The primary focus of this review is tropical-extratropical interactions and especially the issues involved in determining the response of the extratropical atmosphere to tropical forcing associated with sea surface temperature (SST) anomalies. The review encompasses observations, empirical studies, theory and modeling of the extratropical teleconnections with a focus on developments over the Tropical Oceans-Global Atmosphere (TOGA) decade and the current state of understanding. In the tropical atmosphere, anomalous SSTs force anomalies in convection and large-scale overturning with subsidence in the descending branch of the local Hadley circulation. The resulting strong upper tropospheric divergence in the tropics and convergence in the subtropics act as a Rossby wave source. The climatological stationary planetary waves and associated jet streams, especially in the northern hemisphere, can make the total Rossby wave sources somewhat insensitive to the position of the tropical heating that induces them and thus can create preferred teleconnection response patterns, such as the Pacific-North American (PNA) pattern. However, a number of factors influence the dispersion and propagation of Rossby waves through the atmosphere, including zonal asymmetries in the climatological state, transients, and baroclinic and nonlinear effects. Internal midlatitude sources can amplify perturbations. Observations, modeling, and theory have clearly shown how storm tracks change in response to changes in quasi-stationary waves and how these changes generally feedback to maintain or strengthen the dominant perturbations through vorticity and momentum transports. The response of the extratropical atmosphere naturally induces changes in the underlying surface, so that there are changes in extratropical SSTs and changes in land surface hydrology and moisture availability that can feedback and influence the total response. Land surface processes are believed to be especially important in spring and summer. Anomalous SSTs and tropical forcing have tended to be strongest in the northern winter, and teleconnections in the southern hemisphere are weaker and more variable and thus more inclined to be masked by natural variability. Occasional strong forcing in seasons other than winter can produce strong and identifiable signals in the northern hemisphere and, because the noise of natural variability is less, the signal-to-noise ratio can be large. The relative importance of tropical versus extratropical SST forcings has been established through numerical experiments with atmospheric general circulation models (AGCMs). Predictability of anomalous circulation and associated surface temperature and precipitation in the extratropics is somewhat limited by the difficulty of finding a modest signal embedded in the high level of noise from natural variability in the extratropics, and the complexity and variety of the possible feedbacks. Accordingly, ensembles of AGCM runs and time averaging are needed to identify signals and make predictions. Strong anomalous tropical forcing provides opportunities for skillful forecasts, and the accuracy and usefulness of forecasts is expected to improve as the ability to forecast the anomalous SSTs improves, as models improve, and as the information available from the mean and the spread of ensemble forecasts is better utilized.

/pdf/progress-during-toga-in-understanding-and-modeling-global-bqacmww353.pdf

Progress during TOGA in understanding and modeling global teleconnections associated with tropical sea surface temperatures

A local quasi-geostrophic energetics analysis indicates that within the jet core, low-frequency (LF) eddies behave baroclinically essentially the same as high-frequency (HF) eddies. They both have a westward tilting vertical structure and both grow baroclinically by transporting heat poleward and by converting eddy potential energy to kinetic energy. However, the difference in the horizontal orientations of HF and LF eddies has several important implications to their amplitude and peak locations, as well as their interaction with stationary waves. The barotropic decay of meridionally elongated HF eddies tends to terminate the growth of HF eddies beyond the jet exit region. The barotropic growth of the zonally elongated LF eddies not only ensure a continuous growth of LF eddies in the jet exit region, but also results in a new baroclinic growth of LF eddies farther downstream due to the presence of the west–east temperature contrast associated with stationary waves. The continuous growth of LF eddies due to both barotropic and baroclinic processes in the jet exit region is consistent with the facts that LF eddies reach maximum variability farther downstream of the two major jet streams and that the LF variability is much stronger than HF eddies. The results of energetics analysis are confirmed by the feedback analysis, showing that HF eddies, being dominated by meridional orientations, mainly act to maintain (damp) stationary waves by locally enhancing (reducing) north–south gradient of the height (temperature) field near the jet core regions. The zonally elongated LF eddies, on the other hand, act to primarily reduce the zonal gradient associated with stationary waves both barotropically and baroclinically.

Dynamical implications of the orientation of atmospheric eddies: a local energetics perspective

Snow precipitation is the primary mass input to the Antarctic ice sheet and is one of the most direct climatic indicators, with important implications for paleoclimatic reconstruction from ice cores. Provenance of precipitation and the dynamic conditions that force these precipitation events at four deep ice core sites (Dome C, Law Dome, Talos Dome, and Taylor Dome) in East Antarctica were analysed with air mass back trajectories calculated using the Lagrangian model and the mean composite data for precipitation, geopotential height and wind speed field data from the European Centre for Medium Range Weather Forecast from 1980 to 2001. On an annual basis, back trajectories showed that the Atlantic-Indian and Ross-Pacific Oceans were the main provenances of precipitation in Wilkes Land (80%) and Victoria Land (40%), respectively, whereas the greatest influence of the ice sheet was on the interior near the Vostok site (80%) and in the Southwest Ross Sea (50%), an effect that decreased towards the coast and along the Antarctic slope. Victoria Land received snowfall atypically with respect to other Antarctica areas in terms of pathway (eastern instead of western), seasonality (summer instead of winter) and velocity (old air age). Geopotential height patterns at 500 hPa at low (>10 days) and high (2–6 days) frequencies during snowfall cycles at two core sites showed large positive anomalies at low frequencies developing in the Tasman Sea-Eastern Indian Ocean at higher latitudes (60–70°S) than normal. This could be considered part of an atmospheric blocking event, with transient eddies acting to decelerate westerlies in a split region area and accelerate the flow on the flanks of the low-frequency positive anomalies.

/pdf/snow-precipitation-at-four-ice-core-sites-in-east-antarctica-4ug947hded.pdf

Snow precipitation at four ice core sites in East Antarctica: provenance, seasonality and blocking factors

In this study we compare the representation of the southern hemisphere midlatitude winter variability in the NCEP-NCAR and ERA40 reanalyses. We use the classical Hayashi spectral technique, recently applied to compare the description of the atmospheric variability in the northern hemisphere on different spectral sub-domains. We test the agreement of the two reanalysis systems in the representation of the atmospheric activity. In the southern hemisphere, even in the satellite period, the assimilated data are relatively scarce, predominately over the oceans, and they provide a weaker constraint to the model dynamics. We find relevant discrepancies in the description of the variability at different spatial and temporal scales. ERA40 is generally characterised by a larger variance, especially in the high frequency spectral region. In the pre-satellite period the discrepancies between the two reanalyses are large and randomly distributed while after the 1979 the discrepancies are systematic. Moreover, a sudden jump in the VTPR period (1973-1978) is observed, mostly in the ERA40 reanalysis. Our results suggest that today we do not have a well-defined picture of the properties of the winter mid-latitude variability in the southern hemisphere to be used in the evaluation of the realism of climate models and demand for an intercomparison study for the assessment of the self-consistency of the IPCC models in the representation of the analysed properties.

https://arxiv.org/pdf/physics/0509145.pdf

Discrepancies in Southern Hemisphere Mid-latitude Atmospheric Variability of the NCEP-NCAR and ECMWF Reanalyses

[1] The Southern Ocean Gas Exchange Experiment (SO GasEx) is the third in a series of
U.S.‐led open ocean process studies aimed at improving the quantification of gas transfer
velocities and air‐sea CO2 fluxes. Two deliberate 3He/SF6 tracer releases into relatively
stable water masses selected for large DpCO2 took place in the southwest Atlantic sector
of the Southern Ocean in austral fall of 2008. The tracer patches were sampled in a
Lagrangian manner, using observations from discrete CTD/Rosette casts, continuous
surface ocean and atmospheric monitoring, and autonomous drifting instruments to study
the evolution of chemical and biological properties over the course of the experiment. CO2
and DMS fluxes were directly measured in the marine air boundary layer with
micrometeorological techniques, and physical, chemical, and biological processes
controlling air‐sea fluxes were quantified with measurements in the upper ocean and
marine air. Average wind speeds of 9 m s−1 to a maximum of 16 m s−1 were encountered
during the tracer patch observations, providing additional data to constrain wind speed/gas
exchange parameterizations. In this paper, we set the stage for the experiment by detailing
the hydrographic observations during the site surveys and tracer patch occupations that
form the underpinning of observations presented in the SO GasEx special section.
Particular consideration is given to the mixed layer depth as this is a critical variable for
estimates of fluxes and biogeochemical transformations based on mixed layer budgets.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2010JC006852

Southern Ocean Gas Exchange Experiment: Setting the stage

A symbiotic relationship between the high- and low-frequency transients in the Southern Hemisphere has been examined by using ECMWF analyses of 500 mb geopotential height and temperature fields over 9 winter seasons. The low-frequency transients organize the high-frequency eddies over the regions where low-frequency westerly anomalies prevail, forming traveling storm tracks that move together with the low-frequency waves. The high-frequency eddies, in turn, reinforce the barotropic component of the low-frequency waves while tending to diminish the baroclinic component of the low-frequency waves. All of these findings give further credence to earlier results for the Northern Hemisphere. As a supplementary to the feedback analyses, the energetics analysis for the winter seasons in both the Northern and the Southern Hemispheres reveals that the forcing of the low-frequency flow by the high-frequency transients in the Southern Hemisphere is much stronger than in the Northern Hemisphere. The contribution of the stationary waves to maintenance of the low-frequency flow through barotropic instability is significantly less in the Southern Hemisphere. DOI: 10.1034/j.1600-0870.1995.t01-2-00004.x

Interaction between the low‐ and high‐frequency transients in the Southern Hemisphere winter circulation

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Interaction between the low‐ and high‐frequency transients in the Southern Hemisphere winter circulation