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Timothy Eichler
and
Wayne Higgins

Abstract

The climatology and interannual variability of North American extratropical cyclones are examined using 6-hourly sea level pressure data from the NCEP–NCAR reanalysis for the period 1950–2002 and ECMWF 40-yr Re-Analysis (ERA-40) data from 1971 to 2000. The climatology includes an evaluation of the seasonal frequency and intensity of storms as well as an analysis of extreme event intensity. ENSO variability is evaluated by ENSO phase with emphasis on boreal winter. Results show an enhanced East Coast storm track during El Niño as well as an equatorward shift in storm tracks in the North Pacific for storms generated from both the NCEP–NCAR reanalysis and ERA-40 datasets. Observed precipitation close to a storm’s center is used to determine which phase of the ENSO cycle is associated with the most productive storms and where they occur. During El Niño winters, a precipitation maximum is located east of the Appalachians and is associated with an enhanced East Coast storm track. During La Niña winters, the precipitation maximum shifts to the Ohio Valley and is associated with an enhanced Great Lakes storm track. Along the U.S. west coast, there is a precipitation maximum in the Pacific Northwest during La Niña winters, which is due to a storm track west of Washington State.

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Wayne Higgins
and
David Gochis

Abstract

An international team of scientists from the United States, Mexico, and Central America carried out a major field campaign during the summer of 2004 to develop an improved understanding of the North American monsoon system leading to improved precipitation forecasts. Results from this campaign, which is the centerpiece of the North American Monsoon Experiment (NAME) Process Study, are reported in this issue of the Journal of Climate. In addition to a synthesis of key findings, this brief overview article also raises some important unresolved issues that require further attention. More detailed background information on NAME, including motivating science questions, where NAME 2004 was conducted, when, and the experimental design, was published previously by Higgins et al.

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Kingtse C. Mo
and
R. Wayne Higgins

Abstract

Atmospheric circulation features and convection patterns associated with two leading low-frequency modes in the Southern Hemisphere (SH) are examined in multiyear global reanalyses produced by NCEP–NCAR and NASA–DAO. The two leading modes, referred to as the Pacific–South American (PSA) modes, are represented by the first two EOF patterns. The two patterns are in quadrature with each other and are dominated by wavenumber 3 in midlatitudes with large amplitudes in the Pacific–South American sector. In the Pacific, anomalies in the subtropics and in the midlatitudes are opposite in phase. Taken together, the two PSA modes represent the intraseasonal oscillation in the SH with periods of roughly 40 days. The evolution of the PSA modes shows a coherent eastward propagation.

A composite analysis was conducted to study the evolution of tropical convection and the corresponding circulation changes associated with the PSA modes. Outgoing longwave radiation (OLR) anomaly composites during the mature phase of the PSA modes resemble the first two leading EOFs of OLR anomalies (OLRA) in the Tropics. Composites of OLRA show an east–west dipole structure roughly 5–10 days prior to the onset of persistent PSA events. The PSA 1 mode is associated with enhanced convection in the Pacific between 140°E and 170°W and suppressed convection over the Indian Ocean. The PSA 2 mode is linked to tropical heating anomalies in the central Pacific extending from 160°E to 150°W just south of the equator and suppressed convection in the western Pacific with a maximum at 20°N. Contributions are from both interannual and intraseasonal bands.

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Shrinivas Moorthi
and
R. Wayne Higgins

Abstract

An efficient, direct, second-order solver for the discrete solution of a class of two-dimensional separable elliptic equations on the sphere (which generally arise in implicit and semi-implicit atmospheric models) is presented. The method involves a Fourier transformation in longitude and a direct solution of the resulting coupled second-order finite-difference equations in latitude. The solver is made efficient by vectorizing over longitudinal wave-number and by using a vectorized fast Fourier transform routine. It is evaluated using a prescribed solution method and compared with a multigrid solver and the standard direct solver from FISHPAK.

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R. Wayne Higgins
and
Kingtse C. Mo

Abstract

A composite analysis of multiyear (1985–93) global reanalyses produced by the NCEP/NCAR and the NASA/DAO is used to show that the development of persistent North Pacific (PNP) circulation anomalies during NH winter is linked to tropical intraseasonal oscillations. The development is initiated over the tropical west Pacific by anomalous convection (characterized by an east–west dipole structure) one to two weeks prior to the extratropical onset time in both reanalyses. As tropical heating moves eastward toward the central Pacific, anomalous divergent outflow associated with the local Hadley circulation generates an anomalous Rossby wave sink (source) in the subtropics, consistent with the retraction (extension) of the Pacific jet. Prior to onset the signature of the forced anomalies is a pair of cyclonic (anticyclonic) circulation anomalies centered near the node of the tropical heating dipole. Wave trains extending from the region of anomalous convection into the extratropics set the stage for the subsequent rapid development of the PNP anomalies. After onset, the mature PNP anomalies extend equatorward to feed back (through modifications to the moisture transport) on the tropical precipitation anomalies. Throughout the evolution, the tropical precipitation anomalies and the extratropical PNP anomalies evolve coherently with tropical intraseasonal oscillations in both reanalyses.

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Kingtse C. Mo
and
R. Wayne Higgins

Abstract

Atmospheric circulation anomalies and hydrologic processes associated with California wet and dry events were examined during Northern Hemisphere winter. The precipitation anomaly over the west coast of North America shows a north–south three-cell pattern. Heavy precipitation in California is accompanied by dry conditions over Washington, British Columbia, and along the southeastern coast of Alaska and reduced precipitation over the subtropical eastern Pacific. The inverse relationship between California and the Pacific Northwest is supported by the transport of moisture flux. During wet events, the southern branch of moisture flux transport strengthens and brings moisture from the North Pacific to California, hence enhanced rainfall. Strengthened moisture flux transport northward to the area north of Washington is consistent with suppressed rainfall in California.

The local precipitation anomaly pattern in the eastern tropical Pacific just north of the equator has a large influence on precipitation events in California. The enhanced precipitation generates strong rising motion. The associated sinking motion is located over California. Strong sinking motion and strong upper-level convergence favor dry conditions in California. Conversely, suppressed rainfall in the eastern Pacific is associated with above-normal precipitation in California.

Precipitation in California is likely below normal during cold ENSO events. When convection in the central Pacific is enhanced, California has heavy precipitation if rainfall in the subtropical eastern Pacific is suppressed. In addition to ENSO, precipitation in California is also modulated by the tropical intraseasonal oscillation. Wet (dry) events are favored during the phase of the oscillation associated with enhanced convection near 150°E (120°E) in the tropical Pacific.

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R. Wayne Higgins
and
Hampton N. Shirer

Abstract

The possibility of global-scale transitions between atmospheric Hadley and Rossby regimes is investigated with a highly idealized, nonlinear, vertically continuous, rotating, spherical system. Low-order spectral versions of the model are used both to calculate ideal Hadley states and to determine their stabilities to certain three-dimensional baroclinic disturbances of any zonal wavenumber. The flow is forced by an idealized axisymmetric heating pattern based on zonally averaged atmospheric data, and is dissipated using an eddy viscosity formulation.

The dominant modes in the heating pattern force a single meridional cell between the equator and the poles that is compatible with the simple boundary conditions. As the heating rate is increased, these states exchange stability with temporally periodic solutions that have the characteristics of Rossby waves. Although Ekman boundary layer and cumulus friction effects are not included, the transports of heat and momentum by the zonally averaged Rossby flow are reasonable. When all combinations of heating and rotation rates are used, a transition curve separating the ideal Hadley and Rossby regimes is found. The critical values of the heating rates are made more realistic through the use of an effective eddy viscosity that represents energy transports arising from the products of the sub-Hadley and sub-Rossby scale perturbations.

It is shown that a transition from Hadley flow to wavenumber 5 Rossby flow is preferred. This result, which agrees with standard baroclinic instability results, gives a reasonable Rossby wave bifurcation from the Hadley solution. For the cases examined, it is found that the upper symmetric Hadley regime does not exist and that the Hadley to Rossby transition depends on the values of the eddy viscosities. Indeed, the dependence of the preferred zonal wavenumber on the values of the eddy viscosities suggests that small changes in the values of these parameters might result in large changes in the Rossby regime.

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Kingtse C. Mo
and
R. Wayne Higgins

Abstract

Large-scale aspects of the atmospheric moisture transport and the overall moisture budget we studied using data from the National Centers for Environmental Prediction (NCEP) reanalysis. Our objective is to critically evaluate the usefulness of the reanalysis products for studies of the global hydrologic cycle. The study period is from January 1985 to December 1993. Monthly mean water vapor transport, evaporation, and precipitation are compared to the NASA Data Assimilation Office (DAO) reanalysis for roughly the same period and with satellite estimates and station observations.

Comparisons of the moisture flux fields form the NCEP and the DAO reanalyses show general agreement in most aspects, but there are regional differences. Discrepancies in tropical moisture transport are largely due to uncertainties in the divergent winds. The DAO reanalysis shows a weaker Hadley circulation and weaker cross-equatorial flow, particularly during the Northern Hemisphere winter.

Global patterns of evaporation from the two reanalyses are similar, but the NCEP values are higher over the oceans and lower over the landmasses. In the eastern Pacific, the DAO has less total precipitable water and less rainfall. While the large-scale features of precipitation from the reanalyses agree with each other and are within the envelope of the satellite rainfall estimates, regional differences are large. Both analyses show questionable features in the moisture flux divergence fields over North and South America that are to a large extent terrain related. Interannual variability related to the 1987–1989 ENSO cycle is well captured by both reanalyses. On intraseasonal timescales, the NCEP reanalysis has difficulty capturing the precipitation signal associated with the 30–60 day oscillation, but the moisture flux divergence from both reanalyses produces a more reasonable signal.

An examination of the overall moisture budget for rectangular regions over North and South America in both reanalyses reveals large differences in the moisture flux divergence. Both reanalyses overestimate rainfall in the southeastern United States. The largest uncertainties during the spring and summer months are directly related to differences in the topographically bound low-level jets.

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Min Wen
,
Song Yang
,
Wayne Higgins
, and
Renhe Zhang

Abstract

During the boreal summer (June–August), vigorous convection appears over the eastern Pacific, southern Mexico, and northern South America, and oscillates on a distinct time scale of 10–20 days. Extended empirical orthogonal function (EEOF) analysis shows that the quasi-biweekly oscillation (QBWO) of the convection has two major modes: a west–east-orientated mode (WEM) and a north–south-orientated mode (NSM). The WEM, which is explained by the first two EEOF modes, originates over the eastern Atlantic, propagates westward along 15°N, and enhances over the Caribbean Sea before disappearing over the central Pacific. The NSM, explained by the third and fourth EEOF modes, originates over the western Pacific, moves eastward, and strengthens over the eastern Pacific. It shifts northward after arriving over the Caribbean Sea. Both modes have notable seasonal dependence, with the WEM more active in July and August and the NSM more active in June or earlier.

The two distinct QBWO modes are linked to different rainfall patterns over the United States and Mexico. When the WEM is active in July and August, wet conditions occur over the southern central United States and dry conditions appear to the north. When the NSM is active in June, northern Mexico, the southwestern United States, the Missouri basin, and the northern Great Lakes are drier than normal, while southern Mexico and the eastern United States are wetter than normal. Significant variations in atmospheric circulation are found to be associated with the interannual variability of the NSM activity in June. However, these variations may not necessarily result from QBWO but, rather, provide a background for QBWO activity instead. In July and August, the association of QBWO with the precipitation pattern over North America may sometimes be related to hurricane activity.

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Michelle L. L’Heureux
and
R. Wayne Higgins

Abstract

There is increasing evidence that the Madden–Julian oscillation (MJO) modifies the mid- to high-latitude circulation and, in particular, appears to have a relationship to the leading mode of extratropical variability, the Arctic Oscillation (AO). In this study, new insights into the observed similarities between the MJO and the AO are explored. It is shown that the eastward progression of the convectively active phase of the MJO is associated with a corresponding shift in the tendency and sign of the AO index. Moreover, the AO and the MJO share several analogous features not only in the global circulation, but also in surface temperature fields. Also, the AO is linked to a pattern of eastward-propagating MJO-like variability in the tropics that is partially reproduced in free runs of the NCEP Climate Forecast System (CFS) model. Finally, it is shown that the structure of the AO, as defined by the leading mode in the 1000-hPa geopotential height field, is significantly altered based on the phase of the MJO.

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