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Warren B. White

Abstract

Year-to-year changes are found in Australian precipitation (APP) covarying with those in sea surface temperature (SST) and troposphere moisture flux (MF) over the three oceans surrounding Australia for 40 yr from 1958 to 1997. Australia’s wet (dry) years are associated with warm (cool) SST anomalies surrounding Australia and convergent (divergent) MF anomalies directly overhead. Differences in APP (SST) between wet and dry years can reach 0.75 m (1.2°C) in northeast Australia (subtropical Indian Ocean). Wet (dry) years often occur during La Niña (El Niño), but significant differences in covarying SST, MF, and APP anomalies from one El Niño to the next are found, indicating that regional climate changes also influence APP. Covarying SST and MF anomalies on basin space scales and interannual timescales are found to take 2–3 yr to propagate eastward from Africa to Australia. This propagation occurs in association with the Antarctic Circumpolar Wave (ACW) in the Southern Ocean, the north branch of the ACW in the Indian Ocean, and the global El Niño–Southern Oscillation wave in the tropical ocean. A statistical climate prediction system based upon the slow eastward propagation of SST anomalies and their nearly one-to-one relationship with APP anomalies is constructed, yielding significant hindcast skill for predicting interannual APP anomalies at lead times of 1 and 2 yr. Best hindcast skill for the extratropical portion of Australia derives from the ACW south of Australia and the north branch of the ACW west of Australia. Eastward propagation of SST anomalies in these two oceanic domains is capable of predicting more than 50% of the total interannual variance over Victoria and New South Wales and over Western Australia poleward of 20°S over the 40-yr record. This percentage is much better than expected from chance or persistence, demonstrating the importance of the ACW upon year-to-year changes in APP at these latitudes.

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Warren B. White

Abstract

Gridded fields of TOPEX/Poseidon sea level height (SLH) from 1993 to 1998 and National Centers for Environmental Prediction sea surface temperature (SST) and meridional surface wind (MSW) anomalies from 1970 to 1998 are used to examine coupled Rossby waves in the Indian Ocean from 10°S to 30°S. Time–longitude diagrams of monthly SLH, SST, and MSW anomalies yield significant peak spectral energy density in propagation wavenumber–frequency spectra for westward propagating waves of >2 yr period and >4000 km wavelength. Subsequent low-pass filtering of SLH, SST, and MSW anomalies for these interannual timescales >2 yr finds them propagating westward over the Indian Ocean in fixed phase with one another at speeds significantly less (0.04–0.07 m s−1) than first-mode baroclinic Rossby waves, taking 3 to 4 years to cross the basin. These coupled Rossby waves display weak beta refraction patterns in all three variables. Significant squared coherence between interannual SLH and SST (SST and MSW) anomalies yield phase differences ranging from 0° to 45° (150° to 180°). Warm SST anomalies overlie high SLH anomalies, suggesting that pycnocline depth anomalies associated with the Rossby waves modify vertical mixing processes to maintain SST anomalies against dissipation. Warm SST anomalies are associated with outgoing latent heat flux anomalies in the eastern and central ocean, indicating that the ocean is capable of forcing the overlying atmosphere. Poleward MSW anomalies occur directly over warm SST anomalies, suggesting that anomalous planetary vorticity advection balances anomalous low-level convergence in response to SST-induced midtroposphere convection. These inferred thermodynamic processes allow a simple analytical model of coupled Rossby waves to be constructed that yields much slower westward phase speeds than for free Rossby waves, as observed. Maintenance of wave amplitude against dissipation occurs for coupled waves that travel westward and poleward, as observed.

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Warren B. White

Abstract

Gridded fields of TOPEX/Poseidon sea level height (SLH) and National Centers for Environmental Prediction sea surface temperature (SST) and meridional surface wind (MSW) anomalies are constructed monthly on a 2° grid over the Pacific Ocean for nearly 6 years from 1993 to 1998. Time–longitude diagrams of monthly SLH, SST, and MSW anomalies from 10° to 22° lat yield significant peak spectral energy density in zonal wavenumber-frequency spectra for periods of 1–2 yr near the free Rossby wave dispersion curve. Subsequently, temporal and spatial filtering of these SLH, SST, and MSW anomalies finds them propagating westward over the interior tropical Pacific Ocean in fixed phase with one another. Significant squared coherence exists between filtered SLH and SST (SST and MSW) anomalies over the entire latitude band, yielding significant phase differences ranging over 90° ± 45° (−70° ± 45°) at 10°S and 14° lat and ranging over 90° ± 45° (0° ± 45°) at 18° and 22° lat. Over the entire latitude domain warm SST anomalies are displaced westward of high SLH anomalies, consistent with anomalous poleward geostrophic heat advection associated with baroclinic Rossby waves. At 18° and 22° lat poleward MSW anomalies occur directly over warm SST anomalies as observed previously for extratropical coupled Rossby waves. On the other hand, at 10°S and 14° lat poleward MSW anomalies are displaced eastward of warm SST anomalies, consistent with Newtonian cooling balancing SST-induced midlevel diabatic heating in the tropical troposphere. The latter relationship allows an analytical model of tropical coupled Rossby waves to be constructed at 10° and 14° lat, different from that of extratropical Rossby waves at 18° and 22° lat and from that of free Rossby waves expected over the entire latitude domain. This tropical model yields coupled Rossby waves that propagate westward at slower phase speeds than expected of free Rossby waves, as observed. Maintenance (growth) of wave amplitude against dissipation occurs for tropical coupled Rossby waves that travel parallel to (poleward of) isotherms in the mean SST distribution, as observed.

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Warren B. White

Abstract

In the Pacific Equatorial Undercurrent downstream (east) from the Galapagos archipelago, an unusual meander pattern was observed in the spring of 1967. Two separate hypotheses present themselves as explanations for the observed wake phenomenon. The wake may have been a variation of the familiar von Kármán wake, or it may have been a form of the Rossby wake, only recently discussed by White. Through a scale analysis, both hypotheses are found to be reasonable, and both give characteristic length scales (500 km) that agree well with the observed wavelengths. A fundamental difference between the two hypotheses is that the Rossby wake is stationary, while the von Kármán wake is time-dependent. However, the time scale for eddy shedding in a von Kármán wake is found to be on the same scale (2 months) as the length of the cruise that observed the wake phenomenon. Therefore, it appears that the observed oceanic wake may have had characteristics of both the von Kármán and Rossby wakes.

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Warren B. White

Abstract

No abstract available.

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Warren B. White

Abstract

Individual seasonal mean maps of temperature at 300 m in the North Pacific Current cast of 180° from 1976 to 1980 were constructed from TRANSPAC XBT data. The long-term annual mean map is relatively smooth, with some weak quasi-stationary meander activity. Most of the total variance was due to large-scale interannual variability (i.e., ∼60%), loss to the mesoscale perturbations (i.e., ∼30%), and least to the annual cycle (i.e., ∼10%). However, individual mesoscale perturbations were significant, clearly wave-like with a wavelength scale of 500–1000 km and a period scale of 1–2 years, and generally coherent in phase over 10° of latitude. These wave-like mesoscale perturbations emanated from the eastern boundary and propagated westward as coherent features at the phase speed of linear, non-dispersive, baroclinic long-waves. The latitudinal reduction in phase speed from 2.7 cm s−1 at 35°N to 1.4 cm s−1 at 45°N was consistent with baroclinic long-wave theory. An increase in time scale of these wave-like perturbations with latitude was consistent with the “critical latitude” concept.

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Warren B. White

Abstract

During the four year period 1976–80, mesoscale anomalies of 300 m temperature in the eastern midlatitude North Pacific were observed propagating westward from the coast of North America to 165°W at approximately 2 cm s−1, with characteristics similar to baroclinic, nondispersive Rossby waves. These mesoscale anomalies had a dominant period scale of approximately two years and a dominant wavelength scale of approximately 1000 km. In this study, a baroclinic, nondispersive Rossby wave model is driven by the observed wind stress curl in an attempt to simulate the amplitude and phase of these observed mesoscale anomalies over this period of time. Model/data intercomparison finds the frequency/zonal wavenumber spectra of model 300 m temperature to be similar in pattern to that observed for waves of periods greater than one year, with peak spectral energy density in both occurring at zonal wavelengths of 1000–1200 km and periods of 2–3 years. These spectral peaks occur on the Rossby wave dispersion curve at the frequency/wavenumber location where peak spectral energy density occurs in the wind-stress curl spectrum. Coherence between model and observed mesoscale anomalies is maximum at the frequency/wavenumber location of these spectral peaks, significant at the 70% confidence level, with approximately 0° phase difference. The response functions in frequency/wavenumber space of both model ocean and real ocean are nearly identical in pattern for periods greater than one year, with maximum values located along the Rossby wave dispersion curve. Therefore, a resonant response of the baroclinic, nondispersive, Rossby waves to the wind stress curl is indicated. These resonant waves begin at the coast of North America and propagate westward with amplitude increasing linearly. The zonal profile of rms differences of observed mesoscale anomalies from 125–165°W is very similar to the model profile, the latter dominated by the resonant response which explains 60–90% of the total model response. The off-resonant model response is small, even though the wind strew curl variability is largest at off-resonant frequencies and wavenumbers. In the real ocean the off-resonant response is much larger than in the model, with the largest off-resonant response occurring in the dispersive portion of the spectral domain where the model is not applicable. This, together with white noise in both model and observed data, accounts for the model's inability to explain more than 20% of the total observed mesoscale variance.

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Warren B. White

Abstract

A mathematical barotropic model based upon the conservation of absolute vorticity is used to determine the effect of the spherical shape of the rotating earth [approximated by the beta (β) effect] on a steady uniform eastward current streaming past a cylindrical island in an unbounded ocean of uniform depth. Upstream far-field conditions are introduced that confine the disturbance pattern produced by the island to the region downstream from the island.

For an initially uniform eastward flow of velocity u 0 streaming past a cylindrical island of radius a, the downstream disturbance consists of a trail of meanders and eddies. The amplitude of these features depends upon the magnitude of the Island number [Is=(βa 2/u 0)½ and the radial wavenumber equals (β/u 0)½, which is, the Rossby wavenumber for stationary planetary waves.

In order to confirm the theoretical results of the beta-plane wake for an eastward flow situation, appeal is made to a laboratory model, consisting of a rotating annulus with a sloping bottom to simulate the beta effect. Dynamic similarity is achieved through the nondimensional Island number. The resulting flow pattern reveals a uniform flow field upstream from the island with the formation of a stationary disturbance downstream that agrees qualitatively with the theoretical results.

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Warren B. White

Abstract

No abstract available.

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