BackgroundThe Equatorial Undercurrent (EUC) is a vital component of the coupled ocean-atmosphere system in the tropical Pacific.

The details of its termination near the Galápagos in the eastern Pacific have an outsized importance to circulation and ecosystems.

Subject to diverse physical processes, the EUC is also a rigorous benchmark for global climate models (GCMs). Until very recently, repeat observations of the EUC in the Pacific suitable for model evaluation have been limited mainly to the moorings of the Tropical Atmosphere Ocean (TAO) array and underway measurements by cruises servicing them. Historical observations east of the terminus of the TAO array at 95°W are scarce and ad hoc.

This study compares the EUC in GCMs to the mean observations from the Repeat Observationsby Gliders in the Equatorial Region (ROGER) campaign—a fleet of autonomous underwatergliders measuring the EUC along 93°W and its subsequent encounter with the Galápagos.

NA13OAR4830216 NA13OAR4830216 NA13OAR4830216

The Pacific Equatorial Undercurrent in Three Generations ofGlobal Climate Models and Glider Observations

Kristopher B. Karnauskas1, Julie Jakoboski2, T. M. Shaun Johnston3, W. Brechner Owens4, Daniel L. Rudnick3, Robert E. Todd41 University of Colorado, Boulder, CO 80309–03112 MetOcean Solutions, Raglan, New Zealand3 Scripps Institution of Oceanography, La Jolla, CA 92093-02134 Woods Hole Oceanographic Institution, Woods Hole, MA 02543

Karnauskas, K. B., J. Jakoboski, T. M. S. Johnston, W. B. Owens, D. L. Rudnick, and R. E. Todd (2020) The Pacific Equatorial Undercurrent in Three Generations of Global Climate Models and Glider Observations. J. Geophys. Res.–Oceans, in revision.Rudnick, D. L., W. B. Owens, T. M. S. Johnston, K. B. Karnauskas, J. Jakoboski, and R. E. Todd (2020) The equatorial current system during the 2014-2016 El Niño as observed by underwater gliders, J. Phys. Oceanogr., in revision.Jakoboski, J. K., R. E. Todd, W. B. Owens, K. B. Karnauskas, and D. L. Rudnick, 2020: Bifurcation and Upwelling of the Equatorial Undercurrent West of the Galápagos Archipelago. J. Phys. Oceanogr., 50, 887–905.

Funding: NSF (OCE–1232971, OCE–1233282) and NOAA (NA13OAR4830216)

Key Points• Recent glider campaign offers unique opportunity to evaluate GCM simulations of equatorial circulation in key region for climate• GCM simulations of the EUC have improved, but a slow bias of ~36% remains in the eastern Pacific relative to ROGER• Details of the encounter of the EUC with the Galápagos are impactful; resolving them well in GCMs is demonstrably important

3. How does the latest generation of GCMs represent the Galápagos; is there any discernable impact of their islands on the EUC comparable to that revealed by ROGER?

Research Questions (and Answers!)

Walker Cell

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Figures 476

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Figure 1. Peak EUC velocity (m s–1) along the equatorial Pacific Ocean averaged across all CMIP3 478

models (red), CMIP5 models (blue) and CMIP6 models (black). Observations of Johnson et al. (2002) 479

are shown in gray circles with thin outlines and the observations by ROGER are shown by a gray 480

circle with a thick outline at 93°W. The dashed line at 91.7°W marks the westernmost shoreline of the 481

Galápagos Archipelago. See Fig. S1 for all profiles from each CMIP. 482

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Figure 2. Peak EUC velocity (m s–1) as a function of equatorial meridional resolution (degrees 484

latitude) of the ocean component of CMIP6 models at three exemplar longitudes (155°W [a], 125°W 485

[b] and 93°W [c]). The height of each black bar represents the mean peak EUC velocity from 486

models within that resolution bin ± 2 standard errors. The X represents the (single) model with 487

equatorial meridional resolution > 2°. The observed peak EUC velocity is indicated by gray bars in 488

each panel (155°W and 125°W from Johnson et al. [2002] and 93°W from ROGER), where the 489

width of each bar is equivalent to the estimated observational uncertainty except for 155°W where 490

only the mean value is presently available. 491

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Figure 7. Histograms of peak EUC velocity (m s–1) at 93°W in CMIP3 models (a), CMIP5 models (b) 513

and CMIP6 models (c). Observations by ROGER are indicated by the gray bar (width ± 0.07 m s–1). 514

In each panel, the multi-model mean is indicated by a vertical line. In the CMIP6 panel, the multi-515

model means for subsets of models with slow and fast biases are also indicated by thin black lines. 516

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Figure 9. Illustration of the minimum depth of islands or seamounts representing the Galápagos 523

Archipelago in CMIP6 models. The contoured field in the background is the CMIP6 multi-model 524

mean zonal velocity (m s–1) at 93°W, averaged between 0.5°S and 0.5°N. Contour interval 0.05 m s–1. 525

Galapagos Islands in CMIP60

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Figure 10. Plan view of zonal velocity (m s–1) in the eastern equatorial Pacific Ocean on the native 527

grids of four exemplar CMIP6 models: the Community Integrated Earth System Model (CIESM) from 528

Tsinghua University (Lin et al., 2019), the Institut Pierre-Simon Laplace (IPSL) Climate Model version 529

6A–Low Resolution (CM6A–LR; Boucher et al., 2020), the Model for Interdisciplinary Research on 530

Climate version 6 (MIROC6; Tatebe et al., 2019), and the NOAA/Geophysical Fluid Dynamics 531

Laboratory (GFDL) Coupled Model version 4 (CM4; Held et al., 2019). The depth shown is that of 532

the peak EUC velocity at 93°W, which varies slightly from model to model between 60 and 80 m. The 533

93°W section from ROGER and the equator are indicated by black lines. 534

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1a. Has there been an overall improvement in the simulation of the EUC, broadly but particularly in the eastern Pacific, across the latest three generations of GCMs?

1b. As a corollary, has the dependence of EUC bias on ocean model resolution identified in an earlier generation of GCMs persisted into the latest generation of GCMs?

• The CMIP3 multi-model mean severely underestimates the peak EUC velocity across the basin.

• Both the CMIP5 and CMIP6 multi-model ensembles show impressive improvement over CMIP3.

• The peak EUC velocity in CMIP5 (CMIP6) is 47% (56%) faster than in CMIP3, and the slow bias is eliminated entirely west of the dateline.

• Despite these marked improvements, the peak EUC in CMIP6 is still 33% shy of the observations.

• The distribution of peak EUC velocity simulated by GCMs within a given CMIP has changed. In CMIP6, the distribution of the 34 GCMs is bimodal; 23 models have a slow bias, only three models have negligible bias (within 0.1 m s–1 of ROGER), and 8 models have a fast bias.

• The strong dependence of peak EUC velocity on model resolution found previously for CMIP3 persists through CMIP6.

• The meridional resolution near the equator of the ocean component of a GCM is a good predictor of simulated EUC velocity.

• An equatorial ocean meridional resolution of about 0.33° still appears necessary (albeit not always sufficient) to obtain the observed peak EUC velocity.

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Figure 3. Mean zonal velocity (m s–1) along 93°W averaged across all CMIP3 models (a), CMIP5 493

models (b), CMIP6 models (c) and the observations by ROGER (d). Contour interval 0.05 m s–1 with 494

0 m s–1 denoted by white line. The maximum eastward velocity is indicated in the title of each panel. 495

(e)–(g), as in (c) but averaged across subsets of CMIP6 models with a slow bias, negligible bias (within 496

0.1 m s–1 of ROGER) and fast bias. 497

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Figure 5. Mean buoyancy frequency squared (N2, s–2, shading) and potential density (σ, kg m–3, black 502

contours every 0.5 kg m–3 with 25 kg m–3 denoted by thick line) along 93°W from ROGER (a). 503

Difference between CMIP6 multi-model mean N2 along 93°W and ROGER observations (b). For 504

reference to the EUC, zonal velocity is contoured in (b) at 0.1 and 0.3 m s–1 from the CMIP6 multi-505

model mean (white) and ROGER (black). 506

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2. How well do GCMs compare to ROGER observations of the EUC (including shear and stratification) in the eastern Pacific, just prior to the EUC encountering the Galápagos?

• The multi-model mean zonal velocity sections at 93°W from all three CMIPs present a clearly identifiable EUC, with a core south of the equator in CMIP6.

• Although there is clear improvement across the CMIPs, the EUC in most of the individual GCMs is substantially slower than the observed peak EUC velocity observed by ROGER at 93°W of 0.50 m s–1.

• The outstanding feature of the CMIP6 models with a fast EUC bias is not the EUC but the structure of the SEC. Particularly, the westward deep lobe of the SEC in the Northern Hemisphere evident in ROGER is only present in the CMIP6 models with a fast bias.

• The difficulty in properly simulating the shear between the EUC and SEC appears relevant to stratification biases near the equatorial front via barotropic instability.

• Vertical stratification in the CMIP6 multi-model mean is too strong above the EUC core from 2°S to 1°N, and too weak near the equatorial front between 1–2°N and across the EUC core (50–100 m).

• The weak bias of stratification (N2) is consistent with a weak bias of shear (∂u/∂z) from consideration of the Richardson number.

• These stratification biases are readily attributable to biases in potential temperature and salinity; the strong N2 bias south of 1°N is caused by too much temperature stratification, and the weak N2 bias north of 1°N is caused by too little salinity stratification.

• The generally higher resolution of ocean models in CMIP6 GCMs has finally afforded the Galápagos a nontrivial representation.

• Of the 34 CMIP6 models considered, 21 have a “Galápagos” island reaching the surface, 6 have a seamount terminating within the deep half of the EUC, and 7 have no island or seamount in the eastern equatorial Pacific Ocean above 300 m.

• The implementation of islands in an ocean model varies considerably between GCMs, generally beginning with a collection of grid points in the land-ocean mask designated as land with no-slip (and zero normal flow) lateral boundary conditions.

• Whether or not the island crosses (or even reaches) the equator matters greatly for reproducing ROGER observations.

• Ocean model grids should include an island crossing the equator, especially if the model simulates a realistic (or too strong) EUC, to avoid a host of new biases such as between the Galápagos and mainland South America.