Benjamin S. Giese

A Reanalysis of Ocean Climate Using Simple Ocean Data Assimilation (SODA)

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.

A Simple Ocean Data Assimilation Analysis of the Global Upper Ocean 1950–95. Part I: Methodology

Abstract The authors describe a 46-year global retrospective analysis of upper-ocean temperature, salinity, and currents. The analysis is an application of the Simple Ocean Data Assimilation (SODA) package. SODA uses an ocean model based on Geophysical Fluid Dynamics Laboratory MOM2 physics. Assimilated data includes temperature and salinity profiles from the World Ocean Atlas-94 (MBT, XBT, CTD, and station data), as well as additional hydrography, sea surface temperature, and altimeter sea level. After reviewing the basic methodology the authors present experiments to examine the impact of trends in the wind field and model forecast bias (referred to in the engineering literature as “colored noise”). The authors believe these to be the major sources of error in the retrospective analysis. With detrended winds the analysis shows a pattern of warming in the subtropics and cooling in the Tropics and at high latitudes. Model forecast bias results partly from errors in surface forcing and partly from limitati...

Decadal and Interannual SST Variability in the Tropical Atlantic Ocean

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 weak...

Understanding ENSO Diversity

El Nino–Southern Oscillation (ENSO) is a naturally occurring mode of tropical Pacific variability, with global impacts on society and natural ecosystems. While it has long been known that El Nino events display a diverse range of amplitudes, triggers, spatial patterns, and life cycles, the realization that ENSO’s impacts can be highly sensitive to this event-to-event diversity is driving a renewed interest in the subject. This paper surveys our current state of knowledge of ENSO diversity, identifies key gaps in understanding, and outlines some promising future research directions.

Enhanced Warming over the Global Subtropical Western Boundary Currents

Subtropical western boundary currents are warm, fast-flowing currents that form on the western side of ocean basins. They carry warm tropical water to the mid-latitudes and vent large amounts of heat and moisture to the atmosphere along their paths, affecting atmospheric jet streams and mid-latitude storms, as well as ocean carbon uptake1, 2, 3, 4. The possibility that these highly energetic currents might change under greenhouse-gas forcing has raised significant concerns5, 6, 7, but detecting such changes is challenging owing to limited observations. Here, using reconstructed sea surface temperature datasets and century-long ocean and atmosphere reanalysis products, we find that the post-1900 surface ocean warming rate over the path of these currents is two to three times faster than the global mean surface ocean warming rate. The accelerated warming is associated with a synchronous poleward shift and/or intensification of global subtropical western boundary currents in conjunction with a systematic change in winds over both hemispheres. This enhanced warming may reduce the ability of the oceans to absorb anthropogenic carbon dioxide over these regions. However, uncertainties in detection and attribution of these warming trends remain, pointing to a need for a long-term monitoring network of the global western boundary currents and their extensions.

Variations in global mean sea level associated with the 1997–1998 ENSO event: Implications for measuring long term sea level change

The TOPEX/POSEIDON satellite has observed variations in global mean sea level with a precision of 4 mm at 10-day intervals since late 1992. During the 1997–1998 ENSO event, a 20 mm rise, and subsequent fall, of mean sea level was observed. These changes are well correlated with global mean sea surface temperature anomalies, which exhibit a similar response for every major ENSO event since 1981, suggesting the observed mean sea level change is mostly caused by thermal expansion. An Empirical Orthogonal Function analysis of the altimeter-derived sea level maps also suggests a connection with ENSO. We observed the same signal in global mean dynamic heights of the MOM2 ocean model and in anomalies of global mean precipitable water vapor. The presence of ENSO-variability in global mean sea level suggests that detecting the much smaller sea level variations associated with climate change will require at least a decade of precise altimeter measurements.

Impact of the extratropical Pacific on equatorial variability

An ocean general circulation model (OGCM) is used with a data assimilation routine to explore how the higher latitudes in the Pacific affect equatorial variability. The assimilation of observed temperature data in the extratropical regions generates decadal variability on the equator, indicating that the subsurface thermal structure of the tropics may be modulated by low-frequency variability in the higher latitudes. While preliminary analyses show that midlatitude temperature anomalies can be advected to the subtropics along isopycnal surfaces, it is also demonstrated that a wave mechanism may be one of the dominant physical processes by which information is carried from the higher latitudes to the equatorial region.

Interannual and Decadal Variability in the Tropical and Midlatitude Pacific Ocean

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 ...

Middepth circulation of the eastern tropical South Pacific and its link to the oxygen minimum zone

There is an incomplete description of the mid-depth circulation and its link to the oxygen minimum zone (OMZ) in the eastern tropical South Pacific. Subsurface currents of the OMZ in the eastern tropical South Pacific are investigated with a focus at 400 m depth, close to the core of the OMZ, using several Acoustic Doppler Current Profiler sections recorded in January and February 2009. Five profiling floats with oxygen sensors were deployed along 85°50’W in February 2009 with a drift depth at 400 m. Their spreading paths are compared with the model flow field from a 1/10° Tropical Pacific model (TROPAC01) and the Simple Ocean Data Assimilation (SODA) model. Overall the mean currents in the eastern tropical South Pacific are weak so that eddy variability influences the flow and ultimately feed oxygen-poor water to the OMZ. The center of the OMZ is a stagnant area so that floats stay much longer in this region and can even reverse direction. In one case of one float deployed at 8°S returned to the same location after 15 month. On the northern side of the OMZ in the equatorial current system, floats move rapidly to the west. Most current bands reported for the near surface layer exist also in the depth range of the OMZ. A schematic circulation flow field for the OMZ core depth is derived which shows the northern part of the South Pacific subtropical gyre south of the OMZ and the complicated zonal equatorial flow field north of the OMZ.

Episodes of surface westerly winds as observed from islands in the western tropical Pacific

We describe the characteristics of surface westerly wind episodes in the tropical Pacific between mid-1957 and 1980, as they can be determined from a collection of daily average wind records from islands. Several types of frequency of occurrence and duration statistics are presented; events in which the maximum westerly anomalies exceed 7 m s−1 are not common, but are prominent in the records. There is strong seasonal and geographical variation in frequency of occurrence. Four types of westerly episodes are found; we call them types N, C, S, or FS, according to whether the maximum westerly wind is found north of, centered on, south of, or “far south” of the equator near the international date line. Sixteen, 45, 47, and 47 events of type N, C, S, and FS are identified, respectively. The strongest westerly winds are associated with type C and S events; maximum daily average values can exceed 20 m s−1. Only type C and S events are found to have substantial equatorial zonal wind anomalies. Several examples of type C and S events are presented. Composite events of each type have been computed, and their characteristics are described. In order to explore relationships between equatorial westerly events and cyclones, all of the named tropical cyclones within a region encompassing the near-date line islands have been identified; various statistics are presented, but no absolute relationships have been identified. The similarity between the composite type C and S anomaly patterns and patterns of the El Nino-Southern Oscillation monthly mean zonal wind anomaly previously documented in the literature is striking; evidently, the westerly events often dominate in the monthly average anomaly.