Premium
The generation of thermal oscillations in an ocean model
Author(s) -
Cai Wenju
Publication year - 1996
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.49712253512
Subject(s) - forcing (mathematics) , oscillation (cell signaling) , thermal , boundary value problem , mechanics , heat flux , physics , atmospheric sciences , environmental science , climatology , geophysics , heat transfer , thermodynamics , geology , chemistry , biochemistry , quantum mechanics
Recently, interdecadal oscillations induced by saline forcing have been extensively investigated using ocean models driven by a Haney relaxation‐condition for temperature and a flux condition for salinity (traditional mixed boundary‐conditions). By contrast, oscillations induced by thermal forcing remain largely unexplored. In this study, the generation of persistent thermal oscillations in the Bryan–Cox–Semtner model is described. Initially the model is spun up by restoring surface temperature and salinity in the model to prescribed climatologies. When a steady state is reached, switching to mixed boundary‐conditions leads to a collapse in the modelled thermohaline circulation. Such a collapse suggests that the freshwater forcing is too strong. A reduction in the forcing of the freshwater flux then prevents the collapse, but no persistent oscillation occurs. When the thermal‐forcing condition is provided by a non‐traditional Schopf boundary‐condition, and when the reduced freshwater‐flux is applied, persistent thermal oscillations occur. Upon switching from the spin‐up state, the Schopf boundary‐condition allows a change only in the coupling strength, with the atmospheric heat transport remaining the same. This leads to a thermally dominant regime with a negative feedback which is crucial for the generation of thermal oscillations. Under mixed boundary‐conditions, such oscillations are less likely because the strong restoring thermal condition suppresses the negative feedback. The ways in which oscillations depend upon various parameters of the model, such as the strength of surface freshwater‐fluxes, vertical mixing coefficients, and timescales for damping thermal anomalies, are examined. It is found that the two most important requirements for the generation of thermal oscillations are a proper representation of the strength of thermal‐coupling and a not‐too‐strong flux of freshwater, and that the period of oscillation is rather insensitive to the choice of vertical mixing coefficient. The possible implication for coupled ocean–atmosphere modelling is discussed.