Publications

Evaluating a Moist Isentropic Framework for Poleward Moisture Transport: Implications for Water Isotopes over Antarctica

Published in Geophysical Research Letters, 2019

Abstract: The ability to identify moisture source regions and sinks, and to model the transport pathways that link them in simple yet physical ways, is critical for understanding climate today and in the past. Using water tagging and isotopic tracer experiments in the Community Earth System Model, this work shows that poleward moisture transport largely follows surfaces of constant moist entropy. The analysis not only provides insight into why distinct zonal bands supply moisture to high- and low-elevation polar sites but also explains why changes in these source regions are inherently linked to changes in temperature and rainout. Moreover, because the geometry, and specifically length, of the moist isentropic surfaces describes how much integrated rainout occurs, the analysis provides a physical framework for interpreting the isotopic composition of water in poleward-moving air, thus indicating how variations in moisture transport might influence Antarctic ice cores.

Recommended citation: Bailey A, Singh HKA, Nusbaumer J. (2018). "Evaluating a Moist Isentropic Framework for Poleward Moisture Transport: Implications for Water Isotopes over Antarctica." Geophysical Research Letters. accepted Jun 2019.

How Asymmetries Between Arctic and Antarctic Climate Sensitivity Are Modified by the Ocean

Published in Geophysical Research Letters, 2018

Abstract: We investigate how the ocean response to CO2 forcing affects hemispheric asymmetries in polar climate sensitivity. Intermodel comparison of Phase 5 of the Coupled Model Intercomparison Project CO2 quadrupling experiments shows that even in models where hemispheric ocean heat uptake differences are small, Arctic warming still exceeds Antarctic warming. The polar climate impact of this evolving ocean response to CO2 forcing is then isolated using slab ocean experiments in a state-of-the-art climate model. Overall, feedbacks over the Southern Hemisphere more effectively dissipate top-of-atmosphere anomalies than those over the Northern Hemisphere. Furthermore, a poleward shift in ocean heat convergence in both hemispheres amplifies destabilizing ice albedo and lapse rate feedbacks over the Arctic much more so than over the Antarctic. These results suggest that the Arctic is intrinsically more sensitive to both CO2 and oceanic forcings than the Antarctic and that ocean-driven climate sensitivity asymmetry arises from feedback destabilization over the Arctic rather than feedback stabilization over the Antarctic.

Recommended citation: Singh HA, Garuba OA, Rasch PJ. (2018). "How Asymmetries Between Arctic and Antarctic Climate Sensitivity Are Modified by the Ocean." Geophysical Research Letters. 45: pp 13031-13040.

On the Relative Roles of the Atmosphere and Ocean in the Atlantic Multidecadal Variability

Published in Geophysical Research Letters, 2018

Abstract: The relative roles of the ocean and atmosphere in driving the Atlantic multidecadal variability (AMV) are investigated by isolating anomalous sea surface temperature (SST) components forced by anomalous surface heat fluxes and ocean dynamics in fully and partially coupled simulations. The impact of the ocean dynamics-forced SST on air-sea interaction is disabled in the partially coupled simulation in order to isolate the atmosphere-forced variability. The atmosphere-forced AMV component shows weak but significant variability on interdecadal timescales (10- to 30-year periods), while the ocean-forced component exhibits a strong multidecadal variability (25- to 50-year periods). When coupled to the atmosphere, this ocean-forced variability weakens and is imprinted on the coupled surface heat fluxes that further act to damp the ocean-forced SST variability, causing a much weaker fully coupled AMV. Our results suggest that the AMV is largely driven by ocean circulation variability, but its power is also affected by the strength of air-sea coupling.

Recommended citation: Garuba OA, Lu J, Singh HA, Liu F, Rasch PJ. (2018). "On the Relative Roles of the Atmosphere and Ocean in the Atlantic Multidecadal Variability." Geophysical Research Letters. 45: pp 9186–9196.

Insights into Atlantic multidecadal variability using the Last Millennium Reanalysis framework

Published in Climate of the Past, 2018

Abstract: The Last Millennium Reanalysis (LMR) employs a data assimilation approach to reconstruct climate fields from annually resolved proxy data over years 0–2000CE. We use the LMR to examine Atlantic multidecadal variability (AMV) over the last 2 millennia and find several robust thermodynamic features associated with a positive Atlantic Multidecadal Oscillation (AMO) index that reveal a dynamically consistent pattern of variability: the Atlantic and most continents warm; sea ice thins over the Arctic and retreats over the Greenland, Iceland, and Norwegian seas; and equatorial precipitation shifts northward. The latter is consistent with anomalous southward energy transport mediated by the atmosphere. Net downward shortwave radiation increases at both the top of the atmosphere and the surface, indicating a decrease in planetary albedo, likely due to a decrease in low clouds. Heat is absorbed by the climate system and the oceans warm. Wavelet analysis of the AMO time series shows a reddening of the frequency spectrum on the 50- to 100-year timescale, but no evidence of a distinct multidecadal or centennial spectral peak. This latter result is insensitive to both the choice of prior model and the calibration dataset used in the data assimilation algorithm, suggesting that the lack of a distinct multidecadal spectral peak is a robust result.

Recommended citation: Singh HKA, Hakim GJ, Tardif R, Emile-Geay J, Noone DC. (2018). "Insights into Atlantic multidecadal variability using the Last Millennium Reanalysis framework." Climate of the Past. 14: pp 157–174.

The Active Role of the Ocean in the Temporal Evolution of Climate Sensitivity

Published in Geophysical Research Letters, 2018

Abstract: The temporal evolution of the effective climate sensitivity is shown to be influenced by the changing pattern of sea surface temperature (SST) and ocean heat uptake (OHU), which in turn have been attributed to ocean circulation changes. A set of novel experiments are performed to isolate the active role of the ocean by comparing a fully coupled CO2 quadrupling community Earth System Model (CESM) simulation against a partially coupled one, where the effect of the ocean circulation change and its impact on surface fluxes are disabled. The active OHU is responsible for the reduced effective climate sensitivity and weaker surface warming response in the fully coupled simulation. The passive OHU excites qualitatively similar feedbacks to CO2 quadrupling in a slab ocean model configuration due to the similar SST spatial pattern response in both experiments. Additionally, the nonunitary forcing efficacy of the active OHU (1.7) explains the very different net feedback parameters in the fully and partially coupled responses.

Recommended citation: Garuba OA, Lu J, Liu F, Singh HA. (2018). "The Active Role of the Ocean in the Temporal Evolution of Climate Sensitivity." Geophysical Research Letters. 45: pp 306–315.

A Source–Receptor Perspective on the Polar Hydrologic Cycle: Sources, Seasonality, and Arctic–Antarctic Parity in the Hydrologic Cycle Response to CO2 Doubling

Published in Journal of Climate, 2017

Abstract: Numerical water tracers implemented in a global climate model are used to study how polar hydroclimate responds to CO2-induced warming from a source–receptor perspective. Although remote moisture sources contribute substantially more to polar precipitation year-round in the mean state, an increase in locally sourced moisture is crucial to the winter season polar precipitation response to greenhouse gas forcing. In general, the polar hydroclimate response to CO2-induced warming is strongly seasonal: over both the Arctic and Antarctic, locally sourced moisture constitutes a larger fraction of the precipitation in winter, while remote sources become even more dominant in summer. Increased local evaporation in fall and winter is coincident with sea ice retreat, which greatly augments local moisture sources in these seasons. In summer, however, larger contributions from more remote moisture source regions are consistent with an increase in moisture residence times and a longer moisture transport length scale, which produces a robust hydrologic cycle response to CO2-induced warming globally. The critical role of locally sourced moisture in the hy- drologic cycle response of both the Arctic and Antarctic is distinct from controlling factors elsewhere on the globe; for this reason, great care should be taken in interpreting polar isotopic proxy records from climate states unlike the present.

Recommended citation: Singh HKA, Bitz CM, Donohoe A, Rasch PJ. (2017). "A Source–Receptor Perspective on the Polar Hydrologic Cycle: Sources, Seasonality, and Arctic–Antarctic Parity in the Hydrologic Cycle Response to CO2 Doubling." Journal of Climate. 30(24): pp 9999-10017.

Increased Ocean Heat Convergence Into the High Latitudes With CO2 Doubling Enhances Polar-Amplified Warming

Published in Geophysical Research Letters, 2017

Abstract: We isolate the role of the ocean in polar climate change by directly evaluating how changes in ocean dynamics with quasi-equilibrium CO2 doubling impact high-latitude climate. With CO2 doubling, the ocean heat flux convergence (OHFC) shifts poleward in winter in both hemispheres. Imposing this pattern of perturbed OHFC in a global climate model results in a poleward shift in ocean-to-atmosphere turbulent heat fluxes (both sensible and latent) and sea ice retreat; the high latitudes warm, while the midlatitudes cool, thereby amplifying polar warming. Furthermore, midlatitude cooling is propagated to the polar midtroposphere on isentropic surfaces, augmenting the (positive) lapse rate feedback at high latitudes. These results highlight the key role played by the partitioning of meridional energy transport changes between the atmosphere and ocean in high-latitude climate change.

Recommended citation: Singh HKA, Rasch PJ, Rose BEJ. (2017). "Increased Ocean Heat Convergence Into the High Latitudes With CO2 Doubling Enhances Polar-Amplified Warming." Geophysical Research Letters. 44: pp 10583-10591.

A Mathematical Framework for Analysis of Water Tracers. Part II: Understanding Large-Scale Perturbations in the Hydrological Cycle due to CO2 Doubling

Published in Journal of Climate, 2016

Abstract: The aerial hydrological cycle response to CO2 doubling from a Lagrangian, rather than Eulerian, per- spective is evaluated using information from numerical water tracers implemented in a global climate model. While increased surface evaporation (both local and remote) increases precipitation globally, changes in transport are necessary to create a spatial pattern where precipitation decreases in the subtropics and in- creases substantially at the equator. Overall, changes in the convergence of remotely evaporated moisture are more important to the overall precipitation change than changes in the amount of locally evaporated moisture that precipitates in situ. It is found that CO2 doubling increases the fraction of locally evaporated moisture that is exported, enhances moisture exchange between ocean basins, and shifts moisture convergence within a given basin toward greater distances between moisture source (evaporation) and sink (precipitation) regions. These changes can be understood in terms of the increased residence time of water in the atmosphere with CO2 doubling, which corresponds to an increase in the advective length scale of moisture transport. As a result, the distance between where moisture evaporates and where it precipitates increases. Analyses of several heuristic models further support this finding.

Recommended citation: Singh HA, Bitz CM, Donohoe A, Nusbaumer J, Noone DC. (2016). "A Mathematical Framework for Analysis of Water Tracers. Part II: Understanding Large-Scale Perturbations in the Hydrological Cycle due to CO2 Doubling." Journal of Climate. 29(18): pp 6765-6782.

The Global Climate Response to Lowering Surface Orography of Antarctica and the Importance of Atmosphere–Ocean Coupling

Published in Journal of Climate, 2016

Abstract: A global climate model is used to study the effect of flattening the orography of the Antarctic Ice Sheet on climate. A general result is that the Antarctic continent and the atmosphere aloft warm, while there is modest cooling globally. The large local warming over Antarctica leads to increased outgoing longwave radiation, which drives anomalous southward energy transport toward the continent and cooling elsewhere. Atmosphere and ocean both anomalously transport energy southward in the Southern Hemisphere. Near Antarctica, poleward energy and momentum transport by baroclinic eddies strengthens. Anomalous southward cross-equatorial en- ergy transport is associated with a northward shift in the intertropical convergence zone. In the ocean, anomalous southward energy transport arises from a slowdown of the upper cell of the oceanic meridional overturning circulation and a weakening of the horizontal ocean gyres, causing sea ice in the Northern Hemisphere to expand and the Arctic to cool. Comparison with a slab-ocean simulation confirms the importance of ocean dynamics in determining the climate system response to Antarctic orography. This paper concludes by briefly presenting a discussion of the relevance of these results to climates of the past and to future climate scenarios.

Recommended citation: Singh HA, Bitz CM, Frierson DMW. (2016). "The Global Climate Response to Lowering Surface Orography of Antarctica and the Importance of Atmosphere–Ocean Coupling." Journal of Climate. 29(11): pp 4137-4153.

Greater aerial moisture transport distances with warming amplify interbasin salinity contrasts

Published in Geophysical Research Letters, 2016

Abstract: The distance atmospheric moisture travels is fundamental to Earth’s hydrologic cycle, governing how much evaporation is exported versus precipitated locally. The present-day tropical Atlantic is one region that exports much locally evaporated moisture away, leading to more saline surface waters in the Atlantic compared to the Indo-Pacific at similar latitudes. Here we use a state-of-the-art global climate model equipped with numerical water tracers to show that over half of the atmospheric freshwater exported from the Atlantic originates as evaporation in the northern Atlantic subtropics, primarily between 10∘N and 20∘N, and is transported across Central America via prevailing easterlies into the equatorial Pacific. We find enhanced moisture export from the Atlantic to Pacific with warming is due to greater distances between moisture source and sink regions, which increases moisture export from the Atlantic at the expense of local precipitation. Distance traveled increases due to longer moisture residence times, not simply Clausius-Clapeyron scaling.

Recommended citation: Singh HKA, Donohoe A, Bitz CM, Nusbaumer J, and Noone DC. (2016). "Greater aerial moisture transport distances with warming amplify interbasin salinity contrasts." Geophysical Research Letters. 43: pp 8677-8684.

A mathematical framework for analysis of water tracers: Part 1: Development of theory and application to the preindustrial mean state

Published in Journal of Advances in Modeling Earth Systems, 2016

Abstract: A new matrix operator framework is developed to analyze results from climate modeling studies that employ numerical water tracers (WTs), which track the movement of water in the aerial hydrological cycle from evaporation to precipitation. Model WT output is related to the fundamental equation of hydrology, and the moisture flux divergence is subdivided into the divergence of locally evaporated moisture and the convergence of remotely evaporated moisture. The formulation also separates locally and remotely sourced precipitation. The remote contribution (also the remote moisture convergence) may be further subdivided into zonal, meridional, intrabasin, and interbasin parts. This framework is applied to the preindustrial climate as simulated by a global climate model in which water has been tagged in 108 latitude bands in each of the major ocean basins, and in which each major land mass has been tagged separately. New insights from the method reveal fundamental differences between the major ocean basins in locally sourced precipitation, remotely sourced precipitation, and their relative partitioning. Per unit area, the subtropical Atlantic is the largest global moisture source, providing precipitable water to adjacent land areas and to the eastern Pacific tropics while retaining the least for in situ precipitation. Subtropical moisture is least divergent over the Pacific, which is the smallest moisture source (per unit area) for global land areas. Basins also differ in how subtropical moisture is partitioned between tropical, midlatitude, and land regions. Part II will apply this framework to hydrological cycle perturbations due to CO2 doubling.

Recommended citation: Singh HA, Bitz CM, Nusbaumer J, Noone DC. (2016). "A mathematical framework for analysis of water tracers: Part 1: Development of theory and application to the preindustrial mean state." Journal of Advances in Modeling Earth Systems. 8: pp 991–1013.

Influence of West Antarctic Ice Sheet collapse on Antarctic surface climate

Published in Geophysical Research Letters, 2015

Abstract: Climate model simulations are used to examine the impact of a collapse of the West Antarctic Ice Sheet (WAIS) on the surface climate of Antarctica. The lowered topography following WAIS collapse produces anomalous cyclonic circulation with increased flow of warm, maritime air toward the South Pole and cold-air advection from the East Antarctic plateau toward the Ross Sea and Marie Byrd Land, West Antarctica. Relative to the background climate, areas in East Antarctica that are adjacent to the WAIS warm, while substantial cooling (several ∘C) occurs over parts of West Antarctica. Anomalously low isotope-paleotemperature values at Mount Moulton, West Antarctica, compared with ice core records in East Antarctica, are consistent with collapse of the WAIS during the last interglacial period, Marine Isotope Stage 5e. More definitive evidence might be recoverable from an ice core record at Hercules Dome, East Antarctica, which would experience significant warming and positive oxygen isotope anomalies if the WAIS collapsed.

Recommended citation: Steig EJ, Huybers K, Singh HA, Steiger NJ, Ding Q, Frierson DMW, Popp T, White JWC. (2015). "Influence of West Antarctic Ice Sheet collapse on Antarctic surface climate." Geophysical Research Letters. 42: pp 4862-4868.

A Heuristic Model of Dansgaard–Oeschger Cycles. Part I: Description, Results, and Sensitivity Studies

Published in Journal of Climate, 2014

Abstract: A simple model for studying the Dansgaard–Oeschger (D-O) cycles of the last glacial period is presented, based on the T. Dokken et al. hypothesis for D-O cycles. The model is a column model representing the Nordic seas and is composed of ocean boxes stacked below a one-layer sea ice model with an energy-balance atmosphere; no changes in the large-scale ocean overturning circulation are invoked. Parameterizations are included for latent heat polynyas and sea ice export from the column. The resulting heuristic model was found to cycle between stadial and interstadial states at times scales similar to those seen in the proxy observational data, with the presence or absence of perennial sea ice in the Nordic seas being the defining characteristic for each of these states. The major discrepancy between the modeled oscillations and the proxy record is in the length of the interstadial phase, which is shorter than that observed. The modeled oscillations were found to be robust to parameter changes, including those related to the ocean heat flux convergence (OHFC) into the column. Production of polynya ice was found to be an essential ingredient for such sustained oscillatory behavior. A simple parameterization of natural variability in the OHFC enhances the robustness of the modeled oscillations. The authors conclude by discussing the implications of such a hypothesis for the state of the Nordic seas today and its state during the Last Glacial Maximum and contrasting the model to other hypotheses that invoke large-scale changes in the Atlantic meridional overturning circulation for explaining millennial-scale variability in the climate system. An extensive time-scale analysis will be presented in the future.

Recommended citation: Singh HA, Battisti DS, Bitz CM. (2014). "A Heuristic Model of Dansgaard–Oeschger Cycles. Part I: Description, Results, and Sensitivity Studies." Journal of Climate. 27(12): pp 4337-4358.