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The North American Monsoon

Modern scatterometry (QuikSCAT) provides nearly daily maps of the near-surface winds over the ocean at ~ 25-km resolution and makes it possibile for the first time to characterize with a high degree of accuracy the dynamical state of the lower troposphere over otherwise poorly sampled regions. In my research work, the QuikSCAT wind data are being used to quantify the changes in the low-level circulation associated with the monsoon and, with a specific focus on the Gulf of California (GoC), to investigate the contribution to the moisture transport by transient processes, namely gulf surges, at the core of the monsoon. Since the width of the GoC ranges from 150 to 300 km, the spatial resolution of QuikSCAT winds provides a considerable number of data points within the span of the gulf, thus allowing unprecedented spatial and temporal resolution of the summertime circulation in the oceanic monsoon region.

Figure 1: Monsoon index as computed as the ration between the JJA to the total annual precipitation from the GPCP 1DD dataset.

 

Supporting past observations mainly collected during dedicated field studies, six summers (1999 - 2004) of QuikSCAT winds show that the onset of the monsoon season is accompanied by a seasonal reversal of the flow along the GoC, with the establishment of a time-mean southerly wind throughout the gulf. In addition to demonstrating robust seasonality of the GoC winds, our analysis shows that this seasonal reversal happens quite abruptly in late spring, preceding the rains and the warming of the GoC SSTs.

Figure 2: Mean streamlines of the 10-m winds over the GoC as derived from the QuikSCAT data, for (left) DJF and (right) JJA. Contours are mean daily precipitation in mm/day.

In the heart of the monsoon season, the time-mean flow is found to be composed of periods of enhanced southerly winds associated with gulf surges. An EOF analysis of the daily summertime along-shore wind anomalies over the GoC identifies gulf surges as the leading mode of synoptic-scale variability in the low-level winds, thus confirming the view of these transients as a dynamical mode distinct from the background flow and motivating our interpretation of the leading principal component time series as an objective index of gulf surge occurrence.


Figure 3: (left) Latitude-time hovmoller diagram of the along-shore wind for JJAS 2000 averaged in longitude accross an oceanic strip along and south the GoC from 20N to 21N. (right) Leading EOF of the summertime wind anomalies over the northeast Pacific.

We use this gulf surge index as a reference time series for regression analysis and compositing of meteorological fields of interest both from the NCEP and the higher-resolution ERA-40 reanalysis, to explore the relationship between gulf surges and precipitation over the core and marginal regions of the monsoon, as well as the manifestation of these transient events in the large-scale circulation. It is found that, although seemingly mesoscale features confined over the GoC, gulf surges are intimately linked to patterns of large-scale variability of the eastern Pacific ITCZ and greatly contribute to the definition of the northward extent of the monsoonal rains.

Click [here] to see an animation of lagged regressions of the QuikSCAT winds onto the leading PC.
Click [here] to see an animation of the lagged regressions of precipitation onto the leading PC.

Further details on our analysis methods and results can be found in our 2004 GRL paper and our 2006 MWR paper.

 

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