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The article of record as published may be found at https://doi.org/10.1029/2011JC006975 Data collected by an autonomous ice-based observatory that drifted into the Eurasian Basin between April and November 2010 indicate that the upper ocean was appreciably fresher than in 2007 and 2008. Sea ice and snowmelt over the course of the 2010 drift amounted to an input of less than 0.5 m of liquid freshwater to the ocean (comparable to the freshening by melting estimated for those previous years), while the observed change in upper-ocean salinity over the melt period implies a freshwater gain of about 0.7 m. Results of a wind-driven ocean model corroborate the observations of freshening and suggest that unusually fresh surface waters observed in parts of the Eurasian Basin in 2010 may have been due to the spreading of anomalously fresh water previously residing in the Beaufort Gyre. This flux is likely associated with a 2009 shift in the large-scale atmospheric circulation to a significant reduction in strength of the anticyclonic Beaufort Gyre and the Transpolar Drift Stream. This work was funded by the National Science Foundation Office of Polar Programs Arctic Sciences Section under awards ARC‐0519899, ARC‐0856479, and ARC‐ 0806306.

[1] We investigate basin-scale mechanisms regulating anomalies in freshwater content (FWC) in the Beaufort Gyre (BG) of the Arctic Ocean using historical observations and data collected in 2003–2007. Specifically, the mean annual cycle and interannual and decadal FWC variability are explored. The major cause of the large FWC in the BG is the process of Ekman pumping (EP) due to the Arctic High anticyclonic circulation centered in the BG. The mean seasonal cycle of liquid FWC is a result of interplay between the mechanical (EP) and thermal (ice transformations) factors and has two peaks. One peak occurs around June–July when the sea ice thickness reaches its minimum (maximum ice melt). The second maximum is observed in November–January when wind curl is strongest (maximum EP) and the salt input from the growing ice has not yet reached its maximum. Interannual changes in FWC during 2003–2007 are characterized by a strong positive trend in the region varying by location with a maximum of approximately 170 cm a � 1 in the center of EP influenced region. Decadal FWC variability in the period 1950–2000 is dominated by a significant change in the 1990s forced by an atmospheric circulation regime change. The center of maximum FWC shifted to the southeast and appeared to contract in area relative to the pre-1990s climatology. In spite of the areal reduction, the spatially integrated FWC increased by over 1000 km 3 relative to climatology.

vertical heat fluxes through the staircase are in the range 0.05–0.3 W m � 2 , only about one tenth of the estimated mean surface mixed layer heat flux to the sea ice. It is thus concluded that the vertical transport of heat from the Atlantic Water in the central basin is unlikely to have a significant impact to the Canada Basin ocean surface heat budget. Icebreaker conductivity-temperature-depth data from the Beaufort Gyre Freshwater Experiment show that the staircase is absent at the basin periphery. Turbulent mixing that presumably disrupts the staircase might drive greater flux from the Atlantic Water at the basin boundaries and possibly dominate the regionally averaged heat flux.

)'?Several 5 to 10 m s-I westerly wind bursts of 10-15 days' duration occurred in the western equatorial Pacific during November 1989 to January 1990. The response to these wind bursts was characterized by a 400- to 600-km-wide eastward jet in the upper 100-150 m along the equator between 135"E and the date line. Flow in this jet accelerated to speeds of over i00 cm s-' within I week after the onset of westerly winds in November 1989 in association with supertyphoon Irma. In addition. a 20 to 40 cm s-' westward counterflow developed between 3"N and 2"s below the surface jet separating it from the eastward flow of the Equatorial Undercurrent in the thermocline. Changes in surface layer zonal volume transports in the western Pacific due to westerly wind bursts were 25-S6 Sv based on comparison of three shipboard velocity transects in November and December 1989, Although fluctuations in current speeds in the thermocline were generally smaller and less directly related to local wind forcing than those in the surface layer. the Equatorial Undercurrent decelerated to less than 20 cm s-' (Le., less than half its speed before the onset of westerlies) by early December 1989. Westerly winds also produced a strong surface convergence in both the meridional and zonal directions west of the date line. In late November 1989, this Convergence was associated with downwelling of 2-3 m d-' along lhe equator, and a 20- to 30-m depression of the thermocline. In addition, sea level rose by 10 dyn cm from mid-November to mid-December between 1"s and 3"N. 16S"E. 'Sea surface temperature dropped over a large region by 0.2"-0.5"C during episodes of high wester(, winds, and some episodes of high winds were associated with 0.2"-0.3"C sea surface temperature inversions supported by salt-stratified buoyancy gradients in the upper 100 m. The near-surface salinity balance was dominated by lateral advection when relatively fresh water from north of a salinity front centered near 3"N was advected southward by Ekman drift in late November 1989. Displacement of this front past a current meter mooring at O". 16S"E. appeared as a local drop of 1 PSU in surface layer salinity in 10 days. These results are discussed in the context of a swing in the climate system towards EI NiiioSouthem Oscillation-like conditions in early 1990.

Twenty-one oceanographic sections made along 165°E during 1984–1988 provide a unique picture of the 1986–1987 El Nino and the subsequent La Nina in the western equatorial Pacific. The mean of six cruises from January 1984 through June 1986, a relatively normal period, provides a reference with which the later sections are compared. The net warm water transport across 165°E within 10° of the equator was small in this mean reference section: 7 × 106 m3 s−1 to the east. In December 1986, strong westerly winds at and to the west of 165°E increased the net eastward transport of warm water to 88 × 106 m3 s−1, and the 1986–1987 El Nino was underway. During the following 2 years the net transport varied widely and rapidly; the extrema were 56 × 106 m3 s−1 to the east and 58 × 106 m3 s−1 to the west. Changes in the stratification along 165°E were correspondingly large, reflecting both the geostrophic balance of the strong zonal currents and the changes in the volume of warm water in the western equatorial Pacific. The anomaly of warm water volume corresponded closely to the time integral of the warm water transport across 165°E. Local wind forcing and remotely forced waves were both important causes of the transport fluctuations. Winds, precipitation, and currents were all important factors determining the depth of the surface mixed layer and the thickness of the underlying barrier layer. The way in which these factors interact is a strong function of latitude.

Medium- and large-scale oceanographic variability in the Sargasso Sea is examined during the Frontal Air-Sea Interaction Experiment (FASINEX), focusing primarily on processes that influence the formation of subtropical fronts. From Fall to Spring the mean meridional gradient of meridional Ekman transport in the Subtropical Convergence Zone (STCZ) enhances the meridional sea surface temperature (Ts) gradients between 26 o and 32oN. In the presence of this enhanced mean gradient, baroclinic eddies with zonal wavelengths of =800 km and periods of =200 days exert the dominant influence on the formation of subtropical fronts at medium and large scales. These eddies generate westward propagating Ts anomaly features with the same dominant wavelengths and periods. They are confined between 26 o and 32oN and have amplitudes that occasionally exceed +_ IoC. Ts fronts tend to be found within bands =200 km wide that roughly follow the periphery of these anomaly features. Deformation in the horizontal eddy current field is primarily responsible for the existence of these frontal bands. The migration of the strong front originally bracketed by the FASINEX moored array was related to the westward propagation of the larger-scale eddy/anomaly/frontal-band pattern. The moored array was located within a warm-anomaly feature during most of the experiment, which produced exceptionally warm conditions in the upper ocean. These anomalies are confined between 26 o and 32oN, not only because the relatively large seasonal mean Tsy there allows horizontal eddy currents to force strong anomalies, but also because the baroclinic eddies with wavelengths of =800 km and periods of =200 days are confined to the STCZ. Large meridional variability exists in many properties of the eddy field, much of which can be traced to the influence of the Sargasso Sea mean current field on eddy variability.

Two recent sections of temperature and salinity along the equator in the western Pacific were examined in the context of the historical hydrographic data. Both sections occurred during the December–February period when climatological mean winds are northwesterly. The historical data indicate that the zonal sea surface slope responds to the seasonal change in wind stress. During the southeasterly monsoon the historical mean surface relative to 500 dbar dynamic height slopes upwards from east to west; during the northwesterly monsoon this slope reverses. Vertical profiles of the mean zonal pressure gradient relative to 500 dbar suggest that the wind influence may extend quite deep in the water column. The two quasi-synoptic sections both indicated that the sea level sloped downward towards the west between 155°E and 140°E. The sign of this slope agrees with the historical seasonal mean; however, the zonally averaged sea level in this westernmost region changed by nearly 0.1 dyn m between sections. Upper ocean water properties and stratification confirmed the importance of salinity in determining mixed layer depth. Comparisons of repeat stations collected during a 16-day period indicated relatively rapid changes in near surface properties and the importance of lateral advection.

An intensive three-dimensional survey of the Antarctic Polar Front was made in the Drake Passage in March 1976. The front, which was imbedded within one of the high-velocity cores of the circumpolar current, is viewed as a water mass boundary demarking the northern extent of near-surface antarctic waters. Within the front, water masses are observed to intrude, one above the other, with characteristic vertical scales of 50–100 m. The intrusions are horizontally anisotropic, being elongated in the alongstream direction and constrained primarily to the upper 800 m of the front. The spatial and temporal persistence of the variability is examined through the analysis of continuous vertical profiles of horizontal velocity, temperature, salinity, and oxygen with discrete sampling of nutrients. Analysis of the velocity data showed the mean current flowing to the NNE with speeds of the order of 30–40 cm s−1 in the upper 600 m, with temporal variability over a 28-hour ‘yo-yo’ due primarily to internal gravity waves. The thermohaline variability was not internal wave induced but rather was associated with nearly isentropic advection of different water masses across the front. Cold fresh and warm salty intrusions did not conserve potential density, however, and double-diffusive transfers are strongly suggested as being crucial to an understanding of the dynamics of the intrusions. Applying a model (Joyce, 1977) for lateral mixing we estimate poleward temperature and salinity fluxes due to interleaving of 0.086°C cm s−1 and 0.069‰ cm s−1, respectively. If these values are typical, interleaving could play a significant role in large-scale balance of salt and, to a lesser extent, heat for the Southern Ocean.

Profiles of currents, density, and microstructure were obtained in the Pacific Ocean on and near the equator at 140°W in late 1984 as part of the Tropic Heat program. During a 4½-day time series on the equator, the shear zone above the core of the undercurrent had very low mean Richardson numbers, Ri, between ¼ and ½. Average turbulent dissipation rates, e, were high in this zone, ≈ 10−7 W kg−1, and varied by a factor of 100 between minima in the early afternoon and maxima at night. The signal reached to 90 m, well into the stratified zone. Eddy coefficients, K, were low at high Ri, and gradually increased as Ri decreased, until they rose dramatically near Ri = ½. The observed K(Ri) relations are specific to time, location, and temporal and spatial resolution of the data.

Four meridional transects of the turbulent dissipation rate e and of shear were taken across the equator along 140°W in late 1984. Averaging the upper 110 m of the transects, we found that the vertically averaged e as well as shear showed maximum values within ±1° latitude. Because a strong diurnal cycle of e aliased the measurements, we cannot extract spatial variations with good accuracy. However, when e is separated into the contributions from the thermocline and from the surface mixed layer, consistent patterns emerge. In the thermocline, e was strongest where the Richardson number was lowest, i.e., near the equator—consistent with the production of turbulence by the shear of the undercurrent. In the surface mixed layer near the equator, e was usually larger than predicted by similarity scaling, but the discrepancy varied with the diurnal cycle of shear. When the mean shear was low, after prolonged mixing by nighttime convection, e was close to the similarity scaling. We therefore attribute the excess dissipation to shear production.

CTD data collected about the equator along 85°W between October 8 and 12, 1982, are used to investigate stratification changes in the upper 1 km associated with the 1982/83 El Nino. With respect to the thermal field observed in November 1981, virtually the entire section exhibited downward displacements in the upper kilometer of the water column. Observed displacements at the depth of the mean thermocline were large (∼150 m) and relatively symmetric about the equator. Displacements around 1 km depth were near zero. This stratification change appears to have been produced by a zonal convergence of mass at temperatures above 15°C, with divergence below. Least square modal decompositions of the vertical displaced profiles revealed significant contributions from modes 1 and 2 with meridional structure consistent with Kelvin waves. The observations thus support current theories that relate El Nino onset in the eastern Pacific to anomalous wind forcing in the west.

The structure of the near surface waters in the Southern Ocean, poleward of the Antarctic Polar Front but away from continental margins, is investigated with a three-dimensional time-dependent numerical model which resolves the annual sea ice cycle. The growth and decay of the ice field is predicted, using one of Semtner's (1976) thermodynamic ice models, in terms of specified atmospheric data and computed thermohaline characteristics of the ocean layers. The ice field is found to be sensitive to the lateral advection of heat by the oceanic circulation as well as vertical heat transports due to deep winter convection. The model treats the temperature minimum layer in the Southern Ocean as the remnant of a deep winter mixed layer which becomes capped by surface heating and precipitation in summer. The predicted thermohaline characteristics of the temperature minimum layer and the surface mixed layer are in good agreement with observations. Finally, the annual air-sea heat exchange predicted by the model is discussed. The model's predicted area averaged heat loss experienced by the ocean south of the Atlantic Polar Front is much smaller than was previously estimated. This is attributed to errors in the southern region of the model domain. It is suggestedmore » that models, which include the interactions between sea ice and surface, deep, and bottom waters along the continental margins, are needed to investigate this region.« less

Over a 1-month period (October 5 to November 6, 1982) during the initial stages of the 1982–1983 El Nino, hydrographic and velocity profiling measurements were made in the eastern equatorial Pacific by two research vessels. Track lines approximated a square, centered near the Galapagos Islands with approximately meridional sections at 95°W and 85°W and nearly zonal sections at about 5°N and 5°S. Water mass characteristics and estimated currents of these sections are discussed. Sea surface temperature nearly everywhere was warmer than 25°C (an anomaly of 2°C–3°C). The equatorial thermocline, represented by the 20°C isotherm, was depressed by nearly 50 m at 85°W and more than 100 m at 95°W relative to a historical mean profile. Across 95°W, estimated geostrophic (relative to 500 dbar) and directly measured near-equatorial velocity profiles indicated significant eastward surface flow. The South Equatorial Current south of the equator had disappeared. Estimated warm water transport (σt < 24.5 kg m−3) indicated an influx into the region which was approximately that required to balance the observed low-frequency sea level change at the Galapagos Islands. Temperature and salinity properties as well as the transport estimates indicated that this warm water came predominantly from the west.