Observations:
Atmosphere and
Surface – part II
Jón Egill Kristjánsson
Contents
• Changes in Hydrological Cycle
• Changes in Extreme Events
• Changes in Atmospheric Circulation and
Patterns of Variability
CHANGES IN
HYDROLOGICAL CYCLE
- How would we expect the hydrological cycle to change with increasing [CO2]?
- Why?
Mostly insignificant trends, despite
significant global warming in this time
period
Exception: Upward trend (statistically
significant) over NH mid-latitude land
regions
Tendency for decrease 1970s-1990s,
followed by increase 1990s-2010s
Why might that be?
Aerosols and NH land precipitation
Observed Annual
Precipitation over
NH land masses
(GHCN data)
Wild (2012: BAMS)
How does CO2 increase influence the
Hydrological Cycle?
Clausius-Clapeyron
relationship
2xCO2
simulations
ΔP
ΔT
Allen & Ingram (2002; Nature)
How does CO2 increase influence the
Hydrological Cycle?
Clausius-Clapeyron
relationship
2xCO2
simulations
ΔP
ΔT
~5% reduction in P
for ΔT=0
Allen & Ingram (2002; Nature)
Energetics of the Hydrological Cycle
• The atmosphere is in Energy Balance, but not
in Radiative Balance
• Radiative Cooling is balanced by Release of
Latent Heat (~80%) + Sensible Heat (~20%)





Qatm  LH  SH  FTOA  FTOA  Fsurf  Fsurf
Change in
atmospheric
heat content
Fluxes of latent
and sensible
heat (from
surface to
atmosphere)
Net downwelling
radiative flux at TOA
(LW + SW)
Net downwelling
radiative flux at the
surface (LW + SW)
What does this mean for the
Hydrological Cycle?
• Doubling of CO2 leads to an increase in
precipitation (due to global warming), but the
increase is smaller than e.g. for an aerosol
forcing of the same magnitude
Upward trend over
NH mid-latitude
land regions
Trends in specific humidity at the
surface
Why is it increasing?
How about Relative Humidity?
What’s this?
Trends in Surface Humidity
Trends in column integrated water
vapor
Approximately +7% per K over
What’s happening here?
oceans, corresponding to
Clausius-Clapeyron expectations
CHANGES IN
EXTREME EVENTS
Trends in Weather Extremes
Cold Nights
Cold Days
Warm Nights
Warm Days
All consistent with expectations due to Global Warming
from Increasing Greenhouse Gas Concentrations?
Trend in frequency of Cold Nights
Trend in frequency of Cold Days
Stronger downward trend for Cold Nights
than Cold Days
Trend in frequency of Warm Nights
Trend in frequency of Warm Days
Trend in annual amount of precipitation
from days > the 95th percentile
Tendency for increased contribution from
the heaviest precipitation events
Trend in daily precipitation intensity
Tendency for increased contribution from
the heaviest precipitation events
Trend in the maximum number of
consecutive dry days
Trend in ‘hydroclimatic intensity’: a
multiplicative measure of length of dry
spell and precipitation intensity
Trends in tropical storm frequency
No robust statistically significant trends in
any region
The same applies for the strength of the
tropical storms (not shown here)
Trends in extratropical wind speed
No clear systematic trends
But, storm tracks are
shifting poleward (robust)
Trends in the Warmest day of the year
Why downward trend?
CHANGES IN
ATMOSPHERIC CIRCULATION
AND
PATTERNS OF VARIABILITY
Trends in pressure fields
Global Average? Why?
Global Average? Why?
Global Average? Why?
Trends in the strength of Icelandic Low,
Siberian High, Azores High, etc.
November - April
IL and AH stronger in 1980s and
1990s than before and after
Interpretation?
May - October
Trend in surface wind speed
Natural variability
or trends?
Low confidence
concerning possible
trends
Trends in strength of upper-air winds
Large natural
variability
The two datasets
disagree
Weakening, recently
reversed
The storm tracks are shifting poleward
(ISCCP) => Positive Cloud Feedback
Bender et al. (2012): Clim.Dyn.
Trends in the width of the tropical belt
Is the tropical belt widening? If so, why?
Trends in indices of variability
Most of the
indices fail to
show any robust
trends, especially
for the longer
period (19512012)
Summary
•
•
•
•
Changes in the Hydrological Cycle: ….…..
Changes in Extreme Events: ………
Changes in the Atmospheric Circulation: ….…..
Changes in Patterns of Variability: ….…..