7-minute read

After I reviewed Joy McCann’s book Wild Sea I became fascinated with the three-dimensional nature of ocean currents. She captivated my imagination with her vivid description of the formation of bodies of heavy, cold water plunging into the abyss around Antarctica. So when Cambridge University Press announced this textbook it seemed like the perfect opportunity to dive deeper into this topic.

Both authors are oceanographers that actively teach courses on ocean circulation, so the material and exercises presented here have been field-tested on undergraduate students. Their preface contains suggestions for teachers on how to build a two-semester course out of this book, while a useful table indicates which chapters to read before others. This thoughtful level of organisation characterises the book as a whole. The three dimensions of ocean circulation are also applied figuratively, with each chapter divided into three sections: observations of ocean movements, concepts to explain these observations, and theory to quantify and apply these concepts.

The first two chapters review the foundations of physical oceanography you need to be familiar with, of which three pillars stand out. First are the physical characteristics of the ocean, specifically the importance of temperature and salinity, but also the influence of Earth’s rotation, which affects ocean circulation through the Coriolis force. Second is how oceanographers gather their data: via submerged sensors attached to stationary or moving platforms, or via satellites. With so much of oceanography now done using computers and remotely collected data sets, it is easy to forget where your data come from. It is equally easy to forget their limitations, which feeds into the third foundational pillar: mathematical models.

The oceans are enormous and largely inaccessible, and datasets are often limited in either time or space. You can collect long time series in one location, or you can cover more ground but only collect snapshots. Satellites and remote sensing have advanced the field tremendously, but these time series are relatively recent, often have some limitation on spatial resolution or on how deep they can probe the ocean, and are of limited duration: satellites only last so long and new space missions require continued funding. As such, physical oceanography, and therefore this book, rely heavily on numerical models to fill in the gaps and make predictions.

“A useful table indicates which chapters to read before others. This thoughtful level of organisation characterises the book as a whole […] with each chapter divided into three sections”

Having explained these basics, Klinger & Haine first consider the upper ocean, then the deep ocean, and finally the unique circulation patterns at the planet’s poles. What these chapters reveal is the beautiful complexity and diversity of circulation mechanisms. I will attempt a brief summary – with advance apologies if I oversimplify or butcher concepts given my limited background knowledge.

So, at the surface, wind drives the formation of gyres (frequently mentioned in news reports on plastic pollution and oceanic garbage patches). But rather than centrally located whirlpools, the Earth’s rotation causes them to be offset to the west, leading to powerful western boundary currents hugging the coasts of various continents. The constant churning of wind and waves turns the top layer of the ocean into a mixed layer with a relatively uniform vertical profile of temperature, salinity, and density. Gyre circulation at depth differs from the surface. As you descend, pressure rises and circulation patterns slow down and weaken, building up vertical shear. At the equator, currents differ again, with converging and diverging surface flows dominating, leading to upwelling and downwelling through so-called Ekman suction and pumping, transporting water vertically. Here, the three-dimensional character of ocean circulation comes to the fore, with surface and subsurface waters flowing in different directions.

An interlude, chapter 7, talks about meanders, eddies, and small-scale mixing processes. The authors then complete the circulation picture by looking at polar overturning cells where water sinks into the abyss, moves at depth over long distances, and wells up again near the equator. This is where the topography of the ocean bottom comes into play, with sills and marginal seas having an influence. Because of the position of the continents, the world’s oceans have several north-south corridors such as the Atlantic, where the direction of flow is along lines of longitude or meridians, hence this is called meridional overturning.

“The oceans are enormous and largely inaccessible, and datasets are often limited in either time or space. […] As such, physical oceanography, and therefore this book, rely heavily on numerical models […]”

The planet’s poles deserve special attention. The main reason is that the South Pole with its raging Antarctic Circumpolar Current connects to and interacts with all of Earth’s major ocean basins. This involves all the processes discussed so far. Together with the difficulty of gathering data here, this is still aqua incognita, so to speak: many questions remain and many details are sketchy. The other reason why the poles deserve attention is that they are, well, polar opposites. While the South Pole is an isolated landmass surrounded by water, the North Pole is a land-locked ocean partially covered with sea ice. Narrow passages such as the Bering Strait and the Barents Sea Opening are the only connection to global ocean circulation.

I found the observation and concept sections of each chapter reasonably accessible, and some of the illustrations particularly helpful, though I would have liked a glossary. Klinger & Haine define important terms, but I had to go back repeatedly to look up basic concepts such as geostrophic flow that remained abstract to me. As far as the book’s level is concerned then, this is an advanced-level text – personally, I would have benefitted from first reading an introductory oceanography textbook. Students will want to tackle this book in a classroom setting, especially once it gets to the theoretical sections. I read through these only to discover mathematical operators that I didn’t even know existed. One of the endorsements printed on the book mentions the option of self-study, though my impression is that this is the preserve of advanced students and professional oceanographers who are both familiar and comfortable with the mathematics and modelling used here.

“The planet’s poles deserve special attention […] the Southern Pole with its raging Antarctic Circumpolar Current connects to and interacts with all of Earth’s major ocean basins.”

The above is, of course, not a criticism of the book, but is good to know going in. Minor suggestions for improvements would be aforementioned glossary. Similarly, an online module with animations to help visualise movements and dynamic processes (such as was included with the CUP textbook Structural Geology) would be nifty, though I recognize the time, skill, and effort involved in creating these. No, what I really felt was missing was a final integrative chapter bringing together all the concepts that have been treated separately up to that point. Clearly, different modes of circulation dominate in different parts of the oceans, and at different times, but none of them occurs in isolation. So, what, ultimately, does ocean circulation in three dimensions look like? A short ocean-by-ocean walkthrough would have been instructive. That said, I certainly would not suggest ditching the current last chapter. Here, Klinger & Haine briefly explore the link between ocean circulation and the global climate, palaeoclimatology, and the possibility of abrupt climate change.

Ocean Circulation in Three Dimensions is one of the first books I know of to tackle this topic head-on. The authors explore competing models and explanations and are mindful to highlight that this is forever work-in-progress. Nevertheless, as an overview of our current state of knowledge, this book will be a valuable reference work for professionals. Similarly, the book’s structure and plentiful exercises make it a useful textbook for students, though they will probably need a classroom or graduate course setting to get the most out of it.

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Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.

Ocean Circulation in Three Dimensions hardback or ebook

Other recommended books mentioned in this review:

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