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Lawrence A. Mysak

Abstract

A new mechanism is proposed for the generation of the annual-period baroclinic Rossby waves which have been observed in the central North Pacific by Kang and Magaard. It is shown that annual north-south fluctuations in the eastern boundary current off Vancouver Island can efficiently generate first baroclinic mode Rossby waves throughout the central North Pacific. In particular, to the southwest of Vancouver Island the direction, wavelength and speed of phase propagation associated with the far field (asymptotic) wave solution agree favorably with observations. Also, along and fixed latitude contained within the observed latitude band (30–40°N), the amplitudes of the vertical displacement at 300 m and surface horizontal current speed both monotonically increase to the west, in agreement with the observed trends for these quantities. However, the existence of this monotonicity in the solution appears to be quite sensitive to the north-south spatial structure of the fluctuating coastal current. For example, the oscillations of a point source (delta function) current excite a wave field with a fairly uniform amplitude across most of the observation region.

Within the context of a reduced gravity, quasi-geostrophic model, the theory presented here is quite general. For example, baroclinic waves of other than annual period could be investigated, and the radiation pattern due to a number of coastal sources could be determined.

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Lawrence A. Mysak

Abstract

The theory of barotropic nondivergent waves trapped on an exponential shelf lying on an equatorial β-plane is presented. The bottom contours are parallel to the equator so that phase propagation is either eastward or westward. according to the following general rule: when the shelf region is entirely in the Northern (Southern) Hemisphere the shallow water is to the right (left) of the direction of the phase velocity. When both the shelf and deep sea regions are located in the same hemisphere (case 1), the results concerning the dispersion curves and eigenfunctions are qualitatively similar to those obtained by Buch-wald and Adams (1968) for shelf waves on a mid-latitude exponential shelf on an f-plane. However, when the shelf region is on one side of the equator and the deep sea region extends across the equator (case 2), the dispersion curves and eigenfunctions are quite different. In case 2 the dispersion curve for each trapped mode has a long-wave cutoff. However, the cutoff for each mode generally does not preclude the existence of a zero group velocity at an intermediate wavelength. a phenomenon which always occurs in case 1. In case 2 the range of oscillation for each eigenfunction is generally much larger than that of the corresponding eigenfunction in case 1. Finally, when the shelf region straddles the equator (case 3), both westward and eastward propagating modes may exist. Further, one set of these modes has a long-wave cutoff (e.g., if the coast is in the Southern Hemisphere with deep water to the north, the westward propagating modes have a long-wave cutoff). In case 3 the oscillations of each eigenfunction tend to be concentrated near the shelf edge.

The theory is applied to the Gulf of Guinea. where a 0.07 cycle per day (cpd) oscillation in the sea surface temperature has been observed to propagate westward along the Ghaina-lyory coast. It is shown that this signal may be due to the presence of a fundamental mode shelf wave of the type discussed in this paper.

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Lawrence A. Mysak

Abstract

Recent satellite infrared imagery and hydrographic maps of the sea surface temperature of the northeast Pacific reveal a fairly regular pattern of cells or eddies that are aligned north-south about 150 km west of Vancouver Island. The wavelength of this pattern lies in the range 80–100 km Current observations taken near the edge of the continental shelf at a depth of 200 m off Tofino, Vancouver Island, show oscillations with periods of days to about two weeks. At intermediate depths off the coast of Vancouver Island and northern Washington recently derived geostrophic flow patterns show a northward current along the continental slope (the California Undercurrent). In this paper the stability of a two-layer model of the California Undercurrent off Vancouver Island and northern Washington is considered in an attempt to explain the source of the observed spatial and temporal variations. It is shown that the model flow is baroclinically unstable, with the amplifying wave disturbances propagating northward along the continental slope. For typical vertical shears associated with the California Undercurrent, the most unstable waves have wavelengths and periods in the ranges 65–94 km and 5–10 days, respectively, in good agreement with the observed fluctuations.

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Silvia A. Venegas
and
Lawrence A. Mysak

Abstract

A frequency-domain singular value decomposition performed jointly on century-long (1903–94) records of North Atlantic sector sea ice concentration and sea level pressure poleward of 40°N reveals that fluctuations on the interdecadal and quasi-decadal timescales account for a large fraction of the natural climate variability in the Arctic. Four dominant signals, with periods of about 6–7, 9–10, 16–20, and 30–50 yr, are isolated and analyzed. These signals account for about 60%–70% of the variance in their respective frequency bands. All of them appear in the monthly (year-round) data. However, the 9–10-yr oscillation especially stands out as a winter phenomenon.

Ice variability in the Greenland, Barents, and Labrador Seas is then linked to coherent atmospheric variations and certain oceanic processes. The Greenland Sea ice variability is largely due to fluctuations in ice export through Fram Strait and to the local wind forcing during winter. It is proposed that variability in the Fram Strait ice export depends on three different mechanisms, which are associated with different timescales: 1) wind-driven motion of anomalous volumes of ice from the East Siberian Sea out of the Arctic (6–7-yr timescale); 2) enhanced ice motion forced by winter wind anomalies when they align parallel to the Transpolar Drift Stream (9–10-yr timescale); 3) wind-driven motion of old, thick, and very low salinity ice from offshore northern Canada into the outflow region (16–20-yr timescale). Also, a marked decreasing trend in ice extent since around 1970 (30–50-yr timescale) is linked to a recently reported warming in the Arctic.

The Barents Sea ice variability is associated with the nature of the penetration of Atlantic waters into the Arctic Basin, which is affected by two distinct mechanisms: 1) changes in the intensity of the northward-flowing Norwegian Current, which is linked to variability in the North Atlantic oscillation (NAO) pattern (9–10-yr timescale); and 2) changes in the upper-ocean temperature of the Norwegian Current waters, which is likely related to the advection of temperature anomalies by the ocean gyres (16–20-yr timescale). Ice variability in the Labrador Sea, on the other hand, appears to be mainly determined by thermodynamical effects produced by the local wind forcing, which is closely related to the NAO pattern (9–10-yr timescale), and by oceanic advection of ice anomalies into this sea from the Greenland–Irminger Sea by the East Greenland Current (6–7-yr timescale).

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Shiling Peng
and
Lawrence A. Mysak

Abstract

The spatial distributions of northern North Atlantic sea surface temperature and the high-latitude Northern Hemisphere sea level pressure anomalies averaged over six consecutive warm SST winters (1951–1956) and six consecutive cold SST winters (1971–1976) are examined. Three SLP anomaly difference (i.e., warm - cold winters) centers, significant at the 5% level, are observed over the northern North Atlantic, Europe, and western Siberia. This anomaly pattern is consistent in principle with what was identified in a related analyses by Palmer and Sun, who used composite data from selected winter months.

The SLP difference centers over the northern North Atlantic and western Siberia are in phase. The impact of the latter center upon the runoff from the underlying Oh and Yenisey rivers and especially the teleconnection between SST anomalies in the northern North Atlantic and runoff of those two rivers via the atmosphere are investigated. The temporal cross-correlation analyses of 50 years (1930–1979) of records of SST, precipitation, and runoff anomalies indicate that the winter SST anomalies in the northern North Atlantic are significantly correlated with the winter and following summer runoff fluctuations of the Ob and Yenisey rivers. Positive (negative) northern North Atlantic SST anomalies are related to less (more) precipitation, and hence, less (more) runoff, over western Siberia.

Discussions of possible physical mechanisms and process that lead to the above relationships are attempted. The analyses of spatial distributions of precipitation in the warm and cold SST winters suggest that precipitation fluctuations over Europe and western Siberia may be affected by shifts of cyclone tracks associated with the SST variations in the northern North Atlantic.

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Zhaomin Wang
and
Lawrence A. Mysak

Abstract

The authors develop a coupled atmosphere–ocean–sea ice–land surface model for long-term climate change studies that incorporates the seasonal cycle. Three ocean basins, the Antarctic Circumpolar Current region, and the major continents are resolved. The model variables are sectorially averaged across the different ocean basins and continents. The atmosphere is represented by an energy–moisture balance model in which the meridional energy and moisture transports are parameterized by a combination of advection and diffusion processes. The zonal heat transport between land and ocean obeys a diffusion law, while the zonal moisture transport is parameterized so that the ocean always supplies moisture to the land. The ocean model is due to Wright and Stocker, and the sea ice model is a zero-layer thermodynamic one in which the ice thickness and concentration are predicted by the methods of Semtner and Hibler, respectively. In the land surface model, the temperature is predicted by an energy budget equation, similar to Ledley’s, while the soil moisture and river runoff are predicted by Manabe’s bucket model.

The above model components are coupled together using flux adjustments in order to first simulate the present-day climate. The major features of this simulation are consistent with observations and the general results of GCMs. However, it is found that a diffusive law for heat and moisture transports gives better results in the Northern Hemisphere than in the Southern Hemisphere. Sensitivity experiments show that in a global warming (cooling) experiment, the thermohaline circulation (THC) in the North Atlantic Ocean is weakened (intensified) due to the increased (reduced) moisture transport to the northern high latitudes and the warmer (cooler) SST at northern high latitudes.

Last, the coupled model is employed to investigate the initiation of glaciation by slowly reducing the solar radiation and increasing the planetary emissivity, only in the northern high latitudes. When land ice is growing, the THC in the North Atlantic Ocean is intensified, resulting in a warm subpolar North Atlantic Ocean, which is in agreement with the observations of Ruddiman and McIntyre. The intensified THC maintains a large land–ocean thermal contrast at high latitudes and hence enhances land ice accumulation, which is consistent with the rapid ice sheet growth during the first 10 kyr of the last glacial period that was observed by Johnson and Andrews. The authors conclude that a cold climate is not responsible for a weak or collapsed THC in the North Atlantic Ocean; rather it is suggested that increased freshwater or massive iceberg discharge from land is responsible for such a state.

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Lawrence A. Mysak
and
Fengting Huang

Abstract

The Pease latent-heat polynya model is coupled to a reduced-gravity, coastal upwelling model in order to simulate the formation and maintenance of the North Water (NOW), the Arctic's largest polynya, located in northern Beffin Bay. In this region, strong northerly winds during winter and spring drive ice southward as fast as it is produced locally (the Pease mechanism for polynya formation), and also produce upwelling of warm subsurface water along the west coast of Greenland. This upwelling provides an upward heat flux that melts ice near the coast (a sensible-host mechanism for polynya formation). This combined latent- and sensible-heat polynya model is formulated as an initial-boundary value problem with uniform winds. Its solution gives the tide evolution of the upper-layer velocity and depth, and also the polynya width as measured southward from the northern boundary of the polynya.

There are two fundamental time scales in the problem: a fast one (of the order of days) that characterizes the polynya opening by the Pease mechanism, and a slow one (of the order of weeks) dust describes the evolution of the upwelling of warn subsurface water along the Greenland coast and hence the gradual southward extension of the ice edge in this region via the sensible-heat mechanism for polynya formation. The steady-state (asymptotic or limiting) polynya width is a strong function of the air temperature, but a weak function of the wind speed. The model results show that in the upwelling region near the Greenland coast, the limiting polynya width is considerably larger than farther offshore, where it is a constant (the limiting Pease width). This ice-edge configuration is in general agreement with that seen in recent Landsat satellite images of the NOW. For a spatially varying wind field. the southern ice edge of the NOW has an offshore profile similar to that of the wind stress forcing. In a north-south channel domain formulation of the model, upwelling occurs in the eastern part (i.e., near Greenland) and downwelling in the western part (near Ellesmere Island). Thus, the polynya is wider near Greenland and narrower near the Canadian islands.

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Rosemary G. Wood
and
Lawrence A. Mysak

Abstract

A steady, coupled model of the upper ocean circulation and ice cover in a meridional channel is presented and applied to the Greenland Sea. The main ocean gyre is driven by wind stress; however a nonzero boundary condition on meridional velocity is applied at the western side to produce a southward-flowing coastal current, representative of the East Greenland Current (EGC). To match the EGC to the interior Sverdrup solution, a Munk-type western boundary layer is introduced. The upper ocean temperature is governed by an advection-diffusion equation, which is forced at the boundaries, and by heat exchange with the atmosphere. The ice model is purely thermodynamic and the ice-edge position is determined iteratively since the streamfunction and temperature fields depend on the ice cover and, in turn, the ice cover is determined by both the ocean and air temperature. The range of behavior of the temperature field and ice-edge position is studied as a function of the ice–ocean drag, the wind stress transmission factor of the ice, the eddy heat diffusion coefficient, the various heat exchange coefficients, and the boundary conditions. The results are compared with observed ice and ocean temperature data.

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Andrew J. Willmott
and
Lawrence A. Mysak

Abstract

A thermodynamic reduced-gravity ocean model forced by the steady-state surface wind stress and a Haney-type heat flux was used to determine the climatological ice-edge position, ice thickness, ocean circulation, and upper-ocean temperature in a high-latitude meridional channel. The ice model used is purely thermodynamic; however, a parameterization is used to allow the surface wind stress to be transmitted to the water below the ice. The temperature distribution of the upper ocean is specified along the southern zonal boundary of the model domain, and the heat equation is integrated from this boundary poleward along streamlines for the mass transport. As a column of warm water moves poleward, its temperature tends to decrease since the air temperature monotonically decreases to the north. At high latitudes the steady-state heat balance between horizontal advection and cooling to the atmosphere can no longer hold in an ice-free ocean. Thus, an ice layer forms to insulate the ocean from the very cold air temperatures at these latitudes.

The model is applied to the Greenland and Norwegian Seas, between 60° and 80°N, and between the east coast of Greenland and 15°E. Exterior to a narrow western boundary layer, the predicted ice-edge position compares favorably with the climatological 90% ice concentration isoline obtained from an analysis of 32 years of Arctic sea ice data.

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Patrick F. Cummins
and
Lawrence A. Mysak

Abstract

A limited-area quasi-geostrophic numerical model with mesoscale resolution is developed to study the circulation in the northeast (NE) Pacific Ocean. The model domain extends from the British Columbia-Alaska coast out to 170°W and down to 45°N, and incorporates the coastline geometry and bottom topography of the region. A long-term integration was conducted using a steady climatological wind stress curl field to drive the circulation. Several statistical properties of the solution are determined and compared with observations.

A cyclonic circulation develops in the model basin with a meandering Alaska Current feeding, at the head of the Gulf of Alaska, into an intense boundary current corresponding to the Alaskan Stream. The head of the Gulf is a region where anticyclonic closed streamline features are occasionally generated with characteristics resembling those of the Sitka Eddy. In the downstream region, the boundary current separates and is subject to lateral meandering due to topographic waves. The occurrence of perturbations with similar characteristics in the Alaska Stream has recently been verified in satellite IR imagery.

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