David D. Houghton

Atmosphere-Ocean Interaction in the North Atlantic: Near-Surface Climate Variability*

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 208 and 608N) 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.

Response of the NCAR General Circulation Model to Prescribed Changes in Ocean Surface Temperature Part I: Mid-Latitude Changes

Abstract Four numerical experiments are analyzed to determine the three-dimensional response of the NCAR general circulation model to large prescribed changes in mid-latitude North Pacific Ocean surface temperature. The ocean surface temperature (OST) boundary conditions are subjected to changes of opposite sign in the eastern and west-central portions of the North Pacific Ocean. The maximum amplitude of the temperature changes is either 12°C or 4°C. The model atmosphere response in the North Pacific sector includes changes in amplitude and vertical tilt of the long waves, an increased direct thermal circulation (i.e., warm air rises over the positive OST change and cold air sinks over the negative OST change), and locally enhanced westerlies to the north of the positive OST change. Cyclones form and/or intensify over the positive OST change and tend to be absent or weak over the negative OST change. The mid-tropospheric response extends downstream from the prescribed change region, and the response both ...

The Coupling of Momentum Between Internal Gravity Waves and Mean Flow: A Numerical Study

Abstract A numerical model of internal gravity waves allows momentum transport by the waves to interact with the mean flow. Momentum deposited at a critical level develops a “shelf” in the mean flow. Mean flow acceleration Doppler-shifts the wave frequency, allowing more penetration of wave energy than expected from linear theory.

Response of a General Circulation Model to a Sea Temperature Perturbation

Abstract Sea temperature anomalies which departed from the December climatic mean by approximately 2C off the coast of Newfoundland were inserted into the NCAR six-layer, 5° mesh, general circulation model of the atmosphere in order to test the models response to small perturbations in sea surface temperature. The response of the model to the anomalies was analyzed with respect to pressure patterns, heat flux, and cyclone frequency, path and intensity. This response was compared with a statistical analysis of the response of the atmosphere to similar sea temperature anomalies based on approximately 80 years of observations as described by Ratcliffe and Murray. Analyses of the anomaly experiments are preceded by an analysis of the basic (control) statistics for both model and atmosphere. The most pronounced discrepancies between the two were noted in cyclone statistics. A calculation with double horizontal resolution greatly improved the model features. Detailed comparison was complicated by the fact that...

The Self-Destructing Internal Gravity Wave

Abstract A simple numerical model is used to demonstrate that momentum exchange between wave and mean flow can substantially modify the process of “breaking” of internal gravity waves at great height. The momentum exchange results in appreciable transfer of energy from wave to mean flow.

Processes Leading to the Formation of Mesoscale Waves in the Midwest Cyclone of 15 December 1987

Abstract On 15 December 1987, several long-lived, large-amplitude mesoscale wave disturbances accompanied a rapidly developing extratropical cyclone in the midwest United States. Previous observational and modeling studies have suggested that the disturbances had large amplitudes and long lifetimes as a result of a wave-CISK-type instability occurring within an imperfect wave duct and were initiated by convection. However. infrared (IR) satellite imagery and radar echoes shortly before the wave disturbances formed suggest that convection was not the primary feature in the wave genesis region at that time. Instead, a meso-β-scale comma-shaped cloud was present and appeared to evolve into the wave disturbances. The origins of the comma cloud can be traced back to a cloud streak and precipitation maximum in the left exit region of an approaching jet streak over northern Mexico 15 h earlier. In this study, satellite observations are examined in conjunction with numerical simulations of the case to explore a n...

Response of the NCAR General Circulation Model to Prescribed Changes in Ocean Surface Temperature. Part II: Midlatitude and Subtropical Changes

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 o...

LONG-TERM INTEGRATION OF THE BAROTROPIC EQUATIONS BY THE LAX-WENDROFF METHOD*

Abstract The system of equations describing the motion of a barotropic fluid with a free surface is solved by using finite-difference methods which are based on the two-step Lax-Wendroff scheme proposed by Richtmyer. The model has rigid boundaries along two latitude circles and the beta-plane approximation is adopted. Initial conditions are given for an ageostrophic jet flow and computations are carried out up to 100 days or 19,200 time steps in order to observe the long-term stability and truncation error properties of the numerical schemes. A series of integrations is performed using different numerical formulations for the Coriolis term and the boundary conditions. A Fickian type smoothing is introduced to control truncation errors for some of the cases. It is found that the numerical results depend critically upon the method for handling the Coriolis term. One scheme gives a relatively constant total energy during the entire 100 days while the rest of the schemes either develop instability or give dam...

An Observation of Banded Convective Development in the Presence of Weak Symmetric Stability

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 atmospheres response to free convective instability, contributing to the highly organized banded structure observed.

THE IMPACT OF INDIAN OCEAN SST ON THE LARGE-SCALE ASIAN SUMMER MONSOON AND THE HYDROLOGICAL CYCLE

Relationships among the large-scale hydrological cycle, Asian summer monsoon and Indian Ocean sea-surface temperature (SST) anomalies are investigated in a series of numerical experiments. Ensembles of integrations with the National Center for Atmospheric Research community climate model, CCM 1, focusing on the summer months (June, July, and August) provide the basic information for analysis. Impacts of SST anomalies in the southern Indian Ocean are evaluated by intercomparison of experiments with and without these anomalies. Systematic changes in the hydrological cycle and monsoon circulation are demonstrated and summarized in the context of a three-dimensional framework consisting of the basic elements of the Asian summer monsoon system. In the negative SST anomaly case, both the hydrological cycle and circulation processes are enhanced by virtue of their close relationship and especially the inherent coupling between the large-scale water vapour transport and the low-level monsoon flow. The overall intensity of the broad-scale monsoon, in this case, is enhanced. In the positive SST anomaly case, the model response is essentially opposite to that of the negative case and the intensity of the broad-scale monsoon is reduced. These enhanced/weakened monsoon scenarios share many similar features to observed interannual variations of the Asian summer monsoon.