Home » Research » Ocean & Climate Physics » Publications

Abstract:
Droughts over the central United States (US) are modulated by sea surface temperature (SST) variations in the eastern tropical Pacific. Many models, however, are unable to reproduce the severity and spatial pattern of the “Dust Bowl” drought of the 1930s with SST forcing alone. We force an atmosphere general circulation model with 1930s SSTs and model-generated dust emission from the Great Plains region. The SSTs alone force a drought over the US similar to observations, but with a weaker precipitation anomaly that is centered too far south. Inclusion of dust radiative forcing, centered over the area of observed wind erosion, increases the intensity of the drought and shifts its center northward. While our conclusions are tempered by limited quantitative observations of the dust aerosol load and soil erosion during this period, our study suggests that unprecedented atmospheric dust loading over the continental US exacerbated the “Dust Bowl” drought.
back to top

The modulation of tropical cyclone activity by the Madden-Julian Oscillation (MJO) is explored using an empirical genesis potential (GP) index. Composite anomalies of the genesis index associated with the different MJO phases are consistent with the composite anomalies in TC genesis frequency which occur in the same phases, indicating that the index captures the changes in the environment which are at least in part responsible for the genesis frequency changes. Of the four environmental variables which enter the genesis potential index, the mid-level relative humidity makes the largest contribution to the MJO composite GP anomalies. The second largest contribution comes from the low-level absolute vorticity, and only very minor contributions come from the vertical wind shear and potential intensity.When basin-integrated MJO composite anomalies of the GP index are regressed against basin-integrated composite anomalies of TC genesis frequency, the results differ quantitatively from those obtained from the analogous calculation performed on the annual climatologies in the two quantities. The GP index captures the MJO modulation of TC genesis to a lesser degree than the climatological annual cycle of genesis (to which it was originally tuned). This may be due to weaknesses of the reanalysis or indicative of the importance of precursor disturbances, not well captured in the GP index computed from weekly data, to the intraseasonal TC genesis frequency fluctuations.

back to top

It has recently been proposed, within the framework of the linear shallow water equations, that tropical Pacific decadal variability (PDV) can be accounted for by basin modes with eigenperiods of 10 to 20 years, amplifying a mid-latitude wind forcing with an essentially white spectrum (Liu 2003; Cessi and Louazel 2001). We question this idea here, using a different formalism of linear equatorial wave theory. We compute the Green's function for the wind forced response of a linear equatorial shallow water ocean, and use the results of Cane and Moore (1981) to obtain a compact, closed form expression for the motion of the equatorial thermocline, which applies to all frequencies lower than seasonal. This expression is new, and allows a systematic comparison of the effect of low and high latitude winds on the equatorial thermocline. At very low frequencies (decadal timescales), we recover the planetary geostrophic solution used by Cessi and Louazel (2001), as well as the equatorial wave solution of Liu (2003), and give a formal explanation for this convergence. Nonetheless, this more general solution leads us to a different interpretation of the results. In contrast to the aforementioned studies, we find that the equatorial thermocline is inherently more sensitive to local than to remote wind forcing, and that planetary Rossby modes only weakly alter the spectral characteristics of the response. Tropical winds are able to generate a strong equatorial response with periods of 10 to 20 years, while midlatitude winds can only do so for periods longer than about 50 years. The results suggest that ocean basin modes are an unlikely explanation of decadal fluctuations in tropical Pacific sea-surface temperature.

back to top

Abstract:
A quantitative analysis of thermal orbits is developed and applied to modeled air
and ground temperatures. Thermal orbits are phase-space representations of air and ground
temperature relationships that are generated by plotting daily or monthly ground
temperatures against air temperatures. Thermal orbits are useful descriptive tools that
provide straightforward illustrations of air and ground temperature relationships in the
presence of land surface processes related to snow cover, soil freezing, and vegetation
effects. The utility of thermal orbits has been limited, however, by the lack of quantitative
analyses that describe changes in orbits across different environments or in time. This
shortcoming is overcome in the present study by developing a linear regression analysis of
thermal orbits that allows changes to be tracked in time and space and as a function of
depth within the subsurface. The theory that underlies the thermal orbit regression analysis
is developed herein, and the utility of the application is demonstrated using controlled
model experiments.
back to top

Abstract:
A quantitative analysis of thermal orbits is developed and applied to modeled air
and ground temperatures. Thermal orbits are phase-space representations of air and ground
temperature relationships that are generated by plotting daily or monthly ground
temperatures against air temperatures. Thermal orbits are useful descriptive tools that
provide straightforward illustrations of air and ground temperature relationships in the
presence of land surface processes related to snow cover, soil freezing, and vegetation
effects. The utility of thermal orbits has been limited, however, by the lack of quantitative
analyses that describe changes in orbits across different environments or in time. This
shortcoming is overcome in the present study by developing a linear regression analysis of
thermal orbits that allows changes to be tracked in time and space and as a function of
depth within the subsurface. The theory that underlies the thermal orbit regression analysis
is developed herein, and the utility of the application is demonstrated using controlled
model experiments.
back to top

Abstract:
The regularized expectation maximization (RegEM) method has been used in recent studies to derive
climate field reconstructions of Northern Hemisphere temperatures during the last millennium. Original
pseudoproxy experiments that tested RegEM [with ridge regression regularization (RegEM-Ridge)] standardized
the input data in a way that improved the performance of the reconstruction method, but included
data from the reconstruction interval for estimates of the mean and standard deviation of the climate
field—information that is not available in real-world reconstruction problems. When standardizations are
confined to the calibration interval only, pseudoproxy reconstructions performed with RegEM-Ridge suffer
from warm biases and variance losses. Only cursory explanations of this so-called standardization sensitivity
of RegEM-Ridge have been published, but they have suggested that the selection of the regularization
(ridge) parameter by means of minimizing the generalized cross validation (GCV) function is the source of
the effect. The origin of the standardization sensitivity is more thoroughly investigated herein and is shown
not to be associated with the selection of the ridge parameter; sets of derived reconstructions reveal that
GCV-selected ridge parameters are minimally different for reconstructions standardized either over both
the reconstruction and calibration interval or over the calibration interval only. While GCV may select ridge
parameters that are different from those that precisely minimize the error in pseudoproxy reconstructions,
RegEM reconstructions performed with truly optimized ridge parameters are not significantly different
from those that use GCV-selected ridge parameters. The true source of the standardization sensitivity is
attributable to the inclusion or exclusion of additional information provided by the reconstruction interval,
namely, the mean and standard deviation fields computed for the complete modeled dataset. These fields
are significantly different from those for the calibration period alone because of the violation of a standard
EM assumption that missing values are missing at random in typical paleoreconstruction problems; climate
data are predominantly missing in the preinstrumental period when the mean climate was significantly
colder than the mean of the instrumental period. The origin of the standardization sensitivity therefore is
not associated specifically with RegEM-Ridge, and more recent attempts to regularize the EM algorithm
using truncated total least squares could theoretically also be susceptible to the problems affecting RegEMRidge.
Nevertheless, the principal failure of RegEM-Ridge arises because of a poor initial estimate of the
mean field, and therefore leaves open the possibility that alternative methods may perform better.
back to top

In recent years, two alarming trends in North Atlantic climate have been noted: an increase in the intensity and frequency of Atlantic hurricanes and a rapid decrease in Greenland ice sheet volume. Both of these phenomena occurred while a significant warming took place in North Atlantic sea surface temperatures (SSTs), thus sparking a debate on whether the warming is a consequence of natural climate variations, anthropogenic forcing, or both; and if both, what their relative roles are. Here models and observations are used to detect and attribute long-term (multidecadal) twentieth-century North Atlantic (NA) SST changes to their anthropogenic and natural causes. A suite of Intergovernmental Panel on Climate Change (IPCC) twentieth-century (C20C) coupled model simulations with multiple ensemble members and a signal-to-noise maximizing empirical orthogonal function analysis are used to identify a model-based estimate of the forced, anthropogenic component in NA SST variability. Comparing the results to observations, it is argued that the long-term, observed, North Atlantic basin-averaged SSTs combine a forced global warming trend with a distinct, local multidecadal "oscillation" that is outside of the range of the model-simulated, forced component and most likely arose from internal variability. This internal variability produced a cold interval between 1900 and 1930, followed by 30 yr of relative warmth and another cold phase from 1960 to 1990, and a warming since then. This natural variation, referred to previously as the Atlantic Multidecadal Oscillation (AMO), thus played a significant role in the twentieth-century NA SST variability and should be considered in future, near-term climate projections as a mechanism that, depending on its behavior, can act either constructively or destructively with the region's response to anthropogenic influence, temporarily amplifying or mitigating regional climate change.

back to top