Bruce T. Anderson

Temperature and vegetation seasonality diminishment over northern lands

Pronounced increases in winter temperature result in lower seasonal temperature differences, with implications for vegetation seasonality and productivity. Research now indicates that temperature and vegetation seasonality in northern ecosystems have diminished to an extent equivalent to a southerly shift of 4°– 7° in latitude, and may reach the equivalent of up to 20° over the twenty-first century.

Investigation of a Large-Scale Mode of Ocean–Atmosphere Variability and Its Relation to Tropical Pacific Sea Surface Temperature Anomalies

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 th...

Feedbacks of Vegetation on Summertime Climate Variability over the North American Grasslands. Part I: Statistical Analysis

Abstract Feedbacks of vegetation on summertime climate variability over the North American Grasslands are analyzed using the statistical technique of Granger causality. Results indicate that normalized difference vegetation index (NDVI) anomalies early in the growing season have a statistically measurable effect on precipitation and surface temperature later in summer. In particular, higher means and/or decreasing trends of NDVI anomalies tend to be followed by lower rainfall but higher temperatures during July through September. These results suggest that initially enhanced vegetation may deplete soil moisture faster than normal and thereby induce drier and warmer climate anomalies via the strong soil moisture–precipitation coupling in these regions. Consistent with this soil moisture–precipitation feedback mechanism, interactions between temperature and precipitation anomalies in this region indicate that moister and cooler conditions are also related to increases in precipitation during the preceding m...

Regional simulation of the low‐level monsoon winds over the Gulf of California and southwestern United States

Using a fine-scale (10–20 km) nested regional modeling system, synoptic variations in climatological summertime low-level wind fields over the Gulf of California and the southwestern United States are described. Under appropriate synoptic conditions, strong surge events can develop. These are characterized by low-level southerly flow over the entire Gulf of California with southerly winds extending into Arizona, California, and southern Nevada. Vertically, these southerly winds are present through the bottom 1–2 km of the atmosphere. Southerly flow is persistent throughout the day with some local diurnal cycling over the foothills of the Sierra Madre Occidental and northwestern Mexico. Under nonsurge conditions the simulated low-level winds have a significantly different geographic structure. Nighttime southerly flow is limited to the northeastern Gulf and small portions of southwestern Arizona. Flow over the central and southern Gulf is northerly with weak, variable winds over the foothills of the Sierra Madre Occidental. During the day, southerly winds are present over the central and northern Gulf of California, extending into southwestern Arizona; however, this southerly wind pattern does not support continuous flow from the mouth of the Gulf into northwestern Mexico. Instead, there is a westerly component associated with predominantly upslope flow over the foothills of the Sierra Madre Occidental and the Sonora Desert.

Large‐scale forcing of summertime monsoon surges over the Gulf of California and the southwestern United States

Synoptic forcing of the Gulf of California summertime low-level wind field is described using a nested regional modeling system. Under appropriate synoptic conditions, strong surge events develop that are characterized by persistent, vertically extensive (1–2 km) southerly flow extending along the entire Gulf of California and into Arizona, California, and even southern Nevada. These surge periods are initiated either by tropical-cyclone activity to the south of Baja California or by the westward propagation of lower tropospheric troughs from over the Sierra Madre Occidental to the eastern Pacific. The flow over the Gulf is primarily geostrophic and is associated with the presence of these low-pressure centers to the south and west of Baja California.

Uncertainties in the timing of unprecedented climates

Arising from C. Mora et al. 502, 183–187 10.1038/nature12540 (2013)The question of when the signal of climate change will emerge from the background noise of climate variability—the ‘time of emergence’—is potentially important for adaptation planning. Mora et al. presented precise projections of the time of emergence of unprecedented regional climates. However, their methodology produces artificially early dates at which specific regions will permanently experience unprecedented climates and artificially low uncertainty in those dates everywhere. This overconfidence could impair the effectiveness of climate risk management decisions. There is a Reply to this Brief Communication Arising by Mora, C. et al. Nature 511, http://dx.doi.org/10.1038/nature13524 (2014).

Examination of the Bouchet–Morton Complementary Relationship Using a Mesoscale Climate Model and Observations under a Progressive Irrigation Scenario

The complementary relationship between actual and potential evaporation over southeastern Turkey was examined using a mesoscale climate model and field data. Model simulations of both actual and potential evaporation produce realistic temporal patterns in comparison to those estimated from field data; as evaporation from the surface increases with increasing irrigation, potential evaporation decreases. This is in accordance with the Bouchet–Morton complementary relationship and suggests that actual evapotranspiration can be readily computed from routine meteorological observations. The driving mechanisms behind irrigation-related changes in actual and potential evaporation include reduced wind velocities, increased atmospheric stability, and depressed humidity deficits. The relative role of each in preserving the complementary relation is assessed by fitting a potential evaporation model to pan evaporation data. The importance of reduced wind velocity in maintaining complementarity was unexpected, and thus examined further using a set of perturbation simulation experiments with changing roughness parameters (reflecting growing cotton crops), changing moisture conditions (reflecting irrigation), and both. Three potential causes of wind velocity reduction associated with irrigation may be increased surface roughness, decreased thermal convection that influences momentum transfer, and the development of anomalous high pressure that counteracts the background wind field. All three are evident in the mesoscale model results, but the primary cause is the pressure-induced local wind system. The apparent necessity of capturing mesoscale dynamical feedbacks in maintaining complementarity between potential and actual evaporation suggests that a theory more complicated than current descriptions (which are based on feedbacks between actual evaporation and temperature and/or humidity gradients) is required to explain the complementary relationship.

ENSO and meridional modes: A null hypothesis for Pacific climate variability

Pacific low-frequency variability (timescale > 8 years) exhibits a well-known El Nino-like pattern of basin-scale sea surface temperature, which is found in all the major modes of Pacific decadal climate. Using a set of climate model experiments and observations, we decompose the mechanisms contributing to the growth, peak, and decay of the Pacific low-frequency spatial variance. We find that the El Nino-like interdecadal pattern is established through the combined actions of Pacific meridional modes (MM) and the El Nino–Southern Oscillation (ENSO). Specifically, in the growth phase of the pattern, subtropical stochastic excitation of the MM energizes the tropical low-frequency variance acting as a red noise process. Once in the tropics, this low-frequency variance is amplified by ocean-atmospheric feedbacks as the pattern reaches its peak phase. At the same time, atmospheric teleconnections distribute the variance from the tropics to the extratropics, where the pattern ultimately decays. In this stochastic red noise model of Pacific climate, the timescale of the extra-tropical/tropical interactions (1–2 years) permits the stochastic excitation of the ENSO-like pattern of decadal and interdecadal variance.

Model dynamics of summertime low-level jets over northwestern Mexico

Using a fine-scale nested regional modeling system, the diurnal forcing of summertime low-level winds over the Gulf of California and northwestern Mexico is investigated. On diurnal timescales, simulated nocturnal low-level jets develop over the northern portion of the Gulf, the foothills of the Sierra Madre Occidental, and parts of southern Arizona. The southerly component of the nocturnal jet is the result of a geostrophic balance involving the Coriolis force and a cross-gulf pressure gradient force associated with nighttime slope cooling over the elevated Sierra Madre Occidental. Additionally, horizontal temperature gradients over the sloped orography produce vertical variations in this cross-gulf pressure gradient force, generating the jet-like vertical shear in wind components above the nocturnal boundary layer; frictional effects are responsible for producing shear in the wind profiles below the boundary layer. This balance is distinctly different from the inertial balance that is believed to be responsible for the low-level jet over the Great Plains region of the United States. Daytime winds are part of a directly driven wind field forced by horizontal pressure gradients associated with slope heating (up the Sierra Madre Occidental) and sea-land temperature gradients (north of the Gulf). During synoptically forced surge events, a similar diurnal cycle in low-level flow is still present; however, the local thermal forcing appears to be superimposed upon the large-scale synoptic forcing, resulting in weaker up-slope flow during the day and stronger along-slope flow at night.

The Summertime Atmospheric Hydrologic Cycle over the Southwestern United States

Abstract In this paper the authors examine the large-scale summertime hydrologic cycle associated with the northwestern branch of the North American monsoon, centered on the southwestern United States, using a suite of surface-and upper-air-based observations, reanalysis products, and regional model simulations. In general, it is found that on an area-averaged basis, seasonal precipitation is balanced predominantly by evaporation; in addition, this evaporation also supports a net, vertically integrated moisture flux divergence from the region of the same magnitude as the precipitation itself. This vertically integrated large-scale moisture flux divergence is the result of an offsetting balance between convergence of low-level moisture and divergence of moisture aloft (<750 mb). Over the western portion of the domain, most of this low-level moisture convergence is related to advection from the Gulf of California and eastern Pacific; over the eastern portion of the domain, low-level moisture convergence is ...