A quantitative assessment of air-sea heat flux trends from ERA5 since 1950 in the North Atlantic basin

Mayer, Johannes; Haimberger, Leopold; Mayer, Michael

doi:https://doi.org/10.5194/esd-2023-8

Preprints

https://doi.org/10.5194/esd-2023-8

Preprints

14 Apr 2023

| 14 Apr 2023

Status: this preprint is currently under review for the journal ESD.

A quantitative assessment of air-sea heat flux trends from ERA5 since 1950 in the North Atlantic basin

Johannes Mayer, Leopold Haimberger, and Michael Mayer

Abstract. This work aims to investigate the temporal stability and reliability of trends in air-sea heat fluxes from ERA5 forecasts over the North Atlantic basin for the period 1950–2019. Driving forces of the trends are investigated using analyzed state quantities from ERA5. Estimating trends from reanalysis data can be challenging as changes in the observing system may introduce temporal inconsistencies. To this end, the impact of analysis increments is discussed. For individual sub-regions in the North Atlantic basin, parametrization formulas for latent and sensible heat fluxes are linearized to quantitatively attribute trends to long-term changes in wind speed, moisture, and temperature. Our results suggest good temporal stability and reliability of air-sea heat fluxes from ERA5 forecasts on sub-basin scale and below. Regional averages show that trends are largely driven by changes in the skin temperature and atmospheric advection (e.g., of warmer or drier air masses). The influence of climate variability modes, such as the North Atlantic Oscillation (NAO) and Atlantic Multidecadal Oscillation on the found patterns is discussed as well. Results indicate a significant impact on trends in the Irminger and Labrador Sea associated with more positive NAO phases during the past 4 decades. Finally, we use basin-wide trends of air-sea heat fluxes in combination with an observational ocean heat content estimate to provide an energy-budget-based trend estimate of the Atlantic meridional overturning circulation (AMOC). A decrease of area-averaged air-sea heat fluxes in the North Atlantic basin suggests a decline of the AMOC over the study period. However, basin-wide flux trends are deemed partially artificial, as indicated by temporally varying moisture increments. Thus, the exact magnitude of change is uncertain, but its sign appears robust and adds complementary evidence that the AMOC has weakened over the past 70 years.

Received: 11 Mar 2023 – Discussion started: 14 Apr 2023

Johannes Mayer et al.

Status: open (until 15 Jun 2023)

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RC1:
'Comment on esd-2023-8', Anonymous Referee #1, 07 Jun 2023 reply
J. Mayer et al. organized a very nice study to assess the air-sea heat flux trends from ERA5 data since 1950 in the North Atlantic. Although the main focus is on the winter months, the annual changes are also investigated. The drivers of air-sea heat flux change are examined with a linearization approach. A very novel aspect is analyzing the analysis increments, which helps to show the impacts of observational system change and assess the reliability of trends. Furthermore, the air-sea flux data from ERA5 are compared with the indirect estimate based on the atmospheric energy budget approach. The ocean meridional heat transport is derived in the North Atlantic Basin. In summary, it is a very thorough analysis and a well-written manuscript. This study is also helpful for the understanding of the Earth’s energy flow. I don’t have any major concerns but only several clarification questions.
Comments:
Page-3, line-90: it is XBT, not CBT.

Page-6, line-175-180: It is not very clear how the residual has been dispersed in the Atlantic Basin, i.e. you may get a global residual, did you disperse a part of this residual (according to the area of the Atlantic Basin) or the global value?

Page-11, line-335-338: this might be arguable: the correlation may largely come from inter-annual variation instead of trend, thus the strong correlations might not necessarily explain the trend?

Section 4.4, the analyses related to NAO, again, the regression of NAO might largely associate with inter-annual fluctuation, so the low-frequency variation has been overlooked. It might be better to try applying a low-pass filter to the NAO index, so the decadal to multi-decadal variation can be highlighted, which is more relevant to the comparison with trends.

9 (right panel): is there an error bar for both RAPID value and your estimate in black line?

It would be better to put Fig. B1 into the main manuscript.

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Citation: https://doi.org/10.5194/esd-2023-8-RC1

Johannes Mayer et al.

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Short summary

This study investigates the temporal stability and reliability of winter-month trends of air-sea heat fluxes from ERA5 forecasts over the North Atlantic basin for the period 1950–2019. Driving forces of trends and the impact of climate variability modes and analysis increments on air-sea heat fluxes are investigated. Finally, a new and independent estimate of the Altantic Meridional Overturning circulation weakening is provided and associated with a decrease of air-sea heat fluxes.


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