atmosphere–ocean interactions · effects of arctic sea ice loss strongly affecting arctic and...

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


The Climate System

Solar radiation is the ultimate driving force for all motions in the atmosphere and ocean, with regular diurnal and seasonal cycles


Wells, 2012

The Climate System

as well as orbital variations with longer timescales


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


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


Tropical Air–Sea Interaction

Wang et al., 2004

SST > 27ºC SST < 27ºCprecipitation > 4 mm d–1

upwellingeasterly trade winds


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


Tropical Air–Sea Interaction

convection and precipitation are shifted north of the equator in the eastern tropical oceans


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


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


The Tropical Pacific


The Tropical Atlantic


The Tropical Indian Ocean


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


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


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


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


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


Teleconnections

Alexander et al., J. Clim. 2002

ENSO modifies the atmosphere and ocean far from the equatorial Pacific


Teleconnections

Wang et al., 2004

three month lag

ENSO modifies the atmosphere and ocean far from the equatorial Pacific

persistenceteleconnections

1950–1999


ENSO Teleconnections

Wang et al., 2004

The anomalous Walker circulation createsatmospheric bridges that convey the ENSO signal

strong lag correlationswith other ocean basins


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


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


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


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


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


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)


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



The Salt Oscillator


evaporation

precipitation



The Salt Oscillator


evaporation

precipitation

cold, salty water sinks, forming North Atlantic deep water


The Salt Oscillator


• 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


The Wind Field Oscillator


• 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


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


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


An Energy Balance Perspective

Singh et al., 2013No changes in ocean circulation included in model!


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


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


noaa.gov

Climate Models

couple the atmosphere and ocean together with the cryosphere and land surface, and even the biosphere


atmosphere–ocean interactions · effects of arctic sea ice loss strongly affecting arctic and...

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