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David D. Houghton
,
Robert G. Gallimore
, and
Linda M. Keller

Abstract

Two 100-year seasonal simulators, one performed with a low resolution atmospheric general circulation model (GCM) coupled to a mixed-layer ocean formulation and the other made with the GCM forced by prescribed ocean conditions, are compared to assess the effects of an interactive ocean and sea-ice component on the stability and interannual variability of a climate system. Characteristics of the time variation of surface temperature, 700 mb temperature and sea-ice coverage are analyzed for selected land and ocean areas. Both simulations showed stable seasonal cycles of basic variables, although small trends were found. These trends were roughly linear in nature and quite distinct from all other components of variability. Detrended time series were used to describe the other aspects of variability.

There was pronounced interannual variability in the simulations from both models as seen in the time series for temperature and sea ice over the entire 100-year time period. Consistent with observations, variations tended to be larger in polar areas and over land. The inclusion of the interactive ocean and sea-ice component produced a red spectrum for surface temperature but not for 700-mb temperature. Using a linearized air-sea model patterned after the coupled models, this result is shown to be linked to the combined effects of the model longwave cooling and ocean-atmosphere energy exchange. The shift towards lower frequency in surface temperature was most evident in polar regions and occurred in conjunction with very low frequency (even decadal-scale) variability in the computed sea-ice coverage. The simulated mean and variability characteristics of sea ice corresponded fairly well with observations. This suggests that the low resolution model is able to represent some relevant aspects of the physics of climate fluctuations and thus provide useful simulations for studies of interannual variability.

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Uma S. Bhatt
,
Michael A. Alexander
,
David S. Battisti
,
David D. Houghton
, and
Linda M. Keller

Abstract

The impact of an interactive ocean on the midlatitude atmosphere is examined using a 31-yr integration of a variable depth mixed layer ocean model of the North Atlantic (between 20° and 60°N) coupled to the NCAR Community Climate model (CCM1). Coupled model results are compared with a 31-yr control simulation where the annual cycle of sea surface temperatures is prescribed. The analysis focuses on the northern fall and winter months.

Coupling does not change the mean wintertime model climatology (December–February); however, it does have a significant impact on model variance. Air temperature and mixing ratio variance increase while total surface heat flux variance decreases. In addition, it is found that air–sea interaction has a greater impact on seasonally averaged variance than monthly variance.

There is an enhancement in the persistence of air temperature anomalies on interannual timescales as a result of coupling. In the North Atlantic sector, surface air and ocean temperature anomalies during late winter are uncorrelated with the following summer but are significantly correlated (0.4–0.6) with anomalies during the following winter. These autocorrelations are consistent with the “re-emergence” mechanism, where late winter ocean temperature anomalies are sequestered beneath the shallow summer mixed layer and are reincorporated into the deepening fall mixed layer. The elimination of temperature anomalies from below the mixed layer in a series of uncoupled sensitivity experiments notably reduces the persistence of year-to-year anomalies.

The persistence of air temperature anomalies on monthly timescales also increases with coupling and is likely associated with “decreased thermal damping.” When coupled to the atmosphere, the ocean is able to adjust to the overlying atmosphere so that the negative feedback associated with anomalous heat fluxes decreases, and air temperature anomalies decay more slowly.

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Stephen D. Jascourt
,
Scott S. Lindstrom
,
Charles J. Seman
, and
David D. Houghton

Abstract

Satellite imagery dramatically portrays a mesoscale organization of deep convection over the south central United States on 5 June 1986. Free convection was expected over the region. The rapid development and organization of the convection simultaneously across a broad area suggests the presence of a mesoscale instability. Analysis of satellite and conventional data suggests that a layer of weak symmetric stability modified the atmosphere's response to free convective instability, contributing to the highly organized banded structure observed.

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Linda M. Keller
,
Michael C. Morgan
,
David D. Houghton
, and
Ross A. Lazear

Abstract

A climatology of large-scale, persistent cyclonic flow anomalies over the North Pacific was constructed using the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) global reanalysis data for the cold season (November–March) for 1977–2003. These large-scale cyclone (LSC) events were identified as those periods for which the filtered geopotential height anomaly at a given analysis point was at least 100 m below its average for the date for at least 10 days. This study identifies a region of maximum frequency of LSC events at 45°N, 160°W [key point 1 (KP1)] for the entire period. This point is somewhat to the east of regions of maximum height variability noted in previous studies. A second key point (37.5°N, 162.5°W) was defined as the maximum in LSC frequency for the period after November 1988. The authors show that the difference in location of maximum LSC frequency is linked to a climate regime shift at about that time. LSC events occur with a maximum frequency in the period from November through January.

A composite 500-hPa synoptic evolution, constructed relative to the event onset, suggests that the upper-tropospheric precursor for LSC events emerges from a quasi-stationary long-wave trough positioned off the east coast of Asia. In the middle and lower troposphere, the events are accompanied by cold thickness advection from a thermal trough over northeastern Asia. The composite mean sea level evolution reveals a cyclone that deepens while moving from the coast of Asia into the central Pacific. As the cyclone amplifies, it slows down in the central Pacific and becomes nearly stationary within a day of onset. Following onset, at 500 hPa, a stationary wave pattern, resembling the Pacific–North American teleconnection pattern, emerges with a ridge immediately downstream (over western North America) and a trough farther downstream (from the southeast coast of the United States into the western North Atlantic). The implications for the resulting sensible weather and predictability of the flow are discussed. An adjoint-derived sensitivity study was conducted for one of the KP1 cases identified in the climatology. The results provide dynamical confirmation of the LSC precursor identification for the events. The upper-tropospheric precursor is seen to play a key role not only in the onset of the lower-tropospheric height falls and concomitant circulation increases, but also in the eastward extension of the polar jet across the Pacific. The evolution of the forecast sensitivities suggest that LSC events are not a manifestation of a modal instability of the time mean flow, but rather the growth of a favorably configured perturbation on the flow.

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Robert M. Chervin
,
Jouhn E. Kutzbach
,
David D. Houghton
, and
Robert G. Gallimore

Abstract

The sensitivity of a six-layer NCAR atmospheric general circulation model (GCM) to a variety of idealized, very large amplitude, midlatitude and subtropical North Pacific Ocean surface temperature (OST) anomalies is analyzed. In the Pacific sector, the model exhibits a differential sensitivity depending on the latitudinal position of the imposed anomaly. Typically, the model response is a combination of a relative direct thermal circulation, an alteration in the pattern of cyclonic activity, and a selective wave response dependent on the planetary waves present in the unmodified control case. The extent to which the background planetary waves affect the model response is dependent on several factors including latitude-dependent features of the control case and the relative position of the anomaly. Analogous experiments with a simple quasi-geostrophic model are useful in isolating important physical and dynamical processes, and thereby assist in the interpretation of the GCM results. An analysis of the correlation between observed tropospheric thickness and North Pacific OST provides some confirmation of the consistent model result of an anomalously warm troposphere over a warm OST anomaly.

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David D. Houghton
,
Todd S. Glickman
,
Jane Dannenberg
, and
Stanley L. Marsh
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EXECUTIVE COMMITTEE
,
Ronald D. McPherson
,
Eugene M. Rasmusson
,
Paul D. Try
,
David D. Houghton
,
Susan K. Avery
,
William H. Hooke
,
Richard E. Hallgren
, and
Kenneth C. Spengler
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EXECUTIVE COMMITTEE2
,
Paul D. Try
,
Ronald D. McPherson
,
David D. Houghton
,
Warren M. Washington
,
Richard S. Greenfield
,
Susan K. Avery
,
Richard E. Hallgren
, and
Kenneth C. Spengler
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EXECUTIVE COMMITTEE2
,
David D. Houghton
,
Paul D. Try
,
Warren M. Washington
,
Robert T. Ryan
,
Margaret A. LeMone
,
Richard S. Greenfield
,
Richard E. Hallgren
, and
Kenneth C. Spengler
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EXECUTIVE COMMITTEE2
,
Warren M. Washington
,
David D. Houghton
,
Robert T. Ryan
,
Donald R. Johnson
,
Margaret A. LeMone
,
Alexander E. MacDonald
,
Richard E. Hallgren
, and
Kenneth C. Spengler
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