Interannual Sea Level Variability in the North Pacific Ocean and Its Mechanisms
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Abstract We use the 15 years (1993~2007) data of monthly‐averaged sea surface height anomalies (SSHA) to analyze their interannual spatial and temporal variability and their thermodynamic and dynamic mechanisms. The result shows that the prominent interannual variability occurred in the Kuroshio Extension and the western Pacific warm pool. According to decomposition using the EOF method, the first mode of interannual SSHA is more likely zonal, while the second is much more meridional. The steric SSHA induced by the heat flux explains more than 40% of interannual SSHA in the middle‐latitudinal northeastern Pacific Ocean. The time‐varying barotropic Sverdrup balance can account for 20%~40% in the western subarctic gyre, whereas their interannual variations are indistinctive. Among the baroclinic mechanisms, the first baroclinic Rossby waves model forced by large‐scale wind stress could explain the interannual SSHA 20%~60% in the tropical Pacific, 20%~40% in the central middle‐latitudes, and 20%~60% in the eastern Alaska gyre and western subarctic gyre, respectively. The interannual SSHA induced by the local Ekman pumping has a local structure, which could explain more than 40% of the observed changes in the northeast Pacific Ocean, likewise in the Bering sea and central tropical Pacific Ocean. The westward propagating Rossby waves, derived from the difference between the Rossby waves model and Ekman pumping model simulating SSHA, could explain 20%–60% of the interannual SSH observations in the central and western subtropical gyre and east of the Hawaiian Islands.Keywords:
Sea-surface height
Ekman transport
Barotropic fluid
Boundary current
Subarctic climate
An eddy‐permitting ocean model of the northeast Pacific is used to examine the ocean adjustment to changing wind forcing in the Gulf of Alaska (GOA) at interannual‐to‐decadal timescales. It is found that the adjustment of the ocean model in the presence of mesoscale eddies is similar to that obtained with coarse‐resolution models. Local Ekman pumping plays a key role in forcing pycnocline depth variability and, to a lesser degree, sea surface height (SSH) variability in the center of the Alaska gyre and in some areas of the eastern and northern GOA. Westward Rossby wave propagation is evident in the SSH field along some latitudes but is less noticeable in the pycnocline depth field. Differences between SSH and pycnocline depth are also found when considering their relationship with the local forcing and leading modes of climate variability in the northeast Pacific. In the central GOA pycnocline depth variations are more clearly related to changes in the local Ekman pumping than SSH. While SSH is marginally correlated with both Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO) indices, the pycnocline depth evolution is primarily related to NPGO variability. The intensity of the mesoscale eddy field increases with increasing circulation strength. The eddy field is generally more energetic after the 1976–1977 climate regime shift, when the gyre circulation intensified. In the western basin, where eddies primarily originate from intrinsic instabilities of the flow, variations in eddy kinetic energy are statistically significant correlated with the PDO index, indicating that eddy statistics may be inferred, to some degree, from the characteristics of the large‐scale flow.
Pycnocline
Ekman transport
Sea-surface height
Eddy
Forcing (mathematics)
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Abstract The temporal evolution of the strength of the Atlantic Meridional Overturning Circulation (AMOC) in the subtropical North Atlantic is affected by both remotely forced, basin-scale meridionally coherent, climate-relevant transport anomalies, such as changes in high-latitude deep water formation rates, and locally forced transport anomalies, such as eddies or Rossby waves, possibly associated with small meridional coherence scales, which can be considered as noise. The focus of this paper is on the extent to which local eddies and Rossby waves when impinging on the western boundary of the Atlantic affect the temporal variability of the AMOC at 26.5°N. Continuous estimates of the AMOC at this latitude have been made since April 2004 by combining the Florida Current, Ekman, and midocean transports with the latter obtained from continuous density measurements between the coasts of the Bahamas and Morocco, representing, respectively, the western and eastern boundaries of the Atlantic at this latitude. Within 100 km of the western boundary there is a threefold decrease in sea surface height variability toward the boundary, observed in both dynamic heights from in situ density measurements and altimetric heights. As a consequence, the basinwide zonally integrated upper midocean transport shallower than 1000 m—as observed continuously between April 2004 and October 2006—varies by only 3.0 Sv (1 Sv ≡ 106 m3 s−1) RMS. Instead, upper midocean transports integrated from western boundary stations 16, 40, and 500 km offshore to the eastern boundary vary by 3.6, 6.0, and 10.7 Sv RMS, respectively. The reduction in eddy energy toward the western boundary is reproduced in a nonlinear reduced-gravity model suggesting that boundary-trapped waves may account for the observed decline in variability in the coastal zone because they provide a mechanism for the fast equatorward export of transport anomalies associated with eddies impinging on the western boundary. An analytical model of linear Rossby waves suggests a simple scaling for the reduction in thermocline thickness variability toward the boundary. Physically, the reduction in amplitude is understood as along-boundary pressure gradients accelerating the fluid and rapidly propagating pressure anomalies along the boundary. The results suggest that the local eddy field does not dominate upper midocean transport or AMOC variability at 26.5°N on interannual to decadal time scales.
Boundary current
Ekman transport
Sea-surface height
Eddy
Gulf Stream
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Subarctic climate
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Abstract. The Coupled Model Intercomparison Project (CMIP) allows assessment of the representation of the Atlantic Meridional Overturning Circulation (AMOC) in climate models. While CMIP Phase 6 models display a large spread in AMOC strength by a factor of three, the multi-model mean strength agrees reasonably well with observed estimates from RAPID1, but this does not hold for its various components. In CMIP6 the present-day AMOC is characterised by a lack of lower North Atlantic Deep Water (lNADW), due to the small-scale of Greenland-Iceland-Scotland Ridge overflow and too much mixing. This is compensated by increased recirculation in the subtropical gyre and more Antarctic Bottom Water (AABW). Deep-water circulation is dominated by a distinct deep western boundary current (DWBC) with minor interior recirculation compared to observations. The future decline in the AMOC to 2100 of 7 Sv under a SSP5-8.5 scenario is associated with decreased northward western boundary current transport in combination with reduced southward flow of upper North Atlantic Deep Water (uNADW). In CMIP6, wind stress curl decreases with time by 14 % so that the wind-driven thermocline recirculation in the subtropical gyre is reduced by 4 Sv (17 %) by 2100. The reduction in western boundary current transport of 11 Sv is more than the decrease in the wind-driven gyre transport suggesting a decrease over time in the component of the Gulf Stream originating in the South Atlantic. 1 RAPID is used here as shorthand for the RAPID-Meridional Overturning Circulation and Heatflux Array-Western Boundary Time Series at 26°N (Moat et al., 2022).
Boundary current
Ekman transport
Gulf Stream
Wind Stress
Antarctic Bottom Water
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The circulation nature in the Subarctic Gyre in the North Pacific, and the water mass modification processes found there are discussed. The circulation path penetrates well inside the Bering Sea, and a part of the water enters into the Okhotsk Sea. The water masses are modified significantly in and near these marginal seas. The Oyashio Water is shown to be formed by mixing between the East Kamchatka Current Water and the Okhotsk Sea Water. The freshest water in the intermediate layers is found in the Okhotsk Sea, and appears to play important role in the formation of the North Pacific Intermediate Water.
Subarctic climate
Circumpolar deep water
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Boundary current
Barotropic fluid
Zonal flow (plasma)
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Subarctic climate
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Subarctic climate
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The Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident in March 2011 resulted in serious radiocesium contamination of the North Pacific Ocean. Most of the radiocesium was dissolved in seawater and transported by surface currents and subduction of mode waters. Within several years after the accident, a high-concentration water plume of the FNPP1-derived radiocesium at the sea surface had been transported from Japan to the North American continent across the subarctic gyre of the North Pacific Ocean. We measured vertical profiles of dissolved radiocesium along the nominal 47°N zonal line across the North Pacific subarctic gyre twice, in summer 2012 and summer 2014. Using these data and published data, we quantitatively discussed the zonal and vertical transports of the water plume until 2014. The FNPP1-derived radiocesium remained in the surface layer shallower than 200 m, which is the approximate winter mixed-layer depth in the western subarctic gyre. The mean penetration depth did not change between 2012 and 2014. The highest concentration was observed at 180°W in 2012 and at 151°W in 2014, which suggests that the zonal transport speed of the water plume in the eastern subarctic gyre was about 3.8 cm s-1. By combining the data from the zonal line in 2014 and a nominal 152°W meridional line in 2015, we elucidated the three-dimensional size of the high-concentration water plume in summer 2014. The total inventory of the FNPP1-derived radiocesium in the subarctic North Pacific Ocean, decay-corrected to the accident date, was estimated to be 12.0 ± 2.4 PBq.
Subarctic climate
Mode water
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Abstract The large-scale circulation of the bottom layer of the Gulf of Mexico is analyzed, with special attention to the historically least studied western basin. The analysis is based on 4 years of data collected by 158 subsurface floats parked at 1500 and 2500 m and is complemented with data collected by current meter moorings in the western basin during the same period. Three main circulation patterns stand out: a cyclonic boundary current, a cyclonic gyre in the abyssal plain, and the very high eddy kinetic energy observed in the eastern Gulf. The boundary current and the cyclonic gyre appear as distinct features, which interact in the western tip of the Yucatan shelf. The persistence and continuity of the boundary current is addressed. Although high variability is observed, the boundary flow serves as a pathway for water to travel around the western basin in approximately 2 years. An interesting discovery is the separation of the boundary current over the northwestern slope of the Yucatan shelf. The separation and retroflection of the along-slope current appears to be a persistent feature and is associated with anticyclonic eddies whose genesis mechanism remains to be understood. As the boundary flow separates, it feeds into the westward flow of the deep cyclonic gyre. The location of this gyre—named the Sigsbee Abyssal Gyre—coincides with closed geostrophic contours, so eddy–topography interaction via bottom form stresses may drive this mean flow. The contribution to the cyclonic vorticity of the gyre by modons traveling under Loop Current eddies is discussed.
Boundary current
Eddy
Anticyclone
Geostrophic current
Current meter
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