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Tropical rainfall variations are of direct societal relevance and drive climate variations worldwide via teleconnections. The convective rainfall tends to occur when sea surface temperature (SST) exceeds a threshold, SSTthr, usually taken to be constant in time and space. We analyze 40-year monthly observations and find that SSTthr varies by up to 4°C in space and with season. Based on local convective instability, we develop a quantitative theory that largely explains the SSTthr variations using the climatological state of the tropical atmosphere. Although it is often assumed that spatial variations of tropical upper tropospheric temperature are small and can be neglected, it is shown that lower climatological values favor a lower SSTthr. Similarly, a small increase in climatological surface relative humidity also leads to a decrease in SSTthr as does a lower climatological air-sea temperature difference. Consequently, efforts to understand and predict natural or forced variations in tropical rainfall must account for, in addition to SST, the temperatures aloft and the near surface humidity and temperature and requires improved understanding of what controls their distribution in space and time.

Atlantic multidecadal variability (AMV) impacts temperature, precipitation, and extreme events on both sides of the Atlantic Ocean basin. Previous studies with climate models have suggested that when external radiative forcing is held constant, the large-scale ocean and atmosphere circulation are associated with sea surface temperature (SST) anomalies that have similar characteristics to the observed AMV. However, there is an active debate as to whether these internal fluctuations driven by coupled atmosphere–ocean variability remain influential to the AMV on multidecadal time scales in our modern, anthropogenically forced climate. Here we provide evidence from multiple large ensembles of climate models, paleoreconstructions, and instrumental observations of a growing role for external forcing in the AMV. Prior to 1850, external forcing, primarily from volcanoes, explains about one-third of AMV variance. Between 1850 and 1950, there is a transitional period, where external forcing explains one-half of AMV variance, but volcanic forcing only accounts for about 10% of that. After 1950, external forcing explains three-quarters of AMV variance. That is, the role for external forcing in the AMV grows as the variations in external forcing grow, even if the forcing is from different sources. When forcing is relatively stable, as in earlier modeling studies, a higher percentage of AMV variations are internally generated.

This paper attempts to enhance our understanding of the causes of Atlantic multidecadal variability (AMV). Following the literature, we define the AMV as the SST averaged over the North Atlantic basin, linearly detrended and low-pass filtered. There is an ongoing debate about the drivers of the AMV, which include internal variability generated from the ocean or atmosphere (or both) and external radiative forcing. We test the role of these factors in explaining the time history, variance, and spatial pattern of the AMV using a 41-member ensemble from a fully coupled version of CESM and a 10-member ensemble of the CESM atmosphere coupled to a slab ocean. The large ensemble allows us to isolate the role of external forcing versus internal variability, and the model differences allow us to isolate the role of coupled ocean circulation. Both with and without coupled ocean circulation, external forcing explains more than half of the variance of the observed AMV time series, indicating its important role in simulating the twentieth-century AMV phases. In this model the net effect of ocean processes is to reduce the variance of the AMV. Dynamical ocean coupling also reduces the ability of the model to simulate the characteristic spatial pattern of the AMV, but forcing has little impact on the pattern. Historical forcing improves the time history and variance of the AMV simulation, while the more realistic ocean representation reduces the variance below that observed and lowers the correlation with observations.

Persistent multiyear cold states of the tropical Pacific Ocean drive hydroclimate anomalies worldwide, including persistent droughts in the extratropical Americas. Here, the atmosphere and ocean dynamics and thermodynamics of multiyear cold states of the tropical Pacific Ocean are investigated using European Centre for Medium-Range Weather Forecasts reanalyses and simplified models of the ocean and atmosphere. The cold states are maintained by anomalous ocean heat flux divergence and damped by increased surface heat flux from the atmosphere to ocean. The anomalous ocean heat flux divergence is contributed to by both changes in the ocean circulation and thermal structure. The keys are an anomalously shallow thermocline that enhances cooling by upwelling and anomalous westward equatorial currents that enhance cold advection. The thermocline depth anomalies are shown to be a response to equatorial wind stress anomalies. The wind stress anomalies are shown to be a simple dynamical response to equatorial SST anomalies as mediated by precipitation anomalies. The cold states are fundamentally maintained by atmosphere-ocean coupling in the equatorial Pacific. The physical processes that maintain the cold states are well approximated by linear dynamics for ocean and atmosphere and simple thermodynamics.

The most prominent feature of the time-mean global meridional overturning circulation (MOC) is the Atlantic MOC (AMOC). However, interannual variability of the global MOC is shown here to be dominated by Pacific MOC (PMOC) variability over the full depth of the ocean at most latitudes. This dominance of interannual PMOC variability is robust across modern climate models and an observational state estimate. PMOC interannual variability has large-scale organization, its most prominent feature being a cross-equatorial cell spanning the tropics. Idealized experiments show that this variability is almost entirely wind driven. Interannual anomalies of zonal mean zonal wind stress produce zonally integrated Ekman transport anomalies that are larger in the Pacific Ocean than in the Atlantic Ocean, simply because the Pacific is wider than the Atlantic at most latitudes. This contrast in Ekman transport variability implies greater variability in the near-surface branch of the PMOC when compared with the near-surface branch of the AMOC. These near-surface variations in turn drive compensating flow anomalies below the Ekman layer. Because the baroclinic adjustment time is longer than a year at most latitudes, these compensating flow anomalies have baroclinic structure spanning the full depth of the ocean. Additional analysis reveals that interannual PMOC variations are the dominant contribution to interannual variations of the global meridional heat transport. There is also evidence of interaction between interannual PMOC variability and El Niño–Southern Oscillation.

North Atlantic sea surface temperatures (SST) exhibit a lagged response to the North Atlantic Oscillation (NAO) in both models and observations, which has previously been attributed to changes in ocean heat transport. Here we examine the lagged relationship between the NAO and Atlantic multidecadal variability (AMV) in the context of the two other major components of the AMV: atmospheric noise and external forcing. In preindustrial control runs, we generally find that after accounting for spurious signals introduced by filtering, the SST response to the NAO is only statistically significant in the subpolar gyre. Further, the lagged SST response to the NAO is small in magnitude and offers a limited contribution to the AMV pattern, statistics, or predictability. When climate models include variable external forcing, the relationship between the NAO and AMV is obscured and becomes inconsistent. In these historically forced runs, knowledge of the prior NAO offers reduced predictability. The differences between the preindustrial and the historically forced ensembles suggest that we do not yet have enough observational data to surmise the true NAO–AMV relationship and add evidence that external forcing plays a substantial role in producing the AMV.

Tropical Pacific decadal variability (TPDV), though not the totality of Pacific decadal variability, has wide-ranging climatic impacts. It is currently unclear whether this phenomenon is predictable. In this study, we reconstruct the attractor of the tropical Pacific system in long, unforced simulations from an intermediate-complexity model, two general circulation models (GCMs), and the observations with the aim of assessing the predictability of TPDV in these systems. We find that in the intermediate-complexity model, positive (high variance, El Niño–like) and negative (low variance, La Niña–like) phases of TPDV emerge as a pair of regime-like states. The observed system bears resemblance to this behavior, as does one GCM, while the other GCM does not display this structure. However, these last three time series are too short to confidently characterize the full distribution of interdecadal variability. The intermediate-complexity model is shown to lie in highly predictable parts of its attractor 37% of the time, during which most transitions between TPDV regimes occur. The similarities between the observations and this system suggest that the tropical Pacific may be somewhat predictable on interdecadal time scales.

From 1875 to 1878, concurrent multiyear droughts in Asia, Brazil, and Africa, referred to as the Great Drought, caused widespread crop failures, catalyzing the so-called Global Famine, which had fatalities exceeding 50 million people and long-lasting societal consequences. Observations, paleoclimate reconstructions, and climate model simulations are used 1) to demonstrate the severity and characterize the evolution of drought across different regions, and 2) to investigate the underlying mechanisms driving its multiyear persistence. Severe or record-setting droughts occurred on continents in both hemispheres and in multiple seasons, with the “Monsoon Asia” region being the hardest hit, experiencing the single most intense and the second most expansive drought in the last 800 years. The extreme severity, duration, and extent of this global event is associated with an extraordinary combination of preceding cool tropical Pacific conditions (1870–76), a record-breaking El Niño (1877–78), a record strong Indian Ocean dipole (1877), and record warm North Atlantic Ocean (1878) conditions. Composites of historical analogs and two sets of ensemble simulations—one forced with global sea surface temperatures (SSTs) and another forced with tropical Pacific SSTs—were used to distinguish the role of the extreme conditions in different ocean basins. While the drought in most regions was largely driven by the tropical Pacific SST conditions, an extreme positive phase of the Indian Ocean dipole and warm North Atlantic SSTs, both likely aided by the strong El Niño in 1877–78, intensified and prolonged droughts in Australia and Brazil, respectively, and extended the impact to northern and southeastern Africa. Climatic conditions that caused the Great Drought and Global Famine arose from natural variability, and their recurrence, with hydrological impacts intensified by global warming, could again potentially undermine global food security.

This paper summarizes advances in research on tropical–polar teleconnections, made roughly over the last decade. Elucidating El Niño–Southern Oscillation (ENSO) impacts on high latitudes has remained an important focus along different lines of inquiry. Tropical to polar connections have also been discovered at the intraseasonal time scale, associated with Madden–Julian oscillations (MJOs). On the time scale of decades, changes in MJO phases can result in temperature and sea ice changes in the polar regions of both hemispheres. Moreover, the long-term changes in SST of the western tropical Pacific, tropical Atlantic, and North Atlantic Ocean have been linked to the rapid winter warming around the Antarctic Peninsula, while SST changes in the central tropical Pacific have been linked to the warming in West Antarctica. Rossby wave trains emanating from the tropics remain the key mechanism for tropical and polar teleconnections from intraseasonal to decadal time scales. ENSO-related tropical SST anomalies affect higher-latitude annular modes by modulating mean zonal winds in both the subtropics and midlatitudes. Recent studies have also revealed the details of the interactions between the Rossby wave and atmospheric circulations in high latitudes. We also review some of the hypothesized connections between the tropics and poles in the past, including times when the climate was fundamentally different from present day especially given a larger-than-present-day global cryosphere. In addition to atmospheric Rossby waves forced from the tropics, large polar temperature changes and amplification, in part associated with variability in orbital configuration and solar irradiance, affected the low–high-latitude connections.

Predictions of changes of the land monsoon rainfall (LMR) in the coming decades are of vital importance for successful sustainable economic development. Current dynamic models, though, have shown little skill in the decadal prediction of the Northern Hemisphere (NH) LMR (NHLMR). The physical basis and predictability for such predictions remain largely unexplored. Decadal change of the NHLMR reflects changes in the total NH continental precipitation, tropical general circulation, and regional land monsoon rainfall over northern Africa, India, East Asia, and North America. Using observations from 1901 to 2014 and numerical experiments, it is shown that the decadal variability of the NHLMR is rooted primarily in (i) the north–south hemispheric thermal contrast in the Atlantic–Indian Ocean sector measured by the North Atlantic–south Indian Ocean dipole (NAID) sea surface temperature (SST) index and (ii) an east–west thermal contrast in the Pacific measured by an extended El Niño–Southern Oscillation (XEN) index. Results from a 500-yr preindustrial control experiment demonstrate that the leading mode of decadal NHLMR and the associated NAID and XEN SST anomalies may be largely an internal mode of Earth’s climate system, although possibly modified by natural and anthropogenic external forcing. A 51-yr, independent forward-rolling decadal hindcast was made with a hybrid dynamic conceptual model and using the NAID index predicted by a multiclimate model ensemble. The results demonstrate that the decadal changes in the NHLMR can be predicted approximately a decade in advance with significant skills, opening a promising way forward for decadal predictions of regional land monsoon rainfall worldwide.

Abstarct In this model study the authors explore the possibility that the internal component of the Atlantic multidecadal oscillation (AMO) sea surface temperature (SST) signal is indistinguishable from the response to white noise forcing from the atmosphere and ocean. Here, complex models are compared without externally varying forcing with a one-dimensional noise-driven model for SST. General analytic expressions are obtained for both unfiltered and low-pass filtered lead–lag correlations. It is shown that this simple model reproduces many of the simulated lead–lag relationships among temperature, rate of change of temperature, and surface heat flux. It is concluded that the finding that at low frequencies the ocean loses heat to the atmosphere when the temperature is warm, which has been interpreted as showing that the ocean circulation drives the AMO, is a necessary consequence of the fact that at long periods the net heat flux (ocean plus atmosphere) is zero to a good approximation. It does not distinguish between the atmosphere and ocean as the source of the AMO and is consistent with the hypothesis that the AMO is driven by white noise heat fluxes. It is shown that some results in the literature are artifacts of low-pass filtering, which creates spurious low-frequency signals when the underlying data are white or red noise. It is concluded that in the absence of external forcing the AMO in most GCMs is consistent with being driven by white noise, primarily from the atmosphere.

Focusing on ENSO seasonal phase locking, diversity in peak location, and propagation direction, as well as the El Niño–La Niña asymmetry in amplitude, duration, and transition, a set of empirical probabilistic diagnostics (EPD) is introduced to investigate how the ENSO behaviors reflected in SST may change in a warming climate. EPD is first applied to estimate the natural variation of ENSO behaviors. In the observations El Niños and La Niñas mainly propagate westward and peak in boreal winter. El Niños occur more at the eastern Pacific whereas La Niñas prefer the central Pacific. In a preindustrial control simulation of the GFDL CM2.1 model, the El Niño–La Niña asymmetry is substantial. La Niña characteristics generally agree with observations but El Niño’s do not, typically propagating eastward and showing no obvious seasonal phase locking. So an alternative approach is using a stochastically forced simulation of a nonlinear data-driven model, which exhibits reasonably realistic ENSO behaviors and natural variation ranges. EPD is then applied to assess the potential changes of ENSO behaviors in the twenty-first century using CMIP5 models. Other than the increasing SST climatology, projected changes in many aspects of ENSO reflected in SST anomalies are heavily model dependent and generally within the range of natural variation. Shifts favoring eastward-propagating El Niño and La Niña are the most robust. Given various model biases for the twentieth century and lack of sufficient model agreements for the twenty-first-century projection, whether the projected changes for ENSO behaviors would actually take place remains largely uncertain.

A suite of empirical model experiments under the empirical model reduction framework are conducted to advance the understanding of ENSO diversity, nonlinearity, seasonality, and the memory effect in the simulation and prediction of tropical Pacific sea surface temperature (SST) anomalies. The model training and evaluation are carried out using 4000-yr preindustrial control simulation data from the coupled model GFDL CM2.1. The results show that multivariate models with tropical Pacific subsurface information and multilevel models with SST history information both improve the prediction skill dramatically. These two types of models represent the ENSO memory effect based on either the recharge oscillator or the time-delayed oscillator viewpoint. Multilevel SST models are a bit more efficient, requiring fewer model coefficients. Nonlinearity is found necessary to reproduce the ENSO diversity feature for extreme events. The nonlinear models reconstruct the skewed probability density function of SST anomalies and improve the prediction of the skewed amplitude, though the role of nonlinearity may be slightly overestimated given the strong nonlinear ENSO in GFDL CM2.1. The models with periodic terms reproduce the SST seasonal phase locking but do not improve the prediction appreciably. The models with multiple ingredients capture several ENSO characteristics simultaneously and exhibit overall better prediction skill for more diverse target patterns. In particular, they alleviate the spring/autumn prediction barrier and reduce the tendency for predicted values to lag the target month value.

The authors investigate a sea surface temperature anomaly (SSTA)-only vector autoregressive (VAR) model for prediction of El Niño–Southern Oscillation (ENSO). VAR generalizes the linear inverse method (LIM) framework to incorporate an extended state vector including many months of recent prior SSTA in addition to the present state. An SSTA-only VAR model implicitly captures subsurface forcing observable in the LIM residual as red noise. Optimal skill is achieved using a state vector of order 14–17 months in an exhaustive 120-yr cross-validated hindcast assessment. It is found that VAR outperforms LIM, increasing forecast skill by 3 months, in a 30-yr retrospective forecast experiment.

By analyzing a set of the Coupled Model Intercomparison Project phase 3 (CMIP3) climate model projections of the twenty-first century, it is found that the shallow meridional overturning of the Pacific subtropical cells (STCs) show contrasting trends between two hemispheres in a warming climate. The strength of STCs and equivalently the STC surface-layer transport tend to be weakening (strengthening) in the Northern (Southern) Hemisphere as a response to large-scale surface wind changes over the tropical Pacific. The STC pycnocline transport convergence into the equatorial Pacific Ocean from higher latitudes shows a robust weakening in the twenty-first century. This weakening is mainly through interior pathways consistent with the relaxation of the zonal pycnocline tilt, whereas the transport change through western boundary pathways is small and not consistent across models. It is found that the change of the western boundary pycnocline transport is strongly affected by the shoaling of the pycnocline base. In addition, there is a robust weakening of the Indonesian Throughflow (ITF) transport in a warming climate. In the multimodel ensemble mean, the response to greenhouse warming of the upper-ocean mass balance associated with the STCs is such that the weakening of the equatorward pycnocline transport convergence is balanced by a weakening of the poleward surface-layer transport divergence and the ITF transport of similar amounts.

Decadal variations of very small amplitude [∼0.3°C in sea surface temperature (SST)] in the tropical Pacific Ocean, the genesis region of the interannual El Niño–Southern Oscillation (ENSO) phenomenon, have been shown to have powerful impacts on global climate. Future projections from different climate models do not agree on how this critical feature will change under the influence of anthropogenic forcing. A number of attempts have been made to resolve this issue by examining observed trends from the 1880s to the present, a period of rising atmospheric concentrations of greenhouse gases. A recent attempt concluded that the three major datasets disagreed on the trend in the equatorial gradient of SST. Using a corrected version of one of these datasets, and extending the analysis to the seasonal cycle, it is shown here that all agree that the equatorial Pacific zonal SST gradient has strengthened from 1880 to 2005 during the boreal fall when this gradient is normally strongest. This result appears to favor a theory for future changes based on ocean dynamics over one based on atmospheric energy considerations. Both theories incorporate the expectation, based on ENSO theory, that the zonal sea level pressure (SLP) gradient in the tropical Pacific is coupled to SST and should therefore strengthen along with the SST gradient. While the SLP gradient has not strengthened, it is found that it appears to have weakened only during boreal spring, consistent with the SST seasonal trends. Most of the coupled models included in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report underestimate the strengthening SST gradient in boreal fall, and show almost no change in the SLP gradient in any season. The observational analyses herein suggest that both theories are at work but with relative strengths that vary seasonally, and that the two theories need not be inconsistent with each other.

The response of the equatorial Indian Ocean climate to global warming is investigated using model outputs submitted to the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report. In all of the analyzed climate models, the SSTs in the western equatorial Indian Ocean warm more than the SSTs in the eastern equatorial Indian Ocean under global warming; the mean SST gradient across the equatorial Indian Ocean is anomalously positive to the west in a warmer twenty-first-century climate compared to the twentieth-century climate, and it is dynamically consistent with the anomalous westward zonal wind stress and anomalous positive zonal sea level pressure (SLP) gradient to the east at the equator. This change in the zonal SST gradient in the equatorial Indian Ocean is detected even in the lowest-emission scenario, and the size of the change is not necessarily larger in the higher-emission scenario. With respect to the change over the equatorial Pacific in climate projections, the subsurface central Pacific displays the strongest cooling or weakest warming around the thermocline depth compared to that above and below in all of the climate models, whereas changes in the zonal SST gradient and zonal wind stress around the equator are model dependent and not straightforward.

It has recently been proposed, within the framework of the linear shallow-water equations, that tropical Pacific decadal variability (PDV) can be accounted for by basin modes with eigenperiods of 10 to 20 yr, amplifying a midlatitude wind forcing with an essentially white spectrum. Here the authors use a different formalism of linear equatorial wave theory. The Green’s function is computed for the wind-forced response of a linear equatorial shallow-water ocean and use the earlier results of Cane and Moore to obtain a compact, closed form expression for the motion of the equatorial thermocline, which applies to all frequencies lower than seasonal. This expression is new and allows a systematic comparison of the effect of low- and high-latitude winds on the equatorial thermocline. At very low frequencies (decadal time scales), the planetary geostrophic solution used by Cessi and Louazel is recovered, as well as the equatorial wave solution of Liu, and a formal explanation for this convergence is given. Nonetheless, this more general solution leads one to a different interpretation of the results. In contrast to the aforementioned studies, the authors find that the equatorial thermocline is inherently more sensitive to local than to remote wind forcing and that planetary Rossby modes only weakly alter the spectral characteristics of the response. Tropical winds are able to generate a strong equatorial response with periods of 10 to 20 yr, while midlatitude winds can only do so for periods longer than about 50 yr. The results suggest that ocean basin modes are an unlikely explanation of decadal fluctuations in tropical Pacific sea surface temperature.

Could the Dust Bowl drought of the 1930s have been predicted in advance if the SST anomalies of the 1930s had been foreknown? Ensembles of model simulations forced with historical observed SSTs in the global ocean, and also separately in the tropical Pacific and Atlantic Oceans, are compared with an ensemble begun in January 1929 with modeled atmosphere and land initial conditions and integrated through the 1930s with climatological SSTs. The ensemble with climatological SSTs produces values for the precipitation averaged over 1932–39 that are not statistically different from model climatology. In contrast, the ensembles with global SST forcing produce a drought centered in the central plains and southwestern North America that is clearly separated from the model climatology. Both the tropical Pacific and northern tropical Atlantic SST anomalies produce a statistically significant model drought in this region. The modeled drought has a spatial pattern that is different from the observed drought, which was instead centered in the central and northern plains and also impacted the northern Rocky Mountain states but not northeastern Mexico. The model error in extending the Dust Bowl drought too far south is attributed to an incorrect response of the model to warm subtropical North Atlantic SST anomalies. The model error in the northern states cannot be attributed to an incorrect response to tropical SST anomalies. The model also fails to reproduce the strong surface air warming across most of the continent during the 1930s. In contrast, the modeled patterns of precipitation reduction and surface air temperature warming during the 1950s drought are more realistic. Tree-ring records show that the Dust Bowl pattern of drought has occurred before, suggesting that while the extensive human-induced land surface degradation and dust aerosol loading of the 1930s drought may have played an important role in generating the observed drought pattern, natural processes, possibly including land interactions, are capable of generating droughts centered to the north of the main ENSO teleconnection region. Despite this caveat, advance knowledge of tropical SSTs alone would have allowed a high-confidence prediction of a multiyear and severe drought, but one centered too far south and without strong cross-continental warming.

The relationship between all-India summer monsoon rainfall (ISMR) and the timing of (El Niño–Southern Oscillation) ENSO-related warming/cooling is investigated, using observational data during the period from 1881 to 1998. The analysis of the evolutions of Indo-Pacific sea surface temperature (SST) anomalies suggests that when ISMR is not below normal despite the co-occurrence of an El Niño event, warming over the eastern equatorial Pacific starts from boreal winter and evolves early so that the western-central Pacific and Indian Ocean are warmer than normal during the summer monsoon season. In contrast, when the more usual El Niño–dry ISMR relationship holds, the eastern equatorial Pacific starts warming rapidly only about a season before the reference summer so that the western-central Pacific and Indian Oceans remain cold during the monsoon season.

The state of the Indian Ocean dipole representing the SST anomaly difference between the western and southeastern regions of the ocean is investigated using historical SST reconstructions from 1880 to 2004. First, the western and eastern poles of the SST-based dipole mode index are analyzed separately. Both the western and eastern poles display warming trends over this period, particularly after the 1950s. The western pole tends to be anomalously colder than the eastern pole from 1880 to 1919, whereas in the interval 1950–2004 the SST anomalies over the western pole are comparable to those over the eastern pole though there are occasional outliers where the eastern pole is anomalously colder than the western pole.The tendencies of the occurrences of positive and negative dipole events in September–November show three distinct regimes during the period analyzed. In 1880–1919, negative dipole events associated with La Niña events occur more frequently than positive events. In 1920–49, some weak positive events occur relatively independently of El Niño events over the Pacific. The period of 1960–2004 is characterized by strong and frequent occurrences of positive events associated with El Niño events.

The seasonal and interannual predictability of ENSO variability in a version of the Zebiak–Cane coupled model is examined in a perturbation experiment. Instead of assuming that the model is “perfect,” it is assumed that a set of optimal initial conditions exists for the model. These states, obtained through a nonlinear minimization of the misfit between model trajectories and the observations, initiate model forecasts that correlate well with the observations. Realistic estimates of the observational error magnitudes and covariance structures of sea surface temperatures, zonal wind stress, and thermocline depth are used to generate ensembles of perturbations around these optimal initial states, and the error growth is examined. The error growth in response to subseasonal stochastic wind forcing is presented for comparison.In general, from 1975 to 2002, the large-scale uncertainty in initial conditions leads to larger error growth than continuous stochastic forcing of the zonal wind stress fields. Forecast ensemble spread is shown to depend most on the calendar month at the end of the forecast rather than the initialization month, with the seasons of greatest spread corresponding to the seasons of greatest anomaly variance. It is also demonstrated that during years with negative (and rapidly decaying) Niño-3 SST anomalies (such as the time period following an El Niño event), there is a suppression of error growth. In years with large warm ENSO events, the ensemble spread is no larger than in moderately warm years. As a result, periods with high ENSO variance have greater potential prediction utility.In the realistic range of observational error, the ensemble spread has more sensitivity to the initial error in the thermocline depth than to the sea surface temperature or wind stress errors. The thermocline depth uncertainty is the principal reason why initial condition uncertainties are more important than wind noise for ensemble spread.

A time-dependent trajectory algorithm is used to determine the sources of the Pacific Ocean Equatorial Undercurrent (EUC) in a global climate model with ¼° (eddy permitting) resolution and forced with realistic winds. The primary sources and pathways are identified, and the transformation of properties in temperature/salinity space is explored. An estimate for the quantity of recirculation, a notoriously difficult property to estimate from observational data, is given. Over two-thirds of the water in the Pacific EUC at 140°W originates south of the equator; 70% of the EUC is ventilated outside of the Tropics (poleward of 13°S or 10°N): three-quarters of these extratropical trajectories travel through the western boundary currents between their subduction and incorporation into the EUC, and one-fifth of the extratropical trajectories enter and leave the tropical band at least once before entering the EUC.

The Zebiak–Cane (ZC) model for simulation of the El Nino–Southern Oscillation is shown to be capable of producing sequences of variability that exhibit shifts in the time-mean state of the eastern equatorial Pacific that resemble observations of tropical Pacific decadal variability. The model's performance in predicting these shifts is compared to two naive forecasting strategies. It is found that the ZC model consistently outperforms the two naive forecasts that serve as a null hypothesis in assessing the significance of results. Forecasts initialized during anomalously warm and anomalously cold decades are shown to have the highest predictability. These modeling results suggest that, to a moderate extent, the state of the tropical Pacific in one decade can predetermine its time-mean state in the following decade. However, even in this idealized context decadal forecasting skill is modest. Results are discussed in the context of their implications for the ongoing debate over the origin of decada...

A bimonthly coral-based record of the postbomb radiocarbon content of Solomon Sea surface waters is interpreted to reflect mixing of subtropical surface water and that advected in from the east by the equatorial branch of the South Equatorial Current (SEC). Annual mean D14C has a dynamic range of nearly 175‰, with a total range of nearly 200‰. Prebomb values average 256‰ and the annual mean postbomb maxima occurs in 1985 with a value of 1117%. Interannual variability in the record reflects surface current variations in

Pacific Ocean oceanic heat transport is studied in an ocean model coupled to an atmospheric mixed-layer model. The shallow meridional overturning circulation cells in the Tropics and subtropics transport heat away from the equator. The heat transport by the horizontal gyre circulation in the Tropics is smaller and directed toward the equator. The response of the Pacific oceanic heat transport to El Nino-like winds, extratropical winds, and variations in the Indonesian Throughflow is studied. Large, opposing changes are found in the heat transport by the meridional overturning and the horizontal gyres in response to El Nino-like winds. Consequently, the change in total heat transport is relatively small. The overturning transport decreases and the gyres spin down when the winds decrease in the Tropics. This compensation breaks down when the Indonesian Throughflow is allowed to vary in the model. A reduced Indonesian Throughflow, as observed during El Nino-like conditions, causes a large reduction of poleward heat transport in the South Pacific and affects the ocean heat transport in the southern tropical Pacific. Extratropical atmospheric anomalies can affect tropical ocean heat transport as the tropical thermocline is ventilated from the extratropics. The authors find that changes in the heat loss in the midlatitudes affect tropical ocean heat transport by driving an enhanced buoyancy-driven overturning that reaches into the Tropics. The results are related to observed changes in the overturning circulation in the Pacific in the 1990s, sea surface temperarture changes, and changes in atmospheric circulation. The results imply that the ratio of heat transport in the ocean to that in the atmosphere can change.

An ensemble local hydrologic forecast derived from the seasonal forecasts of the International Research Institute for Climate Prediction (IRI) is presented. Three-month seasonal forecasts were used to resample historical meteorological conditions and generate ensemble forcing datasets for a TOPMODEL-based hydrology model. Eleven retrospective forecasts were run at Florida and New York sites. Forecast skill was assessed for mean area modeled water table depth (WTD) and compared with WTD simulated with observed data. Hydrology model forecast skill was evident at the Florida site. Persistence of initial hydrologic conditions and local skill of the IRI seasonal forecast contributed to this local hydrologic forecast skill. At the New York site, there was no persistence of initial hydrologic conditions and local skill of the IRI seasonal forecast was poor; these factors precluded local hydrologic forecast skill at this site.

A number of studies have attempted to explain the cause of decadal variability in the tropical Pacific and explore its possible link to decadal variability in the midlatitude Pacific. To investigate some of the current theories of Pacific decadal variability, a linear, wind-driven model, designed to simulate only baroclinic wave dynamics, was forced with wind stress anomalies in the Pacific Ocean basin from 1945 through 1992. An analysis technique designed to isolate the decadal/interdecadal scale variability from interannual ENSO variability was performed on the model’s thermocline depth anomaly (TDA). It was found that the temporal and spatial patterns of the observed tropical decadal sea surface temperatures are consistent with our modeled TDA. Furthermore, restricting the wind forcing to within 58 of the equator does not substantially alter the decadal/interdecadal variability of the equatorial region. The authors conclude that the observed decadal variability in the low-latitude Pacific is primarily a linear dynamical response to tropical wind forcing and does not directly require an oceanic link to the midlatitudes. The question of how tropical wind anomalies are generated is not addressed. In addition, it is shown that in model scenarios where the wind forcing is restricted to the western equatorial Pacific, the 1976‐77 climate shift is still clearly visible as a dominant feature of tropical decadal variability. The temporal decadal signal of the model-generated TDA is more pronounced during the eastern equatorial upwelling season (July‐September) than in the boreal winter. This is consistent with the observed seasonal bias in tracer and SST data from the eastern equatorial Pacific.

Recent developments in Tropical Atlantic Variability (TAV) identify the El Nino-Southern Oscillation (ENSO) as one of the leading factors in the interannual climate variability of the basin. An ENSO event results in Tropic- wide anomalies in the atmospheric circulation that have a direct effect on precipitation variability, as well as an indirect effect, that is, one mediated by sea surface temperature (SST) anomalies generated in the remote ocean basins. In order to separate the relative contributions of the atmospheric and oceanic components of the ENSO teleconnection to the tropical Atlantic Ocean, results from two ensembles of atmospheric general cir- culation model (AGCM) experiments, differing in oceanic boundary conditions, are compared. AGCM integra- tions performed with the Community Climate Model version 3 (CCM3), forced by global, observed SST during 1950-94 reproduce the observed ENSO-related rainfall anomalies over the tropical Americas and adjacent Atlantic. A parallel ensemble of integrations, forced with observed SST in the tropical Atlantic only, and climatology elsewhere, is used to separate the effect of the direct atmospheric teleconnection from the atmo- sphere's response to the ENSO-forced SST anomalies in the Atlantic basin. It is found that ENSO-related atmospheric and oceanic anomalies force rainfall anomalies of the same sign in northeast Brazil, of opposite sign in the Caribbean basin. The direct atmospheric influence of a warm ENSO event reduces model rainfall as a whole over the tropical Atlantic basin. This observation is consistent with the hypothesis that an ENSO-related Tropic-wide warming of the free troposphere forces the vertical stabilization of the tropical atmosphere. ENSO-related atmospheric anomalies are also known to force a delayed (relative to the mature phase of ENSO) warming of tropical North Atlantic SST through the weakening of the northeasterly trade winds and consequent reduction of surface fluxes. It is found that this delayed oceanic component forces a northward displacement of the Atlantic intertropical convergence zone, resulting in increased precipitation over the Caribbean and reduced precipitation over northeast Brazil during the boreal spring following the mature phase of ENSO.

The causes of decadal variations of North Pacific sea surface temperatures (SSTs) are examined using a hindcast performed with an ocean general circulation model thermodynamically coupled to an atmospheric mixed layer model (OGCM–AML model) and forced by the time history of observed surface winds. The “shift” in North Pacific Ocean climate that occurred around 1976/77 is focused on since this is the best observed example available. After the 1976/77 shift the Aleutian low deepened and moved to the southeast of its previous position. This placed anomalous cyclonic flow over the North Pacific. The SST response, as simulated by the ocean model, consisted of two components: a fast and local part and a delayed and remote part. In the central Pacific stronger westerlies cool the ocean by increased equatorward Ekman drift. Here the dynamical cooling is sufficiently large that the surface fluxes damp the SST anomaly. This Ekman response is fast and local and cools the SSTs beginning in 1977 and persistin...

The El Nino–Southern Oscillation (ENSO) phenomenon and variability in the subtropical North Atlantic high sea level pressure (SLP) are known to affect rainfall in the Caribbean region. An El Nino event is associated with drier-than-average conditions during the boreal summer of year (0), and wetter-than-average conditions during the spring of year (+1). Dry conditions during the summer of year (0) of an El Nino are associated with the locally divergent surface circulation engendered by the eastward shift of deep convection in the Pacific Ocean. Wet conditions during the spring of year (+1) of an El Nino are associated with the lagged warming of the tropical North Atlantic Ocean. Variability in the strength of the North Atlantic high is governed mainly by the North Atlantic oscillation (NAO) with a positive NAO phase implying a stronger than normal high and vice versa. The NAO is negatively correlated with Caribbean rainfall indirectly via anomalous sea surface temperatures (SST) associated with a...

Paleoclimatic data are increasingly showing that abrupt change is present in wide regions of the globe. Here a mechanism for abrupt climate change with global implications is presented. Results from a tropical coupled ocean–atmosphere model show that, under certain orbital configurations of the past, variability associated with El Nino–Southern Oscillation (ENSO) physics can abruptly lock to the seasonal cycle for several centuries, producing a mean sea surface temperature (SST) change in the tropical Pacific that resembles a La Nina. It is suggested that this change in SST would have a global impact and that abrupt events such as the Younger Dryas may be the outcome of orbitally driven changes in the tropical Pacific.

A dynamically motivated statistical forecasting scheme for eastern Mediterranean winter rainfall is presented. The scheme is based on North Atlantic sea level pressure precursors. The resulting forecasts are robust and statistically significant at ∼13 months lead time, and improve at ∼7 months lead. It is suggested that these forecasts form a foundation for an operational early-warning system for eastern Mediterranean droughts.

Near-global 4 83 48 gridded analysis of marine sea level pressure (SLP) from the Comprehensive Ocean‐ Atmosphere Data Set for monthly averages from 1854 to 1992 was produced along with its estimated error using a reduced space optimal interpolation method. A novel procedure of covariance adjustment brought the results of the analysis to the consistency with the a priori assumptions on the signal covariance structure. Comparisons with the National Centers for Environmental Prediction‐National Center for Atmospheric Research global atmosphere reanalysis, with the National Center for Atmospheric Research historical analysis of the Northern Hemisphere SLP, and with the global historical analysis of the U.K. Meteorological Office show encouraging skill of the present product and identifies noninclusion of the land data as its main limitation. Marine SLP pressure proxies are produced for the land stations used in the definitions of the Southern Oscillation and North Atlantic Oscillation (NAO) indices. Surprisingly, they prove to be competitive in quality with the land station records. Global singular value decomposition analysis of the SLP fields versus sea surface temperature identified three major patterns of their joint large-scale and long-term variability as ‘‘trend,’’ Pacific decadal oscillation, and NAO.

The observed subtropical gyre in the North Pacific shows a shift in meridional location with depth. At shallow levels the density deviation peaks at around 15°N while at deep levels the peak is more like 30°N. It is argued here using analytical solutions to the beta-plane shallow-water equations that such a shift can be explained by the effects of oceanic dissipation processes. These solutions show that the highly damped solution is approximately proportional to Ekman pumping whereas the lightly damped case tends toward the classical Sverdrup solution. In the North Pacific, Ekman pumping peaks near 15°N while the Sverdrup solution peaks at 30°N. It is further demonstrated that 1) density deviations in the upper ocean are more highly influenced by higher order baroclinic modes than those in the deep, which are influenced by the lower modes, and 2) constant dissipation effectively acts much more strongly on the higher order baroclinic modes because of their slower speeds and smaller Rossby radii. T...

A finite-difference scheme for solving the linear shallow water equations in a bounded domain is described. Its time step is not restricted by a Courant–Friedrichs–Levy (CFL) condition. The scheme, known as Israeli–Naik–Cane (INC), is the offspring of semi-Lagrangian (SL) schemes and the Cane–Patton (CP) algorithm. In common with the latter it treats the shallow water equations implicitly in y and with attention to wave propagation in x. Unlike CP, it uses an SL-like approach to the zonal variations, which allows the scheme to apply to the full primitive equations. The great advantage, even in problems where quasigeostrophic dynamics are appropriate in the interior, is that the INC scheme accommodates complete boundary conditions.

The large-scale ocean‐atmosphere patterns that influence the interannual variability of Caribbean‐Central American rainfall are examined. The atmospheric circulation over this region is shaped by the competition between the North Atlantic subtropical high sea level pressure system and the eastern Pacific ITCZ, which influence the convergence patterns on seasonal and interannual timescales. The authors find the leading modes of interannual sea level pressure (SLP) and SST variability associated with Caribbean rainfall, as selected by canonical correlation analysis, to be an interbasin mode, linking the eastern Pacific with the tropical Atlantic, and an Atlantic mode. North Atlantic SLP affects Caribbean rainfall directly, by changing the patterns of surface flow over the region, and indirectly, through SST anomalies. Anomalously high SLP in the region of the North Atlantic high translates into stronger trade winds, hence cooler SSTs, and less Caribbean rainfall. The interbasin mode, which manifests itself as a zonal seesaw in SLP between the tropical Atlantic and the eastern equatorial Pacific, is correlated with ENSO. When SLP is low in the eastern equatorial Pacific, it is high in the tropical Atlantic: the surface atmospheric flow over the basin is divergent, to the west, toward the eastern Pacific ITCZ, and to the east, toward the tropical North Atlantic. A weakened meridional SLP gradient in the tropical North Atlantic signifies weaker trade winds and the opportunity for SSTs to warm up, reaching peak intensity 2‐4 months after the mature phase of an ENSO event. This SST anomaly is particularly evident in the Caribbean‐western Atlantic basin. The tendency is for drier-than-average conditions when the divergent atmospheric flow dominates, during the rainy season preceding the mature phase of a warm ENSO event. The dry season that coincides with the mature phase of ENSO is wetter than average over the northwestern section of the basin, that is, Yucatan, the Caribbean coast of Honduras, and Cuba, and drier than average over the rest of the basin, that is, Costa Rica and northern South America. The following spring, as the atmospheric circulation transitions to normal conditions, the positive SST anomaly that has built up in the basin takes over, favoring convection. The positive precipitation anomaly spreads southeastward, from the northwest to the entire basin. At the start of a new rainy season, it is especially strong over the Greater Antilles.

El Nino events owe their name to their tendency to be locked to the seasonal cycle. A simple explanation is proposed here for the locking of the peak of ENSO's basin-scale warming to the end of the calendar year. The explanation is based on incorporating a seasonally varying coupled ocean-atmosphere instability strength into the delayed oscillator mechanism for the ENSO cycle. It is shown that the seasonally varying amplification of the Rossby and Kelvin ocean waves by the coupled instability forces the events to peak when this amplification is at its minimum strength, at the end of the calendar year. The mechanism is demonstrated using a simple delayed oscillator model and is further analyzed using the Cane-Zebiak model. Being based on the oversimplified delayed oscillator paradigm of ENSO, the proposed mechanism cannot be expected to fully explain the locking of observed events to the end of the year. However, the wave dynamics perspective it offers to approaching the ENSO phase-locking problem may serve as a first step toward a fuller explanation based on more realistic models and additional data analysis.

The behavior of the solution to a two-layer wind-driven model in a multiply connected domain with bottom topography imitating the Southern Ocean is described. The abyssal layer of the model is forced by interfacial friction, crudely simulating the effect of eddies. The analysis of the low friction regime is based on the method of characteristics. It is found that characteristics in the upper layer are closed around Antarctica, while those in the lower layer are blocked by solid boundaries. The momentum input from wind in the upper layer is balanced by lateral and interfacial friction and by interfacial pressure drag. In the lower layer the momentum input from interfacial friction and interfacial pressure drag is balanced by topographic pressure drag. Thus, the total momentum input by the wind is balanced by upper-layer lateral friction and by topographic pressure drag. In most of the numerical experiments the circulations in the two layers appear to be decoupled. The decoupling can be explained b...

The fastest perturbation growth (optimal growth) in forecasts of El Nino–Southern Oscillation (ENSO) with the Zebiak and Cane model is analyzed by singular value decomposition of forward tangent models along forecast trajectories in a reduced EOF space. The authors study optimal growth in forecast runs using two different initialization procedures and discuss the relationship between optimal growth and forecast skill. Consistent with Part I of this work, one dominant growing singular vector is found. Most of the variation of optimal growth, measured by the largest singular value, for warm events and mean condition is seasonal, attributable to the seasonal variations in the background states. For cold events the seasonal optimal growth is substantially suppressed. The first singular vector is approximately white in EOF space, while its final pattern after a 6-month evolution is dominated by the first EOF. The energy norm amplifies between 5- and 24-fold in 6 months. This indicates that small-scale...

The nature and causes of the recent increase in North Atlantic wave heights are explored by combining a numerical hindcast with a statistical analysis. The numerical hindcast incorporates a 10-yr history (1980–89) of North Atlantic, twice daily wind analyses to generate a monthly averaged significant wave height (SWH) history. The hindcast compares favorably with published monthly averaged SWH observations. The link between model-generated wintertime monthly SWH and monthly averaged sea level pressure (SLP) data is determined by means of a canonical correlation analysis (CCA). Within the analysis domain, most of the variance in SWH and SLP is captured by two pairs of joint patterns. The leading pair consists of a SLP dipole resembling the North Atlantic Oscillation (NAO) and a SWH dipole in spatial quadrature relation to it. Using the CCA results, an extended statistical hindcast of monthly wave fields is generated from sea level pressure data and used to quantitatively estimate the systematic in...

The skill of a coupled ocean‐atmosphere model in predicting ENSO has recently been improved using a new initialization procedure in which initial conditions are obtained from the coupled model, nudged toward observations of wind stress. The previous procedure involved direct insertion of wind stress observations, ignoring model feedback from ocean to atmosphere. The success of the new scheme is attributed to its explicit consideration of ocean‐atmosphere coupling and the associated reduction of ‘‘initialization shock’’ and random noise. The so-called spring predictability barrier is eliminated, suggesting that such a barrier is not intrinsic to the real climate system. Initial attempts to generalize the nudging procedure to include SST were not successful; possible explanations are offered. In all experiments forecast skill is found to be much higher for the 1980s than for the 1970s and 1990s, suggesting decadal variations in predictability.

The Kalman filter is the optimal linear assimilation scheme only if the first- and second-order statistics of the observational and system noise are correctly specified. If not, optimality can be reached in principle by using an adaptive filter that estimates both the state vector and the system error statistics. In this study, the authors compare the ability of three adaptive assimilation schemes at estimating an unbiased, stationary system noise. The adaptive algorithms are implemented in a reduced space linear model for the tropical Pacific. Using a twin experiment approach, the algorithms are compared by assimilating sea level data at fixed locations mimicking the tropical Pacific tide gauges network. It is shown that the description of the system error covariance matrix requires too many parameters for the adaptive problem to be well posed. However, the adaptive procedures are efficient if the number of noise parameters is dramatically reduced and their performance is shown to be closed to optimal, that is, based on the true system noise covariance. The link between those procedures is elucidated, and the question of their applicability and respective computational cost is discussed.

A linear equivalent barotropic (EB) model is applied to study the effects of the bottom topography H and baroclinicity on the total transport and the position of the Antarctic Circumpolar Current (ACC). The model is based on the observation of Killworth that the time mean velocity field of the FRAM Model is self-similar in the vertical. A realistic large-scale topography H is constructed by filtering 5-minute resolution data with an appropriate smoothing kernel. It is shown that the asymptotic behavior of the solution of the barotropic model (a particular case of the EB model) in the limit of very small bottom friction depends on subtle details of topography and basin geometry. Given the uncertainties of the smoothing procedure the authors conclude that the barotropic model is not robust with respect to possible variations of model topography. The authors found that the EB model with a vertical profile function similar to that of Killworth reproduces the major features of the time- and depth-ave...

The role of ocean dynamics in the regulation of tropical sea surface temperatures (SSTs) is investigated using the Zebiak-Cane coupled occan-atmosphere model. The model is forced with a uniform heating, or cooling, varying between ±40 W m−2 into the ocean surface. A new climatological SST pattern is established for which the area-averaged temperature change is smaller in magnitude than the imposed forcing. The forcing is balanced almost equally by a change in the heat flux out of the ocean and by vertical advection of heat in the ocean through anomalous equatorial ocean upwelling. The generation of anomalous upwelling is identified here as a possible mechanism capable of regulating tropical SSTS. This ocean dynamical thermostat mechanism has, a seasonally varying efficiency that causes amplification (weakening) of the seasonal cycle for the heating (cooling). The interannual variability also changes under the imposed forcing. These results suggest that the role of ocean dynamic should he included...

Recent modeling studies of thermohaline variability have imposed rapid damping of modeled sea surface temperature (SST) anomalies equivalent to assuming the atmosphere has an infinite heat capacity. Such surface heat flux parameterizations effectively exclude the possibility of SST playing an active role in the thermohaline circulation. The authors present results of simple thermodynamic modeling of the lower atmosphere that suggest the sensitivity of the surface heat fluxes to variations in SST is much smaller than often assumed. It is found that the flux response is strongly dependent on the scale of the SST anomaly. For the very largest scales the fluxes increase by only a few watts per square meter per kelvin change of SST. For the scales typical of observed anomalies the nonlocality of the response enhances the sensitivity, which may reach up to ∼15 W m−2 K−1. This extreme is still less than half of the values typically assumed in ocean models. The small sensitivity arises from the adjustmen...

In Part II of this study on the application of the interactive Kalman filter to higher-dimensional systems, a modification suited to periodically forced systems is introduced. As in Part I, the object of study here is the ENSO model of Zebiak and Cane, but here the technique of quasi-fixed points is applied to certain Poincare maps of that system that are related to the forcing period of I year. As a result, it is possible to search the model systematically for possible periodic orbits, no matter whether they are stable or unstable. An unstable 4-year cycle is found in the model, and it is argued that this cycle can be traced back to a 4-year limit cycle, which is known to exist under weak atmosphere-ocean coupling. All other quasi-fixed points are related to orbits that do not appear to be periodic. The findings are applied to the modified version of the interactive Kalman filter, which deals with cycles as regimes. Comparing these results with the findings in Part I, it is found that the filter performances improve using, in the following order, the extended filter, the interactive filter with cycles, a seasonal average filter, and the original interactive Kalman filter from Part I.

In an effort to apply the interactive Kalman filter to higher-dimensional systems, the concept of a quasi-fixed point is introduced. This is defined to be a system state where the tendency, in a suitable reduced space, is at a minimum. It allows one to use conventional search algorithms for the detection of quasi-fixed points. In Part I quasi-fixed points of the ENSO model of Zebiak and Cane are found when run in a permanent monthly mode, the reduced space being defined via a multiple EOP projection. The stability characteristics of the quasi-fixed points are analyzed, and it is shown that they are significantly different from the (in)stabilities of the average monthly models. With these quasi-fixed points, assimilation experiments are carried out with the interactive Kalman filter for the Zebiak–Cane model in the reduced space. It is demonstrated that the results are superior to both a seasonal Kalman filter and the extended Kalman filter.

A reduced gravity, primitive equation, ocean GCM with an isopycnal vertical coordinate is developed. A “buffer” layer is introduced to allow the mixed layer to detrain mass at arbitrary densities without the coordinate drift or the heat loss suffered by other isopycnal models. The diapycnal velocity is derived from the thermodynamic equation. Negative layers are removed by a heat- and mass-conserving convective adjustment scheme. The model formulation on a β plane employs an A grid and allows irregular coastlines and local grid stretching. Simulations with climatological winds and surface heat fluxes based on observed sea surface temperatures (SSTs) are presented for the Atlantic, the Pacific, and the Indian Oceans. The surface mixed layer is modeled as a constant depth layer, and salinity effects are neglected in this version. The surface heat flux parameterization used here leads to errors in model SSTs, which are reasonable in the Tropics but are higher in the western boundary current regions....

A scheme is presented for solving the equation for barotropic ocean circulation, taking into account the special character of the problem: nearly inviscid motion following f/H contours in the ocean interior, with viscous effects closing the flow near western boundaries. Using a special compact finite-difference discretization, the scheme generates boundary layers without spurious oscillations and without demanding very high resolution. Sharp changes in topography and closed f/H contours (e.g., in the vicinity of high sea mounts) are also handled by the scheme in a way that localizes errors due to underresolved topographic features. Strategies are formulated for simplifying the connectedness of the domain by “sinking” the islands.