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Steven W. Lyons

Abstract

Empirical orthogonal function analysis was applied to monthly mean rainfall data at 63 stations in Hawaii encompassing a 37-year period. Major rainfall patterns in order to importance (E1–E3) proved to be trade wind, southwest wind and convective rainfall on an annual basis; trade wind, southwest wind and frontal rainfall during winter, spring and fall seasons; and trade wind, tropical disturbance and convective rainfall during summer. Trade wind rainfall (E1) explains most rainfall variance in summer and least variance in winter. Spectral analyses of the time-dependent coefficients for eigenvectors E1–E5 show annual, semi-annual, three-forths year, and 2–2½ year cycles. No spectral peaks relating to the 11- and 22-year sunspot cycles were found. Composite rainfall maps for wet and dry winter and summer half-years indicate the contributions that specific eigenvector patterns make to these anomalies. Comparisons between Hawaiian rainfall and E1 Niños reveal that most (not all) E1 Niño winters in Hawaii are dry. Lack of trade wind rainfall is the primary cause.

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Steven W. Lyons

Abstract

Fourier analysis was applied to outgoing longwave radiation (OLR) and 200 mb vorticity (VOR) during winter 1974–75, over the global tropics from 20°N to 20°S. Significant OLR and VOR zonal variance (33%) is contained in low wavenumbers (1–4) over the equatorial tropics.

Empirical orthogonal function (EOF) analysis was performed on OLR and VOR Fourier coefficients, am, bm (m = 1−4), over the tropical domain. Spectral analyses for the eight largest eigenvectors exhibit marked peaks at periods of about 15–30 days. Only 15–30 day filtered, planetary-scale (1–4) OLR and VOR fields are examined.

OLR standard deviations reveal extremely large values over Indonesia and the equatorial Pacific with a maximum at 5°S, 115°E. VOR standard deviations are minimum along the equator with maxima at 5–10° north and south.

Correlations between OLR at 5°S, 115°E and global OLR reveal a geographically coherent pattern with OLR over Indonesia out of phase with OLR over the equatorial Indian Ocean and central equatorial Pacific. Correlations between OLR at 5°S, 115°E and global VOR show marked coherence, with VOR over equatorial regions exhibiting a wavenumber 1 distribution symmetric about the equator.

Lag correlations (−10 to +10 days) between OLR at 5°S, 115°E and global OLR and VOR reveal systematic eastward movement over equatorial convective regions (70°E–160°W).

EOF analysis of 15–30 day filtered a am, bm (m = 1−4), Fourier coefficients reveals major tropical modes of oscillation in OLR of zonal wavenumber 3 and zonal wavenumber 1 in VOR. Comparison of equatorial wavenumber 1 OLR (forcing) and wavenumber 1 VOR (response) shows close resemblance to Matsuno/s (1966) equatorial Kelvin wave model.

A composite technique was applied to OLR and VOR to investigate the relationship between long-period changes of OLR at a reference point (5°S, 115°E) and those of OLR and VOR over the tropics. Composite maps constructed by considering only the first five eigenvectors indicate distinct eastward propagation (∼4–5° per day) of OLR from 70°E to 160°W and southeast movement over the Bay of Bengal and Malaysia. VOR propagates eastward (∼5–9° day−1) around the globe while traversing from 20°N to 20°S.

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Steven W. Lyons

Abstract

Monthly precipitation at 46 stations located throughout the state of Texas was examined over a continuous 62-year period from 1923 to 1984. Precipitation data were subjected to spectral, empirical orthogonal function (EOF) and correlation analyses. Focus was on the dominant EOF (E1), which explains 30% to 45% of all precipitation variance. The time-dependent coefficient associated with E1 closely resembles a statewide average precipitation index. This time-dependent coefficient undergoes large month-to-month fluctuations; however, these fluctuations are, for the most part, aperiodic. Other than slight month-to-month persistence during winter and spring, monthly precipitation anomalies cannot be predicted or anticipated based on time-series or spectral analysis.

A long-term monthly mean sea level pressure dataset is composited over anomalously wet and anomalously dry months covering the 62 years. A signal is found in the sea level pressure composites, which is best defined during winter months. Anomalously wet (dry) months in Texas are associated with a northward (southward) shift of high pressure.

Simultaneous correlations between monthly statewide precipitation and temperature indicate strong negative (greater than −0.60) correlations during the warm season. However, lag correlations suggest that precipitation is controlling temperature. Methods to forecast monthly precipitation in Texas remain elusive.

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Steven W. Lyons

Abstract

Analysis of NOAA/NESS outgoing longwave radiation (OLR) over the greater Africa region reveals a large area of low OLR (5°–20°S, 20°E–40°E) during three austral summers (November through February; 1982/83, 1983/84 and 1984/85). This low OLR area is consistent with the climatological rainy area and persistent convective activity. Using OLR as a proxy for synoptic and large-scale cloudiness and convection, OLR standard deviations are computed for the three summers. Highest OLR variability is observed across Africa along 15–17°S, which is about five degrees latitude south of the OLR minimum. Based on the region of maximum OLR standard deviations and minimum mean OLR, a box-average OLR index is derived. Time series of the OLR index for November through February indicate large (±40 W m−2), aperiodic OLR fluctuations within each of the three summers.

Outgoing longwave radiation composites are constructed for periods of large OLR changes from negative OLR anomalies (wet conditions) to positive OLR anomalies (dry conditions). Although fluctuations are noncyclic in time, OLR composites reveal propagation of OLR from south of the Cape of Good Hope toward the northeast. The origin of these OLR fluctuations appears to be the Southern Hemisphere midlatitudes. This is consistent with OLR correlation maps derived for each season. However, a large portion of the OLR changes over equatorial southern Africa are of standing character.

Circulation features associated with the large OLR fluctuations are analyzed by compositing NMC wind and temperature fields. It is found that east-northeastward propagation of midlatitude waves into the subtropical western Indian Ocean occurs prior to OLR decreases over equatorial southern Africa. Trough (ridge) intrusions into subtropical and tropical Africa from the southeast are associated with OLR decreases (increases).

The wind circulation and divergence in these equatorward penetrating troughs is strongest in the upper troposphere (300 mb), temperature perturbations are largest at 500 mb, and the wave signature can be seen down to the low levels.

A close examination of circulation features associated with one prominent OLR change indicates that individual events are similar to the composite average, however, they reveal greater temporal detail. The midlatitude upper level trough does not penetrate directly to equatorial Africa. Rather, the midlatitude trough merges with the Tropical Upper Tropospheric Trough (TUTT), which is a persistent feature at 300–200 mb over the southwest Indian Ocean. The TUTT is then instrumental in modifying circulation over equatorial southern Africa, which is favorable for OLR decreases over that region.

Based on these results it appears that a major source of OLR/convective variability over the rainy region of equatorial southern Africa during austral summer is associated with interaction between midlatitude wave disturbances embedded in the westerlies and the quasi-stationary tropical upper tropospheric trough in the vicinity of the southwest equatorial Indian Ocean.

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Steven W. Lyons and Bruce Hundermark

Abstract

The European Centre for Medium-Range Weather Forecasts (ECMWF) 500-mb height analyses and 7-day forecasts are examined during ten winters (1981–90) for oscillations of geostrophic zonal wind over the western hemisphere. A well-defined zonal-wind oscillation is isolated in the first two eigenvectors. This zonal-wind oscillation accounts for about 55% of the total zonal-wind variance over the western hemisphere during this ten-winter period. The oscillation is characterized by zonal-wind anomalies that are in phase between 30° and 70°N and out of phase with zonal-wind anomalies along 50°N. The oscillation clearly displays southward propagation from 85° through 30°N, with standing components along 30°, 50°, and 70°N. The dominant temporal period associated with the oscillation is found to be in the range of 15–35 days with large interannual variability.

Composites of 500-mb heights through 25 cycles of zonal-wind oscillations over ten winters were performed for unfiltered and 15–39-day filtered data. In both filtered and unaltered composites, the zonal-wind oscillation is associated with southward propagation of positive and negative height anomalies from 85°N southward past 30°N. Composite 500-mb height anomalies change from zonal to meridional over North America as positive and negative zonal-wind anomalies pass through the 50°–40°N latitude belt. Characteristics of meridional height anomalies associated with the zonal-wind oscillation are similar to numerically modeled height anomalies produced by orographic forcing from the Rocky Mountains. Preliminary indications show that there does not appear to be a significant association between the zonal-wind oscillation isolated here (15–35 days) and oscillations in tropical convection (40–50 days).

The geostrophic zonal-wind oscillation is also found in 1–7-day 500-mb height forecasts made by the ECMWF global spectral model; however, the temporal character of this oscillation is significantly different in 7-day forecasts as compared to those observed. As a consequence, it is shown that the zonal-wind oscillation is associated with a significant source of 500-mb height forecast errors in the 7-day ECMWF model, since the pattern of 7-day forecast errors is similar to the height-anomaly pattern associated with the observed zonal-wind oscillation.

This study is an extension of preliminary results by Lyons (1989) and gives further evidence for a zonal-eddy relationship over the western hemisphere during winter.

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Paul R. Janish and Steven W. Lyons

Abstract

The National Meteorological Center's Nested Grid Model (NGM) analyses and 24–48-h forecasts of cold-air outbreaks and their associated return flows are examined from January through March 1988 coincident with the Gulf of Mexico Experiment (GUFMEX). Seven episodes of moderate-to-strong cold-air outbreaks and associated return flows are isolated. A good index of these cycles is the 950-mb meridional wind component. Composites and individual cases of the horizontal, vertical, and temporal structure of wind, temperature, and moisture are diagnosed for NGM analyses and 24–48-h forecasts of these events. Primary focus is on air and moisture modification over the Gulf during the return-flow cycle. Comparisons among observed inversion layers capping the low-level moisture and those present in NGM analyses and forecasts are examined. These differences highlight model strengths and weaknesses.

With regard to humidity-field evolution during the return-flow cycle, NGM forecasts are consistently too dry over the Gulf during, both offshore and onshore flow phases. Isentropic trajectory computations suggest that advective processes clearly dominate the moisture modification processes in the NGM forecasts. Comparisons indicate that low-level moisture errors in the model forecasts result more from shortcomings in model physics than from inadequate or poor model initialization.

Recently, changes to the NGM input analyses and physics package have been implemented. While the affects of these changes have yet to be determined, this study illustrates strengths and weaknesses of the NGM during the 1988 cool season and emphasizes the necessity for accurate simulation of boundary-layer processes during return-flow events and times of airmass modification.

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Carlos R. Mechoso and Steven W. Lyons

Abstract

The impact of sea surface temperature (SST) anomalies observed during the Northern Hemisphere spring of 1984, which include the growing phase of an intense Atlantic warm event on the atmospheric circulation over the tropical Atlantic and Pacific is investigated using the nine-layer, low resolution version of the UCLA general circulation model. This impact is contrasted with that for the same period during 1983, when SST anomalies include the decaying phase of the strongest Pacific El Niño on record. Results obtained in control and anomaly simulations, consisting, respectively, of extended integrations with and without the observed SST anomalies, are analyzed.

It is found that simulated anomalies in the atmospheric circulation corresponding to 1984 include low-level westerlies over the equatorial Atlantic and easterlies over the equatorial Pacific. There are centers of anomalous low-level convergence and divergence off the northeast coast of Brazil and equatorial Brazil, respectively, which are associated with positive and negative precipitation anomalies. Differences between results corresponding to 1984 and 1983 show the impact of El Niño over the Pacific. Further, positive precipitation anomalies over the equatorial Atlantic shift from generally north of the equator in 1983 to south of the equator in 1994 (dry and wet years for northeast Brazil, respectively).

These simulated anomalies and interannual differences in the atmospheric circulation are in good general agreement with those observed. This agreement strongly suggests that the atmospheric anomalies observed during the northern springs of 1984 and 1983 over the tropical Atlantic and Pacific were primarily due to the SST anomalies.

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Mary Beth Whitfield and Steven W. Lyons

Abstract

National Meteorological Center 200-mb analyses are used to develop an abridged six-year climatology of the tropical upper-tropospheric trough (TUTT) over the Gulf of Mexico. The climatology reveals large intraseasonal and interannual variability in TUTT axis position. The summer of 1988 is identified as having had an active TUTT near Texas, and is examined in detail. Satellite imagery and 200-mb winds are used to identify and track TUTT lows.

A TUTT low that remained quasi-stationary over Texas is selected for detailed examination. Horizontal and vertical cross sections of wind, temperature, vorticity, and relative humidity illustrate that 1) the maximum circulation around the low occurs near 200-mb, 2) the cold anomaly is largest near 300 mb, and 3) the troposphere moistens as the TUTT low strengthens over Texas. The initial vorticity source for the TUTT low is attributed to positive vorticity advection from midlatitudes, conservation of absolute vorticity, and vorticity convergence along the TUTT axis.

Calculations reveal a net direct circulation in and around the TUTT low, indicating it is an energy-generating system. Vertical motion and temperature fields display warm air rising on the eastern side of the low and cool air sinking near the center and western side of the low in the middle and upper troposphere. Precipitation is maximum in the SE quadrant relative to the TUTT low center.

Examination of the temporal distribution of rainfall over Texas reveals that the TUTT low was one of six synoptic systems that resulted in significant rainfall over the state during July and August 1988. The amount of cold convective cloud as seen from satellite imagery associated with the TUTT low displays significant diurnal variation with a maximum (minimum) observed in afternoon and early evening (late night and early morning) hours, consistent with the diurnal heating cycle over land. It is shown that TUTT lows can be significant sources of summer precipitation in Texas on synoptic space and time scales.

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Sumant Nigam, Isaac M. Held, and Steven W. Lyons

Abstract

The validity of linear stationary wave theory in accounting for the zonal asymmetries of the winter-averaged tropospheric circulation obtained in a general circulation model (GCM) is ascertained. The steady linear primitive equation model used towards this end has the same vertical and zonal resolution as the spectral GCM, but is finite-differenced in the meridional direction. It is linearized about a zonally symmetric basic state and forced by topography and 3-dimensional diabatic heating and transient flux convergence fields, all of which are taken from the GCM. As in Part I, (in which we studied a GCM with a flat lower boundary) we obtained the best correspondence, between the GCM and the linear solutions when strong Rayleigh friction is included in the linear model not only near the surface, but in the interior of the tropical troposphere as well.

There is sufficient quantitative correspondence between the GCM and the linear solution to justify decomposing the linear simulation into parts forced by different processes, although in some regions, such as over North America, the simulation is unsatisfactory. Different fields give different impressions as to the relative importance of orography, heating, and transients. The eddy zonal velocity field in the upper troposphere shows the orographic and thermal plus transient contributions to he nearly equal in amplitude, whereas the eddy meridional velocity field, dominated by shorter zonal scales, shows the orographic contribution to be decisively dominant. Although there is no systematic phase relationship between these two contributions, they are roughly in phase over the cast Asian coast, where each of them is largest. They also contribute roughly equal amounts to the low level Siberian high.

Other findings are that (i) the 300 mb extratropical response to tropical forcing reaches 50 gpm over Alaska (given our frictional parameterization), which is smaller than the response to local thermal forcing, (ii) the responses to sensible heating and lower tropospheric thermal transients are strongly anticorrelated, and (iii) the circulation in the vicinity of the Andes in the GCM is not attributable to direct mechanical forcing by the mountains.

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Isaac M. Held, Steven W. Lyons, and Sumant Nigam

Abstract

A baroclinic stationary wave model linearized about a zonally symmetric flow is used to interpret the extra-tropical atmospheric response to El Niño produced by a general circulation model. When forced by the anomalous diabatic beating and tendency due to transients, the linear model provides a useful simulation of this response. The direct response to anomalous diabatic heating is found to be small in the extratropics; the dominant term is the response to the anomalous transients, particularly the anomalous upper tropospheric transients in the vorticity equation. These results are complementary to those obtained with a nonlinear barotropic model by Held and Kang, and indicate that the anomalous subtropical convergence which plays a key role in that study is itself primarily forced by the anomalous transients. One can distinguish between two distinct parts of the response of the transients to the tropical heating: the movement of the Pacific storm track associated with the anomalous extratropical wave train, and changes in the penetration of Rossby waves into the tropics resulting from the modified tropical winds.

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