Dr. Naomi H. Naik

Naomi's Work Site

[1] An important issue in modeling and predicting upper ocean variability is the nature of the interactions between ocean biology, ocean dynamics, and irradiance penetration. Numerous studies using in situ observations and model simulations to investigate the effects of biota on light penetration have demonstrated that this biological- physical feedback may be significant over a wide range of spatial and temporal scales. Using a general circulation model which takes into account interannual variations in surface chlorophyll for the period September 1997 to May 2003, we investigate the effect of varying chlorophyll concentration on surface temperature. We conclude that, by using climatological monthly mean chlorophyll values, we capture the first-order effect of chlorophyll on light penetration.

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The causes of persistent droughts and wet periods, or pluvials, over western North America are examined in model simulations of the period from 1856 to 2000. The simulations used either (i) global sea surface temperature data as a lower boundary condition or (ii) observed data in just the tropical Pacific and computed the surface ocean temperature elsewhere with a simple ocean model. With both arrangements, the model was able to simulate many aspects of the low-frequency (periods greater than 6 yr) variations of precipitation over the Great Plains and in the American Southwest including much of the nineteenth-century variability, the droughts of the 1930s (the "Dust Bowl") and 1950s, and the very wet period in the 1990s. Results indicate that the persistent droughts and pluvials were ultimately forced by persistent variations of tropical Pacific surface ocean temperatures. It is argued that ocean temperature variations outside of the tropical Pacific, but forced from the tropical Pacific, act to strengthen the droughts and pluvials. The persistent precipitation variations are part of a pattern of global variations that have a strong hemispherically and zonally symmetric component, which is akin to interannual variability, and that can be explained in terms of interactions between tropical ocean temperature variations, the subtropical jets, transient eddies, and the eddy-driven mean meridional circulation. Rossby wave propagation poleward and eastward from the tropical Pacific heating anomalies disrupts the zonal symmetry, intensifying droughts and pluvials over North America. Both mechanisms of tropical driving of extratropical precipitation variations work in summer as well as winter and can explain the year-round nature of the precipitation variations. In addition, land-atmosphere interactions over North America appear important by (i) translating winter precipitation variations into summer evaporation and, hence, precipitation anomalies and (ii) shifting the northward flow of moisture around the North Atlantic subtropical anticyclone eastward from the Plains and Southwest to the eastern seaboard and western Atlantic Ocean.

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