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Steven W. Lyons

Abstract

A zonal wind oscillation with a period of about 15–35 days is isolated over the middle latitudes of the Northern Hemisphere during winter. A systematic relationship between the zonal winds and wind departures from zonal symmetry (eddies) is found for nine prominent zonal wind oscillation cycles taken from six 120-day winters. This zonal/eddy relationship consists of an oscillation between anomalous zonal flow and anomalous meridional flow, which is strongest across North America from 150°W to 0°.

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Steven W. Lyons

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Steven W. Lyons

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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

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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

Abstract

Interannual variations in U.S. tropical storm and hurricane landfalls are examined over the interval 1950– 2002. The 10 highest and 9 lowest U.S. storm and hurricane landfall years are highlighted. U.S. landfalls are 6 times more frequent during the 10 highest than during the 9 lowest years. Nine major hurricanes struck the United States during the 10 highest U.S. landfall years, one struck during the 9 lowest U.S. landfall years. There is a positive correlation between Atlantic basin storm and hurricane frequency and U.S. storm and hurricane landfall frequency, but U.S. landfall variability explained by that relationship is small. Years with high (low) U.S. landfalls have a high (low) frequency of storm and hurricane formation in the Gulf of Mexico and the western Caribbean Sea and a high (low) percentage of them landfall along the U.S. Gulf coast. U.S. landfall frequency of Atlantic storms and hurricanes is much higher (lower) during high (low) U.S. landfall years, implying that Atlantic steering currents are more (less) favorable for U.S. landfall. La Niña conditions occurred 19% more often during high U.S. landfall years than during remaining years. El Niño conditions occurred 10% more often during low U.S. landfall years than during remaining years. Skill of inferring how many storms and hurricanes will landfall in the United States from a forecast of the number of Atlantic basin storms and hurricanes explains an average of 18% or less of U.S. storm and hurricane variability in a hindcast setting. Results indicate that a large portion of U.S. storm and hurricane landfall variability is related to where the storms form and whether steering currents are favorable or unfavorable for bringing them to the United States.

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Steven W. Lyons

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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

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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 and Bruce Hundermark

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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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Mary Beth Whitfield and Steven W. Lyons

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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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