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Variability of poleward atmospheric energy transports as evaluated from reanalyses. Michael Mayer 1,2, * and Leopold Haimberger 2. 1 ASP visiting graduate student , National Center for Atmospheric Research, Boulder, Colorado

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Conclusions

Variability of poleward atmospheric energy transports as evaluated from reanalyses

Michael Mayer1,2,* and Leopold Haimberger2

1ASP visitinggraduatestudent, National Center forAtmospheric Research, Boulder, Colorado

2Department ofMeteorologyandGeophysics, University of Vienna, Austria

[email protected]

Interannual variability

Comparison to satellite data

Introduction

The vertically integrated global energy budget is evaluated, mainly using European Re-Analysis Interim (ERA-Interim) data from the European Centre for Medium-Range Weather Forecasts (ECMWF).

The horizontal energy flux divergence is estimated in two ways, either by evaluating the left side of the total energy equation (1) using six-hourly analysed fields (direct method) or by evaluating the right side of (1) (residual of the radiation at the top of the atmosphere, the net surface fluxes and the energy tendency) using 12-hour forecasts (indirect method).

Results from both methods only make sense when they are corrected for erroneous analysed mass flux divergence (direct method) and erroneous surface pressure forecasts (indirect method). The direct method suffers from the coarse time resolution, the indirect method from spin-up effects.

A new estimate of poleward energy transports is given and the interannual variability of the energy budgets is investigated. Different other datasets have been employed for comparison and validation of the quality of the obtained results.

Fig. 3. 12-months running mean of tropical energy export anomalies (mass corrected indirect and direct estimate from ERA-Interim) and 12-months running mean of Niño 3.4 SST anomaly index.

Fig. 2. Anomalous horizontal energy flux divergence and divergent fluxes 12-months average, centered at November 1997 (corrected indirect estimated)

Large ENSO-related anomalies of ±25Wm-2 can be found over the Tropical Pacific region but strong compensation occurs between Eastern and Western Pacific (Fig. 2). In total, no positive energy export anomaly from the Tropics to the Mid-Latitudes can be found during the peak phase of the 1997/1998 El Niño, rather in the transition phase to La Niña conditions (Fig. 3). Anomalous latent heat flux is dominating vertical flux anomalies (Fig. 4) but tends to be compensated by erroneous energy storage anomalies (black curve in Fig. 5), which makes the magnitude of anomalies questionable.

Meanpolewardenergytransports

Average poleward energy transports from ERA-Interim prove to be in very good agreement with current reference estimates of this quantity (e.g. FT08). Mass correction improves the obtained results which can be seen from the strong reduction of the unrealistically large cross-equatorial energy fluxes shown by the uncorrected estimates. The directly estimated CEF seems to be more realistic.

Results from ERA-40 show much higher variability in time and the indirect estimate suffers from spin-up effects in the hydrological cycle.

Fig. 6. Monthly anomalies of RadTOA from ERA-Interim (left) and CERES (right) 2000/03-2010/02

Anomalies of ERA-Interim and CERES TOA-fluxes show remarkably good agreement (Fig. 6). ENSO-correlated anomalies in the Tropics mostly agree well in magnitude and timing. ERA-Interim shows a general decrease of TOA fluxes from 2006 onwards which leads to a weaker agreement of the La Niña-signal in 2008/09.

In comparison to ERA-Interim, ISCCP-FD TOA-fluxes are very inhomogeneous in time, making ISCCP data less useable for long-term studies of the energy budget (not shown).

Fig. 5. Anomalies of forecasted and analysed energy tendencies as well as (ERA-Interim);Tropical Pacific region (30°N-30°S, 110°E-80°W)

Fig. 4. Anomalies of RadTOA, LH+SH, RadS and vertical flux divergence (ERA-Interim); Tropical Pacific region (30°N-30°S, 110°E-80°W)

Comparison of different atmospheric peak transport estimates [PW] on the Northern (NH) and Southern (SH) Hemisphere as well as cross-equatorial energy fluxes (CEF) [PW] with 2σ-uncertainty from ERA-Interim (1989-2009), ERA-40 (1989-2000) and FT08

  • Conclusions

  • Poleward energy transports from ERA-Interim are in good agreement with current reference estimates (when mass corrected)

  • Tropical energy exports are a very basic and weakly varying (±4%) quantity of the climate system.

  • Strong compensation of ENSO-related positive and negative anomalies occurs between the Eastern and Western Pacific.

  • The magnitude of energy transport anomalies is questionable as the forecast model tends to convert anomalous latent heat fluxes to unrealistic storage anomalies

  • Comparison to current reference data of TOA-fluxes shows very good agreement of flux anomalies.

  • Further investigation of the budgets is planned (ERA-Interim (available from 1979 onwards soon) as well as other recent reanalyses)

  • More insight has to be gained into the uncertainty of surface flux variability by comparison to independent datasets

Fig. 1. Average poleward energy transports from ERA-Interim, using indirect and direct method and applying mass correction (c)

References

Mayer, M. and Haimberger, L., 2011: Poleward atmospheric energy transports and their variability as

evaluated from ECMWF reanalysis data, to be submitted.

Fasullo, J. T. and K. E. Trenberth, 2008: The annual cycle of the energy budget. Part II:

Meridional structures and poleward transports. J. Climate, 21, 2313–2325 (here abbreviated as FT08)


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