Subseasonal-to-interannual variability of rainfall over New Caledonia (SW Pacific)
Résumé
Daily rainfall occurrence and amount at 55 stations over New Caledonia (NC, 20A degrees S, 166A degrees E) are examined throughout the calendar year during 1980-2010 using a Hidden Markov Model (HMM). Daily rainfall variability is described in terms of six discrete rainfall states identified by the HMM. Three states are interpreted as trade wind regimes associated with persistent low level anticyclonic conditions and weak to strong easterlies. The most prevalent state (state 1; 36 % of days) is dry everywhere and is characterized by an elongated anticyclone centered around Australia; NC is located on the eastern edge of this anticyclone. This state is predominant from mid-May to mid-December and peaks in September. The second most prevalent state (state 2; 27 % of days) and the last trade regime (state 4; 12 % of days) are most frequent in austral summer. States 2 and 4 are associated with the subtropical anticyclone centered south of NC, close to its climatological location in austral summer, and light (state 2) to moderate (state 4) rainfall mostly along the windward coast. A distinct state (state 3; 11 % of days) is mostly associated with wintertime extratropical eastward traveling troughs between New Caledonia and New Zealand, inducing light rainfall over the SW of the main island of New Caledonia. The two last states 5 and 6 are infrequent (< 15 % of days) but account for almost 70 % of total rainfall across the archipelago. They occur mostly in austral summer with their frequency of occurrence peaking in late February-early March. These states are associated with synoptic wave traveling eastward from eastern Australia leading to a strong influx of moisture from the equatorial latitudes when the associated cyclonic centre is located west of New Caledonia. The wettest state is also strongly modulated by intra-seasonal 15-80 days variability. These events contribute to intermittent southwestward shifts of the South Pacific Convergence Zone from its mean location northeast of New Caledonia. On interannual timescales, the occurrence of the rainfall states is modulated by El Nio Southern Oscillation (ENSO) events. The ENSO impact is strongest from mid-August to March with more (less) frequent trade regimes 1-2 and less (more) frequent unstable trade regime 4 and states 5-6 during central-Pacific warm (cold) ENSO events. Stochastic simulations of daily rainfall occurrence and amount at the 55 stations are generated by using predictors based on Nio 4 sea surface temperature index and a local intra-seasonal (15-80 days) OLR index superimposed on a climatological annual cycle. The cross-validated skill peaks in September-November.