Christian E. Buckingham

Mesoscale processes

The oceans are turbulent and chaotic. Described by simple analytical equations at mesoscales or larger, these two-dimensional flows have a unique behavior: eddying processes interact with one another in a non-linear manner to create larger and larger vortices. One refers to this as an inverse energy cascade. For example, within a tank with constant rotation rate, \(\Omega = f / 2\), an in the absence of friction, these vortices would coalesce until a single vortex would remain. However, on a rotating sphere such as Earth where \(f\) varies with latitude, there exists a theoretical constraint placed on the north-south extent of eddies. Oddly, this constraint is not present in the zonal extent. This dynamic—referred to as a forward enstrophy cascade—is thought to result in locations of pronounced horizontal shear, or multiple zonal jets. Additionally, the conservation of enstrophy (squared vorticity) plays an important role. It is notable that these arguments have been used to explain banded structures on Jupiter.

A search for multiple zonal jets

My doctoral work pertains to the above subject in some measure. In 2010, a scientific debate existed about the presence of zonal jets in time-averaged satellite measurements (Schlax and Chelton 2008; Maximenko et al. 2005, 2008). The problem was that the striations (or “zonal jets”) could be an artifact of waves and/or eddies moving westward in time so that averaging these features produces an alternating zonal jet-like signal. My major professor and I showed that these jets in observations were principally the result of zonally propagating eddies and/or waves. However, we did find inconsistencies between a model of random eddies and the observed velocity field, suggesting the existence of preferred eddy paths or a “latent” (i.e. weak) jet-like signal (Berloff et al. 2011). This research made use of both sea surface height (SSH; Buckingham and Cornillon 2013) and microwave sea surface temperature (SST; Buckingham et al., 2014). We were also the first to show evidence of jet-like signals in SST (Buckingham and Cornillon 2010). See also Zonal Jets: Phenomenology, Genesis, and Physics. Ed. by B. Galperin and P. L. Read (2019).

Animations

The animation below does not reveal the existence of multiple zonal jets, per se. However, it does suggest the presence of such phenomena. Here, we illustrate the likelihood of detecting a front (front probability) during the period 2002-2010. The front detections were made by applying an edge detection algorithm (Cayula and Cornillon 1989) to microwave sea surface temperature (AMSRE SST) and computing the fraction of pixels classified as having a front during a three week period. The images are then moved over in a sliding time window and updated weekly. While inconclusive, these graphics suggest that mesoscale eddies and/or Rossby waves may be connected by weak zonal jets. Moreover, they reveal the ubiquity of ocean fronts on the globe. Given that ocean frontal dynamics are important for ocean and atmosphere communication, these dynamics may be pivotal for climate. This work was originally published here.

To better understand the link between ocean surface temperatures and dynamics, we investigated how SST and SSH are related. The following animation reveals the relationship between SST and SSH in the mesoscale regime in the open ocean. These results were obtained from satellite measurements over the mid-latitude South Pacific Ocean. SST anomaly is estimated by applying a spatial high-pass filter to microwave SST, while SSH anomaly (or sea level anomaly, SLA) is defined as a deviation from a time mean. SSH data have been interpolated from multiple satellites.

At these horizontal scales (50-200 km), the ocean is nearly or quasi- geostrophic, so that the data reveal the presence of mesoscale eddies and/or Rossby waves propagating to the west (Chelton et al. 2011, Progress in Oc.). Interestingly, there appears to be a strong SST anomaly that is most evident on the westernmost flanks of the SSH anomalies. This most likely reflects the advective nature of the eddies/waves on upper ocean SST (Qiu et al. 2004; Buckingham et al. 2014).

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