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

Abstract

The coupled ocean-atmosphere model of Anderson and McCreary is extended to include two oceans. An advantage of the two-ocean system is that it is not necessary to specify externally convection over land.

For a basin geometry that most resembles the Indian and Pacific Oceans, strong permanent convection exists in the eastern Indian Ocean, and there is an oscillation in the Pacific Ocean with a period of about five years. Associated with this oscillation is a patch of convection that develops in the central and western ocean and propagates into the eastern ocean before dissipating.

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Magdalena A. Balmaseda, Arthur Vidard, and David L. T. Anderson

Abstract

A new operational ocean analysis/reanalysis system (ORA-S3) has been implemented at ECMWF. The reanalysis, started from 1 January 1959, is continuously maintained up to 11 days behind real time and is used to initialize seasonal forecasts as well as to provide a historical representation of the ocean for climate studies. It has several innovative features, including an online bias-correction algorithm, the assimilation of salinity data on temperature surfaces, and the assimilation of altimeter-derived sea level anomalies and global sea level trends. It is designed to reduce spurious climate variability in the resulting ocean reanalysis due to the nonstationary nature of the observing system, while still taking advantage of the observation information. The new analysis system is compared with the previous operational version; the equatorial temperature biases are reduced and equatorial currents are improved. The impact of assimilation in the ocean state is discussed by diagnosis of the assimilation increment and bias correction terms. The resulting analysis not only improves the fit to the data, but also improves the representation of the interannual variability. In addition to the basic analysis, a real-time analysis is produced (RT-S3). This is needed for monthly forecasts and in the future may be needed for shorter-range forecasts. It is initialized from the near-real-time ORA-S3 and run each day from it.

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