Search Results

You are looking at 1 - 3 of 3 items for

  • Author or Editor: Jeroen Oortwijn x
  • Refine by Access: All Content x
Clear All Modify Search
Jeroen Oortwijn

Abstract

Flows with high and low sensitivity with respect to the initial conditions for onset of blocking (BL) and strong zonal flow (SZF) regimes have been analyzed. The author has considered BL and SZF regimes at 20°W (Atlantic region) and at 150°W (Pacific region). The BL and SZF regimes are characterized by the same dipolelike anomaly pattern but with opposite signs. Experiments have been performed with a three-level quasigeostrophic model triangularly truncated at wavenumber 21 (T21QG) and its tangent linear and adjoint versions. The sensitivity is calculated by perturbing the reference flow with perturbations that optimally trigger the onset of a BL or SZF regime after a prescribed forecast time. For forecast times larger than 3 days an iterative technique is used to take into account nonlinear growth of the perturbations.

The flows with a high sensitivity show an intensified jetstream to the west of a diffluent flow. The strong jetstream by itself results in large perturbation growth. The presence of a diffluent flow amplifies the growth and gives perturbations a typical dipolelike character. Idealized experiments with a barotropic T21 model confirm that these properties increase the sensitivity. Sensitive flows are also characterized by an enhanced ridge upstream of the intensified jet stream. This does not directly influence the sensitivity but is associated with an intensified jet stream. The flow patterns of periods with low sensitivity are more zonal and weaker.

The diffluence of the flow also results in an asymmetry between sensitivity for BL and SZF onset in the medium range. Nonlinear feedback mechanisms increase sensitivity toward BL and decrease sensitivity toward SZF. Finally, it is shown that, on average, a transition toward BL corresponds with a larger than average sensitivity and that the sensitivity is larger when the transition is stronger. Transitions toward an SZF correspond, on average, with an average sensitivity independent of the strength of the transition. The precursor patterns of transitions toward BL have similar characteristics as the flows with a high sensitivity. Thus, blocking onset is likely to be in many cases an inherently sensitive phenomenon.

Full access
Jeroen Oortwijn

Abstract

The development of perturbations that optimally trigger the onset of Euro–Atlantic blocking (BL) and strong zonal flow (SZF) regimes has been investigated. These perturbations will be called optimal transition perturbations. First, a T21 three-level quasigeostrophic model (T21QG) including a forward and adjoint tangent propagator, is used to compute the sensitivity in the initial conditions for onset of BL and SZF regimes. The evolution of an optimal transition perturbation during a sensitive 72-h period is extensively studied. Barotropic and baroclinic mechanisms are distinguished by displaying the results in terms of the barotropic and baroclinic modes of the system. Next, the perturbation is decomposed in normal modes. The evolution can be divided in two phases. During the first rapid phase, the growth is strongly nonmodal and baroclinic. After that, the growth is still nonmodal but not as strong and almost barotropic.

In the second part of this paper, the barotropic evolution is studied using a leading-order WKB approximation adopted for nonzonal smooth background flows. This approach is based on the assumptions that the perturbations may be represented by wave packets and that a scale separation between the perturbations and the background flow can be made. The WKB approach is used as a diagnostic tool to interpret the evolution of the optimal perturbations qualitatively. This paper focuses on the evolution of zonally elongated wave packets that are located in or near the jet stream, and propagate into a diffluent area. Because the background flow is nonzonal, total wave action of a packet is not conserved. However, under certain conditions total wave enstrophy of a packet is conserved. The WKB equations predict reasonably well the evolution of the perturbations, although the assumptions are violated in the final stage of the integration period.

Full access
Jeroen Oortwijn
and
Jan Barkmeijer

Abstract

The sensitivity of the onset of two weather regimes with respect to initial conditions is studied. The weather regimes are a Euro–Atlantic blocking regime and a Euro–Atlantic strong zonal flow regime. Both regimes are characterized by the same anomaly pattern but with opposite sign. Using a three-level quasigeostrophic T21 model and its tangent linear and adjoint versions, initial perturbations are computed that have the largest projection on this anomaly pattern at a prescribed forecast time. The tangent linear and adjoint techniques can be used only to describe linear error growth. However, with an iterative procedure, nonlinear error growth can be taken into account. In this way perturbations can be computed that trigger the onset of a weather regime in the linear range (even optimally) as well as in the nonlinear range. It is shown that moderate initial perturbations occasionally trigger a transition from a blocking regime to a zonal flow regime, or vice versa, within 3 days. For an optimization time of 6 days the iteratively computed perturbations generate such transitions for almost all investigated cases.

The perturbations are compared with regional singular vectors, which are the linearly fastest-growing perturbations in the Euro-Atlantic area. In the linear range the perturbations project mainly onto the leading regional singular vectors. In the nonlinear range the projection onto 1inearly slower-growing regional singular vector is stronger. The method can easily be generalized to study the sensitivity for a transition to any weather regime or anomaly pattern. This approach can be useful in generating specific initial conditions for ensemble forecasting.

Full access