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Bruce T. Anderson

Abstract

Previous research has shown that seasonal-mean boreal winter variations in the subtropical/extratropical sea level pressure and wind stress fields over the central North Pacific are significantly related to the state of the El Niño–Southern Oscillation (ENSO) 12–15 months later. Results presented in this note indicate that boreal winter ENSO events are also preceded by increased intraseasonal variance in the antecedent boreal winter atmospheric circulation patterns over the extratropical central North Pacific as well. Low (high) surface pressure anomalies associated with intraseasonal variability in this region are related to intraseasonal wind stress anomalies that represent a weakening (strengthening) of the trade winds over both the north and south subtropical/tropical Pacific. There is also a concurrent increase (decrease) in the central and eastern subtropical North Pacific sea surface temperatures that projects onto the seasonal-mean SST anomalies that precede mature ENSO events by 9–12 months. Overall these results suggest that similar to seasonal-mean subtropical surface pressure and wind stress fields, enhanced transient variability in the midlatitudes can subsequently induce changes in the atmospheric and oceanic structure of the tropical Pacific that may serve as a precursor to ENSO variability.

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Bruce T. Anderson

Abstract

By examining the linearly coupled atmospheric and oceanic signals related to interannual variability in sea surface temperatures and upper-air wind fields, a hemispheric-scale ocean–atmosphere teleconnection mode is isolated that is significantly correlated with equatorial Pacific SSTs 12 months later. The interannual component of this teleconnection mode is related to a basin-scale dipole in the upper-air wind fields stretching across the extratropical Pacific, with additional anomalies extending from the eastern tropical Pacific over North America and into the Atlantic basin. In addition, it is related to variability in the SST field with warm anomalies found over the tropical/subtropical western Pacific as well as the equatorial eastern Pacific; also, there are related cold anomalies over the extratropical central North Pacific that extend down into the central subtropical/tropical Pacific. Diagnostic studies investigating the ocean–atmosphere structure for this mode of variability indicate that the large-scale variations in the upper-air circulation patterns are associated with anomalous equatorward propagation of transient and stationary wave activity over the North Pacific. In addition, they are characterized by vertical circulation patterns over both the subtropical and extratropical Pacific, which are collocated with variations in surface pressure and wind stress fields over the central subtropical and tropical North Pacific. Previous research has shown that modifications of these two fields are significantly related to the evolution of equatorial Pacific SSTs and may provide the dynamic mechanism whereby the ocean–atmosphere teleconnection mode described here influences the development of the ENSO system. This influence appears to be related to a modification of the basin-scale heat content over the central and eastern tropical Pacific; however, significant discussion is provided concerning alternative hypotheses.

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Bruce T. Anderson

Abstract

Using results taken from a finescale (25 km) regional modeling simulation for the summer of 1999, intraseasonal variations in the climatological summertime hydrologic cycle over the southwestern United States are described for two previously identified spatiotemporal precipitation patterns. Over the western portion of the Rocky Mountain plateau, centered on eastern Utah and western Colorado, columnar moisture divergence associated with precipitation is balanced by a combination of seasonal-mean convective moisture convergence and anomalous upper-air (>4 km) large-scale moisture convergence. The actual precipitating events themselves are predicated upon the anomalous upper-level advection of water vapor into the precipitating region; absent this large-scale advection at upper levels, vertical diffusion of moisture into the atmosphere balances large-scale divergence at midlevels, with little precipitation occurring. The anomalous large-scale advection during precipitating events is due primarily to anomalous large-scale vertical fluxes of moisture, with only a slight contribution from large-scale horizontal moisture fluxes. For precipitation located over the eastern portion of the plateau and the elevated orography of eastern New Mexico and southern Colorado, correlated moisture budget terms indicate that precipitation is again related to mean convective moisture convergence and anomalous midtroposphere large-scale moisture convergence. As with the western-plateau precipitation regime, this anomalous convergence is strongly correlated with an anomalous vertical advection of moisture; however, for the eastern-plateau regime, this vertical term is the sole source of large-scale moisture convergence contributing to rainfall in the region. In both cases, the vertical moisture convergence may be associated with previously identified intraseasonal modifications of the upper-level monsoon ridge centered over the Sierra Madre, which results in significant large-scale vertical velocities over the precipitating regions.

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Bruce T. Anderson

Abstract

Previous research has shown that seasonal mean variations in both the subtropical/extratropical sea level pressures over the central North Pacific and the subsurface heat content anomalies in the western equatorial Pacific are significantly related to the state of the El Niño–Southern Oscillation (ENSO) 12–18 months later. Here we find that positive (negative) subsurface temperature anomalies in the western equatorial Pacific during boreal summer/fall, followed by negative (positive) anomalies in the sea level pressure fields over the subtropical central North Pacific during boreal winter, tend to result in positive (negative) mature ENSO events 12–15 months later (i.e., during the following boreal winter). When the intervening sea level pressure anomalies are of the same sign as the preceding heat-content anomalies, the correlation between the heat-content anomalies and the following boreal-winter ENSO state disappears. There is still some relation between the boreal-winter sea level pressure anomalies and the ENSO state the following year when the two precursor patterns are of the same sign; however, the correlation is smaller and the ENSO events tend to be weaker. Additional analysis indicates that the two precursor fields are related to one another; however, the sea level pressure variations contain more unique information about, and provide better predictability of, the state of the following ENSO system than do the heat content anomalies.

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Bruce T. Anderson
and
Eric Maloney

Abstract

This paper describes aspects of tropical interannual ocean/atmosphere variability in the NCAR Community Climate System Model Version 2.0 (CCSM2). The CCSM2 tropical Pacific Ocean/atmosphere system exhibits much stronger biennial variability than is observed. However, a canonical correlation analysis technique decomposes the simulated boreal winter tropical Pacific sea surface temperature (SST) variability into two modes, both of which are related to atmospheric variability during the preceding boreal winter. The first mode of ocean/atmosphere variability is related to the strong biennial oscillation in which La Niña–related sea level pressure (SLP) conditions precede El Niño–like SST conditions the following winter. The second mode of variability indicates that boreal winter tropical Pacific SST anomalies can also be initiated by SLP anomalies over the subtropical central and eastern North Pacific 12 months earlier.

The evolution of both modes is characterized by recharge/discharge within the equatorial subsurface temperature field. For the first mode of variability, this recharge/discharge produces a lag between the basin-average equatorial Pacific isotherm depth anomalies and the isotherm–slope anomalies, equatorial SSTs, and wind stress fields. Significant anomalies are present up to a year before the boreal winter SLP variations and two years prior to the boreal winter ENSO-like events. For the second canonical factor pattern, the recharge/discharge mechanism is induced concurrent with the boreal winter SLP pattern approximately one year prior to the ENSO-like events, when isotherms initially deepen and change their slope across the basin. A rapid deepening of the isotherms in the eastern equatorial Pacific and a warming of the overlying SST anomalies then occurs during the subsequent 12 months.

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Bruce T. Anderson
and
John O. Roads

Abstract

Using results taken from a finescale (25 km), regional modeling simulation for the summer of 1999, along with contemporaneous daily surface observations, synoptic variations in summertime precipitation over the southwestern United States are described and analyzed. Two separate techniques for characterizing and evaluating large-scale summertime precipitation patterns within the observed and simulated systems are presented; in addition, these evaluation/characterization techniques are used to analyze the hydrologic forcings associated with observed and simulated modes of rainfall variability. Overall, two robust spatiotemporal precipitation patterns are identified involving 1) precipitation over the western portion of the Rocky Mountain plateau centered on eastern Arizona and southern Utah, and 2) precipitation located over the eastern portion of the plateau and the elevated orography of eastern New Mexico and southern Colorado. Time series associated with these two precipitation regimes are correlated with low-level and midlevel circulation patterns in order to investigate the related large-scale environmental conditions. It is found that for both regimes intraseasonal precipitation is related to the intrusion of midtroposphere, midlatitude low-pressure anomalies over the southwestern United States, resulting in synoptic-scale shifts in the position of the climatological midtroposphere monsoon ridge. The interaction between the resultant midtroposphere pressure fields and the quasi-stationary monsoon surface pressures found over the Rocky Mountain plateau during the summertime produce large-scale vertical velocities consistent with the observed and simulated rainfall patterns associated with each regime.

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Nicole Feldl
,
Bruce T. Anderson
, and
Simona Bordoni

Abstract

Projections of amplified climate change in the Arctic are attributed to positive feedbacks associated with the retreat of sea ice and changes in the lapse rate of the polar atmosphere. Here, a set of idealized aquaplanet experiments are performed to understand the coupling between high-latitude feedbacks, polar amplification, and the large-scale atmospheric circulation. Results are compared to CMIP5. Simulated climate responses are characterized by a wide range of polar amplification (from none to nearly 15-K warming, relative to the low latitudes) under CO2 quadrupling. Notably, the high-latitude lapse rate feedback varies in sign among the experiments. The aquaplanet simulation with the greatest polar amplification, representing a transition from perennial to ice-free conditions, exhibits a marked decrease in dry static energy flux by transient eddies. Partly compensating for the reduced poleward energy flux is a contraction of the Ferrel cell and an increase in latent energy flux. Enhanced eddy energy flux is consistent with the upper-tropospheric warming that occurs in all experiments and provides a remote influence on the polar lapse rate feedback. The main conclusions are that (i) given a large, localized change in meridional surface temperature gradient, the midlatitude circulation exhibits strong compensation between changes in dry and latent energy fluxes, and (ii) atmospheric eddies mediate the nonlinear interaction between surface albedo and lapse rate feedbacks, rendering the high-latitude lapse rate feedback less positive than it would be otherwise. Consequently, the variability of the circulation response, and particularly the partitioning of energy fluxes, offers insights into understanding the magnitude of polar amplification.

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Daniel J. Short Gianotti
,
Bruce T. Anderson
, and
Guido D. Salvucci

Abstract

Using weather station data, the parameters of a stationary stochastic weather model (SSWM) for daily precipitation over the contiguous United States are estimated. By construct, the model exactly captures the variance component of seasonal precipitation characteristics (intensity, occurrence, and total amount) arising from high-frequency variance. By comparing the variance of the lower-frequency accumulations (on the order of months) between the SSWM and the original measurements, potential predictability (PP) is estimated. Decomposing the variability into contributions from occurrence and intensity allows one to establish two contributing sources of total PP. Aggregated occurrence is found to have higher PP than either intensity or the seasonal total precipitation, and occurrence and intensity are found to interfere destructively when convolved into seasonal totals. It is recommended that efforts aimed at forecasting seasonal precipitation or attributing climate variability to particular processes should analyze occurrence and intensity separately to maximize signal-to-noise ratios. Significant geographical and seasonal variations exist in all PP components.

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Dan Gianotti
,
Bruce T. Anderson
, and
Guido D. Salvucci

Abstract

A generalizable method is presented for establishing the potential predictability for seasonal precipitation occurrence using rain gauge data. This method provides an observationally based upper limit for potential predictability for 774 weather stations in the contiguous United States. It is found that the potentially predictable fraction varies seasonally and spatially, and that on average 30% of year-to-year seasonal variability is potentially explained by predictable climate processes. Potential predictability is generally highest in winter, appears to be enhanced by orography and land surface coupling, and is lowest (stochastic variance is highest) along the Pacific coast. These results depict “hot” spots of climate variability, for use in guiding regional climate forecasting and in uncovering processes driving climate. Identified “cold” spots are equally useful in guiding future studies as predictable climate signals in these areas will likely be undetectable.

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Bruce T. Anderson
,
Jingyun Wang
,
Guido Salvucci
,
Suchi Gopal
, and
Shafiqul Islam

Abstract

In this paper, the authors evaluate the significance of multidecadal trends in seasonal-mean summertime precipitation and precipitation characteristics over the southwestern United States using stochastic, chain-dependent daily rainfall models. Unlike annual-mean precipitation, trends during the summertime monsoon, covering the period 1931–2000, indicate an overall increase in seasonal precipitation, the number of rainfall events, and the coverage of rainfall events in peripheral regions north of the “core” monsoon area of Arizona and western New Mexico. In addition, there is an increasing trend in intense storm activity and a decreasing trend in extreme dry-spell lengths. Over other regions of the domain, there are no discernible trends found in any of the observed characteristics. These trends are robust to the choice of start dates and, in the case of seasonal-mean precipitation, appear to persist into the current century.

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