Observations and empirical model development

The U.S. Pacific Remote Islands Marine National Monument (PRIMNM), which includes Howland, Baker and Jarvis, was established in 2009 by Presidential Proclamation and accounts for the broadest collection of marine protected areas under U.S. jurisdiction. Loggers deployed by NOAA’s Coral Reef Ecosystem Division (CRED) on the west and east sides of the islands recorded reef-level temperatures continuously over the past 10 years (Supplementary Data Table S1). These data allow us to compute the cross-island temperature gradient (δT) directly from observations, analyze its co-variability with the observed EUC, develop well-constrained empirical models and apply them to the latest generation of GCM projections4 as well as observed trends.

The decreasing mean and standard deviation of δT from Jarvis (0.83 ± 0.80 °C) to Baker (0.54 ± 0.45 °C) to Howland (0.48 ± 0.32 °C) is consistent with the geographic context of each island relative to the mean structure of the EUC and thermocline (Fig. 1A–C). Jarvis is situated at 160°W where, on average, the EUC is ~0.2 m/s faster and the thermocline ~20 m shallower than at Baker and Howland (176°W)7. Moreover, Howland is 69 km further north of the equator than Baker, thus further from the core of the EUC (average difference ~0.1 m/s). Despite differences in mean δT and its amplitude of variability at each island, δT is similarly and significantly correlated with the strength of the EUC (Fig. 1D; correlations of 0.59, 0.60 and 0.50 at Jarvis, Baker and Howland, respectively).

Figure 1 Summary of observations. (A) Top: logger measured temperature on the west (blue) and east (red) sides of Jarvis Island (160.02°W, 0.38°S), native sampling resolution and NCEP SST21 from the nearest grid cell (black), weekly averaged (°C). Bottom: cross-island temperature gradient (δT) at Jarvis based on the west logger contrasted with the east logger (gray) and NCEP SST (black), weekly averaged (°C). (B,C) As in (A) but for Baker (176.49°W, 0.19°N) and Howland (176.62°W, 0.81°N) Islands. (D) Top: EUC velocity at 170°W measured by TAO (black) and estimated by SODA (green), weekly averaged (m/s). Bottom row: Commonly used ENSO indices Nino3 (blue) and Nino3.4 (red), monthly averaged (°C). (E) Linear trends in temperature measured on the west and east sides of each island and in the Nino3.4 region (°C per decade) over the period 2002–2011*. NCEP SST is used for the east side; circles indicate equivalent trends based on east loggers at Jarvis and Baker. Also indicated is the trend in the merged 170°W EUC record over the same period*. Error bars indicate 95% confidence intervals of the trend. *Trends at Baker and Howland are computed beginning with 2004, prior to which the west loggers were not deployed at those islands. The EUC trend is computed over the period ending 2010, after which neither SODA nor 170°W TAO zonal velocity data are available. Full size image

Linear trends over the period of measurement reflect the strong control of the EUC on δT. While the broader central equatorial Pacific (Nino3.4) experienced a cooling trend of −1.34 ± 0.46 °C per decade, which is not significantly different from trends on the east side of each island (~2 °C per decade), the west-side cooling trends were stronger, especially at Baker and Jarvis (Fig. 1E). The cooling trend on the west side of Jarvis was, in fact, significantly stronger than that on its east side (−3.15 ± 0.41 °C per decade compared to −1.94 ± 0.33 °C per decade). A simple explanation for these cross-island trends and their inter-island variability is that the EUC was also strengthening over this period by 0.38 ± 0.07 m/s per decade (Fig. 1D), leading to stronger topographic upwelling and enhanced cooling on the west sides of the islands that stand directly in its path.

Records of δT are paired with a merged record of EUC velocity12,13 (see Methods and Supplementary Data Table S1) to develop empirical models by nonlinear regression (Fig. 2A–C, S4). Solutions to the models predict the change in δT for a given percent change in EUC velocity (Fig. 2D). For example, a 20% strengthening of the EUC would result in a 0.5 °C increase in δT at Jarvis (i.e., from 0.8 °C to 1.3 °C). The model for Baker would yield roughly half the increase in δT for the same increase in EUC velocity and half again for Howland. An EUC-related change in δT would be manifest as a change in temperature on the west side of the island. The empirical models describing the dependence of δT on EUC velocity can therefore be applied to GCM future projections of EUC strength and weighed against the projected changes in sea surface temperature (SST) for the region (i.e., oblivious to island influences) in the same GCM simulations. The result can effectively be interpreted as a “mitigation effect” on the west sides of the islands.

Figure 2 Empirical model development. (A) Scatter plot of EUC velocity at 170°W (m/s) and (δT) at Jarvis (°C) with an exponential model estimated by nonlinear least squares regression. (B,C) As in (A) but for Baker and Howland Islands. Adjusted r-squared values and mean values (“x” marks) are provided in each panel. (D) Solutions to the nonlinear regression models shown in (A–C) expressed as change in δT (°C) as a function of change in EUC relative to the time mean EUC (%). Solutions to models estimated using δT based on logger temperatures from alternative depths at west Jarvis (thin lines) indicate sensitivity to depth (note, however, that only 4–5 years of data are available from the alternative west Jarvis loggers). Full size image

Application to GCM projections

The SST and zonal velocity output fields of 35 GCMs from the CMIP5 archive (see Supplementary Data Table S2) were analyzed following a screening process for reasonable simulation of the magnitude, zonal structure and seasonality of the EUC (see Methods and Fig. S5). Although the analogy is not perfect, both the seasonal cycle and long-term climate change are essentially a response to changes in radiative forcing, so simulating a realistic seasonal cycle is a minimum core requirement for placing confidence in climate change projections. It should also be noted that a realistic simulation of the EUC is a particularly rigorous target because its mean properties are determined by integrating several coupled climate processes including the trade winds, the large-scale thermocline structure and vertical mixing of momentum; many CMIP3 models had difficulty capturing the magnitude and zonal structure14. The remaining 14 models show a very reasonable spread about the observed EUC magnitude, zonal structure and seasonality (Fig. 3A,B); those models capture well the zonal structure of the equatorial SST field as well (Fig. 3C).

Figure 3 Climate model fidelity in the equatorial Pacific. (A) Profiles along the equator of the maximum EUC velocity (m/s), (B) mean seasonal cycles of maximum EUC velocity at 170°W (normalized) and (C) as in (A) but for equatorial SST (°C). The 14 models shown (thin black lines) are those that passed the screening process for reasonable simulation of the EUC magnitude, zonal structure and seasonality (see Methods and Fig. S5). Provided in A–B are observational estimates of the mean EUC and its seasonal cycle7 and equatorial SST21. Full size image

The empirical models as expressed in Fig. 2D (or as ordinary differential equations in Fig. S4) are solved for a range of changes in EUC velocity (ΔU) and compared to a range of large-scale changes in SST (ΔT) from historical estimates and CMIP5 model projections (Fig. 4). For example, under the RCP8.5 scenario as prescribed by IPCC AR5, the NCAR CCSM4 climate model15 predicts a warming at Jarvis of 2.6 °C per century (see Methods). The same experiment also predicts a 13–24% (17.7% ensemble mean) per century strengthening of the EUC, which increases δT (cools west-side SST) by an amount equivalent to 10–25% (15% ensemble mean) of the 2.6 °C warming. The complete ensemble mean projection based on all 14 screened CMIP5 models places the mitigation effect for Jarvis at 11% (3.2 °C warming and 16.3% increase in EUC strength). Note that the projected EUC trend grows as the screening criteria become more stringent (Table 1). Results for Baker and Howland are qualitatively similar but yield a smaller mitigation effect due to the weaker sensitivity of their δT to EUC velocity than at Jarvis given the physical mechanisms previously discussed. Note that an additional robust aspect of the projected response of the equatorial Pacific to anthropogenic forcing is a shoaling of the EUC and thermocline, which would presumably enhance the mitigation effect but is not included in this framework.

Table 1 Effect of screening criteria on projected EUC trends under the RCP8.5 scenario. Full size table

Figure 4 Historical and future mitigation effect. (A) Colors: Mitigation effect on the west side of Jarvis Island (%) as a function of the predicted changes in EUC at 170°W (%) and SST for the region surrounding Jarvis Island (°C). (“Region” is defined as the scale of a grid cell of a global climate model that does not include such small islands.) Vertical lines: Four estimates of the historical trends in EUC at 170°W based on the SODA ocean reanalysis: v.2.2.4/1871-2008, v.2.2.4/1910-2008, v.2.2.6/1871-2008 and v.2.2.6/1910-2008 (%/century). Horizontal lines: Historical trends in regional SST over the period 1870–2012 (°C/century) based on three widely use instrumental data sets. Open circle: Simulated trend (per century) in EUC and SST based on the Historical experiments by the NCAR global climate model spanning 1870–2004. Closed black circle: As in open circle but for projections based on RCP8.5 experiments spanning 2006–2100. Small white dots indicate the spread of individual ensemble runs of the NCAR model. Transparent boxes indicate the CMIP5 multi-model mean projections for RCP4.5 and RCP8.5 as labeled, including +/− 2 standard errors inter-model spread about the multi-model mean trends. The multi-model mean is comprised of the 14 models passing the screening for realistic EUC strength, zonal structure and seasonality (see Materials and Methods). (B,C) As in (A) but for Baker and Howland Islands. (D) As in (A–C) but for the Gilbert Islands and based on the model of KC12 rather than in situ data. The “x” symbol marks the Gilberts projection based on CMIP3 models5. Full size image

Observational estimates of historical trends in the essential quantities (EUC velocity and large-scale SST change) provide a very different perspective. Our present best estimates of the historical trend in EUC velocity at 170°W is between 21.4% and 38.7% per century11,13 (Fig. 4A–C; see Methods). Various instrumental SST data sets can be queried16,17,18, which yield an average trend of 0.25 °C per century at Jarvis and 0.31 °C per century at Baker and Howland. If those trends were to continue, then over 100% of the projected regional-scale warming would be mitigated to the west of Jarvis and the observed cooling trend will continue. Extrapolating observed trends in SST and EUC also results in a mitigation effect of over 100% and 50% at Baker and Howland, respectively. Even if the observed trend to date in EUC velocity is halved and that of SST is doubled, the mitigation effect at Jarvis would be ~30%.

Although sustained in situ measurements of comparable quality and spatial distribution have not yet been made at the Gilbert Islands, the relationship between the EUC and δT at the Gilberts developed previously by5 can be cast in the same framework (Fig. 4D). Future projections of EUC and large-scale SST at this site from both GCMs and historical extrapolations are qualitatively similar to those for the other three islands. However, the predicted mitigation effect is considerably stronger. This may be due to the unique geography and geometry of the Gilberts, which are larger than the other three islands, forming a chain across the equator with an obvious blockage effect on the EUC19 that is striking even in satellite data20. The Gilberts will experience a mitigation effect of 28% based on the 14 screened CMIP5 models analyzed, or ~25% based on CMIP3 models5. As with Jarvis, historical trends suggest that well over 100% of the warming has been mitigated by circulation over the past century. Of these four islands or island chains in the central equatorial Pacific, Jarvis and the Gilberts clearly hold the greatest potential for future warming mitigation by a strengthening EUC based on historical estimates and future GCM projections.