Abstract Tropical areas with mean upward motion—and as such the zonal-mean intertropical convergence zone (ITCZ)—are projected to contract under global warming. To understand this process, a simple model based on dry static energy and moisture equations is introduced for zonally symmetric overturning driven by sea surface temperature (SST). Processes governing ascent area fraction and zonal mean precipitation are examined for insight into Atmospheric Model Intercomparison Project (AMIP) simulations. Bulk parameters governing radiative feedbacks and moist static energy transport in the simple model are estimated from the AMIP ensemble. Uniform warming in the simple model produces ascent area contraction and precipitation intensification—similar to observations and climate models. Contributing effects include stronger water vapor radiative feedbacks, weaker cloud-radiative feedbacks, stronger convection-circulation feedbacks, and greater poleward moisture export. The simple model identifies parameters consequential for the inter-AMIP-model spread; an ensemble generated by perturbing parameters governing shortwave water vapor feedbacks and gross moist stability changes under warming tracks inter-AMIP-model variations with a correlation coefficient ∼0.46. The simple model also predicts the multimodel mean changes in tropical ascent area and precipitation with reasonable accuracy. Furthermore, the simple model reproduces relationships among ascent area precipitation, ascent strength, and ascent area fraction observed in AMIP models. A substantial portion of the inter-AMIP-model spread is traced to the spread in how moist static energy and vertical velocity profiles change under warming, which in turn impact the gross moist stability in deep convective regions—highlighting the need for observational constraints on these quantities. Significance Statement A large rainband straddles Earth’s tropics. Most, but not all, climate models predict that this rainband will shrink under global warming; a few models predict an expansion of the rainband. To mitigate some of this uncertainty among climate models, we build a simpler model that only contains the essential physics of rainband narrowing. We find several interconnected processes that are important. For climate models, the most important process is the efficiency with which clouds move heat and humidity out of rainy regions. This efficiency varies among climate models and appears to be a primary reason for why climate models do not agree on the rate of rainband narrowing.
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This content will become publicly available on May 15, 2024
Energetic Constraints on the Pattern of Changes to the Hydrological Cycle under Global Warming
Abstract The response of zonal-mean precipitation minus evaporation ( P − E ) to global warming is investigated using a moist energy balance model (MEBM) with a simple Hadley cell parameterization. The MEBM accurately emulates zonal-mean P − E change simulated by a suite of global climate models (GCMs) under greenhouse gas forcing. The MEBM also accounts for most of the intermodel differences in GCM P − E change and better emulates GCM P − E change when compared to the “wet-gets-wetter, dry-gets-drier” thermodynamic mechanism. The intermodel spread in P − E change is attributed to intermodel differences in radiative feedbacks, which account for 60%–70% of the intermodel variance, with smaller contributions from radiative forcing and ocean heat uptake. Isolating the intermodel spread of feedbacks to specific regions shows that tropical feedbacks are the primary source of intermodel spread in zonal-mean P − E change. The ability of the MEBM to emulate GCM P − E change is further investigated using idealized feedback patterns. A less negative and narrowly peaked feedback pattern near the equator results in more atmospheric heating, which strengthens the Hadley cell circulation in the deep tropics through an enhanced poleward heat flux. This pattern also increases gross moist stability, which weakens the subtropical Hadley cell circulation. These two processes in unison increase P − E in the deep tropics, decrease P − E in the subtropics, and narrow the intertropical convergence zone. Additionally, a feedback pattern that produces polar-amplified warming partially reduces the poleward moisture flux by weakening the meridional temperature gradient. It is shown that changes to the Hadley cell circulation and the poleward moisture flux are crucial for understanding the pattern of GCM P − E change under warming. Significance Statement Changes to the hydrological cycle over the twenty-first century are predicted to impact ecosystems and socioeconomic activities throughout the world. While it is broadly expected that dry regions will get drier and wet regions will get wetter, the magnitude and spatial structure of these changes remains uncertain. In this study, we use an idealized climate model, which assumes how energy is transported in the atmosphere, to understand the processes setting the pattern of precipitation and evaporation under global warming. We first use the idealized climate model to explain why comprehensive climate models predict different changes to precipitation and evaporation across a range of latitudes. We show this arises primarily from climate feedbacks, which act to amplify or dampen the amount of warming. Ocean heat uptake and radiative forcing play secondary roles but can account for a significant amount of the uncertainty in regions where ocean circulation influences the rate of warming. We further show that uncertainty in tropical feedbacks (mainly from clouds) affects changes to the hydrological cycle across a range of latitudes. We then show how the pattern of climate feedbacks affects how the patterns of precipitation and evaporation respond to climate change through a set of idealized experiments. These results show how the pattern of climate feedbacks impacts tropical hydrological changes by affecting the strength of the Hadley circulation and polar hydrological changes by affecting the transport of moisture to the high latitudes.
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- NSF-PAR ID:
- 10410923
- Date Published:
- Journal Name:
- Journal of Climate
- Volume:
- 36
- Issue:
- 10
- ISSN:
- 0894-8755
- Page Range / eLocation ID:
- 3499 to 3522
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Response of Meridional Wind to Greenhouse Gas Forcing, Arctic Sea-Ice Loss, and Arctic AmplificationAbstract Under increasing greenhouse gases, the Arctic warms about twice as fast as elsewhere, known as Arctic amplification (AA). AA weakens meridional temperature gradients and is hypothesized to weaken zonal wind and cause wavier circulation with stronger meridional wind ( υ ) over northern mid-to-high latitudes. Here model simulations are analyzed to examine the υ response to increased CO 2 and AA alone. Total υ changes are found to be dominated by the effect of increased CO 2 without AA, with a zonal wavenumber-4 (wavenumber-3) change pattern over the northern (southern) extratropics that generally enhances current υ and results partly from changes in zonal temperature gradients. The extratropical υ change patterns are quasi-barotropic and are more pronounced during boreal winter. The CO 2 forcing also causes baroclinic υ changes over the tropics tied to convection changes. The impact of AA on υ is mainly over the northern extratropics and is opposite to the effect of increased CO 2 but with smaller magnitude. An eastward shift (∼5° longitude) and an amplitude increase (∼1 m s −1 ) in the climatology of the northerlies over Europe caused mainly by CO 2 forcing contribute to the drying in southern Europe, while both AA and CO 2 forcing enhance the climatology of the northerlies over East Asia. Over the northern mid-to-high latitudes, Arctic sea ice loss and AA enhance the land–ocean thermal contrast in winter, while increased CO 2 alone weakens it, resulting in opposite changes in zonal temperature gradients and thus υ . Different warming rates over land and ocean also contribute to the intermodel spread in υ response patterns among climate models. Significance Statement Meridional wind ( υ ) greatly contributes to thermal and moisture advection due to large meridional gradients in these fields. It is hypothesized that the enhanced Arctic warming under anthropogenic global warming could weaken meridional temperature gradients, decelerate westerly jets, and cause wavier circulation with stronger υ over northern extratropics. Using novel climate model simulations, we found that the effect of increased CO 2 without AA determines the total υ changes. AA generally weakens the climatological υ , contrary to the direct effect of increased CO 2 . The υ changes are small relative to its climatology but may have large impacts on regional climate over central Europe, East Asia, and interior North America. More research is needed to examine the mechanisms causing regional υ changes.more » « less
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