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  • Author or Editor: E. S. Sarachik x
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Igor V. Kamenkovich
and
E. S. Sarachik

Abstract

Restoring boundary conditions, wherein the temperature and salinity are restored to surface target fields of temperature and salinity, are traditionally used for studies of the ocean circulation in ocean general circulation models. The canonical problem with these boundary conditions is that, when the target fields are chosen as the observed fields, accurate simulation of the surface fields of temperature and salinity would imply that the surface fluxes and therefore the ocean heat transports approach zero, a clearly unrealistic situation. It is clear that the target fields cannot be chosen as the observed fields. A simple but effective method of modifying conventional restoring boundary conditions is introduced, designed to keep the calculated values of surface temperature and salinity as close to observations as possible. The technique involves calculating the optimal target fields in the restoring boundary conditions by an iterative procedure. The method accounts for oceanic processes, such as advection and eddy mixing in the derivation of the new boundary conditions. A reduced version of this method is introduced that produces comparable results but offers greater simplicity in implementation. The simplicity of the method is particularly attractive in idealized studies, which often employ restoring surface boundary conditions. The success of the new method is, however, limited by several factors that cannot be easily compensated by the adjustment of the target profiles. These factors include inaccurate model dynamics, errors in the observations, and the too-simplified form of restoring surface boundary conditions themselves. The application of the method in this study with a coarse-resolution model leads to considerable improvements of the simulation of sea surface temperature (SST) and sea surface salinity (SSS). Both amplitude and phase of the annual cycle in SST greatly improve. The resulting magnitudes of surface heat and freshwater fluxes increase on average, and the meridional heat transport gets stronger. However, the fluxes in some regions remain unrealistic, notably the too-strong freshwater forcing of the western boundary currents in the Northern Hemisphere. Southern Ocean cooling and freshening are also likely to be too strong. The subsurface values of temperature improve greatly, proving that a large part of errors in the subsurface temperature distribution in our model can be corrected by reducing errors at the surface. In contrast, the reduction of errors in surface salinity fails to improve uniformly the simulated subsurface salinity values.

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Ying-Quei Chen
,
D. S. Battisti
, and
E. S. Sarachik

Abstract

A 21/2-layer ocean model is developed to investigate the role of the first two baroclinic modes in determining the interannual variations of the sea surface temperature (SST) associated with the El Niño–Southern Oscillation (ENSO) phenomenon. Rather than simply adding an additional mode to the ocean component of the Zebiak–Cane coupled atmosphere–ocean model, it proved necessary to completely rethink all parts of the model. This allowed the external parameters to be specified more realistically. For example, the drag coefficient used in calculating the surface wind stress in the model is now consistent with that empirically derived, and the temperature of the water entrained in the surface layer that affects SST is now more carefully parameterized.

When forced by observed wind stress anomalies for 1961–93, the ocean model reproduces the interannual variations of SST satisfactorily. The quantitative discrepancies between the model hindcast and observed SST anomalies are limited to an excessive cooling of 0.5–1°C in the eastern/central Pacific during the period of 1989 to early 1991, and weaker warm phases in the central/western Pacific than observed. Both of the two gravest baroclinic modes are shown to be important in affecting the interannual variability in SST. A critique of the ocean model is presented at the end of this work.

When the ocean model is coupled with a simple atmosphere model, the resulting model exhibits quasi-periodic ENSO cycles with a period of ∼5 years. The variability in the coupled model is sensitive to the strength of the coupling and to the model parameterization of subsurface temperature. This model provides an opportunity to gain a better insight into the instability and variability of large-scale, low-frequency phenomena in the coupled atmosphere–ocean climate system and to bridge the gap between the simple Zebiak–Cane model and the more complex and computationally intensive coupled general circulation models in which more vertical modes are present.

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Andrew J. Weaver
,
Jochem Marotzke
,
Patrick F. Cummins
, and
E. S. Sarachik

Abstract

The stability and internal variability of the ocean's thermohaline circulation is investigated using a coarse-resolution general circulation model of an idealized ocean basin, in one hemisphere. The model circulation is driven, in addition to wind forcing, by restoring the surface temperature to prescribed values, and by specifying freshwater fluxes in the surface salinity budget (mixed boundary conditions). All forcing functions are constant in time.

The surface freshwater forcing is the dominant factor in determining the model's stability and internal variability. Increasing the relative importance of freshwater flux versus thermal forcing, in turn, one stable steady state of the model, two stable ones, one stable, and one unstable equilibrium, or no stable steady states at all are found. If the freshwater forcing is sufficiently strong, self-sustained oscillations exist in the deep-water formation rate, which last thousands of years. One type of oscillation occurs on the time scale of decades and is associated with the advection of high-latitude salinity anomalies. The other type has a diffusive time scale of centuries or longer and marks periods of complete absence of deep-water formation followed by violent overturning events (flushes).

When a stochastic component is added to the steady freshwater flux forcing, internal decadal variability persists if the background steady freshwater flux is sufficiently strong. Periodic flushes also exist under stochastic forcing; with increasing magnitude of the stochastic term the frequency of the flush events increases while their intensity decreases.

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