(C) PLOS One
This story was originally published by PLOS One and is unaltered.
 . . . . . . . . . .



Whither warming in the Galápagos? [1]

['Kristopher B. Karnauskas', 'Department Of Atmospheric', 'Oceanic Sciences', 'University Of Colorado Boulder', 'Boulder', 'Co', 'United States Of America', 'Cooperative Institute For Research In Environmental Sciences']

Date: 2022-11 

Abstract The Galápagos Islands play host to an iconic ecosystem—a UNESCO World Heritage Site and the second largest marine reserve in the world, home to several endangered species. The waters off the west coast of the Galápagos are also one of the few places in the world ocean that are presently cooling, with potentially significant ecological consequences of this local reprieve from global warming. Here I show, using a recently developed high-resolution ocean state estimate, that the observed cooling in the Galápagos is the result of a strengthening of the wind-driven equatorial ocean circulation. An acceleration and shift of the Equatorial Undercurrent, which can be attributed to a strengthening of the cross-equatorial component of the trade wind in response to the interhemispheric gradient in surface warming, leads to a 54% increase in upwelling velocity along the western Galápagos Islands as well as increased shear-induced mixing. Analogous to other so-called “cold blobs,” such as the one south of Greenland in the North Atlantic, this is an early and important sentinel of a broader change in the tropical ocean circulation. Thus far, and for perhaps the very near future, the western shores of the Galápagos appear to offer refuge from some of the deleterious impacts of anthropogenic climate change including suppressed upwelling and surface warming.

Citation: Karnauskas KB (2022) Whither warming in the Galápagos? PLOS Clim 1(9): e0000056. https://doi.org/10.1371/journal.pclm.0000056 Editor: Andrea Storto, National Research Council, ITALY Received: January 24, 2022; Accepted: July 28, 2022; Published: September 7, 2022 Copyright: © 2022 Kristopher B. Karnauskas. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: All three primary data sets used in this study are freely and publicly available. The NOAA OIv2 SST data set is available for download from NOAA: https://www.esrl.noaa.gov/psd/data/gridded/data.noaa.oisst.v2.highres.html GLORYS-12v1 fields are available for download from the Copernicus Marine Service: https://resources.marine.copernicus.eu/?option=com_csw&view=details&product_id=GLOBAL_REANALYSIS_PHY_001_030 ERA5 fields are available for download from the Copernicus Climate Change Service: https://cds.climate.copernicus.eu/#!/search?text=ERA5&type=dataset High-resolution coastlines plotted in Figs 1, 2, 3, 4 and 6 are from the Global Self-consistent, Hierarchical, High-resolution Geography (GSHHG) database version 2.2.3, available for download from NOAA: https://www.ngdc.noaa.gov/mgg/shorelines/gshhs.html. Funding: K.B.K. acknowledges support from the NASA Sea Level Change Science Program, Award 80NSSC20K1123. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: I have read the journal’s policy and the authors of this manuscript have the following competing interests: K.B.K. is a member of the editorial board of PLOS Climate.

Introduction There are only a few places in the world ocean where the surface temperature is not increasing, and rightfully, they tend to be foci of scientific intrigue. The high latitude North Atlantic Ocean is one of them, where an observed cooling trend may be the fingerprint of a slowdown of the Atlantic Meridional Overturning Circulation [1, 2]. The Galápagos is another [3, 4], and despite its iconic marine ecosystem hosting several endangered species including the Galápagos penguin [5], Galápagos fur seal [6] and Galápagos sea lion [7], its status as a “living museum and showcase of evolution” according to the UNESCO World Heritage Convention [8], and its conspicuous placement within the eastern equatorial Pacific Ocean where subtle fluctuations have profound global impacts (i.e., El Niño [9, 10]), the coupled dynamics of this system and its recent changes have received comparatively little attention. Viewed from space, the Galápagos Archipelago is but a dot in an otherwise enormous ocean basin. However, the very dynamics governing the equatorial oceans also confer upon the Galápagos a unique oceanographic influence significantly beyond that expected from its physical dimensions alone [11–14]. Because the Galápagos straddles the equator in the eastern Pacific, its oceanographic environs are shaped by the large-scale, wind-driven upwelling spanning most of the equatorial Pacific Ocean, intense local upwelling and mixing due to the upward deflection of the Equatorial Undercurrent (EUC) by the islands themselves [15], and a confluence of surface currents and countercurrents originating in both hemispheres. A recent modeling study has also indicated a key role for submesoscale turbulence and shallow overturning cells, the dynamics of which are critically dependent upon latitude, in maintaining the cool and productive waters west of the Galápagos [16]. As such, and purely by accident of geology, the Galápagos hosts a remarkably well-suited environment for marine animals and seabirds typically associated with higher-latitude climates such as penguins (Spheniscus mendiculus), fur seals (Arctocephalus galapagoensis), and sea lions (Zalophus wollebaeki), all of which are endangered [5–7] and acutely sensitive to seasonal, interannual, and longer-term ocean/climate variability [17–19]. Satellite observations [20] indicate that the cold pool situated adjacent to the Galápagos Archipelago at 92°W in the eastern equatorial Pacific has cooled by about a half a degree centigrade over the past four decades (Fig 1A–1C), quite the opposite of the surrounding region and indeed most of the global ocean (Fig 1D). This steady cooling trend represents a doubling of the intensity of the Galápagos cold pool (GCP), taken relative to sea surface temperatures (SST) throughout the broader eastern equatorial Pacific region (Fig 1E). This trend has potentially significant ecological implications; two thirds of the Galápagos penguin population, for example, lives along a roughly 50-km long stretch of the western coastline of Isabela [21]. Like Kiribati in the central Pacific [22, 23], could the western Galápagos Archipelago be another potential marine refuge from global warming? And like the North Atlantic, is this another fingerprint of a changing ocean circulation—albeit subject to different physical mechanisms? Sustained satellite observations, recent advances in data assimilation and the availability of high-resolution oceanic (and atmospheric) state estimates [24, 25] now enable us to answer such questions. PPT PowerPoint slide

PNG larger image

TIFF original image Download: Fig 1. Observed sea surface temperature trends. a, SST trend (°C per 39 years) from Jan. 1982 through Dec. 2020. Black contours are the mean SST, contoured every 0.5°C; the innermost contour is 23°C. b, As in a but with the influence of large ENSO events removed prior to calculating the trend. c, as in a but for the period Jan. 1993 through Dec. 2018. The color scale next to b applies to all three trend maps (a–c). d, Histogram of SST trends throughout the world ocean (one for each square km), with the minimum and mean SST trend in the GCP region (white boxes in a–c) denoted by blue lines. e, Time series of SST averaged within the GCP region relative to the region west of the GCP (extending from the GCP to 100°W) with two (black line) and five (red line) year low-pass filters to remove high-frequency noise. The blue curve outlines the segment of the two year low-pass filtered time series spanning Jan. 1993 through Dec. 2018. https://doi.org/10.1371/journal.pclm.0000056.g001

Materials and methods Sea surface temperature The National Oceanic and Atmospheric Administration (NOAA) Optimal Interpolation version 2 (OIv2) data are used to evaluate SST trends in the eastern equatorial Pacific from Jan. 1982 through Dec. 2020. The NOAA OIv2 data are provided with daily temporal resolution and 0.25° (~25 km) horizontal resolution [20]. The NOAA OIv2 data are a blend of infrared satellite retrievals from the Advanced Very High Resolution Radiometer (AVHRR) and in situ measurements from the International Comprehensive Ocean Atmosphere Data Set (ICOADS) [26]. This SST data set is suitable for distinguishing features at the spatial scale of the GCP because the GCP (and the cooling trend observed there) encompasses at least 56 individual 0.25° grid cells (i.e., the white box drawn on Fig 1A–1C). Linear trends are estimated by least-squares regression. Interannual climate variations in the eastern equatorial Pacific are dominated by the El Niño-Southern Oscillation (ENSO). To ensure that the SST trends are not influenced significantly by the distribution of such events within the 39-year record, an alternate version of the SST trend map is produced wherein data from all days in which the absolute value of the NINO3 index (a typical measure of ENSO, the area averaged SST anomalies between 5°S–5°N, 150°W–90°W) exceeded 2 standard deviations were omitted prior to calculation of the trends (Fig 1B). The trend in that case is almost indistinguishable from the trend computed over the full, continuous record. Moreover, an alternative version of the trend map is computed only over 1993–2018 (Fig 1C), the period for which the GLORYS-12v1 ocean reanalysis is available (see below) and for which the subsequent analyses of changes in ocean circulation are calculated. Again, the SST trend map is not qualitatively different from that computed using the full period. Ocean state and circulation For estimates of changes in the subsurface ocean thermodynamic state and circulation covering two thirds of the period for which the NOAA OIv2 data are available (1993–2018), the GLobal Ocean ReanalYsis and Simulation (GLORYS)-12v1 product [24] is used. GLORYS-12v1 is a relatively new global ocean, eddy-resolving reanalysis covering the altimeter era at 1/12° (~9 km) horizontal resolution on 50 vertical levels. The ocean model component is the NEMO platform driven at surface by the ERA5 reanalyses (see below) for recent years. Oceanographic observations are assimilated by means of a reduced-order Kalman filter, and 3D-VAR provides a correction for the slowly-evolving large-scale biases in temperature and salinity. Along track altimetry, SST, sea ice concentration and in situ temperature and salinity profiles are jointly assimilated. Daily and monthly means are available—monthly means are used here. Unlike most current ocean models and ocean state estimates/reanalyses, GLORYS-12v1 includes an exceptionally well resolved Galápagos Archipelago, and a mean zonal circulation at 93°W that validates very well against recent in situ observations [27, 28] (Fig 2). These two aspects of the GLORYS-12v1 reanalysis render it uniquely well suited for the present study. Although GLORYS-12v1 is available for only 26 years (1993–2018), all trends are expressed as per 39 years simply to maintain consistency with the observed SST trends that are attempted to be diagnosed, not meant as an explicit assumption that the long-term linear trends must be perfectly invariant from one decade to the next. The similarity between the observed SST trend maps for the two periods, 1982–2020 and 1993–2018 (Fig 1), suggests this may be a defensible assumption anyway. PPT PowerPoint slide

PNG larger image

TIFF original image Download: Fig 2. Representation of the EUC and the Galápagos in GLORYS. a, Cross-section of mean zonal velocity (m/s) along 93°W from recent glider observations. Contour interval 0.05 m/s with 0 m/s denoted by the white line. The maximum eastward velocity is indicated in the title b, As in a but from GLORYS. c, Map of mean sea surface temperature (°C) in GLORYS with white grid cells indicating the GLORYS land mask representing the Galápagos Islands compared to real coastlines (black contours). https://doi.org/10.1371/journal.pclm.0000056.g002 As with all numerical models and reanalyses that are based upon them, results can be dependent on parameterizations of sub-grid scale processes such as mixing. The vertical stratification, shear, and their ratio (a proxy for the tendency for turbulent mixing) are estimated using GLORYS-12v1 data by calculating the buoyancy frequency squared (N2), shear squared (Sh2), and the gradient Richardson number (Ri), respectively, as follows: (1) (2) (3) where g is gravity (9.81 m/s2), ρ is potential density, and u is zonal ocean velocity. Small values of the gradient Richardson number generally indicate conditions in which the destabilizing effect of shear is sufficient to overcome the stabilizing effect of stratification. The gradient Richardson number results are thus presented as 1/Ri to describe the tendency for turbulent mixing to occur. It is important to note that, although Ri and its constituent terms N2 and Sh2 are calculated “offline” using basic outputs such as temperature, salinity, and horizontal velocity, which may have reasonable multidecadal tendencies as constrained by assimilated observations, their solutions are not free from the influence of a model’s numerical schemes such as grid scale closures for buoyancy and momentum. Surface winds Finally, for estimates of changes in the surface wind field, the ERA5 reanalysis [25] is used. ERA5 is the latest version of the global atmospheric reanalysis from ECMWF, covering the period 1979 to the present. Monthly mean 10-m wind and sea level pressure fields are provided at 0.25° horizontal resolution. This atmospheric reanalysis is the ideal choice because it was a source of surface forcing to the GLORYS-12v1 ocean reanalysis, thus ensuring physical consistency of the atmospheric and oceanic diagnostic results. However, several other global atmospheric reanalyses are available, albeit not always with such fine horizontal resolution. The most important result derived from ERA5 in this study is the southerly wind trend over the eastern equatorial Pacific from 1982–2020. This result is well reproduced using the NOAA NCEP2 atmospheric reanalysis [29] (Fig 3), thus ensuring this result is robust and not an artifact of how data are assimilated in one reanalysis. PPT PowerPoint slide

PNG larger image

TIFF original image Download: Fig 3. Cross-validation of surface wind trend. a, Trend in 10-m meridional wind (m/s per 39 years) in the ERA5 reanalysis from 1982–2020. b, As in a but for the NCEP2 reanalysis; same time period. https://doi.org/10.1371/journal.pclm.0000056.g003

Discussion and conclusions A cooling trend in the GCP is robust over a period of almost four decades of satellite observations (1982–2020), and indicators and diagnostics of changes in equatorial oceanography from a high-resolution ocean state estimate spanning two thirds of that time period (1993–2018) shed light on a physical mechanism. The mechanism begins with an acceleration of the cross-equatorial component of the trade winds in the eastern Pacific, driven by an interhemispheric gradient in surface warming, which is an emergent feature of the pattern of warming (past and future) in response to anthropogenic radiative forcing [40, 41]. Interestingly, the southerly wind trend is also consistent with the observed cooling trend in the GCP from the perspective of a newly proposed mechanism for upwelling that links local southerly winds with vigorous local submesoscale circulations [16]. Regardless, this is not a direct attribution to anthropogenic forcing. The long-term evolution of the meridional surface winds in the region is also necessarily linked to concurrent changes in the intertropical convergence zone (ITCZ), which has a complex and uncertain response to radiative forcing [42] and is subject to considerable internal climate variability. Whether and the extent to which the changes diagnosed for recent decades can sustain the cooling trend in the GCP in the decades to come, outpacing surface radiative forcing and an increasing tendency for vertical stratification, is uncertain and may require very high-resolution global climate models (GCMs) that are similarly able to capture the complex Galápagos bathymetry and the delicate dynamical balances governing the coupled atmosphere and ocean in the eastern equatorial Pacific. While several studies have indicated that the EUC is projected to accelerate under future climate forcing scenarios in coupled GCMs [22, 23, 43, 44], ensembles of very high-resolution GCMs may be needed to further disambiguate the role of internal variability in the observed SST trends and changes in circulation. Unfortunately, we may be some ways from ensembles of GCMs capable of adequately resolving the Galápagos, or the detailed dynamics of the EUC for that matter. Interestingly, the results of this study have brought forth a mechanism that could also explain the apparent refusal thus far of the broader eastern equatorial Pacific Ocean (not just the GCP) to warm—or to warm by as much as the rest of the tropics including the western equatorial Pacific, since the late 19th century [45–47]. The increased topographic upwelling implied by the changes in the strength and position of the EUC in the eastern Pacific uniquely impact the GCP, but the increase in vertical shear of zonal velocity along the equator extends well into the central Pacific (Fig 8). It is therefore possible that increased shear-induced mixing has played a role in maintaining (or even strengthening) the zonal SST gradient along the equatorial Pacific, along with other mechanisms proposed earlier [48–52], and the GCP is merely an early sentinel of that signal by virtue of its extremely shallow thermocline. Several other physical mechanisms may be important in the open equatorial ocean, west of the GCP. For example, zonal advection appears to play a mechanistic role in the changing circulation within the broader domain (e.g., Fig 6B), but a calculation of the zonal temperature advection term in the mixed layer heat budget within the GCP reveals negligible contribution to its cooling trend, clearly owing to the islands deflecting the SEC around the GCP (not shown). PPT PowerPoint slide

PNG larger image

TIFF original image Download: Fig 8. Estimated changes in equatorial ocean circulation. Trend in zonal ocean velocity (m/s per 39 years) as an equatorial cross-section (averaged from 1°S–0.5°N). Black contours are the mean zonal velocity, contoured every 0.1 m/s; the outermost contour is 0.1 m/s. Dashed contours indicate negative or westward zonal velocity, the white contours denote the 15°C and 20°C isotherms, and white space near 90°W represents the subsurface topography of the Galápagos Islands. This is as in Fig 4B but extending further westward. https://doi.org/10.1371/journal.pclm.0000056.g008 Thus far, and for perhaps the very near future, the western shores of the Galápagos appear to be a safe haven from some of the deleterious impacts of anthropogenic climate change including ocean warming and suppression of upwelling. El Niño events will continue to happen every few years, accompanied by a temporary reduction of EUC velocity [28, 30, 53] and upwelling, extreme SST warming, and a general ravaging of the entire ecosystem from plankton to penguins [54, 55]. This region and its potential status as a safe haven is also quite distinct from other waters within the archipelago, where a cooling trend is not observed and heat-induced coral bleaching is periodically documented [56]. Challenges to the sustainable management and protection of the second largest marine reserve in the world notwithstanding, the recent trends detected and diagnosed here are cause for cautious optimism for some of the Ecuadorian fisheries, and the penguin population—virtually all of which are already clever enough to live along the western shores of Isabela and Fernandina.

Acknowledgments The author acknowledges support from the NASA Sea Level Change Science Program, Award 80NSSC20K1123. The author also thanks the Mercator Ocean Service Desk (Véronique Landes) and Dr. Aneesh Subramanian for assistance with acquiring GLORYS-12v1 vertical ocean velocity fields.

[END]
---
[1] Url: https://journals.plos.org/climate/article?id=10.1371/journal.pclm.0000056

Published and (C) by PLOS One
Content appears here under this condition or license: Creative Commons - Attribution BY 4.0.

via Magical.Fish Gopher News Feeds:
gopher://magical.fish/1/feeds/news/plosone/