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This paper analyzes the relationships between the circulation regimes of the 500-hPa height (z500) and 250-hPa zonal winds (u250) in the Pacific–North America region during boreal winter, and the 45-day Northern Hemisphere oscillation in z500. The regimes were calculated using a k-means clustering applied to the leading 12 principal components of the combined z500–u250 anomaly fields. We divided the oscillation into eight arbitrary phases. The oscillation phase z500 composite maps are spatially well correlated with regime z500 composites: phases 1–2 are best correlated with the Arctic Low, phases 3–5 are best correlated with the Pacific Trough, phase 6 is best correlated with the Arctic High, and phases 7–8 are best correlated with the Alaskan Ridge. We found that these correlations are generally consistent with the regimes that tend to occur during the individual oscillation phases: the Arctic Low occurs above significance in phases 1–2, the Pacific Trough occurs above significance in phase 3, and Alaskan Ridge occurs above significance in phases 7–8. Therefore, the oscillation has a preferred order with respect to the regimes. The regime transitions indicate a pattern that moves through the Pacific Wavetrain, a regime that appears for k = 5 as a mean state. Transitions out of this regime into different regimes are preferred in different phases of the oscillation. These results imply a possible enhancement to regime prediction using the low-frequency oscillations in combination with regimes. Significance Statement: Subseasonal prediction, weather forecasting in the 2–4-week range, is important for many parts of society, e.g., water managers, emergency response units, and farmers. However, current prediction skill in this time range is low. This paper performs an initial analysis of a possible method to increase weather statistic prediction skill beyond 10 days in the winter for the Pacific–North America region. This is done by combining two ways of looking at large, long-lasting patterns of pressure systems in the atmosphere, which are associated with various weather statistics like precipitation extremes and storminess. The results indicate this method holds potential skill for enhancing subseasonal prediction. Further investigation might yield forecasting improvements in this important time range. [ABSTRACT FROM AUTHOR]

The winter response of the atmosphere to El Nino events in the Pacific is studied both from a 14-year integration of the Center for Ocean-Land-Atmosphere GCM using observed SSTs from January 1979 to February 1993 and from the corresponding analyses of ECMWF. Emphasis is put on the shift in the high-frequency transients that define the Pacific storm track during warm events. Warm and normal ensembles are defined on the basis of the GCM's diabatic heating field in the tropical Pacific, which falls in one of two states. During the winters of 1982/83, 1986/87, and 1991/92, the heating averaged between 6 [degrees] S and 6 [degrees] N lies in the range of 100-200 W [m.sup.-2] all the way across the basin. The remaining 10 'normal' years all show no large tropical diabatic heating anomalies in the mid or eastern Pacific. The difference between warm and normal ensembles for the mean fields of zonal wind u and height z indicates an eastward and equatorward extension of the midlatitude Pacific jet, associated with a similar extension of the transient feedback on the mean flow as measured by the convergence of vorticity flux. The increase in high-frequency (periods of [approximately]2 to 10 days) transient kinetic energy in the eastern portion of the Pacific during El Nino extends across Mexico. The high-frequency transient vertical and meridional sensible heat fluxes, and the low-level diabatic heating and baroclinicity of the mean state (measured by [Ri.sup.-1/2]) also indicate an eastward and equatorward shift of the entire storm track complex in the GCM and the analyses. The shift is consistent with the increased midlatitude shear during El Nino that accompanies the overall tropical warming. While the GCM storm track shift has strong similarities to that in the analyses, the GCM's large systematic errors in the mid Pacific (eastward extension of the Pacific jet and the storm track) lead to an underestimation of the response to El Nino, which has a very similar form. However, the GCM also shows a spurious tendency to move the storm tracks equatorward in the far western Pacific, as seen in the large positive anomalies in all dynamical indicators of the storm tracks at latitudes 20 [degrees] - 30 [degrees] N.

Results are presented from a retrospective analysis of 19 months (May 1982-November 1983) of global atmospheric observations. The National Meteorological Center Global Data Assimilation System was used in tandem with the atmospheric general circulation model of the Center for Ocean-Land-Atmosphere Studies to produce four-times-daily representations of the global atmosphere. Statistics were compiled regarding the use of data by the analysis and the decisions of the quality control procedures. Comparison of the reanalyses with both observation and the archived contemporaneous analyses showed substantial improvements in the representation of the global atmospheric circulation, possibly excepting the Southern Hemisphere south of 60 [degrees] S. A list of data products from the reanalysis is given in an appendix.

Subtropical anticyclones and midlatitude storm tracks are key components of the large-scale atmospheric circulation. Focusing on the Southern Hemisphere, the seasonality of the three dominant subtropical anticyclones, situated over the South Pacific, South Atlantic, and south Indian Ocean basins, has a large influence on local weather and climate within South America, southern Africa, and Australia, respectively. Generally speaking, sea level pressure within the Southern Hemisphere subtropics reaches its seasonal maximum during the winter season when the Southern Hemisphere Hadley cell is at its strongest. One exception to this is the seasonal evolution of the South Pacific subtropical anticyclone. While winter maxima are seen in the South Atlantic and south Indian subtropical anticyclones, the South Pacific subtropical anticyclone reaches its seasonal maximum during local spring with elevated values extending into summer. In this study, we investigate the hypothesis that the strength of the austral summer South Pacific subtropical anticyclone is largely due to heating over the South Pacific convergence zone. Using added-cooling and added-heating atmospheric general circulation model experiments to artificially change the strength of austral summer diabatic heating over the South Pacific convergence zone, our results show that increased heating, through increased upper-level divergence, triggers a Rossby wave train that extends into the Southern Hemisphere midlatitudes. This propagating Rossby wave train creates a high and low sea level pressure pattern that projects onto the center of the South Pacific subtropical anticyclone to intensify its area and strength. [ABSTRACT FROM AUTHOR]

Eastward propagating planetary waves of zonal wavenumber one in the zonal wind (u) with phase speeds in the range of 1-10 m [s.sup.-1], and also with frequencies in the 30-60-day range, are studied using 39 boreal winter (austral summer) seasons (each of length 180 days) from the reanalyses of the National Centers for Environmental Prediction. The purpose of the paper is to study the relationship between these low phase speed waves in the extratropics (which are candidates for instabilities) and the low-frequency tropical waves associated with the Madden-Julian oscillation (MJO). Planetary waves dominate the zonal wavenumber spectrum of all (eastward plus westward) transient fluctuations with phase speeds of 1-10 m [s.sup.-1] at upper levels. Using the theory of Y. Hayashi to separate out standing oscillations, it is found that eastward propagating waves for zonal wavenumber one have variance maxima at 63 [degrees] N, 32 [degrees] N, 13 [degrees] N, 32 [degrees] S, and 52 [degrees] S at upper levels. As a percentage of the total variance for phase speeds 1-10 m [s.sup.-1] and zonal wavenumber one, the eastward propagating waves have strong maxima at 200 hPa in the Tropics (13 [degrees] N, 13 [degrees] S) and at lower levels at 13 [degrees] S (indicative of the MJO). The standing wave variance is maximum in northern midlatitudes. The eastward propagating wave variance for zonal wavenumber two has similar properties. For zonal wavenumber one, eastward propagating waves at 52 [degrees] N and 300 hPa are highly coherent with mid-and upper-level waves at 32 [degrees] N, with a nearly perfectly out-of-phase relationship. For a base point at 32 [degrees] N, 200 hPa, we find a strong coherence maximum for phase speeds of 1-10 m [s.sup.-1] (coherence squared greater than 0.7) with upper levels in the Tropics (13 [degrees] N), accompanied by a 180 [degrees] relative phase shift. Coherence and phase plots with a base point at 13 [degrees] N and 200 hPa show not only strong coherence with waves at 32 [degrees] N, but also with waves at 13 [degrees] S at 700 hPa. While results for the MJO averaging (corresponding to periods of 30-60 days) are generally similar, there is in addition a strong cross-equatorial coherence between fluctuations at 13 [degrees] N and 13 [degrees] S at 200 hPa, and stronger coherence with the lower tropical troposphere. The strong coherence between waves in the subtropics (32 [degrees] N) and the Tropics (13 [degrees] N) indicates a potential role for dynamical instability in the organization of the MJO. Coherence and phase diagnostics for base points in the Southern Hemisphere have generally the same character, although the subtropical-tropical coherence is less dramatic (but still significant). Coherence and phase results for eastward propagating zonal wavenumber two are generally similar. These results represent, for midlatitudes, an extension of earlier work of C. R. Mechoso and D. L. Hartmann to phase speeds less than 10 [m s.sup.-1], for which an interpretation in terms of baroclinic instability becomes viable; a much larger dataset is also used here. The strong coherence found between the subtropics and Tropics lend support to the notion that planetary wave baroclinic instability and the MJO are connected with each other, as suggested by J. S. Frederiksen and C. S. Frederiksen. An origin of the MJO in which subtropical jet instability helps to organize tropical convection is suggested.

The linear trends for a number of fields obtained from the reanalyses of the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) are calculated for the Northern Hemisphere winter months (January-March) from the 55-yr period of 1948-2002. The fields include sea level pressure (SLP); geopotential height at 500 and 50 hPa; temperature at 500 and 50 hPa; zonally averaged height; temperature; zonal, meridional, and vertical velocities from 1000 to 50 hPa; and surface air temperature (SAT). The trend fields are expressed in terms of two alternate expansions: (i) contributions from the Arctic Oscillation (AO) and cold ocean-warm land (COWL) patterns, as defined from the leading modes of an empirical orthogonal function (EOF) analysis of sea level pressure; or (ii) contributions from the modified AO (AO*) and modified COWL (COWL*) patterns, defined from the leading EOFs of 500-hPa height. The residuals in each expansion are considered, and the completeness properties of the expansions are discussed. Long-term linear trends of various fields at mid- and lower-tropospheric levels project well onto the AO (AO*) and COWL (COWL*) modes. The AO contribution accounts for most of the SLP falls over the Arctic and half of the SLP rise over the North Atlantic, while the COWL pattern represents the entire negative pressure trend over the Pacific and half of the rise over the Atlantic. In the expansion into AO* and COWL* patterns, the latter represents most of the SLP trend. Similar remarks hold for the height trend at 500 hPa. In each case the residual is a small fraction of the trend. The observed SAT trend (warming over most of North America and Asia, cooling over northeast Canada and the Pacific) is partitioned nearly equally between contributions from the AO and COWL, although the COWL contribution dominates over North America. In the alternate expansion, the COWL* dominates nearly all of the warming over North America and Asia. The midtropospheric (500 hPa) temperature trend is mostly due to the COWL (or COWL*) patterns, with the AO representing only the local cooling over Greenland. The 50-hPa height and temperature trends are not well represented by either set of patterns. The links of the trends in the zonal-mean fields and the AO (AO*) and COWL (COWL*) are weaker than those in the mid- and lower troposphere. [ABSTRACT FROM AUTHOR]