Surface Turbulent Fluxes Research Articles

Characterization of Turbulent Latent and Sensible Heat Flux Exchange between the Atmosphere and Ocean in MERRA

Abstract Turbulent fluxes of heat and moisture across the atmosphere–ocean interface are fundamental components of the earth’s energy and water balance. Characterizing both the spatiotemporal variability and the fidelity of these exchanges of heat and moisture is critical to understanding the global water and energy cycle variations, quantifying atmosphere–ocean feedbacks, and improving model predictability. This study examines the veracity of the recently completed NASA Modern-Era Retrospective Analysis for Research and Applications (MERRA) product in terms of its turbulent surface fluxes. This assessment employs a large dataset of directly measured turbulent fluxes as well as other turbulent surface flux datasets. The spatial and temporal variability of the surface fluxes are examined in terms of their annual-mean climatologies, their seasonal covariability of near-surface bulk parameters, and their representation of extremes. The impact of data assimilation on the near-surface parameters is assessed through evaluation of the incremental analysis update tendencies. It is found that MERRA turbulent surface fluxes are relatively accurate for typical conditions but have systematically weak vertical gradients in moisture and temperature and a weaker covariability between the near-surface gradients and wind speed than found in observations. This results in an underestimate of the surface latent and sensible heat fluxes over the western boundary current and storm-track regions. The assimilation of observations generally acts to bring MERRA closer to observational products by increasing moisture and temperature near the surface and decreasing the near-surface wind speeds.

Impact of imposed drying on deep convection in a cloud‐resolving model

This study uses a cloud‐resolving model under the weak temperature gradient (WTG) approximation to explore the response of deep convection to imposed drying. The drying, intended to mimic horizontal advection of dry air, is imposed as a linear relaxation of the humidity field toward zero within a specified layer. Radiative cooling and sea surface temperature are held constant and there is no mean vertical shear in the integrations. The statistically steady responses to drying in the lower, middle, and upper free troposphere are compared. The rain rate decreases with drying when that drying is imposed in the lower or middle free troposphere, more strongly so for lower tropospheric drying. The decrease in rainfall is linearly proportional to the drying to a good approximation. Upper‐tropospheric drying has almost no impact on the rain rate. For lower‐tropospheric drying, the decrease in rain rate with drying can be well explained by assuming that both an appropriately defined normalized gross moist stability and surface turbulent fluxes remain constant as drying varies. For middle‐ and upper‐tropospheric drying, the normalized gross moist stability decreases with drying, allowing the precipitation to decrease less rapidly or not at all. The variations in normalized gross moist stability in turn result from changes in the large‐scale vertical motion profiles. These are top‐heavy for weak or no drying, becoming somewhat less so as the strength of the drying increases. Over most of the range of drying strengths, the changes to the shape of the vertical motion profile are greatest for upper tropospheric drying and smallest for lower tropospheric drying. For very strong lower tropospheric drying, however, the vertical motion profile becomes bottom‐heavy and the normalized gross moist stability becomes negative.

Interpretation of the positive low-cloud feedback predicted by a climate model under global warming

The response of low-level clouds to climate change has been identified as a major contributor to the uncertainty in climate sensitivity estimates among climate models. By analyzing the behaviour of low-level clouds in a hierarchy of models (coupled ocean-atmosphere model, atmospheric general circulation model, aqua-planet model, single-column model) using the same physical parameterizations, this study proposes an interpretation of the strong positive low-cloud feedback predicted by the IPSL-CM5A climate model under climate change. In a warmer climate, the model predicts an enhanced clear-sky radiative cooling, stronger surface turbulent fluxes, a deepening and a drying of the planetary boundary layer, and a decrease of tropical low-clouds in regimes of weak subsidence. We show that the decrease of low-level clouds critically depends on the change in the vertical advection of moist static energy from the free troposphere to the boundary-layer. This change is dominated by variations in the vertical gradient of moist static energy between the surface and the free troposphere just above the boundary-layer. In a warmer climate, the thermodynamical relationship of Clausius-Clapeyron increases this vertical gradient, and then the import by large-scale subsidence of low moist static energy and dry air into the boundary layer. This results in a decrease of the low-level cloudiness and in a weakening of the radiative cooling of the boundary layer by low-level clouds. The energetic framework proposed in this study might help to interpret inter-model differences in low-cloud feedbacks under climate change.

Probability Distribution Characteristics for Surface Air–Sea Turbulent Heat Fluxes over the Global Ocean

Abstract To analyze the probability density distributions of surface turbulent heat fluxes, the authors apply the two-parametric modified Fisher–Tippett (MFT) distribution to the sensible and latent turbulent heat fluxes recomputed from 6-hourly NCEP–NCAR reanalysis state variables for the period from 1948 to 2008. They derived the mean climatology and seasonal cycle of the location and scale parameters of the MFT distribution. Analysis of the parameters of probability distributions identified the areas where similar surface turbulent fluxes are determined by the very different shape of probability density functions. Estimated extreme turbulent heat fluxes amount to 1500–2000 W m−2 (for the 99th percentile) and can exceed 2000 W m−2 for higher percentiles in the subpolar latitudes and western boundary current regions. Analysis of linear trends and interannual variability in the mean and extreme fluxes shows that the strongest trends in extreme fluxes (more than 15 W m−2 decade−1) in the western boundary current regions are associated with the changes in the shape of distribution. In many regions changes in extreme fluxes may be different from those for the mean fluxes at interannual and decadal time scales. The correlation between interannual variability of the mean and extreme fluxes is relatively low in the tropics, the Southern Ocean, and the Kuroshio Extension region. Analysis of probability distributions in turbulent fluxes has also been used in assessing the impact of sampling errors in the Voluntary Observing Ship (VOS)-based surface flux climatologies, allowed for the estimation of the impact of sampling in extreme fluxes. Although sampling does not have a visible systematic effect on mean fluxes, sampling uncertainties result in the underestimation of extreme flux values exceeding 100 W m−2 in poorly sampled regions.

Mesoscale structures of two types of cold-air outbreaks over the East China Sea and the effect of coastal sea surface temperature

Two types of cold-air outbreaks over the Yellow and East China Seas are investigated using a regional mesoscale model. Distinct patterns of surface turbulent heat fluxes and precipitation are identified between the two cases. The sea surface heat flux and frictional velocity are strongly influenced by mesoscale differences between high- and low-resolution datasets of sea surface temperature (SST). The influence of the SST difference on atmospheric water is spread to the downstream area of the outbreak with the phase transition of water. The air mass transformation around 800 hPa over the Kuroshio is partly influenced by the upstream SST difference. In particular, the SST difference near the mouth of the Yanzi River strongly modifies the air mass around Taiwan. Thus, in addition to the Kuroshio front, the mesoscale Chinese coastal SST structure is also important in the air mass transformation over the East China Sea.

Closing the Global Water Vapor Budget with AIRS Water Vapor, MERRA Reanalysis, TRMM and GPCP Precipitation, and GSSTF Surface Evaporation

Abstract The authors investigate if atmospheric water vapor from remote sensing retrievals obtained from the Atmospheric Infrared Sounder/Advanced Microwave Sounding Unit (AIRS) and the water vapor budget from the NASA Goddard Space Flight Center (GSFC) Modern Era Retrospective-analysis for Research and Applications (MERRA) are physically consistent with independently synthesized precipitation data from the Tropical Rainfall Measuring Mission (TRMM) or the Global Precipitation Climatology Project (GPCP) and evaporation data from the Goddard Satellite-based Surface Turbulent Fluxes (GSSTF). The atmospheric total water vapor sink (Σ) is estimated from AIRS water vapor retrievals with MERRA winds (AIRS–MERRA Σ) as well as directly from the MERRA water vapor budget (MERRA–MERRA Σ). The global geographical distributions as well as the regional wavelet amplitude spectra of Σ are then compared with those of TRMM or GPCP precipitation minus GSSTF surface evaporation (TRMM–GSSTF and GPCP–GSSTF P − E, respectively). The AIRS–MERRA and MERRA–MERRA Σs reproduce the main large-scale patterns of global P − E, including the locations and variations of the ITCZ, summertime monsoons, and midlatitude storm tracks in both hemispheres. The spectra of regional temporal variations in Σ are generally consistent with those of observed P − E, including the annual and semiannual cycles, and intraseasonal variations. Both AIRS–MERRA and MERRA–MERRA Σs have smaller amplitudes for the intraseasonal variations over the tropical oceans. The MERRA P − E has spectra similar to that of MERRA–MERRA Σ in most of the regions except in tropical Africa. The averaged TRMM–GSSTF and GPCP–GSSTF P − E over the ocean are more negative compared to the AIRS–MERRA, MERRA–MERRA Σs, and MERRA P − E.

The Diurnal Behavior of Evaporative Fraction in the Soil–Vegetation–Atmospheric Boundary Layer Continuum

AbstractThe components of the land surface energy balance respond to periodic incoming radiation forcing with different amplitude and phase characteristics. Evaporative fraction (EF), the ratio of latent heat to available energy at the land surface, supposedly isolates surface control (soil moisture and vegetation) from radiation and turbulent factors. EF is thus supposed to be a diagnostic of the surface energy balance that is constant or self-preserved during daytime. If this holds, EF can be an effective way to estimate surface characteristics from temperature and energy flux measurements. Evidence for EF diurnal self-preservation is based on limited-duration field measurements. The daytime EF self-preservation using both long-term measurements and a model of the soil–vegetation–atmosphere continuum is reexamined here. It is demonstrated that EF is rarely constant and that its temporal power spectrum is wide; thus emphasizing the role of all diurnal frequencies associated with reduced predictability in its daylight response. Oppositely, surface turbulent heat fluxes are characterized by a strong response to the principal daily frequencies (daily and semi-daily) of the solar radiative forcing. It is shown that the phase lag and bias between the turbulent flux components of the surface energy balance are key to the shape of the daytime EF. Therefore, an understanding of the physical factors that affect the phase lag and bias in the response of the components of the surface energy balance to periodic radiative forcing is needed. A linearized model of the soil–vegetation–atmosphere continuum is used that can be solved in terms of harmonics to explore the physical factors that determine the phase characteristics. The dependency of these phase and offsets on environmental parameters—friction velocity, water availability, solar radiation intensity, relative humidity, and boundary layer entrainment—is then analyzed using the model that solves the dynamics of subsurface and atmospheric boundary layer temperatures and heat fluxes in a continuum. Additionally, the asymptotical diurnal lower limit of EF is derived as a function of these surface parameters and shown to be an important indicator of the self-preservation value when the conditions (also identified) for such behavior are present.

An Assessment of the Uncertainties in Ocean Surface Turbulent Fluxes in 11 Reanalysis, Satellite-Derived, and Combined Global Datasets

Abstract Ocean surface turbulent fluxes play an important role in the energy and water cycles of the atmosphere–ocean coupled system, and several flux products have become available in recent years. Here, turbulent fluxes from 6 widely used reanalyses, 4 satellite-derived flux products, and 2 combined product are evaluated by comparison with direct covariance latent heat (LH) and sensible heat (SH) fluxes and inertial-dissipation wind stresses measured from 12 cruises over the tropics and mid- and high latitudes. The biases range from −3.0 to 20.2 W m−2 for LH flux, from −1.4 to 6.0 W m−2 for SH flux, and from −7.6 to 7.9 × 10−3 N m−2 for wind stress. These biases are small for moderate wind speeds but diverge for strong wind speeds (>10 m s−1). The total flux biases are then further evaluated by dividing them into uncertainties due to errors in the bulk variables and the residual uncertainty. The bulk-variable-caused uncertainty dominates many products’ SH flux and wind stress biases. The biases in the bulk variables that contribute to this uncertainty can be quite high depending on the cruise and the variable. On the basis of a ranking of each product’s flux, it is found that the Modern-Era Retrospective Analysis for Research and Applications (MERRA) is among the “best performing” for all three fluxes. Also, the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERA-Interim) and the National Centers for Environmental Prediction–Department of Energy (NCEP–DOE) reanalysis are among the best performing for two of the three fluxes. Of the satellite-derived products, version 2b of the Goddard Satellite-Based Surface Turbulent Fluxes (GSSTF2b) is among the best performing for two of the three fluxes. Also among the best performing for only one of the fluxes are the 40-yr ERA (ERA-40) and the combined product objectively analyzed air–sea fluxes (OAFlux). Direction for the future development of ocean surface flux datasets is also suggested.

The Uncertainty and Sensitivity Analysis of Surface Turbulent Fluxes to Remote Sensing Products

The Uncertainty and Sensitivity Analysis of Surface Turbulent Fluxes to Remote Sensing Products

Land use change exacerbates tropical South American drought by sea surface temperature variability

[1] Observations of tropical South American precipitation over the last three decades indicate an increasing rainfall trend to the north and a decreasing trend to the south. Given that tropical South America has experienced significant land use change over the same period, it is of interest to assess the extent to which changing land use may have contributed to the precipitation trends. Simulations of the National Center for Atmospheric Research Community Atmosphere Model (NCAR CAM3) analyzed here suggest a non-negligible impact of land use on this precipitation behavior. While forcing the model by imposed historical sea surface temperatures (SSTs) alone produces a plausible north-south precipitation dipole over South America, NCAR CAM substantially underestimates the magnitude of the observed southern decrease in rainfall unless forcing associated with human-induced land use change is included. The impact of land use change on simulated precipitation occurs primarily during the local dry season and in regions of relatively low annual-mean rainfall, as the incidence of very low monthly-mean accumulations (<10 mm/month) increases significantly when land use change is imposed. Land use change also contributes to the simulated temperature increase by shifting the surface turbulent flux partitioning to favor sensible over latent heating. Moving forward, continuing pressure from deforestation in tropical South America will likely increase the occurrence of significant drought beyond what would be expected by anthropogenic warming alone and in turn compound biodiversity decline from habitat loss and fragmentation.

A relationship between the aerodynamic and physical roughness of winter sea ice

AbstractA bulk flux algorithm predicts the turbulent surface fluxes of momentum and sensible and latent heat from mean measured or modelled meteorological variables. The bulk flux algorithm resulting from data collected over winter sea ice during SHEBA, the experiment to study the Surface Heat Budget of the Arctic Ocean, failed, however, in its first trial to predict the turbulent momentum flux over sea ice in the Antarctic. This result suggests that the main parameter for predicting the momentum flux, the aerodynamics roughness length z0, does not respond just to the friction velocity, as in the SHEBA algorithm, but is closely related to the physical roughness of snow‐covered sea ice and may need to be site‐specific. I investigate this idea with simultaneous measurements of z0 and the physical roughness of the surface, ξ, at Ice Station Weddell. The metric ξ derives from surveys of surface elevation and is related to but always less than the standard deviation in surface elevation. On combining the z0–ξ pairs from Ice Station Weddell with similar data obtained over Arctic sea ice, I show that the Arctic and Antarctic z0–ξ data lie along a continuum such that measuring ξ could provide a means for estimating a site‐specific z0 for any global sea ice surface. Backscatter data from satellite‐borne synthetic aperture radar might provide a remotely sensed estimate of ξ. Copyright © 2011 Royal Meteorological Society

Modeling of Regional Hydroclimate Change over the Indian Subcontinent: Impact of the Expanding Thar Desert

Abstract The Thar Desert between northwestern India and Pakistan is the most densely populated desert region in the world, and the vast surrounding areas are affected by rapid soil degradation and vegetation loss. The impact of an expanded desert (implemented by changing vegetation type and related greenness fraction, albedo, surface roughness length, emissivity, among others) on the South Asian summer monsoon hydroclimate is investigated by means of 7-month, 4-member ensemble sensitivity experiments with the Weather Research and Forecasting model. It is found that extended desertification significantly affects the monsoon at local and large scales. Locally, the atmospheric water cycle weakens because precipitation, evaporation, and atmospheric moisture convergence all decrease; soil moisture and runoff reduce too. Air temperature cools because of an increase in albedo (the desert makes the area brighter) and a reduction of surface turbulent fluxes; the cooling is partially offset by adiabatic descent, generated to maintain thermodynamic balance and originating at the northern flank of the low-level anticyclone forced by desert subsidence. Regionally, an anomalous northwesterly flow over the Indo-Gangetic Plain weakens the monsoon circulation over northeastern India, causing precipitation to decrease and the formation of an anomalous anticyclone in the region. As a result, the middle troposphere cools because of a decrease in latent heat release, but the ground heats up because of a reduction in cloudiness. At larger scale, the interaction between the anomalous circulation and the mountains leads to an increase in precipitation over the eastern Himalayas and Indochina. The findings of this study reveal that the expansion of the Thar Desert can lead to a pronounced and large-scale impact on summer monsoon hydroclimate, with a potential to redistribute precious water over South Asia.

On the Persistence of Cold-Season SST Anomalies Associated with the Annular Modes

Abstract In this study, a simple stochastic climate model is used to examine the impact of the ocean mixed layer depth, surface turbulent energy fluxes, and Ekman currents on the persistence of cold-season extratropical sea surface temperature (SST) anomalies associated with variability in the annular modes of atmospheric circulation in both hemispheres. Observational analysis reveals that during the cold season, SST anomalies associated with the southern annular mode (SSTSAM) persist considerably longer than those associated with the northern annular mode (SSTNAM). Using the simple model, it is shown that the persistence of the cold-season SSTSAM is consistent with the simple stochastic climate paradigm in which the atmospheric forcing is approximated as white noise, and the persistence of SST anomalies can be largely determined by the thermal inertia of the ocean mixed layer. In the North Atlantic, however, the simple climate model overestimates the persistence of the cold-season SSTNAM. It is thought that this overestimate occurs because the NAM-related heat flux forcing cannot be described purely as white noise but must also include a feedback from the underlying SST anomalies.

Observations and Modelling of the Atmospheric Boundary Layer Over Sea-Ice in a Svalbard Fjord

Sonic anemometer and profile mast measurements made in Wahlenbergfjorden, Svalbard Arctic archipelago, in May 2006 and April 2007 were employed to study the atmospheric boundary layer over sea-ice. The turbulent surface fluxes of momentum and sensible heat were calculated using eddy correlation and gradient methods. The results showed that the literature-based universal functions underestimated turbulent mixing in strongly stable conditions. The validity of the Monin-Obukhov similarity theory was questionable for cross-fjord flow directions and in the presence of mesoscale variability or topographic effects. The aerodynamic roughness length showed a dependence on the wind direction. The mean roughness length for along-fjord wind directions was (2.4 ± 2.6) × 10−4 m, whereas that for cross-fjord directions was (5.4 ± 2.8) × 10−3 m. The thermal stratification and turbulent fluxes were affected by the synoptic situation with large differences between the 2 years. Channelling effects and drainage flows occurred especially during a weak large-scale flow. The study periods were simulated applying the Weather Research and Forecasting (WRF) model with 1-km horizontal resolution in the finest domain. The results for the 2-m air temperature and friction velocity were good, but the model failed to reproduce the spatial variability in wind direction between measurement sites 3 km apart. The model suggested that wind shear above the stable boundary layer provided a non-local source for the turbulence observed.

Mathematical Modeling of Transport Processes in Funnel Shaped Mold of Steel Thin Slab Continuous Caster

In this work a three-dimensional fluid flow and heat transfer model was developed to predict the flow pattern and superheat dissipation in funnel shaped mold of a thin slab continuous caster with a novel tetrafurcated design for the submerged entry nozzle. Low Reynolds k−ε turbulent model was adopted to account for the turbulent effect. The transport equations were solved numerically using finite volume method. The results were compared with a full scale water model of the caster. Good agreement between mathematical and physical models was obtained. Parametric studies were carried out to evaluate the effect of casting speed, nozzle submergence depth, and inlet temperature on the superheat dissipation, flow pattern, and surface turbulence in the mold region. The results indicate a special flow pattern and heat distribution in the caster while using a tetrafurcated nozzle. Aiming to achieve more product capacity, in the case of casting with lower superheat temperature, a higher casting speed, together with higher submergence depth, is recommended in order to avoid surface turbulence and high heat flux across the narrow face.

Root zone soil moisture from the assimilation of screen-level variables and remotely sensed soil moisture

[1] In most operational NWP models, root zone soil moisture is constrained using observations of screen-level temperature and relative humidity. While this generally improves low-level atmospheric forecasts, it often leads to unrealistic model soil moisture. Consequently, several NWP centers are moving toward also assimilating remotely sensed near-surface soil moisture observations. Within this context, an EKF is used to compare the assimilation of screen-level observations and near-surface soil moisture data from AMSR-E into the ISBA land surface model over July 2006. Several issues regarding the use of each data type are exposed, and the potential to use the AMSR-E data, either in place of or together with the screen-level data, is examined. When the two data types are assimilated separately, there is little agreement between the root zone soil moisture updates generated by each, indicating that for this experiment the AMSR-E data could not have replaced the screen-level data to obtain similar surface turbulent fluxes. For the screen-level variables, there is a persistent diurnal cycle in the model-observations bias, which is not related to soil moisture. The resulting diurnal cycle in the analysis increments demonstrates how assimilating screen-level observations can lead to unrealistic soil moisture updates, reinforcing the need to assimilate alternative data sets. However, when the two data types are assimilated together, the near-surface soil moisture provides a much weaker constraint of the root zone soil moisture than the screen-level observations do, and the inclusion of the AMSR-E data does not substantially change the results compared to the assimilation of screen-level variables alone.

The Rossby Centre Regional Climate model RCA3: model description and performance

This paper describes the third full release of the Rossby Centre Regional Climate model (RCA3), with an emphasis on changes compared to earlier versions, in particular the introduction of a new tiled land-surface scheme. The model performance over Europe when driven at the boundaries by ERA40 reanalysis is discussed and systematic biases identified. This discussion is performed for key near-surface variables, such as temperature, precipitation, wind speed and snow amounts at both seasonal and daily timescales. An analysis of simulated clouds and surface turbulent and radiation fluxes is also made, to understand the causes of the identified biases. RCA3 shows equally good, or better, correspondence to observations than previous model versions at both analysed timescales. The primary model bias relates to an underestimate of the diurnal surface temperature range over Northern Europe, which maximizes in summer. This error is mainly linked to an overestimate of soil heat flux. It is shown that the introduction of an organic soil component reduces the error significantly. During the summer season, precipitation and surface evaporation are both overestimated over Northern Europe, whereas for most other regions and seasons precipitation and surface turbulent fluxes are well simulated.

Modelling of spatial variability and topographic effects over Arctic fjords in Svalbard

The spatial variability of near-surface variables and turbulent surface fluxes was investigated in three Arctic fjords in Svalbard applying the Weather Research and Forecasting (WRF) mesoscale model. Ten real cases from winter and spring 2008, representing the most common large-scale flow directions, were simulated at 9, 3 and 1 km resolutions for 36 h each. Validation against tower observations and radiosoundings showed fairly good agreement, although a systematic warm and moist bias and slightly overestimated wind speeds were found close to the surface. The spatial variability within a fjord was large and it often reached levels comparable to the temporal variability. The spatial variability of the surface fluxes of sensible and latent heat was mostly controlled by the air and sea surface temperatures instead of wind speed. The same cases were also simulated without any topography over Svalbard. The topography increased the spatial variability but the influence on the mean values was not systematic, except that a clear warming effect was seen in all the fjords studied. The role of surface type increased with increasing air—sea temperature difference and was dominating over topographic effects for the air temperature, specific humidity and turbulent heat fluxes.

Modelling and measurements of the atmospheric boundary layer in Sofia, Bulgaria

The evaluation of mesoscale model simulations against experimental data on the vertical profiles of mean meteorological parameters and surface turbulent momentum and heat fluxes for Sofia, Bulgaria, is presented. The simulation of the vertical profiles is currently a weak point in mesoscale modelling, and needs investigation in view of the numerous practical applications.

A Method for Improving Simulation of PNA Teleconnection Interannual Variation in a Climate Model

The climate modeling community has been challenged to develop a method for improving the simula- tion of the Pacific-North America (PNA) teleconnection pattern in climate models. The accuracy of PNA telecon- nection simulation is significantly improved by consider- ing mesoscale convection contributions to sea surface fluxes. The variation in the PNA over the past 22 years was simulated by the Grid Atmospheric Model of IAP LASG version 1.0 (GAMIL1.0), which was guided by observational SST from January 1979 to December 2000. Results show that heating in the tropical central-eastern Pacific is simulated more realistically, and sea surface latent heat flux and precipitation anomalies are more similar to the reanalysis data when mesoscale enhance- ment is considered during the parameterization scheme of sea surface turbulent fluxes in GAMIL1.0. Realistic heat- ing in the tropical central-eastern Pacific in turn signifi- cantly improves the simulation of interannual variation and spatial patterns of PNA.  Keywords: sea surface turbulent flux parameterization, PNA, climate simulation Citation: Li, Z.-X., T.-J. Zhou, Z.-B. Sun, et al., 2011: A method for improving simulation of PNA teleconnection interannual variation in a climate model, Atmos. Oceanic Sci. Lett., 4, 86-90.