空气净化材料Word文档格式.docx

1、In the eastern South Pacific Ocean, at a depth of about 200 m, a salinity minimum is found. This minimum is associated with a particular water mass, the Shallow Salinity Minimum Water （SSMW）. SSMW outcrops in a fresh tongue （Ssub min） centered at about 45S. The Ssub min appears to emanate from the e

2、astern boundary, against the mean flow. The watermass transformation that creates SSMW and Ssub min is investigated here. The Ssub min and SSMW are transformed from saltier and warmer waters originating from the western South Pacific. The freshening and cooling occur when the water is advected eastw

3、ard at the poleward side of the subtropical gyre. Sources of freshening and cooling are air-sea exchange and advection of water from south of the subtropical gyre. A freshwater and heat budget for the mixed layer reveals that both sources equally contribute to the watermass transformation in the mix

4、ed layer. The freshened and cooled mixed layer water is subducted into the gyre interior along the southern rim of the subtropical gyre. Subduction into the zonal flow restricts the transformation of interior properties to diffusion only. A simple advection/diffusion balance reveals diffusion coeffi

5、cients of order 2000 msup 2 ssup -1. The tongue shape of the Ssub min is explained from a dynamical viewpoint because no relation to a positive precipitation-evaporation balance was found. Freshest Ssub min values are found to coincide with slowest eastward mixed layer flow that accumulates the larg

6、est amounts of freshwater in the mixed layer and creates the fresh tongue at the sea surface. Although the SSMW is the densest and freshest mode of water subducted along the South American coast, the freshening and cooling in the South Pacific affect a whole range of densities （25.0-26.8 kg msup -3）

7、. The transformed water turns northward with the gyre circulation and contributes to the hydrographic structure of the gyre farther north. Because the South Pacific provides most of the source waters that upwell along the equatorial Pacific, variability in South Pacific hydrography may influence equ

8、atorial Pacific hydrography. Because one-half of the transformation is found to be controlled through Ekman transport, variability in wind forcing at the southern rim of the subtropical gyre may be a source for variability of the equatorial Pacific. （ProQuest Information and Learning: . denotes form

9、ulae omitted.） 1. Introduction The tropical/extratropical exchange of water can be viewed as a meridional-vertical or subtropical cell （STC） driven by subduction and upwelling, which are connected via Ekman transport and interior flow. In the extratropics water is subducted from the mixed layer and

10、flows equatorward in interior wind-driven pathways and western boundary currents. Near the equator this water upwells back into the mixed layer and is transported poleward to the subduction sites through the meridional component of the Ekman transport see Schott et al. （2004） for an STC review. STCs

11、 have been identified in all oceans using observational data （e.g., Johnson and McPhaden 1999; Schott et al. 2002; Zhang et al. 2003） and models （e.g., McCreary and Lu 1994; Rothstein et al. 1998）. One interest in studying the STCs is their potential involvement in low-frequency climate variability

12、of the ocean-atmosphere system, because both upwelling and subduction are controlled through air-sea exchange of momentum, heat, and freshwater. In particular the variability of the Pacific Ocean STC is of importance, because it may be related to El Nio-Southern Oscillation （ENSO） with its far-reach

13、ing socioeconomic impacts. Pacific variability has been decomposed into interannual and interdecadal variability （Zhang et al. 1997）. Interannual variability is usually associated with the ENSO phenomenon and may be explained through atmosphere-ocean interplay near the equator. Decadal variability,

14、however, could be the result of variability that is generated in the extratropical ocean and subsequently advected to the equator. Two mechanisms have been proposed to generate variability based on temperature （T） and transport （v）. Kleeman et al. （1999） proposed a model solely based on fluctuations

15、 in STC transport, that is, without fluctuations in the temperature field （vT）. Fluctuations in transport generate sea-surface temperature fluctuations that could feed back on the atmospheric circulation. Observational evidence for transport fluctuations in the near-equatorial flow has been presente

16、d by, for example, McPhaden and Zhang （2002） and Meinen et al. （2001）. Gu and Philander （1997） proposed a mechanism based on the advection of temperature anomalies with an average flow field （vT）. Subducted temperature anomalies in the extratropics appear with a time lag at the equator. Their upwell

17、ing causes temperature anomalies to appear in the Tropics that feed back onto the atmospheric meridional circulation. The appearance of a warm anomaly along the equator strengthens the extratropical wind and, through an increase of evaporation, introduces a cold anomaly in the extratropics. The subd

18、uction of such a cold anomaly then appears with a time lag along the equator where its upwelling initiates a cold anomaly there. However, the persistence of temperature anomalies subducted into the subtropical gyre is currently under debate. Hindcast runs of coupled ocean-atmosphere models suggest t

19、hat equatorial Pacific isothermal depth variability may be generated by the local wind stress （and Ekman pumping） variability at the equator rather than from anomalies of extratropical origin （Schneider et al. 1999）. In particular for the North Pacific little coupling between Tropics and extratropic

20、s was found. However, Yeager and Large （2004） identified sea-surface temperature variability along the equator generated through isopycnal advection of not only temperature but temperature/salinity （T/S） anomalies. They analyzed output of an ocean model forced with 40 years of realistic surface flux

21、es. Their work emphasizes the role of both heat and freshwater anomalies in decadal variability. McCreary and Lu （1994） showed that thermocline water moving equatorward originates from the east and poleward side of the subtropical gyres. This suggests for the South Pacific （SP） that the Southern Oce

22、an （SO） can play a role in ventilating the Pacific equatorial thermocline （Toggweiler et al. 1991; Johnson 2001）. A prominent exchange path from the SO toward the equator in all Southern Hemisphere oceans is the fresh tongue of Antarctic Intermediate Water （AAIW）. AAIW has its origin in the subducti

23、on of polar surface waters （see, e.g., Molinelli 1978） and spreads at the base of the subtropical gyres, at depths between 600 and 1000 m. However, with a core density anomaly of about 27.2 kg msup -3, AAIW lies below the water that upwells along the Pacific equator. Upwelling along the Pacific equa

24、tor is, rather, fed in the density range of waters advected in the Equatorial Undercurrent （EUC） and the subsurface countercurrents （SCC） （Rowe et al. 2000）, the subsurface pathways from west to east （and deep and shallow） along the equator. The equatorial Pacific is fed by about 60% from the SP. Th

25、e densest water that upwells is 26.8 kg msup -3 （Johnson and McPhaden 1999; Rodgers et al. 2003）. A second fresh tongue is found above AAIW in the interior of the southeast SP （at about 26 kg msup -3; see Fig. 1） named （SSMW; Reid 1973; Tsuchiya and Talley 1996） or Eastern South Pacific Intermediate

26、 Water （Emery and Meincke 1986; Schneider et al. 2003）. SSMW is found in both hemispheres. Its formation has been explained through the sinking of subantarctic surface waters below higher-salinity waters （e.g., Reid 1973）. Consequently one expects a corresponding signature in the sea surface salinit

27、y where SSMW is subducted. North Pacific surface waters freshen north of the subtropical gyre and along the eastern boundary （Fig. 1） so that the conceptual model for SSMW formation holds here. In the SP, however, a local meridional fresh tongue is located at about 45S. Consequently, a simple northw

28、ard transfer of fresh surface water with subsequent subduction cannot explain the formation of SSMW in the SP entirely. SSMW outcrops in a fresh surface tongue （Ssub min） （see Fig. 1）. The shape of Ssub min prompted a number of investigators to infer a westward flow at the southern rim of the SP sub

29、tropical gyre （see Deacon 1977 for a review）. Such a flow, in combination with coastal fresh-water input and a positive precipitation-evaporation balance, was thought to generate the surface salinity pattern with lowest salinities in the east （Davila et al. 2002; Schneider et al. 2003）. However, a c

30、lear signal of westward flow between the eastward-flowing subtropical gyre and the eastward-flowing extension of the ACC was never detected. Neshyba and Fonseca （1980） suspected transient eddies contributed to the westward transport but data coverage was too sparse to prove this idea. In this paper

31、we study watermass transformation in the southern part of the SP subtropical gyre to explain the formation of Ssub min and SSMW. After introducing the data, transformation mechanisms for mixed layer and thermocline waters are discussed. A zonal mixed layer budget is used to quantify the importance o

32、f air-sea exchange and advection. Then the role of diffusion in the watermass transformation is discussed. The formation of Ssub min is explained as an advective feature. Last, variability of the watermass transformation and its relation to decadal variability of the hydrographic structure of the SP and Equatorial Pacific are discussed. 2. Data The ocean surface and interior data products used in this study are mainly the 1990-99 average temperature, salinity, and velocity fields from