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Solar radiation management (SRM), a form of geoengineering, might be used to offset some fraction of the anthropogenic radiative forcing of climate as a means to reduce climate change, but the risks and effectiveness of SRM are uncertain. We examine the possibility of testing SRM through sub-scale deployment as a means to test models of climate response to SRM and explore risks prior to full-scale implementation. Contrary to some claims, this could provide meaningful tests of the climate’s response to SRM within a decade. We use idealized simulations with the HadCM3L general circulation model (GCM) to estimate the response to SRM and signal-to-noise ratio for global-scale SRM forcing tests, and quantify the trade-offs between duration and intensity of the test and it’s ability to make quantitative measurements of the climate’s response to SRM forcing. The response at long time-scales would need to be extrapolated from results measured by a short-term test; this can help reduce the uncertainty associated with relatively rapid climate feedbacks, but uncertainties that only manifest at long time-scales can never be resolved by such a test. With this important caveat, the transient climate response may be bounded with 90% confidence to be no more than \(1.5^{\circ} C\) higher than it’s estimated value, in a single decade test that used roughly 1/10th the radiative forcing perturbation of a \(CO_2\)-doubling. However, tests could require several decades or longer to obtain accurate response estimates, particularly to understand the response of regional hydrological fields which are critical uncertainties. Some fields, like precipitation over land, have as large a response to short period forcing as to slowly- varying changes. This implies that the ratio of the hydrological to the temperature response that results from a sustained SRM deployment will differ from that of either a short-duration test or that which has been observed to result from large volcanic eruptions., Engineering and Applied Sciences

Some key elements of ENSO are not consistently well captured in GCMs. However, modifying the wrong parameters may lead to the right result for the wrong reason. We introduce “transfer functions” to quantify the input/ output relationship of individual processes from model output, to compare them to the corresponding observed processes. Two key transfer functions are calculated: first, the relationship between western Pacific Rossby waves and the reflecting Kelvin waves; second, the frequency-dependent relation between Kelvin waves traveling toward the eastern boundary and sea surface temperature response. These are estimated for TAO array data, the Cane-Zebiak model, and the GFDL CM2.1 coupled GCM. Some feedbacks are found to be biased in both models. Re-tuning parameters to fit observed transfer functions leads to a deteriorated solution, implying that compensating errors lead to the seemingly accurate simulation. This approach should be broadly useful in making climate model improvement more systematic and observation-driven., Earth and Planetary Sciences

Accurately capturing the observed mean period of ENSO in general circulation models (GCMs) is often challenging, and it is therefore useful to understand which parameters and processes affect this period. A computationally efficient simulation-based approach is used to extract both the dominant eigenvalues and corresponding eigenvectors of the linearized model from the Zebiak–Cane intermediate-complexity model of ENSO without having to directly construct the linearization. The sensitivity of the period to a variety of parameters is examined, including atmosphere–ocean coupling, atmospheric heating parameterization, thermocline depth zonal profile, western boundary reflection coefficient, atmospheric and ocean wave speeds or Rossby radii of deformation, ocean decay time, and the strength of the annual cycle. In addition to the sensitivity information, the spatial structures of the main fields (SST, thermocline thickness, and more) that are involved in period changes are obtained to aid in the physical interpretation of the sensitivities. There are three main time lags that together compose one-half of a model ENSO period: the Rossby-plus-Kelvin wave propagation time for a wind-caused central Pacific disturbance to propagate to the western ocean and back, SST dynamics that determine the lag between eastern ocean thermocline anomalies and eastern ocean SST anomalies, and the “accumulation” lag of integrating a sufficient delayed wave signal arriving from the western ocean to cancel the eastern ocean anomalies. For any of the parameter changes considered, the eigenvector changes show that the largest contributor to the period change is from changes to the last of these three mechanisms. Physical mechanisms that affect this accumulation delay are discussed, and the case is made that any significant change to ENSO’s period is in turn likely to involve changes to this delay., Earth and Planetary Sciences

The thermohaline circulation (THC) affects the meridional atmospheric temperature gradient and therefore the atmospheric wind and the wind-driven ocean circulation. The wind-driven circulation (WDC), in turn, affects the THC by the advection of salinity anomalies into deep-water formation sites. This paper considers this two-way coupling between the WDC and THC using a simple box-type model and analysis tools from engineering feedback control. The two-way feedback can have a significant effect on the dynamics of the coupled system. For a reasonable choice of parameters, the feedback destabilizes the THC equilibrium for low freshwater forcing. For higher freshwater forcing, the feedback results in a new stable equilibrium instead of the large amplitude oscillation that develops without feedback. It is expected that the analysis approach used here may be broadly applicable to the study of feedback interconnections of other climate systems as well., Earth and Planetary Sciences