Understanding the influence of climate dynamics on subtropical drying
Lead Research Organisation:
University of Oxford
Department Name: Mathematical, Physical&Life Sciences Div
Abstract
Mediterranean climates are typically located on the western edges of continents and at subtropical latitudes. These regions are climatologically dry and receive most of their rainfall in winter. Some of these areas have exhibited a drying trend in recent years, and are projected to get even drier under climate change. This project will study the relative influences of atmospheric circulation, sea surface temperatures and other factors on this drying trend and therefore increase our understanding of climate change projections in these subtropical regions.
The subtropics are defined as the latitudes immediately poleward of the tropics. Over land, many of these regions are climatologically dry, and Mediterranean-type climates (MCs) are often found at the western extent of continents. 5 main MCs exist: central Chile, southwest South Africa, southern Australia, the southwest US, and the Mediterranean itself.i These regions typically have wet winters and dry summers, which makes managing water resources challenging. Since 1900, precipitation observations have indicated a decline in wintertime rainfall in MCs, which has intensified water stress in these inherently arid environments. This decline appears to have accelerated in the 21st century and contributed to several extreme events, such as the ongoing "megadrought" in Chile since 2010,ii and the Cape Town "Day Zero" drought in the late 2010s.
Climate models also robustly project a decrease in precipitation in these regions, however the drivers behind these observed and projected changes are less clear. Multiple dynamic factors, such as Hadley cell expansion and the poleward shift of annular modes have been implicated.iv Thermodynamic moisture transport effects (often termed "wet-get-wetter, dry-get-drier") as described by the Clausius-Clapeyron equation have also been hypothesised to play a significant role.v Anthropogenic emissions impact all of the above, but the picture is complicated by the influence of natural variability. This is true for the dynamic/circulation factors in particular. For example, shifts in the Southern Annular Mode have been attributed to greenhouse gas emissions and ozone depletion, but also Atlantic Multidecadal Variability, a naturally varying shift in sea surface temperatures (SSTs).vi Therefore, understanding the relative importance of dynamic and thermodynamic factors, as well as internally- and externally-forced trends, is key to improving our knowledge of the physical mechanisms behind subtropical drying.
In addition, tropical Pacific SSTs are an important source of dynamic variability (and thus potentially rainfall variability) over MCs. Due to their proximity to the Pacific, the southwest United States and central Chile are particularly sensitive to variations in Pacific SSTs such as ENSO.iv Simulating these SSTs accurately is important for robust rainfall projects in the neighbouring MCs. However, most ocean-atmosphere coupled climate models are currently unable to reproduce the observed pattern of Pacific SST change. Observations over the last century show an increase in the zonal SST gradient, while the majority of climate models predict a decrease.vii This discrepancy underscores the complexity of regional climate projections, and why further investigation into circulation changes, including those potentially caused by SSTs, is needed to refine our understanding of the subtropical drying trend.
Key research questions
Is the observed drying trend, in particular in the Southern Hemisphere, consistent with historical model simulations?
Are the dynamic and thermodynamic influences on the drying trend similar in observations and climate models?
Do any dynamic trends in these MCs project well onto larger scale climate trends, e.g. in the Hadley cell or annular modes, or are they more regional changes?
How much do tropical Pacific SSTs influence precipitation in MCs (central Chile and the US Southwest in particular)?
The subtropics are defined as the latitudes immediately poleward of the tropics. Over land, many of these regions are climatologically dry, and Mediterranean-type climates (MCs) are often found at the western extent of continents. 5 main MCs exist: central Chile, southwest South Africa, southern Australia, the southwest US, and the Mediterranean itself.i These regions typically have wet winters and dry summers, which makes managing water resources challenging. Since 1900, precipitation observations have indicated a decline in wintertime rainfall in MCs, which has intensified water stress in these inherently arid environments. This decline appears to have accelerated in the 21st century and contributed to several extreme events, such as the ongoing "megadrought" in Chile since 2010,ii and the Cape Town "Day Zero" drought in the late 2010s.
Climate models also robustly project a decrease in precipitation in these regions, however the drivers behind these observed and projected changes are less clear. Multiple dynamic factors, such as Hadley cell expansion and the poleward shift of annular modes have been implicated.iv Thermodynamic moisture transport effects (often termed "wet-get-wetter, dry-get-drier") as described by the Clausius-Clapeyron equation have also been hypothesised to play a significant role.v Anthropogenic emissions impact all of the above, but the picture is complicated by the influence of natural variability. This is true for the dynamic/circulation factors in particular. For example, shifts in the Southern Annular Mode have been attributed to greenhouse gas emissions and ozone depletion, but also Atlantic Multidecadal Variability, a naturally varying shift in sea surface temperatures (SSTs).vi Therefore, understanding the relative importance of dynamic and thermodynamic factors, as well as internally- and externally-forced trends, is key to improving our knowledge of the physical mechanisms behind subtropical drying.
In addition, tropical Pacific SSTs are an important source of dynamic variability (and thus potentially rainfall variability) over MCs. Due to their proximity to the Pacific, the southwest United States and central Chile are particularly sensitive to variations in Pacific SSTs such as ENSO.iv Simulating these SSTs accurately is important for robust rainfall projects in the neighbouring MCs. However, most ocean-atmosphere coupled climate models are currently unable to reproduce the observed pattern of Pacific SST change. Observations over the last century show an increase in the zonal SST gradient, while the majority of climate models predict a decrease.vii This discrepancy underscores the complexity of regional climate projections, and why further investigation into circulation changes, including those potentially caused by SSTs, is needed to refine our understanding of the subtropical drying trend.
Key research questions
Is the observed drying trend, in particular in the Southern Hemisphere, consistent with historical model simulations?
Are the dynamic and thermodynamic influences on the drying trend similar in observations and climate models?
Do any dynamic trends in these MCs project well onto larger scale climate trends, e.g. in the Hadley cell or annular modes, or are they more regional changes?
How much do tropical Pacific SSTs influence precipitation in MCs (central Chile and the US Southwest in particular)?
Organisations
People |
ORCID iD |
| Robert Doane-Solomon (Student) |
Studentship Projects
| Project Reference | Relationship | Related To | Start | End | Student Name |
|---|---|---|---|---|---|
| NE/S007474/1 | 30/09/2019 | 29/09/2028 | |||
| 2886919 | Studentship | NE/S007474/1 | 30/09/2023 | 29/09/2027 | Robert Doane-Solomon |