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Biology

Reconstructing Regional Climate and Oceanographic Processes From Tree Rings

Article:

Black, B.A., Sydeman, W.J., Frank, D.C., Griffin, D., Stahle, D.W., Garcia-Reyes, M., Rykaczewski, R.R., Bograd, S.J., Peterson, W.T., (2014). Six centuries of variability and extremes in a coupled marine-terrestrial ecosystem. Science 345(2603), 1498-1502. DOI:10.1126/science.1253209.

 

Background:

Eastern Boundary Current Systems (EBCS) such as the California Current have been the focus of numerous studies (such as Sydeman et al., 2014) due to their importance in marine ecosystem health, supporting the world’s largest fisheries. Variability in the strength of alongshore North winds which drive these currents directly impact the strength of upwelling (Figure 1). Upwelling is the movement of deep, cold, nutrient-packed water towards the surface. The results of strong upwelling in EBCS are phytoplankton (marine primary producers) blooms, which support a rich ecosystem.

The strength of upwelling varies greatly on short timescales (years to tens of years). Observational records of wind and upwelling strength rarely exceed 70 years, making it difficult to determine mechanisms that contribute to the variability of upwelling strength. Additionally, short observational records provide no context on how variability has changed over centuries.

Figure 1.  Top: High sea level pressure system responsible for upwelling favorable, alongshore North winds.  Bottom: Shaded blue regions indicates correlation with the upwelling favorable North wind direction.  Orange shading onshore indicates precipitation.  Tree chronologies are reported as to how well they agree with Blue Oak chronologies.

Figure 1. Top: High sea level pressure system responsible for upwelling favorable, alongshore North winds. Bottom: Correlations of datasets with the California Current Winter Index (CCWI).  Shaded blue regions indicate correlation of upwelling favorable (North) winds with the CCWI. Orange shading onshore indicates precipitation, which is negatively correlated with the CCWI.  Regional tree chronologies are also reported.

The California Current Winter Index (winter index) is a combination of three highly correlated winter season climatic conditions:

  1. The strength of NE Pacific sea level pressure
  2. The strength upwelling in the California current
  3. The sea level in San Francisco

 

Persistent high sea level pressure causes strong upwelling in the California current, consequently lowering regional sea level along the California coast. These same atmospheric and oceanographic conditions are also responsible for wintertime dry conditions onshore in California (Figure 1). The authors of this study recognized how sea level pressure is robustly connected with onshore precipitation and applied it to better understanding the variability of upwelling strength in the California current over the centuries.

It should come as no surprise that the rate of annual tree growth varies with the amount of precipitation. As trees grow, new growth increases the diameter of the trunk, annually adding a characteristic growth ring. Seasonal dry conditions, such as those caused by strong upwelling in the California current yield narrower growth rings. The authors of this paper were therefore able to measure and count annual growth rings from trees in California to reconstruct onshore precipitation and sea level pressure over several centuries. The science of dendrochronology capitalizes on analyzing the growth rings with the understanding that each ring represents one calendar year of time, making it extremely useful in other methods of dating. The authors built a chronology from 16 blue oak trees from California with ages greater than 400 years.

The growth ring chronology was compared to 4 marine biological datasets that covary with upwelling strength. These four datasets spanned at least 35 years and included fish bone growth, seabird mean egg laying dates and breeding success. High rates of fish bone growth, early seabird egg laying and high reproductive success were coincident with reduced tree growth and onshore precipitation (Figure 2). With such a strong connection between upwelling strength and tree growth as an indicator for precipitation over several decades, the authors extended the tree ring chronologies back in time nearly 600 years to understand the variability in upwelling strength over this same timespan.

Figure 2.  Coherence of marine biological datasets with the blue oak tree ring chronology and the  California Current Winter Index.

Figure 2. Coherence of marine biological datasets with the blue oak tree ring chronology and the California Current Winter Index.

 

Results:

Extending tree ring chronologies over nearly six centuries provides scientists with a regional precipitation trend. Since regional precipitation is tightly correlated with upwelling strength, such that dry winter conditions in California are tied to strong California current winter upwelling, upwelling strength can be reconstructed over this six century period. Over this span of time, there was no observable change in the mean upwelling strength. However, changes in variability were evident across the long term trend. Modern variability is high, but not unprecedented in the reconstruction. The frequency of weak winter upwelling (a negative winter index phase) was found to have greatly increased during the 20th century. The most prominent negative winter index values correspond to strong El Niño events. The increased variability of El Niño events during the 20th century is driving the increasingly frequent strong negative winter index values, contributing to high modern variability.

 

Figure 3.  The California Current Winter Index (CCWI) reconstruction from tree ring chronologies (C.) and running standard deviation (D.).  Sample depth (E.) shows how many blue oak tree ring data contribute to the CCWI reconstruction.  Also shown are the number per century of extreme negative California Current Winter Index events, reported as pre and post-1950.

Figure 3. The California Current Winter Index (CCWI) reconstruction from tree ring chronologies (C.) and running standard deviation (D.). Sample depth (E.) shows how many blue oak tree ring data contribute to the CCWI reconstruction. Also shown are the number per century of extreme negative California Current Winter Index events, reported as pre and post-1950 (F).

Significance:

The key difference that distinguishes modern variability from other time spans of high variability is that the frequency of negative anomalies (aka weak winter upwelling) was 3 to 5 times greater from 1950 to present when compared across the entire reconstruction. This finding is important since weak winter upwelling reduces productivity across marine ecosystems, which can have devastating impacts on important fisheries. Understanding regional climate variability and the ties to precipitation in central and southern California is a critical topic as this region is currently experiencing severe droughts and freshwater shortages. This study advances our understanding of how modern climate variability compares to previous centuries, and sheds light on the how the coupled marine-terrestrial systems respond over short and long timescales.

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