Upload
others
View
0
Download
0
Embed Size (px)
Citation preview
Atmosphere–OceanInteractions
13. Climate Change: Past and Present
Saturday, September 28, 13
Outline1. A review of atmosphere–ocean interactions
2. The effects of mixed layer depth
3. Air–sea interactions during the last glacial maximum (Dansgaard–Oeschger cycles)
4. The effects of recent reductions in sea ice cover
Saturday, September 28, 13
The Climate System
Solar radiation is the ultimate driving force for all motions in the atmosphere and ocean, with regular diurnal and seasonal cycles
Saturday, September 28, 13
Wells, 2012
The Climate System
as well as orbital variations with longer timescales
Saturday, September 28, 13
noaa.gov
climate variations are not solely due to regular repeating cycles of solar radiation, but also show internal variability
(e.g., the El Niño–Southern Oscillation)
The Climate System
El Niño (warm phase)
La Niña (cold phase)
strong Walker circulation
weak Walker circulation
Saturday, September 28, 13
The Climate System
Wells, 2012
• The atmosphere and ocean, the cryosphere (ice sheets, glaciers, and sea ice), the land surface, and the biosphere
• All components interact in complex ways with other components on a variety of time scales
thousands of years
10–30 days
months
decades
seasons to yearsdecades to millennia
Saturday, September 28, 13
Tropical Air–Sea Interaction
Wang et al., 2004
SST > 27ºC SST < 27ºCprecipitation > 4 mm d–1
upwellingeasterly trade winds
Saturday, September 28, 13
deep ocean
Changes in the Thermocline
ocean mixed layer
easterly trade winds
shallow and colddeep and warm
easterly trade winds lead to a zonal tilt of the thermocline in the tropical oceans, with a shallower thermocline in the east
Saturday, September 28, 13
Tropical Air–Sea Interaction
convection and precipitation are shifted north of the equator in the eastern tropical oceans
Saturday, September 28, 13
Seasonal Variability
Wang et al., 2004
strong seasonal variability in SSTin eastern tropical oceans
cross-equatorial surface winds weaken and strengthen in response to the seasonal cycle of solar radiation, resulting in a
strong seasonal cycle in upwelling regions
sea surface height
root mean square seasonal variance in SST and SSH
Saturday, September 28, 13
Seasonal Variability
Wang et al., 2004
the monsoons also play a key role in seasonal ocean variability, especially in the Arabian and South China Seas
sea surface height
root mean square seasonal variance in SST and SSH
monsoon regions
Saturday, September 28, 13
The Tropical Pacific
Saturday, September 28, 13
The Tropical Atlantic
Saturday, September 28, 13
The Tropical Indian Ocean
Saturday, September 28, 13
Interannual Variability
Wang et al., 2004
Interannual variability in the tropical oceans is dominated by the El Niño signal in the tropical Pacific
root mean square variance of interannual SST anomalies
El Niño
Saturday, September 28, 13
Interannual Variability
Wang et al., 2004
Interannual variability in the tropical oceans is dominated by the El Niño signal in the tropical Pacific
root mean square variance of interannual SST anomalies
El NiñoAtlantic Niño
Saturday, September 28, 13
Changes in ENSO?
Cobb et al., 2013
MODELSCORAL RECONSTRUCTIONS
• Coral reconstructions of ENSO activity indicate similar variability through the past 7000 years, with no systematic trend
• Twentieth century ENSO activity has been stronger than average but not unprecedented
• Strong internal variability in ENSO activity, so that forced changes in ENSO will be difficult to detect without very long time series
Saturday, September 28, 13
Midlatitude Air–Sea Interaction• More difficult to identify for several reasons:
• meteorology is more complex, with weaker links between SST anomalies and surface winds
• SSTs are cooler and the mixed layer is generally deeper, so that the ocean takes longer to respond to atmospheric conditions
• the Coriolis term is larger, with stronger constraints on momentum
• Coupled variability is still apparent (e.g., the PDO)• Three categories of mid-latitude atmosphere–ocean
interaction theories:• include interactions in both the tropics and mid-latitudes• occur in subtropics/mid-latitudes and involve changes in the gyre
circulations• occur in mid-latitudes and involve changes in the thermohaline
circulation
• The ocean appears to integrate stochastic weather noise into longer-term variability in mid-latitudes
Saturday, September 28, 13
High Latitude Air–Sea Interaction• Primary mode of variability at high latitudes is the annular
modes (northern and southern), but the role of ocean–atmosphere coupling appears to be weak
• Atmosphere–ocean interactions at high latitudes are heavily influenced by sea ice cover
• Co-varying signals in the atmosphere, ocean, and sea ice have been observed in the Antarctic Southern Ocean, with warm SST anomalies associated with poleward meridional surface wind anomalies and vice versa
nlm.nih.gov
takes ~8–10 years to circle the pole
possibly connected to El Niño
Saturday, September 28, 13
Teleconnections
Alexander et al., J. Clim. 2002
ENSO modifies the atmosphere and ocean far from the equatorial Pacific
Saturday, September 28, 13
Teleconnections
Wang et al., 2004
three month lag
ENSO modifies the atmosphere and ocean far from the equatorial Pacific
persistenceteleconnections
1950–1999
Saturday, September 28, 13
ENSO Teleconnections
Wang et al., 2004
The anomalous Walker circulation createsatmospheric bridges that convey the ENSO signal
strong lag correlationswith other ocean basins
Saturday, September 28, 13
ENSO Teleconnections
Wang et al., 2004
The Pacific–North America patternassociated with ENSO
Rossby waves communicate the ENSO state to the mid-latitudes
middle/upper tropospheregeopotential height anomalies
Saturday, September 28, 13
Other Teleconnections
Alexander, 2010
• From the extratropics to the tropics:• on decadal timescales, largest anomalies in mid-latitudes• the subtropical highs link the mid-latitude westerlies with the easterly
trade winds
• Oceanic bridges• SST and wind anomalies also excite large-scale ocean Rossby and Kelvin
waves, which can connect the tropics and mid-latitudes or provide a memory of previous ocean and atmosphere conditions
• the shallow subtropical overturning circulation may link variability in the North Pacific with ENSO activity
Saturday, September 28, 13
Mixed Layer Depth
Donohoe et al., 2013
The amplitude and phase of the seasonal cycle of SST change with the depth of the mixed layer
Saturday, September 28, 13
Mixed Layer Depth
Donohoe et al., 2013
Changes in mixed layer depth have important impacts on the circulation in the overlying atmosphere
DEEP MIXED LAYER SHALLOW MIXED LAYER
Saturday, September 28, 13
Long-term Variability• Observations only go back 100–300 years, so that other
methods must be used to understand past climate• Paleoclimatology and paleooceanography use evidence
from fossils, pollen, lake levels, deep ocean sediments, glaciers, and ice sheets to reconstruct past climate
• Ice cores over 3000 m in length from Greenland and Antarctica give glimpses of climate over 800,000 years
glacial cycles
Saturday, September 28, 13
The Last Glacial Maximum
• Large amounts of water locked into massive ice sheets• Rapid sea level rise after the last glacial maximum
associated with both redistribution of water mass and continental rebound (from the reduced mass of ice pushing down on the continents)
Saturday, September 28, 13
Dokken et al., 2013
Last Glacial Maximum
Dansgaard–Oeschger Cycles• Abrupt, millenial scale climate shifts
in temperature over Greenland• Oscillations between extremely
cold “stadial” conditions that last for about 1000 years and warmer (~15ºC) “interstadial” conditions that last for about 300 years
• A warmer Greenland means warm, wet conditions over Europe, a stronger Indian monsoon, a more northward ITCZ, and many other global changes
Isotopic proxies reveal atmosphere–ocean interactions
42,000 years ago31,000 years agoSaturday, September 28, 13
Schmidt and Herzberg, 2011
The Salt Oscillator
Trade winds carry water vapor from Atlantic to Pacific, currents carry high salinity water from tropics to North Atlantic
Saturday, September 28, 13
Schmidt and Herzberg, 2011
The Salt Oscillator
Trade winds carry water vapor from Atlantic to Pacific, currents carry high salinity water from tropics to North Atlantic
evaporation
precipitation
Saturday, September 28, 13
Schmidt and Herzberg, 2011
The Salt Oscillator
Trade winds carry water vapor from Atlantic to Pacific, currents carry high salinity water from tropics to North Atlantic
evaporation
precipitation
cold, salty water sinks, forming North Atlantic deep water
Saturday, September 28, 13
The Salt Oscillator
Schmidt and Herzberg, 2011
• When salinity in the North Atlantic is high, more North Atlantic deep water forms and the Atlantic meridional overturning circulation is stronger
• A stronger AMOC imports more heat and exports more salt, melting ice and reducing salinity (and density) of surface water in the North Atlantic
• This reduces the amount of NADW formed and the strength of the AMOC• The weaker AMOC transports less heat to high latitudes, allowing ice
sheets to form and the salinity of surface water to increase again
surface current
deep current
ice sheets
Saturday, September 28, 13
The Wind Field Oscillator
Schmidt and Herzberg, 2011
• The jet stream separates cold, sub-polar air masses from warm, subtropical air masses
• The path of the jet across the North Atlantic is strongly affected by mountains and large ice sheets over North America
• Abrupt changes in the topography of North American ice sheets could have led to changes in the path of the jet stream
• The ice sheets may then gradually reform, restoring stadial conditions
Saturday, September 28, 13
Ocean–Sea Ice Interaction
Dokken et al., 2013
COLD WARM
heat release to atmosphere &
moderate seasonal ice cover
fresh surface layer buffered from
warm water by halocline
Saturday, September 28, 13
Ocean–Sea Ice Interaction
Dokken et al., 2013
COLD WARM
heat release to atmosphere &
moderate seasonal ice cover
fresh surface layer buffered from
warm water by halocline
stratification gradually reduces to the point of collapse
Saturday, September 28, 13
An Energy Balance Perspective
Singh et al., 2013No changes in ocean circulation included in model!
Saturday, September 28, 13
Dansgaard–Oeschger Events• Rapid transitions in Northern Hemisphere climate during
the last glacial maximum, with oscillations between two preferred states
• The salt oscillator hypothesis: feedbacks between the salinity of North Atlantic surface water and the strength of the Atlantic Meridional overturning circulation
• The wind field oscillator: abrupt changes in ice sheet topography over North America lead to changes in the position of the jet stream; the ice sheets then reform, restoring stadial conditions
• Ocean–sea ice interactions: changes in the sea ice cover cause changes in the distribution of the ocean heat flux that feedback on sea ice cover, even without changes in the large-scale ocean circulation
Saturday, September 28, 13
Effects of Arctic Sea Ice Loss
Strongly affecting Arctic and global climate:• Increased energy transport from ocean to atmosphere• Enhanced warming and moistening in lower troposphere• Decreased strength of near-surface inversion layer
Screen et al., 2013
OBSERVED CHANGES, 1979–2009
Saturday, September 28, 13
noaa.gov
Climate Models
couple the atmosphere and ocean together with the cryosphere and land surface, and even the biosphere
Saturday, September 28, 13