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David L. T. Anderson

Abstract

The low-level jet which flows across the equator and up the Somali coast is considered as a western boundary current of the East African mountain chain. The jet is assumed to be forced by the low-level divergence in the subtropical high pressure belt of the Southern Hemisphere and convergence in the monsoon trough. A simple model with this type of forcing is proposed and analytic and numerical solutions obtained. These appear to be in reasonable agreement with observation. The sensitivity of the model jet to spatial variation in the forcing, temporal changes in the intensity of the low-level convergence, and nonlinearity are examined.

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Anthony T. Weaver and David L. T. Anderson

Abstract

A four-dimensional variational method is used to examine the extent to which a time sequence of altimeter measurements can determine the subsurface flow in a linear multilayer model of the tropical Pacific Ocean. The experiments are all of the identical-twin type. Complete maps of sea level extracted from the model in a control integration play the role of the altimeter observations in the assimilation experiments. The results of the experiments indicate that, over timescales of months, the sea level information can be effectively propagated into the subsurface, particularly in the dynamically active equatorial region. Several degrees off the equator, however, where waves propagate more slowly, the recovery of the subsurface flow in models containing more than two vertical modes is significantly more difficult. The sensitivity of these results to the lengths of the data sampling and assimilation periods is discussed.

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David L. T. Anderson and Andrew M. Moore

Abstract

The relative information content of mass and velocity measurements for initializing low-frequency equatorially trapped waves is considered using analytical arguments and a numerical model For the Kelvin wave, mass and velocity data are equally useful, but this is not true for the Rossby waves. It is the relative amount of potential energy and kinetic energy possessed by equatorial waves that determines the relative usefulness of mass and velocity data for initializing these waves. The effects of dissipation on the adjustment process is considered. It is shown for the Kelvin wave that mass and velocity data are equally useful even when dissipation is present, but the Rossby wave adjustment is sensitive to the level and form of dissipation used.

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David L. T. Anderson and Robert A. Corry

Abstract

In a previous study Anderson and Corry used a wind-driven two-layer model to study the effects of topography and islands on the seasonal variation of western boundary currents. The work is continued here with topography, geography and winds appropriate to the North Atlantic to examine the seasonal cycle of the Florida Straits transport. A summer maximum of transport is predicted consistent with observations. The area of importance and processes giving rise to the seasonal cycle are considered.

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David L. T. Anderson and Julian P. McCreary

Abstract

Solutions to a coupled atmosphere model are discussed. The model ocean is a generalization of a reduced-gravity model that includes an equation for the temperature of the layer T. The model atmosphere is linear, baroclinic, and assumed to be in equilibrium with a forcing that represents the release of latent heal by convection Q. The wind stress τ used to drive the ocean is proportional to the wind velocity produced by the model atmosphere, while Q over the ocean is a function only of sea surface temperature. Some of the solutions involve land is well as ocean; in that case Q over land is specified externally and is not influenced by ocean temperature.

The atmosphere is always cyclic in longitude, but three different ocean-land configurations are considered: a) a zonally unbounded, cyclic ocean with no land; b) a bounded ocean with convection over land strong to the west; and c) a bounded ocean with convection over land strong to the cast. Case. b resembles the situation in the Pacific Ocean, with the strong land convection to the west representing the convection over Indonesia, whereas case c resembles the situation in the Indian Ocean. Eastward propagating large-amplitude oscillations develop in cases a and b. They are associated with warm-water pools that have a scale comparable with that observed during the 1982-83 El Nino event, but their propagation speed is only half that observed. No oscillations occur in case c, suggesting that Indonesian convection lion a fundamentally different effect on the Indian and Pacific Oceans.

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Julio Sheinbaum and David L. T. Anderson

Abstract

A linear reduced-gravity model of the tropical pacific is used to assimilate XBT data. The model cannot fit the data in the eastern equatorial Pacific for the whole assimilation period. Several experiments with real and simulated data are performed to investigate the source of this deficiency, which may be in the model or the wind stress used to force the model. It is shown that on the basis of the simple model physics we cannot unambiguously partition the error between model and forcing in the real data assimilation experiments although simulated data experiments do permit discrimination between model and forcing errors. Because the data is incomplete and does not permit a unique determination of the initial state, the use of prior information in the form of first-guess fields and/or smoothing constraints is examined. The filtering characteristics of the optimization algorithm are also discussed by looking at the evolution of the initial conditions as a function of the iteration number.

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Julio Sheinbaum and David L. T. Anderson

Abstract

A variations method based on the adjoint equation technique is used to assimilate data in a relatively simple linear reduced gravity model of the tropical Pacific. Real XBT data are used by identifying the depth of the 16°C isotherm depth with the model layer depth. It is shown that the XBT data contain large scale information that corrects the model first guess. However, the model is not capable of fitting the data in the eastern Pacific for the whole assimilation period. Regions not seeded by the data are explicitly shown and the impact of data from different times on the initial state is also discussed.

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Deng-Hua Wu, David L. T. Anderson, and Michael K. Davey

Abstract

Many features of the El Niño–Southern Oscillation (ENSO) phenomenon have been successfully simulated by coupled models during the last decade; however, some fundamental differences in model behavior remain. They can be classified into two categories according to whether the oscillation is self-sustained within the Pacific sector or whether some external impacts are needed to maintain the oscillation. In the first category, the delayed oscillator scenario describes ENSO as an oscillation generated and maintained by the coupled instability and oceanic waves, without the need for any external impacts. In the second category, the system has two steady states of equilibrium and an external forcing is needed to move the system from one state to another. Recent observational analyses suggest possible interactions or connections between external influences and ENSO variability.

The effects of external impacts on ENSO variability are investigated here by using a simple coupled ocean–atmosphere model. The impacts considered are wind-stress anomalies associated with the seasonal monsoonal cycle, and the tropospheric quasi-biennial oscillation in the Indian and western Pacific region. It was found that 1) the external impact plays an important role in triggering ENSO variability when the coupled system in the Pacific could not support the oscillation by itself, 2) the impact regulates the original self-sustained oscillation to a seasonally phase-locked time evolution; and 3) the periods of the resulting oscillations could be three times that of the external forcing, a result of the interaction between the external forcing and the coupled system in the Pacific.

A modified version of the delayed oscillator equation was used to examine further details of the interaction. It was found that the match of half of the period of the external forcing with the delay time of the reflected oceanic waves from the western boundary arriving at the air–sea interaction region to turn off an event is a key factor in determining how they interact. If the time-matching condition is satisfied, the oscillation period will be three times that of the forcing. It is also shown that wind stress associated with the quasi-biennial oscillation could influence significantly the original self-sustained oscillation in the Pacific, making the amplitude and interval between two successive warm or cold phases variable, as observed in ENSO events.

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Magdalena A. Balmaseda, Michael K. Davey, and David L. T. Anderson

Abstract

When forecasting sea surface temperature (SST) in the Equatorial Pacific on a timescale of several seasons, most prediction schemes have a spring barrier; that is, they have skill scores that are substantially lower when predicting northern spring and summer conditions compared to autumn and winter. This feature is investigated by examining predictions during the 1970s and the 1980s, using a dynamic ocean model of intermediate complexity coupled to a statistical atmosphere. Results show that predictions initialized during the 1970s exhibit the typical prominent skill decay in spring, whereas the seasonal dependence in those predictions initialized during the 1980s is rather small. Similar changes in seasonal dependence are also found in predictions based on simple persistence of observed SST anomalies.

This decadal change in the spring barrier is related to decadal variations found in the seasonal phase locking of the SST anomalies, which is largely determined by the timing of El Niño events. The spring barrier was strong in the 1970s, when El Niño was strongly phaselocked to the annual cycle. An analysis of observed SST anomalies from 1900 to 1990 shows several changes in behavior on a decadal scale, with the largest change being from the 1970s to the 1980s.

The seasonal dependence of model heat content predictions is investigated and found to be similar to that for SST, except that it shows a winter barrier rather than the spring barrier evident in SST.

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Julian P. McCreary Jr. and David L. T. Anderson

Abstract

A model of tropical ocean-atmosphere interaction is used to study the El Niño–Southern Oscillation phenomenon. The model ocean consists of the single baroclinic mode of a two-layer ocean. The thermodynamics of the upper layer are highly parameterized; sea-surface temperature is assigned one of two values, warm or cool, according to whether the interface is shallower or deeper than an externally specified depth. The model atmosphere consists of two wind patches of zonal stress that are idealizations of the annual cycle of the equatorial trades, τs, and of Bjerknes' Walker circulation, τw. When the eastern ocean is in its cool state both patches drive the ocean; when it is warm τw is switched off. Solutions compare favorably with observations in several ways. Most importantly, for reasonable choices of parameters solutions oscillate at the long time scales associated with the Southern Oscillation.

The response of the ocean to τw introduces positive feedback into the system, with the result that the system can adjust to one or the other of two equilibrium states: a state with τw switched on, and another with it switched off. The annual wind τs is the “trigger” that switches τw off or on, and thereby prevents the system from ever reaching either equilibrium state.

When τw switches on, equatorial Kelvin waves swiftly propagate from the wind patch into the eastern ocean, and raise the interface there to a shallow level. Rossby waves, also generated by the wind, subsequently reflect from the western boundary as a second set of equatorial Kelvin waves. The arrival of this second set in the eastern ocean begins a gradual deepening of the interface there toward its equilibrium value. It is this overshoot together with slow relaxation of the interface in the eastern ocean that allows the model to oscillate at long time scales. Essentially, the ocean must be sufficiently relaxed toward an equilibrium gate before τs can act to switch τw Off or on.

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