Equatorial Pacific Surface Research Articles

The advance of El Niño phase locking from period 1982–2000 to 2001–2022

The mature phases of El Niño-Southern Oscillation (ENSO) events exhibit a distinct tendency to peak towards the end of the calendar year, a phenomenon commonly referred to as ENSO phase locking. This phase locking is a fundamental property of the ENSO. The observed characteristics of phase locking are intricately tied to the seasonality of ENSO-related Sea Surface Temperature (SST) growth rate. In this study, notable observational evidence is presented: the strength of phase locking of El Niño weakened, and the phase locking advanced to peak at an earlier time during the period 2001 to 2022 compared to 1982 to 2000. The advancement of El Niño Phase Locking is explored by analyzing the contributions of different oceanic feedbacks to the El Niño phenomenon. Specifically, our findings highlight the significant role of nonlinear advective dynamic heating (NDH), which is influenced by the decrease in equatorial pacific surface zonal current anomaly and the equatorial pacific surface zonal gradient of sea SST anomaly. This investigation enhances our understanding of the evolving dynamics of El Niño phase locking, shedding light on the intricate interplay of oceanic feedbacks in influencing this fundamental aspect of ENSO behavior.

Impacts of Low-Frequency Internal Climate Variability and Greenhouse Warming on El Niño–Southern Oscillation

AbstractEl Niño–Southern Oscillation (ENSO) is the dominant mode of interannual climate fluctuations with wide-ranging socioeconomic and environmental impacts. Understanding the eastern Pacific (EP) and central Pacific (CP) El Niño response to a warmer climate is paramount, yet the role of internal climate variability in modulating their response is not clear. Using large ensembles, we find that internal variability generates a spread in the standard deviation and skewness of these two El Niño types that is similar to the spread of 17 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) that realistically simulate ENSO diversity. Based on 40 Community Earth System Model Large Ensemble (CESM-LE) and 99 Max Planck Institute for Meteorology Grand Ensemble (MPI-GE) members, unforced variability can explain more than 90% of the historical EP and CP El Niño standard deviation and all of the ENSO skewness spread in the 17 CMIP5 models. Both CESM-LE and the selected CMIP5 models show increased EP and CP El Niño variability in a warmer climate, driven by a stronger mean vertical temperature gradient in the upper ocean and faster surface warming of the eastern equatorial Pacific. However, MPI-GE shows no agreement in EP or CP standard deviation change. This is due to weaker sensitivity to the warming signal, such that when the eastern equatorial Pacific surface warming is faster, the change in upper ocean vertical temperature gradient tends to be weaker. This highlights that individual models produce a different ENSO response in a warmer climate, and that considerable uncertainty within the CMIP5 ensemble may be caused by internal climate variability.

Central Asian river streamflows have not continued to increase during the recent warming hiatus

The recent warming hiatus started around 1998; however, how Central Asian river streamflows have responded to this warming slowdown remains unclear. Based on the instrumental streamflow records of eight rivers in Central Asian, we showed that the Central Asian streamflow variation is due to the change in precipitation/drought resulting from sea surface temperature changes in the eastern tropical Pacific Ocean and that equatorial eastern Pacific surface cooling has been the main driver of the slowdown in streamflow increase in Central Asia over the past two decades. Temperature is also likely an important driver of streamflow variation on the Pamirs Plateau during the warm season; however, the linkage between temperature and streamflow is not stable due in part to the precipitation-induced cooling. Central Asian river streamflow is very sensitive to variations in climate factors, and the influences of climate change on streamflow should be considered continuously by water resource managers.

El Niño–Like Physical and Biogeochemical Ocean Response to Tropical Eruptions

AbstractThe oceanic response to recent tropical eruptions is examined in Large Ensemble (LE) experiments from two fully coupled global climate models, the Community Earth System Model (CESM) and the Geophysical Fluid Dynamics Laboratory Earth System Model (ESM2M), each forced by a distinct volcanic forcing dataset. Following the simulated eruptions of Agung, El Chichón, and Pinatubo, the ocean loses heat and gains oxygen and carbon, in general agreement with available observations. In both models, substantial global surface cooling is accompanied by El Niño–like equatorial Pacific surface warming a year after the volcanic forcing peaks. A mechanistic analysis of the CESM and ESM2M responses to Pinatubo identifies remote wind forcing from the western Pacific as a major driver of this El Niño–like response. Following eruption, faster cooling over the Maritime Continent than adjacent oceans suppresses convection and leads to persistent westerly wind anomalies over the western tropical Pacific. These wind anomalies excite equatorial downwelling Kelvin waves and the upwelling of warm subsurface anomalies in the eastern Pacific, promoting the development of El Niño conditions through Bjerknes feedbacks a year after eruption. This El Niño–like response drives further ocean heat loss through enhanced equatorial cloud albedo, and dominates global carbon uptake as upwelling of carbon-rich waters is suppressed in the tropical Pacific. Oxygen uptake occurs primarily at high latitudes, where surface cooling intensifies the ventilation of subtropical thermocline waters. These volcanically forced ocean responses are large enough to contribute to the observed decadal variability in oceanic heat, carbon, and oxygen.

Quantification of the responses of equatorial Pacific surface wind to uncertain cloud‐related parameters in GAMIL2

AbstractThe response of surface winds over the equatorial Pacific to cloud‐related parameters is quantified by using a uniform sampling method and conducting a large number of perturbed parameter simulations. The results show that the surface winds are highly sensitive to, and even linearly dependent on some parameters that include the precipitation efficiency for deep convection (C0_deep), the time scale for the consumption rate of shallow CAPE (CMFTAU), and the threshold value for RH for deep convection (RHCRIT), indicating the potential to improve surface wind simulations by tuning these parameters. Parameter changes indirectly affect surface winds through changes in vertical velocity or deduced circulation linked to diabatic heating (DH) above the surface. The relative contribution of static stability changes induced by perturbed parameter to balancing DH and affecting vertical velocity exceeds 10% in most of the troposphere.

Recycled iron fuels new production in the eastern equatorial Pacific Ocean

Nitrate persists in eastern equatorial Pacific surface waters because phytoplankton growth fueled by nitrate (new production) is limited by iron. Nitrate isotope measurements provide a new constraint on the controls of surface nitrate concentration in this region and allow us to quantify the degree and temporal variability of nitrate consumption. Here we show that nitrate consumption in these waters cannot be fueled solely by the external supply of iron to these waters, which occurs by upwelling and dust deposition. Rather, a substantial fraction of nitrate consumption must be supported by the recycling of iron within surface waters. Given plausible iron recycling rates, seasonal variability in nitrate concentration on and off the equator can be explained by upwelling rate, with slower upwelling allowing for more cycles of iron regeneration and uptake. The efficiency of iron recycling in the equatorial Pacific implies the evolution of ecosystem-level mechanisms for retaining iron in surface ocean settings where it limits productivity.

On the Dynamical Relationship between Equatorial Pacific Surface Currents, Zonally Averaged Equatorial Sea Level, and El Niño Prediction

AbstractPrevious work has shown that for large zonal scales and low frequencies, wind-forced sea level, even near the equator, can be described by wind-forced long Rossby waves. In the eastern equatorial Pacific where the interannual wind forcing is small, these waves are essentially locally unforced and propagate westward from the boundary. At the boundary the waves’ sea level is in phase because of geostrophy and no normal flow to the boundary. However, because the waves propagate more slowly with increasing latitude, west of the boundary lag increases as latitude increases. Consequently a northward sea level gradient is like a time derivative, and the zonal geostrophic flow is like a time derivative of the sea level. This implies that the equatorial flow should lead the equatorial sea level by about 9 months on El Niño time scales. However, analysis shows that when dissipation of the large-scale flow is taken into account, this lead is reduced to about 3 months. This lead time is approximately the dissipation time scale of the second vertical mode, which dominates the zonal surface flow. Since the eastern equatorial Pacific sea level ηE is proportional to eastern equatorial thermocline displacement and El Niño, the zonal equatorial flow leads El Niño indices. Analysis also shows that the zonally averaged equatorial Pacific sea level leads El Niño and that this lead is associated with the geostrophic zonal velocity and the long Rossby wave physics.

Northwestern Pacific typhoon intensity controlled by changes in ocean temperatures

Dominant climatic factors controlling the lifetime peak intensity of typhoons are determined from six decades of Pacific typhoon data. We find that upper ocean temperatures in the low-latitude northwestern Pacific (LLNWP) and sea surface temperatures in the central equatorial Pacific control the seasonal average lifetime peak intensity by setting the rate and duration of typhoon intensification, respectively. An anomalously strong LLNWP upper ocean warming has favored increased intensification rates and led to unprecedentedly high average typhoon intensity during the recent global warming hiatus period, despite a reduction in intensification duration tied to the central equatorial Pacific surface cooling. Continued LLNWP upper ocean warming as predicted under a moderate [that is, Representative Concentration Pathway (RCP) 4.5] climate change scenario is expected to further increase the average typhoon intensity by an additional 14% by 2100.

The influence of suboxic diagenesis on the formation of manganese nodules in the Clarion Clipperton nodule belt of the Pacific Ocean

Abstract Polymetallic nodules and a single Fe-Mn crust from the Clarion and Clipperton Zone (CCZ) of the equatorial eastern Pacific have been analyzed to determine their chemical and mineralogical compositions. ICP-OES analyses of bulk nodules from the CCZ indicate that the nodules formed under oxic-diagenetic conditions with typical Mn/Fe ratios of 4–6, Ni + Cu values of 2–3 wt.%, Co contents of 0.12–0.17 wt.%. However, detail analyses of individual growth layers by electron microprobe analyses (EMPA) reveal much larger chemical heterogeneity of individual layers with Mn/Fe ratios ranging from 800. Two different genetic types of layers can be distinguished: Layer type 1: dense layer with average Mn/Fe ratios of 1.80, average Ni + Cu of 0.81 wt.%, Co contents of 0.30 wt.% (hydrogenetic metal precipitation from oxic waters). These layers consist of Fe-vernadite (δ-MnO2) epitaxially intergrown with feroxyhyte (δ-FeOOH) nanoparticles. Layer type 2: growth structures (dendritic growth structures and dense layers) with average Mn/Fe ratios of 96, Ni + Cu of 3.9 wt.%, Co contents of 0.08 wt.%, and a distinct Ni + Cu maximum of 6.51 wt.% at a Mn/Fe ratio of 56 (suboxic-diagenetic, metal precipitation from suboxic pore waters). Type 2 layers mainly consist of turbostratic phyllomanganates such as 10 A vernadite, 7 A vernadite and todorokite are minor compounds. Mixed layer type 3: These layers can occur as zones of low reflectivity in the transition from layer type 1 to 2 or build up inhomogeneous growth structures of nm-thin layers of low or high reflectivity. The Mn/Fe ratios of this material range between 3 and 11, Ni + Cu of 1–4.6 wt.%, and Co contents are between 0.02 and 0.77 wt.%. In most cases these growth structures represent a mixture of type 1 and 2 layers. X-ray photoelectron spectroscopy (XPS) of the recent outermost nm layers of nodules (top, rim and bottom side) indicates hydrogenetic accretion processes under recent oxic conditions both from seawater and pore water. The mixed-type CCZ nodules are interpreted to consist of an oxic-hydrogenetic and a suboxic-diagenetic end member. Calculations of the proportions of the individual layer end members show that suboxic layers make up about 50–60% of the chemical inventory of the CCZ nodules whereas oxic-hydrogenetic layers comprise about 35–40%. The remaining part (5–10%) consists of incorporated sediment particles occurring along cracks and pores. These results demonstrate that suboxic conditions alternate with oxic conditions during the growth of nodules in the eastern CCZ. This is probably due to fluctuating bioproductivity in the equatorial Pacific surface waters during glacial–interglacial periods which led to changing organic carbon flux to the sediment and changing oxygen consumption in near surface sediments. Furthermore, reduced ventilation of the deep ocean during glacial periods may have led to suboxic conditions in near-surface sediments. Our investigations prove that bulk chemistry data of whole nodules only provides an average of individual layers that build up the nodules and is inappropriate for genetic interpretation.

Solar warming of the south-central Pacific

Surface solar radiation is found to have contributed significantly and positively to the record warming event in the south-central Pacific (SCP) that peaked in December of 2009. The SCP region is within a positive teleconnection pattern between sea surface temperature anomalies in the equatorial Pacific and basin-wide surface solar radiation, as revealed by a 24-year time series; the pattern extends southeast from the western equatorial Pacific toward the SCP region. The results are consistent with the ‘atmospheric bridge postulation’ on El Niño teleconnection with extratropical sea surface temperature anomalies, but with the extension to cloud cover and surface solar radiation over the mid-latitude southern oceans.

Natural variability and anthropogenic change in equatorial Pacific surface ocean pCO2 and pH

AbstractThe equatorial Pacific is a dynamic region that plays an important role in the global carbon cycle. This region is the largest oceanic source of carbon dioxide (CO2) to the atmosphere, which varies interannually dependent on the El Niño‐Southern Oscillation (ENSO) and other climatic and oceanic drivers. We present high‐resolution observations of surface ocean CO2 partial pressure (pCO2) at four fixed locations in the Niño 3.4 area with data sets encompassing 10 ENSO warm and cold events from 1997 to 2011. The mooring observations confirm that ENSO controls much of the interannual variability in surface seawater pCO2 with values ranging from 315 to 578 µatm. The mooring time series also capture the temporal variability necessary to make the first estimates of long‐term pH trends in the equatorial Pacific, which suggests that the combination of ocean acidification and decadal variability creates conditions for high rates of pH change since the beginning of the mooring record. Anthropogenic CO2 increases play a dominant role in significant observed seawater pCO2 trends of +2.3 to +3.3 µatm yr−1 and pH trends of −0.0018 to −0.0026 yr−1 across the full time series in this region. However, increased upwelling driven by increased trade winds, a shallower thermocline, and increased frequency of La Niña events also contribute an average of 40% of the observed trends since 1998. These trends are higher than previous estimates based on underway observations and suggest that the equatorial Pacific is contributing a greater amount of CO2 to the atmospheric CO2 inventory over the last decade.

Improved Representation of Tropical Pacific Ocean–Atmosphere Dynamics in an Intermediate Complexity Climate Model

Abstract A new anomaly coupling technique is introduced into a coarse-resolution dynamic climate model [the Liège Ocean Carbon Heteronomous model (LOCH)–Vegetation Continuous Description model (VECODE)–Earth System Models of Intermediate Complexity Climate deBilt (ECBILT)–Coupled Large-Scale Ice–Ocean model (CLIO)–Antarctic and Greenland Ice Sheet Model (AGISM) ensemble (LOVECLIM)], improving the model’s representation of eastern equatorial Pacific surface temperature variability. The anomaly coupling amplifies the surface diabatic atmospheric forcing within a Gaussian-shaped patch applied in the tropical Pacific Ocean. It is implemented with an improved predictive cloud scheme based on empirical relationships between cloud cover and key state variables. Results are presented from a perturbed physics ensemble systematically varying the parameters controlling the anomaly coupling patch size, location, and amplitude. The model’s optimal parameter combination is chosen through calibration against the observed power spectrum of monthly-mean surface temperature anomalies in the Niño-3 region. The calibrated model exhibits substantial improvement in equatorial Pacific interannual surface temperature variability and robustly reproduces El Niño–Southern Oscillation (ENSO)-like variability. The authors diagnose some of the key atmospheric and oceanic feedbacks in the model important for simulating ENSO-like variability, such as the positive Bjerknes feedback and the negative heat flux feedback, and analyze the recharge–discharge of the equatorial Pacific ocean heat content. They find LOVECLIM robustly captures important ocean dynamics related to thermocline adjustment and equatorial Kelvin waves. The calibrated model demonstrates some improvement in simulating atmospheric feedbacks, but the coupling between ocean and atmosphere is relatively weak. Because of the tractability of LOVECLIM and its consequent utility in exploring long-term climate variability and large ensemble perturbed physics experiments, improved representation of tropical Pacific ocean–atmosphere dynamics in the model may more readily allow for the investigation of the role of tropical Pacific ocean–atmosphere dynamics in past climate changes.

Recent global-warming hiatus tied to equatorial Pacific surface cooling

Despite the continued increase in atmospheric greenhouse gas concentrations, the annual-mean global temperature has not risen in the twenty-first century, challenging the prevailing view that anthropogenic forcing causes climate warming. Various mechanisms have been proposed for this hiatus in global warming, but their relative importance has not been quantified, hampering observational estimates of climate sensitivity. Here we show that accounting for recent cooling in the eastern equatorial Pacific reconciles climate simulations and observations. We present a novel method of uncovering mechanisms for global temperature change by prescribing, in addition to radiative forcing, the observed history of sea surface temperature over the central to eastern tropical Pacific in a climate model. Although the surface temperature prescription is limited to only 8.2% of the global surface, our model reproduces the annual-mean global temperature remarkably well with correlation coefficient r = 0.97 for 1970-2012 (which includes the current hiatus and a period of accelerated global warming). Moreover, our simulation captures major seasonal and regional characteristics of the hiatus, including the intensified Walker circulation, the winter cooling in northwestern North America and the prolonged drought in the southern USA. Our results show that the current hiatus is part of natural climate variability, tied specifically to a La-Niña-like decadal cooling. Although similar decadal hiatus events may occur in the future, the multi-decadal warming trend is very likely to continue with greenhouse gas increase.

Statistical constraints on El Niño Southern Oscillation reconstructions using individual foraminifera: A sensitivity analysis

Recent investigations of submillennial paleoceanographic variability have attempted to resolve high‐frequency climate signals such as the El Niño Southern Oscillation (ENSO) using the population statistics of individual planktic foraminiferal δ18O analyses. This approach is complicated by the relatively short lifespan of individual foraminifers (~2–4 weeks) compared to the time represented by a typical marine sediment sample (~decades to millennia). Here, we investigate the uncertainty associated with individual foraminiferal analyses (IFA) through simulations on forward modeled δ18Ocarbonate. First, focusing on the Niño3.4 region of the tropical Pacific Ocean, a bootstrap Monte Carlo algorithm is developed to constrain the uncertainty on IFA‐statistics. Subsequently, to test the sensitivity of IFA to changes in seasonal cycle amplitude, ENSO amplitude, and ENSO frequency, synthetic time series of δ18Ocarbonate with differing variability are constructed and tested with our algorithm. The probabilities of the IFA technique in detecting changes in ENSO amplitude and seasonal cycle amplitude (or a combination of both) for the surface ocean and thermocline at different locations in the tropical Pacific are quantified. We find that the uncertainty in the standard deviation is smaller than the range, that the IFA‐signal is insensitive to ENSO frequency, and at certain locations the seasonal cycle may dominate ENSO. IFA sensitivity towards ENSO is highest at the central equatorial Pacific surface ocean and the eastern equatorial Pacific (EEP) thermocline whereas sensitivity towards the seasonal cycle is highest at the EEP surface ocean. Our results suggest that rigorous uncertainty quantification should become standard practice for accurately interpreting IFA‐data.

Lead, Nd and Sr isotope records of pelagic dust: Source indication versus the effects of dust extraction procedures and authigenic mineral growth

Eolian silicate dust of the 328 cm long piston core KODOS 02-01-02 from the northeast equatorial Pacific was analyzed for chemical compositions and Nd, Sr, and Pb isotopic ratios to evaluate their use as hemispheric dust source discriminator and to examine the potential influence of authigenic smectite on these source signals. Neodymium and Sr isotope ratios of the core determine a change from southern to northern hemisphere dust deposition at about 7.6 Ma. Samples deposited between 15.5 and 7.6 Ma resemble southeast equatorial Pacific surface and down-core dust values, while younger samples correspond to signatures typical for dust derived from central Asia. The 15.5 to 7.6 Ma lower interval of the core is smecite-rich; however, previously proposed chemical fingerprints for authigenic smectite such as positive Eu anomalies or high Fe/Al ratios are not observed in the KODOS samples. The up to 40% smectite in the KODOS core dust are thus interpreted to represent largely detrital smectite, and no evidence is found for authigenic modification of Nd–Sr isotopic or element ratio source signals of the dust records. Lead isotope results of the core lack prominent down-core variation with changing source regimes as reported above. A compilation of Pacific Pb isotope dust records reveals that Pb isotope ratios vary as a function of silicate dust extraction procedures. The step of removing biogenic silica thereby appears critical. Use of NaOH appears most effective, while minimally attacking eolian silicate material. It is concluded that of the Pb isotopic dust records published so far, only a few are useful for dust source identification. However, once the silicate dust extraction procedure is further optimized for retrieving the pristine silicate dust Pb isotopic composition, specific source signatures should become well resolvable.

Estimating iron and aluminum removal rates in the eastern equatorial Pacific Ocean using a box model approach

Iron limitation plays an important role in maintaining the high-nitrate low-chlorophyll (HNLC) condition in the equatorial upwelling zone. The rate and depth of upwelling control Fe supply to the euphotic zone. This study constrains the transport fluxes and budget of two trace metals, Fe and Al, in the upper ocean. They are co-delivered to the eastern equatorial Pacific surface waters via the Equatorial Undercurrent and upwelling but show distinct biogeochemical cycling processes. We combine the results of the in situ measurements of dissolved Fe and Al (dFe and dAl) with the modeled velocity fields to calculate the physical fluxes. The model calculations are evaluated with the conservation of heat, volume transport, NO 3 and Si(OH) 4 budgets for the equatorial Pacific. The vertical flux due to upwelling provides averaged dFe and dAl supply rates of 1.45 μmol m −2 d −1 and 11.51 μmol m −2 d −1, respectively. The sum of the net physical fluxes in the eastern equatorial Pacific for dFe and dAl are 0.41 μmol m −2 d −1 and 2.77 μmol m −2 d −1, respectively. These estimates are equal to the net biological and chemical removal rates of dFe and dAl. The calculated dFe:C net removal ratio is in the range of 3-9 μmol:mol, which agrees with most other estimates. This suggests that the majority of net dFe removal is due to biological uptake in the upper water column. The results of this box model approach illustrate the usefulness of combining the modeled outputs and in situ measurements, which provide additional constraints on Fe transport and cycling in the equatorial Pacific and possibly other HNLC regions.

Impact of Shifting Patterns of Pacific Ocean Warming on North Atlantic Tropical Cyclones

Two distinctly different forms of tropical Pacific Ocean warming are shown to have substantially different impacts on the frequency and tracks of North Atlantic tropical cyclones. The eastern Pacific warming (EPW) is identical to that of the conventional El Niño, whereas the central Pacific warming (CPW) has maximum temperature anomalies located near the dateline. In contrast to EPW events, CPW episodes are associated with a greater-than-average frequency and increasing landfall potential along the Gulf of Mexico coast and Central America. Differences are shown to be associated with the modulation of vertical wind shear in the main development region forced by differential teleconnection patterns emanating from the Pacific. The CPW is more predictable than the EPW, potentially increasing the predictability of cyclones on seasonal time scales.

Significant contribution of large particles to optical backscattering in the open ocean

Abstract. The light scattering properties of oceanic particles have been suggested as an alternative index of phytoplankton biomass than chlorophyll-a concentration (chl-a), with the benefit of being less sensitive to physiological forcings (e.g., light and nutrients) that alter the intracellular pigment concentrations. The drawback of particulate scattering is that it is not unique to phytoplankton. Nevertheless, field studies have demonstrated that, to first order, the particulate beam-attenuation coefficient (cp) can track phytoplankton biomass. The relationship between cp and the particulate backscattering coefficient (bbp), a property retrievable from space, has not been fully evaluated, largely due to a lack of open-ocean field observations. Here, we present extensive data on inherent optical properties from the Equatorial Pacific surface waters and demonstrate a remarkable coherence in bbp and cp. Coincident measurements of particle size distributions (PSDs) and optical properties of size-fractionated samples indicate that this covariance is due to both the conserved nature of the PSD and a greater contribution of phytoplankton-sized particles to bbp than theoretically predicted. These findings suggest that satellite-derived bbpcould provide similar information on phytoplankton biomass in the open ocean as cp.

Geochemistry of dissolved rare earth elements in the Equatorial Pacific Ocean

Seawater samples were collected from four locations in the Equatorial Pacific Ocean during the MR02-K06 cruise of the R/V Mirai and analyzed for dissolved rare earth elements (REEs) using inductively coupled plasma mass spectrometry. According to variations of REE concentrations and Yb/La ratios, the results show that the river input of the Papua New Guinea islands may affect the compositions of REEs in the Equatorial Pacific surface water. The Yb/La values and the REE concentrations in the waters deeper than 3,000 m in the western South Pacific and the Equatorial Pacific Oceans, which represent the characteristics of Antarctic Bottom Water (AABW), demonstrate similar variation trend with depth. This result also indicates that the REEs which originated from the South Pacific Ocean have entered the North Pacific Ocean across the equator with AABW intrusion.

Roles of Indian and Pacific Ocean air–sea coupling in tropical atmospheric variability

Sea surface temperature (SST) variations include negative feedbacks from the atmosphere, whereas SST anomalies are specified in stand-alone atmospheric general circulation simulations. Is the SST forced response the same as the coupled response? In this study, the importance of air–sea coupling in the Indian and Pacific Oceans for tropical atmospheric variability is investigated through numerical experiments with a coupled atmosphere-ocean general circulation model. The local and remote impacts of the Indian and Pacific Ocean coupling are obtained by comparing a coupled simulation with an experiment in which the SST forcing from the coupled simulation is specified in either the Indian or the Pacific Ocean. It is found that the Indian Ocean coupling is critical for atmospheric variability over the Pacific Ocean. Without the Indian Ocean coupling, the rainfall and SST variations are completely different throughout most of the Pacific Ocean basin. Without the Pacific Ocean coupling, part of the rainfall and SST variations in the Indian Ocean are reproduced in the forced run. In regions of large mean rainfall where the atmospheric negative feedback is strong, such as the North Indian Ocean and the western North Pacific in boreal summer, the atmospheric variability is significantly enhanced when air–sea coupling is replaced by specified SST forcing. This enhancement is due to the lack of the negative feedback in the forced SST simulation. In these regions, erroneous atmospheric anomalies could be induced by specified SST anomalies derived from the coupled model. The ENSO variability is reduced by about 20% when the Indian Ocean air–sea coupling is replaced by specified SST forcing. This change is attributed to the interfering roles of the Indian Ocean SST and Indian monsoon in western and central equatorial Pacific surface wind variations.