Warren B. White

Response of global upper ocean temperature to changing solar irradiance

By focusing on time sequences of basin-average and global-average upper ocean temperature (i.e., from 40oS to 60oN) we find temperatures responding to changing solar irradiance in three separate frequency bands with periods of >100 years, 18-25 years, and 9-13 years. Moreover, we find them in two different data sets, that is, surface marine weather observations from 1990 to 1991 and bathythermograph (BT) upper ocean temperature profiles from 1955 to 1994. Band-passing basin-average temperature records find each frequency component in phase across the Indian, Pacific, and Atlantic Oceans, yielding global-average records with maximum amplitudes of 0.04 o _+ 0.01oK and 0.07 o _+ 0.01oK on decadal and interdecadal scales, respectively. These achieve maximum correlation with solar irradiance records (i.e., with maximum amplitude 0.5 W m -2 at the top of the atmosphere) at phase lags ranging from 30 o to 50 o. From the BT data set, solar signals in global-average temperature penetrate to 80-160 m, confined to the upper layer above the main pycnocline. Operating a global-average heat budget for the upper ocean yields sea surface temperature responses of 0.01o-0.03oK and 0.02o-0.05oK on decadal and interdecadal scales, respectively, from the 0.1 W m -2 penetration of solar irradiance to the sea surface. Since this is of the same order as that observed (i.e., 0.04o-0.07oK), we can infer that anomalous heat from changing solar irradiance is stored in the upper layer of the ocean.

Annual and Interannual Variability in the Kuroshio Current System

Abstract Individual, seasonal, 300 m temperature maps were constructed over the Kuroshio Current System from 130°E to 170°W, for a 4-year period from summer 1976 through spring 1980, using TRANSPAC XBT data and JODC temperature/depth data. Quasi-stationary meanders in the Kuroshic Current System occurred at 137°C (i.e., Kuroshio Meander), at 144°E and 150°E (i.e., lee-wave meanders), and near 160°E (i.e., meander over the Shatsky Rise). A composite of the paths of the Kuroshio (i.e., the 12°C isotherm) from the individual seasonal maps, and the total variance map, finds nodes (i.e., minima) and anti-nodes (i.e., maxima) of variability to have existed along the mean Kuroshio path. The anti-nodes coincided with the location of the quasi-stationary meanders, the nodes in between. Zonal propagation of temperature anomalies accounted for 20–30% of the total interannual variance. These temperature anomalies propagated eastward at 0.5–1.5 cm s−1 in the region 140°–155°E, and westward at −1 to −2 cm s−1 in the re...

Design of a global observing system for gyre-scale upper ocean temperature variability

Abstract Three-dimensional covariance matrices are computed at each 5° latitude by 10° longitude location from approximately 30°S – 60°N at depths of 0m, 200m, 400m from temperature anomalies about the mean annual cycle for the 13 years from 1979–1991. Lags are chosen to focus on seasonal-to-interannual time scales resolved on gyre-to-basin space scales. These sample covariance matrices are fitted with a second-order auto-regressive (AR) model, allowing for the detection of horizontal wave propagation in the presence of dissipation. Over most of the ocean and at all depths, ENSO signals (i.e. with zero-crossing scales of 9–12 months) dominate the interannual variability, with eastward propagation found at the sea surface over most of the Pacific and Indian Oceans, and westward propagation found over most of the Pacific Ocean at 200m and 400m, the latter qualitatively consistent with Rossby wave propagation. The noise variance is approximately equal to the signal variance at all depths, while the ratio of zero-crossing scale-to-decay scale (i.e. 1.5–2.5) finds ENSO wave propagation critically dissipated almost everywhere. This effectively reduces the second-order AR model to a first-order AR model, with decay scales and not zero-crossing scales defining decorrelation scales. Analyzing these covariance statistics within the framework of optimum interpolation allows sampling rates to be estimated for detecting seasonal-to-interannual variability to within prescribed error limits. Utilizing the first-order AR covariance model, 1–2 observations per decorrelation scale in each dimension allows the interpolation error to be 0.8-0.6 of the signal standard deviation. A uniform set of decorrelation scales (e.g. 2.5° latitude, 5° longitude, 3 months) and 1.0 for the noise-to-signal ratio are chosen for detecting minimum gyre-scale biennial variability at all depths uniformly over the global domain. Therefore, sampling the global ocean at a rate of 2500–3000 observations per month yields maximum interpolation errors for biennial signals (i.e. ±0.4°C at the sea surface, ±0.5°C at 200m, and ±0.2°C at 400m) that are similar to those for the mean annual cycle. This sampling rate is adequate for detecting the space-time evolution of biennial signals (and, hence, ENSO signals) over the interior global ocean. But, it is inadequate for detecting year-to-year changes in upper ocean heat storage averaged over large portions of the global ocean as required by the WOCE Heat Budget Study.

ENSO and ENSO-related Predictability. Part I: Prediction of Equatorial Pacific Sea Surface Temperature with a Hybrid Coupled Ocean–Atmosphere Model

A hybrid coupled model (HCM) of the tropical ocean–atmosphere system is described. The ocean component is a fully nonlinear ocean general circulation model (OGCM). The atmospheric element is a statistical model that specifies wind stress from ocean-model sea surface temperatures (SST). The coupled model demonstrates a chaotic behavior during extended integration that is related to slow changes in the background mean state of the ocean. The HCM also reproduces many of the observed variations in the tropical Pacific ocean-atmosphere system. The physical processes operative in the model together describe a natural mode of climate variability in the tropical Pacific ocean–atmosphere system. The mode is composed of (i) westward-propagating Rossby waves and (ii) an equatorially confined air–sea element that propagates eastward. Additional results showed that the seasonal dependence of the anomalous ocean–atmosphere coupling was vital to the models ability to both replicate and forecast key features of the tropical Pacific climate system. A series of hindcast and forecast experiments was conducted with the model. It showed real skill in forecasting fall/winter tropical Pacific SST at a lead time of up to 18 months. This skill was largely confined to the central equatorial Pacific, just the region that is most prominent in teleconnections with the Northern Hemisphere during winter. This result suggests the model forecasts of winter SST at leads times of at least 6 months are good enough to be used with atmospheric models (statistical or OGCM) to attempt long-range winter forecasts for the North American continent. This suggestion is confirmed in Part II of this paper.

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

Abstract 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 ...

Slow oceanic teleconnections linking the Antarctic Circumpolar Wave with the tropical El Niño-Southern Oscillation

A case study for the period 1982–1994 shows that a major source for the Antarctic Circumpolar Wave is in the western subtropical South Pacific, where interannual anomalies in sea surface temperature (SST) and precipitable water (PrWat) form. Once established, these interannual anomalies, in tandem with anomalies in sea level pressure (SLP), move south toward the Southern Ocean. The system then migrates east around the globe through a combination of oceanic advection with the Antarctic Circumpolar Current and ocean-atmosphere coupling. The coincidence of interannual anomalies in SST, SLP, and PrWat indicates the extratropical ocean and atmosphere are tightly linked on these timescales. Large portions of the interannual SST anomalies branch advectively northward into the South Atlantic and Indian Oceans, ultimately reaching the tropics in each basin some 6–8 years after appearing in the low-latitude Pacific. This constitutes a slow, oceanic teleconnection that is unique in climate dynamics, made possible by the continuity of Earths oceans via the Southern Ocean. In the tropical Indian Ocean these interannual anomalies move east and arrive at the Indo-Pacific transition in advance of the trans-Pacific propagation of the respective El Nino-Southern Oscillation (ENSO) phases. The interannual SST and PrWat anomalies that appear in the subtropical South Pacific are directly linked with the ENSO cycle on the equator through anomalous vertical convection and a regional overturning cell in the troposphere, the same cell that initiates fast planetary waves in the atmosphere that carry ENSO signals around the southern hemisphere on much shorter timescales.

ENSO Signals in Global Upper-Ocean Temperature

Abstract The time-space evolution of the El Nino-Southern Oscillation in sea surface temperature (SST) and heat storage of the upper 400 m (HS400) for the Pacific, Indian, and Atlantic Oceans is investigated for 13 years (1979–1991). EOF and rotated EOF (Varimax or VRX) analyses are performed using the time series of normalized anomalies for each ocean separately and then for the global ocean. In the Pacific and Indian Oceans, the two dominant EOF modes for both SST and H5400 are associated with ENSO. For SST they account for 49% of the total variance in each mean, while for H5400 they account for over 35% of the total variance in each ocean. In the Pacific Ocean, the first EOF modes for SST and HS400 display peak values during spring-summer of 1983 and 1987. They are characterized by maximum positive loadings (or warmer temperature) in the eastern and central Pacific Ocean straddling the equator. These modes represent the peak phase of El Nino off the west coast of South America. The second modes for SST...

Patterns of coherent decadal and interdecadal climate signals in the Pacific Basin during the 20th century

Two distinct low-frequency fluctuations are suggested from a joint frequency domain analysis of the Pacific Ocean (30°S-60°N) sea surface temperature (SST) and sea level pressure (SLP). The lowest frequency signal reveals a spatially coherent interdecadal evolution, In-phase SST and SLP anomalies are found along the subarctic frontal zone (SAFZ). It is symmetric about the equator, with tropical SST anomalies peaking near 15° latitudes in the eastern Pacific. The other low-frequency signal reveals a spatially coherent decadal evolution. It is primarily a low-latitude phenomenon. Tropical SST anomalies peak in the central equatorial ocean with evidence of atmospheric teleconnections. These interdecadal and decadal signals join the ENSO and quasi-biennial signals in determining dominant patterns of Pacific Ocean natural climate variability. Relative phasing and location of the SST and SLP anomalies for the decadal, ENSO, and the quasi-biennial signals, are similar to one another but significantly different from that of the interdecadal signal.

Quasi-periodicity and global symmetries in interdecadal upper ocean temperature variability

Recent studies find interannual (i.e., 3 to 7 year), decadal (i.e., 9 to 13 year), and interdecadal (i.e., 18 to 23 year) periodicities, and a trend dominating global sea surface temperature (SST) and sea level pressure (SLP) variability over the past hundred years, with the interdecadal signal dominating sub-El Nino-Southern Oscillation (ENSO) frequencies. We isolate interdecadal frequencies in SST and SLP records by band passing with a window admitting 15 to 30 year periods. From 1900 to 1989, the rms of interdecadal-filtered SST and SLP anomalies is largest in the extratropics and eastern boundaries. First-mode empirical orthogonal functions (EOFs) explain about half the interdecadal variance in both variables, with the tropical warm phase peaking near 1900, 1920, 1940, 1960, and 1980. From 1955 to 1994, EOF spatial patterns of interdecadal SST, SLP, and 400m temperature (T400) anomalies reveals global reflection symmetries about the equator and global translation symmetries between ocean basins, with tropical and eastern ocean SSTs warmer (cooler) than normal, covarying with stronger (weaker) extratropical westerly winds, cooler (warmer) SSTs in western-central subarctic and subantarctic frontal zones (SAFZs), stronger (weaker) subtropic and subarctic gyre circulations in North Pacific and North Atlantic Oceans, and warmer (cooler) basin and global average SSTs of 0.1°C or so. Evolution of interdecadal variability from the tropical warm phase to the tropical cool phase is propagative, also characterized by reflection and translation symmetries. During the tropical warm phase, cool SST anomalies along western-central SAFZs are advected slowly eastward to the eastern boundaries and subsequently advected poleward and equatorward by the mean gyre circulation, the latter conducting extratropical SST anomalies into the tropics. A delayed action oscillation model is constructed that yields the quasiperiodicity of interdecadal variability in a manner consistent with these global symmetries in both pattern and evolution.

Short-Term Climatic Variability in the Thermal Structure of the Pacific Ocean during 1979–82

Abstract Short-term climatic variability in both sea surface temperature (SST) and vertically averaged temperature over the upper 400 m of ocean (Tav) is mapped over the Pacific from 20°S to 50°N each bimonth for four years from 1979 to 1982, leading up to the 1982–83 ENSO (El Nino–Southern Oscillation) event. This mapping was made possible by the collection of approximately 85 000 temperature/depth observations in the Pacific Ocean by volunteer observing ships. Anomalies of SST and Tav were approximately the same magnitude at midlatitude as in the tropics, with the exception of large changes occurring in the tropics during the 1982–83 ENSO event. During the ENSO event, (Tav variability was largest in the western tropical North Pacific and SST variability was largest in the eastern equatorial Pacific. Both parameters had spatial patterns which were of opposite phase on either side of the ocean, indicating an eastward shift of warm waters during the ENSO event. Correlation studies determined that on averag...