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David S. Gutzler

Abstract

Oceanic and atmospheric data from the tropical western Pacific are analyzed to describe decadal-scale trends during the last 20 years, and these low-frequency trends are compared with shorter-term Southern Oscillation-related variations. Regional indices of western and central equatorial Pacific SST exhibit significant upward trends in recent decades. The decadal variability in the tropical Pacific is large enough relative to interannual variability to significantly affect the interpretation of standardized SST anomaly indices used to monitor Southern Oscillation phenomena. Specific humidity in the tropical western Pacific boundary layer exhibits a statistically significant upward trend consistent with previously published results based on a shorter data record. The convective instability of the tropical troposphere is increasing, but two indices related to precipitation show no evidence of a trend. These trends cannot be explained as an aggregate of the effects of more frequent El Niño warm events in recent years because the tropical western Pacific response to El Niño includes negative (i.e., dry) boundary-layer humidity anomalies and decreased convective instability. On interannual timescales there seems to be a distinct separation between the processes affecting tropospheric temperature within and above the tropical western Pacific boundary layer.

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David S. Gutzler

Abstract

The annual and semiannual harmonics of the wind field within 25° of the equator at six tropospheric pressure levels are described, based on data from a network of rawinsonde stations. The annual cycle in the zonal wind component is most prominent in the upper troposphere at subtropical latitude with maximum westerlies occurring late in each hemisphere's winter season. Significant amplitudes of the annual cycle in the meridional wind component are found to be confined to layers about 150 mb thick at the bottom and top of the troposphere. The semiannual cycle is significant only in the zonal wind component. At stations near the equator, the semiannual cycle in zonal wind has larger amplitude than the annual cycle.

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David S. Gutzler

Abstract

Seasonal precipitation anomalies associated with the continental North American monsoon system are characterized using a land-based dataset derived from in situ observations across the southwestern United States and northwestern Mexico. Coherent regions of interannual continental precipitation variability are derived from principal component analysis, after defining separate “early” and “late” summer monsoon seasons. The gravest mode of late-season interannual variability captures precipitation anomalies in the core of the continental monsoon domain. A simple spatial average is developed as an index of this core variability. The seasonal separation allows examination of persistence of precipitation anomalies as an indicator of practical late-season predictability. Possible influences of large-scale oceanic interannual fluctuations [ENSO and Pacific decadal oscillation (PDO)] on core index precipitation anomalies are also considered. The core precipitation index exhibits considerably more early-to-late-season persistence than an ocean-centered precipitation index. Implications of these results for monthly/seasonal predictability of warm-season precipitation are discussed.

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David S. Gutzler

Abstract

Interannual fluctuations of observed summer rainfall across the monsoon region of the southwestern United States are analyzed to ascertain their spatial coherence and to test the hypothesis that antecedent spring snowpack anomalies may modulate the monsoon and exhibit an inverse correlation with summer rainfall anomalies. To characterize the spatial coherence of seasonal rainfall anomalies, an objective linear analysis of interannual variability is applied to climate divisional data across the Southwest. Three coherent subregions are identified, broadly representing rainfall anomalies across Arizona, eastern New Mexico/western Texas (the Southwest Plains), and most of New Mexico. Interannual fluctuations of summer rainfall in the New Mexico region exhibit a very significant negative correlation with a large-scale index of the antecedent 1 April snowpack over the southern U.S. Rocky Mountains during the 1961–90 climatic averaging period. This strong relationship seems to break down in the years before and after this period, possibly indicating a shift in climate associated with other forcing factors.

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David S. Gutzler

Abstract

Geographical and seasonal variations of the variance and vertical structure of interannual anomalies of seasonally averaged zonal winds are calculated from rawinsonde records at tropical stations between 80°E and 140°W, the longitudes spanned by the “Walker Circulation.” Wind anomalies are negatively correlated in the upper and lower troposphere only in a latitude band within about 10° of the equator, defining the latitudinal extent of the Walker Circulation; about 70% of the interannual variance of tropospheric zonal winds is accounted for by a single vertical mode within this band. The band shifts seasonally north and south slightly, in conjunction with seasonal shifts in large-scale convection. The level of zonal overturning occurs between 300 and 500 mb, and is highest near the seasonal convective maximum. Other vertical structures, suggestive of interannual variability not associated with thermally forced circulations, are also found near the equator at some longitudes in the boreal winter season.

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David S. Gutzler and Jagadish Shukla

Abstract

A 15-winter sample of daily gridded values of Northern Hemisphere 500 mb heights is examined for the existence of recurrent flow patterns (“analogs”). The analog search is repeated several times after degrees of freedom are successively removed from the data by spatial filtering, temporal averaging, and consideration of smaller sectors of the hemisphere. The root mean square difference (or “rms error”) between the most closely analogous maps, defined over the middle latitudes (30–70°N), is slightly greater than half the average error between randomly chosen maps, with an estimated rms error doubling time of nearly 8 days. If the analog search is conducted using only the longwave component (zonal wavenumbers 0–4) of each map, the rms error between the best analog pairs is reduced to less than half of the rms error between long-wave anomalies on randomly chosen maps, but the doubling time is also reduced to less than 7 days. If the analog search is further restricted to a limited region over North America or Europe, the rms error between the best analog pairs is less than 40% of the rms error (for the sme region) between randomly chosen maps, but the error doubling time is further reduced to 4–5 days. In all cases, the degradation of analog quality is so rapid that a forecasting scheme based on the analogs would fail to produce more skillful forecasts than simple persistence.

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David A. Portman and David S. Gutzler

Abstract

A study was conducted to identify and separate possible signals of volcanic eruptions and of the El Niño–Southern Oscillation (ENSO) in U.S. surface climate records. Anomalies of monthly mean surface air temperature and total precipitation taken from the U.S. Historical Climatology Network were composited (averaged) over years of major explosive volcanic eruptions. ENSO warm events, and ENSO cold events since the year 1900. It was assumed that volcanic eruptions and ENSO events occur independently of each other. All composite anomalies were assessed for significance with regard to several statistical and physical criteria. The composite ENSO-related anomalies were then subtracted from anomalies of temperature and precipitation associated with the volcanic eruptions.

Removal of large magnitude and highly significant anomalies associated with the ENSO warm and cold events is found to facilitate detection of volcanic signals in monthly records of U.S. temperature and precipitation. Volcanic signals are strongly suggested cast of the Continental Divide, for example, where positive monthly temperature anomalies exceeding 1°C occur during the first fall and winter after eruptions. Negative temperature anomalies occur west of the Continental Divide during the first winter and spring after eruptions and in the southern United States during the summer of the first post-eruption calendar year. Positive monthly precipitation anomalies exceeding 15 mm in magnitude are found in the southeastern United States during the first winter and spring after eruptions. Precipitation anomalies that are smaller in magnitude and yet significant, such as positive anomalies in the northwestern United States and negative anomalies in the central and eastern United States, are found during the summer of the first post-eruption calendar year.

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David S. Gutzler and Joshua S. Nims

Abstract

The effects of interannual climate variability on water demand in Albuquerque, New Mexico, are assessed. This city provides an ideal setting for examining the effects of climate on urban water demand, because at present the municipal water supply is derived entirely from groundwater, making supply insensitive to short-term climate variability. There is little correlation between interannual variability of climate and total water demand—a result that is consistent with several previous studies. However, summertime residential demand, which composes about one-quarter of total annual demand in Albuquerque, is significantly correlated with summer-season precipitation and average daily maximum temperature. Furthermore, regressions derived from year-to-year changes in these variables are shown to isolate the climatic modulation of residential water demand effectively. Over 60% of the variance of year-to-year changes in summer residential demand is accounted for by interannual temperature and precipitation changes when using a straightforward linear regression model, with precipitation being the primary correlate. Long-term trends in water demand follow population growth closely until 1994, after which time a major water conservation effort led to absolute decreases in demand in subsequent years. The effectiveness of the conservation efforts can be quantified by applying the regression model, thus removing the year-to-year variations associated with short-term climate fluctuations estimated from the preconservation period. The preconservation regression provides a good fit to interannual summer residential demand in subsequent years, demonstrating that the regression model has successfully isolated the climatic component of water demand. The quality of this fit during a period of sharply reduced demand suggests that the conservation program has effectively targeted the nonclimatically sensitive component of water demand and has sharpened the climatically sensitive component of demand to a level closer to the consumption that is “climatically needed.”

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David S. Gutzler and D. E. Harrison

Abstract

The longitude–height–time evolution of seasonally averaged wind anomalies over the near-equatorial eastern Indian and western Pacific Oceans is examined, using multiyear time series derived from a network of eight rawinsonde stations. Data at six pressure levels, between 850 and 150 mb, are considered. The first two modes of an empirical orthogonal function analysis of zonal wind fluctuations are cross correlated at lag, with spatial structures suggesting that the dominant pattern of variability on seasonal time scales is best described as a propagating oscillation. This space–time structure is confirmed using a complex empirical orthogonal function analysis, which indicates that over half of the interseasonal zonal wind variance at these stations is associated with an eastward-propagating mode (denoted E1). Wind anomalies described by E1 are negatively correlated in the upper and lower troposphere at each station, and are out of phase between the southern tip of India and the central Pacific, so that E1 can be interpreted as an eastward-propagating pattern of convergence/divergence along the equator. Variations in the phase of this mode are “phase-locked” to the annual cycle, and are highly correlated with a conventional Southern Oscillation Index. The wind anomaly field described by E1 evolves through a characteristic life cycle during El Niño events, which begins before the onset of ocean surface warming in the eastern Pacific; the anomaly pattern then propagates eastward during the course of the event.

These results are further confirmed by compositing wind anomalies with respect to the phones of the six most recent El Niño events. During the Northern Hemisphere autumn season prior to the onset of El Niño, anomalous low level convergence and upper level divergence are observed in the vicinity of Indonesia. This pattern subsequently propagates eastward, until the opposite pattern of anomalies is observed during the fully developed phase of El Niño, one year after the initial appearance of the atmospheric anomaly pattern. The eastward phase speed is much slower than an atmospheric Kelvin wave, suggesting that the wind anomalies are part of an air–sea interactive system.

The interevent variability for each phase of the six El Niño events in the data record is substantial; the significance of the composite anomaly pattern varies considerably from phase to phase. The composite is most robust for the Northern Hemisphere autumn season during the year in which ocean surface warming first occurs. It is particularly noteworthy that the evolution of wind anomalies over the far western Pacific prior to the 1982 event was not significantly different from previous events.

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David S. Gutzler and Roland A. Madden

Abstract

Seasonally varying spectral and cross-spectral calculations are carried out on multiyear time series of vertically and zonally averaged daily zonal wind fields to describe the seasonal cycle of the 40–50-day oscillation of atmospheric angular momentum. Intraseasonal variability (including 40–50-day fluctuations) of global momentum is largest in late boreal winter and smallest in boreal autumn; however, the 40–50-day spectral peak is most pronounced in boreal summer when lower-frequency intraseasonal variance is depressed. The 40–50-day spectral peak in global momentum is much less pronounced and apparently is restricted to a narrower frequency band, than corresponding peaks in zonal wind spectra from individual tropical rawinsonde stations. Contributions to global momentum fluctuations from three near-equal-area latitude bands (tropics, Northern Hemisphere, and Southern Hemisphere) are compared, confirming that intraseasonal momentum fluctuations are tropical in origin. The variance of extratropical momentum at this time scale is about an order of magnitude less than the tropical momentum variability. Coherent tropical–extratropical interactions are found principally in boreal winter, with the highest coherence between the tropics and Northern Hemisphere. The corresponding phase difference between tropical and Northern Hemisphere momentum is suggestive of poleward propagation of momentum out of the tropics.

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