EGU2020-18650
https://doi.org/10.5194/egusphere-egu2020-18650
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Extratropical cyclone induced sea surface temperature anomalies in the 2013/14 winter

Helen Dacre1, Simon Josey2, and Alan Grant1
Helen Dacre et al.
  • 1University of Reading, Department of Meteorology, Reading, United Kingdom of Great Britain and Northern Ireland (h.f.dacre@reading.ac.uk)
  • 2National Oceanography Centre, Southampton, United Kingdom

The 2013/14 winter averaged sea surface temperature (SST) was anomalously cool in the mid-North Atlantic region.  This season was also unusually stormy with extratropical cyclones passing over the mid-North Atlantic every 3 days.  However, the processes by which cyclones contribute towards seasonal SST anomalies are not fully quantified. In this paper a cyclone identification and tracking method is combined with ECMWF atmosphere and ocean reanalysis fields to calculate cyclone-relative net surface heat flux anomalies and resulting SST changes.  Anomalously large negative heat flux is located behind the cyclones cold front resulting in anomalous cooling up to 0.2K/day when the cyclones are at maximum intensity.  This extratropical cyclone induced 'cold wake' extends along the cyclones cold front but is small compared to climatological variability in the SST's.  To investigate the potential cumulative effect of the passage of multiple cyclone induced SST cooling in the same location we calculate Earth-relative net surface heat flux anomalies and resulting SST changes for the 2013/2014 winter period.  Anomalously large winter averaged negative heat flux occurs in a zonally orientated band extending across the North Atlantic between 40-60 oN. The 2013/2014 winter SST cooling anomaly associated with air-sea interactions (anomalous heat flux, mixed layer depth and entrainment at the base of the ocean mixed layer) is estimated to be -0.67 K in the mid-North Atlantic (68% of the total cooling anomaly).  The role of cyclones is estimated using a cyclone masking technique which encompasses each cyclone centre and its trailing cold front. The environmental flow anomaly in 2013/2014 sets the overall tripole pattern of heat flux anomalies over the North Atlantic.  However, the presence of cyclones doubles the magnitude of the negative heat flux anomaly in the mid-North Atlantic.  Similarly, the environmental flow anomaly determines the location of the SST cooling anomaly but the presence of cyclones enhances the SST cooling anomaly.  Thus air-sea interactions play a major part in determining the extreme 2013/2014 winter season SST cooling anomaly. The environmental flow anomaly determines where anomalous heat flux and associated SST changes occur and the presence of cyclones influences the magnitude of those anomalies.

How to cite: Dacre, H., Josey, S., and Grant, A.: Extratropical cyclone induced sea surface temperature anomalies in the 2013/14 winter, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18650, https://doi.org/10.5194/egusphere-egu2020-18650, 2020

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  • CC1: Comment on EGU2020-18650, Linda van Garderen, 05 May 2020

    Dear Helen,

    Thank you very much for this interesting presentation! It is not often to see a study on the effect of storm on SST instead of the other way around.

    I have a questions concerning the location of cooling you are pointing out. This is right were the so called SST heating hole is. The location where instead of SST climate warming there is cooling. As far as I know the reason for this is not understood. Do you think this heating gap may have influenced your results or do you think that your results may be the beginning of understanding the heating gap?

    Thank you.

    Linda

    • AC1: Reply to CC1, Helen Dacre, 05 May 2020

      Dear Linda

      I'm glad you enjoyed the presentation.  You're right, most atmospheric studies look at the impact of SSTs on cyclone development, not the other way around.  We thought that given the persistent zonal storm track in the 2013/14 season, the accumulated impact of multiple cyclones might be seen in the SSTs, which it is.  The next step is to see if these results are unique to the 2013/14 season or if there is a link between the presence of cyclones and SSTs in other years.  If there is, then we might hypothesize that a more constrained storm track, extending further into western Europe (as seen in some climate simulations) would lead to enhanced cooling in the mid-North Atlantic due to the passage of multiple cyclones.  Does that answer your question?

      Regards,

      Helen

      • CC2: Reply to AC1, Linda van Garderen, 05 May 2020

        Yes, that answers my question. I am looking forward to the future restuls of your studies. This is a large question to answer and an interesting line of thought to do so.

        Linda

  • CC3: Comment on EGU2020-18650, Miguel Teixeira, 05 May 2020

    Hi.

    I was wondering if there might be a role played in the cooling of the ocean boundary layer by Ekman pumping. It is well-known that Ekman pumping due to the wind stress in a cyclonic disturbance causes divergence of the water mass at the surface, which causes upwelling. Is that effect negligible compared to the others, or is it somehow implicitly accounted for in your calculations (I may have missed that in your poster)?

    • AC2: Reply to CC3, Helen Dacre, 05 May 2020

      Dear Miguel

      Thanks for your question.  I guess we indirectly accounted for Ekman pumping by looking at the contribution of the mixed layer depth to the SST cooling anomaly.  When cyclones are present the mixed layer depth is deeper, due to enhanced mixing.  This is interesting because although the surface heat flux was doubled when cyclones were present, the SST cooling was only increased by 50% because the cooling was over a greater volume.  Does that answer your question?

      Best wishes

      Helen

      • CC4: Reply to AC2, Miguel Teixeira, 05 May 2020

        Thanks for your answer Helen. I do not think it is exactly the same thing. The upwelling associated with Ekman pumping is a net flux of cold water from below the thermocline (not mixing), but the premise underlying the idea that this would lead to cooling is that the temperature decreases as depth increases (which I am not sure makes sense in winter). Anyway, I am convinced this idea would have occurred to Alan, and if he did not suggest it, he must have had his reasons.