Search Results

You are looking at 1 - 10 of 21 items for

  • Author or Editor: J. M.Cook x
  • Refine by Access: All Content x
Clear All Modify Search
O. Koren
and
J. M.Cook

Abstract

Full access
K. H. Cook
,
J-S. Hsieh
, and
S. M. Hagos

Abstract

The influence of heating over Africa on the South American precipitation climatology, and the influence of South America on Africa, is examined through the application of GCM simulations with idealized boundary conditions and perpetual solstice (January and July) conditions.

The presence of Africa is associated with a pronounced (up to 4 mm day−1) decrease in precipitation in Brazil's Nordeste region during austral summer. Low-level moisture divergence and dry-air advection associated with the downbranch of a Walker circulation induced by heating over southern Africa is amplified over the Nordeste due to the response of the land surface. The response is much smaller during austral winter due to differences in the heat source over Africa and a reduced sensitivity in the surface heat balance over tropical South America. Forcing from South America in January shifts the position of the South Indian convergence zone (SICZ) to the southwest over southern Africa in association with the formation of the South Atlantic convergence zone (SACZ). In July, a Rossby wave train generated over South America induces a response in the surface temperature of Africa that leads to stronger precipitation in central and western Africa.

This study suggests a zonal mode of variability for South American and African circulation and precipitation fields. The resulting perturbations depend as much on land surface–atmosphere interactions as on the direct forcing from the adjacent continent, and the mechanisms are highly nonlinear.

Full access
J. David Neelin
,
Isaac M. Held
, and
Kerry H. Cook

Abstract

A mechanism by which feedback between zonal wind perturbations and evaporation can create unstable, low-frequency modes in a simple two-layer model of the tropical troposphere is presented. The modes resemble the 30–50 day oscillation. A series of general circulation model experiments designed to test the effect of suppressing this feedback on low-frequency variability in the model tropics is described. The results suggest that the evaporation-wind feedback can be important to the amplitude of the spectral peak corresponding to the 30–50 day oscillation in the model, but that the existence of the oscillation does not depend on it. The feedback is found to have a much more dramatic effect on low-frequency variability when sea surface temperatures are fixed than when the lower boundary is a zero heat capacity “swamp”.

Full access
J. Song
,
M. L. Wesely
,
D. J. Holdridge
,
D. R. Cook
, and
J. Klazura

Abstract

Estimates of the hydrological budget in the Walnut River Watershed (WRW; ∼5000 km2) of southern Kansas were made with a parameterized subgrid-scale surface (PASS) model for the period 1996–2002. With its subgrid-scale distribution scheme, the PASS model couples surface meteorological observations with satellite remote sensing data to update root-zone available moisture and to simulate surface evapotranspiration rates at high resolution over extended areas. The PASS model is observationally driven, making use of extensive parameterizations of surface properties and processes. Heterogeneities in surface conditions are spatially resolved to an extent determined primarily by the satellite data pixel size. The purpose of modeling the spatial and interannual variability of water budget components at the regional scale is to evaluate the PASS model's ability to bridge a large grid cell of a climate model with its subgrid-scale variation. Modeled results indicate that annual total evapotranspiration at the WRW is about 66%–88% of annual precipitation—reasonable values for southeastern Kansas—and that it varies spatially and temporally. Seasonal distribution of precipitation plays an important role in evapotranspiration estimates. Comparison of modeled runoff with stream gauge measurements demonstrated close agreement and verified the accuracy of modeled evapotranspiration at the regional scale. In situ measurements of energy fluxes compare favorably with the modeled values for corresponding grid cells, and measured surface soil moisture corresponds with modeled root-zone available moisture in terms of temporal variability despite very heterogeneous surface conditions. With its ability to couple remote sensing data with surface meteorology data and its computational efficiency, PASS is easily used for modeling surface hydrological components over an extended region and in real time. Thus, it can fill a gap in evaluations of climate model output using limited field observations.

Full access
M. C. A. Torbenson
,
D. W. Stahle
,
I. M. Howard
,
D. J. Burnette
,
D. Griffin
,
J. Villanueva-Díaz
, and
B. I. Cook

Abstract

Season-to-season persistence of soil moisture drought varies across North America. Such interseasonal autocorrelation can have modest skill in forecasting future conditions several months in advance. Because robust instrumental observations of precipitation span less than 100 years, the temporal stability of the relationship between seasonal moisture anomalies is uncertain. The North American Seasonal Precipitation Atlas (NASPA) is a gridded network of separately reconstructed cool-season (December–April) and warm-season (May–July) precipitation series and offers new insights on the intra-annual changes in drought for up to 2000 years. Here, the NASPA precipitation reconstructions are rescaled to represent the long-term soil moisture balance during the cool season and 3-month-long atmospheric moisture during the warm season. These rescaled seasonal reconstructions are then used to quantify the frequency, magnitude, and spatial extent of cool-season drought that was relieved or reversed during the following summer months. The adjusted seasonal reconstructions reproduce the general patterns of large-scale drought amelioration and termination in the instrumental record during the twentieth century and are used to estimate relief and reversals for the most skillfully reconstructed past 500 years. Subcontinental-to-continental-scale reversals of cool-season drought in the following warm season have been rare, but the reconstructions display periods prior to the instrumental data of increased reversal probabilities for the mid-Atlantic region and the U.S. Southwest. Drought relief at the continental scale may arise in part from macroscale ocean–atmosphere processes, whereas the smaller-scale regional reversals may reflect land surface feedbacks and stochastic variability.

Full access
G. E. Klazura
,
D. R. Cook
,
R. L. Coulter
,
R. L. Hart
,
D. J. Holdridge
,
B. M. Lesht
,
J. D. Lucas
,
T. J. Martin
,
M. S. Pekour
, and
M. L. Wesely
Full access
Kevin J. Anchukaitis
,
Rosanne D. D’Arrigo
,
Laia Andreu-Hayles
,
David Frank
,
Anne Verstege
,
Ashley Curtis
,
Brendan M. Buckley
,
Gordon C. Jacoby
, and
Edward R. Cook

Abstract

Northwestern North America has one of the highest rates of recent temperature increase in the world, but the putative “divergence problem” in dendroclimatology potentially limits the ability of tree-ring proxy data at high latitudes to provide long-term context for current anthropogenic change. Here, summer temperatures are reconstructed from a Picea glauca maximum latewood density (MXD) chronology that shows a stable relationship to regional temperatures and spans most of the last millennium at the Firth River in northeastern Alaska. The warmest epoch in the last nine centuries is estimated to have occurred during the late twentieth century, with average temperatures over the last 30 yr of the reconstruction developed for this study [1973–2002 in the Common Era (CE)] approximately 1.3° ± 0.4°C warmer than the long-term preindustrial mean (1100–1850 CE), a change associated with rapid increases in greenhouse gases. Prior to the late twentieth century, multidecadal temperature fluctuations covary broadly with changes in natural radiative forcing. The findings presented here emphasize that tree-ring proxies can provide reliable indicators of temperature variability even in a rapidly warming climate.

Full access
R. Kwok
,
T. Markus
,
J. Morison
,
S. P. Palm
,
T. A. Neumann
,
K. M. Brunt
,
W. B. Cook
,
D. W. Hancock
, and
G. F. Cunningham

Abstract

The sole instrument on the upcoming Ice, Cloud, and Land Elevation Satellite (ICESat-2) altimetry mission is a micropulse lidar that measures the time of flight of individual photons from laser pulses transmitted at 532 nm. Prior to launch, the Multiple Altimeter Beam Experimental Lidar (MABEL) serves as an airborne implementation for testing and development. This paper provides a first examination of MABEL data acquired on two flights over sea ice in April 2012: one north of the Arctic coast of Greenland and the other in the east Greenland Sea. The phenomenology of photon distributions in the sea ice returns is investigated. An approach to locate the surface and estimate its elevation in the distributions is described, and its achievable precision is assessed. Retrieved surface elevations over relatively flat leads in the ice cover suggest that precisions of several centimeters are attainable. Restricting the width of the elevation window used in the surface analysis can mitigate potential biases in the elevation estimates due to subsurface returns at 532 nm. Comparisons of nearly coincident elevation profiles from MABEL with those acquired by an analog lidar show good agreement. Discrimination of ice and open water, a crucial step in the determination of sea ice freeboard and the estimation of ice thickness, is facilitated by contrasts in the observed signal–background photon statistics. Future flight paths will sample a broader range of seasonal ice conditions for further evaluation of the year-round profiling capabilities and limitations of the MABEL instrument.

Full access
Sonali Shukla McDermid
,
Ensheng Weng
,
Michael Puma
,
Benjamin Cook
,
Tomislav Hengl
,
Jonathan Sanderman
,
Gabrielle J. M. De Lannoy
, and
Igor Aleinov

Abstract

Most agricultural soils have experienced substantial soil organic carbon losses in time. These losses motivate recent calls to restore organic carbon in agricultural lands to improve biogeochemical cycling and for climate change mitigation. Declines in organic carbon also reduce soil infiltration and water holding capacity, which may have important effects on regional hydrology and climate. To explore the regional hydroclimate impacts of soil organic carbon changes, we conduct new global climate model experiments with NASA Goddard Institute for Space Studies ModelE that include spatially explicit soil organic carbon concentrations associated with different human land management scenarios. Compared to a “no land use” case, a year 2010 soil degradation scenario, in which organic carbon content (OCC; weight %) is reduced by a factor of ∼0.12 on average across agricultural soils, resulted in soil moisture losses between 0.5 and 1 temporal standard deviations over eastern Asia, northern Europe, and the eastern United States. In a more extreme idealized scenario where OCC is reduced uniformly by 0.66 across agricultural soils, soil moisture losses exceed one standard deviation in both hemispheres. Within the model, these soil moisture declines occur primarily due to reductions in porosity (and to a lesser extent infiltration) that overall soil water holding capacity. These results demonstrate that changes in soil organic carbon can have meaningful, large-scale effects on regional hydroclimate and should be considered in climate model evaluations and developments. Further, this also suggests that soil restoration efforts targeting the carbon cycle are likely to have additional benefits for improving drought resilience.

Free access
D. W. Stahle
,
R. D. D'Arrigo
,
P. J. Krusic
,
M. K. Cleaveland
,
E. R. Cook
,
R. J. Allan
,
J. E. Cole
,
R. B. Dunbar
,
M. D. Therrell
,
D. A. Gay
,
M. D. Moore
,
M. A. Stokes
,
B. T. Burns
,
J. Villanueva-Diaz
, and
L. G. Thompson

Exactly dated tree-ring chronologies from ENSO-sensitive regions in subtropical North America and Indonesia together register the strongest ENSO signal yet detected in tree-ring data worldwide and have been used to reconstruct the winter Southern Oscillation index (SOI) from 1706 to 1977. This reconstruction explains 53% of the variance in the instrumental winter SOI during the boreal cool season (December–February) and was verified in the time, space, and frequency domains by comparisons with independent instrumental SOI and sea surface temperature (SST) data. The large-scale SST anomaly patterns associated with ENSO in the equatorial and North Pacific during the 1879–1977 calibration period are reproduced in detail by this reconstruction. Cross-spectral analyses indicate that the reconstruction reproduces over 70% of the instrumental winter SOI variance at periods between 3.5 and 5.6 yr, and over 88% in the 4-yr frequency band. Oscillatory modes of variance identified with singular spectrum analysis at ~3.5,4.0, and 5.8 yr in both the instrumental and reconstructed series exhibit regimelike behavior over the 272-yr reconstruction. The tree-ring estimates also suggest a statistically significant increase in the interannual variability of winter SOI, more frequent cold events, and a slightly stronger sea level pressure gradient across the equatorial Pacific from the mid–nineteenth to twentieth centuries. Some of the variability in this reconstruction must be associated with background climate influences affecting the ENSO teleconnection to subtropical North America and may not arise solely from equatorial ENSO forcing. However, there is some limited independent support for the nineteenth to twentieth century changes in tropical Pacific climate identified in this reconstruction and, if substantiated, it will have important implications to the low-frequency dynamics of ENSO.

Full access