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Basin modes of the ocean: their role in the interannual to interdecadal climate variability

Periodic Reporting for period 1 - OceanModes (Basin modes of the ocean: their role in the interannual to interdecadal climate variability)

Reporting period: 2018-01-08 to 2020-01-07

Understanding the effect of global warming on the ocean remains a central objective for the future of humanity.
However the difficulty is that the anthropogenic climate warming signal in the ocean-atmosphere system is superposed with a rich diversity of natural signals, the Atlantic multidecadal oscillation found for scales in excess of 500 km and 10 year period and the very energetic oceanic mesoscale turbulence found at much smaller scales, 100-500 km and monthly periods.
As a prerequisite to separate the global warming signal from the natural signals, the central objective of OceanModes is to understand the causes of the Atlantic multidecadal oscillation and find how the oscillation interacts with the mesoscale turbulence. In short is the mesoscale turbulence a source or a sink for the long period oscillation?
The work performed during the first period can be organised into the following three categories:

1)Influence of eddy-turbulence on the basin modes
Sea surface temperature is the main link of the ocean with the overlying atmosphere and therefore the questions that are raised are at the heart of ocean-atmosphere climate evolutions.
The mode's existence was first shown in a rectangular flat-bottomed single-hemispheric basin, with prescribed surface heat fluxes. In this configuration, a large scale baroclinic instability continuously feeds large scale planetary waves. The planetary waves give rise to SST and Meridional Overturning Circulation variability. Figure 1 shows the variability that spontaneously emerges in this configuration.
Our approach is to simulate the large scale mode using this relatively simple configuration and to add a mesoscale eddy turbulence field by employing the MITgcm (an Ocean General Circulation Model) at eddy-resolving resolution.
The main outcome of this workpackage is that we find a transfer of temperature variance from low to high frequencies associated with meso-scale eddy turbulence which acts as a sink of temperature variance for the low-frequency large-scale mode.

2)Forcing of the basin modes
Identifying sources, sinks and pathway of energy (Kinetic (KE) and Available Potential Energy (APE)) in frequency space is essential to the understanding of low frequency variability mechanisms. We have shown that the non-linear APE transfer is much larger than the non-linear KE transfer and that it is in the opposite direction. The main source of mechanical energy (APE+KE) for the low frequency mode is the large scale baroclinic instability of the mean flow transfering energy between the mean APE and low frequency APE reservoirs. The non-linear transfer of kinetic energy contribution is negligible.
The stratification is controlled by the vertical diffusion parameter. At low resolution (i.e. without resolved meso-scale variability), increasing the stratification increases the mean circulation instability and thus increases the large scale mode amplitude. We showed that when mesoscale variability is permitted, the low frequency mode amplitude decreases with increasing stratification, at odds with previous results from low resolution studies. This is because the mesoscale turbulence intensity also increases with increasing stratification and thus dissipates further the large scale mode.

3) Understanding the QG APE/KE transfers in frequency space

The primitive equation of the MITgcm simulations are too computationally demanding to investigate APE/KE transfers sensibility to external parameters (forcing and friction in that case). Using a simpler doubly-periodic, two layers QG setup we investigate the role of friction and mean flow forcing intensity. Preliminary results show that if the kinetic energy is indeed transferred to lower frequencies, the transfer of mechanical energy is toward higher frequencies because the NL APE transfer is larger than the KE transfer and in the opposite direction.
Although the work described above has shown that the role of mesoscale eddies is to dissipate the low frequency mode, we show in a simple 1 layer QG model, that mesoscale eddies are able to force large scale basin modes. However, in this setup, basin modes are large scale and barotropic and thus high frequency contrary to the low frequency mode described above. We also focus on the inverse cascade of kinetic energy and direct cascade of enstrophy and describe for the first time the differences that appears between the classical doubly periodic QG model and our more realistic boundary enclosed QG model.
It was previously conjectured that mesoscale eddies could act as a forcing for the large scale low frequency basin mode. It is now clear that the turbulence associated with eddies act only to dissipate the large scale mode. The kinetic energy temporal inverse cascade is indeed much smaller than the direct APE temporal cascade. We also clearly show that wind forcing at the surface is essential to obtain a realistic transfer between mean kinetic energy and mean available potential energy. Increasing the level of eddy turbulence (by means of the vertical diffusion parameter and thus stratification) decreases the large scale mode amplitude confirming the dissipation role of eddy turbulence.
In periodic idealized QG simulation forced by a zonal mean shear flow associated with a meridional slope of the isopycnals, the literature was focused on the inverse cascade of KE. Instead when mechanical energy (i.e. kinetic energy and Available Potential Energy ) instead of solely kinetic energy is considered, the cascade is toward high frequencies. Thus in this setup the low frequency variability is primary due to direct baroclinic instability of the mean flow rather than to non-linear transfer of energy from high frequencies.
If it was previously conjectured that eddy turbulence could act as a basin mode forcing there is no concrete example in the literature of such a behavior. We show a simple example where QG eddy turbulence forces a barotropic high frequency basin mode.

The variability of the climate on time scales of a few decades is driven by intrinsic fluctuations as well as by anthropogenic forcing. Our lack of knowledge of the dynamics behind this natural variability prevents us from making accurate climate predictions which are a very strong societal need. Studies suggest that uncertainty in natural climate variability is larger than that in anthropogenic forcing for the predictions of climate changes over the next decades. The slow oceanic circulation likely plays a central role on decadal time scales because it is on these scales that the top 1000 m of ocean adjusts. Inter-decadal oceanic variability has been observed for instance in sea surface temperature and sea level data of the North Atlantic during the last century. This mode of variability has been called the Atlantic Multidecadal Oscillation (AMO) and is well correlated with Climate variations. Indeed, for instance US continental rainfall is less important during a positive AMO phase than during a negative phase, and AMO is highly correlated with Sahel rainfalls and hurricane intensity in the Atlantic. The work carried out in this project improves our understanding of oceanic low frequency variability mechanisms and will allow better climate predictions everywhere and in particular in Europe.
Schematic showing the temperature variance budget in the MITgcm simulation