2019 BSJ Blog featured Spring 2019

Wave Goodbye to that Coastline

by Ethan Ward

Climate change is a topic that is discussed constantly, but rarely from a non-politicized lens. The work of climate scientists is to study global trends in how the atmosphere interacts with itself as well as other systems (hydrosphere, cryosphere, etc.) and make predictions on what they find, not to dictate policy. The field of climate science is a diverse and vital look at our sole shield from the final frontier. In a paper by Omrami et al., the influence of ocean currents (Pacific and Atlantic) on the climate of the Northern Hemisphere (specifically winter interactions of troposphere/stratosphere coupling) is investigated thoroughly.

It’s important to start by understanding the structure of the 2-fluids that are interacting in this study, the hydrosphere and the atmosphere. The atmosphere is our guardian from the many dangers of space. Not only is it the air we breathe, but also our shield from harmful radiation and even meteorites. The troposphere is the air we know and love and the layer above it is called the stratosphere which bounces UV radiation back off into space with a bunch of ozone. Intuitively, the hydrosphere is the Earth’s reservoir of water (just as the atmosphere is its reservoir of air). Now, virtually all of climate-science comes down to the interactions between these 2 systems, which can be thought of as 2 separate fluids. In this particular study, the focus was on how the Ocean Western Boundary Currents (OWBCs) interact with the Northern Annular Mode (NAM) and the Northern Atlantic Oscillation (NAO). The Northern Annular Mode (NAM) explains variances -over weeks, months, and years -of climate phenomenon. Some of these phenomena include effects on ocean circulation, surface climate, and ecosystems (aquatic and terrestrial), which also have the NAO to thank for being the way that they are. The Northern Atlantic Oscillation (NAO) is an index which gauges temperature based off changes in sea level pressure, and therefore intensities in the jet stream. This basic framework will give the insight needed to understand the significance of probing these interactions.

There are 2 main proposals that this paper sought out to develop. The first of which being that OWBCs provide energy that is carried through deep ocean currents (baroclinic waves) and give rise to jetstreams and NAM. Moreover, that OWBCs have an influence on the stratosphere by weakening the polar vortex, a low-pressure region near the poles, which in turn affects both the troposphere and stratosphere Northern Annular Modes. The former is important to climate fluctuations in the troposphere (and enhances the annularity of NAM) so it gives a basis to what we have everyday experience with. On the other hand, the latter can have a relatively surprising impact on that same everyday experience through coupling between the troposphere and stratosphere. The study was split into 2 experiments, NF and BCF, which were conducted to gain insight into how feasible these claims were through the use of used MEACHAM5 simulation codes (common in atmospheric modeling). NF signifies a non-front Sea Surface Temperature (SST) profile (i.e. SSTs were averaged zonally), whereas the BCF has extra-tropical SST fronts (i.e. not averaged zonally) in the Atlantic and Pacific to account for Boundary Current Fronts. Then, from known observations, initial boundary conditions were entered at corresponding SSTs. The simulation data was then analyzed to extract a mean from the period 1950-2008 and was used as a comparison to define the action of Ocean Western Boundary Currents. It was found that this zone averaged SST gradient relaxed to constant values north of 33.5N (north pole) and south of 33.5S (south pole) with an assumed polar SST constant at freezing (Omrami et al.). By averaging zonally the OWBCs are discounted, which makes for a useful comparison when they are accounted for as one can see which simulation is likely to explain reality and which is not. In a sense, it can be thought of as having the NF simulation as a control (dep.) and the BCF simulation as the experimental group (indep.) as a way to find what impact the OWBCs have on the atmosphere.

From this analysis, it was observed that Ocean Western Boundary Currents have a noticeable effect of precipitation spread (i.e. moving storms around) and even warm the stratosphere by weakening the polar vortex, which is pretty cool. This result is especially interesting when looking at the coupling effects between the troposphere and stratosphere, such as Sudden Stratospheric Warming (SSW), that can cause dramatic shifts in our own backyard. SSW are rare occurrences (about 6 times a decade) marked by temperature spikes in the stratosphere by about 80K in the short period couple days which punctures the polar vortex (Clark). We actually had an example of this phenomenon earlier this year that anyone from the Chicago/midwest area won’t soon forget (Freund). Coupling between the troposphere and stratosphere is mediated by Rossby waves, which are quick westerly winds that carry energy the same way OWBCs do, through baroclinic waves (Clark).  Finally, it was found that these OWBCs can enhance the annularity of NAM and its importance in climate variation at the surface (Omrami et al.). The importance of this study can be seen not only as furthering our understanding of how the world works, but also showing how global warming increases the frequency of these extreme climate events since more heat translates to stronger OWBCs.

Climate change is not an easy problem to solve, or even gauge for that matter. The complexities of the Earth’s interacting cooling and heating systems lead to many different answers to how these systems work together as they are not entirely linear/codependent. The importance of work like this cannot be overstated. The world desperately needs people that want to look at these problems from an objective point of view and work to develop the foundation to tackle larger and larger questions in the field. Aside from its practical aspects (like figuring out a way to make sure our atmosphere doesn’t end up ending us), climate science is a beautiful look at how harmonious even the most basic of things that we take for granted on this earth really are.

 

References

Omrani, et al. “Key Role of the Ocean Western Boundary Currents in Shaping the Northern Hemisphere Climate.” Scientific Reports, vol. 9, no. 1, 2019, doi:10.1038/s41598-019-39392-y.

Clark, Simon. Quasi-Geostrophic Influence of the Polar Stratosphere on the Troposphere. 15 Oct. 2018, www.simonoxfphys.com/blog/2018/10/15/my-phd-thesis.

Freund, Sara. “Chicago Weather: Record-Breaking Cold, Brutal Subzero Windchill Arrives.” Curbed Chicago, Curbed Chicago, 30 Jan. 2019, chicago.curbed.com/2019/1/28/18200581/chicago-weather-forecast-polar-vortex-cold-storm-arctic-air.