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James A. Carton and Benjamin S. Giese

Abstract

This paper describes the Simple Ocean Data Assimilation (SODA) reanalysis of ocean climate variability. In the assimilation, a model forecast produced by an ocean general circulation model with an average resolution of 0.25° × 0.4° × 40 levels is continuously corrected by contemporaneous observations with corrections estimated every 10 days. The basic reanalysis, SODA 1.4.2, spans the 44-yr period from 1958 to 2001, which complements the span of the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis (ERA-40). The observation set for this experiment includes the historical archive of hydrographic profiles supplemented by ship intake measurements, moored hydrographic observations, and remotely sensed SST. A parallel run, SODA 1.4.0, is forced with identical surface boundary conditions, but without data assimilation. The new reanalysis represents a significant improvement over a previously published version of the SODA algorithm. In particular, eddy kinetic energy and sea level variability are much larger than in previous versions and are more similar to estimates from independent observations. One issue addressed in this paper is the relative importance of the model forecast versus the observations for the analysis. The results show that at near-annual frequencies the forecast model has a strong influence, whereas at decadal frequencies the observations become increasingly dominant in the analysis. As a consequence, interannual variability in SODA 1.4.2 closely resembles interannual variability in SODA 1.4.0. However, decadal anomalies of the 0–700-m heat content from SODA 1.4.2 more closely resemble heat content anomalies based on observations.

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Benjamin S. Giese and James A. Carton

Abstract

Forty-four years of mechanical and expendable bathythermograph observations are assimilated into a general circulation model of the Pacific Ocean. The model is run from 1950 through 1993 with forcing at the surface from observed monthly mean wind stress and temperature. The resulting analysis is used to describe the spatial and temporal patterns of variability at interannual and decadal periods. Interannual variability has its largest surface temperature expression in the Tropics and decadal variability has its largest surface temperature expression in the midlatitude Pacific. However, there are important interannual surface temperature changes that occur in the midlatitude Pacific and there are important decadal surface temperature changes in the Tropics.

An empirical orthogonal function (EOF) analysis of model data that has been bandpass filtered to retain energy at periods of 1–5 yr and at periods greater than 5 yr is presented. The results suggest that interannual variability is dominated by a positive feedback mechanism in the Tropics and a negative feedback mechanism in the midlatitude ocean, resulting in larger anomalies in the Tropics. A second EOF analysis of model data that has been low-pass filtered to retain periods greater than 5 yr reveals patterns of wind and surface temperature anomalies that have strikingly similar structure to the interannual patterns; however, the sequencing between the first and second EOFs is different. Even though there are large decadal anomalies of wind stress in the Tropics, the largest anomalies of surface temperature and ocean heat content occur at mid- and high latitudes. The EOF analysis indicates that decadal variability has a positive feedback mechanism that operates in the midlatitude ocean and a negative feedback mechanism that operates in the Tropics, so that the largest temperature anomalies are in midlatitudes. Previous studies have cited the contribution of heat flux anomalies as the primary cause of decadal surface temperature anomalies. These model studies indicate that meridional advection of heat is at least as important. The timing of interannual and decadal changes in the atmosphere and in the ocean suggests that the atmosphere plays an important role connecting these phenomena. One interpretation of the results is that interannual and decadal variability are manifestations of the same climate phenomena but have crucially different feedback mechanisms.

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Benjamin S. Giese and James A. Carton

Abstract

A coupled ocean-atmosphere model is used to investigate the seasonal cycle of sea surface temperature and wind stress in the Tropics. A control run is presented that gives a realistic annual cycle with a cold tongue in the eastern Pacific and Atlantic Oceans. In an attempt to isolate the mechanisms responsible for the particular annual cycle that is observed, the authors conducted a series of numerical experiments in which they alter the solar forcing. These experiments include changing the longitude of perihelion, increasing the heat capacity of land, and changing the length of the solar year. The results demonstrate that the date of perihelion and land heating do not, by themselves control the annual cycle. However, there is a natural timescale for the development of the annual cycle. When the solar year is shortened to just 6 months, the seasonal variations of climate remain similar in timing to the control run except that they are weaker. When the solar year is lengthened to 18 months, surface temperature in the eastern Pacific develops a prominent semiannual cycle. The semiannual cycle results from the ITCZ crossing the equator into the Southern Hemisphere and the development of a Northern Hemisphere cold tongue during northern winter. The meridional winds maintain an annual cycle, while the zonal winds have a semiannual component. The Atlantic maintains an annual cycle in all variables regardless of changes in the length of the solar year. A final experiment addresses the factors determining the season in which upwelling occurs. In this experiment the sun is maintained perpetually over the equator (simulating March or September conditions). In this case the atmosphere and ocean move toward September conditions, with a Southern Hemisphere cold tongue and convection north of the equator.

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D. E. Harrison, William S. Kessler, and Benjamin S. Giese

Abstract

Five different analyses of 1982–83 monthly average surface wind stress fields have been used to force an ocean general circulation model of the tropical Pacific, in a series of El Niño hindcast experiments, like the one reported by Philander and Seigel. Although there were prominent common departures from climatology in the surface wind stress field during 1982–83 according to each wind analysis, there are also very substantial differences between analyses. This study was done to investigate the sensitivity of such hindcasts to our uncertain knowledge of the surface wind stress field. We concentrate here on the behavior along the Pacific ship-of-opportunity tracks.

According to the ship-of-opportunity XBT data, the ocean underwent major changes during this period. The vertical temperature gradients and mixed layer temperatures, as well as the depth of the thermocline, underwent substantial changes. There were also major changes in the geostrophic flow of the major current systems, as revealed by upper ocean dynamic height differences. Comparing the hindcasts with observations, we find that the gross large-scale changes of the ENSO event—surface warming in the second half of 1982, continued warmth into 1983 and cooling in mid-1983, together with major thermocline depth changes—are found in each hindcast. However, major quantitative differences exist between each hindcast and the observations in at least some region for some time and some variable.

Within the waveguide, dynamic height changes generally are hindcast with quantitative skill using each wind stress field and the best hindcasts differ from the observations by only a few dyn-cm more than the estimated uncertainty in the observations. Such hindcast skill is unlikely to be fortuitous: evidently the major elements of the waveguide variability are forced by the 1982–83 surface wind stress field rather than evolving out of some aspect of the state of the ocean during late 1981. Sea surface temperature changes are generally hindcast with qualitative skill, but rms errors of 2–3°C are frequent. Subsurface temperature variability skill varies with hindcast, location and depth; skill is greatest in the thermocline.

Outside the waveguide, hindcast skill tends to be reduced, and varies greatly with location and hindcast. Quantitative hindcast skill is found near 10°S and 10°N in some hindcasts in the WP, and near 10°S in most hindcasts in the CP, but there is never quantitative skill in the NECC region. The most striking inconsistency found involves the behavior of the NMC hindcast in the region of the North Equatorial Counter Current. Wind stress curl-forced Ekman pumping appears to be a significant factor in the variations in the more successful hindcasts.

In almost every comparison, the range of hindcast results brackets the observations, suggesting that the model physics is plausible. Overall, the special research effort wind fields produced better dynamic height results than did the operational wind product fields, but the operational fields produced generally better waveguide SST results. Improved knowledge of the surface wind stress field (and its curl) is a minimum requirement if we are to assess more critically model performance, and to identify needed model improvements.

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Boris Dewitte, Sara Purca, Serena Illig, Lionel Renault, and Benjamin S. Giese

Abstract

Intraseasonal equatorial Kelvin wave activity (IEKW) at a low frequency in the Pacific is investigated using the Simple Ocean Data Assimilation (SODA) oceanic reanalyses. A vertical and horizontal mode decomposition of SODA variability allows estimation of the Kelvin wave amplitude according to the most energetic baroclinic modes. A wavenumber–frequency analysis is then performed on the time series to derive indices of modulation of the IEKW at various frequency bands. The results indicate that the IEKW activity undergoes a significant modulation that projects onto baroclinic modes and is not related in a straightforward manner to the low-frequency climate variability in the Pacific. Linear model experiments corroborate that part of the modulation of the IEKW is tightly linked to change in oceanic mean state rather than to the low-frequency change of atmospheric equatorial variability.

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Benjamin S. Giese, Gennady A. Chepurin, James A. Carton, Tim P. Boyer, and Howard F. Seidel

Abstract

Historical bathythermograph datasets are known to be biased, and there have been several efforts to model this bias. Three different correction models of temperature bias in the historical bathythermograph dataset are compared here: the steady model of Hanawa et al. and the time-dependent models of Levitus et al. and Wijffels et al. The impact of these different models is examined in the context of global analysis experiments using the Simple Ocean Data Assimilation system. The results show that the two time-dependent bias models significantly reduce warm bias in global heat content, notably in the 10 years starting in the early 1970s and again in the early 1990s. Overall, the Levitus et al. model has its greatest impact near the surface and the Wijffels et al. model has its greatest impact at subtropical thermocline depths. Examination of the vertical structure of temperature error shows that at thermocline depths the Wijffels et al. model overcompensates, leading to a slight cool bias, while at shallow levels the same model causes a slight warm bias in the central and eastern subtropics and at thermocline depths on the equator in the Pacific Ocean as a result of reduced vertical entrainment. The results also show that the bias-correction models may alter the representation of interannual variability. During the 1997/98 El Niño and the subsequent La Niña the Levitus et al. model, which has its main impact at shallow depths, reduces the 50-m temperature anomalies in the eastern equatorial Pacific by 10%–20% and strengthens the zonal currents by up to 50%. The Wijffels et al. correction, which has its main impact at deeper levels, has much less effect on the oceanic expression of ENSO.

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Soumi Chakravorty, Renellys C. Perez, Bruce T. Anderson, Benjamin S. Giese, Sarah M. Larson, and Valentina Pivotti

Abstract

During the positive phase of the North Pacific Oscillation, westerly wind anomalies over the subtropical North Pacific substantially increase subsurface heat content along the equator by “trade wind charging” (TWC). TWC provides a direct pathway between extratropical atmospheric circulation and El Niño–Southern Oscillation (ENSO) initiation. Previous model studies of this mechanism lacked the ocean–atmospheric coupling needed for ENSO growth, so it is crucial to examine whether TWC-induced heat content anomalies develop into ENSO events in a coupled model. Here, coupled model experiments, forced with TWC favorable (+TWC) or unfavorable (−TWC) wind stress, are used to examine the ENSO response to TWC. The forcing is imposed on the ocean component of the model through the first winter and then the model evolves in a fully coupled configuration through the following winter. The +TWC (−TWC) forcing consistently charges (discharges) the equatorial Pacific in spring and generates positive (negative) subsurface temperature anomalies. These subsurface temperature anomalies advect eastward and upward along the equatorial thermocline and emerge as like-signed sea surface temperature (SST) anomalies in the eastern Pacific, creating favorable conditions upon which coupled air–sea feedback can act. During the fully coupled stage, warm SST anomalies in +TWC forced simulations are amplified by coupled feedbacks and lead to El Niño events. However, while −TWC forcing results in cool SST anomalies, pre-existing warm SST anomalies in the far eastern equatorial Pacific persist and induce local westerly wind anomalies that prevent consistent development of La Niña conditions. While the TWC mechanism provides adequate equatorial heat content to fuel ENSO development, other factors also play a role in determining whether an ENSO event develops.

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James A. Carton, Xianhe Cao, Benjamin S. Giese, and Arlindo M. Da Silva

Abstract

The mechanisms regulating interannual and decadal variations of sea surface temperature (SST) in the tropical Atlantic are examined. Observed variations of sea surface temperature are typically in the range of 0.3°–0.5°C and are linked to fluctuations in rainfall on both the African and South American continents. The authors use a numerical model to simulate the observed time series of sea surface temperature for the period 1960–1989. Based on the results, experiments are conducted to determine the relative importance of heat flux and momentum forcing. Two dominant timescales for variability of SST are identified: a decadal timescale that is controlled by latent heat flux anomalies and is primarily responsible for SST anomalies off the equator and an equatorial mode with a timescale of 2–5 years that is dominated by dynamical processes. The interhemispheric gradient of anomalous SST (the SST dipole) is primarily linked to the former process and thus results from the gradual strengthening and weakening of the trade wind system of the two hemispheres.

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Benjamin S. Giese, Gilbert P. Compo, Niall C. Slowey, Prashant D. Sardeshmukh, James A. Carton, Sulagna Ray, and Jeffrey S. Whitaker

Abstract

El Niño is widely recognized as a source of global climate variability. However, because of limited ocean observations during the early part of the twentieth century, little is known about El Niño events prior to the 1950s. An ocean model, driven with surface boundary conditions from a recently completed atmospheric reanalysis of the first half of the twentieth century, is used to provide the first comprehensive description of the structure and evolution of the 1918/19 El Niño. In contrast with previous descriptions, the modeled El Niño is one of the strongest of the twentieth century, comparable in intensity to the prominent events of 1982/83 and 1997/98.

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Yangxing Zheng, George N. Kiladis, Toshiaki Shinoda, E. Joseph Metzger, Harley E. Hurlburt, Jialin Lin, and Benjamin S. Giese

Abstract

The annual mean heat budget of the upper ocean beneath the stratocumulus/stratus cloud deck in the southeast Pacific is estimated using Simple Ocean Data Assimilation (SODA) and an eddy-resolving Hybrid Coordinate Ocean Model (HYCOM). Both are compared with estimates based on Woods Hole Oceanographic Institution (WHOI) Improved Meteorological (IMET) buoy observations at 20°S, 85°W. Net surface heat fluxes are positive (warming) over most of the area under the stratus cloud deck. Upper-ocean processes responsible for balancing the surface heat flux are examined by estimating each term in the heat equation. In contrast to surface heat fluxes, geostrophic transport in the upper 50 m causes net cooling in most of the stratus cloud deck region. Ekman transport provides net warming north of the IMET site and net cooling south of the IMET site. Although the eddy heat flux divergence term can be comparable to other terms at a particular location, such as the IMET mooring site, it is negligible for the entire stratus region when area averaged because it is not spatially coherent in the open ocean. Although cold-core eddies are often generated near the coast in the eddy-resolving model, they do not significantly impact the heat budget in the open ocean in the southeast Pacific.

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