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Yochanan Kushnir

Abstract

Retrograding (westward-moving) features over the middle and high latitudes of the Northern Hemisphere during winter are examined in observations and data from a GCM simulation, using complex principal component analysis. The first eigenvector of the low-frequency middle troposphere geopotential height over the North Pacific has a zonal scale of 120–160 deg longitude and a dipole-like meridional structure, with maximum amplitude over the Gulf of Alaska and the Bering Sea. These disturbances propagate westward and go through a life cycle of growth and decay over a period of about three weeks.

The temporal evolution of the energy and its conversions during the life cycle of the simulated Pacific disturbances are composited, using the time-dependent coefficient of the principal component analysis as an index for their occurrence. The composite energy cycle indicates that the kinetic and available potential energy of these disturbances grow and decay together. Both baroclinic and barotropic conversions contribute to the growth of disturbance energy.

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Yochanan Kushnir

Abstract

Evidence is presented for a distinct Pattern Of ocean-atmosphere relationship associated with interdecadal variability in the North Atlantic region. Using a century of surface marine observations it is shown that middle- and high-latitude sea surface temperature (SST) display a long-term fluctuation with negative anomalies before 1920, and during the 1970s and 1980s. Positive SST conditions prevailed from about 1930 to 1960. The pattern of interdecadal SST variability is constructed by subtracting the average field during 15 cold years from that during a similar interval of warm years. The early-century warming and the more recent cooling display a similar spatial pattern. In both cases the pattern is basin scale and largely of one polarity, with maxima in the vicinity of Iceland, in the Labrador Sea, and northeast of Bermuda.

The corresponding differences in surface atmospheric conditions are determined by averaging and subtracting, in the same manner, fields of sea level pressure (SLP) and surface winds. The results display a circulation anomaly in the middle of the ocean basin, centered at about 45°N and 35°W. In this midocean area, an anomalous cyclonic circulation prevailed during years with warm SST, and an anticyclonic anomaly dominated during years with cold SST. These circulation anomalies are strongest during the winter months.

To emphasize the distinct nature of interdecadal variability, short-term, interannual variability is examined in a similar analysis. The resulting patterns display bands of zonally elongated centers of action that are negatively correlated with one another. These anomalies underlay similarly banded features in the zonal wind distribution. The mutual relationship between interannual fluctuations of SST and surface wind conditions suggests that the former are maintained by the latter through a local thermodynamic interaction.

The interdecadal patterns of SST on one hand and SLP and winds on the other lack a similar coherent relationship. This fact, and the unique horizontal distribution of interdecadal SST anomalies, suggest that interdecadal variability may be governed by a basin-scale dynamical interaction between the large-scale oceanic circulation and the atmosphere.

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Nadav Levanon and Yochanan Kushnir

Abstract

An approximate analytical solution of the vertical equation of motion for constant density superpressure balloons is obtained. It is shown how this solution can be used to filter out neutrally buoyant oscillations in balloon records.

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Yochanan Kushnir and John M. Wallace

Abstract

Two sets of 15-day numerical forecasts are performed with a general circulation model to examine aspects of the mutual interaction between high-frequency, baroclinic-wave variability and the low-frequency components of the atmospheric flow. A control run based on an initial field, arbitrarily chosen from the history tapes of a previous model integration and a forecast based on a time-filtered version of the same initial state are compared. The results indicate that the high-frequency variability of the flow in the latter forecast returns to normal amplitudes about one week after the initialization time, at which state it is only weakly correlated in space with the high-frequency component of the flow in the control run. The low-frequency components of the flow seems to behave differently depending on their zonal scale: Ultralong waves (wavenumber 1–3) are only weakly affected by the removal of the baroclinic activity from the initial conditions, while long waves (wavenumber 4–6) react to the removal of the baroclinic waves by drifting eastward faster than their counterparts in the control run.

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Yochanan Kushnir and Steven K. Esbensen

Abstract

The energetics of large-scale disturbances of the wintertime, Northern Hemisphere circulation are studied with the OSU two-level general circulation model. The behavior of simulated eddies with short time-scale (2.5 to 10 days) is found to be consistent with observations and with baroclinic instability theory. Eddies with long time-scales (>10 days) appear to be maintained primarily by high-latitude baroclinic energy conversions. Energy conversions characteristic of barotropic processes are found at jet stream latitudes.

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Yochanan Kushnir and Ngar-Cheung Lau

Abstract

A general circulation model was integrated with perpetual January conditions and prescribed sea surface temperature (SST) anomalies in the North Pacific. A characteristic pattern with a warm region centered northeast of Hawaii and a cold region along the western seaboard of North America was alternately added to and subtracted from the climatological SST field. Long 1350-day runs, as well as short 180-day runs, each starting from different initial conditions, were performed. The results were compared to a control integration with climatological SSTs.

The model's quasi-stationary response does not exhibit a simple linear relationship with the polarity of the prescribed SST anomaly. In the short runs with a negative SST anomaly over the central ocean, a large negative height anomaly, with an equivalent barotropic vertical structure, occurs over the Gulf of Alaska. For the same SST forcing, the long run yields a different response pattern in which an anomalous high prevails over northern Canada and the Alaskan Peninsula. A significant reduction in the northward heat flux associated with baroclinic eddies and a concomitant reduction in convective heating occur along the model's Pacific storm track. In the runs with a positive SST anomaly over the central ocean, the average height response during the first 90-day period of the short runs is too weak to be significant. In the subsequent 90-day period and in the long run an equivalent barotropic low occurs downstream from the warm SST anomaly. All positive anomaly runs exhibit little change in baroclinic eddy activity or in the patterns of latent heat release. Horizontal momentum transports by baroclinic eddies appear to help sustain the quasi-stationary response in the height field regardless of the polarity of the SST anomaly. These results emphasize the important role played by baroclinic eddies in determining the quasi-stationary response to midlatitude SST anomalies. Differences between the response patterns of the short and long integrations may be relevant to future experimental design for studying air-sea interactions in the extratropies.

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Yochanan Kushnir and Steven K. Esbensen

Abstract

The properties of atmospheric Northern Hemisphere wintertime variability, simulated by the Oregon State University two-level general circulation model are examined. Time series of the dependent variables and diabatic heating components are extracted from ten simulated Northern Hemisphere winters. Variance and covariance analyses are performed to determine the geographical distribution of the intensities and transport properties of eddies of high-frequency (periods between 2.5 and 10 days) and low-frequency (periods between 10 days and a season).

In agreement with observations the simulated high-frequency fluctuations are caused by rapidly propagating, eastward-moving disturbances with structures that are consistent with baroclinic instability theory. The regions of strong high-frequency variability (storm tracks) and the associated transports of beat and momentum in the OSU model show a good correspondence with regions where the time-mean circulation has pronounced vertical shear and a weak gradient of absolute vorticity. Thus, discrepancies between the observed and simulated positions of the storm tracks appear to be related to systematic errors in the simulated time-mean circulation.

like their observed counterparts, low-frequency fluctuations are found to be due to disturbances that am almost stationary in phase and show indications of eastward energy dispersion in qualitative agreement with the theory of Rossby-wave dispersion on a sphere. The patterns of low-frequency variability of the geopotential height field are in good agreement with observations except over the Atlantic Ocean.

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Yochanan Kushnir and Isaac M. Held

Abstract

The equilibrium general circulation model (GCM) response to sea surface temperature (SST) anomalies in the western North Atlantic region is studied. A coarse resolution GCM, with realistic lower boundary conditions including topography and climatological SST distribution, is integrated in perpetual January and perpetual October modes, distinguished from one another by the strength of the midlatitude westerlies. An SST anomaly with a maximum of 4°C is added to the climatological SST distribution of the model with both positive and negative polarity. These anomaly runs are compared to one another, and to a control integration, to determine the atmospheric response. In all cases warming (cooling) of the midlatitude ocean surface yields a warming (cooling) of the atmosphere over and to the east of the SST anomaly center. The atmospheric temperature change is largest near the surface and decreases upward. Consistent with this simple thermal response, the geopotential height field displays a baroclinic response with a shallow anomalous low somewhat downstream from the warm SST anomaly. The equivalent barotropic, downstream response is weak and not robust. To help interpret the results, the realistic GCM integrations are compared with parallel idealized model runs. The idealized model has full physics and a similar horizontal and vertical resolution, but an all-ocean surface with a single, permanent zonal asymmetry. The idealized and realistic versions of the GCM display compatible response patterns that are qualitatively consistent with stationary, linear, quasigeostrophic theory. However, the idealized model response is stronger and more coherent. The differences between the two model response patterns can be reconciled based on the size of the anomaly, the model treatment of cloud-radiation interaction, and the static stability of the model atmosphere in the vicinity of the SST anomaly. Model results are contrasted with other GCM studies and observations.

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Yochanan Kushnir and John M. Wallace

Abstract

Low-frequency variability in wintertime 500 mb height is examined, with emphasis on its structure, geographical distribution, and frequency dependence. A 39-year record of 500 mb geopotential height fields from the NMC analyses is time filtered to partition the fluctuations into frequency bands corresponding to periods of 10–60 days, 60–180 days and > 180 days. Winter is defined as the six month period November through April. Variance, teleconnectivity, and anisotropy fields, and selected loading vectors derived from orthogonal and oblique rotations of the eigenvectors of the temporal correlation matrix for each band are shown and discussed.

The variability in all frequency bands exhibits substantial anistropy, with meridionally elongated features arranged as zonally oriented wave trains prevailing over the continents and zonally elongated features organized in the form of north–south oriented dipole patterns prevailing over the oceanic sectors of the hemisphere. The wave trains are most pronounced in the 10–60 day variability, while the dipoles are most pronounced at lower frequencies. Eastward energy dispersion is apparent in the wave trains, but there is no evidence of phase propagation.

Most of the “teleconnection patterns” identified in previous studies appear among the more prominent loading vectors. However, in most cases the loading vectors occur in pairs, in which the two patterns are in spatial quadrature with one another and account for comparable fractions of the hemispherically integrated variance. It is argued that such patterns should be interpreted as basis functions that can be linearly combined to form a continuum of anisotropic structures. Evidence of the existence of discrete “modal structures” is found only in the interannual (> 180-day period) variability, where two patterns stand out clearly above the background continuum: the Pacific–North American (PNA) pattern and the North Atlantic Oscillation (NAO). These patterns leave clear imprints upon the climatological mean variance of the 500 mb height field and the anisotropy tensor of the 500 mb wine field. The western Atlantic (WA) pattern stands out somewhat above the background continuum in the month-to-month (60–180 day period) variability.

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Richard Seager, Yochanan Kushnir, and Mark A. Cane

Abstract

Recent modeling studies of thermohaline variability have imposed rapid damping of modeled sea surface temperature (SST) anomalies equivalent to assuming the atmosphere has an infinite heat capacity. Such surface heat flux parameterizations effectively exclude the possibility of SST playing an active role in the thermohaline circulation. The authors present results of simple thermodynamic modeling of the lower atmosphere that suggest the sensitivity of the surface heat fluxes to variations in SST is much smaller than often assumed. It is found that the flux response is strongly dependent on the scale of the SST anomaly. For the very largest scales the fluxes increase by only a few watts per square meter per kelvin change of SST. For the scales typical of observed anomalies the nonlocality of the response enhances the sensitivity, which may reach up to ∼15 W m−2 K−1. This extreme is still less than half of the values typically assumed in ocean models. The small sensitivity arises from the adjustment of the lower atmosphere to the underlying ocean in accord with its relatively much smaller ability to store heat and moisture. The increase in fluxes with SST is dominated by the latent heat flux but offset significantly by reduced net longwave radiative cooling of the surface.

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