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E. Beier

Abstract

The observations of sea level at the annual frequency in the Gulf of California are reproduced both in amplitude and phase with a horizontal two-dimensional linear two-layer model. The main forcing agents through which variability is explained are wind stress and the action of the Pacific Ocean, which excites an internal wave in the mouth of the gulf. The surface heating is shown to play a secondary role. The response of the basin is qualitatively similar to that observed, that is, an energetic circulation in the upper layer (cyclonic in summer and anticyclonic in winter) compared to a weaker and opposite circulation in the bottom layer, as well as a transversely averaged horizontal heat flux equal both in amplitude and phase to that calculated with historical hydrographic data. The results of the simulation show that variability across the gulf is as important as the longitudinal variability.

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E. Beier
and
P. Ripa

Abstract

The circulation pattern in the northern Gulf of California, based on drifting buoys and hydrographic observations, can be explained using the results of a linear two-layer primitive equations model forced, at the annual frequency, by the Pacific Ocean, wind stress, and heat flux through the surface. The modeled surface circulation consists of a cyclonic gyre from June to October and an anticyclonic gyre from December to April, both located in the central region of the northern Gulf of California, which includes Ángel de la Guarda Island. The maximum intensities of the gyres occur in August and February, respectively, with values of surface velocities of 65 cm s−1 (in agreement with the observations) and very low opposite velocities in the bottom layers. May and November are transition months in which both gyres can be observed. Finally, in June/July or December/January the growing gyre is still connected with the rest of the Gulf of California, through the narrows between Tiburón Island, San Esteban Island, and the Baja California coast, whereas from August through October and from February through April the respective gyre is isolated. The vertical structure of the model results indicates a mainly baroclinic signal both in the southern and central regions of the Gulf of California. In the northern gulf, however, the velocities in the annual signal are a combination of barotropic and baroclinic movements, with similar intensities, coupled by topography effects. Thus, only part of the dynamics is associated to great movements of the interface, which shows maximum values of 40 m.

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E. Palacios-Hernández
,
E. Beier
,
M. F. Lavín
, and
P. Ripa

Abstract

Direct current measurements reveal that the circulation of the northern Gulf of California in the annual timescale consists of a cyclonic basinwide gyre (∼0.35 m s−1) that lasts from June to September (4 months), and an anticyclonic gyre (∼0.35 m s−1) lasting from November to April (6 months). The transitions between regimes take about three weeks each. The hypothesis that the difference in duration of the two circulation regimes is due to the seasonal variation of stratification of the water column is explored by simulations with a nonlinear two-layer numerical model of circulation and thermodynamics that includes vertical mixing, parameterized as an entrainment velocity. The model results agree remarkably well with the observations, considering its simplified vertical structure. In addition, the model predicts a net circulation consisting of an anticyclonic gyre of ∼0.05 m s−1, with a corresponding average concavity of the interface, and a two-layer exchange through the main channels of the archipelago.

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R. P. Matano
,
E. J. Beier
,
P. T. Strub
, and
R. Tokmakian

Abstract

In this article the authors examine the kinematics and dynamics of the seasonal cycle in the western Indian Ocean in an eddy-permitting global simulation [Parallel Ocean Circulation Model, model run 4C (POCM-4C)]. Seasonal changes of the transport of the Agulhas Current are linked to the large-scale circulation in the tropical region. According to the model, the Agulhas Current transport has a seasonal variation with a maximum at the transition between the austral winter and the austral spring and a minimum between the austral summer and the austral autumn. Regional and basin-scale mass balances indicate that although the mean flow of the Agulhas Current has a substantial contribution from the Indonesian Throughflow, there appears to be no dynamical linkage between the seasonal oscillations of these two currents. Instead, evidence was found that the seasonal cycle of the western Indian Ocean is the result of the oscillation of barotropic modes forced directly by the wind.

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