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Warren B. White

Abstract

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

Abstract

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

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
and
Shyh-Chin Chen

Abstract

Tropospheric temperature and vorticity budgets for the Antarctic Circumpolar Wave (ACW) are diagnosed utilizing the National Centers for Environment Prediction–National Center for Atmospheric Research reanalysis datasets from 1983 to 1992, focusing on the eastern Atlantic, Indian, and western and central Pacific sectors of the Southern Ocean where remote forcing from the Tropics has been observed to be weak. There, warm sea surface temperature (SST) anomalies are found in the ACW propagating eastward together with anomalous upward latent heat flux, positive precipitation, low-level convergence, upper-level divergence, midlevel ascent, and poleward surface wind. Diagnosing the anomalous temperature budget finds SST-induced latent heat flux instigating anomalous mid- to upper-level diabatic heating and low-level diabatic cooling in the absence of significant eddy heat flux divergence. This diabatic heating profile is balanced by a combination of vertical and horizontal heat advection, giving rise to anomalous ascent and poleward wind throughout the column. The thermodynamics of this deep diabatic heating scenario are different from those of Palmer and Sun. An intrinsic feedback from atmosphere to ocean is indicated by reduced sensible-plus-latent heat flux displaced 45° to 90° of phase to the east of warm SST anomalies, yielding an anomalous SST warming tendency that contributes both to eastward phase propagation and amplitude maintenance of the ACW. Diagnosing the anomalous potential vorticity budget finds the vertical gradient of anomalous diabatic heating, negative over most of the column, balanced by the anomalous advection of planetary vorticity, the mean advection of anomalous relative vorticity, and net vortex tube advection, together yielding a poleward equivalently barotropic wind response to warm SST anomalies. This deep diabatic heating scenario is contrasted against the remote forcing scenario in the eastern Pacific and western Atlantic sectors of the Southern Ocean where remote forcing associated with the El Niño–Southern Oscillation (ENSO) in the Tropics can now be seen to drive the SST tendency in the ACW.

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Warren B. White
and
Neil J. Cherry

Abstract

Autumn–winter temperature and precipitation records at 34 stations over New Zealand from 1982 to 1995 are found by empirical orthogonal function (EOF) analysis to fluctuate together with 3–6-yr quasi periodicity similar to that associated with the Antarctic Circumpolar Wave (ACW), which propagates slowly eastward past New Zealand in its global traverse around the Southern Ocean. By allowing these EOF time sequences to represent New Zealand temperature and precipitation indices, both the positive temperature index related to warm sea surface temperature (SST) anomalies around New Zealand and the positive precipitation index related to warm (cool) SST anomalies north and east (south and west) of New Zealand are found. These warm (cool) SST anomalies are associated with poleward (equatorward) meridional surface wind (MSW) anomalies, the same as observed in association with the ACW. When warm (cool) SST and poleward (equatorward) MSW anomalies are located north (south) of New Zealand, then anomalous low-level wind convergence occurs over New Zealand, and when they are located east (west) of New Zealand, then anomalous cyclonicity occurs over New Zealand, both during years of anomalously high autumn–winter precipitation over New Zealand. Regular eastward propagation of the ACW past New Zealand suggests that covarying SST and MSW anomalies (and New Zealand autumn–winter temperature and precipitation) can be predicted 1–2 yr into the future. The authors test for this by utilizing the eastward propagation of the ACW contained in the dominant extended EOF mode of SST anomalies upstream from New Zealand to predict SST indices in the western South Pacific that are linked statistically to New Zealand temperature and precipitation indices. At 0-yr lead, this statistical climate prediction system nowcasts the observed sign of New Zealand temperature (precipitation) indices 12 (12) years out of the 14-yr record, explaining 50% (62%) of the interannual variance for each index. At 1-yr lead, it hindcasts the observed sign of New Zealand temperature (precipitation) indices 12 (13) years out the 14-yr record, explaining 24% (74%) of the interannual variance. At 2-yr lead, hindcasting is insignificant. This hindcast skill at 1-yr lead suggests that prediction of interannual climate variability over New Zealand may depend more upon predicting the amplitude and phase of the ACW than upon predicting it for tropical ENSO.

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Warren B. White
and
Youhai He

Abstract

Interannual variability in vertically averaged temperature over the upper 400 m of ocean (i.e., upper ocean heat content) in the vicinity of the Kuroshio Extension experienced significant changes during the 1982 ENSO year, as compared to the preceding three years. Interannual variability was dominated by a mesoscale anomaly pattern, whose variance increased by 40%. on average, in early 1982 over what it had been the previous three years. This was accompanied by a general anomalous cooling south of the Kuroshio Extension and an anomalous warming north of the current, associated with a decrease in the intensity of the Kuroshio Extension east of 150°E. This activity occurred at the same time that the tropical and equatorial western North Pacific was experiencing a rapid reduction in upper ocean heat content. Mesoscale anomalies of vertically averaged temperature in the Kuroshio Extension during the 1982 ENSO year were comparable in magnitude (i.e., 3°C) with maximum anomalous vertically averaged temperatures reported in the equatorial Pacific off the coast of Ecuador and Peru.

The variance of the mesoscale anomaly pattern did not change seasonally, but the average absolute time rate of change in the mesoscale anomaly pattern was maximum in winter, minimum in summer. The variance of the mesoscale anomaly pattern was relatively uniform along the axis of the Kuroshio Extension from 140°–160°E, but the absolute time rate of change was much larger near the bottom bathymetry features of the continental slope of Japan (140°E) and the Shatsky Rise (160°E). Therefore, the mesoscale anomaly pattern in the Kuroshio Extension was relatively constant through most of the year, with rapid changes in winter, intensified near the Japan coast and the Shatsky Rise. These results lead to a hypothesis of mesoscale formation that is wind-driven in nature, intensified by the presence of bottom bathymetry features.

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Keisuke Mizuno
and
Warren B. White

Abstract

Individual, seasonal, 300 m temperature maps were constructed over the Kuroshio Current System from 130°E to 170°W, for a 4-year period from summer 1976 through spring 1980, using TRANSPAC XBT data and JODC temperature/depth data. Quasi-stationary meanders in the Kuroshic Current System occurred at 137°C (i.e., Kuroshio Meander), at 144°E and 150°E (i.e., lee-wave meanders), and near 160°E (i.e., meander over the Shatsky Rise). A composite of the paths of the Kuroshio (i.e., the 12°C isotherm) from the individual seasonal maps, and the total variance map, finds nodes (i.e., minima) and anti-nodes (i.e., maxima) of variability to have existed along the mean Kuroshio path. The anti-nodes coincided with the location of the quasi-stationary meanders, the nodes in between. Zonal propagation of temperature anomalies accounted for 20–30% of the total interannual variance. These temperature anomalies propagated eastward at 0.5–1.5 cm s−1 in the region 140°–155°E, and westward at −1 to −2 cm s−1 in the region 155°E–175°W. In addition to this wave propagation, 31% of the interannual variance in temperature could be explained by two empirical standing-wave modes. Within these two modes, spatial coherency in variability existed between the Kuroshio Meander, the two lee-wave meanders east of Japan and the meander over the Shatsky Rise. Both spatial patterns of variability fluctuated with a 1-year decorrelation time scale, with maximum interannual variability occurring in fall/winter and minimum interannual variability in spring/summer.

In the latter part of the 4-year period (1979–80), the Kuroshio Meander became weak and the Kuroshio Extension was displaced southward, from 36–37°N during the first 2 years to 34°N during the latter two years. Associated with these large scale changes, the quasi-stationary meander pattern in the Kuroshio Extension became unstable, associated with increased eddy activity and ring production. In fact, ring production doubled, i.e., from 5 per year to 10 rings per year, from what it was during the previous 3 years. Prior to this regimal change, the Kuroshio Extension bifurcated near the Shatsky Rise (160°E) with a secondary branch of the Kuroshio Extension extending northeastward along the Shatsky Rise to 40°N, where it turned east, and with the main branch extending eastward along 36°N. After the regional change, this bifurcation occurred much farther to the west near 150°E.

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Hideo Nishida
and
Warren B. White

Abstract

Sequential monthly-mean temperature fields in the mid-latitude western North Pacific (30–45°N, 140–180°E), constructed from TRANSPAC XBT (expendable bathythermograph) data over a two-year period, are employed to analyze the interaction between mesoscale variability and the mean flow of the Kuroshio Extension. An empirical temperature–dynamic height regression, and the geostrophic approximation, lead to the computation of mean and eddy horizontal velocity components over the region, from which horizontal eddy processes in both the mean momentum and the mean kinetic energy balances of the Kuroshio Extension are estimated.

The Kuroshio Extension generally loses mean kinetic energy as it travels from the coast of Japan from 140 to 180°E, with much larger eddy kinetic energy west of the Shatsky Rise (160°E) than east of there. Concentrating upon zonally-averaged quantities in both subregions east and west of the Shatsky Rise, horizontal eddy momentum fluxes due to transient mesoscale variability (i.e., u′v′ ) tend to converge the mean eastward momentum. However, in the western subregion, quasi-stationary meanders in the mean flow are associated with a horizontal eddy momentum flux (i.e., u′v′ ) that reduces this tendency. The horizontal kinetic energy exchange between mean flow and mesoscale variability (e.g., u;′v′ u/∂y〉) tends to increase the mean kinetic energy on the north side of the axis of mean flow and to decrease it on the south side, both east andwest of the Shatsky Rise. On the other hand, the horizontal mean kinetic energy redistribution by horizontal eddy processes (e.g., 〈−∂/∂y( u′v′ u ) tends to increase mean kinetic energy overall in the subregion east of the Shatsky Rise, showing little effect west of there. The net tendency upon the mean kinetic energy balance by these two horizontal eddy processes (i.e. exchange and redistribution) is to increase mean kinetic energy in the region east of the Shatsky Rise, with no significant effect west of there. Therefore, the reduction in mean kinetic energy from west to east along the Kuroshio Extension is not due to horizontal eddy processes.

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Warren B. White
and
Gerard McNally

Abstract

The idea that Ekman transport driven by the mean synoptic wind stress on the f 0-plane is a robust result is true only for infinite scale lengths of the wind stress forcing. For finite scale wavelengths [O(100 km)] and for a range of subinertial frequencies ranging from 2–10 pendulum days, Ekman transport must compete with the transport associated with pressure gradient flow associated with wind-driven evanescent waves that develop in the pycnocline below the mixed layer. This is demonstrated in a schematic model of the upper ocean at midlatitude where the surface mixed layer of depth H lies above a pycnocline of uniform stratification N 2, driven by schematic wind stress that approximates that associated with the passage of storm fronts. It is shown that subinertial response to transient wind forcing at these periods even at infinite wavelength scales favors the anticyclonic response over the cyclonic one; this tendency is maximum at a period of 1 pendulum day, disappearing at 10 pendulum days as the transient subinertial response approaches the exact equilibrium state Ekman response. However, for transient wind stress of finite wavelength, due to the propagation of atmospheric frontal disturbances of wavelength L and period T at speed C pz, the wind-driven response of the mixed layer penetrates downward into the pycnocline. Divergence in mixed layer motions at subinertial frequencies produce evanescent (i.e., vertical attenuating) motion within the upper portion of the pycnocline. The evanescent horizontal motions are directly out of phase with those in the mixed layer; the evanescent vertical motion alters the density field of the pycnocline and, hence, the pressure field over the entire upper ocean. The resulting transient pressure gradient (i.e., geostrophic) response in the mixed layer is directed in the downwind direction, lagging the cross-wind transient Ekman response by 90°. At subinertial periods of 2–10 pendulum days and wavelengths of O(100 km), the magnitude of this transient geostrophic response is on the same order as that of the transient wind-driven Ekman response. Approaching equilibrium state at 10 pendulum days, the ratio of the equilibrium state geostrophic response to the equilibrium state Ekman response in this schematic model is NH/C pz; i.e., the ratio of the internal wave speed scale to the speed scale of propagation of the atmosphere frontal disturbance. Therefore, in the real ocean drive by wind stress of finite wavelength at subinertial frequencies, the transient Ekman response to wind stress forcing represents only part of the total response; a transient geostrophic response also exists that ca be as large or larger than the transient Ekman response.

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