EGU2020-10022
https://doi.org/10.5194/egusphere-egu2020-10022
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Influence of mid-latitude oceanic fronts on the atmospheric water cycle

Fumiaki Ogawa1,2 and Thomas Spengler1,2
Fumiaki Ogawa and Thomas Spengler
  • 1University of Bergen, Geophysical Institute, Bergen, Norway (fumiaki.ogawa@gfi.uib.no)
  • 2Bjerknes Centre for Climate Research, Bergen, Norway

      Midlatitude oceanic fronts play an important role in the air-sea coupled weather and climate system. Created by the confluence of warm and cool oceanic western boundary currents, the strong sea-surface temperature (SST) gradient is maintained throughout the year. The climatological mean turbulent air-sea heat exchange maximizes along these SST fronts and collocates with the major atmospheric storm tracks. A recent study identified that the air-sea heat exchange along the SST front mainly occurs on sub-weekly time scales, associated with synoptic atmospheric disturbances. This implies a crucial role of air-sea moisture exchange along the SST fronts on the atmospheric water cycle through the intensification of atmospheric cyclones and the associated precipitation.  

      In this study, we investigate this influence of the SST front on the atmospheric water cycle by analyzing the atmospheric response to different prescribed SST in the Atmospheric general circulation model For the Earth Simulator (AFES). Changing the latitude of the prescribed zonally symmetric SST in aqua-planet configuration, we find a distinctive response in convective and large-scale precipitation, surface latent and sensible heat fluxes, as well as diabatic heating and moistening with respect to the latitude of SST front. Upward surface latent heat flux and convective precipitation always maximize along the equatorward flank of SST front. On the other hand, large-scale precipitation is always located on the poleward flank of the SST front, in correspondence with the maximum atmospheric moisture flux convergence. The moisture flux convergence is mainly associated with midlatitude eddies and not with the time mean transport. This highlights the influence of mid-latitude SST fronts on the atmospheric water cycle through the organization of atmospheric storm track.

How to cite: Ogawa, F. and Spengler, T.: Influence of mid-latitude oceanic fronts on the atmospheric water cycle , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10022, https://doi.org/10.5194/egusphere-egu2020-10022, 2020

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Presentation version 1 – uploaded on 28 Apr 2020
  • CC1: Comment on EGU2020-10022, Annu Panwar, 17 May 2020

    Hey, Nice work!

    Few questions from not an ocean researcher!

    How SST fronts are shaped by the continents or landmass? If they are does it have any effect on your experiment which considers the earth as an aqua planet?

    I would be also interested since it evaporates a lot near the tropics. If there is no evaporation can you comment/ discuss, how warm it can be SST?

    Thanks

    • AC1: Reply to CC1, Fumiaki Ogawa, 18 May 2020

      Hello Annu,
      I am glad that you are interested in this work.

            Regarding your first question, the observed SST fronts are indeed shaped by the continents. The lateral boundary of the ocean by the landmasses, spherical shape of the earth, and the rotation of the earth generates western boundary currents in the westside of the ocean as the warm currents from the low latitudes and cool currents from the high latitudes. The SST front is shaped as the confluent region of the warm and cool currents in the western side of the individual ocean basin in the midlatitudes in the Northern Pacific and Atlantic, and Southern Atlantic and Indian ocean, respectively. In the midlatitudes, the observed evaporation, which is the supply of the moisture from the surface, is one order smaller from the land than the ocean. Over the ocean, the evaporation maximises along the SST front. Thus in this study we investigated how potentially important the SST front is for the atmospheric water cycle. Considering the amount of land mass, aqua-planet condition mimics the observed Southern Hemisphere. We took observed SST profile of the South Indian ocean, and prescribed as the lower-boundary condition in the atmospheric general circulation model. Since it is prescribed, the SST front is not affected by the atmospheric circulation in our study. In our aqua-planet, the SST front exists in every longitude around the observed latitudes. Since the observed SST front exists only in the western part of the ocean basin, our experiments may indeed exaggerates the impact of SST front on the atmospheric water cycle. Nevertheless, our experiments reproduce the intensity of the eddy activities which was shown to be important for the observed atmospheric water cycle in a previous study. Our study demonstrates that the cyclone activities, which is important for the atmospheric water cycle, is considerably shaped by the SST front.

            Regarding your second question, the tropical SST would be much warmer if no evaporation occurs in the tropical latitudes because an important role of evaporation is the heat transport from the ocean to the atmosphere. The poleward heat transport by the western boundary currents would be stronger if the ocean heat is not released to the atmosphere in the tropical latitudes, thus the SST front in the midlatitude would show a stronger meridional temperature gradient. Also, the absence of evaporation in the tropical latitudes would cause a drastic change of the atmospheric water cycle. The tropical climate would become completely dry because the absence of moisture supply must be balanced by the absence of the moisture supply from the atmosphere (i.e. precipitation). The evaporation in the tropical latitude is also essential for the tropical atmospheric mean circulation. Without evaporation, the atmospheric circulation in the tropical latitudes would become very unrealistic. The midlatitude atmospheric circulation is maintained through the momentum transport from the low latitudes. Therefore, the corruption of the tropical circulation by the absence of the evaporation would also corrupts the midlatitude atmospheric circulation. Therefore, the evaporation in the tropical latitudes is important not only for the tropical SST but also for the global atmospheric circulation. Our study shows that the tropical evaporation is hardly affected by the alternation of midlatitude SST profile, which in turn demonstrates the importance of SST front for the atmospheric water cycle mainly in the extratropical latitudes.