Subsurface Warming Research Articles

Upper ocean response to typhoon Kalmaegi (2014)

AbstractTyphoon Kalmaegi passed over an array of buoys and moorings in the northern South China Sea in September 2014, leaving a rare set of observations on typhoon‐induced dynamical and thermohaline responses in the upper ocean. The dynamical response was characterized by strong near‐inertial currents with opposite phases in the surface mixed layer and in the thermocline, indicating the dominance of the response by the excitation of the first baroclinic mode. The thermohaline response showed considerable changes in the mean fields in addition to a near‐inertial oscillation. In particular, temperature and salinity anomalies generally exhibited a three‐layer vertical structure, with the surface layer becoming cooler and saltier, the subsurface layer warmer and fresher, and the lower layer cooler and saltier again. The response in the surface and subsurface layers was much stronger to the right of the typhoon track, while that in the lower layer was stronger along the track and to the left. These features of the upper ocean response were grossly reproduced by a three‐dimensional numerical model. A model‐based heat budget analysis suggests that vertical mixing was mainly responsible for the surface cooling and subsurface warming, while upwelling was the cause of cooling from below. Both observations and model results indicate that the whole upper ocean experienced an overall cooling in the wake of typhoon Kalmaegi.

Atmosphere-Ocean Coupling Effect on Intense Tropical Cyclone Distribution and its Future Change with 60 km-AOGCM

Atmosphere-ocean coupling effect on the frequency distribution of tropical cyclones (TCs) and its future change is studied using an atmosphere and ocean coupled general circulation model (AOGCM). In the present climate simulation, the atmosphere-ocean coupling in the AOGCM improves biases in the AGCM such as the poleward shift of the maximum of intense TC distribution in the Northern Hemisphere and too many intense TCs in the Southern Hemisphere. Particularly, subsurface cold water plays a key role to reduce these AGCM biases of intense TC distribution. Besides, the future change of intense TC distribution is significantly different between AOGCM and AGCM despite the same monthly SST. In the north Atlantic, subsurface warming causes larger increase in frequency of intense TCs in AOGCM than that in AGCM. Such subsurface warming in AOGCM also acts to alter large decrease of intense TC in AGCM to no significant change in AOGCM over the southwestern Indian Ocean. These results suggest that atmosphere-ocean coupling characterized by subsurface oceanic structure is responsible for more realistic intense TC distribution in the current climate simulation and gives significant impacts on its future projection.

Heat advection processes leading to El Niño events as depicted by an ensemble of ocean assimilation products

AbstractThe oscillatory nature of El Niño‐Southern Oscillation results from an intricate superposition of near‐equilibrium balances and out‐of‐phase disequilibrium processes between the ocean and the atmosphere. The main objective of the present work is to perform an exhaustive spatiotemporal analysis of the upper ocean heat budget in an ensemble of state‐of‐the‐art ocean assimilation products. We put specific emphasis on the ocean heat advection mechanisms, and their representation in individual ensemble members and in the different stages of the ENSO oscillation leading to EN events. Our analyses consistently show that the initial subsurface warming in the western equatorial Pacific is advected to the central Pacific by the equatorial undercurrent, which, together with the equatorward advection associated with anomalies in both the meridional temperature gradient and circulation at the level of the thermocline, explains the heat buildup in the central Pacific during the recharge phase. We also find that the recharge phase is characterized by an increase of meridional tilting of the thermocline, as well as a southward upper‐ocean cross‐equatorial mass transport resulting from Ekman‐induced anomalous vertical motion in the off‐equatorial regions. Although differences between data sets are generally small, and anomalies tend to have the same sign, the differences in the magnitude of the meridional term are seen to be key for explaining the different propagation speed of the subsurface warming tendency along the thermocline. The only exception is GECCO, which does not produce the patterns of meridional surface Ekman divergence (subsurface Sverdrup convergence) in the western and central equatorial Pacific.

Assessing climate impacts and risks of ocean albedo modification in the Arctic

AbstractThe ice albedo feedback is one of the key factors of accelerated temperature increase in the high northern latitudes under global warming. This study assesses climate impacts and risks of idealized Arctic Ocean albedo modification (AOAM), a proposed climate engineering method, during transient climate change simulations with varying representative concentration pathway (RCP) scenarios. We find no potential for reversing trends in all assessed Arctic climate metrics under increasing atmospheric CO2 concentrations. AOAM only yields an initial offset during the first years after implementation. Nevertheless, sea ice loss can be delayed by 25(60) years in the RCP8.5(RCP4.5) scenario and the delayed thawing of permafrost soils in the AOAM simulations prevents up to 40(32) Pg of carbon from being released by 2100. AOAM initially dampens the decline of the Atlantic Meridional Overturning and delays the onset of open ocean deep convection in the Nordic Seas under the RCP scenarios. Both these processes cause a subsurface warming signal in the AOAM simulations relative to the default RCP simulations with the potential to destabilize Arctic marine gas hydrates. Furthermore, in 2100, the RCP8.5 AOAM simulation diverts more from the 2005–2015 reference state in many climate metrics than the RCP4.5 simulation without AOAM. Considering the demonstrated risks, we conclude that concerning longer time scales, reductions in emissions remain the safest and most effective way to prevent severe changes in the Arctic.

Tropical North Atlantic subsurface warming events as a fingerprint for AMOC variability during Marine Isotope Stage 3

AbstractThe role of Atlantic Meridional Overturning Circulation (AMOC) as the driver of Dansgaard‐Oeschger (DO) variability that characterized Marine Isotope Stage 3 (MIS 3) has long been hypothesized. Although there is ample proxy evidence suggesting that DO events were robust features of glacial climate, there is little data supporting a link with AMOC. Recently, modeling studies and subsurface temperature reconstructions have suggested that subsurface warming across the tropical North Atlantic can be used to fingerprint a weakened AMOC during the deglacial because a reduction in the strength of the western boundary current allows warm salinity maximum water of the subtropical gyre to enter the deep tropics. To determine if AMOC variability played a role during the DO cycles of MIS 3, we present new, high‐resolution Mg/Ca and δ18O records spanning 24–52 kyr from the near‐surface dwelling planktonic foraminifera Globigerinoides ruber and the lower thermocline dwelling planktonic foraminifera Globorotalia truncatulinoides in Southern Caribbean core VM12‐107 (11.33°N, 66.63°W, 1079 m depth). Our subsurface Mg/Ca record reveals abrupt increases in Mg/Ca ratios (the largest equal to a 4°C warming) during the interstadial‐stadial transition of most DO events during this period. This change is consistent with reconstructions of subsurface warming events associated with cold events across the deglacial using the same core. Additionally, our data support the conclusion reached by a recently published study from the Florida Straits that AMOC did not undergo significant reductions during Heinrich events 2 and 3. This record presents some of the first high‐resolution marine sediment derived evidence for variable AMOC during MIS 3.

North Pacific deglacial hypoxic events linked to abrupt ocean warming.

Marine sediments from the North Pacific document two episodes of expansion and strengthening of the subsurface oxygen minimum zone (OMZ) accompanied by seafloor hypoxia during the last deglacial transition. The mechanisms driving this hypoxia remain under debate. We present a new high-resolution alkenone palaeotemperature reconstruction from the Gulf of Alaska that reveals two abrupt warming events of 4-5 degrees Celsius at the onset of the Bølling and Holocene intervals that coincide with sudden shifts to hypoxia at intermediate depths. The presence of diatomaceous laminations and hypoxia-tolerant benthic foraminiferal species, peaks in redox-sensitive trace metals, and enhanced (15)N/(14)N ratio of organic matter, collectively suggest association with high export production. A decrease in (18)O/(16)O values of benthic foraminifera accompanying the most severe deoxygenation event indicates subsurface warming of up to about 2 degrees Celsius. We infer that abrupt warming triggered expansion of the North Pacific OMZ through reduced oxygen solubility and increased marine productivity via physiological effects; following initiation of hypoxia, remobilization of iron from hypoxic sediments could have provided a positive feedback on ocean deoxygenation through increased nutrient utilization and carbon export. Such a biogeochemical amplification process implies high sensitivity of OMZ expansion to warming.

The potential role of urban green areas for controlling ground surface and subsurface warming

Subsurface warming in urban areas is higher in magnitude than increases in surface air temperatures. However, little evidence exists on the effects of urbanization on subsurface environments, and there are few quantitative estimates of the effectiveness of adaptation measures. We analyzed the relationship between ground surface warming and the extent of landscape change using subsurface temperature anomalies as an indicator of surface warming in five urban areas in Japan. To interpret these results for urban planning, we presented the percentages of green areas that would be needed to achieve certain reductions in ground surface temperatures for areas with different urbanization levels. Accordingly, a 0.5 °C reduction in average ground surface temperatures can be achieved by an increased Normalized Difference Vegetation Index (NDVI) value of 0.035, which accounts for approximately a 17% increase in natural green areas in an area with 75% urbanization. This study provides quantitative estimates to cope with urban warming at the local scale in the face of climate change.

Subsurface urban heat island and its effects on horizontal ground-source heat pump potential under climate change

Recent urban air temperature increase is attributable to the climate change and heat island effects due to urbanization. This combined effects of urbanization and global warming can penetrate into the underground and elevate the subsurface temperature. In the present study, over-100 years measurements of subsurface temperature at a remote rural site were analysed, and an increasing rate of 0.17 °C per decade at soil depth of 30 cm due to climate change was identified in the UK, but the subsurface warming in an urban site showed a much higher rate of 0.85 °C per decade at a 30 cm depth and 1.18 °C per decade at 100 cm. The subsurface urban heat island (SUHI) intensity obtained at the paired urban–rural stations in London showed an unique ‘U-shape’, i.e. lowest in summer and highest during winter. The maximum SUHII is 3.5 °C at 6:00 AM in December, and the minimum UHII is 0.2 °C at 18:00 PM in July. Finally, the effects of SUHI on the energy efficiency of the horizontal ground source heat pump (GSHP) were determined. Provided the same heat pump used, the installation at an urban site will maintain an overall higher COP compared with that at a rural site in all seasons, but the highest COP improvement can be achieved in winter.

Shallow groundwater thermal sensitivity to climate change and land cover disturbances: derivation of analytical expressions and implications for stream temperature modeling

Abstract. Climate change is expected to increase stream temperatures and the projected warming may alter the spatial extent of habitat for cold-water fish and other aquatic taxa. Recent studies have proposed that stream thermal sensitivities, derived from short-term air temperature variations, can be employed to infer future stream warming due to long-term climate change. However, this approach does not consider the potential for streambed heat fluxes to increase due to gradual warming of the shallow subsurface. The temperature of shallow groundwater is particularly important for the thermal regimes of groundwater-dominated streams and rivers. Also, recent studies have investigated how land surface perturbations, such as wildfires or timber harvesting, can influence stream temperatures by changing stream surface heat fluxes, but these studies have typically not considered how these surface disturbances can also alter shallow groundwater temperatures and streambed heat fluxes. In this study, several analytical solutions to the one-dimensional unsteady advection–diffusion equation for subsurface heat transport are employed to estimate the timing and magnitude of groundwater temperature changes due to seasonal and long-term variability in land surface temperatures. Groundwater thermal sensitivity formulae are proposed that accommodate different surface warming scenarios. The thermal sensitivity formulae suggest that shallow groundwater will warm in response to climate change and other surface perturbations, but the timing and magnitude of the subsurface warming depends on the rate of surface warming, subsurface thermal properties, bulk aquifer depth, and groundwater velocity. The results also emphasize the difference between the thermal sensitivity of shallow groundwater to short-term (e.g., seasonal) and long-term (e.g., multi-decadal) land surface-temperature variability, and thus demonstrate the limitations of using short-term air and water temperature records to project future stream warming. Suggestions are provided for implementing these formulae in stream temperature models to accommodate groundwater warming.

Subsurface North Atlantic warming as a trigger of rapid cooling events: evidence from the early Pleistocene (MIS 31–19)

Abstract. Subsurface water column dynamics in the subpolar North Atlantic were reconstructed in order to improve the understanding of the cause of abrupt ice-rafted detritus (IRD) events during cold periods of the early Pleistocene. We used paired Mg / Ca and δ18O measurements of Neogloboquadrina pachyderma (sinistral – sin.), deep-dwelling planktonic foraminifera, to estimate the subsurface temperatures and seawater δ18O from a sediment core from Gardar Drift, in the subpolar North Atlantic. Carbon isotopes of benthic and planktonic foraminifera from the same site provide information about the ventilation and water column nutrient gradient. Mg / Ca-based temperatures and seawater δ18O suggest increased subsurface temperatures and salinities during ice-rafting, likely due to northward subsurface transport of subtropical waters during periods of weaker Atlantic Meridional Overturning Circulation (AMOC). Planktonic carbon isotopes support this suggestion, showing coincident increased subsurface ventilation during deposition of IRD. Subsurface accumulation of warm waters would have resulted in basal warming and break-up of ice-shelves, leading to massive iceberg discharges in the North Atlantic. The release of heat stored at the subsurface to the atmosphere would have helped to restart the AMOC. This mechanism is in agreement with modelling and proxy studies that observe a subsurface warming in the North Atlantic in response to AMOC slowdown during Marine Isotope Stage (MIS) 3.

On the dynamical mechanisms explaining the western Pacific subsurface temperature buildup leading to ENSO events

AbstractDespite steady progress in the understanding of El Niño–Southern Oscillation (ENSO) in the past decades, questions remain on the exact mechanisms explaining the heat buildup leading to the onset of El Niño (EN) events. Here we use an ensemble of ocean and atmosphere assimilation products to identify mechanisms that are consistently identified by all the data sets and that contribute to the heat buildup in the western Pacific 18 to 24 months before the onset of EN events. Meridional and eastward heat advection due to equatorward subsurface mass convergence and transport along the equatorial undercurrent are found to contribute to the subsurface warming at 170°E–150°W. In the warm pool, instead, surface horizontal convergence and downwelling motion have a leading role in subsurface warming. The picture emerging from our results highlights a sharp dynamical transition at 170°E near the level of the thermocline.

Atlantic–Pacific asymmetry of subsurface temperature change and frontal response of the Antarctic Circumpolar Current for the recent three decades

For the 32-year period from 1979 to 2010, trends of surface and subsurface temperature and meridional motion of the current system in the Antarctic Circumpolar Current (ACC) region are studied with in situ observations and an eddy-resolving general circulation model. The observed and simulated surface temperature shows a similar pattern between the Atlantic and Pacific: warming to the north of the Subantarctic/Subtropical Fronts in the Atlantic and of the Subtropical Front in the Pacific and cooling to the south of those fronts. The subsurface temperature trend, again from both observation and model, reveals an asymmetric pattern between the Atlantic and Pacific: subsurface warming is dominant over the whole ACC region in the Atlantic, while both warming and cooling are significant in the Pacific, the former located to the north of the Subantarctic Front and the latter to the south. The model reveals that the ACC has generally shifted poleward in the Atlantic, while it has shifted equatorward around Subantarctic Front and Polar Front in the Pacific. The ACC shift is consistent with the overall subsurface temperature trend. The basin-scale difference of the ACC response can be related to the different regime of the trend in meridional gradient of the zonal wind stress to the north and south of 50–55°S and suggests a coupling of the ACC and overlying westerly on the multi-decadal time scale.

Heaving modes in the world oceans

Part of climate changes on decadal time scales can be interpreted as the result of adiabatic motions associated with the adjustment of wind-driven circulation, i.e., the heaving of the isopycnal surfaces. Heat content changes in the ocean, including hiatus of global surface temperature and other phenomena, can be interpreted in terms of heaving associated with adjustment of wind-driven circulation induced by decadal variability of wind. A simple reduced gravity model is used to examine the consequence of adiabatic adjustment of the wind-driven circulation. Decadal changes in wind stress forcing can induce three-dimensional redistribution of warm water in the upper ocean. In particular, wind stress change can generate baroclinic modes of heat content anomaly in the vertical direction; in fact, changes in stratification observed in the ocean may be induced by wind stress change at local or in the remote parts of the world oceans. Intensification of the equatorial easterly can induce cooling in the upper layer and warming in the subsurface layer. The combination of this kind of heat content anomaly with the general trend of warming of the whole water column under the increasing greenhouse effect may offer an explanation for the hiatus of global surface temperature and the accelerating subsurface warming over the past 10–15 years. Furthermore, the meridional transport of warm water in the upper ocean can lead to sizeable transient meridional overturning circulation, poleward heat flux and vertical heat flux. Thus, heaving plays a key role in the oceanic circulation and climate.

“Category‐6” supertyphoon Haiyan in global warming hiatus: Contribution from subsurface ocean warming

AbstractWith the extra‐ordinary intensity of 170 kts, supertyphoon Haiyan devastated the Philippines in November 2013. This intensity is among the highest ever observed for tropical cyclones (TCs) globally, 35 kts well above the threshold (135kts) of the existing highest category of 5. Though there is speculation to associate global warming with such intensity, existing research indicate that we have been in a warming hiatus period, with the hiatus attributed to the La Niña‐like multi‐decadal phenomenon. It is thus intriguing to understand why Haiyan can occur during hiatus. It is suggested that as the western Pacific manifestation of the La Niña‐like phenomenon is to pile up warm subsurface water to the west, the western North Pacific experienced evident subsurface warming and created a very favorable ocean pre‐condition for Haiyan. Together with its fast traveling speed, the air‐sea flux supply was 158% as compared to normal for intensification.

Tropical Indian Ocean subsurface temperature variability and the forcing mechanisms

The first two leading modes of interannual variability of sea surface temperature in the Tropical Indian Ocean (TIO) are governed by El Nino Southern Oscillation and Indian Ocean Dipole (IOD) respectively. TIO subsurface however does not co-vary with the surface. The patterns of the first mode of TIO subsurface temperature variability and their vertical structure are found to closely resemble the patterns of IOD and El Nino co-occurrence years. These co-occurrence years are characterized by a north–south subsurface dipole rather than a conventional IOD forced east–west dipole. This subsurface dipole is forced by wind stress curl anomalies, driven mainly by meridional shear in the zonal wind anomalies. A new subsurface dipole index (SDI) has been defined in this study to quantify the intensity of the north–south dipole mode. The SDI peaks during December to February (DJF), a season after the dipole mode index peaks. It is found that this subsurface north–south dipole is a manifestation of the internal mode of variability of the Indian Ocean forced by IOD but modulated by Pacific forcing. The seasonal evolution of thermocline, subsurface temperature and the corresponding leading modes of variability further support this hypothesis. Positive wind stress curl anomalies in the south and negative wind stress curl anomalies in the north of 5°S force (or intensify) downwelling and upwelling waves respectively during DJF. These waves induce strong subsurface warming in the south and cooling in the north (especially during DJF) and assist the formation and/or maintenance of the north–south subsurface dipole. A thick barrier layer forms in the southern TIO, supporting the long persistence of anomalous subsurface warming. To the best of our knowledge the existence of such north–south subsurface dipole in TIO is being reported for the first time.

Rapid subsurface warming and circulation changes of Antarctic coastal waters by poleward shifting winds

AbstractThe southern hemisphere westerly winds have been strengthening and shifting poleward since the 1950s. This wind trend is projected to persist under continued anthropogenic forcing, but the impact of the changing winds on Antarctic coastal heat distribution remains poorly understood. Here we show that a poleward wind shift at the latitudes of the Antarctic Peninsula can produce an intense warming of subsurface coastal waters that exceeds 2°C at 200–700 m depth. The model simulated warming results from a rapid advective heat flux induced by weakened near‐shore Ekman pumping and is associated with weakened coastal currents. This analysis shows that anthropogenically induced wind changes can dramatically increase the temperature of ocean water at ice sheet grounding lines and at the base of floating ice shelves around Antarctica, with potentially significant ramifications for global sea level rise.

The Initiation and Developing Mechanisms of Central Pacific El Niños

Abstract The initiation and developing mechanisms of four major central Pacific (CP) El Niño events in 1994, 2002, 2004, and 2009 were investigated by analyzing oceanic and atmospheric reanalysis data. A mixed layer heat budget analysis was conducted and the result shows that the initiation mechanism of the 1994 CP El Niño is very different from other CP El Niños in 2000s, while the developing mechanisms are similar among these events. The initial sea surface temperature (SST) warming of the 1994 El Niño was caused by enhanced solar radiation, which was related to atmospheric meridional overturning circulation in association with positive SST anomaly forcing in the subtropical Pacific. The subtropical SST anomalies also induced anticyclonic surface wind stress curl anomalies, which caused the formation of subsurface warmer waters in the off-equatorial regions. The off-equatorial subsurface warmer waters were transported farther equatorward by the mean subsurface ocean currents, leading to the subsurface warming in the central equatorial Pacific. The deepened thermocline anomaly at the equator further promoted a positive advective and thermocline feedback so that the SST anomaly grew. During the initiation phase of the 2000s El Niños, ocean dynamics played a dominant role, while the effect of surface heat flux anomalies was minor. Preexisting subsurface warmer waters appeared in the equatorial region during their initiation phases. Such subsurface anomalies can cause the SST warming in the central Pacific through induced anomalous eastward zonal currents that advect high mean SST eastward. This positive zonal advective feedback, along with a positive thermocline feedback, continued to warm the local SST throughout the developing phase of the 2000s El Niño events.

Millennial-scale variability in Antarctic ice-sheet discharge during the last deglaciation.

Our understanding of the deglacial evolution of the Antarctic Ice Sheet (AIS) following the Last Glacial Maximum (26,000-19,000years ago) is based largely on a few well-dated but temporally and geographically restricted terrestrial and shallow-marine sequences. This sparseness limits our understanding of the dominant feedbacks between the AIS, Southern Hemisphere climate and global sea level. Marine records of iceberg-rafted debris (IBRD) provide a nearly continuous signal of ice-sheet dynamics and variability. IBRD records from the North Atlantic Ocean have been widely used to reconstruct variability in Northern Hemisphere ice sheets, but comparable records from the Southern Ocean of the AIS are lacking because of the low resolution and large dating uncertainties in existing sediment cores. Here we present two well-dated, high-resolution IBRD records that capture a spatially integrated signal of AIS variability during the last deglaciation. We document eight events of increased iceberg flux from various parts of the AIS between 20,000 and 9,000years ago, in marked contrast to previous scenarios which identified the main AIS retreat as occurring after meltwater pulse 1A and continuing into the late Holocene epoch. The highest IBRD flux occurred 14,600years ago, providing the first direct evidence for an Antarctic contribution to meltwater pulse 1A. Climate model simulations with AIS freshwater forcing identify a positive feedback between poleward transport of Circumpolar Deep Water, subsurface warming and AIS melt, suggesting that small perturbations to the ice sheet can be substantially enhanced, providing a possible mechanism for rapid sea-level rise.

Tropical cyclones in two atmospheric (re)analyses and their response in two oceanic reanalyses

In this paper, we first evaluate the ability of the European Centre for Medium Range Forecast operational analysis and the ERA-Interim reanalysis to capture the surface wind signature of tropical cyclones (TCs). In those products, the error on the TC position is typically ∼150km, cyclones are too big (∼250km in ERA-Interim and > 100km in the operational analysis against ∼50km in observations) and the maximum wind speed is on average underestimated by 15–27m·s−1 for strong TCs. These biases are generally reduced with the increase of horizontal resolution in the operational analysis, but remain significant at T1279 (∼16km).We then assess the TCs oceanic temperature signature in two global eddy-permitting ocean reanalyses (GLORYS1 and GLORYS2) forced by the above atmospheric products. The resulting cold wake is on average underestimated by ∼50% in the two oceanic reanalyses. This bias is largely linked to the underestimated TCs strength in the surface forcing, and the resulting underestimated vertical mixing. The overestimated TC radius also tends to overemphasize the Ekman pumping response to the cyclone. Underestimating vertical mixing without underestimating Ekman pumping results in the absence of the observed subsurface warming away from the TC tracks in the two reanalyses. Data assimilation only marginally contributes to reducing these errors, partly because cyclone signatures are not well resolved by the ocean observing system. Based on these results, we propose some assimilation and forcing strategies in order to improve the restitution of TC signatures in oceanic reanalyses.

Interdecadal Amplitude Modulation of El Niño–Southern Oscillation and Its Impact on Tropical Pacific Decadal Variability*

Abstract The amplitude of El Niño–Southern Oscillation (ENSO) displays pronounced interdecadal modulations in observations. The mechanisms for the amplitude modulation are investigated using a 2000-yr preindustrial control integration from the Geophysical Fluid Dynamics Laboratory Climate Model, version 2.1 (GFDL CM2.1). ENSO amplitude modulation is highly correlated with the second empirical orthogonal function (EOF) mode of tropical Pacific decadal variability (TPDV), which features equatorial zonal dipoles in sea surface temperature (SST) and subsurface temperature along the thermocline. Experiments with an ocean general circulation model indicate that both interannual and decadal-scale wind variability are required to generate decadal-scale tropical Pacific temperature anomalies at the sea surface and along the thermocline. Even a purely interannual and sinusoidal wind forcing can produce substantial decadal-scale effects in the equatorial Pacific, with SST cooling in the west, subsurface warming along the thermocline, and enhanced upper-ocean stratification in the east. A mechanism is proposed by which residual effects of ENSO could serve to alter subsequent ENSO stability, possibly contributing to long-lasting epochs of extreme ENSO behavior via a coupled feedback with TPDV.