KLAUS WYRTKI AND THE MODERN ERA OF THE MARITIME CONTINENT OCEANOGRAPHY

The December 2019 issue of Marine Research in Indonesia on “Oceanography of the Indonesian Seas” is dedicated to Klaus Wyrtki. In many ways, Klaus Wyrtki’s contribution to the study of the Indonesian seas served as a smooth transition from the era of the great expeditions such as the Dutch Willebrord Snellius Expedition (1929-1930; see Wüst, 1964; van Aken, 2005) of the early and mid-20th century to the modern era. In the NAGA Report (Wyrtki, 1961; also see Wyrtki, 2005), Klaus used existing ocean observations, supplemented with data he helped collect on the Indonesian research ship Samudera during his sojourn in Indonesia (1954-1957) as Head of the Institute of Marine Research in Jakarta to map out a broad view of the Southeast Asian waters (the waters of the Maritime Continent including the Indonesian seas). He presented the geography, the configuration of the seas and basins of the Southeast Asian waters, the surface circulation, and governing dynamics, including the tides and monsoonal driven seasonality, and the temperature/salinity surface layer patterns, as well as aspects of the subsurface stratification. The Plates 1-44 beautifully reveal the oceanographic condition of the Southeast Asian waters as resolved by the pre-1960 observations. As Klaus Wyrtki says in the NAGA Report Preface: “It is hoped that workers in the region, whether in oceanography or other branches of science may find it a source of information and a stimulus to undertake further research in these waters” and “The scientific publications dealing with this region show not so much a lack of observations as a lack of an adequate attempt to synthesize these results to give a comprehensive description of the region.” “I soon decided to devote most of my time during my three years’ stay in Indonesia to the preparation of a general description of the oceanography of these waters.” He succeeded.


THE INDONESIAN SEAS AND ITS THROUGHFLOW
The Indonesian seas are at the center of the ascending branch of the atmospheric Walker Circulation, a critical component of the El Niño-Southern Oscillation (ENSO) and Asian monsoon systems. The complex array of basins and passages provide an oceanic pathway for tropical Pacific water masses to reach into the Indian Ocean, representing only tropical interocean link, enabling the Indonesian Throughflow (ITF) − a key component of the larger-scale ocean and climate systems (Gordon, 2005;Gordon et al., 2010a;Sprintall et al., 2014). Energetic wind and tides elevate the mixing within the Indonesian seas, boosting vertical heat, freshwater and nutrient fluxes, with impact on the climate and ecosystem Gordon, 1992, 1996;Gordon et al., 2010a;Koch-Larrouy, 2010;Ray and Susanto, 2016). The ITF varies across a wide range of time scales, from intraseasonal to seasonal (monsoonal) and interannual (ENSO), reaching to century and longer time scales, and will likely play a key role in the nature of the changing climate and marine ecosystem for the Maritime Continent and larger regional to global scale.
Beginning in the 1980s, there has been increasing interest in the oceanography of the Indonesian seas, as its global significance was more widely recognized (Gordon, 1986a,b). There was the Lombok Strait research of Murray and Arief (1988) and Arief and Murray (1996); the Snellius II Expedition 1984-1985 focused on the eastern Indonesian seas (van Aken et al., 1988;van Bennekom, 1988). The Arlindo 1993-1994, mapped the patterns of the ITF during winter and summer monsoon (Gordon and Fine, 1996;Ilahude and Gordon, 1996) Gordon et al. (2010b), serves as an excellent overview of recent progress and points the way for continued observational and model-based research. The first comprehensive simultaneous measurements of the velocity and property variability in the major inflow and outflow ITF passages were collected over 3 years (2004)(2005)(2006) as part of the International Nusantara Stratification and Transport (INSTANT) program. INSTANT measurements provided an estimate of ITF transport about 30% higher than estimates made from non-simultaneous measurements prior to INSTANT. The observations also confirmed the Arlindo results that the strongest velocity, the velocity-maximum (V max ), was not within the warm sea layer, but within the cooler upper thermocline. There was the extensive INDOMIX program in the eastern seas (Koch-Larrouy et al., 2010, 2015, and time series of the inflow to the eastern seas via the Maluku Channel (Yuan et al., 2018).
As mentioned above, the longest, sustained monitoring of the ITF is from Makassar Strait. The Makassar Strait throughflow  of ~12 Sv, representing ~80% of the total ITF, displays fluctuations over a broad range of time scales, from intraseasonal (Madden Julian Oscillations, Rossby and Kelvin Waves; Napitu et al., 2019;Pujiana et al., 2019) to seasonal and interannual (ENSO) scales. We now have 13.3 years of Makassar throughflow: November 1996 -early July 1998, January 2004 -August 2011, and August 2013 -August 2017. The next mooring rotation may take place in mid to late 2019. Southward transport displays a strong seasonal signal that is strongest in boreal summer as well as interannual variability that scales roughly to ENSO with the weak southward flow with a deeper velocity-maximum during El Niño and stronger southward flow with shallower V max during La Niña. The southward flow relaxed in 2014 and more so in 2015/16, similar though not as extreme as during the El Niño event of 1997. In summer 2017, there is a return to the non-El Niño state. Since 2016, the deep layer 300-760 m (Makassar Strait sill depth is ~680 m) southward transport increases, almost doubles to ~7.5 Sv. From mid-2016 into early 2017, the transport above 300 m and below 300 m is about equal, where they usually have a ratio of 2:1. In early 2017, the total Makassar transport increases to 'historical' highs of over 20 Sv.
Observing, as well as to simulate within numerical models the complex oceanography of the Indonesian seas is a challenge must be met so as to better predict large scale ocean and climate conditions, as well as regional oceanography within the Indonesian seas. Coordinated international collaborations will move us forward. Sprintall et al. (2019) provide in the section "Final Thoughts and Recommendations" ideas for future observations using new technologies, stating: "So, while the convoluted bathymetry of the Indonesian seas means that moorings will likely continue to be the workhorse of any backbone transport array for the near future, our sustained array will propose a multi-platform approach including additional surveys, process studies and new technology that will go some ways to fill in the gaps that cannot be accomplished through moored arrays alone."

KLAUS WYRTKI CONTRIBUTION
Klaus Wyrtki has made broad, insightful contributions to our understanding of the global ocean (see the appended Biographic Summary and Publication Citations from Google Scholar). Many know of Klaus for his groundbreaking ENSO research, but his reach is far greater. His 1961 article on ocean thermohaline circulation and his 1962 article on the ocean oxygen minimum relationship to circulation, are remarkable insightful studies, trailblazers. I encourage the new generation of oceanographers to read them. His work on the tropical ocean goes beyond that directly related to ENSO. Of particular note is his work on the oceanography of the Maritime Continent. Even before his famous NAGA Report of 1961, he published 6 papers on the Southeast Asian waters between 1956 and 1960. In 1971, he published the "Oceanographic Atlas of the International Indian Ocean Expedition," as well as identifying an equatorial jet in the Indian Ocean (Wyrtki, 1973)  Quotes from the interview that are worth remembering: "Regarding the causes of scientific progress: funding, opportunity, people and coincidence. Gelegenheit ist Zufall (Opportunity is chance; ALG: I call this serendipity), it is certainly not planned. The progress in science, I do not think is planned. It happens when certain problems are ripe for a solution. Most people will say that progress in the sciences happens through logical thinking. This is certainly an important ingredient, but I strongly believe that most progress is due to imagination and intuition, much like art is being created. Logical thinking and experimentation are of course very important in confirming and solidifying the ideas born by intuition and imagination." Klaus heard of a job opening in Indonesia. He says: "I wrote to Indonesia, a few months later I was on the way to Indonesia. This went all pretty easy. When I arrived in Indonesia, they were phasing out the Dutch at that time and they were looking for other people. Since Germany had no colonial attachments, we were somewhat welcome in these countries. In Indonesia, I found myself not only the only scientist in the institute because all the Dutch had left, but I was also the director of it. I had a research vessel of about 200 tons, a nice yacht type vessel, the Samudera. I made many voyages with it, with very little instrumentation. We did a few surveys with Nansen bottles down to a few hundred meters but could not reach the deep-sea basins in Indonesia because of a lack of a long wire, and that restricted us to the surface layers. I discovered there was a lot of actual information about these waters that had never been summarized. I started to work on a book, the physical oceanography of the Southeast Asian waters; it became known as the NAGA Report later on when it was published at Scripps. I wrote that book on many long voyages through the Indonesian waters." When analyzing the data from both the Dana and the Snellius expeditions, the Snellius expedition was not completely published by that time. I could analyze existing sea level data, I could make dynamic calculation, both in the Pacific and in the Indian Ocean. I could identify the fact that there was a pressure difference between the two. I analyzed surface circulation, which indicated that there was a monsoon dependent throughflow. That was the start of that type of research.
I found this quote to be particularly interesting: "What came nearest to a book was the NAGA Report, which you may call a monograph; also the Indian Ocean Atlas is a big piece of work. I intended to write a book with the title "The Water Masses and Circulation of the Indian Ocean," and I gave it up since it takes about five to six years to write and by that time much of the information is superseded by new knowledge. Knowledge is accumulating these days at a rate that you can say after a decade, things are old. That's too short a lifetime for a book." Klaus Wyrtki said in the 1999 interview: "At Scripps, I would belong to a tuna research program that stretched all the way from California to Peru, throughout the eastern tropical Pacific investigating the environment of the tuna population. At Columbia, I would be assigned to a new research ship, the Eltanin, and I would go into the Antarctic Ocean. Arnold Gordon eventually got the job, because I said, no, no. No Antarctic Ocean, no seasickness, no roaring forties, I stay in the tropics." After Indonesia I was spoiled, I didn't want to go back to the cold climate, so Scripps institution won. "Wüst was disappointed, of course, but he got Arnold Gordon. That was fine." I thank Klaus Wyrtki for opening up an opportunity for my career. I find it interesting that after my Southern Ocean work, I too moved to the warmer climate of Indonesia. Klaus was right, as usual.
In the 1999 interview, Klaus says: "You asked what I learned from Wüst. It's basically the general overview, to look at large connections, not at the details, but to integrate things, to see the big picture. Somehow that was the message I too received from Wüst. Though I must say, I am glad that many oceanographers do closely look at the details.

CONCLUDING COMMENTS
Klaus Wyrtki: "I have no regrets about the things I have done. I have enjoyed the scientific career that I have made. I would do the same thing, it may not turn out the same way because we are subject to chance, you know, but basically I would do the same. I agree -Well Done, Klaus! Born: February 7, 1925in Tarnowitz, Germany Married. Children: daughter born 1954son born 1962Naturalized U.S. citizen, January 5, 1977 Education University of Marburg, Germany, 1945-48 Mathematics, physics, geography University of Kiel, Germany, 1948-1950Oceanography, physics, mathematics May 20, 1950