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J. S. Godfrey

Abstract

The spindown of an ocean is investigated, using a two-layer numerical model. At the initial instant, a pool of warm water the size of the Coral Sea occupies the subtropical portion of the top layer; the pool is initially in geostrophic balance, with depth-averaged flow equal to zero. The ocean is allowed to develop freely in the absence of wind and heating. Horizontal temperature contrasts are very weak, so the flow is accurately linear.

The warm pool first moves to the western boundary, via internal Rossby waves. A progressive wave of downwelling then develops, which transports heat up to the equator, along the equator, and around the eastern and poleward boundaries; the region of warm water generated as a result near the eastern boundary is separated from the colder ocean interior by a “front,” which spreads inward via internal Rossby waves.

The net effect of all these motions is to spread the pool of warm water, which was initially concentrated in a small area, out over the rest of the ocean; the potential energy released in this process is dissipated by friction, mainly against the western boundary.

An analytic theory of this spindown process shows that the downwelling waves around the boundaries are similar to Kelvin waves, though there are important differences due to friction and the β effect.

For linear, first internal mode disturbances of the kind considered here, the pressure and the heat content are proportional to one another; it is shown that this allows a simple physical interpretation of many of the phenomena in the model, which may also be useful in the real ocean.

Application is made to the El Niñe, and the associated global fluctuations in the atmosphere and ocean. Bjerknes hypothesized that these fluctuations are generated spontaneously by interaction between the ocean Ekman layer and the atmosphere near the equator; it is suggested here that Bierknes'instability mechanism may be strengthened if deeper oceanic effects are also included.

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A. Schiller and J. S. Godfrey

Abstract

The impact of atmospheric intraseasonal variability on the tropical Indian Ocean is examined with an ocean general circulation model (OGCM). The model is forced by observation-based wind stresses and surface heat fluxes from an atmospheric boundary layer model. Composites of 26 well-defined boreal spring and summer intraseasonal events from 1985 to 1994 are used to explore surface and subsurface impacts of intraseasonal oscillations in the ocean. The phase and amplitude of simulated intraseasonal sea surface temperature (SST) variations agree well with observations. The net surface heat flux dominates the composite mixed layer heat budget on intraseasonal timescales, while entrainment through the base of the mixed layer contributes locally. Horizontal advection is of secondary importance in the composite heat balance. However, inspection of individual events suggests that in individual intraseasonal events different processes may control their dynamics.

A characteristic feature of equatorial intraseasonal variability is the formation of a shallow mixed layer caused by a surface freshwater cap associated with strong freshwater fluxes into the ocean. This “barrier-layer” formation in association with mean temperature inversions significantly impacts the heat transfer across the bottom of the mixed layer during the transition from calm and clear to windy and cloudy conditions of an event, such that strong entrainment at the peak of an intraseasonal event warms rather than cools the surface.

The intraseasonal mixed layer salinity budget is about equally determined by entrainment, surface freshwater fluxes, and horizontal advection. The latter is due to notable horizontal salinity gradients in the central and eastern Indian Ocean in combination with equatorial jetlike velocity anomalies that develop in response to the intraseasonal atmospheric wind forcing.

Use of equatorial mooring data in 1994 was useful for understanding model phenomena on several timescales. However, the observations contained no representative intraseasonal events.

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J. S. Godfrey and T. J. Golding

Abstract

The observed distribution of the depth-integrated steric height (mass transport function) in the Indian Ocean compares favorably with the distribution computed from the Sverdrup relation and annual average wind stresses, provided the integration starts from observed values of mass transport function near the eastern boundary. These eastern boundary values increase substantially from 9 to 115°S, presumably because the Pacific-Indian through flow is geostrophically balanced. This flow is estimated to have an order of magnitude of 10 × 106 m3 s−1—rather stronger than previous estimates. The flow may [as suggested by Cox (1975)] contribute to the observed anomalous weakness of the East Australian Current, though southwards flow along the South American coast may also contribute to its weakness. If the Pacific-Indian throughflow did not occur it is shown that the Sverdrup circulation pattern in the Indian ocean south of the equator would be similar to that in the other tropical oceans, in that the flow would be generally northwestwards over most of the ocean. It also is quite likely that blocking the Pacific-Indian flow would lower water temperatures off Western Australia by an amount of order 3°C in the upper 300 m; thus the throughflow may be responsible for the observed anomalous lack of upwelling (or at least for the lack of cold, nutrient-rich water) along the Western Australian coast.

It is suggested that internal Kelvin waves may propagate south from the equatorial Pacific along the West Australian coast, resulting (with internal Rossby wave propagation) in high values of mass transport function throughout the Indian Ocean from 15 to 35°S. The fact that internal Rossby waves propagate westward may explain why the Pacific-Indian throuhflow continues into the Indian Ocean as an essentially zonal jet.

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J. S. Godfrey and J. L. Wilkin

Abstract

No abstract available

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Anthony C. Hirst and J. S. Godfrey

Abstract

The effect of the Indonesian Throughflow on the World Ocean circulation is examined by a series of experiments with a global ocean GCM. The principal objective is to gain an understanding of how ocean flows respond to the throughflow, and how these changes result in changes in the pattern of surface heat flux and sea surface temperature. Four model runs are conducted. The first run features an open Indonesian passage through which a nonzero net throughflow is permitted. The second run features a complete blockage of the Indonesian passage. The third run is designed to isolate the effects of purely buoyancy-driven throughfiow: the Indonesian passage is open but the net volume transport is required to be zero. The fourth run is designed to isolate the effects of nonzero net throughflow on the Indian Ocean, independent of interocean buoyancy differences: the Indonesian passage is open but the throughflow water is cooled and salted toward profiles characteristic of the east Indian Ocean in the absence of throughflow.

Comparison of the first and second runs shows that the throughflow generally warms the Indian Ocean and cools the Pacific. However, large changes in the surface temperature and heat flux are restricted to certain well-defined regions: the Agulhas Current/outflow, the Leeuwin Current region off western Australia, the Tasman Sea, the equatorial Pacific, and two bands in the midlatitude South Pacific. In contrast, large subsurface temperature changes are widespread across both oceans. Heat budget analysis indicates that the large surface responses are dependent on the subsurface temperature change being brought to the surface, either by strong wind-forced upwelling (as in the equatorial Pacific) or by vigorous mixing in convective mixed layers (as in the other regions). Over most of both oceans, such mechanisms are absent and surface heat-flux changes are small (a few W m−2). There, subsurface temperature perturbations are largely insulated from the surface and may extend via direct advection or baroclinic wave propagation. The additional beat is released upon encounter with upwelling or a convective mixed layer, which may be far removed from the source of the perturbation. Atlantic and far Southern Ocean effects are mostly very small, possibly because of our use of restoring upper boundary conditions. The third and fourth runs break the throughflow into its baroclinic and barotropic components. The baroclinic (buoyancy-driven) component affects surface beat flux strongly in the Leeuwin Current region but relatively weakly in the Agulhas Current and Tasman Sea. The barotropic component has the converse effect. Interocean heat exchange is discussed; the full throughflow transports a net 0.63 petawatts out of the Pacific Ocean, which represents about one-third of the total heat input into the model's equatorial Pacific.

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Anthony C. Hirst and J. S. Godfrey

Abstract

The timescale and mechanisms of remote response in a global ocean GCM is investigated in the case of a sudden change in the rate of Indonesian Throughflow. In one experiment, the model is run to equilibrium with the Indonesian passage completely closed off. The passage is then opened, and the evolution of the system toward a new equilibrium is examined. In a second experiment, the model equilibrium solution with passage open is slightly perturbed by application of a body force to the water in the passage. The force is such that the change in throughflow (an increase of about 5%) has vertical profile almost identical to that of the original throughflow. The changes that evolve in the second experiment are, after appropriate scaling, quantitatively similar to those in the first, thereby verifying the approximate linearity of the response. The dynamics of this response are investigated with the aid of several idealized small-perturbation experiments, in which the model is reconfigured with a flat bottom and to be initially at rest with horizontally homogeneous density fields. It is shown that the extensive subsurface temperature responses in both the Indian and Pacific Oceans primarily result from a process of adjustment akin to baroclinic wave propagation of the first and second internal modes. The model's (approximate) first internal mode response is fairly similar to that expected from viscous linear theory. However, temperature perturbations associated with the second internal mode response are strongly distorted, in part by advection associated with the background currents. Temperature advection by the perturbation barotropic mode is unimportant except locally in the Tasman Sea and Agulhas Retroflection regions. Large differences in the patterns of response obtained previously for shallow and deep Indonesian sills, and for full versus buoyancy-driven-only throughflow, are interpreted in terms of preferential excitement of internal modes. Thus the model's baroclinic wave properties, and the spectrum of baroclinic modes excited by the throughflow change, appear very important to the pattern and timing of the subsurface (and hence surface) temperature response.

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J. S. Godfrey and K. R. Ridgway

Abstract

A formula is first given for the error in a 2-harmonic seasonal curve of best fit through a set of N oceanographic data points, assuming the departures from the true mean are independent random numbers.

Departures of actual oceanographic measurements from the mean seasonal cycle are in fact correlated with one another, owing to long-period nonseasonal variability: hence the error estimate from the formula will generally be too small. If the data set can be split into two sets that are statistically independent, a method is given for estimating (in an averaged sense) the factor by which the formula should be multiplied, to account for the effect of correlations on the error estimate. Results from four ways of splitting the data into two sets for steric height data off Western Australia suggest that the results are reasonably independent of the method of splitting.

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J. S. Godfrey and K. R. Ridgway

Abstract

Steric heights have been estimated using TS relationships for all available XBTs within 10–40°S, 105–130°E to 1980, and merged with steric heights from hydrology data. These data were subdivided into bins following the Western Australian coast, and a two-harmonic best fit to the seasonal cycle obtained in each bin. Seasonal departures from the annual mean of surface steric height are combined with similar departures of pressure-corrected coastal tide gage data to obtain maps of seasonal departure in geostrophic flow, both on and off the continental shelf. These are added to annual mean patterns to give total seasonal flows, for comparisons with earlier work. Wind stress vectors, wind stress curls and longshore wind stress components are also obtained, using data from a marine climate altas. The results give an improved picture of the broad-scale environment surrounding the Leeuwin Current (a narrow, rapid poleward current along the Western Australian continental shelf edge) and of the wind stresses that may partly or wholly drive the Current.

On annual average, the Leeuwin Current accelerates into the wind from 22.5 to 32.5°S; its greatest speeds are at the surface. Below 300 m there is a slow deep equatorward undercurrent with a mass transport comparable in magnitude to the nearsurface polewards flow. These and other results support Thompson's contention that the Leeuwin Current is primarily a convection current, driven by longshore steric height gradients at the continental shelf edge. Sverdrup balance may be violated near the shelf edge. It is suggested that throughflow from the Pacific to the Indian Ocean may create the large steric height gradient along the Western Australian continental shelf, which in turn results in the uniquely large poleward nearsurface flow in the Leeuwin Current. In the depth-integrated longshore momentum equation at the continental shelf edge, the seasonal cycles of the pressure gradient and wind stress terms reinforce one another, resulting in a strong net southward force in May, when the Leeuwin Current is strongest; this behavior is specific to the shelf edge, and does not occur at the coast. This seasonal variation seems to be part of a wavelike pattern; the “wave” originates north of Australia during the Northwest Monsoon and progress rather slowly anticlockwise around Western Australia.

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J. S. Godfrey, D. J. Vaudrey, and S. D. Hahn

Abstract

Results are presented from an oceanographic cruise along Australia's south coast in June–July 1982. A narrow shelf-edge current flowed eastwards along the shelf edge for the entire distance. Features of particular interest were (i) various eddies on the offshore side of the current, (ii) very sharp surface fronts, at which geostrophic Richardson numbers were less than 1.0 in 10 out of 34 cases (iii) the formation and offshore transport of saline water in the Great Australian Bight, and (iv) the strongly nonlinear dynamics in the neighborhood of Cape Leeuwin.

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J. S. Godfrey, J. Wilkin, and A. C. Hirst

Abstract

Most available wind products show a mean wind stress curl across the Pacific at 2°N that is close to zero, so a Sverdrup model predicts the western boundary current at the Asian coast to be nearly zero at this latitude. Thus, the Australasian circulation, or Indonesian Throughflow-which is estimated by a Sverdrup model to be (16 ± 4) × 106 m3 s–is predicted to flow westward as a zonal jet from the northern tip of Irian Jaya into the Indonesian seas. If this Sverdrup flow pattern were valid, the throughflow would be supplied by (salty) South Equatorial Current water from the northern tip of Irian Jaya, while the (relatively fresh) Mindanao Current would all return eastward to feed the North Equatorial Countercurrent. Observed salinities in the Indonesian Throughflow are close to those of the Mindanao Current, suggesting that they are inconsistent with the Sverdrup flow pattern.

The outputs of two recent ocean general circulation models are examined; in both models, the Indonesian throughflow is supplied by Mindanao Current water because the South Equatorial Current (SEC) retroflects into the nearby North Equatorial Countercurrent-Equatorial Undercurrent system. In one model, this retroflection is due to the action of a rather large horizontal eddy diffusivity. In a second, fine-resolution model the depth-integrated time-mean flow departs from the Sverdrup prediction within about 3° on either side of the equator; almost complete retroflection of the SEC occurs, associated with a feature similar to the Halmahera Eddy. In both models the retroflected current travels a large distance eastward in the North Equatorial Countercurrent or the undercurrent and then returns west in the North Equatorial Current before entering the Indonesian throughflow. Thus, although the throughflow does originate in the South Equatorial Current, the high-salinity signal of the SEC is probably obliterated by rainfall along this long path so that the throughflow appears to originate from the North Pacific. These results suggest that retroflection processes may play an important role in controlling the supply of fresh water from the Pacific to the Indian Ocean.

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