What's happening in Monterey Bay on seasonal to interdecadal time scales

Abstract

Daily observations of sea-surface temperature (SST) have been acquired at the southern end of Monterey Bay in Pacific Grove, California since 1919. It is one of the longest oceanographic records off the west coast of North America. The record is examined to determine the major sources of variability in Monterey Bay and beyond, on time scales from seasonal to interdecadal. On seasonal time scales, the spring transition to coastal upwelling, often a major event along the coast of central California, is not well-expressed inside the bay but is detectable, occurring, on average, between mid-March and mid-April. The onset of the Davidson Current in Monterey Bay is well-defined, occurring, on average, in mid-October, ±2–3 weeks. Intraseasonal changes also occur during the spring and summer that may correspond to intrusions of warmer offshore waters into Monterey Bay. Intraseasonal oscillations with periods in the range of 40–50 days occur in Monterey Bay, but compared to their signature along the open coast, their event-like behavior is modified. The annual cycle of surface temperature in Monterey Bay is asymmetric with seasonal warming occurring during the spring and summer, and cooling during the fall. This asymmetry is primarily due to the net surface heat exchange which is positive for most of the year, and, to a lesser extent, the influence cold upwelled waters that are advected into the bay during the spring and summer, observations supported by a simple model that combines both the net surface heat exchange and thermal advection. On interannual time scales, the influence of El Niño warming events is strong. A comparison with the Northern Oscillation Index (NOI) using Singular Spectrum Analysis (SSA), shows that the El Niño signal is often as strong in SST at Pacific Grove as it is in the NOI. On interdecadal time scales, the influence of the Pacific Decadal Oscillation (PDO) is also relatively strong in Monterey Bay, again based on SSA. The integrated anomaly was calculated from the record and reveals regime shifts in Monterey Bay that occurred in 1929, an event that was apparently regional in scale, reflecting a transition from unusually cold to warmer conditions, and the regime change in the PDO that occurred in 1976. Each regime change can be approximated by a step-wise increase in temperature. Finally, linear trends were estimated for the entire record (∼+0.01 °C/year), and for the 72-year period from 1930 to 2001 (+0.0042 °C/year), i.e., following the regional regime change in 1929. The estimated trend for the last 72 years is not statistically significant; however, it is in close agreement with the long-term trend for the Intergovernmental Panel on Climate Change (IPCC) record of global surface temperature that spans almost 140 years (∼+0.005 °C/year). Although the long-term increase in SST at Pacific Grove appears to be consistent with global warming, the integrated anomaly suggests that temperature increases in Monterey Bay have occurred rather abruptly and thus it becomes more difficult to invoke the global warming scenario. Finally, based on the monthly averaged data, the annual cycle, El Niño warming episodes, the PDO, the long-term trend, and the semiannual cycle account for approximately 44%, 18%, 6%, 4%, and 3% of the total variance, respectively, in SST at Pacific Grove.