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Tom Murphree
and
Huug Van Den Dool

Abstract

The time-mean tropical surface momentum balance is investigated with a simple model that calculates tropical surface winds from time mean sea level pressure fields. The model domain is the global tropical strip centered on the equator with lateral boundaries at ±30° latitude. Steady state surface winds are numerically calculated from the nonlinear horizontal momentum equations, with forcing from observed climatological monthly mean sea level pressures and prescribed lateral boundary winds. Dissipation is parameterized by linear damping and diffusion. Comparisons of model winds with observed climatological monthly mean winds show realistic simulations in most regions and in all months. The poorest simulations occur in the meridional component of the wind in near-equatorial areas of strongly convergent or weak winds. In these areas, and in the near-equatorial region generally, diffusion processes make a significant positive contribution to the realism of the model winds. Horizontal nonlinear advection also improves the simulation near the equator, though to a smaller degree. The generally skillful model winds refute the conventional idea that weak gradients make the tropical pressure field a poor tool for calculating tropical winds. To the contrary, tropical pressure fields contain substantial information about associated winds. Thus, a relatively complete momentum balance can be identified for the major features of the time-mean tropical wind field.

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Huug M. Van Den Dool
and
Robert M. Chervin

Abstract

The output of a 20-year integration of an annual cycle (AC) version of the NCAR Community Climate Model in which the external conditions went through 20 prescribed identical annual cycles is used to study mouth-to-month persistence of anomalies in monthly mean atmospheric circulation fields on a global and a hemispheric scale. Of all fields considered, the height fields (1000–300 mb) are the most persistent and the transient eddy flux fields the least persistent. Persistence in height field anomalies is largest in winter and small throughout the rest of the year. For the area north of 20°N, a comparison is made with the persistence of months mean height and temperature fields observed in the real world (RW) during a 28-yeu interval. On a pooled all month-pairs basis, RW height anomaly fields are significantly more persistent than those appearing in AC but, from a practical point of view, the difference is small. The differences in persistence are larger for temperature anomalies (500–1000 mb thickness) than for height. Differences between RW and AC monthly persistence over the area north of 20°N are largest in summer when the RW has a local maximum in persistence. On the assumption that the model and atmosphere have the same internal dynamics, the differences just described can be attributed to the interaction of the atmosphere with external or boundary conditions (e.g., ocean surface temperature), which was purposely omitted from the AC integration. Interaction with the lower boundary in summer seems, therefore, to be quite important to explain the observed level of month-to-month persistence in circulation anomalies. In winter, however, the internal dynamics of the atmosphere alone produces the required observed level of month-to-month persistence. The output of a 15-year integration of the same model in which the sea surface temperature, on a global scale, had realistic interannual variability, is used to interpret further the differences between RW and AC.

As a by-product of this study we have calculated the spatial degrees of freedom (dof) associated with time mean anomaly fields. The dof for global monthly mean anomaly height fields in the AC model are quite low, i.e., 25–35 on a yearly pooled basis. Over the area north of 20°N, the dof associated with monthly mean anomaly height fields of the AC model and the RW are quite close, varying from 15–20 in winter months to about 40 in summer.

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Ming Cai
and
Huug M. van den Dool

Abstract

We have documented with the observed Northern Hemispheric 500 mb geopotential height data for ten winter seasons that traveling storm tracks exist downstream of the troughs of traveling low-frequency waves. The relation between the low-frequency flow and the traveling storm tracks is discovered with a novel observational technique that records high-frequency activity in a framework traveling along with an identifiable low-frequency structure. The vorticity flux of the high-frequency eddies associated with the traveling storm tracks acts both to reinforce the low-frequency waves and to retard their propagation.

These findings strongly indicate that a substantial amount of the low-frequency variability of the midlatitude atmospheric circulation is attributable to the forcing of the high-frequency eddies. These low-frequency waves organize the high-frequency eddies in such a way that the latter tend to intensify preferentially downstream of the troughs of the former. The symbiotic relation between the low-frequency flow and the traveling storm tracks is dynamically equivalent to the relation between the stationary waves and the stationary storm tracks. This mutual relationship is a necessary although not sufficient condition to parameterize high-frequency eddies in terms of low-frequency flow.

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Wilbur Y. Chen
and
Huug M. van den Dool

Abstract

A substantial asymmetric impact of tropical Pacific SST anomalies on the internal variability of the extratropical atmosphere is found. A variety of diagnoses is performed to help reveal the dynamical processes leading to the large impact. Thirty-five years of geopotential heights and 29 years of wind fields analyzed operationally at the National Centers for Environmental Prediction (NCEP), formerly the National Meteorological Center, and three sets of 10-yr-long perpetual January integrations run with a low-resolution NCEP global spectral model are investigated in detail for the impact of the SST anomalies on the blocking flows over the North Pacific. The impact on large-scale deep trough flows is also examined.

Both the blocking and deep trough flows develop twice as much over the North Pacific during La Niña as during El Niño winters. Consequently, the internal dynamics associated low-frequency variability (LFV), with timescales between 7 and 61 days examined in this study, display distinct characteristics: much larger magnitude for the La Niña than the El Niño winters over the eastern North Pacific, where the LFV is highest in general.

The diagnosis of the localized Eliassen–Palm fluxes and their divergence reveals that the high-frequency transient eddies (1–7 days) at high latitudes are effective in forming and maintaining the large-scale blocking flows, while the midlatitude transients are less effective. The mean deformation field over the North Pacific is much more diffluent for the La Niña than the El Niño winters, resulting in more blocking flows being developed and maintained during La Niña by the high-frequency transients over the central North Pacific.

In addition to the above dynamical process operating on the high-frequency end of the spectrum, the local barotropic energy conversion between the LFV components and the time-mean flows is also operating and playing a crucial role. The kinetic energy conversion represented by the scalar product between the E vector of the low-frequency components and the deformation D vector of the time-mean flow reveals that, on average, the low-frequency components extract energy from the time-mean flow during La Niña winters while they lose energy to the time-mean flow during El Niño winters. This local barotropic energy conversion on the low-frequency end of the spectrum, together with the forcing of the high-frequency transients on blocking flows on the high-frequency end, explain why there is a large difference in the magnitude of low-frequency variability between the La Niña and the El Niño winters.

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Ming Cai
and
Huug M. Van Den Dool

Abstract

A nearly complete vorticity equation is used to diagnose the tendency components of the low-frequency variations of the 500-mb streamfunction induced by various internal linear-nonlinear interaction processes. With the aid of a special composite technique (“phase-shifting” method) that effectively records the observations in a coordinate system moving with an identifiable low-frequency pattern, the authors are able to separate the internal interactions that primarily act to make low-frequency waves propagate from those that are mostly responsible for development/maintenance/decay (“maintenance” for brevity) of low-frequency transients. It is found that the low-frequency transients are maintained primarily by two nonlinear interaction processes: one is the vorticity flux of high-frequency eddies and the other is the interaction of low-frequency transients and stationary waves. It is also found that an individual propagation tendency component may be much larger than a maintenance tendency component. In particular, the beta effect and the advection of the low-frequency vorticity by the zonally averaged climatological wind are the dominant terms among the propagation tendency components. But there is a great deal of cancellation among the propagation tendency components. As a result, the net magnitude of the tendency components describing propagation is only slightly larger than those relating to maintenance of low-frequency waves. From a forecast point of view, both propagation and forcing terms are equally important if an accurate forecast beyond a few days is required.

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Ming Cai
and
Huug M. Van Den Dool

Abstract

A special composite technique (“phase shifting” method) that records both the low- and high-frequency transient activity throughout the troposphere in a framework moving with an individual low-frequency wave of 500-mb geopotential height at 50°N was used to document the three-dimensional structure of the planetary-scale low-frequency waves as well as the attendant traveling storm tracks from the NMC twice-daily analyses of geopotential height and temperature at pressure levels 850, 700, 500, 300, and 200 mb for the ten winters 1967/68 through 1976/77.

The following are the main characteristics of the Northern Hemisphere midlatitude planetary-scale low-frequency waves (zonal wavenumber m = 1, 2, 3, and 4) in winter: (i) The amplitude of the planetary scale low-frequency waves is nearly constant with the zonal wavenumber m, and has a maximum at 300 mb for geopotential height and at 850 mb for temperature; (ii) All low-frequency waves have a nearly equivalent barotropic structure (much more so than the stationary waves); (iii) The instantaneous zonal phase speed of an individual low-frequency wave is nearly independent of height and latitude so that we may identify the three-dimensional structure of a low-frequency wave by following that wave at just one pressure level and one latitude in either geopotential height or temperature.

The traveling storm tracks, defined as the local maxima on the rms map of the phase-shifted high-frequency eddies, are identifiable from both geopotential height and temperature data throughout the troposphere. They are located over the trough regions of the low-frequency waves. The barotropic feedback (i.e., the geopotential tendency due to the vorticity flux) of the traveling storm tracks tends to reinforce the low-frequency waves and to retard their propagation throughout the troposphere. The baroclinic feedback (i.e., the temperature tendency due to the heat flux) of the traveling storm tracks appears to have an out-of-phase relation with the low-frequency waves in temperature from 850 mb to 300 mb. At 200 mb, the baroclinic feedback is nearly in phase with the low-frequency waves in the temperature field.

The mutual dependence between the low-frequency flow and their attendant traveling storm tracks dynamically resembles that between the climatological stationary waves and the climatological storm tracks. Therefore, our observational study seems to lend support for the local instability theory that accounts for the existence of the stationary/traveling storm tracks as the consequence of the zonal inhomogeneity of the climatological mean/low-frequency flow.

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Yueyue Yu
,
Ming Cai
,
Rongcai Ren
, and
Huug M. van den Dool

Abstract

This study investigates dominant patterns of daily surface air temperature anomalies in winter (November–February) and their relationship with the meridional mass circulation variability using the daily Interim ECMWF Re-Analysis in 1979–2011. Mass circulation indices are constructed to measure the day-to-day variability of mass transport into the polar region by the warm air branch aloft and out of the polar region by the cold air branch in the lower troposphere. It is shown that weaker warm airmass transport into the upper polar atmosphere is accompanied by weaker equatorward advancement of cold air in the lower troposphere. As a result, the cold air is largely imprisoned within the polar region, responsible for anomalous warmth in midlatitudes and anomalous cold in high latitudes. Conversely, stronger warm airmass transport into the upper polar atmosphere is synchronized with stronger equatorward discharge of cold polar air in the lower troposphere, resulting in massive cold air outbreaks in midlatitudes and anomalous warmth in high latitudes. There are two dominant geographical patterns of cold air outbreaks during the cold air discharge period (or 1–10 days after a stronger mass circulation across 60°N). One represents cold air outbreaks in midlatitudes of both North America and Eurasia, and the other is the dominance of cold air outbreaks only over one of the two continents with abnormal warmth over the other continent. The first pattern mainly corresponds to the first and fourth leading empirical orthogonal functions (EOFs) of daily surface air temperature anomalies in winter, whereas the second pattern is related to the second EOF mode.

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Huug M. Van Den Dool
,
Peter J. Lamb
, and
Randy A. Peppler

Abstract

The procedure to calculate the active layer depth of the upper ocean, as proposed by Van den Dool and Horel (DH), was applied to the Atlantic Ocean from 20°S to 70°N. In this method, the observed climatological annual cycle in SST is employed to invert a simple linear energy balance. The results for the Atlantic are similar to those for the Pacific Ocean in several ways. The active layer is considerably shallower than the annual mean mixed layer (which is calculated from in situ sea temperature profiles). Just as for the Pacific, however, the patterns of active and mixed layer depth show a remarkable spatial match.

Using Bunker's datasets for SST and heat transfer over the Atlantic Ocean, the forcing used in the energy balance equation was made increasingly more realistic, from (i) astronomical solar radiation, through (ii) empirical estimates of absorbed solar radiation including the modifying effect of clouds to (iii) the complete empirically determined net ocean surface heat gain. No matter what forcing was used, the calculated active layer is always much shallower than the mixed layer depth. The best pattern match was found using the simplest forcing of all—the astronomical solar forcing.

Increasingly, atmospheric models are being coupled to an oceanic slab in which the SST evolves in response to local heat gains and losses. The key question is how deep that slab should be. Our study implies that, in order to match the observed annual cycle in SST, the oceanic stab should be quite shallow, and certainly shallower than the mixed layer depth. The shallowness of the active layer implies that ocean heat transport contributes to the forcing of the annual cycle in SST in the midlatitudes of the Atlantic Ocean.

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Åke Johansson
,
Anthony Barnston
,
Suranjana Saha
, and
Huug van den Dool

Abstract

This study examines the level and origin of seasonal forecast skill of surface air temperature in northern Europe. The forecasts are based on an empirical methodology, canonical correlation analysis (CCA), which is a method designed to find correlated patterns between predictor and predictand fields. A modified form of CCA is used where a prefiltering step precedes the CCA as proposed by T. P. Barnett and R. Preisendorfer. The predictive potential of four fields is investigated, namely, (a) surface air temperature (i.e., the predictand field itself), (b) local sea surface temperature (SST) in the northern European area on a dense grid, (c) Northern Hemisphere 700-hPa geopotential height, and (d) quasi-global SST on a coarse grid. The design is such that four contiguous predictor periods (of 3 months each) are followed by a lead time and then a single predictand period (3 months long). The shortest lead time is 1 month and the longest is 15 months. The skill of the CCA- based forecasts is estimated for the 39-yr time period 1955–93, using cross-validated hindcasting. Skill estimates are expressed as the temporal correlation between the forecasts and the respective verifying observations.

The forecasts are most skillful in the winter seasons with a secondary weaker skill maximum during summer. During winter the geopotential height field produces the highest skill scores of the four predictor fields. The dominant predictor pattern of the geopotential height field is confined to the predictor period that is closest to a preceding core winter season and resembles the North Atlantic Oscillation (NAO) teleconnection pattern. The time series of the expansion coefficients of this dominant predictor pattern correlates well with a low-pass filtered time series of an NAO index. The obtained skill is similar to what is found in the United States, both with regard to seasonal distribution and level of skill. The origin of skill is however different. In the United States it is the El Niño–Southern Oscillation (ENSO) with its predominantly interannual character that is the main source of skill in winter. In northern Europe it is instead the NAO that contributes the most, and especially the lower frequency part of the NAO (periods between 4 and 10 yr).

Spatially sparse station data of surface pressure extending back to the middle of the nineteenth century suggests a nonstationarity in the NAO behavior. The implications of this nonstationarity for the obtained results of this study is briefly discussed. Because finely resolved field data are not readily available for this earlier period, the level of skill realizable for that period using a pattern relationship technique such as CCA remains an open question.

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