Local Mode Analysis (LMA)
In meteorology and oceanography, intermittent oscillations generally exhibit various spatial structures that succeed one another in time and that are difficult to isolate in a simple and comprehensive manner using conventional approaches such as Empirical Orthogonal Function (EOF) or composite analyses. In most cases, this variability must be considered as typical of the studied oscillation and not as a random noise hiding a hypothetical ideal structure.
The main objective of the LMA approach is to identify the different patterns (see the example below) of a given intermittent oscillation in order to understand better its physical origin and in order to test the applicability of different theoretical hypotheses. The LMA also makes it possible to test the representativity of a mean structure in regard to actual modes that succeed one another in time. The LMA was introduced by Goulet and Duvel (2000).
Multivariate LMA was introduced and used to study the coupled characteristics of the intraseasonal oscillation in Duvel and Vialard (2007).
The LMA may also be used to assess the representation of an intermittent phenomenon by GCMs, as reported in Xavier et al. (2008). An approach was designed to compute the "Realism" and the "Reproducibility" of an intermittent meteorological phenomena and applied to the study of the intraseasonal oscillation in IPCC models (Xavier et al., 2009).
A package to compute these metrics and more explanations are available here. If you use this package, please send us a copy of your paper and aknowledge this site.
Example of a pattern obtained for the intraseasonal (20-60 day) perturbation in the OLR field. The LMA gives the spatial pattern of amplitude A(x) (length and colour of the segment) and relative phase f(x) (angle of the segment, a full rotation for one period) of the quasi periodic event. The LMA also gives the temporal chracteristics B(t) and c(t). Each event can be thus represented in a simple mathematical form and then compared to other events to produce new statistics on the studied intermittent phenomenon.
This pattern shows a northward propagation of a convective perturbation over the Arabian Sea, in relation with the monsoon onset in 2006. This lage-scale convective perturbation is related to another convective event over the south China sea.
Goulet, L. and J.P. Duvel, 2000: A new approach to detect and characterise intermittent atmospheric oscillations: Application on the Intraseasonal Oscillation. J. Atmos. Sci., 57, No. 15, pp. 2397-2416.
Duvel, J.P. and J. Vialard, 2007: Indo-Pacific Sea Surface Temperature Perturbations Associated with Intraseasonal Oscillations of the Tropical Convection, Journal of Climate, 20, 3056-3082. (pdf)
Xavier, P.K., J.P. Duvel, and F.J. Doblas-Reyes, 2008: Boreal Summer Intraseasonal Variability in Coupled Seasonal Hindcasts. J. Climate, 21, 4477–4497. (pdf)
Xavier, P.K., J.P. Duvel, P. Braconnot and F.J. Doblas-Reyes, 2009: An evaluation metric for intraseasonal variability in climate models. Submitted to J. Climate. (pdf)