Abstract We tested the capability of a high-resolution ocean general circulation model called LLC4320, with 2.3-km horizontal grid spacing at the equator, to replicate hourly measurements of currents recorded at 5-m depth intervals in the Pacific Equatorial Undercurrent (EUC) with an acoustic Doppler current profiler (ADCP) moored at 0°, 140°W. The LLC4320 simulations of the mean and variability of the horizontal currents and the vertical shear above the depth of the EUC core speed were poor compared to ADCP observations. The LLC4320 EUC core speed was slower, deeper, and less variable than ADCP measurements. The ADCP and LLC4320 currents were captured, with varying degrees of agreement: (i) meridional current oscillations with approximate 19-day period, (ii) a Kelvin wave–like pulse, and (iii) EUC surfacing. Significance Statement Currents in the interior of the ocean are very sparsely measured, and models are expected to provide information on ocean currents to analyze past and future characteristics of global ocean circulation. Ocean current models are severely challenged at the equator where current and density fields are not in balance with the rotation of Earth, unlike over the remainder of the global ocean. We examined a particular ocean general circulation model called LLC4320 because of its high spatial resolution (horizontal grid spacing of 2.3 km at the equator). The LLC4320-simulated currents in the Pacific Equatorial Undercurrent, a feature related to the El Niño and La Niña phenomena, require further refinement.

The impact of data assimilation on the transports of eastward-flowing Equatorial Undercurrent (EUC) and North Equatorial Countercurrent (NECC) in the Pacific Ocean from 145 degrees E to95 degrees Wduring 2004-05 and 200911 was assessed. Two Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2), solutions were analyzed: one with data assimilation and one without. Assimilated data included satellite observations of sea surface temperature and ocean surface topography, in which the sampling patterns were approximately uniform over the 5 years, and in situ measurements of subsurface salinity and temperature profiles, in which the sampling patterns varied considerably in space and time throughout the 5 years. Velocity measurements were not assimilated. The impact of data assimilation was considered significant when the difference between the transports computed with and without data assimilation was greater than 5.5 X 10(6) m(3) s(-1) (or 5.5 Sv; 1 Sv equivalent to 10(6) m(3) s(-1)) for the EUC and greater than 5.0 Sv for the NECC. In addition, the difference of annual-mean transports computed from 3-day-averaged data was statistically significant at the 95% level. The impact of data assimilation ranged from no impact to very substantial impact when data assimilation increased the EUC transport and decreased the NECC transport. The study's EUC results had some correspondence with other studies and no simple agreement or disagreement pattern emerged among all studies of the impact of data assimilation. No comparable study of the impact of data assimilation on the NECC has been made.

Satellite ocean vector wind measurements are used to describe onshore‐offshore Ekman transport and Ekman pumping/suction (i.e., downward/upward velocity) in the coastal ocean at 15°S off Peru, where upwelling is the dominant physical process. Normal and El Niño conditions are defined for May 1992 – April 1997 and May 1997 – May 1998, respectively. During normal conditions, both Ekman suction and offshore Ekman transport produced upwelling. During the El Niño, the May–August speed of Ekman pumping (−9 × 10−6 m s−1) was nearly 4 times larger than the normal speed of Ekman suction and offshore Ekman transport nearly doubled. The strong Ekman pumping may be the source for the deepened coastal thermocline during El Niño, although the evidence is not conclusive because of the absence of in situ observations.

An intertropical convergence zone in the southeast Pacific Ocean is described from monthly mean 1° × 1° ocean vector wind, rainfall, sea surface temperature, and integrated water vapor measurements recorded from satellites. Time interval of the investigation was January 1993 to December 1998, when ocean and atmosphere conditions during 1993–1996 were normal compared to the 1997–1998 El Niño. The southeast Pacific intertropical convergence zone (SITCZ) occurred in March‐April at 8°S–2°S, 130°W–90°W. During 1993–1996 the average March‐April values of SITCZ surface wind convergence, rain rate, sea surface temperature, and integrated water vapor were 3.3 × 10−6 s−1, 3.3 mm d−1, 27.3°C, and 45.3 mm, respectively. A statistical model of the monthly mean observations predicted rainfall greater than the 2 mm d−1 threshold when surface wind convergence was greater than the 1.5 × 10−6 s−1 threshold and sea surface temperature was above 27°C. During non‐El Niño conditions, SITCZ sea surface temperature was greater than sea surface temperature in the 2°S–2°N equatorial band, with maximum difference in March–April which would create a surface wind convergence larger in March‐April compared to other months. The Intertropical Convergence Zone (ITCZ), which occurred north of the equator throughout the year, and the SITCZ were separated by divergent wind, low (<2 mm d−1) rainfall, and cold (<27°C) surface water, except during the intense 1998 El Niño. The rain rate of the ITCZ was minimum in March‐April during 1993–1996 and comparable to the SITCZ rain rate.

Interannual variations of the Somali Jet in the Arabian Sea during 1988-99 were linked to El Nino and La Nina episodes and to India west coast rainfall. Onset dates and monthly mean strengths of the Somali Jet were described with Special Sensor Microwave Imager surface wind speeds. Each year the Somali Jet formed in a similar area in the western Arabian Sea, and always before the onset of monsoon rainfall in Goa. The average date of Somali Jet onset was two days later in El Nino events in comparison with La Nina conditions. Monthly mean strength of the Somali Jet was 0.4 m s(-1) weaker during El Nino episodes than during La Nina intervals. When the monthly mean intensity of the Somali Jet was above (below) normal, there was an excess (deficit) of rainfall along the Indian west coast.

The annual cycle of monsoon-driven variability in primary productivity was studied in 1995 during the Arabian Sea Expedition as part of the United States Joint Global Ocean Flux Studies (US JGOFS). This paper describes the seasonal progression of productivity and its regulation on a section which ran from the coast of Oman to about 1000km offshore in the central Arabian Sea at 65°E. During the SW Monsoon (June–mid-September), the coolest water and highest nutrient concentrations were close to the coast, although they extended offshore to about 800km; during the January NE Monsoon, deep convective mixing provided nutrients to the mixed layer in the region 400 – 1000km offshore. As expected, the SW Monsoon was the most productive season (123±9mmol Cm−2d−1) along the southern US JGOFS section from the coast to 1000km offshore, but productivity in the NE Monsoon was surprisingly high (112±7mmol C m−2d−1). There was no onshore/offshore gradient in primary productivity from 150 to 1000km off the Omani coast in 1995, and there was no evidence of light limitation of either primary productivity or photosynthetic performance (PoptB) from deep convective mixing during the NE Monsoon, deep wind mixing during the SW Monsoon or offshore Ekman downwelling during the SW Monsoon. Productivity during the Spring Intermonsoon (86±6mmol Cm−2d−1) was much higher than in oligotrophic regions such as the tropical Pacific Ocean (29±2mmol Cm−2d−1) or the North Pacific gyre region (32±8mmol Cm−2d−1). The 1995 annual mean productivity (111±11mmol Cm−2d−1) along this section from the Omani coast to the central Arabian Sea was about equal to the spring bloom maximum (107±23mmol Cm−2d−1) during the 1989 North Atlantic Bloom Experiment (NABE) and the equatorial, 1°N–1°S wave guide maximum (95±6mmol Cm−2d−1) in the Pacific Ocean during the 1992 EqPac study. The 1995 SW Monsoon primary productivity was similar to the mean value observed in the same region in 1994 by the Arabesque Expedition (127±14mmol Cm−2d−1) and in 1964 by the ANTON BRUUN Expedition (115±27mmol Cm−2d−1). During the 1995 SW Monsoon, strong, narrow and meandering current filaments extended from the region of coastal upwelling to about 700km offshore; these filaments had levels of biomass, primary productivity, chlorophyll-specific productivity and diatom abundance that were elevated relative to other locations during the SW Monsoon. The SW Monsoon was the most productive period, but SW Monsoon primary productivity values were lower than predicted because efficient grazing by mesozooplankton kept diatoms from accumulating the biomass necessary for achieving the high levels of primary productivity characteristic of other coastal upwelling regions. The high rates of chlorophyll-specific productivity (PoptB>10mmol C mg Chl−1 d−1) observed in the 1995 SW Monsoon, together with the observed dust flux and iron concentrations, indicate that the Arabian Sea was more iron replete than the equatorial Pacific Ocean or the Southern Ocean.

Forecasting the time of onset of monsoon wind in the western Arabian Sea, which is believed to precede the onset of rainfall along the west coast of India, is an important unsolved problem. Prior to measurements of the surface wind field by satellite, there was an absence of suitable surface wind observations. NASA scatterometer (NSCAT) surface wind vectors revealed that the time of the 1997 onset of 12 m/s southwest monsoon wind speeds in the western Arabian Sea preceded the onset of monsoon rainfall in Goa, India, by 3 - 4 days. Wind speed and direction data were necessary to establish a dynamical mechanism between times of onset of 12 m/s wind speed off Somalia and rainfall in Goa. Except for NSCAT, no satellite scatterometer wind product recorded adequately sampled 2-day 1deg x 1deg averaged wind vectors, which are the required space and time scales, to examine the wind-rain relationship in other years. However, the greater-than-95% steadiness of summer monsoon winds allows an opportunity to use satellite measurements of surface wind speed. The Special Sensor Microwave Imager (SSMI) recorded surface wind speed with adequate sampling to produce a 1-day, 1deg x 1deg data product during 1988 - 1998. SSMI data had been uniformly processed throughout the period. Times of onset of 12 m/s wind speed off Somalia determined with the SSMI data set were 21 May 1988, 24 May 1989, 17 May 1990, 28 May 1991, 8 June 1992, 28 May 1993, 30 May 1994, 7 June 1995, 29 May 1996, 12 June 1997, and 15 May 1998. Uncertainty of the 1992 and 1996 times of onset were increased because of the absence of SSMI data on 6 and 7 June 1992 and on 30 May 1996. Correlations of timing of monsoon wind onset with El Nino will be described. Variability of the time difference between times of onset of 12 m/s wind speed and Goa rainfall will be discussed. At the time of submission of the abstract, the Goa rainfall data have not arrived from the India Meteorological Department.

The National Aeronautics and Space Administration scatterometer surface wind vectors are used to describe the rapid onset of the Somali Jet throughout the Arabian Sea. In June 1997 the time of Somali Jet onset varied over the Arabian Sea, with June 17–18 the average time. The Somali Jet appeared first in the western Arabian Sea, expanded over 2 weeks to encompass the Arabian Sea, and produced a threefold increase in surface wind convergence in the eastern Arabian Sea. The onset time of the 12 m s−1 isotach preceded by 3–4 days the onset of monsoon rainfall in Goa. For Somali Jet wind speeds above 10 m s−1 the 2 day 1° × 1° sea surface temperature decreased 0.5°C per 1 m s−1 increase in collocated wind speed. The Somali Jet created a north‐south distribution of Ekman pumping and suction in the central Arabian Sea to enhance the eastward surface current by 0.1 m s−1. The Somali Jet doubled the southward Ekman transport across the southern boundary of the Arabian Sea. In June 1997, when the most intense El Niño episode of the century was in its onset phase, the southward Ekman transport across the southern boundary of the Arabian Sea was one half that observed since 1992.

Until a decade ago, an often-quoted expression in oceanography is that very few observations are recorded throughout the ocean. Now, the sentiment is no longer valid in the uppermost 10% of the tropical Pacific Ocean nor at the surface of the global ocean. One of the remarkable legacies of the 1985-1994 Tropical Oceans Global Atmosphere (TOGA) Program is an in situ marine meteorological and upper oceanographic measurement array throughout the equatorial Pacific to monitor the development and maintenance of El Nino episodes. The TOGA Observing System, which initially consisted of moored- and drifting-buoy arrays, a network of commercial ships, and coastal and island stations, now includes a constellation of satellites and data-assimilating models to simulate subsurface oceanographic conditions. The El Nino and La Nina tropical Pacific Ocean observing system represents the initial phase of an integrated global ocean observing system. Remarkable improvements have been made in ocean model simulation of subsurface currents, but some problems persist. For example, the simulation of the South Equatorial Current (SEC) remains an important challenge in the 2S-2N Pacific equatorial wave guide. During El Nino the SEC at the equator is reduced and sometimes the direction is reversed, becoming eastward. Both conditions allow warm water stored in the western Pacific to invade the eastern region, creating an El Nino episode. Assimilation of data is a tenet of faith to correct simulation errors caused by deficiencies in surface fluxes (especially wind stress) and parameterizations of subgrid-scale physical processes. In the first of two numerical experiments, the Pacific SEC was simulated with and without assimilation of subsurface temperature data. Along the equator, a very weak SEC occurred throughout the eastern Pacific, independent of assimilation of data. However, as displayed in the diagram, in the western Pacific there was no satisfactory agreement between the two simulations. To help determine reliability of the simulated SEC in the western Pacific, current measurements recorded during the 9-19 October 1994 voyage of the French research vessel L'Atalante are also shown in the diagram. With data assimilation, the simulated SEC was in much better agreement with L'Atalante observations. The simulated SEC with data assimilation was far from perfect, in part because of the sparsity of subsurface temperature observations. In the next experiment, TOPEX/POSEIDON sea surface height data in combination with subsurface temperatures will be assimilated to assess further improvement of the simulation of the SEC.

The origin of the Tropical Oceans-Global Atmosphere (TOGA) Program was closely related to the response of global atmospheric circulation to sea surface temperature variations in the tropical Pacific Ocean, which is evident by the El Nino phenomenon.During the two decades before the 1985 start of TOGA, advancements in scientific understanding of the tropical ocean and global atmosphere and advancements in technology provided strong foundations for TOGA.By the early 1980s, research had demonstrated a strong linkage between tropical SST variations and global atmospheric circulation, and discussions of an international ocean-atmosphere program had begun.Probably the single most important event leading to the creation of TOGA was the unannounced arrival in 1982 of the largest El Nino in a century.

Although the distribution of sunshine is symmetrical about the equator, the earth's climate is not. Climatic asymmetries are prominent in the eastern tropical Pacific and Atlantic Oceans where the regions of maximum sea surface temperature, convective cloud cover, and rainfall are north of the equator. This is the result of two sets of factors: interactions between the ocean and atmosphere that are capable of converting symmetry into asymmetry, and the geometries of the continents that determine in which longitudes the interactions are effective and in which hemisphere the warmest waters and the intertropical convergence zone are located. The ocean-atmosphere interactions are most effective where the thermocline is shallow because the winds can readily affect sea surface temperatures in such regions. The thermocline happens to shoal in the eastern equatorial Pacific and Atlantic, but not in the eastern Indian Ocean, because easterly trade winds prevail over the tropical Atlantic and Pacific whereas monsoons, with a far larger meridional component, are dominant over the Indian Ocean. That is how the global distribution of the continents, by determining the large-scale wind patterns, causes climatic asymmetries to be prominent in some bands of longitude but not others. The explanation for asymmetries that favor the Northern rather than Southern Hemisphere with the warmest waters and the ITCZ involves the details of the local coastal geometries: the bulge of western Africa to the north of the Gulf of Guinea and the slope of the western coast of the Americas relative to meridians. Low-level stratus clouds over cold waters are crucial to the maintenance of the asymmetries.

Spatial variations of the east-west and north-south components of surface wind stress are critical in studies of ocean circulation and biological-physical interactions because surface wind stress curl produces a vertical velocity in the upper ocean at the bottom of the Ekman Layer.The ERS-1 scatterometer provides reasonable coverage and direct measurements of vector of winds. Three schemes are evaluated relative to high-quality moored-bouy wind observations recorded in the central Arabian Sea, where high surface waves and high atmospheric water content during the southeast monsoon adversely affect the estimation of satellite-derived winds.

The Pacanowski-Philander (PP) and Mellor-Yamada (MY) parameterization models of vertical mixing by turbulent processes were embedded in the Geophysical Fluid Dynamics Laboratory high-resolution ocean general circulation model of the tropical Pacific Ocean. All other facets of the numerical simulations were the same. Simulations were made for the 1987-1988 period. At the equator the MY simulation produced near-surface temperatures more uniform with depth, a deeper thermocline, a deeper core speed of the Equatorial Undercurrent, and a South Equatorial Current with greater vertical thickness compared with that computed with the PP method. Along 140 degrees W, between 5 degrees N and 10 degrees N, both simulations were the same. Moored-buoy current and temperature observations had been recorded by the Pacific Marine Environmental Laboratory at three sites (165 degrees E, 140 degrees W, 110 degrees W) along the equator and at three sites (5 degrees N, 7 degrees N, 9 degrees N) along 140 degrees W. Simulated temperatures were lower than those observed in the near-surface layer and higher than those observed in the thermocline. Temperature simulations were in better agreement with observations compared to current simulations. At the equator, PP current and temperature simulations were more representative of the observations than MY simulations.

The following monthly mean global distributions for 1993 are presented with a common color scale and geographical map: 10-m height wind speed estimated from the Special Sensor Microwave Imager (SSMI) on a United States (U.S.) Air Force Defense Meteorological Satellite Program (DMSP) spacecraft; sea surface temperature estimated from the Advanced Very High Resolution Radiometer (AVHRR/2) on a U.S. National Oceanic and Atmospheric Administration (NOAA) satellite; 10-m height wind speed and direction estimated from the Active Microwave Instrument (AMI) on the European Space Agency (ESA) European Remote Sensing (ERS-1) satellite; sea surface height estimated from the joint U.S.-France Topography Experiment (TOPEX)/POSEIDON spacecraft; and 10-m height wind speed and direction produced by the European Center for Medium-Range Weather Forecasting (ECMWF). Charts of annual mean, monthly mean, and sampling distributions are displayed.

The following variables along the Pacific equator from 145°E to 95°W were employed: surface layer phytoplankton pigment concentrations derived from Nimbus 7 coastal zone color scanner (CZCS) measurements of ocean color radiances; vertical velocities simulated at the 90‐m bottom of the euphotic layer from a wind‐driven ocean general circulation model; and nitrate concentrations estimated from model‐simulated temperature. The upward flux of nitrate into the euphotic layer was calculated from the simulated vertical motion and nitrate concentration. The CZCS‐derived phytoplankton pigment concentration was uniform from 175° to 95°W. Longitudinal profiles of upwelling, phytoplankton biomass, and 90‐m nitrate flux were of different shapes. The small annual cycles of the phytoplankton pigment concentration and nitrate flux were in phase: increased phytoplankton biomass was associated with increased upward nitrate flux, but the phase was not consistent with the annual cycles of the easterly wind or of the upwelling intensity. Variation of phytoplankton pigment concentration was greater during El Niño than during the annual cycle. The substantially reduced phytoplankton pigment concentration observed during El Niño was associated with smaller upward nitrate flux. Phytoplankton biomass during non‐El Niño conditions was not related to nitrate flux into the euphotic layer.

Sea-surface temperatures (SSTs) are strongly correlated with surface nitrate concentrations in coastal upwelling regions. Upwelling also occurs in the equatorial Pacific; correlations between temperature and nitrate concentration are strong. The University of Miami weekly averaged Advanced Very High Resolution Radiometer (AVHRR) SST data for October 1986 through June 1989 have been used to compute surface nitrate concentrations from 90° – 180°W and from 15°S - 15°N. The surface areas with nitrate above detection limits are combined with existing nitrate uptake data to give weekly estimates of equatorial new production.

Simulations with the UCLA atmospheric general circulation model (AGCM) using two different global sea surface temperature (SST) datasets for January 1979 are compared. One of these datasets is based on COADS (SSTs) at locations where there are ship reports, and climatology elsewhere; the other is derived from measurements by instruments onboard NOAA satellites. In the former dataset (COADS SST), data are concentrated along shipping routes in the Northern Hemisphere; in the latter dataset (HIRS SST), data cover the global domain. Ensembles of five 30-day mean fields are obtained from integrations performed in the perpetual-January mode. The results are presented as anomalies, that is, departures of each ensemble mean from that produced in a control simulation with climatological SSTs.Large differences are found between the anomalies obtained using COADS and HIRS SSTs, even in the Northern Hemisphere where the datasets are most similar to each other. The internal variability of the circulation in the control simulation and the simulated atmospheric response to anomalous forcings appear to be linked in that the pattern of geopotential height anomalies obtained using COADS SSTs resembles the first empirical orthogonal function (EOF 1) in the control simulation. The corresponding pattern obtained using HIRS SSTs is substantially different and somewhat resembles EOF 2 in the sector from central North America to central Asia.To gain insight into the reasons for these results, three additional simulations are carried out with SST anomalies confined to regions where COADS SSTs are substantially warmer than HIRS SSTs. The regions correspond to warm pools in the northwest and northeast Pacific, and the northwest Atlantic. These warm pools tend to produce positive geopotential height anomalies in the northeastern part of the corresponding oceans. Both warm pools in the Pacific produce large-scale circulation anomalies with a pattern that resembles that obtained using COADS SSTs as well as EOF 1 of the control simulation; the warm pool in the Atlantic does not. These results suggest that the differences obtained with COADS SSTs and HIRS SSTs are mostly due to the differences in the datasets over the northern Pacific.There was a blocking episode near Greenland in late January 1979. Both simulations with warm SST anomalies over the northwest and northeast Pacific show a tendency toward increased incidence of North Atlantic blocking; the simulation with warm SST anomalies over the northwest Atlantic shows a tendency toward decreased incidence. These results suggest that features in both SST datasets that do not have a counterpart in the other dataset contribute significantly to the differences between the simulated and observed fields.The results of this study imply that uncertainties in current SST distributions for the world oceans can be as important as the SST anomalies themselves in terms of their impact on the atmospheric circulation. Caution should be exercised, therefore, when linking anomalous circulation and SST patterns, especially in long-range prediction.

A 10 m height wind vector data set, named CMODFD, from ERS-1 Active Microwave Instrument (AMI) measurements, was compared with moored buoy wind observations. The CMODFD wind vectors compared favorably with moored buoy wind observations at about 60 sites. Satisfactory results were obtained from intercomparisons between CMODFD and moored buoy estimates of horizontal wind divergence and wind stress curl. The CMODFD positions of the Intertropical Convergence Zone (ITCZ) at 28 deg west were nearly equal to those determined from satellite images of clouds. The CMODFD data were employed to compute the Ekman vertical velocity in the region of the Pacific North Equatorial Countercurrent (NECC), and a correlative relation was found between the annual cycles and longitudinal variations of the NECC and Ekman vertical motion.