Day 1 - March 12, 2025

Abstract:

In 1961, Klaus Wyrtki published a landmark study [1], in which he lays out the foundation of our modern understanding of the thermohaline circulation.  The only problem was that this study was largely ignored by the international oceanographic community, or in Klaus Wyrtki’s own words “Nobody at the East Coast of the United States cared about what someone from Australia had to say” (personal communication, 2011). This apparent bias, which is manifested also in the citation report to this study, has delayed research on this important issue, arguable, by several decades. Klaus Wyrtki’s fundamental study on the thermohaline circulation, or Atlantic Meridional Overturning Circulation (AMOC), as we call it now, is a treasure trove of ideas and insights.  Using observational data, theoretical and heuristic arguments, the study highlights – among many other things – the role of Southern Ocean upwelling as a key component of the AMOC; an idea that was later popularized through Robby Toggweiler’s very influential study on the effect of Drake passage on the AMOC [2]. Furthermore, Klaus Wyrtki’s multi-layer model of the AMOC  predicts a total mass transport at 45oN of 10 Sv [1 Sv=106 m3/s]. Modern day observations of the RAPID-MOCHA array and of ocean reanalysis datasets suggest an amplitude of about 14-22 Sv-26.5oN with substantial variations on interannual to multidecadal timescales. When comparing the schematics of modern AMOC review papers (e.g., [3]) with Figure 4 in [1], it is striking to see that almost all of the main components of our modern-day understanding of relevant AMOC processes are already described in detail in Klaus Wyrtki’s seminal 1961 paper. One may wonder, what would have happened to the research on this important topic, if the oceanographic community had read Klaus Wyrtki’s paper and recognized its relevance already in the 1960s.

In 1961– the same year as [1]  - Henry Stommel wrote an interesting conceptual paper [4] about the potential bi-stability of a thermohaline frictionally controlled flow between two boxes. Also, this paper’s relevance lay dormant for at least 2 decades, until the first GFDL model simulations by Manabe and Stouffer in 1988 [5] documented evidence for a bistable AMOC. This match between CGCM and box model triggered a world-wide quest to understand the stability and variability of the AMOC. Stommel’s highly idealized non-rotational model and his proposed salt feedback became immediately a key paradigm in the abrupt climate change community and inspired numerous studies on the issue of AMOC-related tipping points and Dansgaard-Oeschger Events. It is probably fair to say, that we know now more about the instability and variability of the AMOC, than we know about its longterm mean state and the underlying physical processes.

In 2007 a team of oceanographers from the International Pacific Research Center at the University of Hawaii assembled to develop a more comprehensive dynamical understanding of how the really AMOC operates. Together with Prof. Julian McCreary, Assistant Researcher Dr. Ryo Furue and PhD student Fabian Schloesser, we focused on elucidating the basic mechanisms of the combined 3-dimensional thermohaline and wind-driven flow of the North Atlantic. Our main research findings from this study [6] will be presented in this presentation and contextualized with earlier research. I will in particular highlight how - in the presence buoyance forcing  - reduced gravity wave dynamics conspires with diapycnal processes to create a 3 dimensional flow that captures all the main features of the observed AMOC, including the 2 dimensional structure - already known to Klaus Wyrtki more than 60 years ago.

This presentation is an homage to one of the greatest minds in the field of oceanography, as well as tale of the dangers and threats of scientific ignorance and bias.

References:

[1] Klaus Wyrtki, The thermohaline circulation in relation to the general circulation in the oceans, Deep-Sea Research, 8, 39-64. (1961). https://www.sciencedirect.com/science/article/pii/0146631361900144

[2] J.R. Toggweiler, B. Samuels, Effect of Drake passage on the global thermohaline circulation, Deep Sea Research Part I: Oceanographic Research Papers 42 (4 ), 477-500 (1995).

[3] Kuhlbrodt, T., A. Griesel, M. Montoya, A. Levermann, M. Hofmann, and S. Rahmstorf,  On the driving processes of the Atlantic meridional overturning circulation, Rev. Geophys., 45, RG2001, doi:10.1029/2004RG000166, (2007).

[4] Henry Stommel, Thermohaline Convection with Two Stable Regimes of Flow". Tellus. 13 (2): 224–230 (1961).

[5] Manabe and Stouffer, S. Manabe and R. J. Stouffer, Two Stable Equilibria of a Coupled Ocean-Atmosphere Model, J. Climate 1, 841-866 (1988).

[6] F. Schloesser, R. Furue, J.P. McCreary, A. Timmermann, Dynamics of the Atlantic meridional overturning circulation. Part 1: Buoyancy-forced response, Progress in Oceanography, Progress in Oceanography,  120, 154-176 (2014).

Abstract:

Forty years ago, Klaus Wyrtki concluded that “The main result of this analysis of sea level fluctuations in the Pacific during the 1982–83 El Niño is recognition of the very large, basin-wide coherence of sea level fluctuations and of their long time scales.” His insight—that sea levels respond dynamically to changes in the wind field—became foundational to using data from tide gauges and satellite altimetry missions for monitoring the oceanic response to climate variability. Moreover, it provided a benchmark for testing the performance of climate models by comparing the simulated and observed sea levels. Since his publication in Geophys. Res. Lett. (1985), significant progress has been made observing and modeling sea level variability in the tropical Pacific, which has led to a deeper understanding of the El Niño-Southern Oscillation (ENSO) and its impacts. One of the most transformative advancements has been the development of skillful ENSO forecasts, which enable predictions of sea level changes months-to-seasons in advance. This capability builds directly on Wyrtki’s conclusions about ocean dynamics during the 1982–83 El Niño, including about the meridional asymmetry of sea level anomalies that he observed in the western Pacific basin. We will revisit how seasonal forecasting systems capture the sea level patterns Wyrtki described and assess the current state of sea level prediction in the tropical Pacific. Finally, we will explore the challenges and opportunities for enhancing forecasts using tide gauge observations, a legacy of Wyrtki’s pioneering work.

Abstract:

Theory for the steady, wind-driven ocean circulation predicts upwelling at the equator under easterly winds, easily confirmed by observations of temperature and chlorophyll. The time-mean upwelling in the eastern equatorial Pacific is critical to the surface heat balance and the global carbon cycle and has been proposed to play a key role in shaping the response of the tropical Pacific to anthropogenic radiative forcing. Estimating the time-mean of any variable in a noisy system from direct measurements, not to mention one at the limit of measurement uncertainty like vertical velocity, is fraught with challenges. Since the 1960s, oceanographers have attempted to circumvent the latter problem by applying the continuity equation to measured horizontal divergence. A systematic review of published estimates reveals a remarkable range explained by differences in sampling strategy, leaving one unable to reject the highest of the range. After accounting for sampling, even Wyrtki’s (1981) estimated 1 m/day is consistent with both extrema. Careful examination of modern observations and techniques indicates a time-mean upwelling in the equatorial Pacific of 13.1 ± 6.9 m/day—an order of magnitude faster than previously assumed and with a very sharp equatorial peak. The latest generation of climate models simulates time-mean upwelling velocities of ~3.1 m/day, the spread of which is well explained by resolution and the remaining offset is probably related to parameterizations. It is proposed that the relatively slow simulated upwelling contributes to the mismatch between simulated and observed trends in the equatorial Pacific with important implications for future climate projections.

Abstract:

Balancing the volume, heat, and salt budgets in the equatorial Pacific Ocean is critical for understanding interannual variability in the upper ocean. Dubbed the “Wyrtki Challenge,” the problem of quantifying the contributions of upwelling, horizontal transport, and surface fluxes to regional budgets remains a priority for the Tropical Pacific Observing System. A recent data-assimilation product, the Tropical Pacific Ocean State Estimate, provides an opportunity to address this challenge. Budgets are analyzed between 5S and 5N in the upper 300 m, inside a box defined to represent the central and eastern equatorial Pacific. Transports through the faces of this box are quantified to understand the processes responsible for variability in box-mean properties. The onset and recovery of the 2015/2016 El Niño event is found to be dominated by anomalous surface fluxes and horizontal advection. During the onset phase, weaker trade winds cause the shallow meridional overturning circulation to slow down, which reduces the poleward transport of heat and leads to upper ocean warming. Anomalous precipitation and advection of fresh water from the western Pacific drive the net freshening of the region. Relaxation from El Niño conditions is dominated by wind-driven meridional advection at 5N. As the meridional advection regains strength, Ekman advection efficiently exports the warm, fresh surface water out of the equatorial region. Quantifying the heat and salt transport changes in response to wind variability strengthens our understanding of global ocean heat transport.

Abstract:

The warmest year on record was 2023 and 2024 is on track to be even warmer. This record warmth was accompanied by extraordinary weather and climate extremes around the globe including historic droughts and floods, intense marine heatwaves, and widespread coral bleaching. Coincidentally, after nearly a decade of near-neutral or unusually cold conditions in the tropical Pacific, an El Niño that ranked among the strongest of the past 75 years began to develop in the boreal spring of 2023 and lasted through the spring of 2024. This presentation will discuss three ways in which El Niño Southern Oscillation (ENSO) and climate change have conspired to produce the spate of extremes that have occurred in the past two years and what this bodes for the future.

Abstract:

Sharp and rapid changes in the sea surface temperature (SST) associated with fronts and the diurnal cycle can drive changes in the atmospheric boundary-layer stability and circulation. Here we show how a one-dimensional surface ocean model forced with either high-resolution or daily averaged surface fluxes can be used to distinguish diurnal versus frontal SST anomalies observed from an uncrewed surface vehicle. The model, forced with daily satellite fluxes, shows that the diurnal warming is largest within the equatorial Pacific cold tongue of SST. The strong persistent SST front north of the cold tongue is evident in both the oceanic and atmospheric boundary-layer stability scales and, as a consequence, in the magnitude of the diurnal ocean warming. Using SST, barometric pressure and surface wind measurements from moorings at 0°, 95° W and 2° N, 95° W, we show that the front in the SST diurnal warming results in a weakened SST front in the afternoon and a corresponding reduced meridional gradient in the barometric pressure that appears to contribute to a diurnal pulsing of the surface meridional winds. To the extent that these modulate the surface branch of the Hadley cell, these diurnal variations may have remote impacts.

Abstract:

In 1962 Wyrtki assessed the biological and physical processes that lead to the formation of oxygen minima throughout the world’s oceans. Here we assess the processes by which the Pacific Ocean is ventilated: from the initial oxygen concentration and saturation state during water mass formation, modification along circulation pathways, and the eventual end of pathways in the oxygen deficient zones of the Eastern Tropical Pacific. Mode waters that are high in oxygen form in sub-polar winter waters, when intense heat loss and high winds drive deep mixing and high rates of gas exchange. As these mode waters transit the ocean interior, the densest varieties bring oxygen equatorward, losing oxygen to mixing (~60%) and respiration (~40%) along the way. Finally, we are able to define the extent of oxygen deficient zones using a combined float and ship dataset, finding evidence of significant changes between 2000 and 2020.