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Gary Meyers

Abstract

The annual variation in the depth of the 14°C isotherm is described on the basis of the mechanical and expendable bathythermographic data in the archives of the National Oceanographic Data Center. The variations are representative of variations in the depth of the main thermocline. The 14°C isotherm slopes downward toward the west between 100°W and the dateline throughout the year. The slope is small during May and June, and large during October and November. However, variations in isothermal depth with a smaller spatial scale lead to considerable changes in phase of the slope at different longitudes. The easterly wind stress along the equator is weak during March, April and May, while the Intertropical Convergence Zone is located close to the equator. The westward slope of 14°C relaxes during this period, leading to the minimum slope in June. The easterly wind stress is strong during the winter of each hemisphere with maxima in July and December. Strong winds precede and follow the maximum slope in October. Eastward propagation of energy along the equator is suggested by the appearance of second harmonic (2 cycles per year) variations in isothermal depth in the eastern Pacific, remote from wind stress forcing at this frequency in the central Pacific. The phase of variations in isothermal depth and wind stress increases toward the west, suggesting a forced wave propagating toward the west.

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Gary Meyers

Abstract

Geostrophic transports in the upper 300 m relative to 1000 db are computed from observations along four meridional sections occupied monthly during the Trade Wind Zone Oceanography Study, February 1964 to June 1965. Transports varied from a minimum of about 11×106 m3 s−1 in February of both 1964 and 1965 to a maximum of 23×106 m3 s−1 in September 1964. The ridge in the thermocline at the southern edge of the current rose or fell simultaneously as the transport increased or decreased. Theoretical vertical displacements were computed from the wind stress curl using a formula derived by Yoshida and Mao. The observed vertical displacements of the thermocline are similar to the computed displacements. Thus, the observations of the Trade Wind Zone Oceanography Study are in agreement with the simple model of variation in the strength of equatorial currents that was developed by Yoshida and by Fedorov. According to the model, the divergence of Ekman transports cause variation in the meridional slope of the thermocline and in the geostrophic current speed.

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Gary Meyers

Abstract

Annual variation in the depth of the 14°C isotherm (i.e., the main thermocline) throughout the tropical Pacific Ocean between 30°N and 30°S is studied on the basis of 156 000 bathythermographs. Large-amplitude variations are confined in the region between 4 and 15°N. Near 6°N the variations in depth propagate westward. Near 12°N they have almost the same phase across the ocean from the American coast to 145°E. These variations are approximately consistent with a simple model that permits an oceanic response to local Ekman pumping, modified by nondispersive, baroclinic Rossby waves forced by the wind. Near 12°N, the rate of change in thermocline depth is nearly in phase with the Ekman pumping velocity, with only a minor but significant contribution coming from Rossby wave propagation. This type of response depends critically on variations in the eastern boundary region. Near 6°N, the westward propagating variations are generated by relatively large variability in Ekman pumping in the eastern Pacific, and apparently travel into the western Pacific as free nondispersive Rossby waves. Deficiencies of the model are also discussed.

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Gary Meyers

Abstract

The relationship between typical seasonal and interannual variations in Truk sea level is discussed. Periods of extreme low sea level occur episodically, at the same time that El Niño occurs in the eastern tropical Pacific. The episodes have a well-defined phase relative to the seasons and a persistence of one year. They mimic an annual cycle, but they do not repeat every year. Nevertheless, they distort the long-term mean seasonal cycle because they are very energetic. Complex demodulation shows that the amplitude of the annual oscillation is bimodal, the larger amplitude occurring during the episodes of low sea level. The phase is stable only when the amplitude is large. The amplitude and phase of the semiannual oscillation are stable; the range is approximately one-third as large as that of the interannual episodes. Thus, the seasonal cycle is bimodal in the sense that it is dominated by either the annual episode or the stable semiannual oscillation. The climate-related sea-level variation at Truk and other tropical islands is shown to be largely due to vertical displacement of the thermocline.

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Klaus Wyrtki and Gary Meyers

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The trade wind field over the Pacific Ocean between 30°N and 30°S has been studied on the basis of five million wind observations made from ships. Data were sorted by quadrangles of 2° latitude and 10° longitude to resolve north-south gradients in the wind field adequately. Maps of the surface wind stress vector for February and August are presented and the development of the field throughout the year is discussed. The trade wind regime in each hemisphere is largest and strongest during the respective winter and spring. The area covered by northeast trades is smaller than the area covered by southeast trades, but the northeast trades have a stronger mean wind stress and a larger annual variation both in area and mean stress.

The computed divergence of the wind velocity revealed a little known area of convergence in the southeastern Pacific near the equator. The curl of the wind stress and the meridional profile of zonal wind stress vary considerably during the year. The minimum in zonal stress between the northeast and the southeast trades is a poorly developed feature from January to March while the maximum northeast trades are south of their mean position. Averaging the zonal stress over the South Pacific is deceptive because of large zonal variations.

Time series of the Pacific Ocean area covered by northeast and southeast trades, the mean zonal wind stress within the area, and the mean zonal wind stress in large, fixed areas are presented for the period from 1947 to 1972. The southeast trades covered an anomalously large area during 1955–56, 1964, 1966–67 and 1970–71. Interannual variations of the northeast trades are smaller than those of the southeast trades. There is no apparent relationship between fluctuations in the strength and areal extent of the northeast and southeast trades, except with the annual cycle.

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Tangdong Qu and Gary Meyers

Abstract

The circulation in the southeastern tropical Indian Ocean is studied using historical temperature and salinity data. A southward shift of the subtropical gyre at increasing depth dominates the structure of the annual mean circulation. Near the southern Indonesian coast the westward South Equatorial Current (SEC) is at the sea surface and strongest near 10°–11°S, reflecting strong influence of the Indonesian Throughflow (ITF). In latitudes 13°–25°S the SEC is a subsurface flow and its velocity core deepens toward the south, falling below 500 m at 25°S. The eastern gyral current (EGC) is a surface flow overlying the SEC, associated with the meridional gradients of near-surface temperature and salinity. The ITF supplies water to the SEC mainly in the upper 400 m, and below that depth the flow is reversed along the coast of Sumatra and Java. Monsoon winds strongly force the annual variation in circulation. Dynamic height at the sea surface has a maximum amplitude at 10°–13°S, and the maximum at deeper levels is located farther south. Annual variation is also strong in the coastal waveguides, but is mainly confined to the near-surface layer. Although the South Java Current at the sea surface is not well resolved in the present dataset, semiannual variation is markedly evident at depth and tends to extend much deeper than the annual variation along the coast of Sumatra and Java.

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Susan Wijffels and Gary Meyers

Abstract

Temperature and sea level variability within the Indonesian seas and southeast Indian Ocean are described based on expendable bathythermograph deployments along volunteer merchant shipping lines under way since 1983. These data resolve variability at time scales ranging from the intraseasonal to the interannual. A lagged partial regression technique reveals that anomalies from a mean seasonal cycle of temperature and sea level for seasonal to interannual time scales can be largely understood in terms of free Kelvin and Rossby waves generated by remote zonal winds along the equator of the Indian and Pacific Oceans, with local wind forcing appearing to play a minor role. About 60%–90% of sea level variability and 70% of thermocline temperature variability can be accounted for in this way. Variations in zonal Pacific equatorial winds force a response along the Arafura/ Australia shelf break through Pacific equatorial Rossby waves exciting coastally trapped waves off the western tip of New Guinea, which propagate poleward along the Australian west coast. The signature of this Pacific energy radiating westward across the Banda Sea and into the subtropical south Indian Ocean within 1500 km of the coast is also prevalent. Equatorial Indian Ocean wind energy propagates along the Sumatra–Java–Nusa Tenggara waveguide to penetrate the Savu Sea, the western Banda Sea and Makassar Strait, thus having an impact on the western internal seas. Hence the region comprises the intersection of two ocean waveguides, as first predicted by Clarke and Liu.

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Gaël Alory and Gary Meyers

Abstract

In the equatorial Indian Ocean, sea surface has warmed by 0.5°–1°C over the 1960–99 period, while waters have cooled at thermocline depth and the net atmospheric heat flux has decreased. Among a set of twentieth-century climate simulations from 12 coupled models, the Centre National de Recherches Météorologiques Coupled Global Climate Model version 3 (CNRM-CM3) reproduces key observed features of these changes. It is used to investigate changes in the heat budget of the upper equatorial Indian Ocean and identify mechanisms responsible for the warming. By comparing twentieth-century and control simulations, significant shifts in the mean balance of the heat budget between the preindustrial and the 1960–99 periods can be identified. The main cause of the surface warming is a decrease in the upwelling-related oceanic cooling. It occurs in the thermocline dome region because of a slowdown of the wind-driven Ekman pumping. The observed decrease in net heat flux is a negative feedback driven by evaporation, which is enhanced by the equatorial warming and associated strengthening of trade winds.

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Stephen E. Pazan and Gary Meyers

Abstract

Linear regression analysis of tropical Pacific winds versus a Southern Oscillation Index (SOI) show departures from the seasonal mean wind associated with the phase of SOI. When the SOI is low the largest departures are westerlies on the equator and in the subtropics west of the dateline. The Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ) shift equatorward. Departures from mean wind are strongest from September through February and weakest from March to May. The usual pattern of cyclonic wind stress curl in the tropics and anticyclonic wind stress curl in the subtropics intensifies when the SOI is low. Effects of these changes on ocean circulation are discussed and compared to changes in ocean circulation data.

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Andreas Schiller, Gary Meyers, and Neville R. Smith

Abstract

No abstract available.

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