In climatology, how does the La Nina phenomenon primarily affect sea surface temperatures in the Pacific Ocean?

Introduction / Context:
La Nina is a major component of the El Nino Southern Oscillation system and plays a significant role in global climate variability. Competitive exams often ask conceptual questions about how La Nina and El Nino affect ocean temperatures and weather patterns. Understanding these phenomena helps in topics such as monsoon variability, droughts, floods and global climate anomalies. This question focuses specifically on the effect of La Nina on sea surface temperatures in the Pacific Ocean, which is a key point in climatology.

Given Data / Assumptions:

• The phenomenon under discussion is La Nina.
• The region concerned is the Pacific Ocean, particularly equatorial and eastern Pacific.
• The options describe changes in salinity or temperature of ocean water.
• We assume a basic understanding of normal, El Nino and La Nina conditions in the tropical Pacific.

Concept / Approach:
Under La Nina conditions, trade winds strengthen and push warm surface waters further towards the western Pacific. This increases upwelling of cold, deep water in the central and eastern Pacific. As a result, sea surface temperatures in these regions fall below average, leading to a cooler than normal Pacific. The phenomenon is therefore associated with cooling of sea surface temperatures rather than changes in salinity. The approach to solve this is to connect La Nina with enhanced cold water upwelling and recognise that the primary effect is a drop in temperature in the eastern and central Pacific.

Step-by-Step Solution:
Step 1: Recall that in normal conditions, warm water accumulates in the western Pacific and cooler water exists in the eastern Pacific due to upwelling. Step 2: During La Nina, the trade winds become even stronger than usual. Step 3: These stronger winds push more warm surface water westward, increasing upwelling of cold deep water in the eastern and central Pacific. Step 4: This increased upwelling causes sea surface temperatures in those regions to be cooler than the long term average. Step 5: Therefore, the principal effect of La Nina on the Pacific Ocean is that it cools down the temperature of the surface waters.

Verification / Alternative Check:
Meteorological agencies such as the World Meteorological Organization and national weather services define La Nina events based on sustained below average sea surface temperatures in the central and eastern equatorial Pacific. They monitor temperature anomalies and declare La Nina when these anomalies fall below a certain negative threshold. There is no such standard definition based on salinity changes. Climate impact discussions also emphasise that La Nina is associated with cooler Pacific waters and related shifts in rainfall and atmospheric circulation, confirming that cooling is the primary effect described in the question.

Why Other Options Are Wrong:
Decreases salinity of ocean and increases salinity of ocean are not standard descriptors of La Nina. While local salinity can vary due to rainfall and evaporation, La Nina is defined by temperature anomalies, not salinity changes. Maintains stable temperature of water is also incorrect because La Nina specifically indicates a departure from average conditions, namely cooler than normal sea surface temperatures in the equatorial Pacific. Therefore, only the option stating that it cools down the temperature of water aligns with the climatological definition of La Nina.

Common Pitfalls:
Students sometimes confuse La Nina with El Nino or mix up which event causes warming and which causes cooling in the Pacific. Another pitfall is focusing on secondary effects, such as rainfall patterns, while forgetting the primary criterion used to define these events. To avoid these errors, remember a simple rule: El Nino is associated with warming of the eastern and central Pacific, while La Nina is associated with cooling. Creating a comparison table of normal, El Nino and La Nina conditions for winds, sea surface temperature and rainfall can greatly enhance understanding.

Final Answer:
The La Nina phenomenon primarily cools downs the temperature of water in the Pacific Ocean, so that is the correct option.