D. E. Harrison

El Niño‐Southern Oscillation sea surface temperature and wind anomalies, 1946–1993

El Nino-Southern Oscillation (ENSO) periods, which occur irregularly every few years, are a major perturbation of the Earths climate system that involves large-scale changes in winds, rainfall, sea surface temperature (SST), and surface pressure. In some areas of the world there are disastrous droughts, and in others there is serious flooding. North American weather patterns are also affected. It is important to develop skillful forecasts for ENSO periods. Our goal here is to provide an overview of the global ocean and atmosphere surface changes that typically occur during ENSO periods. Knowledge of these anomaly patterns is needed in order to improve our understanding and forecasts of ENSO. With a global surface data set we describe the statistically significant patterns of SST, surface wind, and surface pressure changes that on average are associated with the 10 post-World War II ENSO periods. We present these average anomaly results as an “ENSO composite.” It is useful to identify phases of the typical ENSO and examine the statistically significant elements phase by phase. An ENSO by ENSO period search indicates that about two thirds of these elements occur in 90% or more of the ENSO periods: we define a “Robust ENSO Composite” from these frequently occurring elements and find it to be an Indo-Pacific phenomenon. Limitations in the surface data set make it possible that this study has not identified all the important aspects of ENSO periods; data are very sparse in both space and time over much of the tropics and the southern hemisphere. However, we suggest that any theory or model of ENSO should at least exhibit the features of this robust composite, and is unlikely to able to represent adequately the large-scale environmental impacts of ENSO unless it does so.

Monsoon Breaks and Subseasonal Sea Surface Temperature Variability in the Bay of Bengal

Abstract The Indian southwest monsoon directly affects the lives of over one billion people, providing almost 90% of the annual precipitation to the Indian subcontinent. An important characteristic of the southwest monsoon is variability on subseasonal timescales, with “active” periods of heavy rain interrupted by drier “break” periods. Both the number of monsoon breaks in a season and the timing of these breaks profoundly impact agricultural output from the Indian subcontinent. Most research on monsoon breaks has emphasized possible atmospheric mechanisms. However, new satellite data reveal large-amplitude basin-scale subseasonal sea surface temperature (SST) variability in the Bay of Bengal (BoB), in which northern BoB cooling precedes monsoon breaks by about 1 week. The relationship is statistically significant at the 95% level over the 3 yr examined, and so offers a potential statistical predictor for short-term monsoon variability. The basinwide averaged amplitude of SST changes is 1°–2°C and local c...

ENSO Warm (El Niño) and Cold (La Niña) Event Life Cycles: Ocean Surface Anomaly Patterns, Their Symmetries, Asymmetries, and Implications

Abstract Previous studies by the authors have described the composite global marine surface anomalies of ENSO warm (El Nino) events and cold (La Nina) events. Here the similarities and differences in these life cycles are examined. Qualitatively different behavior between warm events and cold events exists in the tropical Indian and Atlantic Oceans and in the extratropical Pacific. Even in the tropical Pacific statistically significantly different behavior is found in some variables for particular regions and phases of the life cycles. A single-mode regression analysis of the ENSO signal is done; the patterns are very similar to those of previously published ENSO EOF and regression analyses. The authors describe how the regression patterns obscure many of the interesting life cycles and life cycle differences of cold events and warm events. Most of the regression structures outside of the tropical Pacific are not statistically significant because of such differences. ENSO models should be evaluated agains...

Tropical Pacific Sea Surface Temperature Anomalies, El Niño, and Equatorial Westerly Wind Events*

Abstract The authors examine global statistical relationships between westerly wind events (WWEs) and sea surface temperature anomaly (SSTA) variability, using a compositing technique for the period 1986–98. The authors describe the extent to which equatorial WWEs are associated with central and eastern equatorial Pacific waveguide warming and with local SSTA changes under the WWE. Their goal is to quantify the extent to which equatorial WWEs are fundamental to the onset and maintenance of warm El Nino–Southern Oscillation conditions. In order to understand the effect of WWEs on SSTA evolution, they begin by examining how SSTA changes in the absence of equatorial WWEs. They find that SSTA tends toward mean climate values in the absence of equatorial WWEs, whether the eastern equatorial Pacific has close to normal SSTA or warmer than normal SSTA. The two equatorial WWE types whose main surface wind anomalies are west of the date line are associated with weak local surface cooling. The equatorial WWE type t...

Westerly Wind Events in the Tropical Pacific, 1986–95*

Abstract Based on examination of 10 yr of 10-m winds and wind anomalies from European Centre for Medium-Range Weather Forecasts (ECWMF) analysis, definitions for westerly wind events (WWEs) of eight different types are proposed. The authors construct a composite for each type of event, show that a simple propagating Gaussian model satisfactorily describes the evolution of zonal wind anomaly for each type of event, and determine the scales of each composite event by fitting the model to each composite. The authors discuss the WWEs that occurred during the Tropical Oceans Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) intensive observing period (IOP) and show the extent to which these composite events are able to reproduce the major westerly wind features of the IOP. The frequency of occurrence of each type of WWE for each year of this record and by calendar month are described; the authors find several types of events are negatively correlated with the annual mean troup Souther...

On the termination of El Niño

A feature of the end-phase of recent El Nino periods is thermocline shallowing to normal depths, in the central and eastern equatorial Pacific, before sea surface temperature (SST) returns to normal. Also characteristic of El Nino periods is a late-in-the-year shift of the El Nino equatorial westerly wind anomalies, from symmetric about the equator to centered south of the equator. We show that the directly forced ocean response to this shift produces thermocline shallowing like that observed. The wind shift may result from the normal seasonal evolution of Pacific waters warmer than 28°C. These shift from being relatively symmetric about the equator in October–November, to concentrated south of the equator in December–January. Thus, the seasonal cycle may be a central part of the mechanism of El Nino termination.

The Pacific decadal oscillation, air‐sea interaction and central north Pacific winter atmospheric regimes

Prominent and persistent anomalies in the atmospheric flow (troughs and ridges) occur sporadically over the central North Pacific, and can have profound consequences for the weather of North America. We have examined how these events are associated with large scale central North Pacific sea surface temperature (SST) anomalies, using an index for the Pacific Decadal Oscillation (PDO). The anomalies in turbulent air-sea heat fluxes and low-level baroclinity associated with the PDO are manifested differently during troughs than during ridges in their effects on the transient eddies (storms). These effects may help explain why prominent troughs (ridges) occur about 3 (2.5) times more frequently during periods when the PDO is significantly positive (negative) than of opposite sign. Our results suggest that the state of the mid-latitude Pacific Ocean more fundamentally affects the atmosphere than has been thought.

January 1999 Indian Ocean Cooling Event

Unexpectedly strong sub-seasonal SST variability over the tropical Indian Ocean has been observed by the TRMM microwave imager, which provides unprecedented tropical SST information in the presence of clouds. In January-February 1999 SST averaged over an area 20° longitude by 10° latitude cooled by ∼1.5°C over 10 days and warmed by ∼1.25°C in the following week. Local changes were up to ±3°C. There were strong variations in atmospheric convection and surface wind stress associated with the SST variability. The evolution of the convection suggests a feedback from the SST changes into atmospheric convection. Net air-sea heat-flux cannot account for the observed cooling; oceanic processes are fundamental to this event. The roles of such sub-seasonal ocean-atmosphere interactions in the evolution of the Madden-Julian Oscillation, the seasonal and the interannual variability of Indian Ocean SST deserve further investigation.

Assessment of the air‐sea exchange of CO2 in the south Pacific during austral autumn

Measurements of CO2 concentrations in the atmosphere and in the surface waters of the South Pacific Ocean were made by NOAA scientists between 1984 and 1989. These basin-wide measurements were all taken during austral autumn and provide data for evaluation of the seasonal flux of CO2 from this region. The sensitivity of this flux to the uncertainty in the CO2 gas exchange coefficient was evaluated using four different wind data sets and two formulations for the wind dependence of gas transfer velocity. The integrated net flux of CO2 to the atmosphere during austral autumn (February to May) ranges from −0.03 (ocean influx) to +0.09 (ocean efflux) GT of carbon depending on the combination of wind field and wind-dependent exchange coefficient used.

Darwin sea level pressure, 1876–1996: Evidence for climate change?

It has been argued that there was a period of prolonged ENSO conditions between 1990–95 so anomalous that it is “highly unlikely” to be due to “natural decadal-timescale variation” [Trenberth and Hoar, 1996]. This conclusion follows from their study of the Darwin sea level pressure anomaly record, which found that the 1990–95 period would occur randomly about once every 1100–3000 yrs. Taking into account the uncertainty in number of degrees of freedom in the Darwin time series, we find that conditions like those of 1990–95 may be expected as often as every 150–200 yrs at the 95% confidence level. Students-t, ARMA, and Bootstrap/Monte Carlo tests of the time series all yield similar results. We therefore suggest that the 1990–95 period may plausibly be an aspect of the natural variability of the tropical Pacific.