La Nina deepens in Negative Pacific Decadal Oscillation

One of the more interesting natural oscillations of climate is the Pacific decadal oscillation,This is a phase reversal in climatic events and sees a return of cooler then normal SST to the Pacific in its negative phase as opposed to the higher then normal sst during the positive phase. Anomalous responses are seen in deeper or more persistent climate regimes following phase reversals of the PDO.

As we see here with the WMO La Nina outlook.

La Niña conditions are now present in the central and eastern Equatorial Pacific, where sea surface temperatures are now about 1.5 degrees Celsius colder than normal. The atmosphere in this region is now closely coupled to this sea surface temperature pattern, with trade winds strengthened and cloudiness reduced.

Below the surface of the central and eastern Equatorial Pacific, conditions also reflect the presence of La Niña. Water just below the surface is typically 1 to 3 degrees Celsius colder than normal, and seems primed to reinforce the already cold waters at the ocean surface. Almost all forecast models interpret the current situation as one that is ready to sustain a La Niña event for the next 3-6 months into the first quarter of 2008.

While the increasing possibility of this La Niña development has been reported throughout 2007, the actual development occurred in two phases. Developments early in the year hinted at a La Niña event, but the situation stalled during April-June. Then, during July-September, developments surged again toward La Niña conditions. The pause during April-June suggests that the development of this event has been unusual and somewhat delayed compared to the majority of La Niña events that show initial indications earlier in the year.

The Pacific Decadal Oscillation and Pacific Regime Shifts

Mantua et al. (1997) coined the term “Pacific Decadal Oscillation” (PDO) to describe the interdecadal climate variability associated with regime shifts initiated in 1925, 1947, and 1977. The canonical pattern of PDO sea surface temperature (SST) variations indicating an east-west see-saw in anomalies: when SSTs are above average in the northeast Pacific, they tend to be below average in the central and western North Pacific, and vice versa. This pattern is clearly evident in the decadal changes observed following the 1976/77 regime shift.

The PDO is often described as a long-lived El Niño-Southern Oscillation (ENSO)-like pattern of Pacific climate variability (Zhang et al. 1997). As seen with ENSO, extremes in the PDO pattern are marked by widespread variations in Pacific Basin and North American climate Viewed from another perspective, extremes in the (tropical)ENSO cycle often influence North Pacific climate in PDO-like ways. The exceptional tropical El Niño event of 1997-1998 is a clear case in point, wherein changes in tropical rainfall and atmospheric circulation “forced” strong and persistent climate anomalies over the North Pacific (Barnston et al. 1999).Two main characteristics distinguish the PDO from ENSO. First, typical PDO “events” have shown remarkable persistence relative to that attributed to ENSO events.

In this century, major PDO regimes have persisted for 20 to 30 years. Second, the climatic fingerprints of the PDO are most visible in the North Pacific/North American sector, while secondary signatures exist in the tropics. The opposite is true for the year-to-year climate changes associated with ENSO.

Speculation suggests that 1998 may have witnessed the latest PDO climate regime shift (Hare and Mantua 2000, Schwing and Moore 2000), in this case shifting from warm (positive) to cool (negative) PDO conditions. Coincident with the demise of the extreme 1997-98 (tropical) El Niño event, SSTs along the Pacific coast of North America and in the Bering Sea cooled to below average values while SSTs warmed to above average values in the interior north Pacific. This pattern of SST anomalies bears some resemblance to the cool PDO pattern, and the PDO index has been mostly negative from mid-1998 to mid-2002

Mantua 2002

As this simplistic cartoon shows we see persistence in each regime as the ball remains in the well of each phase for 20-30 years before returning over the lisp into the other regime. This of course reduces the predictability of GCM as the colder then normal sst effectively returns the energy exchange component to zero.