Mark Carson

Oceanography, University of Washington

Upper Ocean Warming: Spatial Patterns of Trends and Uncertainty Estimates

Long-term temperature trends in the upper layers of the ocean are calculated from gridded observed data to explore spatial and temporal trend variability.  Depths between 50 m and 1000 m are examined.  Most of the ocean does not have 50-year temperature trends that are significant at the 90% level.  At 100 m only about 35% of the ocean have such trends and the coverage declines dramatically with increasing depth. There is much spatial structure in 50-year trends, with areas of strong warming and others of strong cooling.  There is also strong interdecadal variability shown in 20-year trends; almost every region studied shows both warming and cooling trends over a 50-year period. 50-year trend results are compared and contrasted with trends calculated from data interpolated to standard levels and from a dataset used in previous heat content trend studies.  These comparisons show that different data treatments can yield significantly different trends.  Thus, the uncertainty in heat content integrals is large and such integrals may not be truly representative of the global ocean heat content.  This situation prevails because of the presence of strong interdecadal and spatial trend variability in conjunction with strong temporal and spatial sampling issues.  There is also the new XBT problem involving interdecadal bias to consider.  It is suggested that calculating a world ocean residual heat content trend is highly problematic over this period.

Natalia Stefanova1 & Michael J. McPhaden2

1Oceanography, University of Washington, 2NOAA/PMEL

Interannual Variability of the Pacific EUC

The Pacific Equatorial Undercurrent (EUC), which is a fast subsurface current flowing east in the upper thermocline along the equator, is a key component of the complex equatorial current system.  Understanding its dynamics in a climate context is crucial because the EUC feeds equatorial upwelling and affects the sea surface temperature, which in turn is an important driver of the climate system.  In addition, because the EUC varies significantly on ENSO time scales, further insights into its dynamics might help improve El Niño predictions, especially in a changing climate.  In this study, we examine the interannual variability of the Pacific Equatorial Undercurrent (EUC).  We analyze direct measurements of zonal velocity from the Tropical Atmosphere Ocean (TAO) buoy array to understand how the wind stress, pressure gradient, and vertical diffusive processes combine to produce the observed variability of the EUC and how the low-frequency variability of the EUC relates to the ENSO cycle.   

Debra Tillinger

Lamont-Doherty Earth Observatory, Columbia University

Makassar Strait Transport During the 1997 El Niño Event

The Indonesian Throughflow is the only low latitude connection between the Pacific and Indian Oceans and is an important contributer to the meridional heat transport budget of the climate system. The flow brings North Pacific thermocline water to the Indian Ocean and travels primarily through the Makassar Strait. This flow through the Makassar Strait is investigated via a hydrodynamic model that incorporates pressure and density profiles from the inflow region of the Pacific Ocean and the outflow region of the Indian Ocean. The model is forced with both in situ data and interpolated data from the Simple Ocean Data Assimilation project. The flow was modeled for 1997, a year with strong El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) signals. The modeled flow shows the same profile and magnitude as that derived from current meters in the Makassar Strait. Empirical Orthogonal Function (EOF) analysis of the resultant flow field identifies two overlapping but distinct climatic signals. During that year, the ENSO NINO3.4 index shows a 35% correlation with the magnitude of the ITF. The IOD mode index for 1997 is strongly correlated with the ENSO signal and shows a 53% correlation with the ITF. Both indices are correlated with regional climate including rainfall patterns in Australia and India.

Robert E. Nicholas

Atmospheric Sciences, University of Washington

Empirical Rainfall Prediction for Rice Agriculture in Jiangxi Province, China

A variety of empirical methods have previously been used to make seasonal precipitation forecasts for water-sensitive agricultural regions in developing counties.  These have included ENSO-based predictions of wintertime-total rainfall applied to wheat farming and reservoir management in northwest Mexico, and forecasts of monsoon onset date applied to rice farming in Indonesia.  Both precipitation mechanisms function in China’s Jiangxi Province and are important for the production of rice, which is typically grown in both late spring and early autumn.  This situation provides a unique opportunity to build upon the scientific knowledge developed in our studies of Mexico and Indonesia for the possible benefit of millions of small farmers in southeastern China whose livelihoods are profoundly influenced by the timing and quantity of seasonal rainfall.

The annual cycle of rainfall in Jiangxi Province contains a number of features that make it an interesting subject of inquiry.  Wintertime precipitation arrives via synoptic disturbances traveling along the Asian storm track and is strongly influenced by the El Niño-Southern Oscillation. Late spring and early summer precipitation is associated with a monsoonal circulation that brings bursts of intense rainfall on weekly timescales.  However, the springtime transition between these two regimes is indistinct and poorly understood.  Furthermore, in most years, the northward passage of a feature known as the Baiu front brings an abrupt end to monsoon rainfall in late June and is often followed by an extended dry period. In this work, we seek to further understand the dynamics of the mechanisms responsible for rainfall in Jiangxi Province and develop one or more empirical downscaling models for the prediction of seasonal rainfall based on patterns of tropical Pacific sea surface temperatures and features in the large-scale atmospheric circulation. This work will be combined with the efforts of U.S. and Chinese collaborators in hydrology, agriculture, and economics within a cross-disciplinary modeling framework to be used by the planning community in Jiangxi province.

Thomas Connolly

Oceanography, University of Washington

Interannual Variability in Dissolved Oxygen Over the Continental Shelves of Washington and Oregon

Regions of enhanced coastal upwelling are often characterized by low concentrations of dissolved oxygen near the bottom of the water column.  Upwelling favorable winds drive surface water offshore, causing advection of nutrient rich and oxygen poor water onto the continental shelf, while decomposition of sinking organic matter causes further oxygen depletion.  This low oxygen state, called "hypoxia," develops during summer over the continental shelves of Washington and Oregon, though its severity and spatial extent vary from year to year, sometimes with profound ecosystem impacts.  In the summer of 2006, fish and crab mortalities, associated with record low dissolved oxygen concentrations in near shore coastal waters, drew the attention of scientists and other concerned members of the public.  Much of this attention focused on 1) whether hypoxic events are becoming more frequent and intense along the Pacific Northwest coast and 2) the sensitivity of coastal dissolved oxygen concentrations to shifts in climate, via oceanic and atmospheric forcing mechanisms.

In order to quantify the severity and extent of hypoxia during the summer upwelling season, and to identify dominant processes, this study uses physical and biological data from historical archives and from recent intensive interdisciplinary programs.  Historical data shows that dissolved oxygen concentrations have reached hypoxic levels throughout the period of record, including on the inner shelf (30m or less water depth), where sensitive habitats are located.  Recent hydrographic data from ships and fixed moorings resolves a wide range of spatial and temporal scales, permitting analysis of processes such as local wind patterns, water column stratification, surface chlorophyll, and anomalies in oxygen and nutrients in the deep source of upwelled water.  Seasonal development of dissolved oxygen off Washington and Oregon is compared in three distinct years: 2003, when physical and biological properties were close to historical means, 2005, when the onset of upwelling winds was delayed, and 2006, when dissolved oxygen was almost completely depleted near the bottom of the water column. This view of the factors driving coastal hypoxia is an important step towards assessing the impacts of climate change on coastal regions.