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Steven K. Esbensen

Abstract

Intermonthly and interannual teleconnection patterns in the 700 mb geopotential height field during the Northern Hemisphere winter are computed by the method of digital filtering and spatial correlation analysis. A comparison of the teleconnection patterns of the intermonthly signal with the patterns described by Wallace and Gutzler shows that the western Pacific, western Atlantic and Pacific-North American patterns are clearly defined by the intermonthly signal; weak evidence for the eastern Atlantic pattern is also found. A northern Asian pattern, having one center over Mongolia and another over the Kara Sea near 70°N, 70°E, is also found to be a prominent feature of the intermonthly signal. The one-point correlation maps show that the spatial correlation function is dominated by the intermonthly band.

The Pacific-North American pattern is well-defined in both the intermonthly and interannual bands. Three additional patterns—the North Pacific, the Eurasian and the zonally-symmetric seesaw—appear in the interannual band. The North Pacific and Pacific-North American patterns qualitatively resemble patterns obtained by Horel and Wallace and by Chen in their studies of interannual teleconnections with the equatorial Pacific, while the, zonally-symmetric seesaw resembles the pattern identified by van Loon and Rogers. However the North Pacific, Pacific-North American and zonally-symmetric seesaw patterns are correlated with each other and with an index of the Southern Oscillation, suggesting that they are not independent modes of atmospheric variability.

The fact that some teleconnection patterns can be separated into either the intermonthly band or the interannual band suggests that there may be some important differences in the dynamical mechanisms responsible for the variability. The results also suggest that digital filtering may be a useful method for isolating the low-frequency disturbances in observed data or in the mutts of climate change simulations.

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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 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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Dean Vickers and Steven K. Esbensen

Abstract

Bulk aerodynamic formulas are applied to meteorological data from low-altitude aircraft flights to obtain observational estimates of the subgrid enhancement of momentum, sensible heat, and latent heat exchange at the atmospheric–oceanic boundary in light wind, fair weather conditions during TOGA COARE (Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment). Here, subgrid enhancement refers to the contributions of unresolved disturbances to the grid-box average fluxes at the lower boundary of an atmospheric general circulation model. The observed subgrid fluxes increase with grid-box area, reaching 11%, 9%, 24%, and 12% of the total sensible heat, latent heat, scalar wind stress, and vector wind stress magnitude, respectively, at a grid-box size of 2° × 2° longitude and latitude.

Consistent with previous observational and modeling studies over the open ocean, most of the subgrid flux is explained by unresolved directional variability in the near-surface wind field. The authors find that much of the observed variability in the wind field in the presence of fair weather convective bands and patches comes from contributions of curvature and speed variations of simple larger-scale structure across the grid box.

Inclusion of a grid-scale-dependent subgrid velocity scale in the bulk aerodynamic formulas effectively parameterizes the subgrid enhancement of the sensible heat flux, latent heat flux, and vector stress magnitude, and to a lesser degree the subgrid enhancement of the scalar wind stress. An observational estimate of the subgrid velocity scale derived from one-dimensional aircraft flight legs is found to be smaller than that derived from a two-dimensional grid-box analysis. The additional enhancement in the two-dimensional case is caused by the nonhomogeneous and nonisotropic characteristics of the subgrid-scale wind variability. Long time series from surface-based platforms in the TOGA COARE region suggest that measures of convective activity, in addition to geometric grid-scale parameters, will be required to more accurately represent the subgrid velocity scales.

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Edward I. Tollerud and Steven K. Esbensen

Abstract

A large-amplitude asymmetric vorticity couplet has been observed in the upper troposphere near a lame cloud cluster in GATE (GARP Atlantic Tropical Experiment) on 5 September 1974. The couplet consists of a cyclonic center to the north of the cluster and an anticyclonic center to the south. The couplet is a result of the deceleration of the upper-tropospheric easterly winds in a region near the center of the cluster. These features also appear on a composite of large slow-moving clusters occurring during Phase 3 of GATE.

Vorticity budget analysis shows that the couplets are produced by subcluster-scale process and, to a lesser degree, by cluster-scale twisting. On the basis of this finding and on the basis of the three-dimensional structure of horizontal momentum fields, it is suggested that a part of the deceleration producing the couplet is a result of momentum redistribution by subcluster-scale circulations (such as cumulonimbi or mesoscale cloud lines).

The composite wind fields of the large slow-moving cloud clusters during Phase 3 of GATE are found to be similar to horizontal composites made within easterly wave phase categories near the wave trough.

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Steven K. Esbensen and Richard W. Reynolds

Abstract

Air-sea transfers of sensible heat, latent heat and momentum are computed from 25 years of middle-latitude and subtropical ocean weather ship data in the North Atlantic and North Pacific using the bulk aerodynamic method. The results show that monthly averaged wind speeds, temperatures and humidities can be used to estimate the monthly averaged sensible and latent heat fluxes from the bulk aero-dynamic equations to within a relative error of ∼10%. The estimates of monthly averaged wind stress under the assumption of neutral stability are shown to be within ∼5% of the monthly averaged non-neutral values.

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Eric D. Maloney and Steven K. Esbensen

Abstract

Intraseasonal precipitation variability over the northeast Pacific warm pool during June–October in the National Center for Atmospheric Research Community Atmosphere Model 2.0.1 with a relaxed Arakawa–Schubert convection parameterization is found to be strongly sensitive to wind-induced variations in surface latent heat flux. A control simulation with interactive surface fluxes produces northeast Pacific warm pool intraseasonal wind and precipitation variations that are of similar magnitude and structure to those associated with the observed intraseasonal oscillation (ISO). Periods of low-level westerly intraseasonal wind anomalies are associated with enhanced surface latent heat fluxes and enhanced precipitation, as in observations. Variations in surface wind speed primarily control the surface flux anomalies.

A simulation in which eastern North Pacific oceanic latent heat fluxes are fixed produces intraseasonal precipitation variations that are significantly weaker than those in the control simulation and in observations. These results support the observational findings of Maloney and Esbensen, who suggested that wind-induced latent heat flux variability is a significant driver of ISO-related convective variability over the northeast Pacific warm pool during Northern Hemisphere summer. East Pacific ISO-related convection in this model, thus, appears to be forced by an analogous wind-induced surface heat exchange mechanism to that proposed by Maloney and Sobel to explain the forcing of west Pacific ISO-related convection. The surface exchange mechanism is apparently active within regions of mean westerly low-level flow.

In contrast, summertime eastern North Pacific intraseasonal wind variance and spatial structure does not differ significantly between the control and fixed-evaporation simulations. A strong coupling between the east Pacific flow and precipitation over Central America may be responsible for the relatively small changes in wind variability between the simulations. Interactions among the coarsely resolved Central American orography, the large-scale flow, and the convection parameterization in the model likely contribute to this anomalous coupling.

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Steven K. Esbensen and Michael J. McPhaden

Abstract

The enhancement of monthly averaged evaporation by atmospheric mesoscale systems is estimated from long-term hourly observations of surface meteorological data from the Tropical Ocean Global Atmosphere (TOGA) Tropical Atmosphere Ocean (TAO) buoy moorings over the equatorial Pacific Ocean and a bulk aerodynamic flux algorithm developed as a result of the TOGA Coupled Ocean–Atmosphere Response Experiment (COARE). It is shown that mesoscale enhancement is due primarily to the lack of wind steadiness on subsynoptic timescales and is associated with periods of significant precipitation.

The magnitude of the mesoscale enhancement of monthly averaged sea surface evaporation is found to be ∼10% or less of the total. During occasional periods with weak and variable winds over the western Pacific warm pool and the other major precipitation zones in the equatorial Pacific, the mesoscale enhancement of monthly averaged evaporation can reach 30% of the total evaporation.

A similar result is obtained for mesoscale enhancement of diffusive air–sea sensible heat transfer using data from TOGA TAO moorings. However, a comparison of results from the colocated TAO and Improved METeorological measurements (IMET) moorings during TOGA COARE, and results previously reported from a pre COARE cruise in the western Pacific warm pool region, indicate that processes in addition to mesoscale wind variability may be important contributors to the mesoscale enhancement of the sensible heat flux.

It is suggested that the most important effects of atmospheric mesoscale systems on tropical ocean evaporation and sensible heat flux are represented in existing climatologies.

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Eric D. Maloney and Steven K. Esbensen

Abstract

Tropical intraseasonal variability in the eastern North Pacific during June–September of 2000–03 is analyzed using satellite and buoy observations. Quick Scatterometer ocean vector winds and the Tropical Rainfall Measuring Mission (TRMM) precipitation indicate that periods of anomalous surface westerly flow over the east Pacific warm pool during a summertime intraseasonal oscillation (ISO) life cycle are generally associated with an enhancement of convection to the east of 120°W. An exception is a narrow band of suppressed precipitation along 8°N that is associated with negative column-integrated precipitable water anomalies and anticyclonic vorticity anomalies. Periods of surface easterly anomalies are generally associated with suppressed convection to the east of 120°W. Summertime wind jets in the Gulfs of Tehuantepec and Papagayo exhibit heightened activity during periods of ISO easterly anomalies and suppressed convection. Strong variations in east Pacific warm pool wind speed occur in association with the summertime ISO. Anomalous ISO westerly flow is generally accompanied by enhanced wind speed to the east of 120°W, while anomalous easterly flow is associated with suppressed wind speed. Intraseasonal vector wind anomalies added to the climatological flow account for the bulk of the wind speed enhancement in the warm pool during the westerly phase, while the easterly phase shows strong contributions to the negative wind speed anomaly from both intraseasonal vector wind anomalies and suppressed synoptic-scale eddy activity. An analysis using Tropical Atmosphere Ocean buoys and TRMM precipitation suggests that wind–evaporation feedback is important for supporting summertime intraseasonal convection over the east Pacific warm pool. A statistically significant correlation of 0.6 between intraseasonal latent heat flux and precipitation occurs at the 12°N, 95°W buoy. Correlations between precipitation and latent heat flux at the 10°N, 95°W and 8°N, 95°W buoys are positive (0.4), but not statistically significant. Intraseasonal latent heat flux anomalies at all buoys are primarily wind induced. Consistent with the suppressed convection there during the ISO westerly phase, a negative but not statistically significant correlation (−0.3) occurs between precipitation and latent heat flux at the 8°N, 110°W buoy.

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Edward I. Tollerud and Steven K. Esbensen

Abstract

The wind fields associated with cloud clusters observed during the Global Atmospheric Research Program's Atlantic Tropical Experiment (GATE) are investigated. A compositing procedure is devised to isolate the cluster circulations. Satellite-observed cloud cover estimates by Cox and Griffith form the basis for the identification and classification of clusters and for the determination of their life cycles. The compositing criteria focus on the upper-tropospheric portions of anvil clouds that are a prominent feature of cloud clusters. The compositing procedure is applied to a set of objectively analyzed upper-air winds for Phase 3 of GATE prepared by K. V. Ooyama and J.-H. Chu.

The results show that slow-moving cloud clusters tend to form in regions of relatively small vertical wind shear and that the shear at the cluster center decreases during the cluster life cycle. Squall clusters, on the other hand, have significantly larger lower-tropospheric shear.

Changes in the total horizontal wind field in the middle and lower troposphere during cluster evolution appear to be primarily due to the advance of easterly waves relative to the slower-moving clusters. However, the horizontal divergence field is centered on the cluster during its lifetime. The maximum value of the upper-tropospheric divergence at the cluster center lags the maximum boundary layer convergence by 3–6 hours. During the mature and dissipating stages, a layer of convergence develops in the middle troposphere.

The vertical motion diagnosed in the growing, mature and dissipating stages of the clusters is found to be qualitatively similar to the life cycle hypothesized for the smaller-scale individual mesoscale precipitation features that are found within the clusters. Strong upward motion develops in the upper troposphere from the growing to the mature stage; in the lower troposphere, the upward velocities decrease dramatically from the mature to the dissipating stage.

The cluster-scale vertical motion in the mature stage of the slow-moving clusters is compared with large-scale values in the trough of a composite easterly wave, with Phase-3 averaged vertical velocities, and with vertical motion estimates in squall lines. In general, vertical motion increases rapidly with decreasing space and time scales. The squall and nonsquall vertical motion profiles are qualitatively similar.

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