Warming Perturbations Research Articles

Summer Convective Precipitation Changes Over the Great Lakes Region Under a Warming Scenario

AbstractTo understand future summer precipitation changes over the Great Lakes Region (GLR), we performed an ensemble of regional climate simulations through the Pseudo‐Global Warming (PGW) approach. We found that different types of convective precipitation respond differently to the PGW signal. Isolated deep convection (IDC), usually concentrated in the southern domain, shows an increase in precipitation to the north of the GLR. Mesoscale convective systems (MCSs), usually concentrated upwind of the GLR, shift to the downwind region with increased precipitation. Thermodynamic variables such as convective available potential energy (CAPE) and convective inhibition energy (CIN) are found to increase across almost the entire studied domain, creating a potential environment more favorable for stronger convection systems and less favorable for weaker ones. Meanwhile, changes in the lifting condensation level (LCL) and level of free convection (LFC) show a strong correlation with variations in convective precipitation, highlighting the significance of these thermodynamic factors in controlling precipitation over the domain. Our results indicate that the decrease in LCL and LCF in areas with increased convective precipitation is mainly due to increased atmospheric moisture. In response to the prescribed warming perturbation, MCSs occur more frequently downwind, while localized IDCs exhibit more intense rain rates, longer durations, and larger rainfall area.

Rectified Ocean Heat Uptake from Oscillatory Surface Forcing

Abstract Ocean heat uptake is asymmetric with respect to the sign of radiative forcing. It is already known that surface cooling anomalies penetrate into the ocean faster than surface warming anomalies. Because of this asymmetry, the time-variable component of radiative forcing can induce a long-term, rectified cooling trend in ocean heat content, which is this work’s primary focus. Here, we explore this asymmetry and rectification on global and interannual scales, its implications, and its possible dependence on model parameters. We do so using a full-complexity global ocean–sea ice general circulation model and an idealized one-dimensional vertical mixing model, both forced with idealized abrupt and oscillatory surface forcing anomalies. In both models, the ocean heat uptake response to an abrupt cooling perturbation is larger than an equal-magnitude warming perturbation. This asymmetry is shown to be larger when the background vertical diffusivity is smaller, and is therefore likely model dependent. Sinusoidal oscillatory forcing with zero time mean induces a rectified cooling trend in both models, whose magnitude depends on both the diffusivity and the frequency of the interannual oscillatory forcing. The net rectified cooling can reach a rate of approximately −0.11 W m−2, which is substantial relative to the estimated anthropogenic warming rate of 0.47 W m−2 (von Schuckmann et al.). We discuss this rectification effect in the context of volcanic forcing in climate models, whose time-variable component may cause model-dependent ocean cooling in CMIP6 historical simulations, reaching 5%–30% the size of the total impact of volcanic forcing in our one-dimensional model. Correcting for this cooling may help reduce uncertainties in modeled ocean heat content evolution.

Sensitivity of a calving glacier to ice–ocean interactions under climate change: new insights from a 3-D full-Stokes model

Abstract. Iceberg calving accounts for between 30 % and 60 % of net mass loss from the Greenland Ice Sheet, which has intensified and is now the single largest contributor to global sea level rise in the cryosphere. Changes to calving rates and the dynamics of calving glaciers represent a significant uncertainty in projections of future sea level rise. A growing body of observational evidence suggests that calving glaciers respond rapidly to regional environmental change, but predictive capacity is limited by the lack of suitable models capable of simulating calving mechanisms realistically. Here, we use a 3-D full-Stokes calving model to investigate the environmental sensitivity of Store Glacier, a large outlet glacier in West Greenland. We focus on two environmental processes: undercutting by submarine melting and buttressing by ice mélange, and our results indicate that Store Glacier is likely to be able to withstand moderate warming perturbations in which the former is increased by 50 % and the latter reduced by 50 %. However, severe perturbation with a doubling of submarine melt rates or a complete loss of ice mélange destabilises the calving front in our model runs. Furthermore, our analysis reveals that stress and fracture patterns at Store's terminus are complex and varied, primarily due to the influence of basal topography. Calving style and environmental sensitivity vary greatly, with propagation of surface crevasses significantly influencing iceberg production in the northern side, whereas basal crevasses dominate in the south. Any future retreat is likely to be initiated in the southern side by a combination of increased submarine melt rates in summer and reduced mélange strength in winter. The lateral variability, as well as the importance of rotational and bending forces at the terminus, underlines the importance of using the 3-D full-Stokes stress solution when modelling Greenland's calving glaciers.

ITCZ Width Controls on Hadley Cell Extent and Eddy-Driven Jet Position and Their Response to Warming

The impact of global warming–induced intertropical convergence zone (ITCZ) narrowing onto the higher-latitude circulation is examined in the GFDL Atmospheric Model, version 2.1 (AM2.1), run over zonally symmetric aquaplanet boundary conditions. A striking reconfiguration of the deep tropical precipitation from double-peaked, off-equatorial ascent to a single peak at the equator occurs under a globally uniform +4 K sea surface temperature (SST) perturbation. This response is found to be highly sensitive to the SST profile used to force the model. By making small (≤1 K) perturbations to the surface temperature in the deep tropics, varying control simulation precipitation patterns with both single and double ITCZs are generated. Across the climatologies, narrower regions of ascent correspond to more equatorward Hadley cell edges and eddy-driven jets. Under the global warming perturbation, the experiments in which there is narrowing of the ITCZ show significantly less expansion of the Hadley cell and somewhat less poleward shift of the eddy-driven jet than those without ITCZ narrowing. With a narrower ITCZ, the ascending air has larger zonal momentum, causing more westerly upper-tropospheric subtropical wind. In turn, this implies 1) the subtropical jet will become baroclinically unstable at a lower latitude and 2) the critical (zero wind) line will shift equatorward, allowing midlatitude eddies to propagate farther equatorward. Both of these mechanisms modify the Hadley cell edge position, and the latter affects the jet position.

Response of Southern Ocean Convection and Abyssal Overturning to Surface Buoyancy Perturbations

AbstractThis study explores how buoyancy-driven modulations in the abyssal overturning circulation affect Southern Ocean temperature and salinity in an eddy-permitting ocean model. Consistent with previous studies, the modeled surface ocean south of 50°S cools and freshens in response to enhanced surface freshwater fluxes. Paradoxically, upper-ocean cooling also occurs for small increases in the surface relaxation temperature. In both cases, the surface cooling and freshening trends are linked to reduced convection and a slowing of the abyssal overturning circulation, with associated changes in oceanic transport of heat and salt. For small perturbations, convective shutdown does not begin immediately, but instead develops via a slow feedback between the weakened overturning circulation and buoyancy anomalies. Two distinct phases of surface cooling are found: an initial smaller trend associated with the advective (overturning) adjustment of up to ~60 yr, followed by more rapid surface cooling during the convective shutdown period. The duration of the first advective phase decreases for larger forcing perturbations. As may be expected during the convective shutdown phase, the deep ocean warms and salinifies for both types of buoyancy perturbation. However, during the advective phase, the deep ocean freshens in response to freshwater perturbations but salinifies in the surface warming perturbations. The magnitudes of the modeled surface and abyssal trends during the advective phase are comparable to the recent observed multidecadal Southern Ocean temperature and salinity changes.

The impact of the Atlantic cold tongue on West African monsoon onset in regional model simulations for 1998–2002

ABSTRACTThe Atlantic cold tongue (ACT) develops during spring and early summer near the Equator in the Eastern Atlantic Ocean and Gulf of Guinea. The hypothesis that the ACT accelerates the timing of West African monsoon (WAM) onset is tested by comparing two regional climate model (RM3) simulation ensembles. Observed sea surface temperatures (SST) that include the ACT are used to force a control ensemble. An idealized, warm SST perturbation is designed to represent lower boundary forcing without the ACT for the experiment ensemble. Summer simulations forced by observed SST and reanalysis boundary conditions for each of five consecutive years are compared to five parallel runs forced by SST with the warm perturbation. The article summarizes the sequence of events leading to the onset of the WAM in the Sahel region. The representation of WAM onset in RM3 simulations is examined and compared to Tropical Rainfall Measuring Mission (TRMM), Global Precipitation Climatology Project (GPCP) and reanalysis data. The study evaluates the sensitivity of WAM onset indicators to the presence of the ACT by analysing the differences between the two simulation ensembles. Results show that the timing of major rainfall events and therefore the WAM onset in the Sahel are not sensitive to the presence of the ACT. However, the warm SST perturbation does increase downstream rainfall rates over West Africa as a consequence of enhanced specific humidity and enhanced northward moisture flux in the lower troposphere.

Impacts of global warming perturbation on water resources in arid zone: Case study of Kaidu River Basin in Northwest China

The main goal of this study was to assess the long-term impacts of global warming perturbation on water resources of the Kaidu River Basin in Northwest China. Temperature, precipitation and hydrology data during the past 29 years from 1979 to 2007 were collected and analyzed using parametric and non-parametric methods, the connection between temperature and precipitation by the combination of grey correlation analysis method and the hypothesis testing for trend of climate change. The results show a high increase in temperature in the study area as well as an extreme and highly variable hydrological regime in this region, where flash floods can exceed the total runoff from a sequence of years. These variations may be due to the geographical location of the Kaidu River Basin in arid zone. It also reveals that precipitation has a much greater impact on stream flow than that of temperature. The development of new approaches was proposed as responses to climate change in this arid region.

Development and Tropical Transition of an Alpine Lee Cyclone. Part II: Orographic Influence on the Development Pathway

Abstract The development and subsequent tropical transition of a subsynoptic-scale cyclone over the Gulf of Genoa (GoG) on 15 November 2007 led to the rapid onset of tropical storm-force winds near the islands of Corsica and Sardinia. This study evaluates the influence of two key ingredients on the cyclogenesis event: a near-surface warm potential temperature perturbation in the lee of the Alps and a mountain-scale potential vorticity (PV) banner. A high-resolution modeling system is used to perform a set of attribution tests in which modifications to the Alpine orography control the presence of the cyclogenetic ingredients. When either feature exists in the initial state, a GoG cyclone develops even when the Alpine barrier is removed; however, when neither the warm perturbation nor the PV banner is present, there is insufficient lower-level PV to couple with the upper-level trough to promote cyclogenesis. A conceptual model involving the complimentary interaction of the two PV features is presented that accurately describes the development location of the cyclone beneath a midlevel vorticity maximum. Despite development in most of the attribution tests, the energy sources for the cyclones vary widely and represent a spectrum of cyclogenetic pathways from baroclinically to convectively dominant. Removal of the Alpine barrier allows for a stronger thermal wave and a baroclinic mode of development, rather than the diabatically generated hurricane-like vortex seen in the control and available observations. Similarly, insufficient flow interaction with the low-resolution representation of the Alps in the global-driving model is shown to favor a baroclinic mode of cyclogenesis in that integration. Adequate resolution of both the Alpine terrain and the incipient cyclone itself are shown to be important to correctly predict the evolution of the system from both structural and energetic perspectives.

Simulation of long-term future climate changes with the green McGill paleoclimate model: the next glacial inception

The multi-component “green” McGill Paleoclimate Model (MPM), which includes interactive vegetation, is used to simulate the next glacial inception under orbital and prescribed atmospheric CO2 forcing. This intermediate complexity model is first run for short-term periods with an increasing atmospheric CO2 concentration; the model's response is in general agreement with the results of GCMs for CO2 doubling. The green MPM is then used to derive projections of the climate for the next 100 kyr. Under a constant CO2 level, the model produces three types of evolution for the ice volume: an imminent glacial inception (low CO2 levels), a glacial inception in 50 kyr (CO2 levels of 280 or 290 ppm), or no glacial inception during the next 100 kyr (CO2 levels of 300 ppm and higher). This high sensitivity to the CO2 level is due to the exceptionally weak future variations of the summer insolation at high northern latitudes. The changes in vegetation re-inforce the buildup of ice sheets after glacial inception. Finally, if an initial global warming episode of finite duration is included, after which the atmospheric CO2 level is assumed to stabilize at 280, 290 or 300 ppm, the impact of this warming is seen only in the first 5 kyr of the run; after this time the response is insensitive to the early warming perturbation.

Toward understanding the response of the tropical atmosphere-ocean system to increased CO 2 using equilibrium asynchronous coupling

We investigate the response of the tropical Pacific climate system to a tripling of CO2 in a coupled atmosphere–ocean model. Equilibrium asynchronous coupling (EAC) is used to diagnose the climatic forcing mechanisms. Our results corroborate those from most other coupled GCMs, which generally simulate an El Nino-like mean climate state under increased CO2. This pattern is characterized by enhanced warming of the ocean surface in the equatorial Pacific in conjunction with a reduction in the zonal SST gradient, stronger atmospheric convection and precipitation in the central equatorial Pacific, a stronger Subtropical Westerly Jet, and a deepened wintertime Aleutian Low in association with a PNA-like geopotential pattern aloft. We show that these climate changes are triggered by a weakening of the tropical Walker Circulation, which leads to weaker surface easterly winds along the equator and therefore less oceanic upwelling. Furthermore, the main El Nino-like atmospheric anomalies do not depend critically on the existence of an east-west equatorial gradient either in the simulated modern SSTs or in the SST anomalies under increased CO2. Regardless of the details of the SST boundary conditions, the atmosphere seems to respond similarly to a CO2 warming perturbation and in a manner that favors the evolution of the climate system toward an El Nino-like state. An enhanced meridional atmospheric circulation in the Pacific is linked to increased convection along the ITCZ, coinciding with subtropical anomalies of higher surface pressure and stronger subsidence aloft. This anomalous surface pressure pattern is much more pronounced in the Northern Hemisphere, presumably due to a PNA-like atmospheric teleconnection. These surface pressure anomalies produce a stronger meridional outflow of wind toward the equator and weaker easterly winds along the equator. The weaker equatorial easterlies reduce oceanic upwelling in the central and eastern equatorial Pacific, thereby promoting a reduced east-west SST gradient and a maximum warming in the central equatorial Pacific (around 140–170°W).