Abstract Author: Yan Xue
Abstract Title: The 2007/08 La Niña Cycle: Evolution, Prediction and Remote Oceanic Influences
Abstract: La Niña is defined by negative anomalies in the NINO3.4 region less than or equal to -0.5°C. According to the NOAA operational definition, the 2007/08 La Niña cycle started in August 2007, peaked in February 2008 with a monthly value in the NINO3.4 region of -1.9°C, and diminished in June 2008. Based on NCEP’s Optimal Interpolation SST analysis, the February 2008 NINO3.4 value was the coldest since February 1989. This strong La Niña event was noteworthy because ENSO variability has been relatively weak during 2001-2006.
The evolution of the La Niña cycle is illustrated using typical atmospheric and oceanic observational data, and compared to the canonical evolution of La Niña. The onset, growth, and decay of La Niña are analyzed using a heat budget analysis for the mixed layer temperature. Zonal/meridional advection and vertical entrainment of temperature are calculated with variables from the NCEP’s Global Ocean Data Assimilation System (GODAS), and surface heat fluxes from the NCEP CDAS and CDAS2, and JRA-25. Pentad GODAS fields are used to examine oceanic impacts due to the strong Madden-Julian Oscillation (MJO) observed during November 2007 - February 2008, and its influence on the SST forecast is quantified by the NCEP Climate Forecast System (CFS). The sudden weakening of La Niña from February to March 2008 and its association with the sudden warming in the far eastern Pacific and recharge of the equatorial heat content are also investigated.
Although the NCEP CFS has a competitive hindcast skill for ENSO, it performed poorly during the onset and decay phase of the La Niña. Although the CFS predicted La Niña to emerge during 2007, the CFS SST forecast was systematically too cold during spring/summer 2007, and the model completely missed the decay phase of the La Niña during early spring 2008.
Significant SST variability beyond the tropical Pacific was observed during the La Niña cycle. A mini positive Indian Ocean Dipole (IOD) event occurred during September-October 2007, which contributed to suppressed atmospheric convection over the Maritime Continent at the time. In the North Pacific, the pattern of SSTs projected onto a strong negative Pacific Decadal Oscillation (PDO) pattern from September 2007 to at least July 2008 (the present time). Impacts of La Niña on the PDO and tropical Indian and Atlantic Ocean are also investigated.