Todd P. Mitchell

The annual cycle in equatorial convection and sea surface temperature

Abstract The coupled atmosphere–ocean system in the equatorial eastern Pacific and Atlantic exhibits a distinct annual cycle that is reflected in contrasting conditions at the times of the two equinoxes. The contrasts are so strong that they dominate the annual march of zonally averaged outgoing longwave radiation for the equatorial belt. The March equinox corresponds to the warm season when the equatorial cold tongues in the eastern Pacific and Atlantic are absent. With the onset of summer monsoon convection over Colombia, Central America, and West Africa in May–June, northward surface winds strengthen over the eastern Pacific and Atlantic, the equatorial cold tongues reappear, and the marine convection shifts from the equatorial belt to the intertropical convergence zones (ITCZs) along 8°N. As the northern summer program the ITCZs remain strong and shift northward to new 10°N, while sea surface temperature (SST) continues to drop over the cold tongues and the southern tropics, perhaps in response to the...

On the structure and evolution of ENSO‐related climate variability in the tropical Pacific: Lessons from TOGA

Improved observations in the tropical Pacific during the Tropical Ocean-Global Atmosphere (TOGA) program have served to corroborate preexisting notions concerning the seasonally dependent relationships between sea surface temperature, sea level pressure, wind stress, rainfall, upper tropospheric circulation, and ocean thermal structure anomalies in the El Nino-Southern Oscillation (ENSO) phenomenon. However, the paradigm of a quasiperiodic “ENSO cycle,” phase locked with the annual march, does not capture the complexity of the evolution of the anomalies. The inadequacy of this model was particularly apparent during the second half of TOGA when the variability was highly aperiodic. Also, a single modal structure or empirical orthogonal function does not appear to be capable of representing the range of spatial patterns of ocean-atmosphere interaction in the tropical Pacific. These results suggest the need for a more inclusive phenomenological description of ENSO. Data collected during TOGA serve to confirm the influence of tropical Atlantic sea surface temperature anomalies upon rainfall in northeast Brazil.

The Influence of Sea-Surface Temperature on Surface Wind in the Eastern Equatorial Pacific: Seasonal and Interannual Variability

Abstract The climate of the eastern Pacific exhibits a pronounced equatorial asymmetry. Boundary layer air originating in the Southern Hemisphere trades crosses the equator and flows into the intertropical convergence zone (ITCZ), whose southern limit is nearly always located at least 4° to the north of the equator. The sea-surface temperature (SST) distribution is characterized by a prominent “cold tongue” centered ∼ 1°S, a strong frontal zone centered ∼ 2°N, and a warm eastward current centered near 5°N. The surface wind field exhibits a pronounced horizontal divergence as the air flows northward across the oceanic frontal zone. These features vary in strength in response to the annual cycle and the El Nino/Southern Oscillation phenomenon. The northward cross-equatorial surface winds, the cold tongue and the frontal zone all tend to be strongest during the cold season (July through November). During the cold season of the coldest years, when the cold tongue is most prominent, the cross-equatorial flow t...

How are climate and marine biological outbreaks functionally linked

Since the mid-1970s, large-scale episodic events such as disease epidemics, mass mortalities, harmful algal blooms and other population explosions have been occurring in marine environments at an historically unprecedented rate. The variety of organisms involved (host, pathogens and other opportunists) and the absolute number of episodes have also increased during this period. Are these changes coincidental? Between 1972 and 1976, a global climate regime shift took place, and it is manifest most clearly by a change in strength of the North Pacific and North Atlantic pressure systems. Consequences of this regime shift are: (1) prolonged drought conditions in the Sahel region of Africa; (2) increased dust supply to the global atmosphere, by a factor of approximately four; (3) increased easterly trade winds across the Atlantic; (4) increased eolian transport of dust to the Atlantic and Caribbean basins; and (5) increased deposition of iron-rich eolian dust to typically iron-poor marine regions. On the basis of well-documented climate and dust observations and the widely accepted increase in marine outbreak rates, this paper proposes that the increased iron supply has altered the micronutrient factors limiting growth of opportunistic organisms and virulence of pathogenic microbes, particularly in macronutrient-rich coastal systems.

ENSO Seasonality: 1950–78 versus 1979–92

Abstract ENSO-related seasonal-to-interannual variability in the Pacific basin is documented, based on marine surface observations of monthly mean sea surface temperature, sea level pressure, and wind, together with satellite-based estimates of rainfall and mean tropospheric temperature. Anomalies in these fields are linearly regressed onto simultaneous values of an index of equatorial Pacific SST anomalies. The analysis is performed separately on the data for earlier (1950–78) and later (1979–92) epochs of the record. The analyses are further stratified in terms of the climatological-mean warm and cold seasons in the equatorial Pacific, which correspond to January–May and July–November, respectively. Composite SST, wind, and rainfall fields for he warm and cold seasons that fall within typical warm and cold ENSO episodes are also presented. Despite the dramatic differences in the sequencing of ENSO warm episodes with respect to the annual march in the two epochs, the anomaly patterns are found to be rema...

Correlation Structure of the El Niño/Southern Oscillation Phenomenon

Abstract Relationships among the atmospheric phenomena associated with the Southern Oscillation and El Nino are investigated, using the Comprehensive Ocean-Atmosphere Data Set (COADS) of marine surface observations from ships of opportunity and the World Monthly Surface [Land] Station Climatology (WMSSC) for the period 1950-79. Annual mean (April–March) sea level pressure at Darwin, Australia is used as an index of the Southern Oscillation. Results are based on simple linear correlation techniques stratified by season as in the Rasmusson and Carpenter (1982) composite. Correlations on the order of +0.9 are observed between Darwin pressure, sea surface temperature (SST) and rainfall in the equatorial central Pacific, and zonal wind in the equatorial western Pacific. Relations among these variables are strongest from July through November, when the month to month autocorrelation is also at its strongest. Sea surface temperature along the Peruvian coast and pressure in the eastern Pacific are also most stron...

The variability of wintertime precipitation in the region of California

Abstract Rotated principal component (RPC) analysis, subject to the varimax criterion and including area weighting, is applied to a 58-yr record (1931–88) of monthly- and seasonal-mean Climatic Division precipitation anomalies for the contiguous United States to document wintertime precipitation variability in the region of California. Rotated principal components (time series) derived from this analysis are related to anomalies of seasonal-mean global sea surface temperature, and monthly mean Northern Hemisphere 500-hPa geopotential height and sea level pressure (SLP). Wintertime seasonal-mean precipitation in California is captured by two RPCs. The first RPC documents coherent precipitation anomalies centered in northern California, Oregon, southern Idaho, and eastern Washington, and explains the largest portion of area-averaged variance of any of the patterns in the decomposition. A second RPC captures coherent precipitation variability in the south coast and southeast desert regions of California, sou...