Editorial Type:
Article
Article Type:
Research Article

Predictability of the Onset of Blocking and Strong Zonal Flow Regimes

Jeroen Oortwijn Royal Netherlands Meteorological Institute, De Bilt, the Netherlands

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Print Publication:
01 Mar 1998
DOI:
https://doi.org/10.1175/1520-0469(1998)055<0973:POTOOB>2.0.CO;2
Page(s):
973–994
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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.

Corresponding author address: Jeroen Oortwijn, Royal Netherlands Meteorological Institute, P.O. Box 201, 3730 AE De Bilt, the Netherlands.

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.

Corresponding author address: Jeroen Oortwijn, Royal Netherlands Meteorological Institute, P.O. Box 201, 3730 AE De Bilt, the Netherlands.

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