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An isolated, volcanic archipelago at the confluence of several major ocean currents, the Galápagos Archipelago (GA) is among the most biologically diverse places on Earth. There remain many open questions concerning evolution and speciation in the GA, with the details of the geologic formation of the islands over the past millions of years representing a key source of uncertainty. Paleoceanographic sea surface temperature (SST) proxy records from the far eastern equatorial Pacific (EEP) indicate that the modern gradient of SST across the GA (the cross-island SST gradient, or CIΔT) emerged relatively abruptly ∼1.6 Ma. As the modern CI is the result of a blockage and subsequent upwelling of the Equatorial Undercurrent (EUC) by the GA, we infer from these paleoceanographic data that the modern period during which the GA is arranged such that the islands constitute a significant topographic barrier to the EUC began ∼1.6 Ma. An extensive suite of ocean circulation model experiments—new and previously published—confirms that the sign and magnitude of the change in CI captured in paleoceanographic records can be explained by the islands impinging upon the EUC. Implications for the geologic history of the Galápagos and related biogeographical questions are discussed. Additionally, these results suggest that investigations of the Pan-Pacific SST gradient (PPΔT) should use one of the available proxy sites in the EEP that is not influenced by regional, geologically forced oceanographic changes; such an analysis supports recent suggestions of a more gradual development of the modern over the Plio-Pleistocene.

Karnauskas, K. B., E. Mittelstaedt, and R. Murtugudde, 2017: Paleoceanography of the eastern equatorial Pacific over the past 4 million years and the geologic origins of modern Galápagos upwelling. Earth Planet. Sci. Lett., 460, 22–28, doi: 10.1016/j.epsl.2016.12.005.

Contrary to the above title, a more typical question might be ‘How does climate variability and change impact atolls (and associated ecosystems and civilizations)?’ While several studies have focused on the effects of island topography on local circulation, equatorial currents and their dynamics play a vital role in the tropical heat balance and therefore global climate through atmospheric teleconnections. The Gilbert Islands, which straddle the equator in the western Pacific, block 55% of the corridor between 2°S and 2°N. Here we explore the potential role of relatively small open-ocean topographic features, in particular equatorial atolls such as the Gilbert Islands, on the large-scale climate system. Observations and high-resolution ocean model simulations spanning the full width of the Pacific basin indicate that, whereas the Gilbert Islands have only a local impact on the Equatorial Undercurrent (EUC), the South Equatorial Current (SEC) is substantially reduced downstream of the islands, resulting in a surface warming and freshening over a ∼1000-km wide region along the sharp temperature and salinity front defining the eastern edge of the Indo-Pacific warm pool (IPWP), the heat engine that is a critical player in global climate processes. The simulated mean thermohaline structure in the central and western Pacific, which has previously been shown to be crucial for El Nino-Southern Oscillation (ENSO) dynamics, is improved by including the Gilbert Islands. Implications for coupled model biases, ENSO simulation and paleoclimate are discussed.

Karnauskas, K. B., G. C. Johnson, and R. Murtugudde, 2016: On the climate impacts of atolls in the central equatorial Pacific. Int. J. Climatol., doi: 10.1002/joc.4697.

The NOAA Tropical Atmosphere Ocean (TAO) moored array has, for three decades, been a valuable resource for monitoring and forecasting El Niño–Southern Oscillation and understanding physical oceanographic as well as coupled processes in the tropical Pacific influencing global climate. Acoustic Doppler current profiler (ADCP) measurements by TAO moorings provide benchmarks for evaluating numerical simulations of subsurface circulation including the Equatorial Undercurrent (EUC). Meanwhile, the Sea Education Association (SEA) has been collecting data during repeat cruises to the central equatorial Pacific Ocean (160°–126°W) throughout the past decade that provide useful cross validation and quantitative insight into the potential for stationary observing platforms such as TAO to incur sampling biases related to the strength of the EUC. This paper describes some essential sampling characteristics of the SEA dataset, compares SEA and TAO velocity measurements in the vicinity of the EUC, shares new insight into EUC characteristics and behavior only observable in repeat cross-equatorial sections, and estimates the sampling bias incurred by equatorial TAO moorings in their estimates of the velocity and transport of the EUC. The SEA high-resolution ADCP dataset compares well with concurrent TAO measurements (RMSE = 0.05 m s−1; R2 = 0.98), suggests that the EUC core meanders sinusoidally about the equator between ±0.4° latitude, and reveals a mean sampling bias of equatorial measurements (e.g., TAO) of the EUC’s zonal velocity of −0.14 ± 0.03 m s−1 as well as a ~10% underestimation of EUC volume transport. A bias-corrected monthly record and climatology of EUC strength at 140°W for 1990–2010 is presented.

Leslie, W. R., and K. B. Karnauskas, 2014: The Equatorial Undercurrent and TAO sampling bias from a decade at SEA. J. Atmos. Oceanic Technol., 31(9), 2015–2025, doi: 10.1175/JTECH-D-13-00262.1. Data doi: 10.1575/1912/6746.

Leslie, W. R., K. B. Karnauskas, and J. H. Witting, 2014: CORRIGENDUM. J. Atmos. Oceanic Technol., 31(12), 2871–2871, doi: 10.1175/JTECH-D-14-00187.1.

Several recent studies utilizing global climate models predict that the Pacific Equatorial Undercurrent (EUC) will strengthen over the twenty-first century. Here, historical changes in the tropical Pacific are investigated using the Simple Ocean Data Assimilation (SODA) reanalysis toward understanding the dynamics and mechanisms that may dictate such a change. Although SODA does not assimilate velocity observations, the seasonal-to-interannual variability of the EUC estimated by SODA corresponds well with moored observations over a ~20-yr common period. Long-term trends in SODA indicate that the EUC core velocity has increased by 16% century−1 and as much as 47% century−1 at fixed locations since the mid-1800s. Diagnosis of the zonal momentum budget in the equatorial Pacific reveals two distinct seasonal mechanisms that explain the EUC strengthening. The first is characterized by strengthening of the western Pacific trade winds and hence oceanic zonal pressure gradient during boreal spring. The second entails weakening of eastern Pacific trade winds during boreal summer, which weakens the surface current and reduces EUC deceleration through vertical friction. EUC strengthening has important ecological implications as upwelling affects the thermal and biogeochemical environment. Furthermore, given the potential large-scale influence of EUC strength and depth on the heat budget in the eastern Pacific, the seasonal strengthening of the EUC may help reconcile paradoxical observations of Walker circulation slowdown and zonal SST gradient strengthening. Such a process would represent a new dynamical “thermostat” on CO2-forced warming of the tropical Pacific Ocean, emphasizing the importance of ocean dynamics and seasonality in understanding climate change projections.

Drenkard, E. J., and K. B. Karnauskas, 2014: Strengthening of the Pacific Equatorial Undercurrent in the SODA ocean reanalysis: Mechanisms, ocean dynamics, and implications. J. Climate, 27(6), 2405–2416, doi: 10.1175/JCLI-D-13-00359.1.

[1] The biological response in the western equatorial Pacific Ocean during El Niño/La Niña transitions and the underlying physical mechanisms were investigated. A chlorophyll a bloom was observed near the Gilbert Islands during the 2010 El Niño/La Niña transition, whereas no bloom was observed during the 2007 El Niño/La Niña transition. Compared to the previously observed bloom during the 1998 El Niño/La Niña transition, the 2010 bloom was weaker, lagged by 1–2 months, and was displaced eastward by ~200 km. Analysis suggested that the occurrence, magnitude, timing, and spatial pattern of the blooms were controlled by two factors: easterly winds in the western equatorial Pacific during the transition to La Niña and the associated island mass effect that enhanced vertical processes (upwelling and vertical mixing), and the preconditioning of the thermocline depth and barrier layer thickness by the preceding El Niño that regulated the efficiency of the vertical processes. Despite the similar strength of easterly winds in the western equatorial Pacific during the 1998 and 2010 transitions to La Niña, the 2009–2010 El Niño prompted a deeper thermocline and thicker barrier layer than the 1997–1998 El Niño that hampered the efficiency of the vertical processes in supplying nutrients from the thermocline to the euphotic zone, resulting in a weaker bloom.

Gierach, M. M., M. Messié, T. Lee, K. B. Karnauskas, and M.–H. Radenac, 2013: Biophysical Responses near Equatorial Islands in the Western Pacific Ocean during El Niño/La Niña Transitions. Geophys. Res. Lett., 40(20), 5473–5479, doi: 10.1002/2013GL057828.

The Equatorial Undercurrent (EUC) is a major component of the tropical Pacific Ocean circulation. EUC velocity in most global climate models is sluggish relative to observations. Insufficient ocean resolution slows the EUC in the eastern Pacific where nonlinear terms should dominate the zonal momentum balance. A slow EUC in the east creates a bottleneck for the EUC to the west. However, this bottleneck does not impair other major components of the tropical circulation, including upwelling and poleward transport. In most models, upwelling velocity and poleward transport divergence fall within directly estimated uncertainties. Both of these transports play a critical role in a theory for how the tropical Pacific may change under increased radiative forcing, that is, the ocean dynamical thermostat mechanism. These findings suggest that, in the mean, global climate models may not underrepresent the role of equatorial ocean circulation, nor perhaps bias the balance between competing mechanisms for how the tropical Pacific might change in the future. Implications for model improvement under higher resolution are also discussed.

Karnauskas, K. B., G. C. Johnson, and R. Murtugudde, 2012: An equatorial ocean bottleneck in global climate models. J. Climate, 25(1), 343–349, doi: 10.1175/JCLI-D-11-00059.1.

Although sustained observations yield a description of the mean equatorial current system from the western Pacific to the eastern terminus of the Tropical Atmosphere Ocean (TAO) array, a comprehensive observational dataset suitable for describing the structure and pathways of the Equatorial Undercurrent (EUC) east of 95°W does not exist and therefore climate models are unconstrained in a region that plays a critical role in ocean–atmosphere coupling. Furthermore, ocean models suggest that the interaction between the EUC and the Galápagos Islands (∼92°W) has a striking effect on the basic state and coupled variability of the tropical Pacific. To this end, the authors interpret historical measurements beginning with those made in conjunction with the discovery of the Pacific EUC in the 1950s, analyze velocity measurements from an equatorial TAO mooring at 85°W, and analyze a new dataset from archived shipboard ADCP measurements. Together, the observations yield a possible composite description of the EUC structure and pathways in the eastern equatorial Pacific that may be useful for model validation and guiding future observation.

Karnauskas, K. B., R. Murtugudde, and A. J. Busalacchi, 2010: Observing the Galápagos–EUC interaction: Insights and challenges. J. Phys. Oceanogr., 40(12), 2768–2777, doi: 10.1175/2010JPO4461.1.

A reduced-gravity ocean general circulation model of the tropical Pacific Ocean is used to determine potential improvements to the simulated equatorial Pacific cold tongue region through choices in horizontal resolution and coastline geometry—in particular, for the Galápagos Islands. Four simulations are performed, with identical climatological forcing. Results are compared between model grids with and without the Galápagos Islands, with coarse and fine resolutions. It is found that properly including the Galápagos Islands results in the obstruction of the Equatorial Undercurrent (EUC), which leads to improvements in the simulated spatial structure of the cold tongue, including a basinwide warming of up to 2°C in the east-central Pacific. The obstruction of the EUC is directly related to the improvements east of the Galápagos Islands, and for the basinwide reduction of the tropical cold bias through an equatorial dynamical adjustment. The pattern of SST warming resulting from the inclusion of the Galápagos Islands is similar to that of the known cold biases in ocean models and the current National Oceanic and Atmospheric Administration Climate Forecast System. It is thought that such an improvement would have a considerable impact on the ability of coupled ocean–atmosphere and ocean–ecosystem models to produce realistic clouds, precipitation, surface ocean bioproductivity, and carbon cycling in the tropical Pacific Ocean.

Karnauskas, K. B., R. Murtugudde, and A. J. Busalacchi, 2007: The effect of the Galápagos Islands on the equatorial Pacific cold tongue. J. Phys. Oceanogr., 37(5), 1266–1281, doi: 10.1175/JPO3048.1.