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Warren B. White

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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

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Warren B. White

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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

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From an analysis of the bathythermograph (BT) data files for the entire tropical North Pacific Ocean, the annual vertical displacement in the main thermocline from 10°−20°N is observed to propagate Westward as baroclinc long waves, with a phase speed and wavelength twice that of baroclinc Rossby waves, the latter emanating from the coast of Central America. For a closer investigation into the behavior of these long waves, a unique set of data is considered, consisting of BT observations taken monthly for 16 months in a rectangular grid from 11°−18°N, 148°−157°@. In this data set the vertical displacement in the main thermocline of the North Equatorial Current also displays a westward propagating annual signal; the annual signal (with an rms vertical displacement of 34 m) did-not appear in phase everywhere over the grid, but rather indicated a wave propagating through the observational BT network toward the northwest, with an average wavelength of about 2700 km and an average speed of 8 cm s−1. The zonal wave speed was faster at the southern end of the grid than on the northern, with a zonal wavelength that was nearly three times as large. This indicates the wave was being refracted from the west direction into the north direction.

These observations are consistent with theory where the annual forcing on the general circulation by the wind stress is found to generate baroclinic Rossby waves emanating from the eastern boundary which, when superimposed upon the local forced response, yields a baroclinic long wave that propagates westward at a phase speed of cpx = 2gH 0β/f 2, or twice the speed of nondispersive Rossby waves. Because the zonal phase speed of these long waves is latitude dependent, the quasi-meridional wave front is refracted as it travels westward, the wavenumber vector progressively rotating anticyclonically into the northward direction. The magnitude of the zonal phase speed at 11°N is calculated to be approximately 40 cm s−1 and at 18°N is ∼15 cm s−1. This is in quantitative agreement with observation. In addition, the zonal wavelength is determined by the distance traveled to the west during the annual cycle of wind forcing. At 11°N this is calculated to be approximately 12 000 km, at 18°N it is ∼5000 km, both also in quantitative agreement with observation.

From refraction principles inherent within the theory, the annual baroclinic long waves observed in the observational BT network near 153°W, 15°N were traced eastward to their point of origin. These waves were found to have originated at the coast of Central America nearly one year earlier, with the wave crest aligned parallel to the coast. This was what is expected from the generation theory. On the basis of this understanding, the expected phase and relative amplitude of these waves over the entire tropical North Pacific from 10°−20°N are mapped schematically.

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Warren B. White

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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

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A wind-driven model experiment is designed to test the effectiveness of the classical wind-driven theory of Veronis and Stommel (1956) in explaining the phase and amplitude of the seasonal cycle of the main thermocline in the interior portion of the midlatitude North Pacific. The model is a two-layer quasi-geostrophic model on the β-plane. driven by the observed seasonal cycle of the wind-stress curl. It allows the first-mode baroclinic response to Ekman pumping by the wind stress curl, modified by baroclinic planetary (Rossby) wave effects. The horizontal phase and amplitude distributions of the seasonal cycle of temperature at 200 m over the interior midlatitude North Pacific from 25–45°N, 130°W–150°E, are determined based on XBT data collected by ship-of-opportunity from 1968–74. Quantitative comparison between these observed distributions of phase and amplitude with those of the model shows that the phase distributions have a pattern correlation of 0.72, while the amplitude distributions have a correlation of only −0.15. Even worse is the fact that the rms magnitude for the model is 5–10 times smaller than that observed. There is some evidence that a portion of this amplitude mismatch may be due to an underestimation of the magnitude of the wind-stress curl by a factor of 2, but this does not even begin to explain the order of magnitude difference. Therefore, it must be concluded that the simple dynamical concepts put forth by Veronis and Stommel (1956) cannot account for the seasonal cycle of the main thermocline in the midlatitude North Pacific.

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Warren B. White

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The annual cycle in TOPEX/Poseidon sea level height (SLH) and National Centers for Environmental Prediction (NCEP) sea surface temperature (SST) and meridional surface wind (MSW) from 1993 to 1998 in the trade wind zone over the Indo-Pacific Ocean from 5° to 25° latitude in both hemispheres is examined. Zonal wavenumber–frequency spectra of monthly mean SLH, SST, and MSW residuals about the annual mean find peak spectral energy density for westward-propagating waves of annual period and basinscale wavelengths in all three variables. Moreover, spectral coherence and phase between the three variables are significant, with warm SST residuals displaced 0°–90° of phase to the west of high SLH residuals, and poleward MSW residuals displaced 0°–90° of phase to the east of warm SST residuals, similar to those observed for coupled Rossby waves on interannual period scales. Removing zonal means at each latitude from the annual cycle in SLH, SST, and MSW allows westward phase propagation to be observed in each variable, together with poleward refraction stemming from the reduction in westward phase speed with latitude. The fact that the latter occurs in both oceanic and atmospheric variables indicates that annual Rossby waves in the ocean are coupled to the overlying atmosphere. The reduction in westward phase speed with latitude (and, hence, the poleward refraction) is less than expected for free Rossby waves, with the apparent coupled Rossby waves traveling faster (slower) poleward (equatorward) of ∼12° latitude.

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Warren B. White

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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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