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Lynne D. Talley

Abstract

A set of simple analytical models is presented and evaluated for interannual to decadal coupled ocean–atmosphere modes at midlatitudes. The atmosphere and ocean are each in Sverdrup balance at these long timescales. The atmosphere’s temperature response to heating determines the spatial phase relation between SST and sea level pressure (SLP) anomalies. Vertical advection balancing heating produces high (low) SLP lying east of warm (cold) SST anomalies, as observed in the Antarctic circumpolar wave (ACW), the decadal North Pacific mode, and the interannual North Atlantic mode. Zonal advection in an atmosphere with a rigid lid produces low SLP east of warm SST. However, if an ad hoc equivalent barotropic atmospheric response is assumed, high SLP lies east of warm SST. Relaxation to heating produces behavior like the observed North Atlantic decadal pattern, with low SLP over warm SST. Meridional advection in the atmosphere cannot produce the observed SST/SLP patterns.

The dominant balance in the ocean’s temperature equation determines the phase speed of the modes. The coupled mode is nondispersive in all models examined here, indicating the need for additional processes. For modes with an SST–SLP offset as observed in the ACW and North Pacific, Ekman convergence acting as a heat source causes eastward propagation relative to the mean ocean flow. Sverdrup response to Ekman convergence, acting on the mean meridional temperature gradient, causes westward propagation relative to the mean ocean flow. When the ocean temperature adjusts through surface heat flux alone, the mode is advected by the mean ocean flow and is damped.

Relaxation to heating in the atmosphere, when operating with Sverdrup response in the ocean, produces the only complete solution presented here that exhibits growth, with an e-folding timescale of order (100 days). This solution appears appropriate for the North Atlantic decadal mode.

In Northern Hemisphere basins, with meridional boundaries, the same sets of dynamics create the observed SST–SLP phase relation. An additional factor is the creation of SST anomalies through variations in the western boundary current strengths, which are related to the zonally integrated wind stress curl over the whole basin. If barotropic and hence fast adjustment is assumed, the resulting positive feedback can maintain or strengthen the coupled anomalies in the North Pacific and interannual North Atlantic modes.

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Lynne D. Talley

Abstract

The North Pacific Intermediate Water (NPIW), defined as the main salinity minimum in the subtropical North Pacific, is examined with respect to its overall property distributions. These suggest that NPIW is formed only in the northwestern subtropical gyre; that is, in the mixed water region between the Kuroshio Extension and Oyashio front. Subsequent modification along its advective path increases its salinity and reduces its oxygen.

The mixed water region is studied using all bottle data available from the National Oceanographic Data Center, with particular emphasis on several winters. Waters from the Oyashio, Kuroshio, and the Tsugaru Warm Current influence the mixed water region, with a well-defined local surface water mass formed as a mixture of the surface waters from these three sources.

Significant salinity minima in the mixed water region are grouped into those that are directly related to the winter surface density and are found at the base of the oxygen-saturated surface layer, and those that form deeper, around warm core rings. Both could be a source of the more uniform NPIW to the east, the former through preferential erosion of the minima from the top and the latter through simple advection. Both sources could exist all year with a narrowly defined density range that depends on winter mixed-layer density in the Oyashio region.

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Lynne D. Talley

Abstract

The shallow salinity minimum of the subtropical North Pacific is shown to be a feature of the ventilated, wind-driven circulation. Subduction of low salinity surface water in the northeastern subtropical gyre beneath higher salinity water to the south causes the salinity minimum. Variation of salinity along surface isopycnals causes variations in density and salinity at the minimum.

A model of ventilated flow is used to demonstrate how the shallow salinity minimum can arise. The model is modified to account for nonzonal, realistic winds; it is also extended to examine the three-dimensional structure of the western shadow zone. The boundary between the subtropical and subpolar gyres is given by the zero of the zonal integral of Ekman pumping. The western shadow zone fills the subtropical gyre at the base of the ventilated layers and decreases in extent with decreasing density. For parameters appropriate to the North Pacific, the eastern shadow zone is of very limited extent.

Observations of salinity and potential vorticity within and below the ventilated layer bear out model predictions of the extent of the western shadow zone.

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Lynne D. Talley

Abstract

Vertical sections and maps of potential vorticity ρ−1 f∂ρ/∂z for the North Pacific are presented. On shallow isopycnals, high potential vorticity is found in the tropics, subpolar gyre, and along the eastern boundary of the subtropical gyre, all associated with Ekman upwelling. Low potential vorticity is found in the western subtropical gyre (subtropical mode water), in a separate patch near the sea surface in the eastern subtropical gyre and extending around the gyre, and near sea-surface outcrops in the subpolar gyre; the last is analogous to the subpolar mode water of the North Atlantic and Southern Ocean.

Meridional gradients of potential vorticity are high between the subtropical and subpolar gyres at densities which outcrop only in the subpolar gyre; lateral gradients of potential vorticity are low in large regions of the subtropical gyre on these isopycnals. On slightly denser isopycnals which do not outcrop in the North Pacific, there are large regions of low potential vorticity gradients which cross the subtropical-subpolar gyre boundary. These regions decrease in area with depth and vanish between 2500 and 3000 meters. Regions of low lateral gradients of potential vorticity are surrounded by and overlie regions where the meridional gradient of potential vorticity is approximately β. In the abyssal waters, below 3500 meters, meridional potential vorticity gradients again decrease, perhaps associated with slow geothermal heating. The depth and shape of the region wheel potential vorticity is relatively uniform or possesses closed contours is noted and related to theories of wind-driven circulation.

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Lynne D. Talley

Abstract

Initial mixing between the subtropical and subpolar waters of Kuroshio and Oyashio origin occurs in the mixed water region (interfrontal zone) between the Kuroshio and Oyashio. The relatively fresh water that enters the Kuroshio Extension from the Mixed Water Region is this already mixed subtropical transition water. Subtropical transition water in the density range 26.64–27.4 σ θ can be considered to be the newest North Pacific Intermediate Water (NPIW) in the subtropical gyre; this density range is approximately that which is ventilated in the subpolar gyre with significant influence from the Okhotsk Sea. Freshening of the Kuroshio Extension core occurs between 140° and 165°E in the upper part of the NPIW (26.64–27.0 σ θ), with the greatest freshening associated with the eastern side of the first and second Kuroshio meanders. Kuroshio Extension freshening in the lower part of the NPIW (27.0–27.4 σ θ) occurs more gradually and farther to the east. There is nearly no distinction in water properties north and south of the Kuroshio Extension by 175°W. The upper part of the NPIW in the Mixed Water Region progresses from very intrusive and including much freshwater in the west, to much smoother and more saline water in the east. The lower part of the NPIW in the mixed water region progresses from very intrusive and fresh in the far west, to noisy and more saline at 152°E, to smooth and fresher in the east. These suggest a difference between the two layers in both advection direction and possibly transport across the Subarctic Front. Assuming that all waters in the region are an isopycnal mixture of subtropical and subpolar water, the zonal transport of subpolar water in the subtropical gyre at 152°E is estimated at about 3 Sv (Sv ≡ 106 m3 s−1). This could be approximately one-quarter of the Oyashio transport in this density range.

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Lynne D. Talley

Abstract

The ocean's overturning circulation and associated heat transport are divided into contributions based on water mass ventilation from 1) shallow overturning within the wind-driven subtropical gyres to the base of the thermocline, 2) overturning into the intermediate depth layer (500–2000 m) in the North Atlantic and North Pacific, and 3) overturning into the deep layers in the North Atlantic (Nordic Seas overflows) and around Antarctica. The contribution to South Pacific and Indian heat transport from the Indonesian Throughflow is separated from that of the subtropical gyres and is small. A shallow overturning heat transport of 0.6 PW dominates the 0.8-PW total heat transport at 24°N in the North Pacific but carries only 0.1–0.4 PW of the 1.3-PW total in the North Atlantic at 24°N. Shallow overturning heat transports in the Southern Hemisphere are also poleward: −0.2 to −0.3 PW southward across 30°S in each of the Pacific and Indian Oceans but only −0.1 PW in the South Atlantic. Intermediate water formation of 2 and 7 Sv (1 Sv ≡ 106 m3 s−1) carries 0.1 and 0.4 PW in the North Pacific and Atlantic, respectively, while North Atlantic Deep Water formation of 19 Sv carries 0.6 PW. Because of the small temperature differences between Northern Hemisphere deep waters that feed the colder Antarctic Bottom Water (Lower Circumpolar Deep Water), the formation of 22 Sv of dense Antarctic waters is associated with a heat transport of only −0.14 PW across 30°S (all oceans combined). Upwelling of Circumpolar Deep Water north of 30°S in the Indian (14 Sv) and South Pacific (14 Sv) carries −0.2 PW in each ocean.

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Xiaojun Yuan and Lynne D. Talley

Abstract

CTD/STD data from 24 cruises in the North Pacific are studied for their vertical salinity structure and compared to bottle observations. A triple-salinity minimum is found in two separated regions in the eastern North Pacific. In the first region, bounded by the northern edge of the subarctic frontal zone and the 34°N front between 160° and 150°W, a middle salinity minimum is found below the permanent pycnocline in the density range of 26.0 and 26.5 σθ. This middle minimum underlies Reid's shallow salinity minimum and overlies the North Pacific Intermediate Water (NPIW). In the second region, southeast of the first, a seasonal salinity minimum appears above the shallow salinity minimum at densities lower than 25.1 σθ. The shallow salinity minimum and the NPIW can be found throughout year, while the seasonal minimum only appears in summer and fall.

The middle and shallow salinity minima, as well as the seasonal minimum, originate at the sea surface in the northeast Pacific. The properties at the minima depend on the surface conditions in their source areas. The source of the middle minimum is the winter surface water in a narrow band between the gyre boundary and the subarctic front west of 170°W. The shallow salinity minimum is generated in winter and is present throughout the year. The seasonal salinity minimum has the same source area as the shallow salinity minimum but is formed in summer and fall at lower density and is not present in winter.

A tropical shallow salinity minimum found south of 18°N does not appear to be connected with the shallow salinity minimum in the eastern North Pacific. South of 20°N, the shallow salinity minimum and the NPIW appear to merge into a thick, low salinity water mass. When an intrusion of high salinity water breaks through this low salinity water mass south of 18°N, this tropical salinity minimum appears at the same density as the shallow salinity minimum. Though the water mass of the tropical minimum is deprived from the water in the shallow salinity minimum, the formation of the vertical minimum is different.

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Lynne D. Talley and Roland A. DeSzoeke

Abstract

A closely spaced hydrographic section from Oabu, Hawaii to 28°N, 152°W and then north along 152°W shows strong eddy or current features with dynamic height signatures of about 30 dyn cm across 150 km and associated geostrophic surface velocities of approximately 60 cm s−1. Two such features are found between Hawaii and the Subtropical Front, which is located at 32°N. Similar features have been observed on a number of other hydrographic and XBT sections perpendicular to the Hawaiian Ridge. It is hypothesized that the features are semipermanent, are due to the presence of the Ridge, and are related to the North Hawaiian Ridge Current of Mysak and Magaard.

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Frederick M. Bingham and Lynne D. Talley

Abstract

No abstract available

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Lynne D. Talley and Jae-Yul Yun

Abstract

The top of the North Pacific Intermediate Water (NPIW) in the subtropical North Pacific is identified with the main salinity minimum in the density range σθ = 26.7–26.8. The most likely source of low salinity for the NPIW salinity minimum is the Oyashio winter mixed layer, of density σθ = 26.5–26.65. The Oyashio waters mix with Kuroshio waters in the broad region known as the Mixed Water Region (MWR), between the separated Kuroshio and Oyashio Fronts just east of Japan. It is shown that cabbeling during mixing of the cold, fresh Oyashio winter mixed layer water with the warm, saline Kuroshio water increases the density of the mixture by up to σθ = 0.07 at densities around σθ = 26.6–26.65, regardless of the mixing mechanism. Thus cabbeling accounts for about half of the observed density difference between the Oyashio winter mixed layer water and the top of the NPIW.

Double diffusion during mixing of the interleaving layers of Oyashio and Kuroshio waters in the MWR can also change the density of the mixing intrusions. Density ratios favorable to double diffusion are shown to be especially prominent in Oyashio intrusions into a Kuroshio warm core ring in the 1989 data examined here. The average potential temperature–salinity profile of the new subtropical NPIW just east of the MWR, with its nearly uniform salinity, suggests the dominance of salt fingering over diffusive layering. Using the observed salinity and density differences between Oyashio surface water and the NPIW salinity minimum, after subtracting the density difference ascribed to cabbeling, an effective flux ratio of about 0.8 is estimated for possible double diffusive processes in the MWR.

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