Are the Waters Along the Central California Coast and Monterey Bay Getting Colder?
- Larry Breaker
Moss Landing Marine Laboratories, California State University
- Richard Starr
California Sea Grant
There are indications that waters along the California coast are getting colder. In this essay, we briefly examine three questions related to this change. First, how evident is this cooling along the central California coast and in Monterey Bay? Second, when did the change to cooler conditions occur locally? And third, why is it happening?
Increased upwelling between April and September has occurred primarily along the central California coast between 32°N and 40°N (Mendelssohn and Schwing, 2002). According to Peterson and Schwing (2003), the climate of the North Pacific experienced a major transition in late 1998 that resulted in a cooling of the coastal waters in the California Current by several degrees, and that this change was concurrent with a sign reversal in the phase of the Pacific Decadal Oscillation (PDO). This event may not have been related to the PDO, however, since the patterns of sea level pressure and sea surface temperature (SST) anomalies showed little resemblance to those normally associated with the PDO, according to Bond et al. (2003). Consistent with a sustained change in the climate system of the North Pacifc, however, Garcia-Reyes and Largier (2010) have observed stronger upwelling-favorable winds, increased coastal upwelling and cooler sea surface temperatures (SSTs) along the California coast from 1982 to 2008.
Linear regressions of daily sea surface temperature (SST) have shown that temperature at Pacific Grove in Monterey Bay has decreased by approximately 0.7°C over the past decade and by almost 0.5°C over the past five years. These changes are indeed significant when we compare them to the mean annual cycle which has a range of only 3°C (Breaker, 2005). To further elucidate this cooling trend, we examine temperature data from Pacific Grove and the Farallon Islands off San Francisco from 1975 to 2010 to provide a broader framework within which to interpret this trend. This period includes the major 1976-77 regime shift that signaled a change from cooler to warmer conditions, and four major El Nino episodes occurring in 1976-77, 1982-83, 1992-93 and 1997-98, and one moderate episode in 1986-87. According to McGowan et al. (1998), the frequency of warm events has increased since 1977, which, together with the regime change in 1976-77, initially led to a period of rapidly increasing temperatures.
Figure 1 shows smoothed versions of monthly-averaged SSTs from 1975 through 2010 at Pacific Grove and from 1975 through 2009 at the Farallon Islands. The blue curves show greater smoothing and suppress the influence of the El Nino warming episodes that have occurred during the period of observation. The red curves display less smoothing and so the influence of El Nino warming has not been completely removed. Following the blue curves, temperatures gradually increase from the mid-1970’s up to the mid-1990’s and then gradually decrease up to the present time, generally consistent with the climatic change in late 1998. The red curves show greater cooling starting in 2003 at Pacific Grove and in 2005 at the Farallones. The observed rates of cooling range from about –0.02°C/year since the mid-1990’s, to about –0.06°C/year since the early-to-mid 2000’s. By comparison, Garcia-Reyes and Largier (2010) observed cooling rates of –0.04°C/year to –0.06°C/year for the same the region between 1982 and 2008, but with no indication as to when significant cooling within this period began. To add further support to our observations, Booth (2010) found a significant long-term decrease in temperature at a depth of 17 m since 1996 near the Monterey Bay Aquarium.
To gain more insight into the nature of the cooling process, we have stratified the data by month in order to see if any seasonal patterns exist. Linear trends at each location were calculated over the length of record for each month. The slopes were extracted and found to be negative between October and April, and positive between May and August. Positive slopes during the upwelling season were surprising and so we took a closer look to see what might explain this unexpected result. Although the scatter in the monthly data is high, repeating patterns did emerge. In most cases, SSTs increased rapidly from 1975 to the early 1980’s, followed by a slight decrease in temperature between the mid-1980s and the mid-1990’s. Between October and April, temperatures generally decreased starting in the mid-to-late 1990’s, and between 2000 and 2005, temperatures decreased more rapidly up through 2009. Between May and August, however, we found that temperatures often increased starting as early as the late 1990’s. Fig. 2 shows February and July at Pacific Grove. The smoothed versions shown in red are generally representative of the patterns we have just described for both locations. Based on the slopes of the linear trends, cooling clearly outweighs warming since cooling occurs for seven months of the year and warming for only four. If we combine these results for all 12 months we reproduce the pattern of cooling shown in Fig. 1.
Summary to DateReturning to the three questions that were initially posed, we now we address the third question - why are temperatures along the central California coast and in Monterey Bay cooling? According to the Bakun hypothesis (1990), global warming should cause continental low pressure systems adjacent to regions of coastal upwelling to become more intense, increasing the onshore-offshore pressure gradient and thus the alongshore winds that produce coastal upwelling. To our knowledge, however, this hypothesis has not been verified. Also, this explanation has not been universally embraced (P. Mote, personal communication).
That temperatures could be increasing during the height of the upwelling season was unexpected, based on the results of previous studies. Thus, our results, although limited to two locations, do not appear to be consistent with those obtained by Garcia-Reyes and Largier (2010). We note that the cooling they observed is based on data acquired from NDBC buoys that are located approximately 20km from the coast. Strictly speaking, these buoys lie slightly beyond the expected region of active coastal upwelling based on dynamical considerations. Also, the model results of Capet et al. (2004), show that the drop-off of the alongshore winds due to frictional effects next to the coast favor Ekman pumping (i.e., shear-driven upwelling) rather than coastal upwelling per se. Thus, processes other than coastal upwelling may be important. To specifically address warming during the summer, we refer to the model results of Di Lorenzo et al. (2005), who found that large-scale changes in surface heat fluxes over the eastern North Pacific can outweigh the effects of upwelling along the coast in causing changes in temperature. Thus, we could be witnessing the growing importance of global warming whose influence would be expected to be greatest during the summer. If global warming does play a role, then we need to address the question of how this influence is partitioned between local and remote (i.e., advective) forcing.
The Pacific Decadal Oscillation (PDO) appears to be an important factor that is contributing to lower temperatures off central California. Within two degrees of the coast and north of 31°N, the PDO signal is a dominant source of variability (Lluch-Cota et al., 2001). Thus, it is not surprising that the PDO index is highly correlated with temperatures at the Farallones and Pacific Grove. Fig. 3 shows the correlations between the Farallones, Pacific Grove, and the PDO for the yearly-averaged data. Lagging did not produce higher correlations using the monthly values. Correlation coefficients of 0.81 and 0.71 for the Farallones and Pacific Grove, respectively, were obtained. Cross-correlations between the Farallones, Pacific Grove, and the PDO were also calculated using a 12-month moving window and showed that the strength of the relationships vary significantly over the 35-year period from 1975 to 2009. To summarize, the climate shift that occurred in the late 1990’s resulted in a change from warmer to cooler conditions, and the yearly-averaged PDO index has been negative for the past four years (2007-2010). These results are consistent with the overall cooling we have observed, but not necessarily with indications of summer warming over the past decade. Finally, it is noteworthy that there have been fewer and weaker El Nino warming events since 1998.
At this point, we consider why greater cooling has taken place between October and April. Over the past five decades, winter storms in the North Pacific have increased both in frequency and intensity, according to Graham and Diaz (2001). At mid-latitudes, relatively large latent and sensible heat fluxes occur in the cold sectors of winter storms that are major contributors to the cooling process (Cayan, 1992). The surface winds associated with these storms also increase mixing in the surface layer bringing cooler waters to the surface.
- To conclude, based on the observations from Pacific Grove and the Farallones, surface temperatures are indeed cooling and the overall cooling trend begins in the mid-to-late 1990’s at rates approaching –0.07°C/year. More intense cooling began in the early-to-mid 2000’s at rates approaching –0.10°C/year, and continues up to the present time. Unexpected warming trends were found between May and August at both locations that are not necessarily consistent with increased coastal upwelling but instead may reflect the growing importance of global warming and its effect on surface heat fluxes over the eastern North Pacific. Significant cooling has occurred between October and April since the late 1990’s, consistent with the overall cooling trend that has been observed. Increased winter storm activity may be primarily responsible for this cooling.
- Finally, the Pacific Decadal Oscillation appears to be an important factor in contributing to cooler temperatures along the central California coast. SSTs at the Farallones and Pacific Grove are highly correlated with the PDO, consistent with its importance off central California. In closing, the results presented here should not be considered in any sense conclusive but rather a starting point for further study to help improve our understanding of how and why the waters along the central California coast are cooling.
DiscussionReferences: Bakun, A., 1990. Global climate change and intensification of coastal ocean upwelling. Science, 247, 198-201. Bond, N.A., J.E. Overland, M. Spillane, and P. Stabeno, 2003. Recent shifts in the state of the North Pacific. Geophysical Research Letters, 30, 2183, doi:10.1029/2003GL018597. Booth, J.A.T., E.E. McPhee-Shaw, P. Chua, M. Denny, R. Phillips, S.J. Bograd, L.D. Zeidberg, and W.F. Gilly, 2010. Hypoxic, acidic intrusions into nearshore environments on the California coast. (in preparation). Breaker, L.C., 2005. What’s happening in Monterey Bay on seasonal to interdecadal time scales. Continental Shelf Research, 25, 1159-1193. Capet, X.J., P. Marchesiello, and J.C. McWilliams,2004. Upwelling response to coastal wind profiles. Geophysical Research Letters, 31, L13311, doi:10.1029/2004GL020123. Cayan, D.R., 1992. Latent and sensible heat flux anomalies over the northern oceans: driving the sea surface temperature. Journal of Physical Oceanography, 22, 859-881. Di Lorenzo, E., A.J. Miller, N. Schneider, and J.C. McWilliams, 2005. Garcia-Reyes, M., and J. Largier, 2010. Observations of increased wind- driven coastal upwelling off central California. Journal of Geophysical Research, 115, C04011, doi: 10.1029/2009JC005576. Graham, N.E., and H.F. Diaz, 2001. Evidence for intensification of North Pacific winter cyclones since 1948. Bulletin of the American Meteorological Society, 82, 1869-1893. Lluch-Cota, D.B., W.S. Wooster, and S.R. Hare, 2001. Sea surface temperature variability in coastal areas of the Northeastern Pacific related to the El Nino-Southern Oscillation and the Pacific Decadal Oscillation. Geophysical Research Letters, 28, 2029-2032. McGowan, D.R. Cayan, and L.M. Dorman, 1998. Climate-ocean variability and ecosystem response in the Northeast Pacific. Science, 281, 210-217. Mendelssohn, R., and F.B. Schwing, 2002. Common and uncommon trends in SST and wind stress in the California and Peru-Chile current systems. Progress in Oceanography, 53, 141-162. Peterson, W.T., and F.B. Schwing, 2003. A new climate regime in northeast Pacific ecosystems. Geophysical Research Letters, 30, 1896, doi: 10.1029/2003GL017528.
Study MethodsPacific Grove and South Farallon Island were selected as study sites since they have been shown to be representative of coastal temperatures elsewhere. Data were (and continue to be) collected daily at 8:00am PST, using a bucket sampler and a hand-held thermometer. Two or three samples are often taken and then averaged, although the variation is usually small (within a tenth of a degree). These data records extend to the 1920s, and unlike records from NDBC buoys are not subject to frequent gaps in the data. The year 1975 was chosen as a starting point since it includes the major regime shift that occurred in 1976-77 as well as several major El Niños. This time horizon allowed for a broader framework in which to interpret the recent cooling pattern.
Figures and Images
Figure 1. Monthly-averaged sea surface temperatures at Pacific Grove for the period from 1975 through 2010 (upper panel) and the Farallon Islands (lower panel) for the period from 1975 through 2009. The red and blue curves represent smoothed versions of the data.
Figure 2. The months of February and July at Pacific Grove are shown for the period from 1975 through 2009. Smoothed versions of the data are shown in red.
Figure 3. Correlations between the Farallones (red), Pacific Grove (blue), and the PDO for the yearly-averaged data from 1975 through 2009.