Pacific Thermocline Research Articles

Correction of Systematic Errors in Coupled GCM Forecasts

The prognostic tendency (PT) correction method is applied in an attempt to reduce systematic errors in coupled GCM seasonal forecasts. The PT method computes the systematic initial tendency error (SITE) of the coupled model and subtracts it from the discrete prognostic equations. In this study, the PT correction is applied only to the three-dimensional ocean temperature. The SITE is computed by calculating a climatologically averaged difference between coupled model initial conditions and resulting forecasts at very short lead times and removing the observed mean seasonal tendency. Two sets of coupled GCM forecasts, one using an annual mean SITE correction and the other using a SITE correction that is a function of season, are compared with a control set of uncorrected forecasts. Each set consists of 17 12-month forecasts starting on 1 January from 1980 through 1996. The PT correction is found to be an effective method for maintaining a more realistic forecast climatology by reducing systematic ocean temperature errors that lead to a relaxation of the tropical Pacific thermocline slope and a weak tropical SST annual cycle in the control set. The annual mean PT correction, which allows the model to freely generate its own seasonal cycle, leads to increased prediction skill for tropical Pacific SSTs while the seasonally varying PT correction has no impact on this skill. Physical mechanisms responsible for improvements in the coupled model’s annual cycle and forecast skill are investigated. The annual mean structure of the tropical Pacific thermocline is found to be essential for producing a realistic SST annual cycle. The annul mean PT correction helps to maintain a realistic thermocline slope that allows surface winds to impact the annual cycle of SST in the eastern Pacific. Forecast skill is increased if the coupled model correctly captures dynamical modes related to ENSO. The annual mean correction leads to a model ENSO that is best characterized as a delayed oscillator mode while the control model appears to have a more stationary ENSO mode; this apparently has a positive impact on ENSO forecast skill in the PT corrected model.

Kelvin wave and Rossby wave interaction in the extratropical‐tropical Pacific

The adjustment of the equatorial Pacific thermocline to an initial extratropical thermocline anomaly is studied using analytical and high‐resolution (1/16°) primitive equation numerical models. It is found that the fast adjustment due to the coastal Kelvin wave and equatorial Kelvin and Rossby waves, and the slow adjustment due to the eastern boundary planetary wave play important roles in the redistribution of the initial anomalous mass, thereby determining the phase and magnitude of the equatorial thermocline transition induced by the initial extratropical thermocline transition. For a given extratropical anomaly, both analytical and numerical models show that the equatorial thermocline transition occurs about 2–5 years later, and the magnitude is about 5–10% of the initial anomaly. The mean circulation in the North Pacific intensifies the transition of the equatorial thermocline and reduces the phase‐lag of extratropical‐tropical thermocline transition.

The bomb 14C transient in the Pacific Ocean

A modeling study of the bomb 14C transient is presented for the Pacific Ocean. A primitive equation ocean circulation model has been configured for a high‐resolution domain that accounts for the Indonesian Throughflow (ITF). Four separate runs were performed: (1) seasonal forcing with 20 Sv of ITF transport, (2) seasonal forcing with 10 Sv of ITF transport, (3) seasonal forcing with no ITF transport, and (4) interannual forcing with 15 Sv of ITF transport. This study has two main objectives. First, it is intended to describe the time evolution of the bomb 14C transient. This serves as a tool with which one can identify the physical processes controlling the evolving bomb 14C distribution in the Pacific thermocline and thus provides an interpretive framework for the database of Δ14C measurements in the Pacific. Second, transient tracers are applied to the physical oceanographic problem of intergyre exchange. This is of importance in furthering our understanding of the potential role of the upper Pacific Ocean in climate variability. We use bomb 14C as a dye tracer of intergyre exchange between the subtropical gyres and the equatorial upwelling regions of the equatorial Pacific. Observations show that while the atmospheric Δ14C signal peaked in the early to mid‐1960s, the Δ14C levels in the surface water waters of the subtropical gyres peaked near 1970, and the Δ14C of surface waters in the equatorial Pacific continued to rise through the 1980s. It is shown that the model exhibits skill in representing the large‐scale observed features observed for the bomb 14C transient in the Pacific Ocean. The model successfully captures the basin‐scale inventories of bomb 14C in the tropics as well as in the extratropics of the North Pacific. For the equatorial Pacific this is attributed to the model's high meridional resolution. The discrepancies in the three‐dimensional distribution of bomb 14C between the model and data are discussed within the context of the dynamical controls on the Δ14C distribution of bomb 14C in the Pacific.

Surface ocean density gradients during the Last Glacial Maximum

We have compiled published planktonic foraminiferal oxygen isotope values from the tropical to subtropical Atlantic and Pacific for the late Holocene and Last Glacial Maximum (LGM). We interpret the spatial distribution of δ18O values in terms of meridional density gradients in the LGM surface ocean and discuss implications on tropical ocean thermocline structure. In both oceans, glacial δ18O values display a pattern of interhemispheric asymmetry evidenced by increasingly higher glacial δ18O values toward northern latitudes. The δ18O‐derived density estimates suggest that the northern subtropical surface ocean density was higher during the LGM than today, which has significant implications on LGM thermocline ventilation. In the Atlantic, steeper density gradients in the northern subtropics during the LGM are consistent with a more compressed subtropical gyre and thermocline water formation farther south than today. In the Pacific the steeper density gradients suggest a significant reduction in freshwater fluxes to the northern tropical and subtropical regions, which raises the possibility of a more significant component of Northern Hemisphere source waters to the tropical Pacific thermocline. These predictions could be tested with numerical ocean models that assess the structure of the tropical thermocline in the glacial ocean.

A decadal spiciness mode in the tropics

Analysis of a complex climate model suggests the existence of a mode of coupled ocean‐atmosphere variability in the tropical Pacific with the following dynamics. Anomalous strong Pacific trade winds accelerate the North Equatorial current and Countercurrent. At 10° to 15°N in the central Pacific thermocline this causes anomalous advection across mean gradients of temperature and salinity on isopycnal surfaces and generates cool/fresh spiciness anomalies of the order of 0.3°K. These are advected by the mean circulation via the western boundary region to the equatorial Pacific in approximately five years. At the equatorial outcrops of the isopycnals, the cool/fresh anomalies affect the surface heat budget and are hypothesized to initiate a relaxation of the trade winds that is reinforced by positive feedbacks with the slope of the thermocline and with air‐sea fluxes of heat and freshwater. At 5° to 10°S the decrease of the trades generates by anomalous advection cool/fresh spiciness anomalies that migrate to the equator in one to two years and provide a positive feedback. At 10° to 15°N, the decrease of the trades decelerates North Equatorial current and Countercurrent and causes warm/salty anomalies on isopycnals that arrive at the equator five years later and close the cycle.

The role of the Indonesian Throughflow in equatorial Pacific thermocline ventilation

The role of the Indonesian Throughflow (ITF) in the thermocline circulation of the low‐latitude Pacific Ocean is explored using a high‐resolution primitive equation ocean circulation model. Seasonally forced runs for a domain with an open Indonesian passage are compared with seasonally forced runs for a closed Pacific domain. Three cases are considered: one with no throughflow, one with 10 Sv of imposed ITF transport, and one with 20 Sv of ITF transport. Two idealized tracers, one that tags northern component subtropical water and another that tags southern component subtropical water, are used to diagnose the mixing ratio of northern and southern component waters in the equatorial thermocline. It is found that the mixing ratio of north/south component waters in the equatorial thermocline is highly sensitive to whether the model accounts for an ITF. Without an ITF, the source of equatorial undercurrent water is primarily of North Pacific origin, with the ratio of northern to southern component water being approximately 2.75 to 1. The ratio of northern to southern component water in the Equatorial Undercurrent with 10 Sv of ITF is approximately 1.4 to 1, and the ratio with 20 Sv of imposed ITF is 1 to 1.25. Estimates from data suggest a mean mixing ratio of northern to southern component water of less than 1 to 1. Assuming that the mixing ratio changes approximately linearly as the ITF transport varies between 10 and 20 Sv, an approximate balance between northern and southern component water is reached when the ITF transport is approximately 16 Sv. It is also shown that for the isopycnal surfaces within the core of the equatorial undercurrent, a 2°C temperature front exists across the equator in the western equatorial Pacific, beneath the warm pool. The implications of the model results and the temperature data for the heat budget of the equatorial Pacific are considered.

Pacific thermocline bridge revisited

The coupling on decadal time scales of the mid‐latitude and tropical Pacific via an oceanic ‘bridge’ in the thermocline is investigated using ocean general circulation model hindcasts and a coupled ocean atmosphere model. Results indicate that in the tropics decadal anomalies of isopycnal depth are forced by Ekman pumping and are largely independent of the arrival of subducted anomalies in the thermocline that originate in the mid‐latitudes of either hemisphere. In the coupled model, temperature anomalies on isopycnals show little coupling from the tropics to the northern hemisphere, but are lag correlated between southern hemisphere mid‐ and low‐latitudes. However, anomaly magnitudes on the equator are small. These results suggest that the oceanic ‘bridge’ to the northern hemisphere explains only a small part of the observed decadal variance in the equatorial Pacific. Coupling to the southern mid‐latitudes via temperature anomalies on isopycnals remains an intriguing possibility.

Thermohaline Stratification of the Indonesian Seas: Model and Observations*

The Indonesian Throughflow, weaving through complex topography, drawing water from near the division of the North Pacific and South Pacific water mass fields, represents a severe challenge to modeling efforts. Thermohaline observations within the Indonesian seas in August 1993 (southeast monsoon) and February 1994 (northwest monsoon) offer an opportunity to compare observations to model output for these periods. The simulation used in these comparisons is the Los Alamos Parallel Ocean Program (POP) 1/6 deg (on average) global model, forced by ECMWF wind stresses for the period 1985 through 1995. The model temperature structure shows discrepancies from the observed profiles, such as between 200 and 1200 dbar where the model temperature is as much as 38C warmer than the observed temperature. Within the 58‐288C temperature interval, the model salinity is excessive, often by more than 0.2. The model density, dominated by the temperature profile, is lower than the observed density between 200 and 1200 dbar, and is denser at other depths. In the model Makassar Strait, North Pacific waters are found down to about 250 dbar, in agreement with observations. The model sill depth in the Makassar Strait of 200 m, rather than the observed 550-m sill depth, shields the model Flores Sea from Makassar Strait lower thermocline water, causing the Flores lower thermocline to be dominated by salty water from the Banda Sea. In the Maluku, Seram, and Banda Seas the model thermocline is far too salty, due to excessive amounts of South Pacific water. Observations show that the bulk of the Makassar throughflow turns eastward into the Flores and Banda Seas, before exiting the Indonesian seas near Timor. In the model, South Pacific thermocline water spreads uninhibited into the Banda, Flores, and Timor Seas and ultimately into the Indian Ocean. The model throughflow transport is about 7.0 Sv (Sv [ 106 m3 s21) in August 1993 and 0.6 Sv in February 1994, which is low compared to observationally based estimates. However, during

水温躍層調節とENSO位相変化における北西太平洋風変動の役割

Analysis of 17 years (1980-1996) of tropical Pacific thermocline variations indicates that short period (about 18 months) cycles in the early 1990s exhibit a relatively small meridional scale. The long period (4-5 year) cycles in the 1980s, associated with major basic-wide warming/cooling events, have much wider meridional scales. The dominant mode of interannual variation of thermocline depth can be approximately described by a basin-scale east-west seesaw oscillation, with an eastward propagation of thermocline depth anomalies occurring in the equatorial waveguide during the transition phases. The long-period ENSO cycles associated with the major basic-wide warming/cooling events involve a substantial change of heat storage in the western North Pacific (WNP) (5-15°N, 130-170°E). The tendency of thermocline displacement and local wind stress curl in the WNP exhibit a coherent, broad spectral peak on a 8-20 month time scale. The deepening (rising) of the thermocline occurs in phase with the local anticyclonic (cyclonic) wind stress forcing, suggesting the essential roles of the in situ wind forcing in thermocline adjustment. The surface wind variation in the western Pacific is found to play a critical role in the phase transition of ENSO cycles. During the mature phases of major warm episodes, there is a rapid establishment of an anomalous anticyclonic wind stress curl over the WNP. The anticyclonic wind stress deepens the WNP thermoline and starts a recharge of heat content in the warm pool. Meanwhile, the easterly anomalies to the south of the anticyclonic center elevate the thermocline in the equatorial western Pacific, and trigger an eastward migration of the rising thermocline along the equator, leading to cooling in the east. The transition from cooling to warming during the mature phases of cold episodes involves a similar process, but with opposite anomalies.

A Westward-Intensified Decadal Change in the North Pacific Thermocline and Gyre-Scale Circulation

From the early 1970s to the mid-1980s, the main thermocline of the subarctic gyre of the North Pacific Ocean shoaled with temperatures at 200–400-m depth cooling by 1°–4°C over the region. The gyre-scale structure of the shoaling is quasi-stationary and intensified in the western part of the basin north of 30°N, suggesting concurrent changes in gyre-scale transport. A similar quasi-stationary cooling in the subtropical gyre south of 25°N is also observed but lags the subpolar change by several years. To explore the physics of these changes, the authors examine an ocean model forced by observed wind stress and heat flux anomalies from 1970–88 in which they find similar changes in gyre-scale thermocline structure. The model current fields reveal that the North Pacific subpolar and subtropical gyres strengthened by roughly 10% from the 1970s to the 1980s. The bulk of the eastward flow of the model Kuroshio–Oyashio Extension returned westward via the subpolar gyre circuit, while the subtropical gyre return flow along 20°N lags the subpolar changes by several years. The authors demonstrate that the model thermocline cooling and increased transport occurred in response to decadal-scale changes in basin-scale wind stress curl with the quasi-stationary oceanic response being in a time-dependent quasi-Sverdrup balance over much of the basin east of the date line. This wind stress curl driven response is quasi-stationary but occurs in conjunction with a propagating temperature anomaly associated with subduction in the central North Pacific that links the subpolar and subtropical gyre stationary changes and gives the appearance of circumgyre propagation. Different physics evidently controls the decadal subsurface temperature signal in different parts of the extratropical North Pacific.

Silica depletion in the thermocline of the glacial North Pacific: corollaries and implications

Over much of the tropical ocean, late Quaternary productivity patterns show maxima associated with glacial periods, minima during warm times. Also, biogenic opal deposition tends to parallel the accumulation rates of organic carbon and related productivity proxies. This pattern is found in the eastern equatorial Pacific, for example. However, in the western equatorial Pacific, while productivity is normally high during glacials, opal deposition is reduced. In Santa Barbara Basin, both organic matter supply and opal supply are reduced during the last glacial period, showing a reversed productivity pattern. Silicate/phosphate ratios at 100 m depth in Pacific waters and elsewhere suggest that present circulation greatly favors silicate enrichment of the thermocline in the North Pacific, presumably through large-scale deep upwelling, which prevents segregation of silicate from phosphate through partial recycling in the uppermost water column. If so, it appears that the opal supply to Santa Barbara Basin may indicate the nutrient content of the North Pacific thermocline. We suggest that during the last glacial period the nutrient content of the thermocline was low, and large-scale deep upwelling was absent. The records of organic matter supply and laminations show patterns that call for additional factors, presumably changing wind strength, changing shelf exposure, and local feedback effects.

Why Is There a Tritium Maximum in the Central Equatorial Pacific Thermocline?

Abstract It is suggested that the tritium maximum in the central Pacific is caused by two water pathways across the North Equatorial Countercurrent (NECC), one from the central Pacific and the other from the Mindanao Current. It is argued that an interior pathway exists, by which tritium-rich thermocline waters from the subtropical North Pacific cross the NECC in the central Pacific. The transport in this pathway, however, is small compared with that from the Mindanao Current.

North Pacific Thermocline Variations on ENSO Timescales

The North Pacific thermocline (250 to 400 m) is studied using XBT observations acquired during the 1970s and 1980s. Interannual variations (3–5 yr timescales) in thermocline temperature, with O(0.1°C) amplitude at 400 m, are found to exhibit westward propagation throughout the extratropical North Pacific up to 45°N. Southward of 30°N, the features propagate intact across the basin from the eastern boundary to the western boundary. Northward of 30°N, the features can be observed to propagate only as far as the date line. The observed midlatitude thermocline anomalies are often related to tropical ENSO events in that they occur most strongly after the development of tropical El Niño or La Niña conditions and propagate westward from near the eastern boundary in the midlatitudes. But it is found that the observed midlatitude thermocline anomalies have larger phase speeds than theoretically predicted free baroclinic Rossby waves. Also, the observed anomalies have larger wavelength and faster propagation speeds than baroclinic Rossby waves that radiate from coastal Kelvin-like waves near the eastern boundary in well-known high-resolution models. Large-scale thermocline fluctuations that have spatial scale and phase speeds similar to the observations are also found in a coarse-resolution model of the Pacific Ocean forced by observed wind and heat flux anomalies over the 1970–88 period. In the midlatitudes, north of 30°N, large-scale Ekman pumping by interannual wind stress curl variations provides a significant driving mechanism for the modeled large-scale thermocline anomalies. The modeled ocean response is a combination of the static thermocline response to large-scale Ekman pumping plus a train of westward traveling Rossby waves, which accounts for part of the propagating temperature fluctuations. A tropical, remotely forced component is prominant near the eastern boundary, but this only contributes weakly in the model open ocean.

Tropical Pacific Ocean thermocline depth reconstructions for the Last Glacial Maximum

We evaluate the relationship between ten surface ocean (0–300 m) hydrographic parameters and the spatial distribution of factor‐analyzed core top planktonic foraminiferal abundances in the tropical Pacific Ocean (24°N–24°S) for core tops <3800 m. The spatial distribution of the first three faunal factor loadings (88% of the variance) are most highly correlated to subsurface variability (mixed layer depth, thermocline depth) and resistant species percent (RSP). However, RSP is not related to dissolution but is related to thermocline depth. Factor I (mixed layer species G. glutinata, G. ruber) and factor III (G. ruber) can be distinguished from each other by low abundances of G. glutinata in factor III. Both assemblages spatially comprise the deep mixed layer region of the western tropical and equatorial Pacific Ocean, but are associated with distinct water mass properties. A combination of Factor I and III loadings shows a higher correlation to thermocline depth (R² = 0.70). Factor II loadings (dominated by thermocline dwelling species N. dutertrei) are most significantly correlated with the thermocline depth (R² = 0.73). Most factors show only marginally significant correlation to sea surface temperatures (SSTs), indicating that SST is not the primary forcing factor on the planktonic foraminiferal species distributions in the tropical Pacific. A new transfer function was calculated to predict tropical Pacific thermocline depth from planktonic foraminifera abundances using the Imbrie‐Kipp Method (IKM) (standard deviation of residuals ±22 m (1σ)). An additional ±5‐m error is attributed to low species counts in the core top database. The modern analog technique (MAT) was also used to predict thermocline depth (standard deviation of residuals ±21 m). While last glacial maximum (LGM) thermocline depth changes by IKM and MAT were generally within error, estimated changes were geographically uniform, suggesting an oceanographic response to climate forcing. We estimate that the thermocline depth of the LGM was shallower than present by ∼20 m south of 8°S, possibly due to a shift in the South Pacific anticyclone to the northeast. Both the IKM and MAT estimate a steeper east‐west thermocline slope along the equator, suggesting that zonal wind stress (Walker circulation) was intensified during the LGM. Collectively, the thermocline estimates for the LGM suggest an equatorward compression of the climate zones in both hemispheres.

An Equatorial Ocean Recharge Paradigm for ENSO. Part I: Conceptual Model

A new conceptual model for ENSO has been constructed based upon the positive feedback of tropical ocean‐ atmosphere interaction proposed by Bjerknes as the growth mechanism and the recharge‐discharge of the equatorial heat content as the phase-transition mechanism suggested by Cane and Zebiak and by Wyrtki. This model combines SST dynamics and ocean adjustment dynamics into a coupled basinwide recharge oscillator that relies on the nonequilibrium between the zonal mean equatorial thermocline depth and wind stress. Over a wide range of the relative coupling coefficient, this recharge oscillator can be either self-excited or stochastically sustained. Its period is robust in the range of 3‐5 years. This recharge oscillator model clearly depicts the slow physics of ENSO and also embodies the delayed oscillator (Schopf and Suarez; Battisti and Hirst) without requiring an explicit wave delay. It can also be viewed as a mixed SST‐ocean dynamics oscillator due to the fact that it arises from the merging of two uncoupled modes, a decaying SST mode and a basinwide ocean adjustment mode, through the tropical ocean‐atmosphere coupling. The basic characteristics of this recharge oscillator, including the relationship between the equatorial western Pacific thermocline depth and the eastern Pacific SST anomalies, are in agreement with those of ENSO variability in the observations and simulations with the Zebiak‐Cane model.

Fractal dimension as an indicator for turbulent mixing in the thermocline

The complexity of small‐scale density fluctuations is investigated experimentally, with special emphasis on their scale‐similar (fractal) facets. Using empirical observations, it is shown that the fractal dimension of density at scales between 0.1 and 3 m is closely related to the turbulent kinetic dissipation rate ε, the local buoyancy frequency N, and the buoyancy Reynolds number ε/νN2 in the upper equatorial Pacific thermocline.

Dianeutral Motion, Water Mass Conversion, and Nonlinear Effects on the Density Ratio in the Pacific Thermocline

Abstract The averaged hydrography of Levitus has been used to form maps of various quantities that contribute to dianeutral advecion in the Pacific thermocline. On much of the “26.50” neutral surface cabbeling and thermobaricity contribute between −0.2 × 10−7 m s−1 and −2 × 10−7 m s−1 to the total dianeutral velocity (assuming a lateral diffusivity of 103 m2 s−1). The dianeutral advection caused by vertical turbulent mixing is also mapped on several surfaces in the Pacific Ocean. Assuming a vertical diffusivity of 10−5 m2 s−1, the typical dianeutral velocity caused by vertical mixing is 10−7 m s−1. This upwelling across the “26.20” neutral surface, at an average depth of about 200 m, is positive and quite strong throughout the whole equatorial region. There is a striking pattern of strong dianeutral upwelling in the central Pacific on the “26.50” neutral surface, while in the eastern Pacific on the same surface there is a large region of downwelling. Subsurface water mass conversion is defined as the rate...

The Peru upwelling and the ventilation of the south Pacific thermocline

A reconstruction of the prebomb Δ14C distribution in the tropical Pacific using data from old coral heads shows that surface waters with the lowest Δ14C content are found distinctly south of the equator. Prebomb, low‐Δ14C surface water appears to owe its origin to the upwelling of ∼15°C water off the coast of Peru. The low‐Δ14C water upwelling off Peru is shown to be derived from the “13° Water” thermostad (11°–14°C) of the Equatorial Undercurrent. Untritiated water in the lower part of the undercurrent had nearly the same Δ14C content during the Geochemical Ocean Sections Study (GEOSECS) as the prebomb growth bands in Druffel's (1981) Galapagos coral. Similar Δ14C levels were observed in 9°–10°C water in the southwest Pacific thermocline in the late 1950s. We suggest that the low‐Δ14C water upwelling off Peru and the thermostad water in the undercurrent both originate as ∼8°C water in the subantarctic region of the southwest Pacific. This prescription points to the “lighter variety” of Subantarctic Mode Water (7°–10°C) as a possible source. Because prebomb Δ14C is so weakly forced by exchange of carbon isotopes with the atmosphere, thermocline levels Of Δ14C should be particularly unaffected by diapycnal mixing with warmer overlying water types. We argue that successively less dense features of the South Pacific thermocline, like the Subantarctic Mode Water, the equatorial 13°C Water, and the Peru upwelling, may be part of a single process of thermocline ventilation. Each evolves from the other by diapycnal alteration, while prebomb Δ14C is nearly conserved. Detailed comparisons are made between the coral Δ14C distribution and a model simulation of radiocarbon in Toggweiler et al. (1989). While the Δ14C data suggest a southern hemisphere thermocline origin for the equatorial Δ14C minimum, the model produces its Δ14C minimum by upwelling abyssal water to the surface via the equatorial divergence. In an appendix to the paper we present a new set of coral Δ14C measurements produced over the last 10 years at Lamont‐Doherty Geological Observatory and compile a post‐1950 set of published coral Δ14C measurements for use in model validation studies.

Southern Oscillation simulation in the OSU coupled upper ocean-atmosphere GCM

The Southern Oscillation is a major component in the interannual variations of global climate. The Oregon State University global climate model, with a dynamically interactive upper ocean, reproduces in qualitatively correct fashion some of the major characteristics of the Southern Oscillation. This model simulates the observed anti-correlation of annually averaged sea-level pressure (SLP) between the eastern Pacific and the Indonesian region, the primary atmospheric signal of the Southern Oscillation. In the composite of the simulated warm events positive sea-surface temperature (SST) anomalies expand eastward towards South America from the tropical western Pacific during the first half of the calendar year. The SST anomalies develop in conjunction with eastward mixed layer current anomalies in the tropical Pacific. In the late summer and early fall anomalously warm water near South America develops and moves westward to merge with the central Pacific anomalies. This lagged development in the eastern Pacific is analogous to the evolution of the 1982/83 and 1986/87 El Ninos. The temperature of the thermocline layer also increases, with the slope of the equatorial Pacific thermocline decreasing in response to the relaxation of the surface forcing. Enhanced precipitation occurs in the mid-Pacific while in the Indian and Australian monsoon regions a deficit occurs. The peak of the warm phase occurs in late northern fall/early winter, somewhat earlier than during observed El Ninos. The cold phase of the Southern Oscillation, enhancement of the zonal circulation, evolves in a fashion similar to the warm phase with the signs of the anomalies reversed, similar to observations. Occurrence of Southern Oscillation in this coarse resolution GCM indicates that high resolution ocean waves do not play a crucial role in the generation of this phenomenon as suggested by Pacific basin models. These results also show that ocean-atmosphere global climate models are useful tools for investigation of time dependent changes on the interannual timescale in addition to their hitherto accepted use for studying equilibrium properties of climate.

On the Difference in Strength between Atlantic and Pacific Undercurrents

Abstract From an earlier study of the longitudinal evolution of the equatorial zonal momentum balance in a nonlinear stratified simulation of the Atlantic equatorial undercurrent it was shown that the annual mean simulated undercurrent reaches its maximum zonal velocity as it emerges from an accelerated deep inertial regime extending over about 15 degrees of longitude off the western boundary. Further east, frictional forces overcome the accelerating effect of the zonal pressure gradient form and the undercurrent keeps losing eastward momentum as it flows eastward. If the accelerating deep western regime extended over a wider range of longitudes the maximum emerging velocity would be larger. This is shown to occur in a nonlinear stratified simulation of the Pacific equatorial undercurrent because the western Pacific thermocline is thicker due to the basin size and to the longitudinal structure of the zonal winds. This is offered as an explanation for the larger observed zonal velocities of the Pacific und...