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Investigaciones marinas

versión On-line ISSN 0717-7178

Investig. mar. v.30 n.1 supl.Symp Valparaíso ago. 2002 

The precipitation regimes along Chile
and California at the extremes of the
ENSO and ENSO-like cycles*

José A. Rutllant

Department of Geophysics. Universidad de Chile,
Casilla 2777, Santiago, Chile,

Most of the annual precipitation in the mediterranean climates along the Humboldt and California eastern boundary current systems falls in winter, particularly in the subtropics and low-mid-latitudes. There, precipitation events are associated with the passage of frontal disturbances stemming from extratropical low-pressure systems embedded in the belt of the westerlies that strengthens and shifts equatorward in the low-sun season.

Superimposed on this synoptic-scale variability (roughly weekly weather rhythms) characteristic large-scale atmospheric circulation anomaly patterns develop in connection with atmospheric heating anomalies near the equatorial Pacific at intraseasonal to interdecadal time-scales. At interannual time-scales the signal of the ENSO cycle dominates those rainfall anomaly patterns, particularly over the eastern Pacific basin.

The interannual "warm" phase of the atmospheric heating anomalies over the central equatorial Pacific is associated with the warm phase of the ENSO cycle (El Niño).

The warm extreme of the ENSO cycle coincides with a depressed subtropical anticyclone in the SE Pacific and with Rossby-like standing wave-trains of alternating circulation anomaly patterns extending from the tropics eastward towards subpolar latitudes over the winter hemisphere (atmospheric teleconnection patterns). Conversely, the "cold" phase of the ENSO cycle (La Niña) is associated with suppressed convection over the central equatorial Pacific, a strong subtropical anticyclone in the SE Pacific and the poleward shift of the westerlies and associated frontal disturbances. These patterns at the extremes of the ENSO cycle result into a tendency to enhanced /suppressed winter-spring rainfall at the subtropics and low-mid latitudes during El Niño/La Niña events in central Chile and California.

The winter hemisphere atmospheric teleconnection patterns are very similar in terms of their equivalent-barotropic nature and the alternation of cyclonic circulation anomalies at the subtropics and low-mid-latitudes (enhanced and eastward shifted South Pacific Convergence Zone and Aleutian low) and anticyclonic ones at high-latitudes (Bellingshausen Sea and Alaska). These teleconnection patterns have been named PSA (Pacific South America) and PNA (Pacific North America) in the SE and NE Pacific, respectively.

The high-latitude anticyclonic PSA anomaly pattern over the Bellingshausen Sea area favours the blocking of the westerlies (blocking highs) and the subsequent diversion of the storm tracks towards mid-latitudes. This phenomenon, together with a weakened subtropical anticyclone, explains the excess precipitation in central Chile during the southern winter/spring coinciding with the development of the warm phase of the ENSO cycle. The culmination of this El Niño event usually occurs during the following austral summer, when the PNA anomaly pattern (northern winter) often results in excess rainfall in California.

Polewards of the area where excess precipitation has its larger impact, gradual rainfall deficit is to be expected. Satellite-derived precipitation data show this feature at high latitudes over the eastern Pacific, with a stronger signal in the Southern Hemisphere. Local rain-gauge data confirm this tendency in southern Chile, although the precipitation deficit at about 40 ×S tends to occur in austral summer. High latitude blocking associated with PNA and PSA anomaly circulation patterns produces warm advection along the Alaska coast and cold advection along the western sector of the Antarctic Peninsula respectively. As a result of the persistency of blocking highs over the Bellingshausen Sea, the cold advection west of the Antarctic Peninsula results in the equatorward shift of the ice edge and negative anomalies in the SST, consistent with the corresponding phase of the Antarctic Circumpolar Wave.

In order to illustrate the austral winter persistence of the PSA anomaly pattern during the development phase of the last three major El Niño, 500 hPa zonal geopotential anomalies are presented, highlighting a meridional dipole off the Chilean coast consistent with the PSA teleconnection pattern.

The general tendency towards wet winters during the warm extreme of the ENSO cycle is strongly modulated by intraseasonal variability. In fact Madden - Julian Oscillations (MJO's) in the period range of 30-60 days produce "ENSO-like" conditions in the equatorial Pacific. The warm/cold phase of the ENSO cycle is mimicked here by the convective/subsident phase of the MJO in the central equatorial Pacific and the subsident/convective phase in the western equatorial Pacific. These MJO together with submonthly-scale variability, as identified for California, are responsible for wet and dry spells during a particularly wet winter. The enhanced activity of the MJO often observed in November-February, preceding the onset of the El Niño (e.g. 1996 and 2001), can lead to excess rainfall in California and early-winter strong precipitation events in central Chile separated by relatively dry periods. Examples of this behaviour include the austral rainy winters of 1987 and 1997 in central Chile when dry spells occurred in June 1987 and July 1997

In order to investigate the connection between the MJO and blocking events in the far southeastern Pacific, an MJO Index near the dateline at the equatorial Pacific based on outgoing long-wave radiation data and a blocking index centred about 90¥ W and 50¥ S have been cross-correlated without a significant covariance at intraseasonal time scales, probably reflecting other sources and scales of variability including the position of the blocking highs, the strength and latitudinal position of the enhanced subtropical jet-stream associated with the anomalous convection over the equatorial Pacific, and the phase of the Antarctic Circumpolar Wave.

At interdecadal time-scales, after excluding the interannual ENSO signal, ENSO-like circulation anomaly patterns appear again but mainly in spring at mid to high latitudes in both hemispheres. These regime transitions, although well correlated with the abundance of some fish particularly in the NE Pacific, seem to be the result of a combination of interannual variability with long-term positive trends in global average SST, as evidenced by recent research results.

* Keynote presentation.

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