Large-scale Low-frequency Variability Research Articles

On data-driven induction of the low-frequency variability in a coarse-resolution ocean model

This study makes progress towards a data-driven parameterization for mesoscale oceanic eddies. To demonstrate the concept and reveal accompanying caveats, we aimed at replacing a computationally expensive, standard high-resolution ocean model with its inexpensive low-resolution analogue augmented by the parameterization. We considered eddy-resolving and non-eddy-resolving double-gyre ocean circulation models characterized by drastically different solutions due to the nonlinear mesoscale eddy effects. The key step of the proposed approach is to extract from the high-resolution reference solution its eddy field varying in space and time, and then to use this information to improve the low-resolution analogue model.By interactively coupling both the continuously supplied history of the eddy field and the explicitly modeled low-resolution large-scale flow, we obtained the additional eddy forcing term which modified the low-resolution model and significantly augmented its solutions. This eddy forcing term represents the action of the eddy field, its coupling with the large-scale flow and is a key dynamical constraint imposed on the augmentation procedure.Although the augmentation drastically improved the low-resolution circulation patterns, it did not recover the robust, intrinsic, large-scale low-frequency variability (LFV), which is an important feature of the high-resolution solution. This is by itself an important (negative) result that has significant implication for any data-driven eddy parameterization, especially, given the fact that we used the most complete information about the space–time history of the eddy fields. Note, when we supplied the reference (true) eddy forcing, rather than just the eddy field, the LFV was recovered. This suggests that the LFV is crucially dependent on the details of the space–time eddy forcing/large-scale flow correlations, which are not fully respected by the proposed augmentation procedure.In order to overcome the deficiency and recover the LFV, we statistically filtered the augmented low-resolution model solution by projecting it onto the leading Empirical Orthogonal Functions (EOFs) of the large-scale component of the high-resolution reference solution. This operation allowed us to remove spurious effects associated with higher EOFs. We tested and confirmed that without using the data-driven eddy information this filtering alone cannot augment the low-resolution solution; but in conjunction with the eddy information, it produced desirable outcome.Moreover, as a natural step towards parameterization, we took advantage of data-driven stochastic inverse modeling to obtain inexpensive emulators of the eddy field and showed generally promising results of augmenting the coarse-resolution model with the obtained emulators. Our results showed that obtaining the LFV characteristics for the eddy parameterization, which is already capable of reproducing the large-scale flow pattern, should become a standard parameterization requirement, but it can be challenging to meet.

On low-frequency variability of the midlatitude ocean gyres

This paper studies the large-scale low-frequency variability of the wind-driven midlatitude ocean gyres and their western boundary currents, such as the Gulf Stream or Kuroshio, simulated with the eddy-resolving quasi-geostrophic model. We applied empirical orthogonal functions analysis to turbulent flow solutions and statistically extracted robust and significant large-scale decadal variability modes concentrated around the eastward jet extension of the western boundary currents. In order to interpret these statistical modes dynamically, we linearized the governing quasi-geostrophic equations around the time-mean circulation and solved for the corresponding full set of linear eigenmodes with their eigenfrequencies. We then projected the extracted decadal variability on the eigenmodes and found that this variability is a multimodal coherent pattern phenomenon rather than a single mode or a combination of several modes as in the flow regimes preceding developed turbulence.

Modes of internal thermohaline variability in a single-hemispheric ocean basin

In this study, the internal modes of variability of the thermohaline ocean circulation in an idealized single-hemispheric basin are investigated. The modes are determined systematically by performing linear stability analyses of fully three-dimensional thermohaline flows. Relevant for the large-scale low-frequency variability of the flows are two kinds of oscillatory modes. The modes in one of these classes, which have an interdecadal time scale, are truly three-dimensional phenomena and are characterized by westward propagating temperature anomalies, while modes in the other class have a centennial time scale. The latter modes are shown to persist as internal modes of variability of two-dimensional flows and are therefore related to overturning (or loop) oscillations. The physical mechanism of propagation of these centennial modes is described and the relevance of each class of internal modes is investigated through analysis of stochastically forced transient flows.

1979年から1988年までの熱帯太平洋の海上風についてのインド物理学研究所の大気大循環モデルによるシミュレーション

The annual cycle and the large-scale low-frequency variability of surface winds over the tropical Pacific simulated by an atmospheric general circulation model (AGCM) are examined. The AGCM was integrated for 10 years with the observed sea surface temperatures (SSTs) from 1979 to 1988 and the model outputs were compared with National Center for Environmental Prediction (NCEP) reanalysis.The AGCM successfully simulates the climatological features associated with the annual mean and annual cycle of the surface winds. The amplitudes of the simulated annual harmonics are nearly comparable with observations over the south Pacific convergence zone (SPCZ). On the interannual time scale, the AGCM realistically captures two warm events (1982-83 and 1986-87), and one cold event (1988). The first two empirical orthogonal functions (EOFs) and their principal components (PCs) of simulated wind anomalies reproduce the important features, which are crucial for coupled air-sea interaction, like westerly anomalies associated with warm events, eastward propagation of westerly anomalies during 1982-83 and realistic convergence pattern in the first EOF mode over tropical eastern Pacific. Also, the simulated wind anomalies do not exhibit eastward migration during 1986-87 and the strength of anomalies is much larger during 1982-83 than 1986-87. Thus, the AGCM succeeds in reproducing the distinct characteristics of different warm events. It is found that the PCs of wind anomalies are strongly correlated to the PCs of precipitation anomalies. Also, the correlation between PCs of observed and modeled precipitation anomalies is very high (0.93 for both PC1 and PC2) at zero lag. Therefore, it appears that the AGCM's capability to represent the large-scale low-frequency variability of convective heating realistically contributes to its success in simulating the large-scale low-frequency part of surface winds.The most apparent deficiencies in the AGCM simulation include the incorrect location of westerly maximum, and less westward extension of westerly anomalies in EOF1. Also, the mean south-westerlies over Peruvian coast are weaker and not extended up to 170°W, especially during January. To get more insight into the AGCM simulated winds, the response of a steady-state linear Matsuno-Gill type model to two type of forcing (AGCM simulated precipitation anomalies, and AGCM simulated precipitation anomalies+SST anomalies) is compared with AGCM simulated winds. The linear model, forced only by the AGCM precipitation anomalies, can reproduce the AGCM simulated surface winds. The results of the linear model, when it is forced by both AGCM precipitation anomalies and SST anomalies, suggest that the deficiencies of AGCM in the western Pacific are mainly related to the insufficient treatment of surface turbulent fluxes.

Patterns of interannual variability defined by a cluster analysis and their relation with ENSO

Results of the COADS data processing aimed at a classification of the structure of the large-scale lowfrequency variability of the sea surface temperature, the sea-level atmospheric pressure and the surface wind-speed components are presented. Using cluster analysis, patterns of interannual variability in the temporal and spatial domains are obtained. Clusters consisting of points possessing similar temporal evolution are compared with the known dipole-type patterns of interannual variability, and special attention is given to selection of the global ENSO-related patterns. of particular interest are the teleconnection links between the equatorial and the extratropical regions in the Pacific Ocean. Clusters consisting of times with similar spatial patterns, as well as extended canonical correction analysis patterns, show the typical sequence of the spatial patterns during the ENSO cycle, especially for the ‘mature’ warm and cold phases. For these cases, typical features of the field anomalies, including the extratropical peculiarities in the Pacific Ocean, are considered and summarized.