipole structure. With a weaker (stronger) SST front, the eddy heat transport is increased in the eastern (western) Pacific region, consistent with reduced (enhanced) low- level baroclinicity. The response of the large-scale atmospheric circulation is a barotropic blocking-type pattern in the east Pacific, which is interpreted in terms of the barotropic “eddy-straining” mechanism and eddy-mean flow interaction.