Interannual variations of North Equatorial Current transport in the Pacific Ocean during two types of El Niño
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Keywords:
Ekman transport
Sea-surface height
Anomaly (physics)
Wind Stress
Sea surface height (SSH) anomalies from the Geosat altimeter for the northeast Pacific Ocean were analysed to determine their annual and interannual fluctuations over a 2.5-year period. The interannual anomalies suggested large-scale changes in the intensity of the California and Alaska currents, with a weak California Current for the first year (1986–1987), which strengthened during the second year, partly by a diversion of flow from the Alaskan gyre into the California Current and partly by a decrease in SSH along the coast. In the California Current between about 36° and 46°N, the annual fluctuations in SSH showed westward phase propagation. These observations were modeled using a linearized potential vorticity equation with one active layer, forced by realistic wind stress curl, which resembled a standing wave. The annual fluctuations in SSH were produced primarily by Ekman pumping, because Rossby waves are coastally trapped poleward of about 37°N. The predicted response had excellent phase agreement with the data but underestimated the magnitude of the observations by nearly a factor of 2. This simple analysis suggests that the California Current core propagates offshore during the year not due to Rossby waves but rather due to a combination of spatial variations in the wind stress curl field and the meridional variation of the Coriolis parameter. Some qualifications to these conclusions are discussed along with an examination of errors in both SSH and wind stress.
Sea-surface height
Wind Stress
Ekman transport
Kelvin wave
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Hindcast
Wind Stress
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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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15 years (1993～2007) monthly-averaged sea surface height anomalies (SSHA) are used to analyze their interannual spatial and temporal variability, also their thermodynamic and dynamic mechanisms are studied.The prominent interannual variability is located in the Kuroshio Extension and the western Pacific warm pool.By 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 midlatitudinal northeastern Pacific Ocean.The time-varying barotropic Sverdrup balance could explain 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 midlatitudes, 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.
Sea-surface height
Ekman transport
Barotropic fluid
Subarctic climate
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The large-scale ocean circulation is fuelled by a combination of winds and buoyancy (or heat) fluxes acting on the ocean’s surface. Gyres are central features of large-scale ocean circulation and are involved in the transport of many tracers like heat, nutrients, carbon-dioxide and so on within and across ocean basins. Traditionally, the gyre circulation is explained by the relationship between meridional transport and wind stress curl, known as the Sverdrup balance. However, it has been proposed that surface buoyancy fluxes may also contribute to the formation of gyres, although such a theoretical relationship is lacking in oceanographic literature. Through a series of eddy-permitting global ocean model simulations, we aspire to better understand the relative contribution of wind stress and surface buoyancy fluxes on large-scale ocean circulation. We perturb the atmospheric forcing by spatially varying the wind stresses and/or surface buoyancy fluxes, while minimising the associated changes in mixed layer dynamics. We compare perturbed forcing simulations with a control simulation in an attempt to decompose the large- scale ocean circulation into buoyancy and wind-driven components.
Wind Stress
Forcing (mathematics)
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Countercurrent exchange
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In this study, the dynamic mechanisms of interannual sea surface height (SSH) variability are investigated based on the first-mode baroclinic Rossby wave model, with a focus on the effects of different levels of wind stress curl (WSC). Maximum covariance analysis (MCA) of WSC and SSH anomalies displays a mode with significant WSC anomalies located primarily in the mid-latitude eastern North Pacific and central tropical Pacific with corresponding SSH anomalies located to the west. This leading mode can be attributed to Ekman pumping induced by local wind stress and the westward-propagating Rossby wave driven by large- scale wind stress. It is further found that in the middle latitudes, the SSH anomalies are largely determined by WSC variations associated with the North Pacific Gyre Oscillation (NPGO), rather than the Pacific Decadal Oscillation (PDO). The sensitivity of the predictive skill of the linear first-mode baroclinic model to different wind products is also examined.
Wind Stress
Ekman transport
Sea-surface height
Middle latitudes
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The dominant processes affecting sea surface height (SSH) variability observed by the TOPEX/Poseidon altimeter vary regionally in the Pacific; baroclinic Rossby waves, equatorially trapped Kelvin waves, steric response to seasonal heating, and the response to wind stress curl forcing are all important. The steric response to surface heating dominates seasonal SSH variability in the subpolar gyre and the eastern subtropical gyre. South of the Kuroshio Extension and south of 20°N in the eastern Pacific, the dominant contribution to SSH is from near‐annual period Rossby waves. To quantify the wave energy, observed SSH was assimilated into a kinematic model of westward propagating waves. These waves account for >70% of SSH variance between 10°S and 10°N but only ∼30% between 10°N and 30°N. Although wave energy in the eastern Pacific is correlated with SSH anomalies at the equator, the much larger wave energy in the western Pacific is correlated with wind stress curl, suggesting that the Rossby waves there are locally forced. In addition to these planetary waves, the ocean response to wind forcing via Ekman pumping is observed in several places, specifically in the North Equatorial Current. A quasi‐steady topographic Sverdrup balance is detectable over most of the North Pacific at latitudes as low as 10–15°N, as well as in the South Pacific, where it is seen north of 50°S. The decomposition of the SSH signal into propagating waves, an Ekman pumping response, and Sverdrup transport is consistent with the results from an isopycnal numerical model.
Sea-surface height
Equatorial waves
Ekman transport
Kelvin wave
Isopycnal
Wind Stress
Sverdrup
Forcing (mathematics)
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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.
Sea-surface height
Ekman transport
Barotropic fluid
Boundary current
Subarctic climate
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A reduced-gravity, primitive equation, upper-ocean GCM is used to study subduction pathways in the Atlantic subtropical and tropical gyres. In order to compare the different responses in the pathways to strong and weak wind stress forcings, Hellerman and Rosenstein (HR) and da Silva (DSV) climatological annual-mean and monthly wind stress forcings are used to force the model. It is shown that subtropical–tropical communication is dependent on both the strength and structure of the wind forcing. A comparison between the two experiments shows two results for the North Atlantic: 1) the full communication window between the subtropical and tropical gyres is similar in width despite the difference in the intensity of the winds and 2) the interior exchange window width is substantially larger in the weak forcing experiment (DSV) than the strong forcing experiment (HR), accompanied by a larger transport as well. The South Atlantic exhibits a similar communication between the subtropics and Tropics in both cases. The annual-mean of the seasonally varying forcing also supports these results. A two-layer ventilated thermocline model is developed with a zonally varying, even though idealized, wind stress in the North Atlantic, which includes the upward Ekman pumping region absent from the classical ventilated thermocline model. The model shows that the communication window for subduction pathways is a function of the zonal gradient of the Ekman pumping velocity, not the Ekman pumping itself, at outcrop lines and at the boundary between the subtropical and tropical gyres. This solution is validated using three additional GCM experiments. It is shown that the communication windows are primarily explained by the ventilated thermocline model without considering the buoyancy effects. From the GCM experiments, the interior exchange window, which is a part of the communication window and cannot be explained by the ventilated thermocline model, is widened by two factors: 1) eliminating part of the positive Ekman pumping region in the eastern North Atlantic and 2) weakening the Ekman pumping over the whole region. The implications of these results suggest that changes in the wind forcing on the order of the difference in the wind products used here can have a significant effect on the attributes of the communication window and, hence, the thermocline structure at lower latitudes.
Ekman transport
Wind Stress
Forcing (mathematics)
Tropical Atlantic
Subtropical front
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Citations (38)