Seasonal Features of Global Meridional Heat Transports

1
Seasonal Features of Global Meridional Heat
Transports

• Ho-Jeong Shin
• Dept. of Atmospheric Sciences,
• Yonsei University

2
Introduction

• Meridional heat transport (MHT) is a fundamental
  framework to explain the climate system and thus
  is essential for understanding of the system.
• The latitudinal differential heating due to
  radiative energy imbalance is balanced by the
  poleward heat transports fulfilled both by the
  atmospheric and oceanic circulations.
• This balance, however, is not exact. Some
  previous studies corroborated that an oceans
  heat uptake resulted from the climate change
  during the 20th century.

3
Introduction

• It cannot be overemphasized to say that the
  annual mean transport is a small sum of the two
  large canceling transports particularly by the
  ocean of the extreme seasons on most latitudes.
• Minor latitudinal shifts in the seasonal mean may
  lead to large difference in the annual mean and
  thus its interannual variability.
• Therefore, it is necessary for a proper
  understanding of long-term trend and variability
  to assess seasonal heat transports with
  clarifying their uncertainty.

4
Introduction

• In this study, seasonal meridional heat
  transports by the atmosphere and ocean have been
  examined for their climatological mean,
  interannual variability, and long-term trend on
  the global scale.

5
Data

• Simulation ensemble dataset
• WGNE/WCRP CMIP3 multi-model ensemble (contributed
  to IPCC AR4)
• 20th Century Coupled Climate Model (20C3M)
  experiment standard outputs
• 14 out of 25 models holds a complete dataset
  required for this study
• Monthly MHTs from 1901 to 2000

6
Estimation of MHT

• Global constraints
• global mean
• S energy storage
• F? downward net energy flux
• T vertically integrated energy transport

• Estimation of MHT

7
Climatological Annual Mean MHT
8
Climatological Seasonal Mean MHT
9
Annual Variation
10
Annual Mean
11
Annual Variation (FTOA)
12
Annual Variation (SA)
13
Annual Variation (SO)
14
Time Evolution (ANN)
15
Time Evolution (JJA vs. DJF)
16
Time Evolution (MAM vs. SON)
17
Time Evolution (AMHT vs. OMHT)
18

• Results and Discussion (continued)
• 3.3. Long-term Trend net energy fluxes over
  the global ocean
• It is evident that the planetary energy imbalance
  is almost entirely due to the net energy fluxes
  at TOA and surface over the global ocean. This
  can be interpreted as that the ocean heat storage
  induces the planetary energy imbalance as
  confirmed by Hansen et al. (2005).

APCC summertime lecture on Climate Dynamics and
AGCM
19

• Results and Discussion (continued)
• 3.4. Long -term Trend comparison with a few
  recent researches using observational data
• Levitus et al. (2005) convince the ensemble
  result using the in-situ observation data that
  the planetary energy imbalance has increased
  during the late 20th century due to the world
  oceans heat uptake (See the figures below).
  Hansen et al. (2005) estimated that the imbalance
  corresponds to the atmospheric forcings by the
  increased green house gases and aerosols during
  the century (Hansen et al. 2005).

APCC summertime lecture on Climate Dynamics and
AGCM
20
Inter-annual Standard Deviation
21
Summary and Conclusion

• Annual mean structure of AMHT is dominant for all
  seasons.
• Annual mean of OMHT is a small residual of larger
  but compensating seasonal transports.
• The annual mean AMHT is more than twice greater
  than the annual mean OMHT especially in the
  middle and high latitudes. However, the seasonal
  mean MHTs by the atmosphere and ocean are
  compatible.

22
Summary and Conclusion

• Inter-annual variability of annual and seasonal
  MHTs is relatively very large over the tropical
  ocean especially during DJF.
• However, the inter-model difference for DJF OMHTs
  is also large over that region. It is necessary
  to make an endeavor to minimize this large
  difference among the climate models because the
  tropical ocean plays an important role in OMHT.
• AMHT shows a positive anomaly since 1970s, which
  seems to have occured by global warming during
  the 20th century.

23
Importance of Oceanic Heat Storage for MHTs
24
Long-term Trend (EOF)
25
Annual Variation (FTOA)