The western equatorial Pacific is a crossroads for thermocline and intermediate waters formed at higher latitudes. The role of the equatorward flowing, low‐latitude western boundary currents (LLWBCs) in advecting well‐ventilated (with respect to atmospheric gases), higher‐latitude waters varies with density. At densities <26.5 σθ the Mindanao Current (MC) (Wyrtki, 1961; Masuzawa, 1969) advects recently ventilated water observed as tracer maxima predominantly from the North Pacific subtropical gyre (tropical water is <3 years old and the remnant subtropical mode water is <5 years); it branches into the southern Celebes Sea feeding the Indonesian throughflow and toward the east north of the equator. Between 26.5 and 26.8 σθ the MC advects predominantly North Pacific Intermediate Water (having a component that is <20 years old) mainly into the southern Celebes Sea; there is also some indication of a tracer maximum extending eastward north of the equator. However, below 26.8 σθ, South Pacific water masses appear to be stronger, so that they are the major ventilation source for the western equatorial region, including the Celebes Sea. At 27.2 σθ the New Guinea Coastal Undercurrent advects Antarctic Intermediate Water (having a component that is <25 years) into a background of older water. The presence of subtropical mode water in the western tropical North Pacific and Celebes Sea is attributed to an equatorward LLWBC in the North Pacific (and suggests a reason for the absence of 18° water in the tropical North Atlantic). The absence of a LLWBC in the North Atlantic highlights a basic difference between the circulation of the two oceans, which may be due to the different ways they import and export water. At the western boundary in the North Atlantic, warm water is imported and cold water is exported as part of the global thermohaline circulation, whereas at the western boundary in the North Pacific, warm water (above 26.8 σθ) is mainly exported to the Indian Ocean via the Indonesian throughflow and cold water is imported.
A time‐averaged description of the formation and spreading of North Pacific subtropical mode water (NPSTMW) is presented. The formation of NPSTMW is studied using TRANSPAC expendable bathythermograph data. The data are transformed into a stream coordinate system and averaged. The coordinate system uses the position of the Kuroshio front as its origin. Maps are presented of the temperature structure and mixed layer distribution in winter relative to the Kuroshio front. The formation of NPSTMW occurs in two separate stages, cooling and thickening. As water flows at the surface in the Kuroshio in winter, the water is cooled by the intense ocean‐atmosphere heat flux taking place in the area. When water leaves the Kuroshio to the south, the thermocline underneath becomes deeper, convection is able to penetrate deep into the water column, and the heat flux acts more to thicken the surface mixed layer rather than reduce the temperature. The area south of the Kuroshio, where relatively homogeneous waters are found, is the source region for NPSTMW. At the end of the winter the thick mixed layers are assumed to be covered over by a seasonal thermocline and become NPSTMW. The Levitus (1982) data are used to study the way in which NPSTMW is spread away from its source region by the subtropical circulation. The distribution of potential vorticity (PV) on four NPSTMW isopycnals is studied, with low PV used as a tracer for NPSTMW. The PV reveals a pattern of “differential spreading,” whereby NPSTMW formed in the western part of the source region is advected to the west after its formation. NPSTMW formed farther east is advected more to the south or east after formation. Differential spreading allows NPSTMW to be found in a large part of the northwestern subtropical gyre, well away from its source region. The distribution of NPSTMW described by Masuzawa (1969), with NPSTMW getting colder from west to east, is explained as a combination of the distribution of NPSTMW at its source and the way it is spread after it is formed.
[1] The characteristics of the principal barotropic diurnal and semidiurnal tides are examined for the South Atlantic Bight (SAB) of the eastern United States coast. We combine recent observations from pressure gauges and ADCPs on fixed platforms and additional short-term deployments off the Georgia and South Carolina coasts together with National Ocean Service coastal tidal elevation harmonics. These data have shed light on the regional tidal propagation, particularly off the Georgia/South Carolina coast, which is perforated by a dense estuary/tidal inlet complex (ETIC). We have computed tidal solutions for the western North Atlantic Ocean on two model domains. One includes a first-order representation of the ETIC in the SAB, and the other does not include the ETIC. We find that the ETIC is highly dissipative and affects the regional energy balance of the semidiurnal tides. Nearshore, inner, and midshelf model skill at semidiurnal frequencies is sensitive to the inclusion of the ETIC. The numerical solution that includes the ETIC shows significantly improved skill compared to the solution that does not include the ETIC. For the M 2 constituent, the largest tidal frequency in the SAB, overall amplitude and phase error is reduced from 0.25 m to 0.03 m and 13.8°to 2.8°for coastal observation stations. Similar improvement is shown for midshelf stations. Diurnal tides are relatively unaffected by the ETIC.
Abstract. The seasonal variability of surface layer salinity (SLS) is examined in the Pacific Ocean between 40 • S and 60 •
Abstract. The seasonal variability of surface layer salinity (SLS), evaporation (E), precipitation (P ), E − P , advection and vertical entrainment over the global ocean is examined using in situ salinity data, the National Centers for Environmental Prediction's Climate System Forecast Reanalysis and a number of other ancillary data. Seasonal amplitudes and phases are calculated using harmonic analysis and presented in all areas of the open ocean between 60 • S and 60 • N. Areas with large amplitude SLS seasonal variations include: the intertropical convergence zone (ITCZ) in the Atlantic, Pacific and Indian Oceans; western marginal seas of the Pacific; and the Arabian Sea. The median amplitude in areas that have statistically significant seasonal cycles of SLS is 0.19. Between about 60 • S and 60 • N, 37 % of the ocean surface has a statistically significant seasonal cycle of SLS and 75 % has a seasonal cycle of E −P . Phases of SLS have a bimodal distribution, with most areas in the Northern Hemisphere peaking in SLS in March/April and in the Southern Hemisphere in September/October.The seasonal cycle is also estimated for surface freshwater forcing using a mixed-layer depth climatology. With the exception of areas near the western boundaries of the North Atlantic and North Pacific, seasonal variability is dominated by precipitation. Surface freshwater forcing also has a bimodal distribution, with peaks in January and July, 1-2 months before the peaks of SLS. Seasonal amplitudes and phases calculated for horizontal advection show it to be important in the tropical oceans. Vertical entrainment, estimated from mixedlayer heaving, is largest in mid and high latitudes, with a seasonal cycle that peaks in late winter.The amplitudes and phases of SLS and surface fluxes compare well in a qualitative sense, suggesting that much of the variability in SLS is due to E − P . However, the amplitudes of SLS are somewhat different than would be expected and the peak of SLS comes typically about one month earlier than expected. The differences of the amplitudes of the two quantities is largest in such areas as the Amazon River plume, the Arabian Sea, the ITCZ and the eastern equatorial Pacific and Atlantic.
Subfootprint variability (SFV), variability within the footprint of a satellite measurement, is a source of error associated with the validation process, especially for a satellite measurement with a large footprint such as those measuring sea surface salinity (SSS). This type of error has not been adequately quantified in the past. In this study, I have examined SFV using in situ ocean data from the SPURS-1 (Salinity Processes in the Upper ocean Regional Studies-1) and SPURS-2 field campaigns in the subtropical North Atlantic and eastern tropical North Pacific respectively. I computed SFV from these data over two one-year periods of intense sampling. The results show that SFV is highly seasonal. I have computed SFV errors in several different forms, a median value of the weekly snapshot error, a total snapshot error, an absolute error of the Aquarius and SMAP (Soil Moisture Active Passive) measurement, a part of that error associated with SFV and a bias due to the skewness of the distribution of SSS. These results are characteristic only of the particular regions studied. However, comparison of the results with high resolution models, and in situ data from moorings gives the possibility of getting global estimates of SFV from these other more common sources of SSS data.
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